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To determine whether xylem sap and leaf cytokinin levels were associated with live root length density (LRLD) and leaf senescence, these variables were measured in intact (control) plants and those from which florets were removed, in hybrids of contrasting post-anthesis senescence patterns. Field experiments were carried out during two consecutive seasons using sunflower hybrids (Paraiso75, stay-green [SG]; and Paraiso65, fast dry down [FDD]) of contrasting canopy senescence patterns. Crop carbon partitioning was altered by removing all florets (-F treatment) before anthesis. At crop level, the dynamics of LRLD, xylem sap flow rate, cytokinins in the xylem sap, and green leaf area index (GLAI) were measured, while at leaf level, dynamics of total chlorophyll content, trans-Zeatin content, and net photosynthesis were recorded for leaf positions 17, 20, 22, 24 and 27. In both hybrids, the –F treatment had higher LRLD, cytokinins in the xylem sap, and leaf trans-Zeatin concentration at anthesis and during the grain filling period than control plants. All measured variables decreased earlier in the control treatment than the -F treatment (in both hybrids), and in the FDD hybrid than the SG hybrid. The dynamics of xylem sap cytokinins were related to the dynamics of LRLD and that of leaf trans-Zeatin levels, contributing to contrasting canopy senescence patterns and underlying the responses to defruiting in both hybrids.

Winter mechanical hedge pruning of olive trees can result in a strong post-pruning shoot growth response and a low return flowering intensity the following year. However, post-pruning practices including tip heading, watersprout thinning, and the application of the growth inhibitor mepiquat chloride (MC) may positively reduce subsequent vegetative growth and promote flowering. Two experiments were carried out with the aim of addressing the following questions: (1) Do these post-pruning practices applied on trees that received simulated mechanical pruning during the winter reduce growth and increase flowering in the spring of the following year?; and (2) What is the optimal MC dose in unpruned trees needed to increase flowering the following spring? In the first experiment, five-year-old olive trees (cv. Arbequina) growing in a fairly low density orchard (208 trees ha−1) were hedge pruned at the end of winter using manual clippers on the east and west sides to simulate mechanical disk pruning, and the post-pruning treatments were applied at the pit hardening stage (early summer). In the second experiment, foliar or soil application of MC was performed at pit hardening in unpruned trees. The results show that tip heading of new shoots led to a large number of lateral shoots that flowered little, or not at all, the following spring. In contrast, the watersprout thinning treatment had sufficient flowering to significantly increase fruit number and yield the following year compared to tip heading. Foliar application of MC (1500 ppm) after winter pruning did not inhibit new shoot growth, return flowering was low, and yield was less than the watersprout thinning treatment. The lack of a post-pruning response to foliar MC was likely related to the dose used (1500 ppm). In the second experiment, a foliar application of MC at 3000 ppm was associated with greater return flowering and yield than an untreated control. In conclusion, thinning of vigorous watersprouts at the pit hardening stage three months after simulated winter mechanical hedge pruning can improve fruit number and yield the next year, especially on a per canopy volume basis. Furthermore, the responses to foliar MC application in unpruned trees suggest that more detailed post-pruning studies with MC application doses in the 2000–3000 ppm range and at additional phenological stages should be performed over several consecutive years to assess whether this practice could be a suitable management tool. Fil: Albarracin, Valeria Mariel. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Regional de Investigaciones Científicas y Transferencia Tecnológica de La Rioja. - Universidad Nacional de La Rioja. Centro Regional de Investigaciones Científicas y Transferencia Tecnológica de La Rioja. - Universidad Nacional de Catamarca. Centro Regional de Investigaciones Científicas y Transferencia Tecnológica de La Rioja. - Secretaría de Industria y Minería. Servicio Geológico Minero Argentino. Centro Regional de Investigaciones Científicas y Transferencia Tecnológica de La Rioja. - Provincia de La Rioja. Centro Regional de Investigaciones Científicas y Transferencia Tecnológica de La Rioja; Argentina Fil: Hall, Antonio Juan. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Parque Centenario. Instituto de Investigaciones Fisiológicas y Ecológicas Vinculadas a la Agricultura. Universidad de Buenos Aires. Facultad de Agronomía. Instituto de Investigaciones Fisiológicas y Ecológicas Vinculadas a la Agricultura; Argentina Fil: Searles, Peter Stoughton. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Regional de Investigaciones Científicas y Transferencia Tecnológica de La Rioja. - Universidad Nacional de La Rioja. Centro Regional de Investigaciones Científicas y Transferencia Tecnológica de La Rioja. - Universidad Nacional de Catamarca. Centro Regional de Investigaciones Científicas y Transferencia Tecnológica de La Rioja. - Secretaría de Industria y Minería. Servicio Geológico Minero Argentino. Centro Regional de Investigaciones Científicas y Transferencia Tecnológica de La Rioja. - Provincia de La Rioja. Centro Regional de Investigaciones Científicas y Transferencia Tecnológica de La Rioja; Argentina Fil: Rousseaux, Maria Cecilia. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Regional de Investigaciones Científicas y Transferencia Tecnológica de La Rioja. - Universidad Nacional de La Rioja. Centro Regional de Investigaciones Científicas y Transferencia Tecnológica de La Rioja. - Universidad Nacional de Catamarca. Centro Regional de Investigaciones Científicas y Transferencia Tecnológica de La Rioja. - Secretaría de Industria y Minería. Servicio Geológico Minero Argentino. Centro Regional de Investigaciones Científicas y Transferencia Tecnológica de La Rioja. - Provincia de La Rioja. Centro Regional de Investigaciones Científicas y Transferencia Tecnológica de La Rioja; Argentina

Crop floret number per unit area and, to a lesser extent grain set, are the determinants of yield that are associated with the increase in sunflower oil yield at crop densities greater than the most frequently used in Argentina of 3–5.1 pl.m−2. A better understanding of the responses of these two yield determinants to crop density and their intra-specific variability could contribute to the development of higher yielding cultivars suited to higher crop densities. In an initial experiment, we measured the aboveground biomass partitioning to leaves and stems between emergence and the end of floret differentiation, the weight and length of the basal internodes whose growth overlapped with the floret differentiation phase, plant height at the star visible (R1) and anthesis (R5.5) stages, and florets per plant in response to crop density across a 0.7–14.3 pl.m−2 range. In Exps. 2 and 3, designed to explore intra-specific variability for responses to crop density we measured, in five hybrids, the last four variables plus oil yield in crops grown at 5.1, 10.2 and 14.3 pl.m−2. Lastly, in Exp. 4, we measured the responses of the basal internode of four of the hybrids used in Exps. 2 and 3 to three levels of red/far red ratio (R/FR) in the light incident on spaced plants. Plant height at the R1 stage and above-ground biomass partitioning to the stem increased significantly with crop density, contrasting with a fall in floret number per plant in Exp. 1. The length and weight of the fourth internode, the internode whose growth period overlapped the most with the floret differentiation phase, increased with crop density, while floret number per plant showed a significant decrease with increasing weight of the fourth internode. The hybrids examined in Exps. 2 and 3 exhibited intraspecific variability in their responses to crop density for plant height at R1 stage, fourth internode length, floret number per plant, in the relationships between floret number per plant with fourth internode length and plant height at R1 stage, and in oil yield. The length of the first internode of the four hybrids examined in Exp. 4 increased consistently with reductions in the R/FR ratio of the light incident on the plants, suggesting that perception of the R/FR ratio by phytochrome may play an important part in the responses of floret number per plant to crop density and its intraspecific variability. The lack of association in some hybrids of the responses of floret number and oil yield per square meter to crop density showed that grain set and unit grain weight are additional yield determinants that require attention when considering intraspecific variability in yield responses to high density.

