In higher plants, abscisic acid and xanthoxin are two potent growth regulators. Although similar properties in both substances have been demonstrated in several biological tests including biochemical interconversion of the substances, evidence is available that in the plant as a whole, xanthoxin has regulatory functions other than those of abscisic acid. Several environmental factors, such as water supply, photoperiod and low temperature, which affect growth and development also greatly change the level of abscisic acid in the plant; however, only small variations in the xanthoxin level have been observed in response to changes in the environmental conditions. On the other hand, a strong enhancement of the xanthoxin level can be induced when dark-grown seedlings are briefly illuminated; this treatment, however, has no influence on the abscisic acid level. This observation supports the hypothesis that light-induced inhibition of growth may be mediated by an increased formation of the growth inhibitor xanthoxin. Light-induced enhancement of the xanthoxin level may also contribute to the phototropic bending in dictyledonous seedlings. Evidence has been obtained from experiments in this laboratory that xanthoxin may be involved in the regulation of root branching. Decapitation of root tips causes a significant increase in the number of lateral root primordia. Chromatographic studies reveal the presence of two substances in the root, which, in a specific bioassay, are active inhibitors of the development of root primordia. The activity of these root inhibitors in the basal part decreases when the root tip is removed. They are probably produced in the root tip and are transported to the base. One of these inhibitors has been identified as xanthoxin, the other is cytokinin. The hormonal regulation of abscission is another process where xanthoxin may have a regulatory function. Senescent, abscinding petioles contain a factor called ‘senescence factor’ which promotes the abscission of leaves. In an attempt to identify its chemical nature, it was found that at least three different abscission accelerating substances, including xanthoxin, participate in the composition of the senescence factor.
Distribution of Lateral Root Primordia in Root Tips of Musa
