Study On The Relationship Between Wax Characteristics Of Watermelon Leaves Anddrought Resistance

Watermelon has high nutritional value and economic value,and occupies an important position in the melon and fruit industry in China.Drought is one of the most important environmental hazards that adversely affects fruit quality and yield and subsequently restrict the development of watermelon industry.Epidermal wax is closely related to drought resistance,and its accumulation is beneficial to plant resistance to drought stress.However,the changes of wax content in watermelon leaf epidermis and the response mechanism to drought stress remain unclear.In this study,six watermelon materials at seedling stage were treated with drought stress by potted water control.The relationship between leaf epidermal wax and drought resistance was analyzed by drought resistance identification,the determination of wax and physiological indexes.On this basis,the mechanism of exogenous ABA and melatonin(MT)pretreatment on wax accumulation and drought resistance was further explored with drought-sensitive watermelon material “Y34” as the research object.The main results are as follows:1.Six kinds of watermelon materials were treated with drought stress for 8 days,and the drought injury index was calculated.The average subordinate function value was calculated by measuring the relative water content,relative electrical conductivity,photosynthetic characteristics and chlorophyll fluorescence parameters.According to the average drought injury index and the subordinate function value,the results of drought resistance identification of the six kinds of watermelon seedlings were M20 > M08 > Hong Xiaoyu > Jin 5 Father > Jin X-4 > Y34,M20 > Hong Xiaoyu > M08 > Jin 5 Father > Jin X-4 > Y34,respectively.The results of the two identification methods were basically the same.Under drought stress,drought-resistant materials had higher photosynthesis and lighter membrane damage.2.Twenty-five kinds of wax compositions were identified by gas chromatography-mass spectrometry(GC-MS)analysis on the epidermal wax of six kinds of watermelon materials.They were classified into four categories: alkanes,primary alcohols,aldehydes and fatty acids.Alkanes and primary alcohols were the two main compositions,and C28 primary alcohols were the highest wax composition.After drought stress,the wax synthetic genes were up-regulated,both the wax content and crystals increased significantly in the leaves of watermelon.However,there was no significant difference in wax composition and content among different drought-resistant materials.These indicated that the epidermal wax of watermelon leaves responds to drought stress.3.The regulation mechanism of ABA and melatonin pretreatment on epidermal wax accumulation and drought resistance of watermelon leaves was studied.The results showed that suitable concentration of ABA or melatonin pretreatment alleviated the decline of photosynthesis and cell membrane damage caused by drought stress,and improved drought resistance of watermelon seedlings.Under drought stress,ABA or melatonin pretreatment induced up-regulation of wax synthesis-related genes and leaf epidermal wax accumulation,indicating that ABA and melatonin-induced drought resistance may be closely related to its regulation of wax synthesis.In addition,melatonin pretreatment significantly increased the level of endogenous ABA by regulating the up-regulation of ABA synthesis gene and down-regulation of degradation gene under drought stress,while ABA pretreatment had no significant effect on the content of endogenous melatonin.Thus,melatonin may function upstream of the ABA pathway in inducing wax synthesis and accumulation,and ultimately improve drought resistance of watermelon seedlings.