Ecophysiological implications of vascular differentiation and plant evolution

Environmental cues regulate plant vascular differentiation and plant evolution through simple hormonal mechanisms of a single or a few moving signals. Mechanisms regulating the responses of plants and their vascular tissues to environmental stimuli are mediated by continuously moving hormonal signals that enable continuous response to ecological cues. Auxin from young leaves is the primary hormonal signal that can induce vascular differentiation by itself. Its concentrations determine whether phloem or xylem is induced. Auxin produced in a parasitic plant induces continuous vessel system into its host with open perforation at their junction. Polar auxin gradients along trees regulate the gradual widening of vessel diameter and decrease in vessel density from leaves to roots. This basic mechanism also regulates vascular adaptation to the plant's environment. Gibberellin from mature leaves, in the presence of auxin, promotes cambial activity and woodiness, and is the specific signal inducing fibers. The evolutionary development of vessels and fibers from tracheids reflects their hormonal specialization; from the combined mechanism of auxin and gibberellin for tracheids in gymnosperms, to the specialized mechanisms of auxin inducing vessels, and gibberellin inducing fibers in angiosperms. Cytokinin from root tips promotes cambial activity and sensitivity enabling the extreme differentiation of ring-porous wood in temperate deciduous hardwood trees. These mechanisms are discussed for clarifying the role of the environment in vascular adaption and evolution.