Foliar application of selenium promotes starch content accumulation and quality enhancement in foxtail millet grains

The northern regions of China are the core production areas for foxtail millet. However, the lack of selenium in the soil constrains local agricultural development. Foliar application of selenium is a significant method to improve selenium levels, yet few studies have systematically analyzed the effects of selenium on the quality of millet, especially its starch content. This study aims to assess the effects of selenium foliar spray on the starch composition and overall content within millet grains, specifically by elucidating the mechanisms through examining changes in chlorophyll content in the flag leaves during the grain-filling stage and the activity of enzymes related to starch synthesis in the grains. Additionally, this research seeks to determine the optimal rate of selenium application that would maximize the quantity and quality of starch in millet grains. A two-year field experiment on foxtail millet was conducted with five selenium treatments (Se rates of 0, 20, 40, 60, and 100 g Se/ha, defined as T0, T1, T2, T3, T4 in this study, respectively). Grain starch content, activities of starch synthesis-related enzymes, flag leaves chlorophyll content, and antioxidant status of the flag leaves were measured at seven time points during grain filling. The starch content in foxtail millet grains increased quadratically with the application of selenium, reaching a maximum under the T3 treatment, which enhanced the content by 15.27 % and 11.97 % over two years, respectively. The addition of selenium at low concentrations (T1-T3 treatments) not only enhanced the activities of the three starch synthesis-related enzymes, AGPase, SuSy, and SSS, but also delayed the senescence of the flag leaves, thereby promoting grain starch synthesis. In contrast, excessive selenium application (T4 treatment) expedited the aging of the flag leaves, grain starch synthesis was inhibited compared to the T3 treatment. Moreover, foliar application of selenium led to an increase in the SSS activity in the grains, which in turn reduced the content of straight-chain starch. Under the T3 treatment, the grain straight-chain starch content decreased by 22.30 % and 28.35 % over two years, respectively. Foliar application of selenium enhanced the activities of enzymes related to grain starch synthesis and had a dual effect on the senescence process of flag leaves, regulating the content and composition of starch in foxtail millet grains. The spraying concentration of 60 g Se/ha was most favorable for the accumulation and quality improvement of grain starch. The results of this study will aid in enhancing the value-added potential of foxtail millet starch products through precise fertilization, improving the yield and quality of foxtail millet in northern China.