Key message: More vigorous watersprouts and fewer low vigor shoots form as wood age increases at the cutting points with greater canopy pruning depth in olive trees, which reduces return flowering. Such information is relevant to long-term olive orchard pruning strategies in hedgerows. Abstract: Demographic analysis of growth responses to pruning in fruit trees seeks the quantification of the typology of new shoots originating from the remaining branches. Pruning of hedgerows using mechanical discs is becoming increasingly common in orchards, but little information is available as to how such pruning, which does not discriminate between branch size, wood age at the cutting point, branch type, or position, modifies subsequent new shoot demography. Hence, the experiment described in this study in young olive trees (cv. Arbequina) assessed the following questions: (1) Is the type and growth of new shoots associated with the intensity of mechanical pruning and/or the wood age on which they grow? (2) How many growing seasons does it take for new shoots formed after pruning to flower? The principal hypothesis was that a greater proportion of vigorous watersprouts form compared to low vigor shoots as wood age at the cutting points increases with canopy pruning depth, and that the watersprouts have low flowering potential. Both new shoot growth and return flowering were monitored on exposed supporting wood over several growing seasons after implementing three winter (25W, 50W, and 75W) pruning levels of increasing intensity and one summer pruning (75S) treatment along with an unpruned control (CON). As hypothesized, a greater number and elongation of vigorous watersprouts were found as wood age increased at the cutting points with greater winter pruning intensity, and the watersprouts had low levels of return flowering even 3 full years after pruning compared to the CON. Growth of low vigor shoots was relatively more important than watersprout growth in the severe summer pruning treatment, although 3 years after the summer pruning flowering was not fully recovered. In contrast, the more lightly pruned winter treatments (25W, 50W) did not show significant differences in flowering with the CON at the end of 3 years. Thus, mechanical hedge pruning in olive trees should be light-to-moderate to avoid the formation of watersprouts on older wood, which leads to long-term reductions in flowering. Fil: Albarracin, Valeria Mariel. Centro Regional de Investigaciones Cientificas y TRansferencia Tecnológica La Rioja (CRILAR) - Provincia de La Rioja - UNLaR - SEGEMAR - UNCa - CONICET. ; Argentina. Ministerio de Agroindustria - Subsecretaria de Agricultura, Bs As ; Argentina Fil: Hall, Antonio Juan. Universidad de Buenos Aires, Facultad de Agronomía (IFEVA), CONICET; Argentina Fil: Searles, Peter Stoughton. Centro Regional de Investigaciones Cientificas y TRansferencia Tecnológica La Rioja (CRILAR) - Provincia de La Rioja - UNLaR - SEGEMAR - UNCa - CONICET; Argentina Fil: Rousseaux, Maria Cecilia. Centro Regional de Investigaciones Cientificas y TRansferencia Tecnológica La Rioja (CRILAR) - Provincia de La Rioja - UNLaR - SEGEMAR - UNCa - CONICET; Argentina

Mechanical pruning has become increasingly common in olive orchards, particularly under high tree densities. Large cutting disks make heading cuts at a single canopy depth without discriminating between branch thickness, size, or type of branch. The objectives of this study were to: (i) quantify the responses of vegetative growth over two growing seasons and yield components over three seasons following different intensities and moments of application of mechanical pruning; and (ii) evaluate some leaf morphology and gas-exchange characteristics of the remaining leaves after pruning. Five year-old olive trees with high crop load (cv. Arbequina) were pruned towards the end of the winter (W) or early summer (S). Three intensities of winter pruning representing different distances (0.25, 0.50, 0.75 m) from the outer canopy surface were applied, while there was only a single summer pruning treatment (0.75 m). The vegetative growth variables measured after pruning included new branch number and length, new leaf number, and increase in trunk cross sectional area. Reproductive variables included fruit and oil yield, fruit number, fruit weight, and oil content per fruit. Growth of new branches increased significantly with winter pruning intensity while delaying pruning to early summer reduced regrowth to the level of the unpruned control. Despite differences in yield in individual years between the unpruned control and the winter pruning treatments, the average yield over the three years after the winter pruning event was similar between all trees. Delaying the intense pruning to summer was associated with some reduction in yield, and moderate winter pruning (0.50 m) appeared to partially reduce alternate bearing. When measured shortly after winter pruning, specific leaf mass of the remaining leaves decreased steadily as the level of winter pruning increased, which is consistent with prior shading within the tree. The leaf net photosynthetic rate per unit mass was also different between pruning treatments. In conclusion, our results contribute to filling the gaps in knowledge related to important aspects of olive tree responses to the intensity and timing of mechanical pruning. Fil: Albarracin, Valeria Mariel. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Regional de Investigaciones Científicas y Transferencia Tecnológica de La Rioja. - Universidad Nacional de La Rioja. Centro Regional de Investigaciones Científicas y Transferencia Tecnológica de La Rioja. - Universidad Nacional de Catamarca. Centro Regional de Investigaciones Científicas y Transferencia Tecnológica de La Rioja. - Secretaría de Industria y Minería. Servicio Geológico Minero Argentino. Centro Regional de Investigaciones Científicas y Transferencia Tecnológica de La Rioja. - Provincia de La Rioja. Centro Regional de Investigaciones Científicas y Transferencia Tecnológica de La Rioja; Argentina Fil: Hall, Antonio Juan. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Parque Centenario. Instituto de Investigaciones Fisiológicas y Ecológicas Vinculadas a la Agricultura. Universidad de Buenos Aires. Facultad de Agronomía. Instituto de Investigaciones Fisiológicas y Ecológicas Vinculadas a la Agricultura; Argentina Fil: Searles, Peter Stoughton. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Regional de Investigaciones Científicas y Transferencia Tecnológica de La Rioja. - Universidad Nacional de La Rioja. Centro Regional de Investigaciones Científicas y Transferencia Tecnológica de La Rioja. - Universidad Nacional de Catamarca. Centro Regional de Investigaciones Científicas y Transferencia Tecnológica de La Rioja. - Secretaría de Industria y Minería. Servicio Geológico Minero Argentino. Centro Regional de Investigaciones Científicas y Transferencia Tecnológica de La Rioja. - Provincia de La Rioja. Centro Regional de Investigaciones Científicas y Transferencia Tecnológica de La Rioja; Argentina Fil: Rousseaux, Maria Cecilia. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Regional de Investigaciones Científicas y Transferencia Tecnológica de La Rioja. - Universidad Nacional de La Rioja. Centro Regional de Investigaciones Científicas y Transferencia Tecnológica de La Rioja. - Universidad Nacional de Catamarca. Centro Regional de Investigaciones Científicas y Transferencia Tecnológica de La Rioja. - Secretaría de Industria y Minería. Servicio Geológico Minero Argentino. Centro Regional de Investigaciones Científicas y Transferencia Tecnológica de La Rioja. - Provincia de La Rioja. Centro Regional de Investigaciones Científicas y Transferencia Tecnológica de La Rioja; Argentina

Leaf senescence during grain filling can reduce crop yield. We studied, under field conditions and during grain-filling, the association between leaf cytokinin levels and the onset of leaf senescence in sunflower hybrids of contrasting canopy senescence patterns (Paraiso75, stay-green [SG] and Paraiso65, fast dry down [FDD]). At crop level, dynamics of live root length density (LRLD) and green leaf area index (GLAI) were followed, while at leaf level dynamics of total chlorophyll content, trans-Zeatin content, net photosynthesis and PSII quantum yield, were followed in leaf positions 17, 20, 22 and 24. Responses of these leaf variables to exogenous cytokinin applications to leaves at position 17 were also followed. SG exhibited greater (p

Root system size and functionality during the grain-filling phase are critical for water uptake and grain yield realization. In spite of its potential importance, root system functionality during this phase has received little attention. The study reported here aimed at documenting root system functionality dynamics between anthesis and physiological maturity, and its responses to water deficit, in two sunflower ( Helianthus annuus L.) hybrids of contrasting intrinsic patterns of canopy senescence. In experiments repeated in two separate years, crops of the hybrids Aguara 6 (stay green [SG]) and CF 101 (fast dry down [FDD]) were exposed to two levels of soil water availability (irrigation and drought) during grain filling, and the temporal dynamics of root and crop system variables (live root length density [LRLD], root respiration, sap flow rate, green leaf area index [GLAI] and the percentage of live roots [LR%]) were followed. The highest values for all variables were observed close to anthesis, and these differed little or not at all between hybrids within water regimes; but decreased thereafter at rates that differed between hybrids, water supply treatment and response variable. Drought hastened the beginning of root and leaf senescence in of both hybrids. The start of the decline in GLAI in the SG hybrid was significantly ( p p p p p

Genetic improvement of crop yields under potential (Yp) and water scarce conditions (Yw) will be an important avenue to improved food security over the next four decades, at the end of which projected demand for food, feed and biofuel feedstock is expected to level out. Current measured relative rates of improvement in Yp and Yw for the three main cereal crops (maize, wheat and rice) in many cropping systems in the world are mostly well below the 1.16–1.31% y−1 rates required to meet projected demand for cereals in 2050. These relative rates can be expected to fall further if the current absolute rates of yield improvement continue unchanged and/or the current indications of stagnation in yield improvement for some crops in some regions of the world become widespread. This review assesses the available evidence for unexploited opportunities for enhancing current rates of genetic improvement for Yp and Yw, and examines some substantive proposals for achieving the same end through genetic engineering of photosynthesis, above-ground ideotype design, and improvement of root capacity for water uptake. Because time is of the essence, special attention is paid to the time scales required to progress potentially useful traits through to proof of concept under field conditions, from there to farmer-ready cultivars, and for widespread adoption by farmers of the improved cultivars. The requirements of breeders for inclusion of potentially important traits into the conventional breeding process are outlined and the value of molecular breeding tools as aids to genetic improvement of simple and complex traits is considered. Intellectual property and regulatory requirements are taken as additional potential drags on the rates of adoption into the breeding process of useful traits and/or to the free flow of information between research teams. Main conclusions are: (a) there are some unexploited opportunities to improve Yp or Yw in some crops and/or cropping systems; (b) exploitation of these opportunities could be hastened by increasing funding for focussed research and by identifying and eliminating or reducing drags at various stages of the idea to farmer-ready cultivar chain; (c) the time-scales required for major improvements in yield in farmer-ready cultivars through genetic engineering or ideotype realisation are likely to be measured in decades rather than years; (d) current and expected future relative rates of progress in Yp and Yw are a matter of real concern and are insufficient to meet projected demand for cereals by 2050. Possible step changes in Yp or Yw powered by genetic improvements such as exploitation of hybrid vigour in rice or hypothetical successes in the genetic engineering of photosynthesis are unlikely to change this outlook. The pessimistic assessments embodied in the last two conclusions should not obscure the fact that without continued investment in the search for sources of genetic improvement and development of aids to breeding, the unmet demands for cereals in 2050 will be even greater.

