The Role And Mechanism Of Melatonin In Grafting-enhanced Watermelon Tolerance To Cold Stress

Watermelon?Citrullus lanatus?Thunb.?matsum.et Nakai.?is an important horticultural crop in China and even in the world,ranking first among the production of melon and fruit crops in China.However,watermelon is native to tropical areas of Africa and is sensitive to low temperature.In early spring,it is easily affected by low temperature when cultivated in facilities.Especially the cold snap during the spring,the seeding groeth of watermelon would be seriously affected,thus damaging the yield and quality.Grafting is an effective means to alleviate the stress of temperature and other adverse conditions,which can improve the low temperature resistance of plants.But the signal transduction mechanism of graft and regulation of watermelon's cold resistance is still unclear.As such,in this experiment,watermelon was used as scion and grafted onto white seed pumpkin or black seed pumpkin which could resist low temperature.This was followed by the study of the effect of resistant rootstock on low-temperature stress resistant of watermelon.The signal transduction mechanism of graft and regulation of low temperature resistance of watermelon,was discussed from the perspective of signal regulation such as plant hormone,melatonin and H₂O₂signal regulation.The main results are as follows:1. Graft watermelon onto white seed pumpkin or black seed pumpkin for whole low temperature treatment. Low temperature conditions,compared with the self-root grafting,black or white seed pumpkin rootstock increased net photosynthetic rate Pn,maximum photochemical efficiency Fv/Fm,and actual quantum yield?PSII of watermelon scion,and reduced the relative conductivity REC.In terms of antioxidant,both pumpkin rootstocks can improve the activity of superoxide dismutase?SOD?,peroxidase?POD?and catalase?CAT?in leaves,and increase the activity of glutathione reductase?GR?,dehydroascorbic acid reductase?DHAR?and ascorbic acid?As A?in GSH-As A system.In addition,the expressions of CBF1,CBF2 and CBF4 in CBF pathway were up-regulated in both pumpkin rootstocks.These results indicate that resistant rootstocks can improve the cold resistance of watermelon scions by regulating photosynthetic system,antioxidant system and transcriptional regulation pathway of CBF.2. The role of melatonin?MT?as a long-distance transport signal and jasmonic acid?JA?in improving low-temperature resistance of pumpkin rootstocks was studied.It was found that,compared with self-root grafting,the MT content in leaves and roots of pumpkin grafted plants and the expression of MT synthesis related gene ASMT in leaves were significantly increased under low temperature stress,while the JA content in the leaves significantly increased and the expression of JAZ1 related gene of JA signal pathway were significantly decreased.When MT and JA were treated exogenously,it was found that the appropriate concentration of MT and JA could increase Fv/Fm of watermelon self-root grafted seedlings and reduce REC.However,the low temperature resistance induced by pumpkin rootstock was weakened when treated with MT inhibitor CPA and JA inhibitor DIECA.These results indicate that MT and JA are involved in the regulation of low temperature resistance of watermelon scion by resistant rootstocks.3. The effect of hydrogen peroxide?H₂O₂?on improving cold resistance of pumpkin rootstock was studied.At low temperature,the expression of RBOH gene encoding NADPH oxidase in pumpkin seedlings was significantly up-regulated and the content of H₂O₂was significantly increased compared with that of self-root grafting.Exogenous spraying of H₂O₂with the appropriate concentration increased the low temperature resistance of watermelon self-root grafted seedlings,while the low temperature resistance of grafted pumpkin plants was weakened after treated with H₂O₂inhibitor DPI.These results indicate that H₂O₂is involved in induction of low temperature resistance of pumpkin rootstock.4. The correlation of melatonin?MT?,jasmonic acid?JA?and hydrogen peroxide?H₂O₂?in pumpkin rootstock induced low temperature resistance was studied.Under low temperature conditions,the content of JA and H₂O₂in self-root grafted seedlings was increased by exogenous MT at the appropriate concentration.Exogenous treatment of appropriate concentration of JA promoted the production of H₂O₂,exogenous treatment of H₂O₂also promoted the accumulation of JA,while JA inhibitor DIECA pretreatment reduced the effect of MT on the improvement of H₂O₂content.These results showed that MT regulated JA accumulation and promoted the production of H₂O₂,and H₂O₂also regulated JA content,thus participating in the induction of low temperature resistance of pumpkin rootstock.