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

Abstract

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 (R²) 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.

Keywords: abscisic acid; jasmonic acid; limited nitrogen application; neighboring competition; photosynthesis; stomatal conductance.

Figure 1

Effects of competition and nitrogen supply on shoot water potential (ψ_shoot) 14 days after transplanting. Mean values and error bars of standard error (SE) are presented (n = 6). N15 and N1 mean 15 mmol·L⁻¹ and 1 mmol·L⁻¹ supply, respectively; 4 plants and 1 plant mean four grouped plants or single plants grown in one pot, respectively. The table summarizes the significance (p-value) of competition treatment (C), nitrogen treatment (N), and their interaction (C × N) by two-way analysis of variance (ANOVA), Duncan’s multiple-range test.

Figure 2

Net photosynthetic rate (A), stomatal conductance (B), transpiration rate (C), and intercellular CO₂ concentration (D) in response to neighbors in two nitrogen applications 14 days after transplanting. Mean values and SE (n = 6) are presented. The table summarizes the significance (p-value) of competition treatment (C), nitrogen treatment (N), and their interaction (C × N) by two-way ANOVA, Duncan’s multiple-range test. Different letters indicate a significant difference at the p < 0.05 level by Duncan’s multiple-range test.

Figure 3

Changes of ABA (A), JA (B), and shoot-to-root ratio in response to neighbors in two nitrogen applications 14 days after transplanting. Mean values and SE (n = 3) are presented. Different letters indicate a significant difference at the p < 0.05 or p < 0.01 level by Duncan’s multiple-range test.

Figure 4

Relationships of stomatal conductance (g_s) to shoot ABA concentration (A), g_s to shoot JA concentration (B), under N15 and N1 treatments, and the g_s to leaf N content under all treatments (C). In (A) and (B), the open squares indicate plants under N15 treatment and closed squares indicate plants under N1 treatment. A fit curve is presented with the fitted equation (n = 8 or 16); ** indicates that the regression lines are statistically significant at the p < 0.01 level.

Figure 5

Chlorophyll index (SPAD) (A) and limitation contribution to photosynthesis (B) in response to neighbors in two nitrogen applications 14 days after transplanting. Mean values and SE (n = 6) are presented. Different letters indicate a significant difference at the p < 0.05 level by Duncan’s multiple-range test.

Figure 6

Relationships of chlorophyll index (SPAD) to leaf N concentration (A), P_n to SPAD (B), under N15 and N1 treatments. Open squares indicate plants under N15 treatment; closed squares indicate that plants under N1 treatment. Fit curve is presented with the fitted equation (n = 8), ** indicates the regression lines are statistically significant at the p < 0.01 level.

Figure 7

Proposed model of neighboring competition induced plant morphological and physiological response under N deficiency conditions. Lines ending in arrowheads indicate positive impacts, while lines ending in a bar indicate negative impacts.