Mitigation Of Alfalfa Under Salt Stress By Bacillus Subtilis

Alfalfa（Medicago sativa L.）is a perennial leguminous forage grass of the genus Alfalfa with high yield,good quality,versatile use and good palatability,and is now widely cultivated worldwide as the most economically valuable and excellent forage grass.Alfalfa is highly resistant to adversity,but salt stress is one of the main factors limiting its growth and can even lead to plant death in severe cases.Bacillus subtilis is a highly resistant strain that is enriched in the soil microbial community and can both promote and protect plants from salt stress in the soil.In this paper,we investigated the effects of five concentrations of 50 mmol·L-1,100 mmol·L-1,150 mmol·L-1,200 mmol·L-1,250 mmol·L-1 and 300 mmol·L-1 NaCl on alfalfa seed germination and seedling growth,physiological indicators and the addition of five gradients of 0.5×108 cfu/g,1×108 cfu/g,2×108 cfu/g,4×108 cfu/g and 8×108 cfu/g of Bacillus subtilis on the growth and physiological parameters of alfalfa seedlings under salt stress.The main findings are as follows:1.NaCl treatment produced a promotion followed by inhibition of alfalfa biomass and plant morphology.NaCl at low concentrations（50 mmol·L-1）promoted plant height,root length,and above-and below-ground biomass of alfalfa;as NaCl concentrations increased,NaCl at medium to high concentrations（100-50 mmol L-1）significantly reduced plant height,root length,and above-and below-ground biomass of alfalfa（P<0.05）;addition of 8×108 cfu/g of Bacillus subtilis significantly promoted the elongation of plant height and root length as well as the accumulation of above-and below-ground biomass in alfalfa（P<0.05）,and at high concentrations（250 mmol·L-1）salt stress significantly increased the accumulation of plant height,root length and above-and below-ground biomass（P<0.05）.2.NaCl treatments differentially affected the photosynthetic system of alfalfa leaves.Low concentration（50 mmol·L-1）of NaCl treatment promoted the growth of chlorophyll content in alfalfa,while high concentration of NaCl（100-250 mmol·L-1）treatment significantly inhibited the accumulation of chlorophyll content in leaves（P<0.05）,drastically reduced the chlorophyll a fluorescence OJIP transient curve,and disrupted chlorophyll fast and slow fluorescence parameters;addition of certain The addition of Bacillus subtilis at a certain concentration（2-8×108 cfu/g）significantly increased the chlorophyll b content and total chlorophyll content of alfalfa under salt stress（P<0.05）,improved the chlorophyll a fluorescence OJIP transient curve and fluorescence parameters,and effectively alleviated the damage of salt stress on alfalfa.3.NaCl treatment reduced soluble sugar and soluble protein contents in alfalfa leaves and roots.The accumulation of soluble sugars and soluble proteins was inhibited to different degrees by different NaCl concentrations,and decreased with increasing NaCl concentration.250 mmol·L-1 NaCl treatment significantly reduced soluble sugars in leaves（P<0.05）,200-250 mmol·L-1 NaCl treatment significantly reduced soluble sugars in roots and soluble proteins in leaves（P<0.05）.The addition of Bacillus subtilis at a certain concentration（8×108 cfu/g）significantly increased the soluble sugar content in the roots of alfalfa under salt stress（P<0.05）and the soluble protein content in the leaves and roots（P<0.05）,and soluble protein content in leaves and roots of alfalfa（P<0.05）.3.NaCl treatment increased the relative conductivity and malondialdehyde（MDA）content in the leaves and roots of alfalfa,with 100-300 mmol·L-1 NaCl treatment significantly increasing the relative conductivity and MDA content in the leaves and 150-250 mmol·L-1 NaCl treatment significantly increasing the MDA content in the leaves;the addition of different concentrations of Bacillus subtilis significantly increased the MDA content in the leaves and roots of alfalfa.The addition of different concentrations of Bacillus subtilis had different effects on the relative conductivity and MDA in leaves and roots,where the addition of 4×108 cfu/g and 8×108 cfu/g of Bacillus subtilis significantly reduced the relative conductivity of leaves（P<0.05）and 2×108 cfu/g significantly reduced the MDA content in the roots（P<0.05）.5.NaCl treatment significantly reduced CAT,POD and SOD activities in alfalfa leaves and roots（P<0.05）,and the addition of Bacillus subtilis increased CAT and SOD activities and reduced POD activity.50 mmol·L-1 NaCl treatment increased SOD activity in leaves,and under salt stress,the addition of 4×108 cfu/g and 8×108 cfu/g of Bacillus subtilis significantly increased CAT and SOD activities in leaves and roots（P<0.05）.In conclusion,different concentrations of NaCl treatment have different effects on the growth and physiology of alfalfa.Low salt stress can promote the growth of some alfalfa leaves and roots,while high salt stress has toxic effects on the growth and physiology of alfalfa.Adding certain concentration of Bacillus subtilis can effectively alleviate the toxic effects of salt stress on alfalfa leaves and roots,and the changes of various indexes in leaves and roots are basically consistent.This conclusion provides a theoretical basis for improving the salt tolerance of alfalfa by using rhizosphere growth-promoting bacteria.