Olive cultivation has expanded considerably beyond the Mediterranean Basin to include new areas in the Southern Hemisphere. This study evaluated the response functions of yield determinants and components to photosynthetically active radiation (PAR) in a large olive hedgerow (cv. ‘Arbequina’) in subtropical Argentina, and assessed whether mechanical pruning altered these relationships. Mechanical pruning was applied at the end of the winter dormant period to the east side and top of hedgerows oriented North–South, while the other side was left unpruned. The PAR distribution and leaf density at different positions within the hedgerow were measured over the two subsequent growing seasons with inflorescence density, fruit set, fruit density, individual fruit dry weight, and oil content (%) being assessed the second growing season. Hedgerow size increased substantially during the first season, especially on the pruned side and top, reducing transmitted PAR in much of the canopy to less than 20% of full Sun incident PAR values above the orchard. The relationship between leaf density (and thus number of axillary buds) and PAR was bilinear for the canopy positions within the unpruned side with a threshold of about 15% of full Sun PAR, above which leaf density did not increase. No relationships between leaf density and PAR were found within the pruned side and top. In contrast, inflorescence density exhibited a threshold of 18% of PAR for canopy positions within both the pruned and unpruned sides, although no relationship was apparent for the pruned top. Neither fruit set nor yield component relationships with PAR were affected by pruning. Fruit set had a linear response to PAR for the observed range of PAR values within the hedgerow (approx. 0–50% of full Sun). Individual fruit dry weight and oil content (%) also showed linear responses to PAR. The results suggest that oil production in large olive hedgerows (height × width) is limited by low solar radiation within the canopy, and that substantial vegetative growth triggered by mechanical pruning may be an important contributor to this limitation under current management practices in subtropical climates. Fil: Cherbiy Hoffmann, Silvana Ursula. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Regional de Investigaciones Científicas y Transferencia Tecnológica de la Rioja. Universidad Nacional de la Rioja. Centro Regional de Investigaciones Científicas y Transferencia Tecnológica de La Rioja. Universidad Nacional de Catamarca. Centro Regional de Investigaciones Científicas y Transferencia Tecnológica de la Rioja. Secretaria de Industria y Mineria. Servicio Geológico Minero Argentino. Centro Regional de Investigaciones Científicas y Transferencia Tecnológica de La Rioja; Argentina Fil: Searles, Peter Stoughton. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Regional de Investigaciones Científicas y Transferencia Tecnológica de la Rioja. Universidad Nacional de la Rioja. Centro Regional de Investigaciones Científicas y Transferencia Tecnológica de La Rioja. Universidad Nacional de Catamarca. Centro Regional de Investigaciones Científicas y Transferencia Tecnológica de la Rioja. Secretaria de Industria y Mineria. Servicio Geológico Minero Argentino. Centro Regional de Investigaciones Científicas y Transferencia Tecnológica de La Rioja; Argentina Fil: Hall, Antonio Juan. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Parque Centenario. Instituto de Investigaciones Fisiológicas y Ecológicas Vinculadas A la Agricultura. Universidad de Buenos Aires. Facultad de Agronomía. Instituto de Investigaciones Fisiológicas y Ecológicas Vinculadas A la Agricultura; Argentina Fil: Rousseaux, Maria Cecilia. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Regional de Investigaciones Científicas y Transferencia Tecnológica de la Rioja. Universidad Nacional de la Rioja. Centro Regional de Investigaciones Científicas y Transferencia Tecnológica de La Rioja. Universidad Nacional de Catamarca. Centro Regional de Investigaciones Científicas y Transferencia Tecnológica de la Rioja. Secretaria de Industria y Mineria. Servicio Geológico Minero Argentino. Centro Regional de Investigaciones Científicas y Transferencia Tecnológica de La Rioja; Argentina

Delayed leaf senescence during the grain filling phase, or stay green (SG), may be functional or cosmetic; the first being considered a valuable trait in breeding of many crop species. To establish whether canopy senescence patterns exhibited by two sunflower (Helianthus annuus L.) hybrids visually selected for slow post-anthesis canopy leaf area index (LAI) loss reflected functional, rather than cosmetic, SG, LAI dynamics and total biomass increase between anthesis and physiological maturity were followed in three separate experiments in which the putative SG hybrids were compared with standard (i.e., non stay green, NSG) hybrids exhibiting normal rates of canopy senescence under non- or minimal-water stress conditions. In two experiments, pairwise (i.e., one SG vs. one NSG) comparisons were made at two crop population densities. In the third experiment, grown at a single crop population density, four NSG hybrids were contrasted with the two SG hybrids. Canopy senescence dynamics were well described by fitted bilinear functions which discriminated between an initial, slow phase of leaf area loss and a second phase of rapid canopy senescence. No differences between hybrids in the rate of senescence during the first phase were found, but the putative SG hybrids exhibited a significantly slower rate of senescence during the second phase and a significantly higher LAI at physiological maturity (as % of LAI at anthesis). One NSG hybrid showed a greater rate of second-phase senescence than the remaining three hybrids in this category. Across experiments, the anthesis-physiological maturity increment in total oil-corrected biomass and radiation use efficiency (RUE) were significantly greater in the SG hybrids. Cluster analysis based on these four attributes, using data from Exp. 3, clearly discriminated between SG and NSG hybrids. Examination of specific leaf N (SLN) dynamics suggests that the lower RUE values observed in one of the NSG hybrids could be attributable to lower SLN values, but this was not the case for the remaining NSG hybrids. It is also possible that very rapid canopy LAI loss in another NSG hybrid may underlie its lower RUE. The failure of SG hybrids to translate greater post-anthesis biomass increment into grain yield across experiments was associated with their slightly shorter times to anthesis and their significantly lower biomass at anthesis. Grain number, the component of yield most strongly associated with yield, was associated with biomass at anthesis. A further contributing factor was that the duration of grain-filling tended to be shorter in SG hybrids. In the one experiment in which this variable was measured, resistance to stalk breakage was greater in the SG hybrids than the NSG one to which they were compared. We conclude that the SG observed in the hybrids with slower canopy senescence is functional and not cosmetic, and that for this to be translated into a yield advantage all hybrids need to reach anthesis at the same time and grain-filling duration also has to be the same across hybrids. Stay green is an interesting secondary trait to select for and should lead to higher and more stable yields in environments in which stem breakage (lodging) is a problem.

Stem lodging is a risk to sunflower production in Argentina, and may contribute to fixing the upper limit to commercially viable crop population density, since yield is known to increase up to densities higher than those currently used. Reputedly, crops are particularly susceptible to stem lodging during grain filling and at harvest maturity, but the temporal and spatial unpredictability of lodging events under field conditions has hampered systematic research on this issue. In this study we used mechanical lodging to examine the relationships between stem failure moment of force and stem structure in plants of two sunflower hybrids of contrasting susceptibility to stem lodging grown at each of three crop population densities (5.6 plants m-2, 10 plants m-2 and 16 plants m-2). Measurements were performed at mid- and near-end of grain filling and at harvest maturity in crops grown in three separate seasons at two locations. Stem failure moment of force at all three developmental stages was significantly (p < 0.05) greater in the lodging-resistant hybrid than in the lodging-susceptible hybrid used in these experiments at 5.6 plants m-2, and fell with increasing crop population density and between 90% grain filling and harvest maturity in both hybrids. At harvest maturity differences in stem failure moment of force between hybrids were significant (p < 0.05) at all three crop population densities. Stem flexural rigidity (ability of the stem to resist bending) and stem deformation (horizontal displacement from the vertical of the stem immediately prior to breakage) exhibited responses to hybrid, crop population density and developmental stage that were broadly consistent with those of stem failure moment of force. Stem diameter at the breakage point fell with crop population density, but there was little difference between hybrids except at the harvest maturity, when the susceptible hybrid at the two highest crop population densities had smaller diameters than the resistant hybrid. Measured at the breakage point, the thickness of the epidermis plus cortex tissues (i.e., the effective thickness of the stem wall) which surround the friable stem pith fell with crop population density and between end of grain filling and harvest maturity in both hybrids. Differences in effective thickness of the stem wall between hybrids were small and often not significant, but there was a tendency for the susceptible hybrid to exhibit greater thickness of the effective stem wall, particularly at the two higher population densities. Stem failure moment of force was linearly related to thickness of the effective stem wall in both hybrids, the slope of the relationship being significantly greater for the resistant hybrid. We conclude that, within limits, the thickness of the effective stem wall may prove to be useful as a guide to genotype susceptibility to stem lodging in breeding programs and may offer a simple approach to modelling susceptibility. This work has also served to highlight the need to investigate the origins (presumably related to stem anatomy and/or cell wall properties) of the genotypic effects on the stem failure moment of force/the effective stem wall relationship. Finally, the demonstration of the existence of genotype differences in tolerance to stem lodging should encourage the execution of a broader survey aimed at identifying sources of tolerance to stem lodging at high crop population densities in sunflower. © 2009 Elsevier B.V. All rights reserved. Fil: Hall, Antonio Juan. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Parque Centenario. Instituto de Investigaciones Fisiológicas y Ecológicas Vinculadas a la Agricultura. Universidad de Buenos Aires. Facultad de Agronomía; Argentina Fil: Sposaro, Mariano M.. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Parque Centenario. Instituto de Investigaciones Fisiológicas y Ecológicas Vinculadas a la Agricultura. Universidad de Buenos Aires. Facultad de Agronomía; Argentina Fil: Chimenti, Claudio Alejandro. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Parque Centenario. Instituto de Investigaciones Fisiológicas y Ecológicas Vinculadas a la Agricultura. Universidad de Buenos Aires. Facultad de Agronomía; Argentina

