Quantifying pre- and post-anthesis heat waves on grain number and grain weight of contrasting wheat cultivars

Abstract

Context or problem: Further increasing wheat productivity is more challenging due to consistently rising temperatures. Heat stress negatively affects both grain number per m2 (GN) and average grain weight (AGW). It is not clear whether the sensitivity of these components differ, nor whether it would be related to the relevance of the component for determining unstressed yield in the particular cultivar. Objective or research question: The aim of this study was to elucidate the plasticity of GN and AGW in response to heat waves at pre-anthesis; and post-anthesis under different source-sink ratios. Methods: The field experiments were carried out in two different locations of Catalonia (NE Spain). We selected two contrasting genotypes (Pistolo and Sublim) for the relevance of GN and AGW and analysed in detail (i) the causes for their differences in AGW and (ii) their GN and AGW responses to heat wave stress (HT). The HT treatments were imposed in the field using portable tents with transparent polyethylene films at either at booting (pre-AN HT) or 15 days after anthesis (post-AN HT) for 7 days, with a delta in maximum temperature of c. 5ºC. In addition, we increased source-sink ratios in the control and post-AN HT treatments by removing all spikelets on one side of the spike (de-graining) 14 days after anthesis. Results: Pistolo had always less but heavier grains than Sublim. The differences in AGW were constitutive, rather reflecting a trade-off with GN, as there was no scarcity of resources to satisfy the demands of the growing grains in any of the two cultivars. Exposing the crops to heat stress at pre- and post-anthesis resulted in yield penalties, but yield was more sensitive to pre-AN HT than to post-AN HT, consistently across both locations and genotypes. The reduction of GN per spike was mainly mediated through grain abortion rather than through mortality of floret primordia. The reduction of AGW caused by post-AN HT was not related to source strength changes, but due to a direct effect reducing potential grain weight (PGW) per se. There seemed to be a trend for the cultivar constitutively producing more grains having less sensitivity of GN to pre-AN HT, and for the cultivar constitutively having heavier grains having less sensitivity of AGW to post-AN HT. Conclusions: The reduction of AGW caused by post-AN HT was not related to source strength changes, but due to a direct effect reducing potential grain weight. The plasticity of a yield component in response to heat stress was inversely related to the relevance of that component for yield determination in unheated conditions. Implications or significance: Our result would open an opportunity for breeders to select for improved sink-strength through either higher GN or higher PGW depending on the timing and frequency of the most damaging heat waves.

We thank Albert Morera at University of Lleida who provided advanced R coding for data analysis. Funding was provided by projects AGL2015-69595R and RTI2018-096213-B-100 funded by the Agencia Estatal de Investigación (AEI) of Spain. Jinwook Kim held a pre-doctoral research contract from AGAUR (the Agency for Management of University and Research Grants of Catalonia).