Effects Of Grafting On Uptake And Assimilation Of Nitrate In Watermelon Seedling

Grafting is a routine technique used widely in watermelon production. The main aim of cucurbit grafting is to overcome biotic stress, such as soilborne diseases and pests. Additionally, grafting can also improve plant resistance to abiotic stress, increase plant biomass, improve water and nutrient use efficiency, prolong the harvest period, as well as improve the yield and quality of fruit and so on. It has been known that grafting affected nutrient absorption characteristics of watermelon and melon, however, the specific absorption mechanism is not clear. In this study, we grafted watermelon(Citrullus lanatus) onto bottle gourd(Lagenaria siceraria) rootstock (Wm/BG), while the control was self-grafted watermelon(Citrullus lanatus)(Wm/Wm). Wm/BG and Wm/Wm were grown under normal (7.5mM) and low (0.75mM) nitrate conditions to investigate the effect of grafting on nitrate uptake and utilization. The main results are as follows:1) Physiological and biochemical effects of grafting on watermelon seedlings under two different levels of nitrate supply.Wm/BG and Wm/Wm seedlings were cultured in normal and low nitrate nutrient solution until eight-leaves stage for12d, subsequently, we measured scion biomass, leaf nitrate content and the activity of major enzymes in nitrate assimilation process including nitrate reductase (NR) and glutamine synthetase (GS). It was found that under both N levels, Wm/BG grafting significantly increased plant height, dry weight and fresh weight as compared to Wm/Wm, and nitrate content in leaves of Wm/BG were significantly higher than those of Wm/Wm, suggesting that grafting can promote watermelon seedlings growth, increase the accumulation of nitrate in plant leaves. In addition, we found that low nitrate treatment significantly reduced NR activity while GS activity was increased. Under normal nitrate conditions, grafting induced leaf NR activity, but GS activity was unaffected. On the other hand, no effect of grafting could be found under low nitrate conditions.2) Effect of grafting on regulating nitrate metabolism-related genes under two different levels of nitrate supply. In order to clarify the impact of grafting on nitrogen metabolism, we determined the stress response of related genes in nitrate transport and assimilation process under short-term and long-term low nitrate condition. Under short term low nitrate condition, grafting upregulated NITRATE TRANSPORTER2.3(NRT2.3), NITRATE REDUCTASE2(NR2), GLUTAMINE SYNTHETASE1-1(GS1-1), GLUTAMINE SYNTHETASE1-2(GSI-2) and GLUTAMINE SYNTHETASE2(GS2), and GLUTAMATE SYNTHETASE (NADH-GOGAT). Compared with normal N supply, long term treatment of low nitrate significantly upregulated the expression of NRT2.3, GSI-2and NADH-GOGAT genes while downregulated the expression of NR1and GS1-1genes. In addition, we found GSI-2gene significantly differentially expressed in Wm/BG and Wm/Wm under long term low nitrate treatment, and their relative expression levels were folded by12.4and6.7times respectively, and Wm/BG was significantly higher than Wm/Wm. Therefore, we deduced that grafting can affect the plant responese to low N stress, while GSI-2gene might play an important role in the process.