Alleviation Effects And Mechanisms Of Exogenous Silicon On Salt Stress-induced Damage In Alfalfa

Soil salinization causes stunted plant growth and decreased biomass yield,which has become a prominent problem threatening agricultural production in arid and semi-arid regions.In recent years,silicon,as a beneficial element,can regulate the plant growth and development under a range of adverse conditions.Silicon has proved to enhance plant stress tolerance to some extent.Studies on silicon-mediated plant salt tolerance have received attention.However,systematic studies on silicon-induced salt tolerance in alfalfa are still rare.In this study,silicon-priming experiment was taken to investigate the effect of silicon on the germination process of alfalfa seeds under salt stress.Hydroponic experiment was taken to study the effect of exogenous silicon on the growth,antioxidant system activities,photosynthesis capability,ion metabolism and carbon metabolism of alfalfa with exogenous silicon application under salt stress.The objective of this experiment was to elucidate the regulatory mechanism about exogenous silicon enhanced alfalfa salt resistance.Another experiment was taken on sandy pot to investigate the changes and physiological mechanisms of root-irrigated exogenous silicon on salt tolerance of alfalfa.These studies provide a theoretical basis for the use of exogenous silicon to enhance salt tolerance of alfalfa.The main findings were as follows:（1）Germination test showed that seed germination,seedlings root vigor and dry weight were significantly affected by salt stress（150 mM NaCl）.The inhibitory effect of salt stress on seed germination was effectively alleviated when the alfalfa seed was silicon-priming at a proper concentration（2 mM）.The amylase activity,soluble sugar content,soluble protein content,proline content and starch content were significantly increased by silicon-priming.The germination rate,germination potential,germination index and vigor index were significantly improved.Alfalfa seedlings root vigor and dry weight were also significantly increased.（2）Results based on hydroponic experiment showed that the peroxidase,superoxide dismutase,catalase and ascorbate peroxidase activities were maintained with exogenous silicon application during salt stress.The accumulation of superoxide anion and hydrogen peroxide was effectively reduced in alfalfa seedlings.This can contribute to alleviating salt stress-induced oxidative damage to alfalfa seedlings,significantly reducing malondialdehyde content in alfalfa.（3）Results based on hydroponic experiment showed that the chlorophyll a content of alfalfa was significantly increased with exogenous silicon application under salt stress.The activity and electron transfer efficiency of PSⅡreaction center was maintained.The electron transfer capacity of PSⅡwas enhanced.The excess excitation energy of PSⅡwas decreased.This could effectively protect the photosynthetic structure of leaves with higher photosynthetic capacity under salt stress.Exogenous silicon alleviated the effects of salt stress on alfalfa growth inhibition by increasing the net photosynthetic rate.（4）Results based on hydroponic experiment showed the Na⁺content of alfalfa roots and stems was significantly reduced with exogenous silicon application under salt stress.The content of K⁺,Ca²⁺ and Mg²⁺ in alfalfa roots,stems and leaves was increased.The antagonistic effect between Na⁺and K⁺,Ca²⁺ and Mg²⁺ ions was decreased under exogenous silicon addition.This could improve the uptake of major inorganic ions and effectively regulate the transport and transport selectivity ratios of major inorganic ions.The ionic homeostasis of cells was maintained with exogenous silicon during salt stress.（5）Results based on hydroponic experiment showed the activities of acid invertase and sucrose phosphate synthase in leaves was significantly increased with exogenous silicon.The activity of amylase in leaves was also significantly decreased.The accumulation of carbon metabolites was reduced in leaves.Meanwhile,the alkaline invertase,acid invertase and amylase activities was significantly increased with exogenous silicon.The sucrose phosphate synthase and sucrose synthase activities were also reduced with exogenous silicon.The accumulation of carbon metabolites was increased in roots.Exogenous silicon could regulate the activities of carbon metabolism-related enzymes.This decreased the negative photosynthetic feedback in leaves while increasing the accumulation of stored substances in roots.（6）Results based on pot experiment showed that the SOD,POD and CAT activities were maintained with silicon-root application.The accumulation of reactive oxygen species was decreased with lower malondialdehyde content in leaves under salt stress.Salt-induced oxidative damage was decreased with silicon root application.The Si content and K⁺content was increased,while the Na⁺content was decreased in alfalfa leaves with silicon-root application under salt stress.In summary,alfalfa seed germination and seedlings growth were increased with exogenous silicon under salt stress.When exogenous silicon was applied under salt stress,the antioxidant enzymes activity was enhanced.The major inorganic ions uptake,transport and distribution were effectively regulated.The carbon metabolism and photosynthetic capacity were improved with exogenous silicon.These changes could contribute to improving salt tolerance of alfalfa.