Nitrogen Metabolism in Adaptation of Photosynthesis to Water Stress in Rice Grown under Different Nitrogen Levels

Abstract

To investigate the role of nitrogen (N) metabolism in the adaptation of photosynthesis to water stress in rice, a hydroponic experiment supplying with low N (0.72 mM), moderate N (2.86 mM), and high N (7.15 mM) followed by 150 g⋅L^-1 PEG-6000 induced water stress was conducted in a rainout shelter. Water stress induced stomatal limitation to photosynthesis at low N, but no significant effect was observed at moderate and high N. Non-photochemical quenching was higher at moderate and high N. In contrast, relative excessive energy at PSII level (EXC) was declined with increasing N level. Malondialdehyde and hydrogen peroxide (H₂O₂) contents were in parallel with EXC. Water stress decreased catalase and ascorbate peroxidase activities at low N, resulting in increased H₂O₂ content and severer membrane lipid peroxidation; whereas the activities of antioxidative enzymes were increased at high N. In accordance with photosynthetic rate and antioxidative enzymes, water stress decreased the activities of key enzymes involving in N metabolism such as glutamate synthase and glutamate dehydrogenase, and photorespiratory key enzyme glycolate oxidase at low N. Concurrently, water stress increased nitrate content significantly at low N, but decreased nitrate content at moderate and high N. Contrary to nitrate, water stress increased proline content at moderate and high N. Our results suggest that N metabolism appears to be associated with the tolerance of photosynthesis to water stress in rice via affecting CO₂ diffusion, antioxidant capacity, and osmotic adjustment.

Keywords: nitrogen assimilation; nitrogen level; photosynthesis; rice (Oryza sativa L.); water deficit.

FIGURE 1

Comparison in xylem secretion rate (A), transpiration rate (B), and leaf relative water content (RWC) (C) in rice plants grown under different nitrogen and water conditions. Data refers to mean ± SE (n = 4). P-values of the two-way ANOVAs of nitrogen, water, and their interaction are indicated: ns, not significant; ^∗P < 0.05; ^∗∗P < 0.01; ^∗∗∗P < 0.001. Bars with the same letter are not significantly different by LSD test. WD, water deficit; WW, well-watered.

FIGURE 2

Comparison in whole plant dry weight (A), net photosynthetic rate (P_n, B), stomatal conductance (g_s, C), and intercellular CO₂ concentration (C_i, D) in rice plants grown udner different nitrogen and water conditions. Data refers to mean ± SE (n = 4). P-values of the two-way ANOVAs of nitrogen, water, and their interaction are indicated: ns, not significant; ^∗P < 0.05; ^∗∗P < 0.01; ^∗∗∗P < 0.001. Bars with the same letter are not significantly different by LSD test. WD, water deficit; WW, well-watered.

FIGURE 3

Comparison in maximum carboxylation rate of Rubisco (V_c,max, A), maximum electron transport rate (J_max, B), triose phosphate utilization (TPU, C), and mesophyll conductance (g_m, D) of rice plants grown under different nitrogen and water conditions. Data refers to mean ± SE (n = 4). P-values of the two-way ANOVAs of nitrogen, water, and their interaction are indicated: ns, not significant; ^∗P < 0.05; ^∗∗P < 0.01; ^∗∗∗P < 0.001. Bars with the same letter are not significantly different by LSD test. WD, water deficit; WW, well-watered.

FIGURE 4

Comparison in chlorophyll fluorescence parameters in rice plants grown udner different nitrogen and water conditions. (A) Maximum quantum yield of PSII photochemistry. (B) Effective quantum yield of PSII photochemistry. (C) Photochemical quenching coefficient. (D) Non-photochemical quenching coefficient. (E) Electron transport rate at PSII level. (F) Relative excessive energy at PSII level. Data refers to mean ± SE (n = 4). P-values of the two-way ANOVAs of nitrogen, water, and their interaction are indicated: ns, not significant; ^∗P < 0.05; ^∗∗P < 0.01; ^∗∗∗P < 0.001. Bars with the same letter are not significantly different by LSD test. WD, water deficit; WW, well-watered.

FIGURE 5

Comparison in the contents of total leaf nitrogen (A) and soluble protein (B) in rice plants grown udner different nitrogen and water conditions. Data refers to mean ± SE (n = 4). P-values of the two-way ANOVAs of nitrogen, water, and their interaction are indicated: ns, not significant; ^∗P < 0.05; ^∗∗P < 0.01; ^∗∗∗P < 0.001. Bars with the same letter are not significantly different by LSD test. WD, water deficit; WW, well-watered.

FIGURE 6

Comparison in the contents of ammonium (A), nitrate (B), proline (C), and free amino acid (D) in rice plants grown udner different nitrogen and water conditions. Data refers to mean ± SE (n = 4). P-values of the two-way ANOVAs of nitrogen, water, and their interaction are indicated: ns, not significant; ^∗P < 0.05; ^∗∗P < 0.01; ^∗∗∗P < 0.001. Bars with the same letter are not significantly different by LSD test. WD, water deficit; WW, well-watered.

FIGURE 7

Comparison in activities of glutamine synthase (GS, A), NADH-glutamate synthase (NADH-GOGAT, B), and NADH-glutamate dehydrogenase (NADH-GDH, C) in rice plants grown under different nitrogen and water conditions. Data refers to mean ± SE (n = 4). P-values of the two-way ANOVAs of nitrogen, water, and their interaction are indicated: ns, not significant; ^∗P < 0.05; ^∗∗P < 0.01; ^∗∗∗P < 0.001. Bars with the same letter are not significantly different by LSD test. WD, water deficit; WW, well-watered.

FIGURE 8

Comparison in leaf lipid peroxidation (A) and hydrogen peroxide (B) in rice plants grown udner different nitrogen and water conditions. Data refers to mean ± SE (n = 4). P-values of the two-way ANOVAs of nitrogen, water, and their interaction are indicated: ns, not significant; ^∗P < 0.05; ^∗∗P < 0.01; ^∗∗∗P < 0.001. Bars with the same letter are not significantly different by LSD test. WD, water deficit; WW, well-watered.

FIGURE 9

Comparison in activities of ascorbate peroxidase (APX, A), superoxide dismutase (SOD, B), catalase (CAT, C), and glycolate oxidase (GO, D) in rice plants grown udner different nitrogen and water conditions. Data refers to mean ± SE (n = 4). P-values of the two-way ANOVAs of nitrogen, water, and their interaction are indicated: ns, not significant; ^∗P < 0.05; ^∗∗P < 0.01; and ^∗∗∗P < 0.001. Bars with the same letter are not significantly different by LSD test. WD, water deficit; WW, well-watered.