Chapter 14 - Apical Dominance and Some Other Phenomena Illustrating Correlative Effects of Hormones

Abstract

There is abundant evidence from the application of auxin to defoliated or disbudded stems, cortical or pith explants, callus tissues in culture, and parenchyma in stem flaps that auxins promote xylogenesis. If the vascular tissues are severed in a stem or petiole, basipetal transport of IAA induces the regeneration of xylem and phloem tissues in parenchyma cells of pith and/or cortex to reestablish vascular continuity. Similarly, in blocks of parenchyma cells from storage tissue or pith cylinders, the basipetal flow of IAA induces the differentiation of vascular strands. It appears from these and other experiments that IAA acts as an inducing signal to set up its own channels for polar transport, a phenomenon known as canalization. IAA also acts as a morphogenetic signal for the induction of xylem strands. In conifers and hardwoods growing in temperate climates, IAA is a major factor responsible for the activation of vascular cambium in the spring. In young shoots, cambial activity depends on the de novo synthesis of IAA in expanding buds, but in old trunks, sufficient IAA may already be present in the cambial zone for the initiation of cell division activity. To maintain cell divisions and xylem and phloem production, the cambium must be supplied with IAA by polar transport and/or localized synthesis. The production of secondary xylem, like that of primary xylem, is favored by high concentrations of IAA or synthetic auxin.