Comparison of sudden heat stress with gradual exposure to high temperature during grain filling in two wheat varieties differing in heat tolerance .1. Grain growth

Two wheat varieties differing in heat tolerance were exposed to four heat treatments in order to determine if a sudden rise from ca 20-40 degrees C caused a greater reduction of individual kernel mass than a gradual (6 degrees C h(-1)) rise over the same temperature range. For the heat sensitive variety (Oxley), the reduction of individual kernel mass following sudden heat stress (26%) was greater than that resulting from a gradual heat stress of equivalent thermal time (13%) or equal days of treatment (18%). By contrast, for the heat tolerant variety (Egret), the reduction of individual kernel mass following rapid exposure to heat stress (12%) was not significantly greater than that caused by a gradual treatment of equal days duration (10%). Nevertheless, for Egret, sudden heat stress significantly reduced mature kernel mass compared with high temperature treatment of equivalent thermal time (6%). We conclude that heat acclimation may help to mitigate wheat yield losses due to high temperature and that the ability to acclimate to high temperature varies between wheat genotypes. Comparison of wheat varieties for yield tolerance to high temperature should therefore occur under conditions that allow gradual acclimation to elevated temperature.