The post-anthesis dynamics of the water content of whole sunflower achene and its major parts (pericarp, embryo) were examined for seven genotypes that spanned a broad range of final achene size (30–100 mg achene −1 ). Objectives were: (i) to establish the relative contributions of pericarp and embryo to whole-achene water content dynamics, (ii) to determine the relationship between maximum water content of the pericarp and final achene size, and (iii) to examine the effect of final achene size (as affected by genotype and environment) on achene dry-down dynamics after physiological maturity (=maximum achene weight). Four experiments were conducted over 2 years under field and glasshouse conditions. Across genotypes and growth conditions, whole-achene and pericarp water contents peaked earlier and more sharply during grain filling (ca. 35% of grain filling duration, or 30% of final achene weight), maximum embryo water content was achieved somewhat later and declined less sharply. Although the pericarp was a minor (17–35%) component of final achene dry weight, it contained 65–70% of achene maximum water content. Absolute pericarp water content did not fall to values close to those of the embryo until after physiological maturity. Final achene and embryo dry weights were closely ( r 2 0.90 and 0.85, respectively) associated with maximum pericarp water content. After maximum achene water content, rates of whole-achene dry-down were linear (ca. 1.35% d −1 ), and absolute rates of water loss per achene (range = 1.1–3.7 mg H 2 O achene −1 d −1 ) were strongly associated with achene maximum water content and final achene dry weight ( r 2 0.86 and 0.75, respectively). Excluding the inbred line HA89, the remaining genotypes achieved harvest (17%) and storage (11%) achene water concentrations at about 15 and 20 d, respectively, after physiological maturity, largely because absolute rates of achene water loss increased with achene size. We conclude that the pericarp is the dominant component of whole-achene water content dynamics, and that pericarp and achene maximum water contents are good indicators of potential final achene and embryo sizes and achene dry-down rates. Present results also provide a first approximation to the quantification of post-physiological maturity dry-down in this crop.

Root lodging is an important adversity affecting sunflower (Helianthus annuus L.) production in Argentina under current husbandry practices, and may limit progress towards the achievement of higher yields via increased plant population density. Although there are perceptions that lodging susceptibility varies across developmental stages, crop population densities, genotypes and soil types, these perceptions have not been tested for sunflower using a standardized experimental protocol. This study aimed at: (1) identifying the sources of the variation in root lodging susceptibility in response to variations in crop population density in two genotypes of reputedly different susceptibility; (2) detecting the crop developmental stages most susceptible to root lodging; and (3) examining the relationships between root failure moment, root plate diameter and soil shear strength. We mechanically induced lodging at three developmental stages in plants rooted in pre-wetted plots. The crops were grown at 5.6 plants m−2 over 3 years on either Typic Argiudoll or Typic Hapludoll soils and at 3, 5.6, 10 and 16 plants m−2 on a Typic Argiudoll. The force needed to induce root lodging (root failure moment) and root plate diameter varied across genotypes, plant densities and developmental stages. Root failure moment and root plate diameters were greater (p

Published analyses of the changes in yield potential of the sunflower crop in Argentina across cultivars released between 1930 and 1995 suggested that part of the observed increase in yield potential could have arisen from a larger contribution from non-structural carbohydrates (NSC) present in the crop at anthesis. To test this idea, stems and receptacles were harvested at intervals over the anthesis–post-physiological maturity phase from crops, grown under good water and nutrient availability conditions, of three open-pollinated (OP) cultivars and five hybrids released at various times between 1930 and 1995. Stem NSC and structural biomass (SB) at anthesis diminished in the hybrids as compared with the OP cultivars (and across year of cultivar release), in spite of the fact that some hybrids had a significantly greater (p

Visual methods are commonly used to estimate physiological maturity (PM) in sunflower crops. These methods, while simple and less laborious than following grain dry weight dynamics, are subjective and results can be affected by environmental conditions. The objective of the present work was to study the relationship between the dynamics of dry matter and water concentration (WC) of sunflower fruits, with the aim of identifying the timing of physiological maturity on the basis of fruit WC. The effects of brief periods of high temperature stress (lasting 4 or 6 days) during grain-filling were also explored. Eight different sunflower genotypes (inbred lines and hybrids) were studied in four separate experiments conducted under different growth conditions (greenhouse and field) and contrasting dates of sowing (autumn, spring and summer), generating a broad range of grain-filling durations (28–41 days) and final fruit weight (30–105 mg fruit−1). In these experiments the evolution of fruit fresh and dry weights were followed in fruit from the peripheral and intermediate positions on the capitulum from anthesis to harvest maturity. Tri-linear functions were fitted to the relative (to final) fruit dry weight (RFDW) to fruit WC relationships (R2 from 0.94 to 0.99). Across experiments, genotypes and fruit positions on the capitulum, PM in non-stressed plants was attained when fruits exhibited 38% WC. This model was validated against independent data, successfully simulating the dynamics of fruit dry weight based on fruit WC (r = 0.99; P 20% with respect to controls, raised fruit WC at PM to ca. 50%. Simulations were performed to explore the effects of variations in timing of anthesis that occur between positions on the capitulum and among plants in a crop on fruit WC at maximum yield. At a crop level, 38% WC in these simulations corresponded to grain yield >95% of the maximum attainable when harvest was delayed until last growing fruits reached 38% WC. Simulations also showed that in crops exposed to high temperature during grain-filling, there would be no risk of shortfall with respect to potential yield using 38% WC as an indicator of PM. We conclude that this simple, fast and non-subjective method based on fruit WC would be useful to determine PM in sunflower.

Higher than normal rainfall during grain filling of sunflower ( Helianthus annuus L.) has been shown to be associated with reductions in grain yield. This yield loss has been variously attributed to low radiation (cloudy weather) or to the increased incidence of diseases. Impaired physiological processes directly linked to transient waterlogging arising from excess rainfall could also reduce yield. The aim of this work was to evaluate the existence and nature of physiological responses to waterlogging in sunflower. Four experiments were conducted during the 1999–2000, 2000–2001, 2002–2003 and 2003–2004 seasons (Experiments 1–4), using plants of the hybrid Paraiso 20 grown under non-limiting conditions in 30 L pots arrayed as a crop. Waterloggings of 24 or 48 h duration were applied in each of three separate occasions (Experiments 1 and 2) or for several days on a single occasion (Experiments 3 and 4) during grain filling. Leaf area dynamics were followed from anthesis to physiological maturity and grain yield and its components were measured in all four experiments. In Experiment 3 two different soil mixtures were used and leaf photosynthesis was measured during and after waterlogging. In Experiment 4 the response of shaded plants to waterlogging was studied. Rate of oxygen diffusion in the rooting substrate, leaf water potential and leaf wilting were measured during and after waterlogging. Root density during grain filling and stem plus petiole biomass at physiological maturity were also measured in this experiment. Across these experiments grain yield was reduced by 5–60% ( p

The value of the capacity for osmotic adjustment (OA) as a trait, which can contribute effectively to yield maintenance under drought, has not yet been established for maize ( Zea mays L.) using cultivars of similar genetic background. Here we report results obtained using crops of two S4 populations derived from a cross between inbred lines exhibiting the highest and lowest capacities for osmotic adjustment in a screening applied to 20 inbred lines. The mean values of OA for the two S4 populations were 0.47 MPa for the high OA population (HOA) and 0.06 MPa for the low (LOA). Crops of these populations were grown under a rain-out shelter and subjected to 30-day droughts either before or during flowering. In both experiments, exposure to drought evoked a significant ( p

