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Grain weight (GW) is an important component of grain yield and quality of sunflower and the improvement of this trait has been proposed as an aim for breeding of this crop. There is strong evidence suggesting the pre-flowering period as key to determining GW in different crops; however, the key traits and mechanisms controlling GW during this phase are still largely unknown in sunflower. This study conducted a detailed survey of how the pre-flowering period determines grain growth dynamics by assessing the early developmental process during the growth of reproductive organs and the sensitivity of GW, grain number (GN) and grain oil concentration to different source–sink manipulations immediately before flowering. Two experiments were carried out under field conditions in two growing seasons. In Exp. 1 two sunflower oilseed hybrids contrasting in GW (Alybro and SG) were sown at the Experimental Station of the Universidad Austral de Chile in Valdivia (39°47′S, 73°14′W), Chile. In Exp. 2, the same hybrids plus a confectionery genotype (RHA) were evaluated in the Experimental Station where two source–sink treatments were performed: a control (without manipulation) and a shading treatment starting 16 days previous to anthesis, i.e. during the R2-R5 period. Crop development, average and individual GW, GN and oil concentration of grains were assessed at harvest. Also, flower and grain traits and their dynamics were measured from early reproductive stage (ovary at R3) to harvest in different grain positions from the capitulum (e.g., fresh and dry weight and dimensions of grains, pericarp and embryo and grain oil concentration). GW was significantly affected by genotype and shading treatments imposed at pre-anthesis, but no statistical effect of the lower source–sink ratio was found on GN. Contrary to GW, grain oil of oilseed genotypes was highly conservative under the source–sink reduction in our study. Positive associations were found between final GW and: ovary weight at R3, at anthesis (R5), ovary growth rate, grain dimensions and water content. Final GW also showed a linear association with ovary growth rate. This study is the first to evaluate the ovary weight from the early development stage and the ovary growth rate during the pre-anthesis period in sunflower and the effect of source reduction at pre-anthesis on grain oil concentration, supporting the importance of the maternal tissues on the setting of potential and actual GW in sunflower.

Lentil is a cool season grain legume produced primarily in the Mediterranean and temperate regions of the world where water and heat stress in critical developmental windows constrain yield. The effect of stress on yield depends on the timing, intensity and duration of stress; here we focus on timing relative to phenological development. To determine the critical period for grain yield and grain protein, we compared unshaded controls and crops exposed to sequential 10–14 d shading periods using two locally adapted varieties at Roseworthy, south-eastern Australia, and Valdivia, southern Chile. Yield of unstressed controls varied from 1 t ha−1 at Roseworthy to 7 t ha-1 at Valdivia, and grain protein from 22.0 % at Valdivia to 27.6 % at Roseworthy. Irrespective of growing conditions, the most sensitive period was at 50–126 °Cd after flowering, around pod emergence. Grain number and biomass accounted for most of the variation in yield. Grain protein concentration varied according to a bi-linear model, with minor reduction (

Grain yields in wheat can be limited by the assimilate supply (source) or by the carbohydrate demand of the grains (sink). Recently, there have been questions regarding the capability of crop models to simulate the physiology of source-sink interactions in crops; however, crop models have never been tested with source-sink manipulated data. We tested the Nwheat model with detailed measured field experimental data with treatments of manipulated source (i.e., assimilate supply), sink (i.e., kernel number), and their combinations. In general, the model could reproduce observed effects of shading before and after anthesis as well as the additional impact of halving the spikes. A 90% shading during grain filling reduced individual grain weights drastically, with the remaining yield mostly determined by carbohydrate remobilisation, which the model reproduced. The model also reproduced the decline of biomass accumulation due to shading, but was not sensitive enough to simulate the observed reduction of kernels per m2 from a 90% reduction in solar radiation between booting and the beginning of grain filling, resulting in an overestimated grain yield. The model reproduced the positive impact of a 7% genetically increased radiation use efficiency (RUE) on growth and yield. A sensitivity analysis indicated that the yield response to increased RUE can vary among environments. The yield impact can be positive in many environments, but negative in terminal drought environments. In these environments, stimulated early growth from higher RUE can cause accelerated water deficit during grain filling and reduced yields. The model adequately simulated source-sink interactions of most of the treatments, but there were obvious shortcomings in simulating kernel set and final grain size. Improving these will be critical for estimating crop-environmental interactions that affect assimilate supply, including breeding, industrialisation-induced or geo-engineered solar dimming, genetically and atmospheric CO2-related increased RUE, and source manipulations, such as pest and disease impacts.

Annual crops accommodate environmental variation through grain number, whereas grain weight is more stable. Grain number is determined in a species-specific window which has been established for many crops, but not for oat. Field trials were established at two sites in southern Australia and in one site in southern Chile where successive, single 14-d shading periods were applied from crop establishment to maturity to identify the developmental window when the crop is most responsive to stress. Three oat varieties were compared in Australia (Mitika, Williams and Wintaroo) and two in Chile (Mitika and Yallara). Unshaded controls yielded from 327 g m −2 in Australia to 747 g m −2 in Chile. The overall pattern of yield response to time of stress was similar to that of wheat; it spanned the period from stem elongation (GS31) to about 10 days after anthesis. In line with theory, most of the yield response was mediated through response in grain number; further, the two environments in Australia where reduction in grain number in response to stress shortly before anthesis was larger, individual grain weight increased with shading. Grains per panicle was more responsive to stress than panicles per m 2 , in contrast to other cereals. The critical period is often assumed to be species-specific. However, our limited comparison of varieties suggests that there might also be varietal differences in oat. Interaction between time of shade and variety was significant for harvest index in all locations. Hence, we propose genotype-dependent response to time of stress is worth exploring.

International audience; Silicon (Si) exerts beneficial effects in mitigating aluminum (Al) toxicity in different plant species. These include attenuating oxidative damage and improving structural strengthening as a result of the increased production of secondary metabolites such as phenols. The aim of this research was to evaluate the effect of Si on phenol production and composition in two barley cultivars under Al stress. Our conceptual approach included a hydroponic experiment with an Al-tolerant (Sebastian) and an Al-sensitive (Scarlett) barley cultivar treated with two Al doses (0 or 0.2 mM of Al) and two Si doses (0 or 2 mM) for 21 days. Chemical, biochemical and growth parameters were assayed after harvest. Our results indicated that the Al and Si concentration decreased in both cultivars when Al and Si were added in combination. Silicon increased the antioxidant activity and soluble phenol concentration, but reduced lipid peroxidation irrespective of the Al dose. Both barley cultivars showed changes in culm creep rate, flavonoids and flavones concentration, lignin accumulation and altered lignin composition in Si and Al treatments. We concluded that Si fertilization could increase the resistance of barley to Al toxicity by regulating the metabolism of phenolic compounds with antioxidant and structural functions.

