A dynamic model of leaf photosynthesis for C3 plants has been developed for examination of the role of the dynamic properties of the photosynthetic apparatus in regulating CO2 assimilation in variable light regimes. The model is modified from the Farquhar‐von Caemmerer‐Berry model by explicitly including metabolite pools and the effects of light activation and deactivation of Calvin cycle enzymes. It is coupled to a dynamic stomatal conductance model, with the assimilation rate at any time being determined by the joint effects of the dynamic biochemical model and the stomatal conductance model on the intercellular CO2 pressure. When parametrized for each species, the model was shown to exhibit responses to step changes in photon flux density that agreed closely with the observed responses for both the understory plant Alocasia macrorrhiza and the crop plant Glycine max. Comparisons of measured and simulated photosynthesis under simulated light regimes having natural patterns of lightfleck frequencies and durations showed that the simulated total for Alocasia was within ±4% of the measured total assimilation, but that both were 12–50% less than the predictions from a steady–state solution of the model. Agreement was within ±10% for Glycine max, and only small differences were apparent between the dynamic and steady–state predictions. The model may therefore be parametrized for quite different species, and is shown to reflect more accurately the dynamics of photosynthesis than earlier dynamic models.

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Dynamic responses of stomatal conductance to lightflecks (15 s to 5 min) were observed in the tropical understory plant Alocasia macrorrhiza.Brief observations were also made on step changes in light, flashing light and short low-light periods (darkflecks) after stomata had reached steady state conductances in high light. Stotnata opened substantially even in response to very short lightflecks, with maximal opening being reached about 20 min after the end of the lightfleck. These responses are compared to those of a semi-mechanistic stomatal model consisting of three steps in series: a biochemical signal which responds directly to light, producing osmolic changes within the guard cells which, in turn, drive the movement of water into the guard cells. The rate of change of each component was assumed to be given by the difference between the current state of that component and the state of the previous component in the series divided by a characteristic time constant for each step. This model allowed excellent description of observed responses to lightflecks, including the fleck-length dependence of stomatal opening, the lag between the end of the lightfleck and the time of maximal stomatal conductance and the response to tnultiple flecks. However, agreement with predicted responses to flashing light and darkflecks was poor. Good agreement could be restored by using dilTerent titne Abbreviations: A = Net CO, assimilation rate; c, = mole Traction of COj at the leal" surface; g, = stomatal conductance; g,,f,,,,i, = steady-state conductance in flashing light; g».iri»/r ~ Steady-State conductance in high light; g,.i,,w = steadystate contkictancc in low light; h, = relative humidity at the leaf surface; I = photon llux density; L = lightfleck length; S = initial bioehemical signal in the guard eells; S^,, = equilibrium value of S at different photon llux densities; S,,,,,, = extrapolated minimum of S^.j, in darkness; S,,,, = the light dependent variable part of S^^; 1 = time; w = guard cell water content; a = initial slope of S^,, as a function of light; I) = lenn to describe the curvature in the light dependence of S,,,,; n = guard cell osmotic potential; r,,, T,, t^, T^ = time constants for decrease and increase of S, and changes in 71 and w. constants for the initial biochemical step, suggesting that the speed of the initial biochemical response is not constant but dependent on the current physiological state of the stomata. For leaves experiencing predotninantly low light, punctuated by relatively short and infrequent periods of high light, this model allows good description of the dynamic changes in stomatal conductance.