Effects Of Exogenous Salicylic Acid On Physiological Characteristics And Resistance Gene Expression Of Salix Matsudana Koidz.under Drought And Cadmium Stress

Heavy metal cadmium（Cd）is the most common toxic pollutant in soil,which inhibits growth and development after absorption by plants and threatens human health through food chain accumulation.In arid and semi-arid regions,drought is the most common environmental stress factor affecting the growth and development of plants.Therefore,plants growing in this region usually face the stress of drought and Cd pollution.Salix matsudana Koidz.as the most widely cultivated Salix species in China,has strong drought tolerance and heavy metal tolerance.Salicylic acid（SA）is a plant growth regulator and a signal molecule,which plays an important role in the physiological metabolism of plant growth and stress resistance.In order to explore the effects of drought,Cd single or compound stress on Salix matsudana and the effects of salicylic acid on Salix matsudana and its mechanism under drought,Cd single or compound stress,the hydroponic Salix matsudana was used as the material in this experiment.SA pretreatment and polyethylene glycol（PEG）were used to simulate drought stress.The effects of drought,Cd single or compound stress on physiological characteristics and stress resistance gene expression of Salix matsudana and the regulation of SA on them were discussed.A total of eight treatments were set up in the experiment,including CK,PEG（5%）,Cd(50μmol·L^-1),PEG+Cd,SA(50μmol·L^–1),PEG+SA,Cd+SA,PEG+Cd+SA.The treatment containing SA was pretreated for three days before stress treatment,and then all treatments were treated according to CK,PEG,Cd,PEG+Cd（PEG and Cd exist alone or simultaneously in nutrient solution）.The responses of Salix matsudana to growth,chlorophyll content,reactive oxygen species metabolism and proline metabolism were analyzed at 7 d and14 d,respectively.The specific research results are as follows:（1）The growth rate of Salix matsudana was slowed down after 7 d and 14 d of drought,Cd single or combined stress.The plant height increment,root length increment and leaf area increment of Salix matsudana were significantly lower than those of the control.The contents of chlorophyll a,b and total chlorophyll were significantly decreased.The production rate of superoxide radical（O₂.^-）,hydrogen peroxide（H₂O₂）and malondialdehyde（MDA）content were increased.Compared with CK,under the three treatments,the activities of superoxide dismutase（SOD）,catalase（CAT）,ascorbate peroxidase（APX）and glutathione reductase（GR）in Salix matsudana leaves increased,the content of ascorbic acid（As A）decreased,and the content of glutathione（GSH）increased.The gene expression levels of SOD,CAT,APX and GR were significantly up-regulated,and the gene expression levels related to proline（Pro）metabolism in plants were changed.Among them,the expression levels ofΔ~1-pyrroline-5-carboxylic acid synthase（P5CS）andΔ~1-pyrroline-5-carboxylic acid reductase（P5CR）genes in Pro synthesis metabolism were significantly up-regulated,and the expression level of proline dehydrogenase（PDH）gene during proline degradation was down-regulated,resulting in the increase of Pro content in plants.From the Cd accumulation point of view,Cd,drought and Cd combined treatment increased the Cd content of Salix matsudana roots,stems and leaves,and the accumulation of Cd in Salix matsudana roots was the most.（2）From the perspective of plant growth rate,chlorophyll content,ROS level,lipid peroxidation degree and Cd content,the negative effects of drought and Cd combined stress on plants were greater than those of drought and Cd single stress,and the combined stress had greater damage to plants.The main manifestations were that compared with drought and Cd single stress,the plant height increment,chlorophyll a,chlorophyll b and total chlorophyll content of Salix matsudana were significantly decreased at 7 d,and the O₂.^-production rate and MDA content were significantly increased.In addition,the Cd content in roots and leaves of Salix matsudana increased significantly under combined drought and Cd treatments compared with single Cd treatment.（3）The application of exogenous SA alleviated the inhibitory effect of drought stress,Cd stress and combined stress on the growth of Salix matsudana,reduced the ROS level and MDA content under drought and Cd single or combined stress,increased the chlorophyll,antioxidant enzyme activity,As A and GSH content,and the expression level of antioxidant enzyme related genes was also up-regulated.The gene expression level of Pro metabolism related enzymes in plants was changed,and the endogenous Pro content was increased.Compared with Cd,drought and Cd combined stress,SA also significantly reduced the Cd content in aboveground parts of Salix matsudana,and reduced the toxic effect of Cd on aboveground parts of Salix matsudana.In summary,the growth of Salix matsudana was inhibited under drought and Cd single or combined stresses.The ROS level and membrane lipid peroxidation increased,the antioxidant system was threatened,and the cells were seriously damaged.Among them,the damage of combined stress on Salix matsudana was the most serious.Under drought and Cd single or combined stresses,exogenous SA restored the growth rate of Salix matsudana seedlings,increased chlorophyll content,antioxidant system activity and antioxidant enzyme-related gene expression,and increased Pro content by regulating Pro metabolism-related gene expression,thereby improving the tolerance of Salix matsudana seedlings to drought and Cd single or combined stresses.