Breeding heavier seeds while maintaining seed number has been proposed as an effective strategy to improve oil crop production. Recent studies assessing rapeseed source-sink (S-Sratio) reduction after flowering suggest a window in the early seed-filling period that can compensate, either partially or totally, for a decrease in the number of seeds. However, little evidence has been reported in relation to the key window for seed plasticity in rapeseed. Furthermore, such studies have only assessed spring rapeseed genotypes and there is a lack of information on winter rapeseed. This study aims to elucidate the occurrence of a narrow window when seed plasticity is greatest, which is hypothesized to occur during the early stages of seed filling after the beginning of flowering in both winter and spring rapeseed genotypes. To asses this hypothesis, two winter and two spring rapeseed hybrids were evaluated under three S-Sratio treatments in two experiments performed in field conditions in Valdivia, Chile. The winter hybrids Trust and Mercedes were assessed under different nitrogen levels, and the spring hybrids Lumen and Solar were assessed using two sowing dates. S-Sratio treatments in both rapeseed genotypes consisted of a control without S-Sratio manipulation, a reduced S-Sratio from the beginning of flowering [BBCH 61] to 15 days after flowering (DAF) and a reduced S-Sratio from 15 to 30 DAF. The S-Sratio was reduced by shading the crop with black nets to intercept 75 % of the incoming solar radiation. Shading treatments decreased seed yield relative to the control between 13 and 42 % for the winter genotypes and from 23 to 44 % for the spring genotypes. The negative effect of shading on seed number under both S-S reduction timings explains the seed yield reduction. Interestingly, different windows of time for seed weight plasticity were found in response to the lower S-Sratio for winter and spring genotypes (i.e., 15–30 and 0–15 DAF, respectively). In the winter-type genotypes, seed weight increased between 28 and 33 % under the 15–30 DAF shading treatment, while seed weight in spring-type genotypes increased from 15 to 39 % during the 0–15 DAF treatment. Seed weight change was almost negligible outside of the sensitivity window as this trait increased only 2% at 0–15 DAF in winter-type genotypes and 7% at 15–30 DAF in spring-type genotypes, across treatments. In parallel, the S-S reduction decreased seed number by 23.6 and 40 %, at the 0–15 and 15–30 DAF treatment, respectively. The study showed statistical differences for seed oil and protein concentration across treatments, but both quality traits remained highly stable to the S-S reduction and showed similar behavior in winter and spring rapeseed types. Seed oil concentration changed little while seed protein concentration was more sensitive to seed weight response. The findings of the present study suggest that seeds per silique and seed weight are determined by the plant assimilate S-Sratio; therefore, seed number and seed weight determinations overlap as has been demonstrated in other crops like wheat, barley, sorghum and sunflower.

The cellular expansion of maternal tissues has been suggested as strongly associated to grain volume determination in wheat and is therefore a key process to understand potential grain weight determination. The dynamics of grain growth in seed size-contrasting diploid, tetraploid and hexaploid genotypes of wheat were evaluated to explore the relationships between the final grain weight and: a) grain volume, b) water content, c) epidermal pericarp cell size and d) the rate of cell area (area per cell) increase. The assessment of cell expansion was reinforced by the study of the expression of the XTH5 gene in pericarp tissues, as this gene codifies a cell wall-remodeling agent. Differences in grain weight across ploidies and experiments (greenhouse and field evaluations) were attributable to the grain-filling rate (ANOVA p

Understanding the response of the number of seeds and seed weight to the availability of assimilates is crucial for designing breeding strategies aimed to increase seed and oil yield in oilseed rape. This study aims to answer the questions: i) do seed number and seed weight in oilseed rape differ in their plasticity in response to the availability of assimilates at flowering? and ii) how sensitive are oil and protein concentrations to the availability of assimilates during flowering? A spring oilseed rape hybrid was sown in two field experiments and the treatments were combinations of i) two plant densities and ii) shading or no shading between the beginning and end of flowering. Seed yield was not affected by plant density or by shading. Lower plant density was compensated by an increase in seed number per plant, without effects on single-seed weight. However, the negative effect of shading during flowering on seed number per area was fully compensated by an increase in single-seed weight by 47–61%. The plasticity of single-seed weight observed in the present study of oilseed rape has never been reported for annual seed crops. Shading at flowering increased both the seed filling rate and the duration of the seed filling period at all positions in the canopy. We also observed that the reduction of the source-sink ratio at flowering increasing seed weight does not necessarily modify oil or protein concentrations. Thus, single-seed weight could be targeted to increase seed yield in oilseed rape without compromising oil content.

Faba bean is a cool-season, indeterminate grain legume grown in Mediterranean and temperate environments where yield loss associated with abiotic and biotic stress is common. Yield responses depend on the timing of stress relative to the species-specific vulnerable stages in the crop lifecycle. To determine the critical period for yield determination in faba bean, we used sequential 14-d shading periods in locally-adapted varieties grown in five environments with yield of unshaded controls from 2.3 to 6.8 t ha−1. Yield of shaded treatments diverged from the controls around 450 °Cd before flowering, reached the largest difference around 100 °Cd after flowering, and converged with controls towards 700–800 °Cd after flowering; the most critical stage aligned with pod emergence. Seed number accounted for most of the variation in yield response to shading. Shading increased seed size when stress severely reduced seed number. Pod number accounted for most of the variation in seed number for shading before flowering, and both pod number and seeds per pod contributed to the reduction in seed number in crops shaded after flowering. The defined critical period is a useful reference for breeding and agronomic solutions to improve yield under stress.

Previous yield gains in wheat have been achieved through increased expression of grain number per unit area, while fruiting efficiency has also been shown to explain improved grain number. However, combining a high grain number and weight in the same genotype is a difficult task in practical breeding. This study evaluated the progeny of a cross between two elite parents that contrast in grain number and grain weight, as a way to better understand how the two desired traits might be combined to achieve a significant boost in yield. The two parents are both high biomass elite spring wheat lines that achieve high yield through contrasting strategies: Bacanora achieves high grain number with a high density of relatively small spikes. Weebil has a lower density of larger spikes in which larger grain size is expressed. The specific objectives of this research were: (i) to evaluate grain yield performance of DH lines and parents over two seasons, (ii) to identify promising physiological traits explaining high yield performance and (iii) to determine whether these traits are also expressed at the low plant densities. Two experiments were carried out under field conditions in southern Chile where the highest yield potential of wheat has ever been recorded. In Exp. 1 the performance of the 105 spring wheat DH lines, their parents and a Chilean spring cultivar (Pandora-INIA) with similar phenology were assessed together. Results showed that it is possible to increase yield potential by combining high grain number and grain weight, thereby reducing the trade-off between both components. 42 and 50% of the DH lines showed transgressive segregation in S1 and S2, respectively. 3 and 4% of the DH lines yielded between 15,000 and 16,000 kg ha−1. Grain yield was highly correlated with above-ground biomass (R2 = 0.78, p

