Grafting Enhances the Photosynthesis and Nitrogen Absorption of Tomato Plants Under Low-Nitrogen Stress

Nitrogen is an essential macronutrient required for plant growth. In this study, the relationships between plant growth, photosynthetic capability, chlorophyll fluorescence characteristics and nitrogen forms in response to low-nitrogen stress were studied in tomato grafted with different nitrogen-use-efficiency tomato seedlings. Using tomato plants grafted with different rootstocks, we found that, under low-nitrogen stress, plant growth, chlorophyll contents, net photosynthetic rate (Pn), maximal photochemical efficiency (F-v/F-m), the total nitrogen (TN) concentration, and nitrate reductase (NR) activity were significantly inhibited but that the ammonium-nitrogen (NH4+-N) content in the roots significantly increased. High-nitrogen-use-efficiency tomato grafted plants exhibited significantly enhanced plant growth but reduced Pn, F-v/F-m, TN content and NR activity under low-nitrogen conditions compared to the grafted seedlings of the low-nitrogen-use-efficiency genotype. In addition, tomato plants grafted onto high-nitrogen efficient rootstock presented reduced damage caused by excessive accumulation of NH4+-N in the roots under low-nitrogen stress. Our results indicate that tomato plants grafted onto high-nitrogen efficient rootstock presented enhanced absorption and utilization of nitrogen and maintained growth by promoting the use efficiency of light energy under low-nitrogen stress.