Pumpkin Resources Stress Resistance Evaluation And The Effects Of Rootstocks On Fruit Quality Of Watermelon By Using Transcriptomics And Metabonomics Analysis

Watermelon(Citrullus lanatus)is an important horticultural crop across the world.The high incidence of soil borne diseases such as root rot and wilt disease are the bottlenecks to restrict the development of watermelon industry.Grafting can improve the resistance of watermelon to soil borne diseases.Pumpkin is a suitable rootstock for grafting of watermelon.However,the research on the resource evaluation and utilization of the pumpkin is limited.In order to screen the excellent germplasm resources and lay the foundation for further rootstock selection,pumpkin germplasms were evaluated for drought resistance and root rot resistance.The pumpkin rootstocks are known to decrease the fruit quality of grafted watermelon.To address this problem,two pumpkin rootstocks with distinct effect on grafted watermelon fruit quality were used in this study.Transcriptomics and metabonomics were used to identify the pathways and metabolites significantly affected by grafting and rootstocks.The main results are as follows.1.A total of 126 pumpkin lines were used in this study,including 33 Cucurbita moschata and 93 Cucurbita Maxima.Two commercial rootstocks ?Qingyan NO.1‘ and ?Jingxin NO.4‘ were used as the control to investigate and evaluate the characters of pumpkin germplasm at the appropriate stage of grafting.Hypocotyl length of 27 lines was equal or shorter than commercial rootstocks and most of the pumpkin germplasm had similar stem diameter as commercial rootstocks or better than commercial rootstocks.Only 13 lines had thinner stem diameter of hypocotyl;31 lines had larger spreading angle than commercial rootstocks;9 lines have cotyledon chlorophyll content equivalent to commercial rootstocks;and 18 lines have similar cotyledon length and width compared with commercial rootstocks.2.94 pumpkin lines,including 29 Cucurbita moschata and 63 Cucurbita maxima were used as plant material.Two commercial rootstocks,?Qingyan NO.1‘ and ?Jingxin NO.4‘ were used as control.More than 70% of the plants in the treatment group showed wilting symptoms at three leaf stage when growing under drought conditions and the drought damage level and the corresponding number of plants were counted.After the drought symptoms were evaluated,the plant material was rehydrated and soaked with the same method.Then the treatment group was treated with drought again,and the control group was still under normal water supply.When more than 70% of the plants in the treatment group had drought damage symptoms,the drought damage grade and the corresponding plant number of the plants with wilting symptoms were counted,plant height,stem diameter,dry and fresh weight were measured,and the drought stress index and plant height stress index were calculated.The stem diameter stress index and dry matter stress index were analyzed by UPMGA method(non-weighted arithmetic average method).According to drought tolerance grade,pumpkin germplasm resources were classified into 6 categories.18 lines were classified as the most drought-tolerant lines;14 lines were classified as sub-drought-tolerant lines;13 lines were grouped as drought tolerant;7 lines were considered as least drought-tolerant materials;and 24 lines were secondary grouped as drought-tolerant lines;and 18 lines were drought resistant lines.3.45 pumpkin germplasm lines,including 25 Cucurbita moschata and 18 Cucurbita maxima were used as plant material.Two commercial rootstocks ?Qingyan NO.1‘ and ?Jingxin NO.4‘ were used as the control.When the seedlings were at three leaf stage,root rot disease resistance of 45 pumpkin rootstock lines was evaluated by the spore suspension method,and the resistance classification of pumpkin germplasm resources was classified according to the method of Nagao(1994).Based on the calculation of disease index,19 lines such as R7S11-4,R7S20 and R7S82 were classified having high root rot resistance and 26 lines such as R7S21-1,R7S28-2 and R7S06 were having medium root rot resistance.4.By comparing the watermelon 97103 grafted onto 10 different pumpkin rootstocks with the self-grafted 97103 as control,the rootstock L9 was selected as the rootstock which has the great influence on the fruit quality of watermelon.Pumpkin line L12 was the rootstock that has the least effect on fruit quality of watermelon after grafting.The transcriptomics and metabonomics data of 97103 fruits at days 18 and 32 after pollination grafted onto pumpkin line L9,L12 was obtained.At the transcriptional level,the metabolic pathways of citric acid metabolism,fructose and mannose and sucrose,and starch were different.The metabolomics also showed that L9 and L12 made different influence in fructose and mannose metabolism and citric acid cycle in watermelon fruit.Finally,the fruit taste of watermelon was altered.5.The effects of L9 and L12 pumpkin rootstocks on the quality of watermelon fruit at transcriptional level were studied.The transcriptional level data showed that pumpkin rootstock L9 affects citric acid metabolism,fructose and mannose metabolism,and sucrose and starch metabolism in the fruit of grafted watermelon,thus the fruit quality of watermelon was poor.While L12 do not affects the fruit of watermelon at transcriptional level and has no effect on the fruit quality of watermelon.6.The effects of L9 and L12 rootstocks on the fruit quality of watermelon at metabolic level were studied.Pumpkin rootstock L9 affects the metabolism of fructose and mannose and citric acid cycle in watermelon fruit that leads to poor quality of watermelon fruit.While the metabolic level of L12 affects the fruit of watermelon only in citric acid metabolism thus it has no significant effect on the fruit quality of watermelon.