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 heated 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 synthase activity was not significantly affected by heating and ADPglucose pyrophosphorylase decreased to a lesser extent and more slowly than soluble starch synthase. Wheat grains which were heated for 1 h at 35-degrees-C and allowed to recover at 20-degrees-C exhibited partial recovery of soluble starch synthase activity within 4 h. These responses in isolated grains to heating resembled effects reported elsewhere in intact 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 activity of soluble starch synthase was highly correlated with the incorporation of C-14 into starch. Control coefficients close to unity indicated that the activity of soluble starch synthase imposed a high degree of control on the rate of starch synthesis in this system. These results with isolated grains support a previous suggestion that the activity of soluble starch synthase at elevated temperature in wheat is an important factor limiting the accumulation of dry matter through effects on the synthesis of starch.