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. [ABSTRACT FROM AUTHOR]