Modelling shoot:root relations:: the only way forward?

The basic transport-resistance (TR) model of shoot:root carbon:nitrogen allocation is described. This approach assumes that the two processes of substrate transport and chemical conversion determine allocation. It is suggested that all allocation models, whether built for the purposes of theoretical investigation or practical application, should start with this irreducible framework. Here it is assumed that the processes operate according to: (a) for substrate sources, dependence on shoot and root sizes with possible product inhibition; (b) for transport, movement down a substrate concentration gradient; and (c) for substrate sinks or utilization, linear bisubstrate kinetics. Some dynamic properties of the model are explored. Failure of this approach to allocation flags the need for additional mechanisms to control the processes. Details of the failure will indicate the modifications needed, which may involve hormones or reflect teleonomy (apparently goal-seeking behaviour), and which are added to the irreducible framework. However, these additions should not replace the irreducible framework of transport and chemical conversions, because they do not in reality. Modifications to the basic model to represent possibilities such as ontogenesis with the transition from exponential growth towards a steady state or with the scaling of within-plant transport resistances with plant size, the influence of hormones, and active transport, are described. (C) 1998 Annals of Botany Company.