Adaptive significance of the relation between root and shoot growth

Abstract

The partitioning of dry matter between the root and shoot tissues of a plant is regulated precisely at a constant value for a given genotype under specified environmental conditions. But individuals of different species or of the same species under different environmental conditions show characteristic variation in the root-io-shoot ratio. we postulate that this ratio is ultimately regulated not by competition berneen loot and shoot of a plant, but by considerations of the maintenance of a proper balance between the functions of root and shoot of an integrated whole plant such that the net carbon fixation by the plant is maximum. A theoretical analysis of this problem shows that under certain conditions the root-to-shoot ratio would be expected to decrease for plants growing under better lighted or more arid conditions, in contradiction to the usually observed and expected trends. A simple mathematical model of the phenomenon is presented which delineates the critical parameters of the system and generates several testable predictions. For example, it is predicted that if the root-to-shoot ratio increases under conditions of greater availability of light, then the cost of maintenance and replacement of unit shoot tissue will be smaller than that for root tissue.