Resource allocation under poor growth conditions. A major role for growth substances in developmental plasticity.

This article argues that the basic function for growth substance is resource allocation under poor growth conditions. The following scheme is suggested. Plants in the wild frequently suffer a paucity of resources which result from interplant competition and ecological and local environmental variation. The strategy adopted by many plants particularly ruderals (from which crops may have evolved) to help mitigate these problems is phenotypic plasticity; the growth of the plant body is adjusted to best exploit the scarce resources and help achieve desirable growth and reproductive goals. Phenotypic plasticity requires decisions to be made concerning the diversion of scarce growth resources to one facet of development rather than another; for example, to height or leaf area rather than thickness; or, between tissues, stem rather than leaves. Growth substances are coupled to these individual facets of development. They represent a simple way in which the extent of resource diversion can be controlled. Cells in specific tissues acquire sensitivity to particular growth substances at a stage in their development when environmental variability often necessitates choices to be made. This acquisition of ontogenetic sensitivity may be all or none. It may reflect acquisition of receptor proteins coupled to specific metabolic events. However in well-nourished plants these phases of development are relatively insensitive to changes in the level of the growth substance/receptor complex. Cells become more sensitive under certain well-defined but specific circumstances, characterized by the general term, poor growth conditions. These are produced by imbalances in one or more of the major environmental (nutritional) requirements for growth, light, nitrogen, water and oxygen. Imbalance in one or more of these produces characteristic and far-reaching metabolic and protein synthesis changes which normally constrain the synthetic processes for growth but amplify metabolic events coupled to growth substances. It is the function of growth substances to circumvent some of these metabolically constraining steps and by applying a constant stimulus to one specific aspect of growth or metabolism permit continued development. The additional input of growth substances into particular facets of development ensures the better maintenance (protection) of that character when competition for resources inside the plant is severe. However competition for scarce resources ensures that continuation of one growth aspect generally leads to relative depletion of others.(ABSTRACT TRUNCATED AT 400 WORDS)