In addition to the many effects on development of individual plant growth hormones, a sizeable number of effects of one hormone on another have been found. For example, IAA alone can restore the full number of normal tracheary cells in the xylem, but to restore the full number of sieve-tube cells in the phloem zeatin is needed in addition to IAA. Similarly, to restore the full number of fibers in the phloem, GA must be added along with IAA.
Hormones affect each other's movement, too. Mentioned above was the decrease in IAA movement from pretreatment with ethylene. Similarly, ab-basipetal toward the scisic acid decreases the basipetal polar movement of IAA in stems and peti-
base oles. Therefore, in view of IAA's role as the primary inhibitor of abscission in plants, the abscisic acid-induced decrease in IAA movement down the leaf stalk toward the abscision zone probably explains at least part of abscisic acid's role as an accelerator of abscission. In other cases, increases in IAA basipetal movement have resulted from GA or cytokinin treatment. The nonpolar movement typical of cytokinins was changed to polar movement when IAA was added, too. see also Differentiation and Development; Embryogenesis; Genetic Mechanisms and Development; Germination and Growth; Hormonal Growth and Development; Photoperiodism; Seedless Vascular Plants; Senescence; Tropisms.
William P. Jacobs
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