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

Effects of nitrogen deficiency on photosynthetic CO2 assimilation, PSII pho tochemistry and photoinhibition were investigated in maize plants grown und er natural illumination. Nitrogen-deficient plants had a significantly smal ler CO2 assimilatory capacity, but they showed little changes in the maxima l efficiency of PSII photochemistry, the rate of Q(A) reduction and the het erogeneity of PSII reaction centers, suggesting that nitrogen deficiency ha d little effects on PSII primary photochemistry and photoinhibition even un der natural illumination. However, modifications in PSII photochemistry und er the steady state of photosynthesis induced by nitrogen deficiency were o bserved. This is reflected in decreases in the quantum yield of PSII electr on transport, the efficiency of excitation energy capture by open PSII reac tion centers, and the photochemical quenching coefficient and an increase i n the non-photochemical quenching coefficient. These results suggest that m odifications of PSII photochemistry under the steady state of photosynthesi s may be a mechanism to downregulate photosynthetic electron transport so t hat 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 def iciency induced no photoinhibition under natural illumination. Our results suggest that such increased susceptibility to photoinhibition was associate d with the increased accumulation of inactivated PSII reaction centers, the decreased capacity of non-photochemical quenching, and the increased fract ion of the reduction state of Q(A). (C) 2000 Published by Elsevier Science Ireland Ltd. All rights reserved.