The effects of shading (to 20% of incident radiation) on grain number (GN) in sunflower were examined by shading the crop in each of the six separate 10-day intervals running from the end of floret differentiation through to early grain growth (roughly, from mid-anthesis minus 15 days to mid-anthesis plus 26 days). An ontogenic index, based on floret morphology, male and female floret organ development, and early embryo development was formulated and used to describe the timing of shading and to categorize floret responses at three positions (periphery, mid-section, centre) on the head. Floret number per head was not affected by treatment, but shading reduced grain set (and, consequently, GN) significantly (P

Understanding of the underlying physiology of the genotype-specific responses to predictable and unpredictable environmental variation would improve the efficiency of selection within a complex target population of environments. Three-mode principal component analysis (PCA) can be used for interpreting the complex three-way (genotypes, environments, attributes) trial datasets from which this understanding should emerge. The efficiency of this method largely depends on the right combination between the biological and statistical models used, especially on the attributes selected to describe the genotypic responses and the centring of the three-way input data. In this study, we assessed the scope of yield determination models and double-centring of input data for generating some physiological understanding of the genotype×environment (G×E) interactions observed in a sunflower genotype–environment system and for developing ideotype-based breeding strategies. Double-centring of the three-way arrays permitted the separation of predictable and unpredictable G×E interactions. This, in combination with the use of models that explain the physiological bases of yield variation among genotypes, has served to identify three relevant sources of genotypic variation for use in a breeding program, namely: (i) attributes that can be selected to achieve specific adaptation to the target environment by emphasising predictable interactions (e.g. duration of grain filling, a trait associated with canopy stay green); (ii) attributes that allow the unpredictable G×E interactions to be accommodated, improving the linkage between managed-environments and target production environments (e.g. grain set); and (iii) genotypes of similar response pattern for yield but contrasting relative behaviour for the primary and secondary yield determinants. Breeding projects involving crosses between these genotypes could generate better opportunities for yield improvements for individual mega-environments.

Sunflower (Helianthus annuus L.) yields are strongly reduced when normal sowing dates are delayed. The objectives of this study were to investigate the physiological bases of the sowing date (S), genotype (G), and G x S interaction effects on sunflower yield, and to contribute to the formulation of ideotype-based selection strategies for improving yield at late plantings. Nine hybrids differentially adapted to northern and central Argentina were evaluated during two seasons in October (normal) and December (late) planting dates at Venado Tuerto, Argentina. Yield was defined as the product of total biomass and harvest index. Sowing date accounted for most of the yield variation. The G × S interaction, in turn, accounted for a portion of the total variability three times higher than the contribution of G. Both S and G x S interaction effects on yield mostly involved the variation of attributes and processes expressed postanthesis. Biomass differences between planting dates were the dominant determinant of the S effect on yield. The genotype-specific responses for harvest index were the dominant determinant of the G x S interaction, and were mostly associated with changes in the rate of harvest index increase. Variations in biomass and harvest index were strongly associated with the amount of intercepted radiation during grain filling which, in turn, was associated to duration of grain filling and green leaf area. Canopy stay green proved to be associated with adaptation to late planting dates. This indirect selection criterion appears to be a more reliable attribute for use in breeding for adaptation to late plantings than some other genotype characteristics linked to yield.

The value of osmotic adjustment (OA) as a trait which can contribute effectively to yield maintenance under drought, has not been established for sunflower ( Helianthus annuus L.). We screened a set of 25 inbred lines reputed to differ for drought tolerance for OA expressed in the 8-leaf stage; crossed the extreme lines (high and low OA); and selected four individuals (two high OA, two low OA) from the F2 population derived from this cross. Crops of F3 families obtained by self-pollination of these individuals were grown under a rain-out shelter and subjected to a 30-day drought ending at anthesis. High OA families expressed greater OA ft [OA at full turgor, estimated as the difference in osmotic potential between droughted and control treatments] than low OA families at the end of the drought period (0.23 MPa vs. 0.04 MPa). Crops of high OA families extracted more water from the profile during the stress period (117 mm vs. 63 mm) had greater shoot biomass and harvest index (HI) at physiological maturity, and greater grain yield (ca. 30%). There was no effect of OA on these variables in the irrigated controls. Grain size, rather than grain number, was the yield component most affected by level of OA. Yield maintenance under drought conditions was attributable to variations in post-drought shoot biomass increase and HI increase. We conclude that OA can contribute to drought tolerance in sunflower and stress the need to develop markers for this trait.

Grain number in sunflower responds to variations in radiation over a broad developmental window running from floral initiation through to grain setting, encompassing several sequential processes (i.e. floret differentiation, micro- and megasporogenesis, flowering, fertilization and grain set). We examined the effect of radiation stress during the floret differentiation process (a period of 5–8 days) using shades (80% interception) to reduce radiation incident on field-grown crops. Light stress reduced the integral of the generative area, inflorescence floret number and grain number per plant, without altering grain set. The effects of stress on floret number were mediated by changes in floret differentiation rate rather than by variations in duration of the floret differentiation interval. These responses were linked to changes in total apex area and dry weight, such that floret cross-section and associated dry weight at the end of floral differentiation were reduced by stress. These differences between treatments tended to become smaller towards anthesis. Although stress reduced plant and organ dry weight, there was little change in biomass partitioning to organs other than the apex. There was no measurable compensation between grain size and grain number, so that yield was determined by grain number. We conclude that light stress during the floret differentiation interval can irreversibly reduce grain number and yield in this species.

Correlative studies in olive using data from different locations or years suggest that temperature can modulate crop oil yield and oil composition. However, there are no published studies of manipulative experiments that demonstrate a direct role for temperature as a regulator of oil yield and oil quality in olive. The objectives of this study were to: i) elucidate the effect of temperature during the fruit oil accumulation phase on fruit dry weight, oil concentration and fatty acid composition; and ii) identify the developmental window within the oil accumulation phase exhibiting the greatest sensitivity to temperature and that with the highest fruit capacity to recover from the temperature treatments. Two branch-level experiments were conducted in a commercial orchard at Los Molinos (La Rioja, Argentina) using var. ‘Arauco’. Both experiments were conducted during the oil accumulation phase by enclosing fruiting branches in transparent plastic chambers with individualized temperature control. The first experiment; known as the four month long experiment, employed four temperature treatments that were applied for a single period of four months: a control at ambient temperature, two heating levels (5 °C and 10 °C warmer than the control), and a cooling level (5 °C cooler than the control). The second experiment consisted of four separate successive one month long treatment periods, in each of which two temperature treatments were applied: control and heating (ca. 7 °C higher than control). In the four month long experiment, fruit dry weight was not affected by average temperatures in the 16–25 °C range, but it was reduced with further increases in temperature. Oil concentration decreased linearly at 1.1% °C−1 across the whole range (16–32 °C) of average seasonal temperatures explored, while oleic acid concentration decreased 0.7% °C−1 over the same range. In the one month long experiment, 30 days of temperatures ca. 7 °C above ambient had a permanent negative effect on oil concentration at final harvest, particularly when the exposure to high temperature took place at the beginning of oil accumulation. By contrast, oleic acid concentration at the end of the treatment interval fell with increasing temperature but it could recover after treatment was removed in all periods except the first one. These results show that high temperatures during the oil accumulation phase may negatively affect olive oil yield and quality in warm regions, particularly if the high-temperature event occurs early in the phase. Additionally, the response of oleic acid concentration (%) to temperature under our experimental conditions was found to be opposite to that of many annual oil-seed crops. Fil: García Inza, Georgina Paula. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Regional de Investigaciones Cientificas y Transferencia Tecnológica de Anillaco; Argentina Fil: Castro, Diego Nicolas. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Regional de Investigaciones Cientificas y Transferencia Tecnológica de Anillaco; Argentina Fil: Hall, Antonio Juan. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Parque Centenario. Instituto de Investigaciones Fisiológicas y Ecológicas Vinculadas A la Agricultura; Argentina. Universidad de Buenos Aires. Facultad de Agronomia; Argentina Fil: Rousseaux, Maria Cecilia. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Regional de Investigaciones Cientificas y Transferencia Tecnológica de Anillaco; Argentina

The number of florets per head in sunflower is a function of the integral of the generative area of the inflorescence primordium during the phase of floret differentiation (FS5–FS8). To understand the effects of temperature during early reproductive stages on grain number, we quantified the effect of temperature on the duration of the FS5–FS8-period, on the maximum generative area, on floret number and viability, and on grain number. Plants of the sunflower (Helianthus annuus L.) inbred line HA89 were exposed to daily temperatures between 14 and 38°C for 14 days from FS4. Increasing temperature significantly reduced the integral of the generative area between floral stages 5 and 8, due to reductions in both the maximum size and the duration of the generative area. These variations in generative area integral translated into significant proportional variations in florets per head. However, the high floret numbers evoked by low temperatures did not result in a high number of filled grains per head. The latter variable exhibiting a convex response to temperature with an optimum at 25°C. Residual effects of early exposures to low and high temperatures significantly reduced the number of functional florets, filled grain, and increased unfilled grain (flats). The curvilinear response of grain numbers per head can be understood as the result of opposing effects of temperature on floret differentiation and functionality and on continuation of grain growth after flowering. Early exposure to low or high temperature also reduced leaf area and dry matter per plant at anthesis, which may have influenced the observed reduction in grain number. Caution is necessary in using the photothermal quotient as an estimator of grain number at low temperature.