SUMMARYAgricultural systems are challenged by global climatic change in a scenario of increasing food demand by a growing population. The increase in average temperature will be the main environmental factor affecting the crop development and productivity worldwide, although changes in carbon dioxide (CO2) concentration and rainfall are also expected. Global warming in the range of moderately high temperatures (15–32°C) is projected for temperate environments such as that of central-southern Chile, where grain crops such as wheat are widely grown. The present study assessed the impact of moderately high temperatures on both yield and quality traits of wheat during key stages for grain number and grain weight determination. Two cultivars of spring wheat (Pandora INIA and Huayún INIA) were grown under field conditions during two cropping seasons (2006/07 and 2007/08) under different thermal regimes, consisting of a combination of three temperatures (a control at ambient temperature and two increased temperature treatments, ranging from 2·6 to 11·7°C above the control) and two (3–15 and 20–32 days after anthesis) or three (booting to anthesis (Bo-At), 3–15 and 20–32 days after anthesis) timing regimes. The data recorded showed that the extent of yield reduction was strongly dependent on the timing of the heat treatments. Increased temperature at pre- (Bo-At) or early post-anthesis (3–15 days after anthesis) affected grain yield the most (reducing it by 8–30%). In light of these results, yield reductions of up to 18% can be expected when the crop undergoes average temperature increase of 2·8°C at Bo-At. In this study, the negative effect of increasing temperature on grain yield was associated with both grain number and grain weight reductions; however, different sensitivities to higher temperatures were found between cultivars. Although protein concentration of grains was not affected by higher temperatures, other negative effects on industrial quality traits are important, considering the impact of thermal treatments on grain size of both cultivars.

There is a noticeable gap in knowledge about the key phases for grain yield determination in temperate legume crop species such as lupin and pea. For this reason, the aim of the present study was to comparatively assess the critical period for grain yield and grain number determination of narrow-leafed lupin ( Lupinus angustifolius L.) and pea ( Pisum sativum L.). Two field experiments were conducted during the 2007–08 (Exp. 1) and 2009–10 (Exp. 2) growing seasons at the experimental station of the Universidad Austral de Chile. Treatments were the combination of (i) two crops (narrow-leafed lupin and pea) and (ii) seven shading treatments (control without shading and six shading treatments imposed for 15 days throughout the crop cycle). Lupin showed a longer ( P P P

Among the yield components, grain weight is considered a conservative trait whose determination is still beyond our complete understanding. Crop physiology uses a whole approach to study this complex trait, which can provide helpful information to plant breeders and molecular biologists. This study emphasizes the understanding of pre- and post-anthesis determinants of final grain weight. A field experiment was carried out in two growing seasons evaluating two wheat cultivars contrasting in grain weight potential. Carpel weight at pollination, grain dimensions, grain water, dry matter and volume dynamics were assessed. Among grain dimensions, grain length was the trait, which explained final grain weight (r2 = 0.78; P

The impacts of acidic soils and Al toxicity on wheat nutrient economy have been scarcely researched under field conditions even though these soils are widely spread in wheat production areas around the world. The main objective of this study was to quantitatively evaluate the element (N, P, K, Ca and Al) economy of an Al-sensitive and an Al-tolerant wheat cultivar growing under different soil Al concentrations at field conditions. To reach this objective, two field experiments were conducted in an Andisol in Valdivia (39°47′18″S, 73°14′05″W), Chile. Treatments were a factorial arrangement of: (i) two spring wheat cultivars (Al-sensitive, Domo.INIA and Al-tolerant, Dalcahue.INIA) and (ii) five exchangeable Al levels (0–2.7 cmol(+) kg−1) with three replicates. At harvest, plant biomass was sampled and divided into 5 organ categories: ears, grains, blade leaves, stems plus sheath leaves and roots. The element content (N, P, K, Ca and Al) in each organ was measured to quantify element uptake and concentration, nutrient uptake efficiency (UPE) and nutrient utilization efficiency (UTE). Element uptake (N, P, K, Ca, and Al) was negatively affected by the increased soil Al concentration in above-ground and root biomass in both cultivars (R2 = 0.61–0.98, p

Since the introduction of mechanized production in both developed and developing countries, crops and their management have undergone significant adaptation resulting in increased productivity. Historical yield increases in wheat have occurred across most regions of the world (20–88 kg ha−1 year−1), but climate trends threaten to dampen or reverse these gains such that yields are expected to decrease by 5–6% despite rising atmospheric CO2 concentrations. Current and projected climatic factors are temporally and spatially variable in dryland cereal production systems throughout the world. Productivity gains in wheat in some locations have been achieved from traditional agronomic practices and breeding. Continued improvement in all cereal production regions and locations of the world requires technical advances, including closer monitoring of soils, water conservation strategies, and multiple sowing times using different crops to reduce risks. The management of disease, pests, and weeds will be an added challenge, especially in areas of higher precipitation. Excellent progress has been achieved in Asia and there is much potential in Sub-Saharan Africa. Technical solutions seem within our grasp but must be implemented in the context of variable social, economic, regulatory, and administrative constraints, providing opportunities for cross fertilization and global collaboration to meet them.

Soil acidity and Al toxicity are highly extended in agricultural lands of Chile, especially where wheat is widely sown. To evaluate quantitatively the response of wheat biomass and its physiological determinants (intercepted radiation and radiation use efficiency) to Al toxicity, two field experiments were conducted in an Andisol in Valdivia (39°47′S, 73°14′W), Chile, during the 2005–2006 and 2006–2007 growing seasons. Treatments consisted of a factorial arrangement of: (i) two spring wheat cultivars with different sensitivity to Al toxicity (the sensitive cultivar: Domo.INIA and the tolerant cultivar: Dalcahue.INIA) and (ii) five exchangeable Al levels (from 0 to 2.7 cmol(+) kg−1) with three replicates. Crop phenology and intercepted radiation (IR) were registered during the entire crop cycle, while 10 samples of above-ground biomass were taken at different stages between double ridge and maturity. Both biomass and leaf area index (LAI) were recorded in these 10 stages. Radiation use efficiency (RUE) was calculated as the slope of the relationship between accumulated above-ground biomass and accumulated photosynthetically active radiation intercepted by the canopy (IPARa). Crop phenology was little affected by soil Al treatments, showing only up to 17 days delay in the Al-sensitive cultivar under extreme Al treatments. Above-ground biomass at harvest was closely associated (R2 = 0.92) with the crop growth rate but no relationship (R2 = 0.14) was found between the crop cycle length. IPARa explained almost completely (R2 = 0.93) the above-ground biomass reached by the crop at harvest under the wide range of soil Al concentrations explored in both experiments. On the other hand, a weaker relationship was found between above-ground biomass and RUE. The effect of soil Al concentration on IPARa was mainly explained by LAI as a single relationship (R2 = 0.93) between IR (%) and LAI at maximum radiation interception showing a common light attenuation coefficient (k = 0.33).

