Eco-Physiological Mechanisms Of Silicon-Mediated Alleviation Of N-and P-Limitations In Alfalfa Plants

Alfalfa（Medicago sativa L.）is a perennial herb in the family Leguminosae and is known as the "King of Forage".As two indispensable nutrients for the growth and development of alfalfa,the limitation of nitrogen and phosphorus in the soil has great influence on the growth and physiology of alfalfa.In order to reduce the environmental pollution caused by artificial nitrogen and phosphorus fertilizer inputs,two alfalfa ’WL-363’ and ’WL-656’ were used as materials to systematically investigate the effects of different nutrient conditions including balanced（N:P=15）,phosphorus limitation（N:P=135）and nitrogen limitation（N:P=1.7）and 1 mmol L^-1 and 2 mmol L^-1 potassium silicate on the growth and physiology of alfalfa under different nutrient conditions using the pot experiment,which consists of nine treatments with four replicates for each treatment.By measuring the effects of different nutrient treatments and silicon addition on the growth and photosynthetic characteristics of alfalfa,physiological metabolic characteristics,nitrogen and phosphorus content and root system characteristics,as well as C:N:P stoichiometry,the effects of silicon application on alleviating nitrogen and phosphorus limitation in alfalfa and related physiological and ecological mechanisms were revealed to provide relevant scientific basis and theoretical guidance for the application of silicon fertilizer in alfalfa cultivation.The main research results are as follows:1.Under nitrogen and phosphorus limitation,alfalfa plant height and biomass were reduced,but not to a significant level,leaf area,net photosynthetic rate as well as stomatal conductance were significantly reduced.The optimal concentration of silicon significantly increased the silicon content of leaves and roots and alleviated the inhibitory effect of nitrogen and phosphorus on the growth and photosynthesis of alfalfa plants,as shown by the improvement of growth index,chlorophyll content,net photosynthetic rate,and stomatal conductance.Moreover,the amount of silicon deposited in leaves was significantly and positively correlated with plant height,biomass,chlorophyll content and net photosynthetic rate.2.Malondialdehyde content of alfalfa leaves under nitrogen and phosphorus limitation showed an increasing trend and indicated impaired plasma membrane integrity.Under phosphorus limitation,the soluble sugar content of WL-656 variety was significantly reduced and the acid phosphatase activity of leaves of both varieties was significantly increased.Leaf glutamine synthetase activity was significantly reduced under nitrogen limitation.There was a decreasing trend of MDA content with the addition of silicon.Under phosphorus limitation,silicon significantly increased leaf soluble sugar and protein content and significantly increased acid phosphatase activity,and leaf silicon content was significantly and positively correlated with soluble sugar,protein content,acid phosphatase,and glutamine synthetase activity.The addition of silicon significantly increased soluble protein content,acid phosphatase and glutamine synthetase activities under nitrogen limitation.3.Nitrogen limitation significantly reduced the root length of alfalfa,while phosphorus limitation significantly inhibited the formation of root nodules.The secretion rates of citric acid,malic acid and oxalic acid in alfalfa roots were increased to different extends under Nand P-limitation.The addition of silicon significantly increased the root to shoot ratio and the secretion rate of four organic acids in alfalfa under nitrogen and phosphorus limitation.The formation of root nodules under nitrogen limitation was promoted by silicon,while the number of root nodules under phosphorus limitation changed somewhat but not significantly.Further correlation analysis showed that silicon increased plant nitrogen and phosphorus content by increasing the exudation rates of citric and oxalic acid under nutrient balanced conditions.Under N-limitation,root nitrogen content was increased by regulating plant biomass partitioning and malic acid exudation rate,and under P-limitation,phosphorus content in the plants was increased by stimulating malic and tartaric acid exudation rates.4.There was no significant change in the carbon content of alfalfa leaves and roots under different nutrient and silicon treatments.The N:P ratio was higher under P-limitation than in the balanced and N-limitation but did not reach a significant level.The addition of silicon significantly reduced the C:N ratio,C:P ratio,and N:P ratio,and this change was significantly correlated with plant silicon content.In summary,the results of this experiment clarify that silicon addition induced silicon deposition in both leaves and roots under nitrogen and phosphorus limitation,which in turn improved plant biomass and dry matter production by increasing photosynthesis and decreasing membrane lipid peroxidation.Silicon also improved the uptake and utilization efficiency of nitrogen and phosphorus elements by increasing nodulation,the rate of organic acid secretion in the root system,and the activity of enzymes related to nitrogen and phosphorus metabolism.Silicon significantly increased the nutrient content in leaves and roots,and reduced the C:N,C:P and N:P ratios,thereby increasing nutrient availability and alleviating stress.