Major unresolved issues in sunflower ecophysiology constrain efforts to improve crop modelling, management, genetic analysis and breeding. Three issues are used here to illustrate this point. Firstly, much of the work on the duration of the emergence to flowering phase has considered the phase as a whole. It is argued that a more detailed analysis based on sub-phases is required, particularly in view of possible intraspecific variability in the durations of the basic vegetative and juvenile phases and evidence that photoperiod responses before, during and after floral initiation may differ between genotypes and even be of opposite sign for the same genotype. Secondly,contrasting responses of grain oil proportion to manipulation of plant population density and incident radiation appear to be linked to variations in kernel oil proportion rather than to kernel: hull ratio, and responses of grain oil proportion to changes in sowing date seem to have a similar origin. More effort should be focused on understanding the controls of oil mass per kernel. It is speculated that there may be a genotype-dependent limit to this variable. A third unresolved issue relates to the nature and strength of the linkage between post-anthesis stay-green and leaf photosynthetic functionality. These variables are poorly related during pre-anthesis senescence of leaves in the lower portion of closed canopies, and for sunflower this linkage appears much weaker than in other crop species. Current interest in post-anthesis stay-green as a possibly useful crop attribute requires clarification of this uncertainty.

The effects of nitrogen (N) availability on cell number and cell size, and the contribution of these determinants to the final area of fully expanded leaves of sunflower (Helianthus annuus L.) were investigated in glasshouse experiments. Plants were given a high (N =315 ppm) or low (N=21 ppm) N supply and were transferred between N levels at different developmental stages (5 to 60% of final size) of target leaves. The dynamics of cell number in unemerged (< 0.01 m in length) leaves of plants growing at high and low levels of N supply were also followed. Maximum leaf area (LAmax) was strongly (up to two-fold) and significantly modified by N availability and the timing of transfer between N supplies, through effects on leaf expansion rate. Rate of cell production was significantly (P

A good association between leaf N and photosynthetic photon flux density (PPFD) canopy distribution profiles has often been found. However, it is not clear whether N export from basal leaves antidpates the reduction in basal leaf PPFD that occurs as the canopy develops; our first objective was to examine this issue. We documented the dynamics of the light environment and specific leaf nitrogen (Nf) for target leaves at a single level of insertion under sunflower (Helianthus annuus L) canopies grown at 0.47, 1.18, and 4.76 plants m -2 and contrasted observed Nf with the estimated value of Nf required to maximize net carbon exchange (NCE) for the actual PPFD impinging on target leaves. The second objective of this work was to define the relationships between Nf and PPFD and between Nf and the spectral composition of light impinging on basal leaves. Paired (red and farred) photodiode sensors placed on the adaxial leaf surface were used to document light quality dynamics of basal leaves. Measured Nf was greater than or similar to that required to maximize NCE for 79% of the observations made between achievement of maximum target leaf area and senescence of these leaves. Nf showed a strong association with both red/far-red ratio of light impinging on target leaves and PPFD, but the association was stronger for the former variable. We conclude that N export from basal leaves does not precede PPFD reduction and that the red/far-red ratio is a better predictor of Nf gradients within the canopy than PPFD.

Potential yield in several crop species is affected by radiation and temperature when they are grown under unstressed conditions. This issue has been not explored for sunflower (Helianthus annuus L.), and the objectives of our experiments were (i) to analyze the dependence of seed number (SN, seeds m -2 ) on the thermal and light conditions during the period from floret initiation to the end of seed-setting and (ii) to determine whether sensitivity of SN to radiation differs between pre- and post-anthesis. Inter and intra-annual variations in sowing dates and artificial shading were used to expose irrigated crops to different conditions of radiation (range 14-24 MJ m -2 d -1 ) and temperature (20-24°C) (temperature/radiation combinations = 18). Significant positive (P < 0.00001) relationships between SN (range 3499-11547 seeds m -2 ) and intercepted (r 2 = 0.65) and incident (r 2 = 0.56) short-wave radiation during the floret initiation to seed-setting period were found. A weaker negative association with temperature (r 2 = 0.29; P < 0.05) was also found. The relationship between SN and a photothermal quotient [PQ = mean radiation/ (mean temperature-base temperature), MJ m -2 d -1 °C -1 ], calculated with intercepted radiation was better than that achieved by either variable alone (SN = -436 + 9214 PQ; r 2 = 0.74; P < 0.000001). We conclude that PQ would be a better tool to predict SN in sunflower than the use of radiation or temperature alone. Crops subjected to shading after anthesis suffered significantly greater (P < 0.05) reductions in SN, at equivalent PQ, than those exposed to shading before anthesis.

Leaf growth responses to N supply and leaf position were studied using widely-spaced sunflower plants growing under field conditions. Both N supply (range 0.25 to 11.25 g added N per plant) and leaf position significantly (p=0.001) affected maximum leaf area (LAmax) of target leaves through variations in leaf expansion rate (LER); effects on duration of expansion were small.

Basal leaves frequently senesce before anthesis in high population density crops. This paper evaluates the hypothesis that quantitative and qualitative changes in the light environment associated with a high leaf area index (LAI) trigger leaf senescence in sunflower (Helianthus annuus L.) canopies. Mean leaf duration (LD, time from achievement of maximum leaf area) of leaf 8 was significantly (P < 0.05) reduced from 51 to 19 days as crop population density was increased from 0.47 to 4.76 plants m−2. High compared to low plant population density was associated with earlier reduction in the photosynthetically active radiation (PAR) and red/far-red ratio (R/FR) reaching the target leaf. However the changes in R/FR preceded those in PAR. When the light environment of individual leaves of isolated plants growing under field conditions was manipulated using filters and FR-reflecting mirrors, LD was positively and linearly related with the mean daily PAR (MDR) received in the FR- (no FR enrichment) treatments. FR enrichment of light reaching the abaxial surface of the leaf significantly (P < 0.05) reduced LD by 9 days at intermediate PAR levels with respect to FR-controls, but did not affect LD at the maximum PAR used in these experiments. However, when light reaching both leaf surfaces was enriched with FR, LD (for leaves receiving maximum PAR) was 13 days shorter than that of the FR- control. These results show that basal leaf senescence in sunflower is enhanced both by a decrease in PAR and by a decrease in R/FR.

We evaluated a series of management strategies for rainfed wheat cropping in a monsoonal climate in the Pampas using the CERES-Wheat model with 24 years of daily weather records. These strategies included the combination of two cultivars of different maturity (intermediate and short) and two sowing dates (early and late), in two different locations (Pergamino and Marcos Juarez). In addition, the impact of not controlling weeds during the fallow preceding each crop was assessed. Simulation experiments showed that without limitations of nitrogen, maximum yields could be achieved by early sowing of the intermediate-cycle cultivar. The highest yield achieved by this strategy was partially associated with greater water consumption during the crop cycle. However, in the lowest-yield years of both sites, but particularly in Marcos Juarez, the intermediate cultivar sown early did not appear to be the best option. An analysis of stability of grain yield indicated that late sowing of the short-cycle cultivar was the most stable system. For weedy fallows, the yield of the short-cycle cultivar was more affected than the intermediate one, particularly in years of low yield.

Loss of nitrogen from the leaves and a reduction in specific leaf nitrogen (SLN, g N m −2 ) is associated with grain filling in sunflower ( Helianthus annuus L.). To explore the relationship between crop radiation-use efficiency (RUE, g MJ −1 ) and SLN, crop biomass accumulation and radiation interception were measured between the bud-visible and physiological-maturity stages in crops growing under combinations of two levels of applied nitrogen (0 and 5 g N m −2 ) and two population densities (2.4 and 4.8 plants m −2 ). Both nitrogen fertilization and density had significant ( P = 0.05) effects on crop biomass yield, nitrogen uptake, leaf area index and SLN, but the nitrogen effects were more pronounced for these and other crop variables. Linear regressions of accumulated biomass (OCdwt, corrected for the energy costs of oil synthesis in the grain) on accumulated intercepted short-wave radiation between bud visible and early grain filling provided appropriate and significantly ( P = 0.05) different estimates of RUE for the pooled 0 g N m −2 (1.01 g OCdwt MJ −1 ) and 5 g N m −2 (1.18 g OCdwt MJ −1 ) treatments. When calculated for each inter-harvest interval, crop RUE varied in a curvilinear fashion during the season, with a broad optimum from 40 to 70 days after emergence of the crops, and with lower values earlier and later in the season. The reduction in RUE toward physiological maturity was particularly marked. A plot of RUE against SLN revealed a reduction in RUE at small SLN values, but the relationship may be confounded by ontogenetic changes in other factors. A published model (Sinclair and Horie (1989), Crop Sci., 29: 90–98) was used to explore the RUE/SLN relationship. The model was unable to reproduce the decline in RUE during the second half of the grain-filling period. It is suggested that an important cause of this failure may be the partition, in the model, of a fixed, rather than a variable, fraction of crop gross photosynthesis to respiration.

