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Background and Aims The pericarp weight comprises Methods Two spring wheat cultivars contrasting in kernel weight (Bacanora and Kambara) were sown in field conditions during seasons 2012–13 and 2014–15. Both genotypes were grown under contrasting plant density (control, 370 plants m–2; and low plant density, 44 plants m–2) and night temperatures, i.e. at ambient and increased (>6 °C) temperature for short periods before and after anthesis. From anthesis onward, grains were harvested every 3 or 4 d. Grain samples were measured and the pericarp was removed with a scalpel. Whole grain and pericarp fresh and dry weight were weighed with a precision balance. At harvest, 20 grains from ten spikes were weighed and grain dimensions were measured. Key Results Fresh weight, dry matter and water content of pericarp dynamics showed a maximum between 110 and 235 °Cd. Maximum dry matter of the pericarp ranged between 4.3 and 5.7 mg, while water content achieved values of up to 12.5 mg. Maximum values and their timings were affected by the genotype, environmental condition and grain position. Final grain weight was closely associated with maximum dry matter and water content of the pericarp. Conclusions Maximum pericarp weight is a determinant of grain weight and size in wheat, which is earlier than other traits considered as key determinants of grain weight during grain filling. Better growing conditions increased maximum pericarp weight, while higher temperature negatively affected this trait.

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.

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.

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.

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

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.

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.

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 increase of UV-B radiation may be a challenge for wheat production systems in Southern Chile. Previous reports had shown that increased UV-B radiation decreased wheat yield by affecting both grain number and grain weight. The objective of the present study was to evaluate the response of grain yield, yield components and grain quality to higher UV-B radiation during key periods for yield component determination. The experiment was carried out at field conditions in the Universidad Austral de Chile (40 ˆ South latitude). Treatments consisted on 2 spring wheat cultivars exposed to 2 periods of supplemented UV-B radiation (280–320 nm): between (i) booting and anthesis and (ii) anthesis and physiological maturity. Ultra-violet radiation was enhanced 3.5 kJ m-2 during 5 hours per day by UV-B lamps (Q panel UV-313, Philips). At harvest, plants were sampled to quantify above-gound biomass, grain yield, grain number and thousand grain weight. In addition, protein and gluten concentration of grains were measured. Grain yield was not affected (p>0.05) by UV-B increase either at pre- nor at post-anthesis treatments. Similar results were found for each yield component. In addition, grain protein and gluten concentration showed similar values between enhanced UV-B and control treatments. Therefore, these results do not support that expected increases of UV-B radiation in Southern Chile could compromise wheat production systems, at least if increases of UV-B radiation take place at latter stages of the crop cycle

The physiological and genetic basis of yield improvement in wheat is only partially understood. Nonetheless, a significant increase in yield and biomass has been observed in several backgrounds when alien chromatin associated with Lr19 was introgressed from Agropyron elongtatum Theoretically, higher yield and biomass may be achieved through (i) greater interception of incident radiation, (ii) increased radiation use efficiency, (iii) a more optimal source-sink balance permitting higher sink demand and/or a higher partitioning of assimilates to yield. The objectives of the current study were to evaluate the performance of near isogenic lines differing in Lr19 to observe the physiological basis of superior performance Lr19 was associated with increases in yield (average 13%), final biomass (10%) and grain number (15%) in all backgrounds studied. Differences were not associated with improved light interception based on measurements of biomass shortly after canopy closure, nor with improved radiation use efficiency (RUE) prior to grain filling based on biomass accumulation rate and direct measurement of flag-leaf photosynthetic rate prior to anthesis. Lr19 was associated with an increased partitioning of biomass to spike growth at anthesis (13%), a higher grain number per spike, and higher RUE and flag-leaf photosynthetic rate during grain filling. The mechanism causing increased partitioning of assimilates to spikes relative to the rest of the plant in Lr19 isolines was apparently not related to phenology or assimilation capacity.

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.

Source limitation during grain filling is important for both management and breeding strategies of grain crops. There is little information on the sensitivity of grain weight of temperate cereals to variations in source–sink ratios, and no studies are available on the comparative behaviour of temperate cereals growing together in the same experiment. The objective of the current study was to evaluate, under field conditions, the response of grain weight to different source–sink ratios during grain filling in high-yielding cultivars of bread wheat, durum wheat, and triticale at 2 contrasting locations. Two experiments were carried out at C. Obregon and El Batan in Mexico. In each location, 6 genotypes (2 bread wheat, 2 durum wheat, 2 triticale) were evaluated. A week after anthesis, 2 source–sink (control and halved spikes) treatments were imposed. Location and genotype significantly (P < 0.01) affected grain yield and components. Significant grain weight increases (P < 0.05) were found only in 2 cases in El Batan. The highest response of 17% was found in triticale, with less than 10% in most of the other genotypes. The effect of genotype and location is discussed.

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.