Understanding the physiology of crop yield is important to inform both agronomy and breeding. In grain crops, there is consensus in the interpretation of data, further supported by theory, to conclude that grain number is source-limited; this accounts for the strong correlation of yield and grain number, and the high phenotypic plasticity of grain number due to source limitation. However, whether grain weight during the effective grain filling period is source- or sink-limited remains debatable. This lack of consensus is commonly interpreted as variation associated with the interaction between genotype and environment. In this opinion paper, we argue that part of the inconsistency in the literature may stem from overinterpretation of experimental results, extreme treatments (e.g., 50-90 % shading), and the assumptions of linearity to conclude that grain weight is sourcelimited during the effective grain filling. A central flaw is the unjustified extrapolation of conclusions from manipulated plants to the unmanipulated real crop. We review the outcomes of both direct and indirect manipulations of source-sink ratios during the effective grain filling across grain crops with a focus on methods and interpretation of results. Indirect approaches that increase or reduce grain number to measure grain weight compensation (e.g., shading or thinning the plots during the critical period of grain number determination) are ill-suited because they influence potential grain size and grain size hierarchies, confounding interpretation of the grain weight-grain number relationship. Direct manipulations of source-sink ratio that do not alter grain weight (e.g., shading or de-graining plants during the effective grain filling), provide strong evidence that grain growth in the intact control is sink-limited. Conversely, when grain weight changes significantly in response to severe manipulation, the only valid conclusion is that the manipulated plants were source-limited; it is not justified to reach conclusions on the intact control crop. These considerations call for a more cautious interpretation of experimental data where direct manipulation of the source-sink ratio leads to a significant change in grain weight, and suggest a re-evaluation of experimental and analytical methods are needed to conclude on the nature of grain weight limitation.

The physiological basis of the trade-off between grain number (GN) and thousand grain weight (TGW) is key to wheat yield improvement. To that end, three wheat line groups were assessed at conventional (300-350 pl m-2) and low (44 pl m-2) planting rates in field experiments: TaExpA6 (EXPANSIN A6 overexpression), TaGW2 (GRAIN WIDTH 2 triple knockout), and TaP1xGW2A (VEGETATIVE TO REPRODUCTIVE TRANSITION 2 ectopic expression, and TaGW2-A knockout), together with their wild types (WT). Lines TaExpA6 and TaGW2 increased TGW over their respective WTs, whereas line TaP1xGW2A showed no effect on this trait. The transgenic TaExpA6 line was the only one to achieve greater grain yield (GY) than its WT, as the TGW increase conferred by the TaGW2 triple mutant was fully offset by reductions in spike number (SpN) and grain number per spike (GNS). Contrasting effects of TaExpA6 and TaGW2 lines were identified on the ovary weight and floret dynamics, likely associated with the trade-off between TGW and GN. The TaExpA6 construct drives targeted expression in developing grain tissues at post-anthesis, avoiding the overlap with GN determination. In contrast, TaGW2 disruption constrains tillering, likely as a pleiotropic effect, and shifts intra-spike resource allocation, promoting early ovary growth at the expense of distal floret development and GNS. Low planting rate increased individual spike dry weight at anthesis across all line groups, mitigating the TGW-GNS trade-off and increasing both traits simultaneously. These results highlight that post-anthesis, grain-specific gene expression plays an important role in mitigating the trade-off between TGW and GN.