Several studies have been conducted to evaluate the response of crops, especially temperate cereals, to different source–sink ratios during grain filling. However, there is much less information about temperate legumes and even less work comparing the two. The objective of this study was to evaluate the response of both grain yield and grain nitrogen concentration of wheat ( Triticum aestivum L.), narrow-leafed lupin ( Lupinus angustifolius L.) and pea ( Pisum sativum L.) to similar source reduction during grain filling. Two field experiments were conducted in a high yielding environment of Southern Chile. In experiment 1 wheat and narrow-leafed lupin were grown for two consecutive years. Experiment 2 evaluated wheat and pea on two sowing dates. In both experiments a reduction in the source–sink ratio was imposed by using black nets that intercepted 90% of the incident solar radiation from the commencement of the linear dry matter accumulation to physiological maturity. Grain yield was differentially ( p p vs. temperate legumes.

Ozone layer depletion increases the level of ultraviolet radiation reaching the earth's surface affecting both natural and agricultural ecosystems, especially in the Southern Hemisphere. Considering the harmful effects UV-B radiation has on plant growth the future productivity of wheat crops in Southern Chile could be challenged by both (i) the forthcoming level of UV-B increase and (ii) the sensitivity of this crop to higher UV-B radiation. In this study the effect of increased UV-B radiation at different phenophases on a spring wheat cultivar (Pandora) was investigated in two experiments at plant and crop levels under out-door conditions. The experiments consisted of controls, increased UV-B radiation at specific phenophases (from 3 leaf stage to booting 3L-Bo, and from booting to maturity Bo-PM), and increased UV-B radiation for the majority of the crop cycle (from 3 leaf stage to maturity). UV-B radiation was increased by Q panel UV-313 lamps set in plastic framed structures. Control plants were grown either without frames or below the same framed structures as those which received increased UV-B treatments. Phenology, above-ground biomass, grain yield, components, grain protein concentration, leaf area index (LAI), Fv / Fm and pigments were measured at booting and/or at harvest. Above-ground biomass and yield decreased by 11–19 and 12–20%, respectively, when UV-B radiation was increased at the 3L-Bo phase, while no effect was observed when irradiation was applied later in the crop cycle (Bo-PM). No additional UV-B effects to those observed at booting were detected in plants irradiated during the majority of the entire crop cycle (3L-PM). Biomass variation was strongly associated ( r = 0.99; P Fv / Fm , chlorophyll, carotenoid concentration and carotenoid:chlorophyll ratio were found at crop level (experiment 2) under higher UV-B in the 3L-Bo and 3L-PM treatments. The flavonoid concentration responded differently in the two experiments, probably due to the optimum responses these pigments had to expose UV-B doses.

Nitrogen (N) uptake at heading and at maturity was evaluated under four N treatments (20, 50, 110 and 160 kgN ha−1 at sowing) in four malting barley cultivars released to the market from 1944 to 1998 in Argentina. Grain N concentration was slightly lower in the newest cultivar than in the oldest one. Grain N yield showed cultivar × N interaction: modern cultivars tended to increase their grain N yield in response to N fertilization more than old cultivars. Grain N yield was correlated to grain yield as well as to total above-ground biomass. Nitrogen fertilization affected total N in above-ground biomass at maturity, but no differences among cultivars were found. Nitrogen harvest index of the most modern cultivar (0.76) was higher than that of the oldest cultivar (0.67). Modern cultivars tended to have a higher N content in ears at heading than old cultivars, and the magnitude of the differences increased with N availability. Physiological nitrogen efficiency for grain yield was significantly higher in the newest than in the oldest cultivar (57 and 43 g grain g N − 1 , respectively).

SUMMARYThe increase of ultraviolet (UV-B) radiation could be a challenge for wheat production systems in Southern Chile, as in other areas. Previous reports have shown that increased UV-B radiation decreases wheat yield by affecting both grain number and grain weight. However, contrasting results have also been published showing no effect on wheat biomass and grain yield. In addition, little is known about the effect of higher UV-B radiation at particular periods of the crop cycle on grain quality traits. The objective of the present study was to evaluate grain yield, yield components and grain quality in response to increased UV-B radiation during key periods of yield component determination. Two experiments were carried out under field conditions in the Universidad Austral de Chile (latitude 39°62′S). Two spring wheat cultivars were exposed to two periods of supplemented UV-B radiation (280–320 nm): (i) between booting and anthesis,c. 20 days, and (ii) from 10 days after anthesis until physiological maturity,c. 40 days. Ultraviolet radiation was increased to levels of 3·8 and 4·9 kJ/m2/day in experimental years 1 and 2, respectively, by using UV-B lamps. At harvest, plants were sampled to quantify aboveground biomass, grain yield, grain number and average grain weight. In addition, protein and gluten concentration of grains were measured. Grain yield was not affected (P>0·05) by the UV-B increase at pre- or at post-anthesis treatments. Similar results were found for each yield component. In addition, grain protein and gluten concentration showed similar values in the increased UV-B and control treatments. Therefore, if increases in UV-B radiation take place during the latter stages of the crop cycle as is expected, the present results do not support the speculation that increases of UV-B radiation in Southern Chile will compromise wheat production systems.

In temperate cereals are commonly accepted that determination of grain number (GN) and grain weight (GW) scarcely overlap during the crop cycle. However, the assumption that GW is determined exclusively after anthesis needs to be critically reviewed in the light of reports published over the few years where temperature treatments imposed before anthesis decreased GW of bread wheat. Although these evidences suggest that both GW and GN could be affected by environmental conditions before anthesis little is known about the effect of pre-anthesis temperature on these two main yield components in wheat, barley and triticale at field conditions. In addition, the effect of temperature on GW and GN at different stages prior to anthesis has been scarcely evaluated. The objectives of the current study were: (i) to evaluate the overall response, and specific differences, of GN and GW to pre-anthesis temperature, and (ii) to study the effect of different timings of high temperature at pre-anthesis on GN and GW in wheat, barley and triticale. Three fully irrigated field experiments were carried out in three successive seasons. At each season, a wheat, barley and triticale high yielding cultivar was evaluated at three temperature regimes: control, and two timings of heating before anthesis. During the first and second seasons, the timings of heating were booting-anthesis and heading-anthesis. In the thirst season, the timings were beginning of stem elongation-booting and booting-anthesis. Plots were arranged in a split-plot design with three replicates, where the main plot was assigned to thermal regime and the sub-plots to crop species. To apply heat, transparent chambers equipped with thermostatically controlled electric heaters were used. The thermal regime was controlled by sensors connected to a temperature regulator and recorded using data loggers. Temperature within the chambers was stable across developmental stages, crops, and seasons; it averaged 5.5 °C higher than air temperature. Thermal treatments consistently reduced grain yield (p < 0.05), the magnitude of the effect ranged between 5 and 52%. The highest effect was found when temperature increased during stem elongation (yield decrease: 46%), lowest when treatments were imposed during heading-anthesis (15%) and intermediate for treatments imposed during booting-anthesis (27%). Most effects of thermal treatments on yield were due to parallel effects on GN. However, thermal treatments significantly (p < 0.05) decreased GW during the three seasons. The most effecting treatment on GW was when the crops were heated during the B-A period, i.e. GW decreased up to 23%.

