Contrasting effects of chronic heat stress and heat shock on kernel weight and flour quality in wheat.

Phytotron studies were conducted to compare the potential effects of chronic high-temperatures (daily maxima of over 20˚C) and heat-shock conditions (a few days of over 32˚C), on wheat (Triticum aestivum L.) yield and quality, to form a basis for the selection of improved high-temperature tolerance in wheat. The series of heat-shock treatments were designed to provide similar heat loads, by varying the duration (number of days) of each treatment. Studies involved two cultivars, Lyallpur and Trigo 1. Both showed a reduction in kernel weight in response to chronic high day temperatures (i.e. above 18˚C), with Trigo 1 more tolerant than Lyallpur. Kernel weight of both cultivars was also reduced by short periods of heat shock, and this was most evident at day/night temperatures above 30/25˚C. There was no reduction in the germination of the lighter weight kernels formed under either chronic high temperature or heat-shock conditions. Dough strength, as judged by mixing time, declined in both genotypes with prolonged chronic high temperature, and also following the most extreme of the heat-shock treatments - Trigo 1 showed an ability to resist these changes better than Lyallpur. The heat-related decreases in dough strength were associated with decreases in the proportion of the larger molecular size glutenin (most 'unextractable'). This change in quality was not however, associated with changes in flour protein content.