Rapeseed ( Brassica napus L.) final grain weight and in turn grain yield, results from the interaction between assimilate supply (source) and sink capacity; however, the extent to which source limitation constrains yield formation during grain filling remains under debate. Understanding how the manipulation of the source–sink ratio (S–S ratio) affects yield and grain traits is critical for elucidating the physiological mechanisms behind yield stability in high-yield environments. This study aimed to evaluate how variations in the S–S ratio during the grain-filling phase influence grain weight and yield, biomass allocation, grain-filling dynamics, and grain quality traits in rapeseed. A field experiment was conducted during two seasons in Valdivia, Chile. One high-yield potential and adapted hybrid (Click CL) was evaluated under three radiation regimes in a randomized complete block design: control, −50% incident radiation (shading), and +50 % incident radiation (reflected radiation pannels, PET). S–S ratio treatments were applied from the beginning of grain filling (BBCH 71) to physiological maturity (BBCH 89) aimed at modify the S-S ratio during the actual grain filling period. The reduced S–S ratio increased thousand-grain weight (TGW), particularly in basal siliques, resulting in yield compensation and demonstrating a strong structural and physiological buffering capacity. Conversely, increasing the S–S ratio enhanced grain number and grain yield, while TGW remained stable. Grain quality traits responded asymmetrically: under reduced S–S, oil concentration slightly declined whereas protein concentration increased. The increased S–S ratio, had no effect on grain oil and protein concentrations, remaining similar to the control. Sieving analyses revealed a shift toward larger grain size classes under reduced S– S, whereas the distribution under increased S–S resembled the control. Overall, these findings indicate that rapeseed maintains yield stability through compensatory adjustments in grain weight and size distribution under contrasting assimilate availabilities. Under high-radiation temperate conditions, rapeseed productivity during grain filling is predominantly governed by sink capacity, highlighting its physiological plasticity and resilience to variations in source–sink balance Highlights 1. In high-yield conditions without structural changes, grain filling depends on sink capacity. 2. A 50% reduction in radiation increases grain weight and maintains grain yield. 3. A 50% increase in radiation raises grain number and yield via more grains per plant. 4. Source reduction shifts grains to larger sizes; source increase maintains stability. 5. Oil in grain is stable with increased radiation, declines when it is reduced. ### Competing Interest Statement The authors have declared no competing interest. FONDECYT, 1170913 ANID, 2017-21171384

Context: Global climate change is driving the temperature increase, which often reduces crop production. Most of the temperature increase studies in rapeseed have been conducted under controlled conditions, limiting their results to true field conditions. Objective: This study aimed to assess the sensitivity of rapeseed to temperature increase during two phases of the early grain filling period in southern Chile, a high yield potential environment. To our knowledge, this is the first field study evaluating the effects of temperature increase at different phases after flowering in rapeseed. Methods: Three field experiments were conducted with two adapted spring rapeseed hybrids, Lumen and Solar CL, under three temperature treatments: a control at ambient temperature, a 5 degrees C increase from the beginning of flowering to 15 days after flowering (DAF), and the same temperature increase from 15 to 30 DAF. Results: Grain yield was minimally affected by increased temperature in Lumen, but grain yield was sensitive to heating in Solar CL (up to - 35.9 %) during the 0-15 DAF period. In this hybrid, grain number decreased 26.8 % in response to higher temperature. On the contrary, grain weight and grain oil concentration were tolerant to higher temperature and grain protein concentration was increased by heating. Conclusions: The higher grain yield resilience of Lumen could be due to the longer period between start of flowering and physiological maturity (11 days, 173.6 degrees Cd) than Solar CL. The lower impact of heating on grain yield and yield components reported in the present study across the genotypes would be attributed to the lower background temperature of southern Chile than in other environments.

Physiological basis of the trade-off between grain number (GN) and thousand grain weight (TGW) is key to improve wheat yield. To that end, three wheat line groups were assessed at conventional (300-350 pl m -2 ) and low (44 pl m -2 ) plant rates in the field: Ta ExpA6 (ectopic expression), Ta GW2 ( TaGW2 triple mutant), and Ta P1xGW2A ( VRT-A2 ectopic expression and TaGW2-A knock-out), together with their wild-types (WT). Except Ta P1xGW2A, the manipulated lines increased TGW over their WTs. However, only the transgenic TaExpA6 line reached higher grain yield (GY) than its WT, whereas increased TGW recorded by the triple knockout of TaGW2 was fully compensated by reduced spike number (SpN) and grain number per spike (GNS). Contrasting effects of Ta ExpA6 and Ta GW2 lines were found on the ovary weight and floret dynamics, associated with the trade-off between GW and GN. Interestingly, the overexpression of TaExpA6 gene acts in grain tissues at post-anthesis avoiding the overlap with GN determination, while TaGW2 disruption constrains tillering, likely as a pleiotropic effect, and shifts intra-spike resource allocation promoting early ovary growth at the expense of distal floret development and GNS. Low plant rate increased resource allocation to the spike, mitigating the GW–GNS trade-off and increasing both traits simultaneously.

