A model for the growth of plants is proposed. It extends the concept of thermal time to include the effects of photosynthetically active radiation and carbon dioxide concentration in a formula that is analogous to the rate of flow through a set of conductances in series. The formula transforms time periods of differing or varying environmental conditions to a consistent plant-related basis. The integrated plant environmental time is used in the expolinear equation of Goudriaan and Monteith (1990) to provide a simple dynamic model for the growth of mutually shading plants in a uniform canopy of an even-aged monoculture. The model is fitted to data from lettuce and cauliflower. Each had been grown, in a glasshouse, at four different times of year and at four densities. The plant environmental time relation allows for the different conditions of temperature and light, and the expolinear equation then predicts the set of growth curves at the different densities. Parameter values differ between the species, as would the predicted values of environmental time. An improved fit is obtained by using the mutual shading relation to modify the radiation term only, rather than the whole environmental growth rate expression. Most of the observed results differ by less than one day in time from the fitted values. Corroboration for the model is provided by values of an effective leaf area ratio, estimated from the effects of plant density and plant weight upon competition for radiation. © 1993 Annals of Botany Company.
