A model describing photosynthesis in terms of gas diffusion and enzyme kinetics

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Abstract
A model predicting net photosynthesis of individual plant leaves for a variety of environmental conditions has been developed. It is based on an electrical analogue describing gas diffusion from the free atmosphere to the sites of CO2 fixation and a Michaelis-Menten equation describing CO2 fixation. The model is presented in two versions, a simplified form without respiration and a more complex form including respiration. Both versions include terms for light and temperature dependence of CO2 fixation and light control of stomatal resistance. The second version also includes terms for temperature, light, and oxygen dependence of respiration and O2 dependence of CO2 fixation. The model is illustrated with curves based on representative values of the various environmental and biological parameters. These curves relate net photosynthesis to light intensity, [CO2], [O2], temperature, and resistances to CO2 uptake. The shape of the [CO2]-net photosynthesis curves depends on the total diffusion resistance to CO2 uptake and the Michaelis constant for CO2 uptake. The curves range from typical Michaelis-Menten to Blackman types. The model is combined with a model of leaf energy exchange permitting simultaneous estimation of net photosynthesis and transpiration. The combined model is illustrated with curves relating transpiration to photosynthesis under a wide variety of environmental conditions. Environmental regimes yielding maximum efficiency of water use are identified for the given assumptions and biological parameters.