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ISHS Acta Horticulturae 727: X International Symposium on Plant Bioregulators in Fruit Production


Author:   J.E. Olsen
Keywords:   abscisic acid, circadian rhythm, dormancy, gibberellin, growth cessation, light quality, phytochrome, photoperiod, plant hormone, temperature
DOI:   10.17660/ActaHortic.2006.727.18
Dormancy involves a temporary suspension of visible growth of plant structures containing a meristem. Dormancy is an important mechanism ensuring a seasonal synchronization of growth and also contributes to the control of plant architecture. Establishment of the dormant condition in time before the unfavourable season requires sensing and processing of a regular and reliable environmental seasonal signal. Although bud dormancy has been extensively studied, knowledge of the physiological and molecular mechanisms involved is still limited. In temperate trees and shrubs a short day control of growth cessation and dormancy induction has been widely demonstrated. Physiological studies, including our recent studies of the effects of monochromatic red, far-red and blue light on growth and bud formation, have pointed to an important role of the phytochrome system as the daylength sensor and might suggest also a role of one or more blue light receptors. Studies of woody plants with changed expression of phytochromes might suggest that perception of photoperiod is related to levels of phytochromes. Also, recent studies have shown that steady state mRNA levels of different phytochromes in woody plants are affected by daylength. The phytochrome system apparently interacts with biosynthesis of plant hormones, particularly gibberellin. Also, the involvement of abscisic acid in control of dormancy-related processes has been shown. Dormancy induction is associated with reduced rates of cell division, and it appears that these hormones act, at least partly, through their interaction with the mechanisms of cell cycle regulation. In some species, like apple and pear, no effect of photoperiod on growth cessation and dormancy induction has been found, whereas low temperature is highly effective. However, it is a paradox that the same temperature regime that induces dormancy in these species also controls its release. Moreover, in a number of species in which growth cessation occurs under short photoperiod, a low night temperature can bring about growth cessation and bud set even under long days. Bud set under a non-inductive photoperiod has been shown even in daylength-insensitive plants of phytochrome A overexpressing Populus when exposed to a low night temperature. These and other studies indicate that phytochrome action is affected by temperature, and is suggestive of the existence of a photoperiod-independent pathway in addition to a photoperiodic control resulting in growth cessation and bud set. Since temperature under dormancy induction has also been shown to affect the rate of dormancy development as well as strength of dormancy, an understanding of dormancy induction might be a key to understanding dormancy.

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