Mechanism Of High Nitrogen Absorption And Utilization Efficiency In Watermelon

Watermelon(Citrullus lanalus) is one of the20most important food and agricultural commodities worldwide, and China is the highest producing country of watermelon and produces more than half the world’s watermelon. The area of watermelon is26.77million acres and it is up to58.66%of the world total area and the production is up to67.87%of the world in China. The annual consumed watermelon is about52.54Kg, and the annual world consumed of watermelon is about15.82kg (FAO,2007). However, as the watermelon production area is increasing, more and more nitrogen is being used to increase the yield, which results in lower nitrogen use efficiency, lower watermelon quality and pollution to environment. For the development of sustainable agriculture, it is importance to study the mechanism of high plant nitrogen use efficiency and explore an effective strategy to use nitrogen fertilization. This study focused on the mechanism and regulating strategies of improving watermelon nitrogen use efficiency, and also investigated the mechanism of grafting effect on the nitrogen absorption, utilization and the plant growth and response to stresses. The following are the major results:1. At ample nitrogen and low nitrogen stress, different genotypes showed significant diversities in terms of N use efficiency. And also the dry matter, plant height and root morphological parameters were genotypic differences. The eight watermelon genotypes were classified into four groups:efficient responsive (ER), inefficient responsice (IER), efficient non-responsive (ENR) and inefficient nonresponsive (IENR) according to N utilization efficiency under low N stress and dry matter at high N supply. XN8, ZCHY and JX1were ER, ZM5and XF were ENR, XF were IER, LF and Zi were IENR. These results may be valuable resources for watermelon breeding and production.2. Nitrogen is taken up by most plant species in the form of nitrate and ammonium. Plants were grown in hydroponic culture with five NO3-/NH4+^ratios (100/0,75/25,50/50,25/75,0/100). When the proportion of NH4+was increased, the leaf number, leaf area, shoot height, net photosynthesis, biomass and root growth were significantly decreased. Higher accumulation of macroelements and microelements were observed with high concentration of NO3-in the solution. We found the nitrogen utilization efficiency was increased with higher NO3-/NH4+ratios. Grower may increase watermelon growth by using a predominantly NO3--N source fertilizer in the early vegetative growth stage.3. We observed that grafting could enhance plant growth and mineral nutrients absorption, and could significantly increased the photosynthetic and day matter by grafting watermelon onto bottle gourd "Yongzhen1" and squash "Xintuzuo" under ample and low nitrogen treatments. Grafting-induced enhancement of plant growth and tolerance to low nitrogen stress were associated with the superior root system of bottle gourd and squash compared with self-grafted watermelon. Furthermore, the nitrogen utilization efficiency of watermelon was increased by45%and10%when watermelon was grafted onto bottle gourd in high and low nitrogen level, respectively. The nitrogen utilization efficiency was increased by69%and39%when watermelon was grafted onto squash in the two nitrogen levels, respectively.4. We performed transcriptome sequencing for watermelon grafted onto bottle gourd and squash, self-grafted watermelon used as control. We totally obtained155,309,206high quality reads and among them about27,733contigs with length more than1000bp. The unigenes were further assigned with gene ontology (GO) terms and mapped to biochemical pathways. Furthermore, we carried out different gene expression analysis of these contigs and identified787genes that were significant different expressed when watermelon was grafted onto bottle gourd (459up-regulated and328down-regulated), and3458genes were significant different expressed when watermelon was grafted onto squash (3332up-regulated and153down-regulated). The up-or down-regulated genes were found to take part in a broad range of physiological processes, including transcription regulation, hormone stimulus, defense against stresses, light signaling pathway, secondary metabolites, starch and sucrose metabolism, et al. Furthermore, some genes involved in nitrate transport and nitrogen metabolism such as nitrate transporter and glutamine synthetase were also regulated by graft watermelon onto different rootstocks. 5. Solexa sequencing has been used to discover small RNA populations and compare miRNAs on genome-wide scale in watermelon grafting system. A total of11,458,476,11,614,094and9,339,089raw reads representing2,957,751,2,880,328and2,964,990unique sequences were obtained from the scions of self-grafted watermelon and watermelon grafted onto bottle gourd and squash at two true-leaf stage, respectively.39known miRNAs belonging to30miRNA families and80novel miRNAs were identified in our small RNA dataset. Compared with self-grafted watermelon,20(5known miRNA families and15novel miRNAs) and47(17known miRNA families and30novel miRNAs) miRNAs were expressed significantly different in watermelon grafted onto bottle gourd and squash, respectively. There were5miRNA families that were co-regulated in both graft systems, including miR396, miR408, miR398, miR477and miR1661. Furthermore, some of the different expressed miRNAs were involved in plant response to nitrate and low nitrogen stress, such as miR160, miR167, miR169, miR171, miR395, miR397, miR398, miR408and miR827. MiRNAs expressed differentially when watermelon was grafted onto different rootstocks, suggesting that miRNAs might play an important role in diverse biological and metabolic processes in watermelon and grafting may possibly by changing miRNAs expressions to regulate plant growth and development as well as adaptation to stresses. The small RNA transcriptomes obtained in this study provided insights into molecular aspects of miRNA-mediated regulation in grafted watermelon.