ABSTRACT
The Role of Phytochrome and Gibberellin Jorunn E. Olsen John B. Jensen
Jorgen A. Molmann Arild Emstsen Olavi Junttila
SUMMARY. Tree species adapted to the climatic conditions of the northern boreal and subarctic vegetation zones have a capacity to de velop a very high level of frost hardiness, even to survive the tempera-
ture of liquid nitrogen in midwinter. Proper timing of hardening, as well as of dehardening, is crucial for winter survival of these species. In northern tree species, cessation of apical elongation growth and bud set is a prerequisite for developmental and metabolic processes leading to hardening, and this chain of events is induced by photoperiod. The northern tree species are closely adapted to the local light climate and display photoperiodic ecotypes. The critical photoperiod is under ge netic control and increases with increasing latitude of origin of the eco type. The photoperiod is probably perceived by the phytochrome system, but the role of other pigment systems, like cryptochrome, has not been studied in woody plants. Phytochrome genes have been cloned from both conifers and deciduous species, but so far we do not have any infor mation about possible differences between photoperiodic ecotypes at the phytochrome level. Northern and southern ecotypes have different re sponses to redrfar red ratios, which could indicate differences in compo sition of their phytochrome systems, for example, the proportions of phytochrome A and B. Both phytochrome A and phytochrome B can be involved in photoperiodic responses. Experiments with transgenic hy brid aspen suggest that responses to photoperiod could be affected by the amount of phytochrome A present in plants. In deciduous species, the plant hormone gibberellin Aj (GAl ) can completely substitute for a long photoperiod, and short day induced cessation of growth is preceded by a significant reduction of GAj levels, particularly in the elongation zone. Photoperiodic control of GA metabolism is supported by several studies, but very little is known about the interaction between phytochrome and GA metabolism and/or responsiveness to GA}. Although our knowledge is still very fragmentary, available results suggest that cessation of growth and initiation of hardening in trees can be controlled both through the phytochrome and the GA mediated systems. Research with tree species is a tedious and slow process, but with the emerging new methods and approaches, we may expect exciting new results in the near future. [Article copies available fo r a fe e from The Haworth Document Delivery Service: 1-800-HAWORTH. E-mail address: <docdelivery@haworthpress.com> Website: <https://www.HawonhPress.com>; © 2004 by The Haworth Press, Inc. All rights reserved.]
KEYWORDS. Critical photoperiod, cold hardening, gibberellin, phyto chrome, plant hormones
INTRODUCTION
The influence of photoperiod on the regulation of flowering in herba ceous plants and seasonal growth in woody plants has been well known
since the discovery by Garner and Allard (1923) early in this century. Generally, photoperiodism is connected to the regulation of processes which are important for reproduction (flowering in seed plants) and sur vival (woody plants) of the plants (Nitsch, 1957). In both cases, the tim ing of these processes is of great importance and must be synchronized with the local climatic conditions. At a given locality, the photoperiod provides an accurate and reliable signalling system for timing of these developmental events. In northern tree species, cessation of apical elon gation growth is a prerequisite for cold hardening and therefore photo period is a decisive factor for timing of development of cold hardiness in these species. Due to this, photoperiodism in northern tree species is a highly adaptive characteristic and the presence of photoperiodic ecotypes in trees related to latitudinal and altitudinal origin is well documented (for references, Junttila and Robberecht, 1998). Photoperiodism in such tree species provides an interesting system for studies of physiological and genetic aspects of climatic adaptation in plants. In addition, photo periodic regulation of growth, cessation of growth and frost hardening has important practical aspects both in horticulture and in forestry.
