Regulatory Effect Of Exogenous Nitric Oxide And 24-epibrassinolide On Kiwifruit Seedlings Under Drought Stress

Drought stress is one of the main abiotic stress factors affecting crop growth and yield.The kiwifruits（Actinidia deliciosa）are more sensitive to water stress in cultivation because of its nature of thermophilic and intolerant to drought.Nitric oxide（NO）and24-epibrassinolide（EBR）are signaling molecules and bioactive substances that effectively mitigate the effects of drought stress on plants.In the experiment,kiwi potted seedlings were used as materials with the most optimal concentration of NO donor sodium nitroprusside solution（SNP）root irrigation and EBR foliar spraying individually or in combination under drought stress.The effects of NO and EBR on the regulation and interaction of kiwifruit seedlings under drought stress were studied by analyzing the changes of physiological metabolism level and drought-related gene expression-related gene expression.The results are as follows:（1）The treatments of 100μmol/L SNP and 0.01 mg/L EBR had the most significant mitigation effect on drought stress,which performed that they significantly lead to a increased the relative water content of kiwifruit leaves and decreased the content of MDA,thus reducing the membrane lipid peroxidation.（2）NO and EBR downregulated the expression of aquaporin genes to increase leaf water and alleviated the inhibition of growth parameters by drought stress.The SNP pretreatment and EBR treatment increased plant fresh weight and improved the relative water content and water potential of leaves The expression level of aquaporin genes,PIP1;1-1,PIP1;1-2,PIP2;2-1 and PIP2;2-2,were down-regulated significantly by SNP pretreatment under day 12 of drought stress.Whereas,EBR treatment only down-regulated the expression of PIP1;1-2 and PIP2;2-1 genes.Moreover,there was a higher level of the overall aquaporin gene expression under the treatment of SNP and EBR in combined.（3）NO and EBR alleviated the inhibition of photosynthesis and improved the photosynthetic ability of kiwifruit seedlings under drought stress.The SNP pretreatment and EBR treatment increased the content of photosynthetic pigments.The gas exchange parameters were improved in varying degrees including transpiration rate（Tr）,intercellular CO₂concentration（Ci）,net photosynthetic rate（Pn）and stomatal conductance（Gs）.In particular,the combination of NO and EBR played a more prominent role.（4）NO and EBR maintained the integrity of cell membrane in leaves of kiwifruit seedlings under drought stress,reduced membrane lipid peroxidation and improved antioxidant capacity.The SNP pretreatment and EBR treatment significantly declined the content of malondialdehyde（MDA）and hydrogen peroxide（H₂O₂）content as well as relative conductivity levels,and improved the activity of catalase（CAT）and superoxide dismutase（SOD）of kiwifruit seedlings under drought stres.（5）NO and EBR reduced the osmotic regulation level of kiwifruit seedlings under drought stress.The SNP pretreatment and EBR treatment reduced the overall level of osmotic regulators such as proline,soluble protein and soluble sugar.The free proline content increased under SNP pretreatment,while it decreased significantly under EBR and SNP combined with EBR treatments in the later stage of drought.The expression of key enzyme genes P5CR1 and P5CS2 in the proline synthesis pathway was significantly down-regulated under SNP and EBR alone or in combination.（6）NO and EBR maintained the level of nitrogen metabolism and promoted the accumulation of NO in kiwifruit seedlings under drought stress.The SNP pretreatment improved the activity of glutamine synthase（GS）,a key enzyme of nitrogen metabolism,by 37.07%,while EBR treatment not only increased the activity of glutamine synthase（GS）,but also decreased the activity of glutamine synthase（GOGAT）and glutamate dehydrogenase（GDH）under day 12 of drought treatment.The combination of them resulted in the highest level of enzymes.The expression level of GS1 gene was remarkably down-regulated under the treatments of EBR and SNP combined with EBR,and the expression of GOGAT1 gene was only down-regulated under SNP pretreatment.In addition,SNP and EBR treatment alone or in combination significantly increased the endogenous NO content.（7）NO and EBR decreased endogenous hormone levels in kiwifruit seedlings under drought stress.The SNP and EBR treatment alone or in combination all reduced the content of zein（ZT）and auxin（IAA）at 12 d of drought treatment.The treatments of EBR and SNP combined with EBR resulted in the decrease in abscisic acid（ABA）content,whereas ABA content under SNP pretreatment increased significantly to 7.49μg·g^-1.