MODELING PHOTOSYNTHESIS IN FLUCTUATING LIGHT WITH INCLUSION OF STOMATAL CONDUCTANCE, BIOCHEMICAL ACTIVATION AND POOLS OF KEY PHOTOSYNTHETICINTERMEDIATES

Photosynthetic carbon gain in rapidly fluctuating light is controlled by stomatal conductance, activation of ribulose-1,5-bisphosphate carbo xylase-oxvgenase, a fast induction step in the regeneration of ribulos e-1,5-bisphosphate, and the build-up of pools of photosynthetic interm ediates that allow post-illumination CO2 fixation. Experimental work o ver recent years has identified and characterised these factors. A phy siologically-based dynamic model is described here that incorporates t hese factors and allows the simulation of carbon gain in response to a ny arbitrary sequence of light levels. The model output is found to co nform well to previously reported plant responses of Alocasia macrorrh iza (L.) G. Don. observed under widely differing conditions. The model shows (i) responses of net assimilation rate and stomatal conductance to constant light levels and different CO2 concentrations that are co nsistent with experimental observations and predictions of a steady-st ate model; (ii) carbon gain to continue after the end of lightflecks, especially in uninduced leaves; (iii) carbon gain to be only marginall y reduced during low-light periods of up to 2 s; (iv) a fast-inducing component in the regeneration of ribulose-1,5-bisphosphate to be limit ing for up to 60 s after an increase in light in uninduced leaves: the duration of this limitation lengthens with increasing CO2 concentrati on and is absent at low CO2 concentration; (v) oxygen evolution to exc eed CO2 fixation during the first few seconds of a lightfleck, but CO2 fixation to continue after the end of the lightfleck whereas oxygen e volution decreases to low-light rates immediately, The model is thus a ble to reproduce published responses of leaves to a variety of perturb ations. This provides good evidence that the present formulation of th e model includes the essential rate-determining factors of photosynthe sis under fluctuating light conditions.