It has been found that the application of epi-brassionolide (epi-BR) at anthesis results in improving photosynthetic character, postponing senescence, and increasing wheat grain weight. However, the physiological effects of epi-BR application at anthesis on starch accumulation and starch quality of grains were poorly understood. The present study reports the effect of epi-BR application on the activities of some key enzymes involved in starch synthesis at grain filling stage. The experiment was carried out on the farm of Henan Agricultural University in the growing season of 2003–2004 with winter wheat cultivar Yumai 49. Four different concentrations (0, 0.1, 1.0 and 10.0 µg/L) of epi-BR were used. Epi-BR was sprayed at a rate of 75 mL/m2 on the leaves and spikes on 3 days after anthesis (DAA). The activities of key starch-synthesizing enzymes in grains, i.e. adenosine diphosphorate glucosepyrophorylase (ADPGPPase), soluble starch synthase (SSS), granule-bound starch synthase (GBSS) and starch branching enzyme (SBE), as well as the total starch content, amylopectin and amylose accumulation rate during grain filling were measured. The total starch content, amylopectin and amylose content of grains, and grain starch quality were also examined at maturing stage. The results indicated that changes in activities of ADPGPPase (Fig.1-A), SSS (Fig.1-B) and SBE (Fig.1-D) were consistent with those in total starch content and the accumulation of amylopectin. Activity of GBSS (Fig.1-C) was consistent with the accumulation of amylose. The activities of ADPGPPase, SSS and SBE were increased in the treatment of 1.0 µg/L epi-BR, thereby the total starch (Fig.2-A) and amylopectin accumulation rate accelerated (Fig.2-B). The activity of GBSS was decreased, thereby the rate of amylose accumulation slowed down by epi-BR treatment (Fig.3). Consequently, contents both of total starch and amylopectin were significantly increased (α＜0.01） compared with those of the control, respectively. Amylose content was significantly decreased (α＜0.05）, and the ratio of amylopectin to amylose increased significantly (α＜0.01 ） by epi-BR application (Table 1). The peak value and breakdown of epi-BR application treatments were 32.4 BU and 13.7 BU, significantly higher (α＜0.01） than those of control, respectively (Table 2). Epi-BR at 10.0 µg/L had similar effects, but the high value duration of the indexes were shorter. Consequently, content of total starch, amylopectin, and amylose was significantly increased (α＜0.05 ）, respectively. However the ratio of amylopectin to amylose was not significantly different with that of control. Compared with control, the peak value and swelling power were significantly higher, respectively, while the setback was significantly lower (α＜0.05）. At 0.1 µg/L, the effects of epi-BR were similar to those at 1.0 µg/L, but weaker than those at either 1.0 or 10.0 µg/L, therefore, the total starch content and starch quality were not significantly different with those of control. The results indicated that epi-BR plays an important role in starch accumulation and starch quality, probably through affecting the activities of ADPGPPase, SSS, SBE and GBSS. Under the experimental condition, epi-BR at 1.0 µg/L had the best regulative effects on total starch content, ratio of amylopectin to amylose, starch pasting properties and starch quality.
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