For many years yield improvement reported in wheat was associated with increased dry matter partitioning to grain, but more recently increases in above-ground biomass have indicated a different mechanism for achieving yield potential. The most likely way of increasing crop biomass is by improving radiation use efficiency (RUE); however there is evidence that sink strength is still a critical yield limiting factor in wheat, suggesting that improving the balance between source and sink (source/sink (SS)) is currently the most promising approach for increasing yield, biomass, and RUE. Experiments were designed to establish a more definitive link of SS traits with yield, biomass and RUE in high-yield environments using progeny deriving from parents contrasting in some of those traits. The SS traits formed three main groups relating to (i) phenological pattern of the crop, (ii) assimilation capacity up until shortly after anthesis, and (iii) partitioning of assimilates to reproductive structures shortly after anthesis. The largest genetic gains in performance traits were associated with the second group; however, traits from the other groups were also identified as being genetically linked to improvement in performance parameters. Because many of these traits are interrelated, principal component analysis (PCA) multiple regression and path analysis were used to expose these relationships more clearly. The trait most consistently associated with performance traits was biomass at anthesis (BMA). The PCA indicated a fairly close association among traits within this group (i.e. assimilation-related traits) while those from the other two groups of SS traits (i.e. phenological and partitioning) appeared to have secondary but independent effects. These conclusions were partially born out by stepwise multiple regression for individual crosses where BMA was often complemented by traits from the two other groups. Taken together, the data suggest that the assimilation traits biomass in vegetative stage (BMV) and BMA have partially independent genetic effects in this germplasm and were complementary to achieving improved performance. The identification of a number of SS traits associated with yield and biomass, which both PCA and multiple regression suggest as being at least partially independent of one another, support the idea that additive gene action could be achieved by adopting a physiological trait based breeding approach where traits from different groups are combined in a single background. A second breeding intervention based on these results would be in selecting progeny for BMA and BMV using spectral reflectance approaches since those traits that lend themselves to large-scale screening. Path analysis confirmed the importance of the spike primordial stage in the genotype by environment interaction for these traits.

Agreement that wheat (Triticum aestivum L.) is scarcely limited by source of assimilates during grain fill has not been confirmed when source was drastically decrease in high yield potential environments. Dry matter (DM) and water dynamics of growing grains being possibly able to explain grain weight (GW) responses to source-sink ratios (S-Sratios) in these conditions. Objectives were to evaluate response of GW to S-Sratios at different phases during grain fill, and relationship between DM and grain water content in response to different S-Sratios. Wheat was sown at field conditions during 20042005 and 2005-2006 growing seasons. Four S-Sratios were assessed: Control without S-Sratios modification (C), S-Sratios from anthesis (At) +12 d to physiological maturity (ShAll), S-Sratios from At +12 d to first half of grain fill (Sh1st) and from second half of grain fill to maturity (Sh2nd). Thousand grain weight (TGW), GW at individual positions (IGW), stabilized grain water content (SGWC) and grain filling rate (GFR) were measured. TGW sensitivity to S-Sratios varied according to length of treatment and its timing, i.e. ShAll, Sh1st, and Sh2nd treatments reduced TGW by 48%, 26%, and 22%, respectively. These reductions were little higher when IGW were evaluated in ShAll (i.e. 53%) and Sh1st (i.e. 33%) treatments and lower in Sh2nd (i.e. 12%). SGWC sensitivity was lower than that of IGW across S-Sratios (e.g. SIiau 27%, Shust 22%, and Sh2nd 5%). However, close association between IGW and SGWC (R² = 0.78, p < 0.001) and between GFR and SGWC (R² = 0.98, p ≤ 0.001) was found regardless of S-Sratios and seasons

Two field experiments were carried out to evaluate the effect of the genetic improvement of barley on leaf and tiller appearance. Four malting barley cultivars were used to represent the main genotypes released from 1944 to 1998 in Argentina. One experiment was carried out with 110 kg N ha −1 of nitrogen availability at sowing, while in the other experiment cultivars were grown with 20, 50, 110 or 160 kg available N ha −1 . There was no difference in final leaf number among cultivars, which ranged from 9.5 to 11.2 leaves per main stem. Differences in phyllochron between old and modern cultivars were only observed for the early leaves and under treatments of low nitrogen availability (20 and 50 kg N ha −1 ). Cultivars began tillering at a similar time. Nitrogen treatments modified the range of maximum tiller number per plant from three (N 20 ) to seven (N 160 ) across cultivars. In addition, higher nitrogen availability delayed the time to maximum tiller number. Differences in the number of tillers per plant were explained by the rate of tiller appearance. High levels of tillering were accompanied by high levels of intercepted radiation and associated with modern cultivars growing under high levels of nitrogen. Accumulated intercepted radiation during the jointing–heading period had a trend to increase with years from release, at a mean rate of 1.17 MJ m −2 per year.

Barley breeding programs have empirically selected for improving grain yield and quality. The objective of this study was to quantify genetic gains in yield in 2-rowed malting barley cultivars released from 1944to 1998 in Argentina, identifying the major physiological traits responsible for them. For this purpose, a field experiment was conducted in absence of biotic and abiotic stressful factors and with lodging being prevented mechanically. Until the 1970's,potential yield was maintained nearly constant at 5.25 mg ha-1 and since then it increased at a rate of 41 kg ha-1 year-1. That bi-linear trend was closely related to the trend of averaged yields obtained by farmers. The contribution made by breeding yield potential to the total yield gains achieved by farmers was estimated in c. One third. Neither time to heading nor time to maturity were systematically modified by breeding. However, the partitioning of the developmental time was modified: time to achieve both maximum number of floret primordia and length of the jointing –heading period were increased with the year of release of the cultivars. The main component associated with yield was the number of grains per m2, due to variations in number of spikes per m2.Total and vegetative biomass at maturity increased with the year of release of the cultivars, at a rate of 45 and 19 kg ha-1 year-1, while both harvest index and stem height remained virtually unmodified. Differences in biomass at heading among cultivars were related to the improvement on the abilities to capture more radiation.

Element concentration in wheat grains is an important objective of plant breeding programs. For this purpose, synthetic hexaploid lines (Triticum durum ×Aegilops tauschii) have been identified as potential sources of high element concentration in grains. However, it is not known if these lines reach higher element concentrations in grains as the consequence of a dilution effect due to lower grain yield. In addition, most of the studies carried out with these lines did not evaluate above-ground element uptake. The objective of this study was to improve understanding of grain element concentrations as a function of grain yield, element uptake and biomass and element partitioning to grains in synthetic and conventional cultivars of wheat. One experiment with two standard sowing dates was carried out under field conditions. Biomass, grain yield, and macronutrient(Ca, Mg, K, P and S) and micronutrient (Cu,Fe, Mn and Zn) concentrations in grains and vegetative tissues were measured in two cultivars and one synthetic (chosen from ten lines). The synthetic showed higher element concentration in grains, e.g. between 25 and 30% for Fe, Mn and Zn across sowing dates, than cultivars while grain yield was similar or lower, depending on the sowing date. On the contrary, the synthetic showed lower concentration of Cain grains. This line showed also higher uptake of Fe, Mn, K and P than cultivars. The superior grain element concentration of the synthetic line was not only due to a dilution effect but also to a higher uptake efficiency. Therefore, synthetics would bea valuable source of germplasm for increasing element grain concentration, at least in this case for Fe, Mn, K and P.

