A coordination model of whole-plant carbon allocation in relation to water stress

Although water is an important determinant of the allocation of material between roots and shoots during growth, and often parallels the effects of nitrogen, few models have explicitly considered allocation in relation to water supply. We use coordination theory to develop a simple exponential model that considers allocation of dry matter between shoots and roots during growth in relation to carbon and water supplies, and accounts for the effects of water stress on growth. We compare coordination us. optimization (global and local) versions of the exponential model by examining similarities and differences in model behaviour obtained under constant and variable environmental conditions, and with drastically changing conditions (mild, moderate and severe water stress). The greatest differences between coordination and optimization exist in the drastically changing conditions. In a second version of the model, we remove the restriction of exponential growth and show how coordination principles can be extended to a more complicated structure. The non-exponential model is used to analyse experimental data on the effects of different pot sizes (and hence water availability) on root restriction and plant growth as reported by Thomas and Strain (Plant Physiology 96: 627-634, 1991). With further refinements, the coordination model has potential as a tool to model plant growth in relation to water supply under various environmental conditions. (C) 1997 Annals of Botany Company.