Abstract:

Grain starch is synthesized from sucrose of leaves, and regulated by a series of enzymes in wheat (Triticum aestivum L.). Previous researches have revealed that sowing date affects wheat quality by changing protein content, wet gluten content, sedimentation value, dough development time and stability time. Grain quality also relates to starch content, especially the component of starch. In order to understand the regulation mechanism of sowing date to the accumulation and component of grain starch, a wheat coultivar, Gaocheng 8901, was employed with different sowing dates. The starch (amylose and amylopectin) content in grain, and the activities of sucrose-phosphate synthase (SPS), sucrose synthase (SS), adenosine diphosphorate glucose pyrophrylase (AGPase), uridine diphosphorate glucose pyrophrylase (UGPase), soluble starch synthase (SSS) and granule-bound starch synthase (GBSS), as well as sucrose content in flag leaf and grain were determined during grain filling. The results showed that the accumulation of amylose was decreased under both early and late sowing treatments, but that of amylopectin increased significantly (P<0.01) (Fig.1). The optimum daily sunlight hour (ASH) for amylose and amylopectin accumulation were 9.06 and 11.07 hours, respectively (Table 1). The amylopectin content was maximal when the average daily air temperature (ATD) was 22.75℃, while amylose content showed a positive correlation with ATD from 17℃ to 25℃. Under late sowing treatment, the activities of SS and SPS increased in flag leaf, but decreased in grain significantly (P<0.05) 14 days after anthesis, while the sucrose accumulation in flag leaf and grain both reduced (Fig.3–6). Late sowing also resulted in the increase of AGPase activity, SSS activity and amylopectin accumulation, but the decrease of UGPase and GBSS activities in grain at mid to late period of grain filling (Fig.7, 8). The above results suggest a feedback system of sucrose metabolism by starch accumulation in wheat, which takes function through reducing the activity of SPS instead of SS in grain. The approach of sucrose decomposition catalyzed by SS is not the unique one for starch synthesis in grain. AGPase, SSS, UGPase and GBSS all play important roles in starch synthesis, of which UGPase and GBSS activities are restrained at certain stages to regulate the transformation of sucrose to starch and the accumulation of starch component, so that a reasonable ratio of amylose to amylopectin can be kept.

Key words: Sucrose, Starch, Starch synthase, Sowing date, Wheat