EFFECT OF HIGH-TEMPERATURE ON STARCH SYNTHESIS AND THE ACTIVITY OF STARCH SYNTHASE

The decrease in yield which is observed when developing storage organs such as cereal grains or potato tubers are exposed to high temperatur es is due to a lower final starch content. The rate of starch synthesi s during the development of these storage organs at high temperature, is either reduced or fails to increase sufficiently to compensate for the shorter developmental period. This effect on the rate of starch sy nthesis does not seem to be due to a reduction in the supply of photos ynthate. One of the enzymes in the pathway of starch synthesis, solubl e starch synthase, is susceptible to heat inactivation at unusually lo w temperatures and may also have a low optimum temperature for maximum activity. In some storage organs, the maximum catalytic activity of s oluble starch synthase is not very much greater than the rate of starc h synthesis. A decrease in the activity of this enzyme is therefore, l ikely to affect the rate of starch synthesis. Thus, the effect of high temperature on the rate of starch synthesis may be due, at least in p art, to the properties of this enzyme. This review discusses the unusu al heat-sensitivity of starch synthase in the context of the effects o f high temperature on starch synthesis in storage organs.