HIGH-TEMPERATURE AFFECTS THE ACTIVITY OF ENZYMES IN THE COMMITTED PATHWAY OF STARCH SYNTHESIS IN DEVELOPING WHEAT ENDOSPERM

Ears of wheat were exposed for up to 10 days during the grain-filling stage to high temperature (35-degrees-C) and activities of five enzyme s in the sucrose to starch pathway were compared to those in ears main tained at lower temperature (21-degrees-C day/16-degrees-C night). Two cultivars of wheat known to differ in their post-anthesis tolerance o f high temperature were compared. On a per grain basis, the activity o f sucrose synthase and of ADPglucose pyrophosphorylase in ears maintai ned at 21/16-degrees-C throughout did not change greatly between days 16 and 32 after anthesis, whereas UDPglucose pyrophosphorylase and sol uble starch synthase activities declined with advancing development. S oluble starch synthase activity in grains of heated ears was decreased within 1 day to about one-half of the value in unheated grains, and 3 days' additional heating did not reduce the activity much further. In soluble starch synthase activity was not significantly reduced by heat ing. Compared to soluble starch synthase, ADPglucose pyrophosphorylase activity was more slowly affected and decreased to a lesser extent by heat. Sucrose synthase and UDPglucose pyrophosphorylase activities we re either not affected or only slightly reduced; part of this reductio n could be due to advanced development at the higher temperature. In r ecovery experiments ears were heated for brief periods and then return ed to 21/16-degrees-C for a few days. ADPglucose pyrophosphorylase and soluble starch synthase activities recovered in the cooler conditions but the other two enzymes generally only maintained or lost further a ctivity. From a comparison of the activities of these enzymes with the rate of starch deposition, and by taking into account the effects of heating, it is proposed that the influence of heating on final grain d ry weight is attributable to the observed reductions of soluble starch synthase activity.