Effects Of Exogenous Melatonin On Growth Physiology Of Grafted Watermelon Seedlings Under Low Temperature Stress

Watermelon is one of the most important vegetable crops in the world.It has continuous obstacles in the production process.The low temperature or sub-optimal temperature in the deep winter and early spring facilities can cause the watermelon seedlings to be thin and even wilting,seriously affecting their growth,yield and quality,and reducing production efficiency.Many studies have shown that grafting can prevent soil-borne diseases and improve stress resistance,and the application of exogenous melatonin can also increase the tolerance of plants to stress.In this experiment,the 'Jingxin No.2' watermelon was used as the scion,and the 'Jiuzhen No.1' pumpkin was used as the rootstock.On the one hand,the effects of eight kinds of grafting methods including insertion grafting,cleft grafting,double root-removed grafting,approach grafting,splice grafting,lie grafting,hook grafting,double root-removed and lie grafting were compared,and the grafting method with high survival rate and high quality of grafted seedlings was selected for subsequent experiments.On the other hand,the effects of different concentrations of exogenous melatonin and grafting on the growth physiology of watermelon seedlings under low temperature stress were studied,and the mitigation effects of the two were discussed.The main findings are as follows:1.The survival rate of the eight grafting methods ranged from 80.67% to 100.00%,of which insertion grafting,double root-removed grafting and splice grafting was 100.00%.Stem diameter,leaf width,leaf area,chlorophyll content,growth amount,stem diameter/plant height,root-shoot ratio,leaf area/plant height,seedling index,and maximum photosynthesis system ? including maximal photochemical efficiency(Fv/Fm),quantum yield of PS ?(Y(?)),relative electron transfer rate(rETR),photochemical quenching(qP),net photosynthetic rate,stomatal conductance,intercellular carbon dioxide concentration and transpiration rate of each grafting treatment were all improved compared with self-rooted seedlings,and plant height and non-photochemical quenching(NPQ)were reduced.In general,the insertion method was the best,followed by double root-removed grafting and splice grafting.2.Within 15 days after grafting,the chlorophyll fluorescence parameters of watermelon seedlings and grafted seedlings varied,but the overall trend was consistent,that is,Fv/Fm,Y(?),rETR,qP all decreased first and then increased.The NPQ increased first and then decreased.The variation of grafted seedlings was greater than that of self-rooted seedlings.The fluorescence parameters processed by insertion method from 10 d to 15 d after grafting were closest to the corresponding values of self-rooted seedlings.3.Under low temperature stress,grafting treatment could increase the growth indexes,Fv/Fm,Y(?),rETR,qP,chlorophyll content,soluble protein content,SOD,POD and CAT activities of watermelon seedlings compared with self-rooted seedlings.At the same time,the chilling injury index,NPQ,relative conductivity and malondialdehyde content of seedlings were reduced to improve the ability of watermelon seedlings to resist low temperature.4.Under low temperature stress,50~400 ?mol/L melatonin treatment could increase the growth indexes,Fv/Fm,Y(?)and rETR,qP,chlorophyll content,soluble protein content,SOD,POD and CAT enzyme activities of watermelon seedlings compared with the self-rooted seedlings with 0 ?mol/L melatonin,while reducing seedling chilling injury index,NPQ,relative conductivity and malondialdehyde content,of which 100 ?mol / L was the best.5.Under low temperature stress,the combination of 100 ?mol / L melatonin and grafting treatment was better than only grafting treatment or melatonin treatment,and there was a significant interaction effect between them.