Measurements of the profiles of leaf area and leaf nitrogen were made on five occasions from midflowering to maturity (53, 61, 70, 78 and 83 days after emergence, DAE) in sunflower crops grown at contrasting density (2.4 and 4.8 plants m-2) and nitrogen supply (0 and 5 g N m-2 at emergence) in the summer in Buenos Aires, Argentina. As the crops matured, nitrogen was withdrawn unequally from all leaf positions and leaves senesced from the bases of the canopies. A model was used to estimate the daytime net photosynthesis (Pc) of canopies of defined leaf area and nitrogen content under the observed conditions of temperature and irradiance. Comparisons were made between the observed profiles of leaf nitrogen and those that would maximise Pc (the optimal profiles). The observed nitrogen profiles were sub-optimal at mid-flowering, except in the low-density, low-nitrogen treatment. The differences were most marked in the high-nitrogen treatments which held 'excessive' nitrogen in their lower canopies. As the canopies matured and nitrogen was mobilised to the grain, leaf area index and total nitrogen content decreased and optimal profiles changed shape from exponential to linear. During this period observed profiles became more optimal. There was, however, little difference in Pc between observed and optimal profiles. The maximum difference was 3.2% observed in the low-density, high-nitrogen treatment at DAE 53. The comparison of actual and optimal profiles as leaf nitrogen content (mg N) in addition to the more commonly used specific leaf nitrogen (SLN, g N m-2 leaf) explains this result because relatively large changes inSLN in the small leaves at the top of canopies have little effect on Pc. The study shows that leaf nitrogen content is an appropriate basis for comparison of canopy nitrogen profiles in sunflower.

A simple framework for describing the partitioning of biomass among the organs (root, stem, laminae, head) of sunflower plants between emergence and anthesis as been developed using data from experiments in which adequate water and nitrogen supplies were assured. It was hypothesized that organ partitioning coefficients (the slope of the organ weight: total weight relationship) remained constant during phenologically defined phases of crop development. A non-linear optimization routine was used to define organ partitioning coefficients and the values of total plant weight at which changes in partitioning took place. It proved possible to relate these values to defined stages of plant development, namely bud visible and commencement of rapid head growth. The dynamics of partitioning to the stem could be described by two coefficients applying to the emergence-bud visible and bud visible-anthesis phases, respectively. Complementary changes in the coefficients for leaf laminae and fine roots took place at the bud visible stage. Once rapid growth of the inflorescence had become established, further changes in partitioning to roots and laminae, but not to stem, took place. Values of the coefficients for each organ changed only slightly between experiments (three) and cultivars (five), in spite of substantial variations, between data sets, of total plant weight at anthesis and, to a lesser extent, in the timing of the anthesis and bud visible stages. A satisfactory description of organ biomass dynamics was obtained when this descriptive framework was tested against data obtained in an independent experiment.

The responses to water stress of the bulk modulus of elasticity (e) and the apoplastic water fraction were examined using six sunflower cultivars of differing capacity for osmotic adjustment (OA). Water stress did not affect the partitioning of water between apoplastic (ca. 20%) and symplastic fractions in leaves which expanded during the exposure to stress in any genotype. Hence, no genotype-linked effects on either the buffering of cell water status during stress or on the estimates of bulk leaf osmotic potential could be expected. Genotypes differed in the degree of change in e (estimated from pressure/volume [P/V] curves) and OA (estimated using both ln RWC/ ln Ψo plots and P/V curves) induced by exposure to stress. In three genotypes e increased significantly (p=0.05) as a consequence of stress, in another three change were small. OA was the only attribute of the three examined that could have contributed to turgor maintenance under stress. There was a strong negative association between leaf expansion and degree of OA across genotypes (r=−0.91) and a strong positive one between OA and e (r=0.94). However all genotypes evidenced some degree of OA. These results are consistent with part of the genotype differences in OA being attributable to variations in leaf expansion during exposure to stress.

In vegetative canopies of many species, the vertical gradient of lamina nitrogen concentration (NW) parallels the profile of light distribution in such a way that the actual nitrogen partitioning approaches the optimum pattern for canopy photosynthesis. This paper evaluates the hypothesis that a strong sink for nitrogen, viz. growing grain, affects the pattern of lamina nitrogen distribution usually described for vegetative canopies. The light and NW profiles of sunflower (Helianthus annuus L.) crops were characterised from anthesis to physiological maturity. The factorial combination of two plant populations (2.4 and 4.8 plants m−2) and two levels of nitrogen supply (0 and 5 g N m−2) were the sources of variation for NW and light profiles. Before the onset of nitrogen accumulation in grain, the pattern of NW was similar to that described for other species and it was related to the distribution of light in the canopy. Important changes in the profile of NW occurred during grain filling that were unrelated to the light regime. Nitrogen was mobilised from leaves in all positions in the canopy and the rate of NW change was greater in leaves closer to the grain, which were also the leaves where nitrogen was more concentrated. It is concluded that the physiological mechanisms involved in determining the distribution of leaf nitrogen in vegetative canopies do not apply to sunflower during grain filling.

Intraspecific variation for osmotic adjustment in sunflower was examined using a collection of 33 genotypes of different origin which were exposed to water stress at the 8-leaf stage. Changes in osmotic adjustment with ontogeny were also evaluated in the pre- and post-anthesis phases using seven genotypes drawn from this collection. Estimates of osmotic adjustment were derived from measurements of leaf relative water content (RWC) and osmotic potential (π) during a period in which the soil was allowed to dry out gradually. The degree of osmotic adjustment, expressed as the value of RWC for a π of −1.7 MPa (RWCe), was derived from the ln RWC/ln π relationship. Both monophasic and biphasic ln RWC/ln π relationships were found. Irrespective of the form of the relationship, all genotypes at the 8-leaf stage showed some degree of osmotic adjustment. This was also true for the cultivars included in the subset examined in pre- and post-anthesis phases. Significant differences (P=0.05) in RWCe were found between extreme genotypes in all three phases. Significant (P=0.05) linear relationships were found between RWCe measured in the 8-leaf stage and that measured in the pre- and post-anthesis phases, establishing the viability of measurements in the 8-leaf stage as a means of selection for osmotic adjustment in later developmental stages. Genotype rank order was stable (P=0.01) across the three ontogenetic phases examined.

Radiation use efficiency (ϵ) values for sunflower were derived using data from periodic determinations of crop aerial or total (aerial + root) biomass and radiation interception in two experiments conducted at Junin and Buenos Aires, Argentina. Analysis showed that three distinct phases, each characterized by a different value of ϵ, could be distinquished. The phases were establishment (0–47 days after emergence ( dae )), rapid growth (47 dae -anthesis) and postanthesis (anthesis-physiologcial maturity). The crops had reached the bud-visible stage and intercepted at least 85% of incident radiation by 47 dae . Efficiency values based on aerial biomass were highest during the rapid growth (ϵrg = 2.4 ± 0.22 g/MJ PAR) phase and lower during the establishment (ϵe = 1.01 ± 0.25 g/MJ PAR) and postanthesis (ϵpa = 1.3 ± 0.15 g/MJ PAR) phases. The differences between ϵpa and ϵrg are attributed to the increased respiration load, high synthesis costs of the oil-rich grains and leaf senescence during grain filling. The value of ϵe was less than of ϵrg even when efficiencies were calculated on the basis of total crop biomass (i.e. including root biomass). Hence, ontogenetic changes in partitioning of biomass between the root and aerial organs do not seem to be the cause of the changes in the value of ϵ. No variations in photosynthetic capacity with leaf position could be found, and it is concluded that the most likely reason for the lower values of ϵe lies in saturation of canopy photosynthesis at less than full sunlight while crop cover is incomplete.

