5 - Physiological processes and response to stress

Summary

Environmental control of carbon dioxide exchange

Methods

A plant survives only where the annual environmental regime includes periods favourable for metabolism and growth and where, in the case of perennials, there are no periods of lethal stress. Most polar plants, vascular and non-vascular, are perennials, and must therefore show resistance to both elastic and plastic stress. Levitt (1980) defines a biological stress as ‘any environmental factor capable of inducing a potentially injurious strain in living organisms’: a plastic stress is one producing irreversible chemical or physical change, e.g. frost damage, while an elastic stress results in a reversible change such as a major reduction in net assimilation rate (NAR) under suboptimal conditions. This chapter begins to examine the features that enable bryophytes and lichens to survive under the apparently severe stress of Arctic and Antarctic environments. Such features include, first, general characteristics of these plants that confer fitness under polar conditions and, second, adaptations specific to polar species or populations which may therefore have evolved in response to local selection pressure.

Carbon dioxide exchange has been widely investigated to determine how polar cryptogams maintain positive carbon and energy budgets under adverse conditions. The results are not fully comparable due to differences in experimental procedure. Most methods have involved infrared gas analysis (IRGA) in the laboratory, either in open systems at ambient CO₂ concentration (Oechel, 1976) or in sealed cuvettes (Larson & Kershaw, 1975a). The latter method allows greater replication but has the disadvantage that CO₂ concentration inevitably fluctuates somewhat during the period of measurement. The relative merits of these systems have been discussed by Lange & Tenhunen (1981) and Kershaw (1985).