Traditionally, barley in Argentina has been cultivated in low-yielding environments. A study was conducted to test whether breeding for improved performance under these conditions would have also improved the responsiveness to nitrogen availability. Four cultivars of two-rowed malting barley (released in 1944, 1960,1982 and 1998) were grown under 4 rates of nitrogen fertilizer at sowing (20, 50, 110and 160 kgN ha-1). All cultivars increased their yield with the increase in soil nitrogen. But yield of modern cultivars responded more strongly than yield of old ones. For modern cultivars, increase in grain yield was of 12 ± 0.6 kgha-1 for each 10 kg ha-1 of increase in the mean yield (environmental index). Absolute values of genetic gain were related to nitrogen availability: 1.59, 2.58, 4.52 and 4.29 g m-2 year-1 for the N20, N50, N110 and N160 treatments, respectively. Grain yield was associated with grain number m-2, which was dependent on spikes m-2 and grains spike-1. Total biomass at maturity also explained the changes in yield. It is concluded that selection under stress conditions was, in this case, beneficial to identify cultivars with high yields under a wide range of nitrogen availabilities.

Although individual grain weight is an important source of variation forgrain yield, there is still poor understanding of the causes determining finalgrain weight. Almost all studies conducted for understanding thedeterminants of grain weight have been focused on the post-anthesis period.However, there is important evidence that pre-anthesis conditions couldalso modify final grain weight. Three experiments including different sowingdates, genotypes and temperature regimes between booting and anthesis,were carried out in Argentina and Mexico to analyse the effect oftemperature and associated traits during the pre- and post-anthesis periodson grain weight under field conditions. In these experiments final grainweight could not be explained by average or maximum temperature duringthe post-anthesis period. However, average temperature between bootingand anthesis was closely related to the observed grain weight differences,probably as a consequence of the effects of this factor on carpel growth.Differences in grain weight between genotypes and grain position weresuccessfully explained by differences in carpel weight at anthesis. Theseresults suggest that our knowledge to determine grain weight could improveif the immediately pre-anthesis period conditions were taken into account.

The understanding of ecophysiological basis of wheat (Triticum aestivum L.) grain yield potential provides a useful framework to complement conventional breeding aimed at achieving genetic gains. This study analyzed the ecophysiological performance of an elite wheat mapping population (105 double-haploid lines derived from two modern cultivars, Bacanora and Weebil, with similar phenology but different and stable combinations of grain number per area unit (GN) and grain weight (GW) resulting in high grain yield) grown in four contrasting high-yielding environments, to determine the most successful strategies to increase grain yield potential. Main effect of environment on grain yield was significant (p < 0.0001) but the genotypic component was larger than genotype × environment interaction (30%). A robust and positive relationship between grain yield and biomass production was observed across all environments (r2 > 0.82, p < 0.0001), and relatively high harvest indexes were expressed (0.39–0.51). While GN was clearly the dominant numerical component in terms of association with grain yield (r2 > 0.51, p < 0.0001), a wide range in both components (i.e., GN and GW) was observed across all environments. This population represents a valuable resource for prebreeding studies, as the transgressive segregation in physiological and numerical yield components in combination with favorable expression of all agronomic traits could allow a fine phenotyping and mapping to identify key traits and quantitative trait loci linked with grain yield. Fil: García, Guillermo Ariel. 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: Hasan, Ahmed K.. Universidad Austral; Chile Fil: Puhl, Laura E.. Universidad de Buenos Aires. Facultad de Agronomia; Argentina Fil: Reynolds, Matthew P.. Centro Internacional de Mejoramiento de Maíz y Trigo; México Fil: Calderini, Daniel F.. Universidad Austral; Chile Fil: Miralles, Daniel Julio. 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

The effect of plant breeding on yield and their physiological determinants has been widely studied in wheat. However, it is poorly understood how, and to what extent, yield stability has been modified. To attempt a direct analysis of changes in absolute and relative yield stability, data of yield of cultivars released in different eras in different environments were obtained from records from our lab and from the literature. Depending on the availability of data, effects of plant breeding on yield stability of cultivars released in Argentina, Australia, Italy and the United Kingdom were evaluated using a quantitative approach. In this paper it was assumed that the slope of yield vs. environmental index estimates the instability of the cultivars. In addition, a more qualitative approach for Mexico, and the former USSR complemented this analysis. There was a clear decrease in yield stability assessed in absolute terms as a consequence of wheat breeding. In Argentina, Australia, Italy and the UK this decrease was related to the magnitude of yield increases. However, the decrease in yield stability in Argentina and Australia was less than for Italy and the UK, particularly so during the last 30 years. Modern cultivars released in Argentina and Australia showed a trend to maintain yield stability as a percentage of their yield similar to that of their predecessors, while the two European countries analysed tended to a slight decrease in yield stability even in relative terms. The complementary, less quantitative evaluation of Mexico and the former USSR appeared to confirm the quantitative trends described for the other countries, i.e. a general decrease in yield stability (assessed in absolute terms) with genetic gains in yield potential.

Field experiments were conducted during 1991 and 1992 to identify the effect of wheat breeding on crop biomass production and its physiological determinants, i.e. radiation interception and radiation-use efficiency. To address this objective, biomass accumulation and its physiological attributes of seven cultivars (including a commercial hybrid) released between 1920 and 1990 were compared in fertilised and irrigated plots. Weeds, pests, diseases and lodging were controlled or prevented. Crop radiation interception was calculated from measurements of incident and transmitted radiation at different times after sowing. Above-ground dry matter was determined at particular stages of development of the cultivars. Radiation-use efficiencies and crop growth rates were calculated for each cultivar for pre-anthesis (in both years) and post-anthesis (in 1992) periods. Biomass at anthesis tended to be less in most recently released cultivars than in older materials. Accumulated intercepted radiation at similar developmental stages also differed between cultivars. These differences, as well as the trend of biomass at anthesis, were caused by differences in the length of developmental phases between cultivars rather than by changes in the architecture of the canopies. The most recently released cultivars had shorter seedling emergence-anthesis periods than older cultivars. Moreover, all cultivars had similar canopy light extinction coefficients ( k ), pre-anthesis radiation-use efficiencies, and crop growth rates. After anthesis, the two oldest cultivars accumulated the least biomass and their radiation-use efficiencies and crop growth rates were smaller than those of modern cultivars. Remarkably, modern cultivars maintained during post-anthesis almost the same levels of crop growth rates and radiation use efficiencies reached during the pre-anthesis period.

