Regulation of Light Utilization for Photosynthetic Electron Transport

Abstract

SummaryIn nature photosynthesis is required to perform efficiently and safely with an energy resource, light, whose supply is remarkably variable. A description of the operational responses of the energy transduction apparatus in C3 leaves is presented in the context of limiting and non-limiting light conditions for photosynthesis. In C3 leaves linear electron transport appears to be the principal route for reducing equivalents. Under natural lightlimiting conditions for CO2 assimilation, the quantum yield for linear electron transport at steady state has been found to be largely uniform in non-stressed C3 leaves, and its high value indicates that plants are able to maintain optimal co-ordination of photosystems at near maximal photochemical efficiency. Under non-limiting light conditions, electron transport operates under a rate restriction occurring at the PQH2/cytochrome b 6 f step which results in a co-ordinated decline of the photochemical quantum efficiencies of both photosystems. Modulation of the rate of electron transfer at the PQH2/chtochrome b 6 f step is also the means by which metabolism exerts feedback control over electron transport. Limitation of electron transport on the donor side of PS I results in the operation of a safe dissipation mechanism for excess excitation energy at PS I. At PS II, only a fraction of excess excitation energy is dissipated safely through a mechanism that is subject to control and whose capacity appears to be dependent on species and growth conditions, in particular irradiance. These general responses are discussed in the framework of an effective co-ordination by feedforward and feedback mechanisms that balance the supply of NADPH and ATP from electron transport with the sink strength of stromal metabolism. The significance for these short term operational responses of the environmental dependency of the stoichiometries of the photosystems and electron transport components is also considered.