Global climate change is driving the temperature increase, which negatively impacts on crop production. Most heat stress studies in rapeseed have been conducted under controlled conditions, limiting their results to true field crops. This study aimed to assess the sensitivity of rapeseed to temperature increase during two phases of the grain filling period in southern Chile, a high-yield potential environment. To our knowledge, this is the first field study evaluating the effects of heat stress at different phases of grain filling in rapeseed. Three field experiments were conducted with two adapted spring rapeseed hybrids, Lumen and Solar CL, under three temperature treatments: a control at ambient temperature, a 5°C increase from the beginning of flowering to 15 days after flowering (DAF), and the same increase from 15 to 30 DAF. Crop and climate variables, including air temperature and solar radiation were recorded along the experiments. Slight effects on grain yield due to heat stress were found, however, the hybrids exhibited different sensitivities, with Lumen being less affected than Solar CL. Grian yield of the last hybrid showed positive association with photothermal quotient and negatively with temperature, while Lumen did not show relationship. The most significant impact on grain yield occurred during the first half of grain filling (0-15 DAF), resulting in a reduction of 26.8% of grain number in Solar CL, compared to a 6.0% reduction in Lumen across experiments. Grain weight remained little affected by thermal stress, indicating its conservative behaviour and tolerance in southern Chile conditions. Grain oil concentration was scarcely sensitive, while grain protein concentration increased under heat stress. The low impact on the crop outcomes of our studies may be attributed to the lower background temperature of southern Chile, suggesting that this environment may confer greater rapeseed heat tolerance during grain filling than in other agroecosystems. ### Competing Interest Statement The authors have declared no competing interest.

Context or problem: Despite its global significance as a highly nutritious food, the critical period for grain yield and grain protein determination of quinoa has not yet been identified.Objective or research question: This study aimed to determine the critical period of grain number, yield, and grain protein of quinoa in contrasting environments of South America.Methods: Five experiments were conducted in contrasting South American environments, including Argentina, Chile, Ecuador, and Peru, from latitudes 4 degrees S to 39 degrees S, and between 19 and 2165 m.a.s.l. Treatments consisted of the factorial combination of two quinoa cultivars (except in Peru, where only one genotype was used) and 4-9 shading treatments (50-85% shade) applied over 10-20 days throughout the crop cycle, depending on the site and the length of the crop cycle. We evaluated how GY, its components, and grain protein responded to the reduction of the source-sink ratio at different phenological stages.Results: Yield of unstressed controls varied from 2.2 t ha-1 in Lima to 9.8 t ha-1 recorded in Valdivia. The grain protein concentration ranged between 12.6% in Lima and 16.8% in Loja. The most sensitive yield-determining period was between -26 and 445 degrees Cd from flowering across genotypes and environments. Grain protein con-centration increased by up to 19% when the source of assimilates was reduced after anthesis; however, this was insufficient to compensate for the protein yield that was affected by the reduction in grain yield due to shading.Conclusions: The critical period for quinoa grain yield was determined throughout the flowering and grain-filling stages across genotypes and environments, mainly due to a decrease in both grain number and above-ground biomass.Implications or significance: Identifying the critical period provides a useful target for breeding and management strategies to increase the quinoa crop yield.

