The Effect of Heat Stress on Wheat Leaf and Ear Photosynthesis

The effect of heat-hardening on carbon exchange rate per unit area (CER) of flag leaves, whole ears, and ears with the awns removed, was measured in hexaploid (Triticum aestivum L.) and tetraploid (T. turgidum L. and T. dicoccoides) wheat varieties. The CER for awns was calculated by the difference. For the non-hardened hexaploid cv. ‘H-895’ the CER for the leaves and glumes had an optimum temperature of 25°C. By contrast, the CER for the awns increased from 25°C to 32°C, indicating an optimum at 32°C or more. Heat-hardening decreased the CER of leaves and glumes at the optimum temperature, but increased the CER especially in leaves at supra-optimal temperatures. Thus, leaf CER in hardened plants became essentially independent of temperatures between 25°C and 32°C. Awn CER was little affected by heat-hardening. For all 12 varieties, leaf and ear CER was smaller in hardened plants at 30°C than in non-hardened plants at 22°C. Leaf and ear CER measured at 30°C differed significantly between varieties within a species. Whole ear CER at 30°C was negative in most varieties although the calculated value for the awns was positive. Thus, the high temperature optimum for CER of the awns was a major factor in the variation among wheat varieties in tolerance of ear CER to heat. The biochemical attributes of the photosynthetic mechanism in awns responsible for the high temperature optimum were already present in wild tetraploid wheat. There was a positive correlation across all varieties between ear CER at 30°C and the percentage of awns in total ear area (r = 0930, P = 0 This together with previous results (Blum, 1985a), suggests that a large amount of awns in the ear is a sensible selection index in wheat for improved production in hot, dry environments.