'Node counting': An internal control of balanced vegetative and reproductive development

Abstract

Plants can complete an entire life cycle in uniform environments. This is evidence that while shoot apices reiterate leaf formation they also change as a function of their own development. These changes occur in individual apices rather than the plant as a whole, and they are most closely correlated with the number of nodes an apex has formed. Evidence about the mechanism of 'node counting' suggests a working hypothesis: apical transitions depend on cumulative information about the distances of an apex from the roots, rather than on any single, critical state. Studies of the ecological roles of plant size and age have neglected the physiological information about 'node counting'. Yet it is evident in field conditions, especially in the morphological location of flowers on branches situated at different heights along the main axis. 'Node counting' affects flower formation, but as a component rather than sole determinant of apical differentiation it displays pronounced phenotypic plasticity. The ecological advantages of 'node counting' could include its being a basis for concurrent vegetative and reproductive development on the same plant. It also allows for environmental responses that are a function of the state of plant sectors without constraining the sizes of leaves and other organs.