The probability of occurrence of high temperatures and cloudiness events increases during the post-flowering period of temperate crops in the southern cone of South America. Several studies have evaluated the effect of high temperature or shading on wheat and rapeseed, but little is known about the impact of the combined effect of both stresses on the yield and grain quality of these crops during the post-flowering phase. This study aims to analyze the sensitivity and stability of wheat and rapeseed grain yield and quality in response to shading, high temperature and the combined stresses during post-flowering in two contrasting environments. Field experiments were carried out using the same wheat and rapeseed genotypes in environments with high (Valdivia, Chile) and medium (Buenos Aires, Argentina) yield potential. The treatments consisted of i) control, ii) shading stress, iii) heat stress and iv) combined shading and heat stress during the central hours of day. Portable chambers were placed on the plots at 7–10 days from anthesis in wheat and 14 days from the beginning of flowering in rapeseed, throughout 10 days. Grain yield was significantly reduced for the combined stress in wheat (−46%) and rapeseed (−39%) in Buenos Aires, but there were no differences between treatments in any crop in Valdivia. Combined stress in rapeseed produced a trade-off between grain number (−40%) and grain thousand weight (+43%) in Valdivia, while grain thousand weight was penalized (−25%) in Buenos Aires. For wheat there were no differences in grain protein in both locations, while for rapeseed protein increased due to the combined stress in Buenos Aires (+15%), no effects were recorded for grain oil in either location. Our results showed a high stability of grain weight in rapeseed to single and combined environmental stresses, with better environmental conditions during grain filling.

Increasing global food demand will require more food production1 without further exceeding the planetary boundaries2 while simultaneously adapting to climate change3. We used an ensemble of wheat simulation models with improved sink and source traits from the highest-yielding wheat genotypes4 to quantify potential yield gains and associated nitrogen requirements. This was explored for current and climate change scenarios across representative sites of major world wheat producing regions. The improved sink and source traits increased yield by 16% with current nitrogen fertilizer applications under both current climate and mid-century climate change scenarios. To achieve the full yield potential—a 52% increase in global average yield under a mid-century high warming climate scenario (RCP8.5), fertilizer use would need to increase fourfold over current use, which would unavoidably lead to higher environmental impacts from wheat production. Our results show the need to improve soil nitrogen availability and nitrogen use efficiency, along with yield potential. Martre et al. found that to achieve the full yield potential of improved wheat varieties, nitrogen fertilizer use would need to increase fourfold over current use, which would unavoidably increase the environmental impacts of wheat production.

Enhancing grain yield is a primary goal in the cultivation of major staple crops, including wheat. Recent research has focused on identifying the physiological and molecular factors that influence grain weight, a critical determinant of crop yield. However, a bottleneck has arisen due to the trade-off between grain weight and grain number, whose underlying causes remain elusive. In a novel approach, a wheat expansin gene, TaExpA6, known for its expression in root tissues, was engineered to express in the grains of the spring wheat cultivar Fielder. This modification led to increases in both grain weight and yield without adversely affecting grain number. Conversely, a triple mutant line targeting the gene TaGW2, a known negative regulator of grain weight, resulted in increased grain weight but decreased grain number, potentially offsetting yield gains. This study aimed to evaluate the two aforementioned modified wheat genotypes (TaExpA6 and TaGW2) alongside their respective wild-type counterparts. Conducted in southern Chile, the study employed a Complete Randomized Block Design with four replications, under well-managed field conditions. The primary metrics assessed were grain yield, grain number, and average grain weight per spike, along with detailed measurements of grain weight and dimensions across the spike, ovary weight at pollination (Waddington’s scale 10), and post-anthesis expression levels of TaExpA6 and TaGW2. Results indicated that both the TaExpA6 and the triple mutant lines achieved significantly higher average grain weights compared to their respective wild types. Notably, the TaExpA6 line did not exhibit a reduction in grain number, thereby enhancing grain yield per spike. By contrast, the triple mutant line showed a reduced grain number per spike, with no significant change in overall yield. TaExpA6 expression peaked at 10 days after anthesis (DAA), and its effect on grain weight over the WT became apparent after 15 DAA. In contrast, TaGW2 gene disruption in the triple mutant line increased ovary size at anthesis, leading to improved grain weight above the WT from the onset of grain filling. These findings suggest that the trade-off between grain weight and number could be attributed to the overlapping of the critical periods for the determination of these traits.

