A Review of Evidence on the Control of Shoot: Root Ratio, in Relation to Models

Four basic models exist for the control of shoot: root ratio (S: R): (a) allometric models, proposing a fixed ratio of shoot growth rate to root growth rate; (b) functional equilibrium models, based on the ratio of shoot activity to root activity; (c) the Thornley model, based on carbon and nitrogen uptake and transport, (d) hormone models, generally suggesting the root produces a hormone that controls the shoot and vice versa. Models (a) and (b) are empirical, and therefore provide no test of the processes operating. Ontogenetic changes in S: R for fibrous-rooted herbs could be fitted by a modified Thornley model. Ontogenetic effects must be excluded in judging other effects. Responses of S: R to deficits of water, major inorganic nutrients, light and carbon dioxide, and to defoliation and root pruning, usually conform to Thornley's model. With current knowledge Thornley's model cannot usefully be applied to minor nutrients, nutrient toxicity or temperature differences. S: R changes at reproduction usually conform to Thornley's model if it is assumed that young reproductive structures are a strong sink, but this begs the question of what determines sink strength. There are apparent exceptions to most of these responses, which should be studied further. Phytohormones can influence S: R, but may not be the control operating in the normal, intact plant. Most of the available evidence is compatible with a source-sink model of Thornley's type, and therefore does not demand a hormonal theory of S: R control. There is a need for more critical tests.