Grafting improves nitrogen-use efficiency by regulating the nitrogen uptake and metabolism under low-nitrate conditions in cucumber

Excessive application of nitrogen (N) fertilizer can inhibit plant growth, reduce the N-use efficiency (NUE), increase fertilizer waste, and leads to secondary salinization. Grafting provides a feasible method for improving N uptake and utilization of plants under abiotic stress, but the mechanism by which grafting regulates N uptake, assimilation, and metabolism under low-nitrate conditions is poorly known. To explore the mechanism of rootstock grafting promoting plant growth and N absorption and utilization, in this study, a commercial cucumber cultivar ('Jinchun No. 4', a relatively N-inefficient cultivar) was self-grafted and grafted onto two pumpkin rootstocks ('Figleaf Gourd', a relatively N-medium efficiency pumpkin cultivar, and 'Ribenjingtiantaimu', a relatively N-efficient pumpkin cultivar, respectively). The grafted plants were exposed to two levels of NO3--N (7 and 4 mM) under substrate culture conditions. Root morphological traits; NH4+, NO3-, and total N contents; N-based enzymatic activities; and N metabolite contents of self-grafted plants (J/J) were all markedly inhibited by low-nitrate treatment, leading to lower N accumulation (NA), N-uptake efficiency (NUpE), N-utilizition efficiency (NUtE) and NUE than rootstock-grafted plants (J/F and J/R). In contrast, the inhibition caused by the low-nitrate treatment was effectively alleviated in J/R-grafted plants, as J/R-grafted plants had the largest root systems and the strongest potential for N absorption and metabolism. Variation in these parameters was determined by the rootstock genotype and contributed greatly to improvements in NUpE, NUtE and NUE. Overall, our findings demonstrate that grafting with N-efficient rootstock can permit plants to make better use of their roots and enhance N absorption and N metabolism potential, thereby promoting the growth and NUE of cucumber seedlings. Furthermore, our findings infer that grafting with N-efficient rootstock may permit high levels of productivity to be maintained under low N supplies during cucumber production.