Photosynthetic CO₂ assimilation, chlorophyll fluorescence and photoinhibition as affected by nitrogen deficiency in maize plants

Effects of nitrogen deficiency on photosynthetic CO₂ assimilation, PSII photochemistry and photoinhibition were investigated in maize plants grown under natural illumination. Nitrogen-deficient plants had a significantly smaller CO₂ assimilatory capacity, but they showed little changes in the maximal efficiency of PSII photochemistry, the rate of Q(A) reduction and the heterogeneity of PSII reaction centers, suggesting that nitrogen deficiency had little effects on PSII primary photochemistry and photoinhibition even under natural illumination. However, modifications in PSII photochemistry under the steady state of photosynthesis induced by nitrogen deficiency were observed. This is reflected in decreases in the quantum yield of PSII electron transport, the efficiency of excitation energy capture by open PSII reaction centers, and the photochemical quenching coefficient and an increase in the non-photochemical quenching coefficient. These results suggest that modifications or PSII photochemistry under the steady state of photosynthesis may be a mechanism to downregulate photosynthetic electron transport so that production of ATP and NADPH would be in equilibrium with the decreased demand in the Calvin cycle in nitrogen-deficient plants. On the other hand, the nitrogen-deficient plants still exhibited increased susceptibility to photoinhibition when exposed to very high irradiance, although nitrogen deficiency induced no photoinhibition under natural illumination. Our results suggest that such increased susceptibility to photoinhibition was associated with the increased accumulation of inactivated PSII reaction centers, the decreased capacity of non-photochemical quenching, and the increased fraction of the reduction state of Q(A). (C) 2000 Published by Elsevier Science Ireland Ltd.