Although it has been generally recognized that the difference in yield potential amongst wheat cultivars released in different eras is related to differences in their reproductive sink strength, there have been few investigations about changes in source–sink ratios as a consequence of wheat breeding. In the present study, two field experiments, in which plots were fertilized and irrigated and lodging and diseases were prevented, were carried out with seven cultivars (including a commercial hybrid) representing different periods of plant breeding in Argentina from 1920 to 1990. The cultivars were defoliated during post-anthesis to analyse the response of grain weight at particular positions within the spike (which have intrinsic differences in potential size).Individual grain weight was virtually unaffected by defoliation in the old cultivars, but modern cultivars exhibited a significant reduction in individual grain weight for several positions within the spike, although this reduction was small (c. 15%) and many grains were unaffected. In addition, no relationship was found between individual grain weight in the controls and its reduction due to defoliation.We concluded that if the source–sink ratio is further reduced, the grain yield of modern wheats will be simultaneously limited by the source and the sink. Future breeding should therefore attempt to improve simultaneously both sink and source strengths.

While an adequate supply of food can be achieved at present for the current global population, sustaining this into the future will be difficult in the face of a steadily increasing population, increased wealth and a diminishing availability of fertile land and water for agriculture. This problem will be compounded by the new uses of agricultural products, for example, as biofuels. Wheat alone provides ≥20% of the calories and the protein for the world's population, and the value and need to increase the production is recognized widely. Currently, the world average wheat yield is around 3 t/ha but there is considerable variation between countries, with region-specific factors limiting yield, each requiring individual solutions. Delivering increased yields in any situation is a complex challenge that is unlikely to be solved by single approaches and a multidisciplinary integrated approach to crop improvement is required. There are three specific major challenges: increasing yield potential, protecting yield potential, and increasing resource use efficiency to ensure sustainability. Since the green revolution, yields at the farm gate have stagnated in many countries, or are increasing at less than half the rate required to meet the projected demand. In some countries, large gains can still be achieved by improvements in agronomy, but in many others the yield gains will only be achieved by further genetic improvement. In this overview, the problems and potential solutions for increased wheat yields are discussed, in the context of specific geographic regions, with a particular emphasis on China. The importance and the prospects for improvement of individual traits are presented. It is concluded that there are opportunities for yield increase but a major challenge will be avoiding a simultaneous increase in resource requirements. Fil: Hawkesford, Malcolm J.. Rothamsted Research; Reino Unido Fil: Araus, Jose Luis. Universitat de Barcelona. Facultat de Biologia. Unitat de Fisiologia Vegetal; España Fil: Park, Robert. University of Sydney. Faculty of Agriculture and Environment. Plant Breeding Institute; Australia Fil: Calderini, Daniel. Universidad Austral de Chile. Facultad de Ciencias Agrarias. Instituto de Producción y Sanidad Vegetal; Chile Fil: Miralles, Daniel Julio. 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 Agronomía. Departamento de Producción Vegetal. Cátedra de Cerealicultura; Argentina Fil: Shen, Tianmin. Henan Tianmin Seed Company Limited; China Fil: Zhang, Jianping. Henan Tianmin Seed Company Limited; China Fil: Parry, Martin A. J.. Rothamsted Research; Reino Unido

Two field experiments were carried out with seven wheat cultivars (three of them, including a commercial hybrid, released during the last 10 years) representing different eras of plant breeding, to evaluate genetic improvement over the last century in grain yield, height, biomass, harvest index and grain yield components. Plots were fertilized and irrigated, and lodging and diseases were prevented. Main culm height was negatively correlated with the year of release of the cultivars, probably as a consequence of selection for increased lodging resistance. There was no significant association between total above-ground biomass and year of release of the cultivars. On the other hand, grain yield increased as newer cultivars were released. Results indicate that during recent years harvest index has been kept as the main attribute responsible for increases in grain yield. In general, number of grains/m2 was associated with increases in grain yield during the century. However, the newest cultivars showed an increased grain weight. In both growing seasons, cultivars released before 1980 showed a trend towards reduced grain weight, but cultivars released after 1987 had a similar number of grains per m2 with a higher grain weight than their predecessors. This was probably because the most modern cultivars have a longer grain-filling duration with a similar length of growth cycle.

Past breeding programs for increased wheat grain yield have increased the number of grains per m 2 , associated with a higher spike dry weight at anthesis. Other workers have shown that shading during the preanthesis period strongly reduced the number of grains per m 2 of the crop via reductions in the spike dry weight at anthesis. However, these effects have not been determined for cultivars of different potential number of grains per m 2 . A factorial field experiment was executed with three bread wheat cultivars released in 1920 (Klein Favorito), 1940 (Eureka FCS) and 1980 (Buck Pucara) in Argetina and two shading treatments (shading to 50% of the incoming solar radiation from the beginning of stem elongation to heading or no shading). Both cultivars and shading significantly affected the number of grains per m 2 , but their interaction was not significant. Cultivars differed only in number of grains per spikelet, while shading reduced both the number of spikes per m 2 and the number of grains per spike. The lack of a significant interaction indicates that the absolute reduction in grain number was similar for all cultivars, but the relative effect of shading was much greater for the old (low number of grains) cultivar than the modern one (62 and 42%, respectively). Preanthesis shading reduced number of grains per m 2 on main shoots less (45%) than on tillers (65%). Therefore, the relative contributions of these shoot categories to total number of grains per m 2 were strongly modified by shading. An evaluation of published data with conflicting findings on this shading effect suggests that the degree of modification observed has been related to the proportions of tiller and main shoot spikes.

The importance of yield improvement at farm conditions is highly dependent on the interaction between genotype and environment. The aim of the present work was to assess the attainable yield of a traditional and a modern malting barley cultivar growing under a wide range of soil nitrogen (N) availabilities and different water scenarios (low, intermediate and high rainfall conditions during the fallow period and throughout the crop cycle) considering a 25-year climate dataset for two sites (a shallow and a deep soil) in the Pampas, Argentina. For that purpose, a barley model was first calibrated and validated and then used to expand field research information to a range of conditions that are not only much wider but also more realistic than experiments on experimental farms. Yield of the modern cultivar was at least equal to (under the lowest yielding conditions) or significantly higher (under most growing conditions) than that of the traditional cultivar. Averaged across all the scenarios, yield was ~20% higher in the modern than in the traditional cultivar. The average attainable yield represented 42% of the yield potential in the shallow and 79% in the deep soil profiles. Yield advantage of the high yielding cultivar was based on using N more efficiently, which not only determined higher attainable yields but also reduced the requirements of soil N to achieve a particular yield level. Farmers would face little risk in adopting higher yielding cultivars in both high and low yielding environments and even in the latter ones N fertilisation could be beneficial in most years. Fil: Abeledo, Leonor Gabriela. Universidad de Buenos Aires. Facultad de Agronomía. Departamento de Producción Vegetal. Cátedra de Cerealicultura; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Parque Centenario; Argentina Fil: Calderini, Daniel F.. Universidad Austral de Chile; Chile Fil: Slafer, Gustavo A.. Universitat de Leida; España

