The Mechanism Of Pumpkin Rootstock Grafting Improve Watermelon Chilling Tolerance

Low temperature is a major environmental factor that severely impairs plant growth and productivity.Watermelon(Citrullus lanatus)is a chilling sensitive crop.Grafting of watermelon onto pumpkin rootstock is an effective technique to induce chilling tolerance of watermelon.However,the mechanism of pumpkin rootstock induced chilling tolerance is still not well understood,therefore,to understand this mechanism is important.In this study,combination of physiological,hormonal,transcriptomic as well as metabolomics analysis was used to understand the mechanism of pumpkin rootstock mediated chilling stress tolerance of watermelon at both physiological and molecular level.The results of this study can be used for breeding programs of rootstock improvement.The main results of this study are as following:1.Pumpkin rootstock increased the photosynthetic efficiency of watermelon under chilling stress conditions.Compared with watermelon,pumpkin seedling showed more chilling tolerance.Under 10/5°C(day/night)chilling stress treatment,proline and ABA responded to chilling stress in both self-grafted and pumpkin grafted watermelon plants.Under chilling stress conditions,the self-grafted watermelon plants showed shriveling and wilting of leaves with high chilling injury index.However,pumpkin rootstock grafted significantly reduced lipid peroxidation and chilling injury index of watermelon scion.Chilling stress induced serious damage in PS II.Grafting of watermelon onto pumpkin rootstock efficiently reduced the light damage by maintaining PS II heat dissipation ability and high level of V_c,max and J_max,and enhanced net photosynthetic rate.In conclusion,pumpkin rootstock grafted reduces damage induced by ROS accumulation and lipid peroxidation,and improves chloroplast stability that ultimately improves photosynthetic ability of watermelon under chilling stress conditions.2.Integrated transcriptome and metabolome analysis results showed that pumpkin rootstock grafted regulated chilling tolerance by regulating differential gene expression and changing metabolite accumulation patterns under chilling stress condition.Pumpkin grafted seedling showed reduced number and expression level of differential genes in watermelon scion,reduced accumulation of carbohydrate,amino acid and other related metabolites,and promoted the accumulation of unsaturated fatty acids under chilling stress condition.It specifically increased the up-regulated ADC(Cla97C11G210580)gene expression,a key gene in the polyamine metabolism pathway;promoted the accumulation of ornithine,the precursor of putrescine synthesis,and provided substrates for putrescine synthesis that ultimately promoted the accumulation of putrescine.Treatment with D-Arg(inhibitor of ADC)increased the electrolyte leakage level of pumpkin grafted watermelon plants.This mechanism may be attributed by inhibition in the expression levels of ADC and ODC genes,reduction in expression levels of spermidine synthase SPDS,and the key PAO gene involved in polyamine metabolism,and change in synthesis and metabolism of polyamine.3.Aquaporins are involved in pumpkin rootstock mediated chilling tolerance by regulating root hydraulic conductivity to maintain hydraulic signals.Enhancing root vitality by either maintaining root optimal temperature or by maintaining high relative humidity(RH)under chilling stress condition decreases the reduction of leaf water potential,reduces electrolyte leakage and improves photosynthesis efficiency,therefore alleviate chilling damage.Grafting of watermelon onto pumpkin rootstock significantly improved water transport ability from root to aboveground plant parts,reduced the fluctuation of water potential in leaves,delayed the reduction of water potential to aboveground plant parts and maintained the plant hydraulic signals.Maintained leaf water potential increased self-grafted watermelon chilling tolerance at similar level to pumpkin grafted watermelon plants.Treatment with aquaporins inhibitors resulted in rapid decrease in root hydraulic conductivity,increase in electrolyte leakage in leaves and decrease in chilling stress tolerance ability of pumpkin grafted watermelon plants,suggesting the important role of aquaporins in maintaining root hydraulic signals and inducing chilling stress tolerance.Furthermore,the chilling damage was mediated by reduced water potential.Whereas,attenuated reduction in leaf water potential reduced electrolyte leakage and improved photosynthetic efficiency,therefore,alleviated chilling damage.ABA pre-treatment increased chilling tolerance under low RH conditions,however,become ineffective in case of high RH conditions.Analysis of endogenous ABA content indicated that water potential mediated chilling tolerance was independent of ABA.In conclusion,aquaporins are involved in pumpkin rootstock grafted induced chilling tolerance of watermelon by regulating the root hydraulic conductivity to maintain hydraulic signals,maintaining plant morphology,provided good cell environment for life activity.Through transcription-associated metabolomic alterations,promoting the accumulation of ornithine,up-regulating the expression level of ADC and promoting the synthesis of putrescine.Retarded the chloroplast degradation,effectively delayed the PS II damage,and enhanced the photosynthetic adaptability,which ultimately improved the chilling tolerance of pumpkin grafted watermelon plants.