Effects of Exposure of Wheat Ears to High Temperature on Dry Matter Accumulation and Carbohydrate Metabolism in the Grain of Two Cultivars. II. Carry-Over Effects

Yield of wheat exposed in the field during the grain-filling stage to brief episodes of high (but sub- lethal) temperature is reduced as a consequence of lower single grain weight due mainly to diminished accumulation of starch. To simulate these effects, ears of wheat were enclosed in a transparent box midway through the grain filling stage and heated (35�C day/25�C night) above ambient conditions (21�C day/16�C night) for periods of up to 7 days. Afterwards the ears were returned to ambient conditions. Two cultivars known to differ in their tolerance of high temperature were compared. Final single grain dry weight of the more tolerant cultivar (Kalyansona) was not significantly affected by exposure to heat, but it was reduced in cv. Sun 9E by approximately 15.5% after 7 days of exposure. Rate of grain filling in the cool conditions following previous exposure to high temperature was also substantially slower in both cultivars than it was in ears that had not been heated. Following transfer from hot to cool conditions, the amounts of fructose and the concentration of UDP-glucose returned to control levels. On the other hand, quantities of sucrose in the grain of ears that had been heated rose higher than in ears that had been kept continuously in cooler conditions. Thus any effects of high temperature at the sucrose synthase step appeared to be readily reversible, and both cultivars responded similarly. Cultivar-dependent responses were observed however in effects on the concentrations of hexose phosphates and ADP-glucose. After only one hot day the concentration of G-6-P, F-6-P and G-1-P returned to control levels following transfer to the cooler environment. After 2 or more days the concentrations of these metabolites did not recover in cv. Sun 9E but recovery was observed in cv. Kalyansona after 2 or 4 days' exposure, but not after 7 hot days. Last, concentrations of ADP-glucose in the cultivar of which starch deposition was reduced most at high temperature (Sun 9E) rose more after transfer from high to low temperature than was the case in the more tolerant cultivar (Kalyansona), and ADP-glucose concentrations fell more in the latter than in the former cultivar as a consequence of 7 days at high temperature. It is suggested that more than one action of temperature could be involved in the cultivar-dependent responses to high temperature: effects on starch synthase activity as well as on the metabolism and possibly transport of hexose phosphates are possibilities in accordance with the information available.