Increasing global food demand will require more food production without further exceeding the planetary boundaries, while at the same time adapting to climate change. We used an ensemble of wheat simulation models, with sink-source improved traits from the highest-yielding wheat genotypes to quantify potential yield gains and associated N requirements. This was explored for current and climate change scenarios across representative sites of major world wheat producing regions. The sink-source traits emerged as climate neutral with 16% yield increase with current N fertilizer applications under both current climate and mid-century climate change scenarios. To achieve the full yield potential, a 52% increase in global average yield under a mid-century RCP8.5 climate scenario, fertilizer use would need to increase fourfold over current use, which would unavoidably lead to higher environmental impacts from wheat production. Our results show the need to improve soil N availability and N use efficiency, along with yield potential.

Climate-change-induced temperature fluctuations pose a significant threat to crop production, particularly in the Southern Hemisphere. This study investigates the transcriptome and physiological responses of rapeseed to post-flowering temperature increases, providing valuable insights into the molecular mechanisms underlying rapeseed tolerance to heat stress. Two rapeseed genotypes, Lumen and Solar, were assessed under control and heat stress conditions in field experiments conducted in Valdivia, Chile. Results showed that seed yield and seed number were negatively affected by heat stress, with genotype-specific responses. Lumen exhibited an average of 9.3% seed yield reduction, whereas Solar showed a 28.7% reduction. RNA-seq analysis of siliques and seeds revealed tissue-specific responses to heat stress, with siliques being more sensitive to temperature stress. Hierarchical clustering analysis identified distinct gene clusters reflecting different aspects of heat stress adaptation in siliques, with a role for protein folding in maintaining silique development and seed quality under high-temperature conditions. In seeds, three distinct patterns of heat-responsive gene expression were observed, with genes involved in protein folding and response to heat showing genotype-specific expression. Gene coexpression network analysis revealed major modules for rapeseed yield and quality, as well as the trade-off between seed number and seed weight. Overall, this study contributes to understanding the molecular mechanisms underlying rapeseed tolerance to heat stress and can inform crop improvement strategies targeting yield optimization under changing environmental conditions.

Grain production must increase by 60% in the next four decades to keep up with the expected population growth and food demand. A significant part of this increase must come from the improvement of staple crop grain yield potential. Crop growth simulation models combined with field experiments and crop physiology are powerful tools to quantify the impact of traits and trait combinations on grain yield potential which helps to guide breeding towards the most effective traits and trait combinations for future wheat crosses. The dataset reported here was created to analyze the value of physiological traits identified by the International Wheat Yield Partnership (IWYP) to improve wheat potential in high-yielding environments. This dataset consists of 11 growing seasons at three high-yielding locations in Buenos Aires (Argentina), Ciudad Obregon (Mexico), and Valdivia (Chile) with the spring wheat cultivar Bacanora and a high-yielding genotype selected from a doubled haploid (DH) population developed from the cross between the Bacanora and Weebil cultivars from the International Maize and Wheat Improvement Center (CIMMYT). This dataset was used in the Agricultural Model Intercomparison and Improvement Project (AgMIP) Wheat Phase 4 to evaluate crop model performance when simulating high-yielding physiological traits and to determine the potential production of wheat using an ensemble of 29 wheat crop models. The field trials were managed for non-stress conditions with full irrigation, fertilizer application, and without biotic stress. Data include local daily weather, soil characteristics and initial soil conditions, cultivar information, and crop measurements (anthesis and maturity dates, total above-ground biomass, final grain yield, yield components, and photosynthetically active radiation interception). Simulations include both daily in-season and end-of-season results for 25 crop variables simulated by 29 wheat crop models.

