Photosystem II photochemistry and its sensitivity to heat stress in maize plants as affected by nitrogen deficiency

Effects of nitrogen deficiency on photosystem II photochemistry and its sensitivity of heat stress (25-44 degrees C) were investigated in maize plants (Zea mays L.) grown under natural illumination. Maize plants were fertilized with 15 mmol/L nitrate (high N, control) versus 0.5 mmol/L nitrate (low N, N deficiency). Compared with the high-N plants, the low-N plants had lower values in the apparent quantum yield of photosynthesis and light-saturated CO2 assimilation capacity expressed either on a basis of leaf area or chlorophyll. In the light-adapted state, the low-N plants showed substantial decreases in the quantum yield of PSII electron transport (Phi(PSII), 28 %), the efficiency of excitation energy capture by open PSII reaction centers (F-v'/F-m', 12 %) and the photochemical quenching coefficient (q(p), 19 %), as well as a significant increase in the non-photochemical quenching coefficient (q(N), 84 %). In the dark-adapted state, the low-N plants showed a small but a significant decrease (4 %) in the maximal efficiency of PSII photochemistry (F-v/F-m). These results suggest that N deficiency induced no substantial damage to PSII apparatus and the changes in PSII photochemistry in the light-adapted state can be seen as a regulatory response to down-regulate the quantum yield of PSII electron transport (Phi(PSII)) that would match with the decreased CO2 assimilation rate under N deficiency conditions. When exposed to high temperatures, a greater decrease in Phi(PSII), F-v'/F-m' and q(p), as well as a larger increase in q(N) and the proportion of the Q(B)-non-reducing PSII reaction centers were observed in the low-N plants than in the high-N plants, indicating that the responses of PSII to heat stress was significantly affected by N status, with N deficiency increasing the sensitivity of PSII to heat stress. These results suggest that nitrogen nutrition plays an important role in the protective adaptation of PSII to heat stress.