Grafting improves growth and nitrogen-use efficiency by enhancing NO3- uptake, photosynthesis, and gene expression of nitrate transporters and nitrogen metabolizing enzymes in watermelon under reduced nitrogen application

Background Excessive application of N fertilizer can inhibit plant growth, reduce N-use efficiency (NUE) and lead to production reduction. Watermelon is an important crop that often restricted by inappropriate N supply. The study aims to test whether grafting with bottle gourd rootstock can improve the NUE and growth performance of watermelon under reduced nitrate application and to clarify the underlying mechanism. Methods Grafted (self-grafted and rootstock-grafted watermelon) and ungrafted (watermelon and bottle gourd) seedlings were treated separately with 9 mM (control) and 4 mM (reduced-nitrate) N concentrations for 18 days under hydroponic conditions. Results The growth and NUE of bottle gourd rootstock-grafted watermelon seedlings increased under reduced-nitrate, while decreased slightly in self-grafted seedlings compared with the control. Rootstock-grafted plants had higher root morphological traits, NO3- accumulation, NR and GS activities, photosynthesis, and NUE traits than self-grafted plants under reduced-nitrate. Reduced-nitrate treatment significantly up-regulated the expression of nitrate transporter genes NRT1.5 and NRT2.1 and N metabolizing enzyme genes NR2, NR3, NiR, GS1 and GS2 in rootstock-grafted plants. Conclusion Under reduced-nitrate treatment, grafted watermelon can make better use of the developed rootstock roots to increase the NO3- absorption and transportation to the shoot, so as to enhance the N metabolism potential and photosynthetic capacity of scions, and finally improve plant growth and NUE. The enhanced NO3- uptake and utilization of rootstock-grafted plants were regulated at the transcriptional level. Grafting with the bottle gourd rootstock may be beneficial to the efficient production of watermelon and economic application of N fertilizer.

Automated summary

This study found that grafting with bottle gourd rootstock can improve the nitrogen use efficiency and growth performance of watermelon under reduced nitrate application. The rootstock-grafted plants showed better root morphology, NO3- accumulation, photosynthesis, and nitrogen use efficiency compared to self-grafted plants under reduced nitrate conditions. The up-regulation of nitrate transporter and nitrogen-metabolizing enzyme genes in the rootstock-grafted plants contributed to the enhanced NO3- uptake and utilization. Grafting with bottle gourd rootstock may be beneficial for the efficient production of watermelon and economical application of nitrogen fertilizer.