Regulations Of Pumpkin Rootstocks On Sugar Accumulations In Fruits Of Grafted Watermelon

Watermelon(Citrullus Lanatus) is a worldwide horticultural crop with significant economic value. Grafting is an efficient approach to overcome the obstacles caused by continuous cropping in watermelon production. Pumpkin(Cucurbita spp.) is one of the most commonly used rootstocks in watermelon grafting. But they always have negative effects on the fruit quality of grafted watermelon. Pumpkin germplasm is very abundant in China. However, evaluation and utilization of pumpkin germplasm as the rootstocks of watermelons are currently very limited. Meanwhile, the underlying mechanism of negative influences of pumpkin rootstocks on the fruit quality of grafted watermelon is still unkonw. To address these problems, we firstly evaluated the suitability of 85 pumpkin germplasms introduced from the National Vegetable Germplasm Genebank as rootstocks of grafted watermelon, then analyzed the regulation mechanism of the pumpkin rootstocs on sugar accumulations in the fruits of grafted watermelon at physiological and transcription regulation levels. The major results are as the following.1. A total of 85 pumpkin germplasms were introduced from the National Vegetable Germplasm Genebank and evaluated for their suitablities as rootstocks of grafted watermelon. Taking into account the phenotypes of grafted watermelons including grafting compatibility and fruit quality, 8 germplasms, namely Y8, Y26, Y27, Y42, Y53, L12, L22, and L29, exhibited excellent performaces and were suitable to be used as rootstocks of grafted watermelon.2. The effects of pumpkin rootstocks on the contents and components of sugar were investigated in the fruits of grafted watermelons. Compared with the nongrafted watermelon, the sugar contents varied from-2.80 ?Brix to 1.77 ?Brix in the fruits of grafted watermelons. There were 9 and 17 pumpkin gremplasms increased or decreased the sugar contents over 1.00 ?Brix in the fruits of grafted watermelons compared to the nongrafted watermelon, respecitvely. Four pumpkin germplasms were selected to further validate the effects of rootstocks on the sugar contents of grafted watermelons. The results were same as the previous experiments that L9 and L29 increased the sugar contents and L6 and L18 decreased it in the fruits of grafted watermelon, indicating that rootstocks have stable influences on the sugar contents of grafted watermelon. Dynamic changes of soluble sugar content were measured by gas chromatography during the fruit development(10 DAP, 20 DAP, and 30 DAP) of watermelon inbred line 97103 grafted onto L29 and L18, respectively. Compared with the nongrafted watermelon, the contents of glucose and fructose were significantly increased in the grafting combination of 97103 and L29, resulting in significant enhancement of sugar contents. While, when L18 was used as rootstock, the sucrose content was significantly decreased, casuing the reduced sugar content in the fruit of grafted watermelon. These results suggested that sugar content in the fruits of grafted watermelon was a rootstock dependent trait.3. The effects of pumpkin rootstocks on the interaction of source and sink and its relationship with sugar accumulation were also assessed in grafted watermelon. Morphological and physiological analyses showed that L29 significantly improved the photosynthetic capacity of leaves in watermelon scion. However, the photosynthetic stability of leaves was significantaly decreased in watermelon grafted onto L18. Meanwhile, compared to the grafting combination of L29 and 97103, the flesh firmness and edge sugar contents were higher when L18 was used as rootstock. These results demonstrated that pumpkin rootstocks could affect the sugar accumulation in the grafted watermelon fruits by regulating the photosynthesis capacity of leaves and allocation of sugar in the fruits of grafted watermelon.4. Transcriptome analysis was conducted to clarify the underlying regulation mechanism of pumpkin rootstocks on the sugar accumulation in the fruits of grafted watermelons. The watermelon inbred line 9710 was used as the sicon. L18 and L29 were used as the rootstocks. Gene expression profiles in fruits were compared by RNAseq analysis at the late stage of fruit enlargement(20 DAP) and maturity(30 DAP). There were 4322 genes that differentially expressed between grafted and non-grafted watermelon, and 2202 differentially expressed genes were found between the two grafted combinations. The Go analysis showed that pumpkin rootstocks could regulate the carbohydrate metabolic process, transporter activity, signal transducer activity, and cell part in grafted watermelon fruits. At the fruit maturity stage(30 DAP), compared with the nongrafted watermelon, the sugar metabolism related genes were significantly enriched when L29 was used as the rootstock. However, when L18 was use as the rootstock, the sugar transportation related genes were significantly enriched in the fuirts of grafted watermelon. Pathway analyses on sugar metabolism and sugar transporters showed that the sucrose synthase gene Cla009124 and sugar transporters genes Cla010725 and Cla011774 were significantly down-regulated in the matured fruits(30 DAP) of watermelon grafted onto L29, which positively correlated with the significant reduction of sucrose contents. Expression level of hexokinase gene Cla008563 were significantly increased at the maturity stage compared with the late fruit enlargement stage in nongrafted watermelon fruits. However, this gene was significantly down-regulated in the matured fruits of watermelon grafted onto L29, which negatively correlated with the enhancement of fluctose and gluctose contents. In the grafting combination of L18 and 97103, Cl SWEET1 and Cl SWEET10 were significantly up-regulated in the matured fruits, which negatively correlated with the changes of sucrose contents. Meanwhile, the significantly down-regulations of the sugar transporters genes Cla010725 and Cla011774 exhibited negative correlation with the decrease of sucrose contents. These results demonstrated that pumpkin rootstock L29 increased the sugar contents in the fruits of grafted watermelon by regulating the expression of sugar synthesis and transportation related genes. While, the rootstock L18 decreased the sugar contents in the fruits of grafted watermelon mainly by regulating the expression of sugar transportor genes.