Leaf Gas Exchange of Tomato Depends on Abscisic Acid and Jasmonic Acid in Response to Neighboring Plants under Different Soil Nitrogen Regimes

High planting density and nitrogen shortage are two important limiting factors for crop yield. Phytohormones, abscisic acid (ABA), and jasmonic acid (JA), play important roles in plant growth. A pot experiment was conducted to reveal the role of ABA and JA in regulating leaf gas exchange and growth in response to the neighborhood of plants under different nitrogen regimes. The experiment included two factors: two planting densities per pot (a single plant or four competing plants) and two N application levels per pot (1 and 15 mmol·L(−1)). Compared to when a single plant was grown per pot, neighboring competition decreased stomatal conductance (g(s)), transpiration (T(r)) and net photosynthesis (P(n)). Shoot ABA and JA and the shoot-to-root ratio increased in response to neighbors. Both g(s) and P(n) were negatively related to shoot ABA and JA. In addition, N shortage stimulated the accumulation of ABA in roots, especially for competing plants, whereas root JA in competing plants did not increase in N15. Pearson’s correlation coefficient (R(2)) of g(s) to ABA and g(s) to JA was higher in N1 than in N15. As compared to the absolute value of slope of g(s) to shoot ABA in N15, it increased in N1. Furthermore, the stomatal limitation and non-stomatal limitation of competing plants in N1 were much higher than in other treatments. It was concluded that the accumulations of ABA and JA in shoots play a coordinating role in regulating g(s) and P(n) in response to neighbors; N shortage could intensify the impact of competition on limiting carbon fixation and plant growth directly.