The Alleviating Effects And Mechanisms Of Exogenous Melatonin On Watermelon Under The Continuous Cropping Obstacle

Continuous cropping obstacle refers to planting the same species or the same family crops in the same field.Although under the strict and standardized planting management mode,plant growth defects,yield reduction,quality deterioration and other phenomena will occur.With the large-scale and industrialized production trend of facility vegetables,continuous cropping obstacles become increasingly prominent.The continuous cropping obstacle of watermelon is the most serious among the melon crops.Watermelon set cultivation scale develops rapidly,successive years of watermelon planting,forming a special soil environment,a large number of harmful microorganisms breeding,affecting the normal growth and development of watermelon plants,resulting in reduced commercial value,yield decline or even no harvest.Melatonin(MT)can improve plants’ resistance to and adaptation to adversity,and at present,few studies have been conducted in response to continuous cropping obstacles in plants.In this experiment,watermelon(Citrullus lanatus(Thunb)Matsum & Nakai),the main vegetable crop,was used as the research object,and varieties of "Nongkeda No.5" and "Jingxin No.2" were used as test materials.Pot culture was adopted to study the effect of melatonin treatment on the growth of watermelon under continuous cropping obstacles.The test results are as follows:1.Exogenous melatonin can significantly increase the plant height,stem diameter,leaf area,aboveground dry mass and underground dry mass of watermelon seedlings under continuous cropping obstacles,significantly improved the robustness index and dry matter content of watermelon plants,and ensured the robust growth of watermelon seedlings.2.Exogenous melatonin can significantly increase the chlorophyll content of watermelon seedlings under continuous cropping obstacles,enhance root vitality,and ensure adequate nutrition of watermelon seedlings during reproductive growth.3.Exogenous melatonin can significantly increase the net photosynthetic rate,promote the photosynthesis of watermelon seedlings under continuous cropping obstacles,and improve the watermelon‘s autotrophic abilities.4.Exogenous melatonin can significantly reduce the relative conductivity(REL)and malondialdehyde(MDA)content of watermelon leaves under continuous cropping obstacles,and effectively alleviate the damage caused by continuous cropping obstacles to the cell membrane of watermelon plants.5.Exogenous melatonin can significantly increase the superoxide dismutase(SOD),catalase(CAT)and peroxidase(POD)and ascorbic acid peroxidase(APX),polyphenol oxidase(PPO)activity of antioxidant enzymes,enhance the ability of watermelon plant antioxidant,thus reduces the continuous cropping obstacle to the watermelon plant stress.6.Exogenous melatonin can significantly increase the content of soluble protein and proline(Pro)in watermelon leaves under continuous cropping obstacles,promote the balance effect of osmotic regulation,and keep the watermelon plant in a relatively stable cell environment.7.Exogenous melatonin can significantly increase the activities of nitrate reductase(NR),glutamine synthase(GS),glutamate dehydrogenase(GDH)and glutamate synthase(GOGAT),and improve the nitrogen metabolism of watermelon seedlings.8.Melatonin pretreatment can significantly increase the single fruit weight of watermelon under continuous cropping obstacles.The soluble protein content was increased,but the soluble sugar content was decreased.