Background and Aims: Sunflower cultivars exhibit a wide range of oil content in the mature achene, but the relationship between this and the dynamics of oil deposition in the achene during grain filling is not known. Information on the progress, during the whole achene growth period, of the formation of oil bodies in the components of the achene and its relationship with variations in final oil content is also lacking. Methods: The biomass dynamics of achene components (pericarp, embryo, oil) in three cultivars of very different final oil concentration (30-56 % oil) were studied. In parallel, anatomical sections were used to follow the formation of oil and protein bodies in the embryo, and to observe pericarp anatomy. Key Results: In all cultivars, oil bodies were first observed in the embryo 6-7 daa after anthesis (daa). The per-cell number of oil bodies increased rapidly from 10-12 daa until 25-30 daa. Oil bodies were absent from the outer cell layers of young fruit and from mature pericarps. In mature embryos, the proportion of cell cross-sectional area occupied by protein bodies increased with decreasing embryo oil concentration. The sclerenchymatic layer of the mature pericarp decreased in thickness and number of cell layers from the low-oil cultivar to the high-oil cultivar. Different patterns of oil accumulation in the embryo across cultivars were also found, leading to variations in ripe embryo oil concentration. In the high-oil cultivar, the end of oil deposition coincided with cessation of embryo growth, while in the other two cultivars oil ceased to accumulate before the embryo achieved maximum weight. Conclusions: Cultivar differences in mature achene oil concentration reflect variations in pericarp proportion and thickness and mature embryo oil concentration. Cultivar differences in protein body proportion and embryo and oil mass dynamics during achene growth underlie variations in embryo oil concentration. © The Author 2006. Published by Oxford University Press on behalf of the Annals of Botany Company. All rights reserved. Fil: Mantese, Anita Ida. Universidad de Buenos Aires. Facultad de Agronomía. Departamento de Recursos Naturales y Ambiente; Argentina Fil: Medan, Diego. Universidad de Buenos Aires. Facultad de Agronomía. Departamento de Recursos Naturales y Ambiente; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina Fil: Hall, Antonio Juan. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Parque Centenario. Instituto de Investigaciones Fisiológicas y Ecológicas Vinculadas a la Agricultura. Universidad de Buenos Aires. Facultad de Agronomía. Instituto de Investigaciones Fisiológicas y Ecológicas Vinculadas a la Agricultura; Argentina

Analyses were performed to evaluate the hypothesis that, under conditions of good water availability, variations in yield in the Argentine wheat belt were related to variations in the photothermal quotient (Pq, the ratio of mean daily incident short-wave radiation to mean daily temperature in excess of 4.5°C). Data relating to spring bread wheat yields collected between 1978 and 1985 at several sites of the national wheat nursery, for which good weather data were available, were filtered to eliminate trials which may have been exposed to drought and other adversities, such as disease and hail. There were significant (P = 0.05) differences in yield, kernel number (Nk, grains m−2) and, to some extent, kernel weight among crops grown at different sites under close to optimum levels of water and nutrient availability. Nk was the most important determinant of yield, variations in Nk accounting for 78% of the total variance in the yield. Cultivar effects, presumably cultivar-specific differences in kernel weight, accounted for a further 14% of this variance. For the filtered data set, the relationship between Nk and Pq for the 25 days prior to anthesis, accounted for 52% of the total variability in Nk. The Pq and cultivar together accounted for 82% of total variability. Associations between Nk and either radiation or temperature alone were weaker, and site effects not accounted for by Pq were very small. The Pq estimated from 12- to 15-year weather records for the currently accepted optimum date of anthesis differed significantly (P = 0.05) between each of three representative sites in the northern, central and southern parts of the wheat belt. Reductions in potential Nk associated with interannual variations in Pq at each site may be as much as 34% of maximum Nk for the site. The Pq for anthesis dates of up to 20 days before or after the currently accepted optimum at any of these sites did not differ with respect to the optimum date, thus no advantage in terms of greater Nk is to be expected from changes in anthesis dates.

Photosynthesis-irradiance response curves and leaf nitrogen contents were measured weekly by destructive sampling over the life cycles of leaves 10, 15, 20 and 25 of sunflower plants (cv. Prosol 35) grown in large pots in the open under optimum conditions of temperature and high irradiance. Individual leaf responses were adequately described by a hyperbola of three parameters, viz. Pmax, the rate of photosynthesis in saturating irradiance; R, the rate of dark respiration adjusted for temperature (30�C); and ε, the apparent quantum efficiency of photosynthesis at low irradiance. Pmax (range 0-40 μmol CO2 m-2 s-1) and R (0-4 μmol CO2 m-2 s-1) were non-linearly related to nitrogen content per unit leaf area (NL) (range 0.3-2.9 g N m-2) across all leaf positions and for all leaf ages. ε (mean value 0.050 mol mol-1, s.e. 0.001) was independent of NL. The equations for net photosynthesis derived from pot studies were shown to explain (r2 =0.80) leaf photosynthesis in a crop of the same cultivar over a wide range of NL and irradiance.

Thirteen maize ( Zea mays L.) genotypes were evaluated for variability with respect to seedling traits reportedly associated with tolerance to water stress. Significant variability was found within this set of cultivars for chlorophyll loss under thermal stress, loss of intracellular electrolytes under thermal and desiccation stresses, and proline accumulation in response to desiccation. These attributes were not mutually correlated in the genotypes examined. A sub-set of seven genotypes was grown in a sand-nutrient solution culture in the field and subjected to stress at tasseling for about eight days. Yield was significantly reduced in all cultivars, but the magnitude of the reduction was variable. Relative kernel number of the uppermost ear (the major yield-determining factor in most cultivars) was strongly associated with duration of exposure to pollen of the same ear. Relative spikelet number of the uppermost ear was significantly reduced intwo cultivars by stress, but this response appeared to be independent of mean exposure to pollen. Relative spikelet number was associated with plant-leaf area: root weight ratio at tasseling. No association was found between seedling traits and relative yield, additional yield-related responses or other, presumably less complex, responses to stress such as loss of leaf area and fall in chlorophyll content.

Hall, A.J., Connor, D.J. and Whitfield, D.M., 1989. Contribution of pre-anthesis assimilates to grain-filling in irrigated and water-stressed sunflower crops. I. Estimates using labelled carbon. Field Crops Res., 20: 95-112. Labelled carbon was used to estimate pre-anthesis assimilate contributions to seed yield in sunflower crops irrigated throughout the season or stressed during grain-filling. At weekly intervals, commencing 23 days before anthesis and continuing through to maturity, previously unlabelled plots were exposed to labelled carbon dioxide. At maturity, the partitioning of label between the seed and the remainder of the crop (including roots) was determined. Estimates of pre-anthesis assimilate contributions to grain carbon were obtained by weighting these partitioning coefficients by seasonal crop gross photosynthesis data (Variant 1 ) and by crop dry-weight changes over the pre- and post-anthesis periods (Variant 2 ). Estimates of absolute contributions varied slightly between treatments and with estimation technique. Taking the synthesis costs of grain oil content into account, about 60 g C m- e (or about 22 or 27 % of the C content of seeds in irrigated and droughted crops, respectively ) originated in pre-anthesis assimilation. Estimates for stressed crops obtained using Variant 1 were lower than those obtained using Variant 2. The amount of water-soluble carbohydrate stored in stem, tap-root and receptacle at anthesis was equivalent to 86 g C m -2. During grain-filling, the losses from the non-seed portion of the crops in dry weight, in nitrogen-associated C, and in water-soluble carbohydrates were equivalent to 73, 14 and 60 g C m -s, respectively. Thus, labile C stored in the crop at anthesis considerably exceeded that required to sustain the estimated transfer to seeds, and observed losses from nonseed organs were similar in magnitude to these estimates.

The effects of daily mean temperature and photoperiod on green leaf per plant (LAp) were studied using seven monthly plantings of four sunflower (Helianthus annus L.) hybrids of varying photoperiodic sensitivity. Time to anthesis ranged from 57 to 145 days fromemergence, and LAp at anthesis from 1375 to 6739 cm2 plant−1, respectively. Time from emergence to anthesis was successfully estimated using the model of Hammer et al. (1982, Aust. J. Agric. Res., 33: 263–274), which requires information on temperature, photoperiod at emergence and cultivar photoperiodic sensitivity. Seasonal dynamics of LAp were successfully described using a thermal-time approach base temperature = 8.5°C) and potential LAp at anthesis (a cultivar characteristic). Analysis of published data showed there is a negative relationship between LAp at anthesis and population density over the 1–8 plants m−2 range. A study of LAp dynamics at three densities in the 2–5 plants m−2 range showed that the basis of this relationship lies in the effect of population on maximum rate of LAp growth and in the linkage between LAp at anthesis and the maximum rate of LAp growth. The functions derived in this paper were successfully used to generate descriptions of the dynamics of LAp during the whole growth cycle for independent data sets, which included four cultivars, one of them at three population densities.

The effects of exposure to water stress at three developments stages (start of tasseling, start of pollen-shedding, and silking) on the viability of pollen and the dynamics of flowering in maize (Zea mays L.) were examined using a single genotype. Viability was not affected by stress at any stage, but alterations in the dynamics of flowering reduced the number of pollen grains available for pollination of the last ears to silk within the stressed populations. In a second experiment the effects of stress at the start of pollen-shedding on the availability of pollen during the presentation of silks was studied in a set of six cultivars. There was considerable variability between cultivars in the number of pollen grains produced per tassel, and water stress decreased the quantity of pollen grains harvested. There was a strong association between pollen availability and kernel number in the apical ear (as a proportion of control values). Two effects of stress on pollen availability were distinguished: a reduction in the duration of exposure of silks of the apical ear to pollen, and a fall in the rate of pollen production during the period in which receptive silks were present. The relative importance of these effects in determining pollen availability varied with cultivar.