Abstract: Low light intensity as a result of cloudy or rainy days often occurs during the grain filling of winter wheat (Triticum aestivum L.) in Huanghuai region of China, largely affecting grain yield and starch accumulation. Many investigators have de-
monstrated the effects of shading on photosynthesis, dry matter accumulation, grain yield and its components of winter wheat. However, there are few reports on the effects of shading on starch synthesis in wheat grain. In the present experiment, two wheat cultivars Jimai 20 and Shannong 1391 were used for the mechanism study on starch synthesis response to shading in grains. The black nets were over the wheat canopy to reduce light intensity to 10% in the early (6–9 d after anthesis, S1) and middle (16–19 d after anthesis, S2) grain filling stages. The starch accumulation, starch granule distribution, and activities of enzymes involved in starch synthesis were examined during grain development of winter wheat. The results showed that shading after anthesis remarkably reduced starch accumulation, and significantly increased the amylose content in ripe grains. The amount of starch accumulation showed an order of control (CK)>S1>S2. Shading significantly increased and reduced the volume percents of A-type starch granule and B-type starch granule, respectively. Logistic simulation and correlation analysis showed that it was mean starch accumulation rate and the starch accumulation rate in actively-increasing period, but not starch accumulation duration affected by shading to reduce grain starch accumulation. The activities of sucrose phosphate synthase (SPS), sucrose synthase (SS) and adnosine diphosphate glucose pyrophosphorylase (AGPase) in shading treatments were significantly lower than those in control. The activities of granule bounded starch synthase (GBSS) and soluble starch synthase (SSS) under shading stress at early grain filling were higher than those in control or no significant difference with those in control. However, the activities of GBSS and SSS under shading stress at middle grain filling were significantly lower than those in control. The results suggested that there are less the substrate for starch accumulation, caused by low activities of SPS, SS, and AGPase in S1 treatment, and both less the substrate and the low activities of starch synthase (GBSS and SSS) in S2 treatment.

Key words: Winter wheat (Triticum aestivum L.), Shading, Starch, Starch granule, Enzyme activity