Silicon Enhanced Salt Tolerance in Foxtail Millet: Role of Photosynthetic Pigments, Phenolic Accumulation and Stress Responsive Enzymes

The intensification of environmental stresses, such as soil salinization, due to climate change has a substantial effect on agricultural output, crop quality and food security. The hardy crop foxtail millet (Setaria italica L.) is renowned for its capacity to withstand extreme conditions, which makes it a good option for salinity prone regions. The phenylpropanoid pathway, which is essential for the synthesis of flavonoids and phenolic compounds can lessen the effects of oxidative stress thus enhancing tolerance. With an emphasis on growth, photosynthetic pigments, phenolic accumulation, and associated enzymes, this study explored the effects of exogenous silicon and NaCl on foxtail millet. Increased NaCl concentration impaired the growth and development of the plant. This included reduced photosynthetic pigments, fresh weight (FW), dry weight (DW), and relative water content (RWC). Foxtail millet responded to stress through an increased accumulation of phenolic compounds and increased activity of associated enzymes. Si supplementation (2 mM and 4 mM), improved the growth parameters, total plant height (up to 59.2%) and leaf area (up to 207%). Si also enhanced the total chlorophyll (40.5%), FW (28.8%), DW (53.7%) and RWC (6.7%) in salt stressed foxtail millet plants. Additionally, Si boosted the activities of phenylalanine ammonia lyase (PAL) (up to 64.9%) and tyrosine ammonia lyase (TAL) (up to 95.7%) which resulted in an increased accumulation of flavonoids and phenolic compounds that help combat stress. Polyphenol oxidase (PPO) activity was also elevated by Si supplementation. This study suggests that Si can enhance salt tolerance in foxtail millet through the regulation of key physiological and biochemical parameters, including the phenylpropanoid pathway and accumulation of phenolic compounds.