ENVIRONMENTAL-EFFECTS ON THE RELATIONSHIP BETWEEN THE QUANTUM YIELDS OF CARBON ASSIMILATION AND INVIVO PSII ELECTRON-TRANSPORT IN MAIZE

The partitioning of light energy absorbed by photosystem (PS) II in the C4 species maize was investigated under various photosynthetic photon flux densities (PPFD), temperatures, and intercellular CO2 concentrations. The relationship between the quantum yield of PSII electron transport (PHI-e) and the quantum yield of CO2 assimilation (PHI-CO2) was generally found to be linear, with similar slopes. This indicates that PSII electron transport is tightly coupled to CO2 assimilation such that measurements of PHI-e may be used to estimate photosynthetic rates in maize. Coefficients of quenching of PSII chlorophyll fluorescence indicated that, under excessive PPFD or when CO2 assimilation was decreased due to suboptimal or supraoptimal temperature or low C(i), the energy in excess of that needed to drive the reduced rate of PSII electron transport was dissipated via a mechanism known to be correlated to the trans-thylakoid proton gradient (high energy quenching, q(E)) and a mechanism believed to arise in the PSII antenna chlorophyll (q(N)(slow)). At suboptimal temperature the energy dissipation was principally at the antenna level and q(E) was low, while at supraoptimal temperature the reverse was true. The results are discussed relative to coupling of PSII activity to CO2 fixation and mechanisms of energy dissipation in this C4 species.