HEAT-STRESS DURING GRAIN FILLING IN MAIZE - EFFECTS ON CARBOHYDRATE STORAGE AND METABOLISM

Heat stress during maize seed development can interfere with endosperm starch biosynthesis and reduce seed size, an important component of y ield. Our objectives were to evaluate the direct influence of temperat ure during grain filling on kernel growth, carbohydrate accumulation, and corresponding endosperm metabolism Kernels of maize were grown in vitro at 25 degrees C until 15 or 16 days after pollination and then s ubjected to various temperatures for the remainder of their developmen t. Mature kernel dry weight declined 45% in a linear fashion between 2 2 and 36 degrees C. The rate of starch accumulation reached a maximum at approximately 32 degrees C, and when measured at frequent intervals , declined only slightly with further temperature increase to 35 degre es C. Reduced seed size resulted from an abbreviated duration of starc h-related metabolism, which did not appear to be limited by endogenous sugars. Instead, a survey of 12 enzymes of sugar and starch metabolis m indicated that ADPglucose pyrophosphorylase and soluble starch synth ase were unique in displaying developmental peaks of activity which we re compressed both in amount and time, similar to the effect of temper ature on starch accumulation. We conclude that decreased starch synthe sis in heat-stressed maize kernels results from a premature decline in the activity of these enzymes.