An Analytical Model for Field Measurement of Photosynthesis

Abstract

A steady state model is developed for the analysis and interpretation of rates of CO2 exchange by terrestrial plants. A system is defined around the substomatal cavity so that the influxes and effluxes of CO2 to and from the cavity are linked through the mean intercellular space CO2 concentration. The rate of net photosynthesis is expressed as a function of radiation flux, leaf temperature, ambient CO2 concentration and stomatal resistance. Dark respiration is expressed as a function of leaf temperature and radiation flux. The parameters in the relationships between CO2 exchange and the environmental variables are estimated from field data by a nonlinear least squares method, and are all physiologically meaningful characteristics of the photosynthetic process. To obtain unbiased parameters, there must be simultaneous measurement of rate of net photosynthesis, stomatal resistance, irradiance, leaf temperature and ambient CO2 concentration, and the data should be uniformly distributed over the variable space. This can be best achieved if the leaf environment is controlled in the field. The model accounted for 90% of the variance in field measurements made on Liriodendron tulipifera in the field. Using the parameters derived, predictions of rate of net photosynthesis were made which agreed well with observations made on other occasions. The model can be used to derive useful photosynthetic characteristics from field data and as a process model for simulation.