Environmental Effects on the Relationship Between the Quantum Yields of Carbon Assimilation and in vivo PsII Electron Transport in Maize

Abstract

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 (¦e) and the quantum yield of CO₂ assimilation (ΦCO₂) 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 ¦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.