THE SYNTHESIS OF [C-14] STARCH FROM [C-14] SUCROSE IN ISOLATED WHEAT GRAINS IS DEPENDENT UPON THE ACTIVITY OF SOLUBLE STARCH SYNTHASE

Soluble starch synthase activity decreased in isolated wheat grains he ated in vials at temperatures between 31 and 40-degrees-C; a decrease of more than 50% occurred at 35-degrees-C within 30 min. Sucrose synth ase activity was not significantly affected by heating and ADPglucose pyrophosphorylase decreased to a lesser extent and more slowly than so luble starch synthase. Wheat grains which were heated for 1 h at 35-de grees-C and allowed to recover at 20-degrees-C exhibited partial recov ery of soluble starch synthase activity within 4 h. These responses in isolated grains to heating resembled effects reported elsewhere in in tact ears. Isolated grains were exposed to a range of temperatures for 1 h prior to incubation in [C-14]sucrose at 20-degrees-C. Within the range 20-40-degrees-C, the effect of pretreatment temperature on the a ctivity of soluble starch synthase was highly correlated with the inco rporation of C-14 into starch. Control coefficients close to unity ind icated that the activity of soluble starch synthase imposed a high deg ree of control on the rate of starch synthesis in this system. These r esults with isolated grains support a previous suggestion that the act ivity of soluble starch synthase at elevated temperature in wheat is a n important factor limiting the accumulation of dry matter through eff ects on the synthesis of starch.