Activities of the Enzymes Involved in Starch Synthesis and Starch Accumulation in Grains of Wheat Cultivars GC8901 and SN1391

Two wheat cultivars, GC8901 (hard winter wheat) and SN1391 (soft winter wheat), were used for investigating the changes of enzyme activities for sucrose metabolism and starch biosynthesis and the accumulation characters of starch composition. Result showed that, the activities of sucrose (SS), sucrose-phosphate synthase (SPS), adenosine diphosphorate glucose pyrophrylase (ADPGPPase) and soluble starch syntheses (SSS) of SN 1391, which has more starch, were significantly higher than GS 8901, that with low starch content. But the changes of granule-bound starch synthase (GBSS) activity were consistent with the amylose content which indicated that amylose contents in grain were determined by GBSS activity, especially the activity at later grain filling stages. Simulating with Richards Equation showed that it was initiating time and accumulation rate, but not accumulation duration that determined the content of starch composition. Furthermore, the changes of sucrose transport capacity were consistent with SSS and GBSS activities, starch accumulation rate was accordant to ADPGPPase and SS/SPS ration, not SS, SPS, SSS or GBSS activities. The results suggested that there is no inevitable relation of starch accumulation rate and starch composition contents with the activity of single enzyme such as SS, SPS, SSS or GBSS, but closely related to ADPGPPase activity and SS/SPS ratio, and it was SPS and ADPGPPase have played a vital role in the biosynthetic pathway. Later polymerization reactions catalyzed by SSS and GBSS do not seem to control the rate of starch accumulation, but do affect starch structure.