Physiological bases of wheat grain weight response to heat waves: Post-anthesis sensitivity and responses to source-sink manipulations in contrasting cultivars

The companion paper reported that two contemporary cultivars differed constitutively in their average grain weight (AGW) and in their sensitivity to heat waves (HW). It is relevant to elucidate whether the effects of HW are direct on the capacity of the grains to grow or indirect through penalizing post-anthesis (AN) growth and consequently restricting the availability of assimilates for the growing grains. Testing HW with changes in source-strength could help further understanding the causes of the sensitivity of AGW to a HW. This may help to identify strategies in crop management and traits to target in breeding. Objectives: To quantify the effects and analyse the causes of post-AN HW on AGW on two contrasting cultivars, considering a double treatment at both stages (pre- + post-AN HW) and whether an increase in source strength through removing alternate rows in the plots by thinning modify the yield penalties. Also, source strength per grain during the effective period of grain filling was either decreased (through a defoliation) or increased (through de-graining) in combination with each of the other treatments. Methods: Experiments were carried out in two locations, Lleida and Bell-lloc (NE, Spain). Main treatments consisted in the factorial combination of two contrasting genotypes (Pistolo of high AGW and Sublim of high GN), and three HW that were imposed in the field using installing tents with transparent polyethylene films either at booting (pre-AN HW) or 15 days after anthesis (post-AN HW). An additional HW (combination of pre- and post-AN HW) was imposed in one location while thinning treatment at the onset of stem elongation in the other. Results: Across both locations, AGW was consistently higher in Pistolo compared to Sublim. Moreover, the entire distribution of individual grain sizes was higher in Pistolo, with both the lightest (bottom decile) and heaviest (top decile) grains significantly heavier in Pistolo. The main reason for the higher AGW of Pistolo was its higher rate of grain growth, as the duration was similar between both genotypes and also higher potential weight. Post-AN HW, reduced AGW, being Pistolo more sensitive than Sublim. The double HW treatment revealed that exposure to a post-AN HW was markedly less detrimental when preceded by a pre-AN HW. This suggests a non-additive effect on AGW, with prior HW exposure inducing a priming response that mitigated the impact of subsequent stress through antagonistic interactions between the two events. Varying source-sink ratios under heated conditions did not change the weight of the grains noticeably suggesting that direct effects of post-AN HW on the capacity of the grains to grow dominated those mediated by assimilate availability during grain filling. Conclusions: The GN–AGW trade-off was not due to limited resources or more small grains. Post-AN HW reduced AGW, but this was lessened in thinned plots, suggesting reduced source strength. HW likely affected AGW via direct (growth capacity) and indirect (assimilate supply) mechanisms. De-graining partially reversed HW effects. Interestingly, combined pre- and post-AN HWs reduced AGW less than post-AN HW alone, suggesting a possible priming effect. Further field studies are needed as multiple heat events may become more common with climate change. Implication: As the HW effects include a direct effect component, breeding programs needs to target traits related to grain growth resilience through profiting on genotypic variation in sensitivity of the capacity of the grains to grow; and agronomic practices should consider strategies relate to escaping HWs.