Increasing genetic wheat yield potential is considered by many as critical to increasing global wheat yields and production, baring major changes in consumption patterns. Climate change challenges breeding by making target environments less predictable, altering regional productivity and potentially increasing yield variability. Here we used a crop simulation model solution in the SIMPLACE framework to explore yield sensitivity to select trait characteristics (radiation use efficiency [RUE], fruiting efficiency and light extinction coefficient) across 34 locations representing the world’s wheat-producing environments, determining their relationship to increasing yields, yield variability and cultivar performance. The magnitude of the yield increase was trait-dependent and differed between irrigated and rainfed environments. RUE had the most prominent marginal effect on yield, which increased by about 45 % and 33 % in irrigated and rainfed sites, respectively, between the minimum and maximum value of the trait. Altered values of light extinction coefficient had the least effect on yield levels. Higher yields from improved traits were generally associated with increased inter-annual yield variability (measured by standard deviation), but the relative yield variability (as coefficient of variation) remained largely unchanged between base and improved genotypes. This was true under both current and future climate scenarios. In this context, our study suggests higher wheat yields from these traits would not increase climate risk for farmers and the adoption of cultivars with these traits would not be associated with increased yield variability.

This research elucidates the dynamic expression of expansin genes during the wheat grain (Triticum aestivum L.) development process using comprehensive meta-analysis and experimental validation. We leveraged RNA-seq data from multiple public databases, applying stringent criteria for selection, and identified 60,852 differentially expressed genes across developmental stages. From this pool, 28,558 DEGs were found to exhibit significant temporal regulation in at least two different datasets and were enriched for processes integral to grain development such as carbohydrate metabolism and cell wall organization. Notably, 30% of the 241 known expansin genes showed differential expression during grain growth. Hierarchical clustering and expression level analysis revealed temporal regulation and distinct contributions of expansin subfamilies during the early stages of grain development. Further analysis using co-expression networks underscored the significance of expansin genes, revealing their substantial co-expression with genes involved in cell wall modification. Finally, qPCR validation and grain morphological analysis under field conditions indicated a significant negative correlation between the expression of select expansin genes, and grain size and weight. This study illuminates the potential role of expansin genes in wheat grain development and provides new avenues for targeted genetic improvements in wheat.

The properties of individual grain weight, protein concentration and protein composition contribute to grain quality of wheat. They are known to be affected by genetics, environment and crop management. This paper reports these properties in two dryland field experiments comparing cultivars with similar yield but with contrasting grain weight and number. A large-seeded cultivar, Baviacora, and a small-seeded cultivar, H45, were managed with different nitrogen (N) fertilizer applications and grown in seasons with different rainfall. The aim is to suggest strategies to produce wheat grain of consistently high quality for specified markets, with either preharvest or post-harvest management. Individual grain weight and protein concentration varied throughout the spikes of the cultivars and responded to crop management and environment. Protein concentration within a spike was closely correlated with individual grain weight, increasing by 0.05 +/- 0.006% per mg of additional grain weight, independent of cultivar or position of grains on the spike. Grain protein composition was related to protein concentration of individual grains but was not as sensitive to individual grain weight. Larger grains were associated with less glutenin per unit of protein, partly offsetting the increase in protein concentration with grain weight. Unextracted polymeric protein (%UPP) was unrelated to grain weight but was negatively related to grain protein concentration in Baviacora. If found to be general, the within-spike relationships between individual grain weight, and the concentrations of grain protein, glutenin and %UPP offer promise for improving grain segregation and post-harvest processing to produce grain of closely defined quality from an unprocessed harvest sample.

Wheat is the most widely grown food crop, with 761 Mt produced globally in 2020. To meet the expected grain demand by mid-century, wheat breeding strategies must continue to improve upon yield-advancing physiological traits, regardless of climate change impacts. Here, the best performing doubled haploid (DH) crosses with an increased canopy photosynthesis from wheat field experiments in the literature were extrapolated to the global scale with a multi-model ensemble of process-based wheat crop models to estimate global wheat production. The DH field experiments were also used to determine a quantitative relationship between wheat production and solar radiation to estimate genetic yield potential. The multi-model ensemble projected a global annual wheat production of 1050 ± 145 Mt due to the improved canopy photosynthesis, a 37% increase, without expanding cropping area. Achieving this genetic yield potential would meet the lower estimate of the projected grain demand in 2050, albeit with considerable challenges.