ISHS Acta Horticulturae 80: Symposium on Growth Regulators in Fruit Production
VEGETATIVE GROWTH IN APPLE WITH REFERENCE TO ABSCISIC ACID |
| Author: | Loyd E. Powell |
| Abstract:
Woody plants of the temperate zone such as the apple follow well defined growth patterns. Growth begins in spring with the coming of warm temperatures. It begins slowly at first, then its rate increases so that after a few weeks the plant is in Sachs' "Grand Period of Growth". This phase lasts a relatively short time, after which the growth rate decreases and terminal growth of the shoot finally ceases. Terminal buds are generally formed. The time at which terminal bud formation occurs varies with species, plant vigor and other factors, but it typically takes place during early summer in many geographical regions. At approximately this time many woody plants enter a state of physiological dormancy which many horticulturalists refer to as the rest period. This dormant condition can at first be rather easily broken by invigorating the plant, e.g. with nitrogen and/or water. As the intensity of the rest period increases it becomes more and more difficult to activate growth, so that by late summer or autumn, such treatments are usually without effect. With the onset of chilling temperatures, the intensity of the rest period begins to decrease, so that after a period of several weeks or months, the chilling requirement is satisfied and growth can once more take place. In many regions an environmental dormancy is imposed by the cold winter temperatures, so that growth cannot begin immediately after the chilling requirement is met. Instead, the resumption of growth must await the moderating temperatures of spring. Physiologists have long been interested in the mechanisms which control these growth phases. What activates bud growth in the spring? Why do shoots cease growth in summer when environmental conditions for growth appear to be ideal? What is the nature of the rest period? What is the role of chilling? These are some of the questions for which answers are being sought. Shoot growth is thought to be under hormonal control. It is easy to show by applications of various growth regulators that such substances can indeed affect shoot growth processes. It is, however, much more difficult to prove that similar naturally occurring substances play specific roles. Abscisic acid (ABA) is a good case in point. This naturally occurring inhibitor aroused much interest after the discovery that it seemed to be involved in the induction of dormancy in Acer pseudoplatenus under short days (Phillips and Wareing, 1958; Eagles and Wareing, 1964; Cornforth et al., 1965). These reports triggered considerable research and further support for an important role for this substance in several | |
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