Effect of short-term heat stress prior to flowering and at early grain set on the utilization of water-soluble carbohydrate by wheat genotypes

In Mediterranean environments, cereal crops are often exposed to short periods of elevated temperatures in spring when crops are approaching flowering and grain filling. Stem water-soluble carbohydrate (WSC) could be an important carbon bank for supporting grain filling in wheat especially when carbon assimilation is hampered by heat stress. However, there is relatively little information available in the literature on the effects of short periods of heat stress on grain growth and the importance of WSC in mitigating the effects of heat stress. Therefore, field and controlled environment studies were undertaken to determine the effects of short-term heat stress on the peduncle WSC content and its contribution to grain size of wheat in the field. Wheat genotypes were exposed to heat stress on a single-day at two different stages: (H1) near flowering or green anther stage and (H2) early grain set or 7–10 days after anthesis (DAA). On each occasion crops were enclosed in a portable heat chamber and the temperature was gradually increased to a maximum of 35°C. This single day heat stress event caused a significant reduction in individual grain mass (IGM) and grain number in both years. There was no discernible change in IGM until 14 days postanthesis, after which time grain growth in the heat-stressed plants was reduced. On average, as compared to the unheated control, the reductions in IGM in wheat genotypes ranged from 10 to 25%. The maximum WSC content was reduced by heat stress in all wheat genotypes. Heat stress (average of H1 and H2) reduced peduncle WSC content by 26% and mobilized WSC content by 15% across all studies. Mobilization of peduncle WSC content was also significantly reduced by the heat stress treatments and mobilization appeared to start at 21 days postanthesis when the grain was about one third of the final mass. IGM of wheat genotypes with higher maximum peduncle WSC content showed lower sensitivity to heat stress in both years of this study (H1: r=−0.82; p<0.001 and H2: r=−0.84; p<0.001). Over both seasons, heat stress treatments (H1, H2) and all genotypes, greater mobilized WSC was associated with increased grain size (r=0.70; p<0.01). It could be argued that selection of wheat genotypes with higher stem WSC reserves and greater mobilization of WSC could be used to buffer grain growth and development under heat stress conditions, which are a common occurrence in the Australian wheat belt.