Soil constraints affect potential grain yield of wheat. Among these constraints acidic soils are especially important due to their combined effect on aluminum (Al) toxicity and phosphorus (P) fixation. The objective of the present study was to evaluate the response of final leaf number (FLN), phyllochron and tillering dynamic of wheat in response to different Al and P concentrations in the soil under field conditions. Two field experiments were conducted in an Andisol in Valdivia (39°47′S, 73°14′W), Chile, during the 2006–07 (Expt 1) and 2007–08 (Expt 2) growing seasons. Treatments in Expt 1 consisted of a factorial arrangement of: (i) two spring wheat cultivars with different sensitivities to Al toxicity (the sensitive cultivar: Domo.INIA and the tolerant cultivar: Dalcahue.INIA) and (ii) five exchangeable soil Al levels (from 0 to 2.7 cmolc kg–1). In Expt 2 treatments consisted of a control, two levels of Al toxicity and two P treatments with three replicates in both experiments. Leaf appearance was measured from seedling emergence to anthesis; their dynamics were recorded according to the scale developed by Haun. FLN and tiller appearance were recorded in the same plants at the same time. Exchangeable Al affected FLN in Expt 1 showing a linear association in both cultivars (r = 0.99). In Expt 2 FLN was unaffected by both Al and P levels because there was a lower soil Al concentration in this experiment. Leaf appearance rate (LAR) was adjusted to bilinear equations, differentiating among early and later leaves. In Expt 1 soil Al concentration affected phyllochron of early leaves, increasing this trait by 14 and 33 degree-days in the Al-sensitive and Al-tolerant cultivars, respectively. Similarly, phyllocron of later leaves was also increased but at a higher extent in the same cultivars (62 and 38 degree-days). Both Al toxicity and P shortage decreased the maximum (MNT) and final number of tillers (FNT). Leaf area index at anthesis was positively associated with FLN (r = 0.77) and MNT (r = 0.95 and 0.99 in the Al-sensitive and Al-tolerant cultivars, respectively), with no regard to Al or P constraints.

Little is known about nutrient distribution within the spike of bread wheat (Triticum aestivum L.). This knowledge is important for determining breeding strategies aimed at increasing grain yield without affecting nutritional quality. The objective of this study was to gain a better understanding of how grain position affects nutrient concentration, dry matter distribution, and water dynamics of grains. An experiment using two sowing dates was performed under field conditions. Dry weight and concentrations of macronutrients (Ca, Mg, K, P, and S) and micronutrients (Cu, Fe, Mn, and Zn) in grains from the basal (BS), central (CS), and apical spikelets (AS) of two cultivars and one synthetic hexaploid line were determined. Grain water dynamics and nutrient and dry matter concentrations were also measured throughout the grain-filling period for the second sowing date. Genotypes showed different distributions of dry matter in different grain positions. Grain macronutrient and micronutrient concentrations in all genotypes decreased at grain positions more distal from the rachis. This reduction was as great as 30% (Ca) but varied by nutrient (e.g., Zn = -18%; S = -10%; K = +1%). Grain water content did not differ between grains. The observed differences in grain weight and nutrient concentration between grain positions could have important implications for wheat breeding. They suggest that it might be more effective to select for higher grain yield by increasing individual grain weight rather than grain number, a strategy that, in addition, would be less likely to affect the balance of nutrient concentrations within the spike.

Grain weight is a trait which has hardly been exploited for raising genetic yield potential of wheat. A clearer understanding of physiological determinants of grain weight potential would be useful in establishing the potential value of this trait in future breeding programs. The objective of this study was to improve understanding of how intra-spikelet competition for assimilates pre- and post-anthesis affect grain weight potential, and to evaluate possible mechanisms determining final grain weight in wheat. Two experiments were carried out under field con-ditions. Proximal or distal grains from the two central spikelets of spikes of three synthetic hexaploid lines were detached at heading or 7 d after anthesis. Synthetic wheats were used since they represent a potential source of genetic variability for grain weight potential. Carpel size at anthesis and grain weight during the grain filling period were measured. The de-graining treatment at heading significantly increased grain weight, especially in distal posi-tions. On the contrary, the de-graining treatment carried out after anthesis caused no increase in final grain weight. The largest response to pre-anthesis de-graining occurred in grain positions with the lowest grain mass. In addition, the effect of de-graining prior to anthesis was associated hyperbolically with weight of carpels at anthesis within each grain position. Therefore, carpel weight at anthesis could be partially associated with the regulation of grain weight potential.

Individual grain weight is an important source of variation for grain yield in wheat. The aim of this study was to investigate the effect of short periods of high temperature immediately pre-anthesis, or during post-anthesis, on grain weight under field conditions. Thus, two wheat cultivars of different grain weight potential were sown on four different sowing dates to provide different temperature conditions during the pre- and post-anthesis periods. In addition, for two sowings, acrylic boxes were installed to increase spike temperature either immediately before anthesis, or during the lineal phase of the grain-filling period. Final grain weight was significantly affected by sowing date, genotype and grain position on the spike. Grain weight showed a clear relationship with the average temperature of the grain filling period, but this relationship was either linear or curvilinear, depending on the cultivar. Both high temperature treatments, i.e. at pre- or post-anthesis, significantly diminished final grain weight, and their effect was similar with the exception of heavier grains, which were unresponsive to the high temperature treatment at pre-anthesis. Finally, a better understanding of final grain weight was reached when temperatures from the pre-anthesis period were included in the analysis of grain weight response to temperature.

Estimation of the time of physiological maturity could be beneficial to avoid yield penalties due to lodging, sprouting, hail, and other harvest risks. The aim of our study was to evaluate a simple empirical relationship (model) to determine physiological maturity by simultaneously analyzing the water and dry matter dynamics of wheat kernels. An experiment was conducted in which two cultivars with different kernel mass potential were sown on four dates. Fresh and dry kernel mass from different positions in the spike were measured twice weekly. To validate the regression model, measured and calculated data from different cultivars, growing seasons, and kernel positions were compared. A negative linear relationship between kernel dry matter (relative to final kernel mass) and kernel water concentration was determined. This showed that in wheat, physiological maturity is reached at 37% of kernel water concentration. Validation of the regression model was done using data from field experiments in Argentina and Mexico, and from controlled-conditions experiments reported in the literature. The regression model successfully simulated results from field experiments (r = 0.98; P < 0.001). In addition, data from controlled-conditions experiments showed the same negative linear relationship between relative kernel dry matter and kernel water concentration (slope = -0.4), and the model achieved a good fit for measured data (r = 0.96; P