A. S. Collinson
5.2 Temperature and tolerance range of plants
It can be observed in the field and laboratory that every organism has a range of tolerance of external temperature in which, during its lifetime, it can remain indefinitely active. The boundaries of this range are set by the minimum and maximum effective temperatures beyond which the organism ceases its activities and may die. Within their normal geographical range, most organisms exhibit adaptations in their life-cycle and metabolism which provide protection against these extremes during periods of growth and reproduction. Unfavourable periods of low or high temperatures may be passed as seeds or in a dormant state or by increasing the resistance of the tissues The effectiveness of cold resistance can be quite remarkable. For example, seeds some 10000 years old found in permafrost silt have been successfully germinated in the laboratory (Porsild et al, 1967).
As noted above, it is practically impossible to give a single 'optimum' temperature to which an organism is adapted. Rather, a species is adapted to a range of temperature optima for different processes, as is shown in Figure 5.1 The optimum temperature for germination, for example, may be quite different from that for photosynthesis. Although exposure to cold might seem at first sight to be deleterious to plant growth, it has become a vital condition to ensure successful germination in some species and for the initiation of the reproductive cycle in many.
|Figure 5.1 Photosynthetic and respiratory rates in two ecotypes of mountain sorrel (Oxyria digyna). (After Mooney and Billings (1961), © Ecological Society of America.)|
For example, temperature has an important role in stimulating germination in some buried seeds. If, for instance, a canopy gap opens, the diurnal soil temperature range increases, and some herbaceous seeds are very sensitive to this and germinate rapidly. The seeds of grasses, for example Poa annua, Poa trivialis, Poa pratensis, Holcus lanatus and Deschampsia cespitosa, in British pastures have all been shown experimentally to respond to temperature fluctuations in darkness. This capacity seems to be widespread in species composing persistent seed banks. However, very small seeds seem to require light to initiate germination (see Sec. 3.4).
Because all climates experienced by individual plants are local climates:, which might vary greatly over even quite short distances, plants rarely if ever meet 'optimum' conditions in middle and high latitudes, even for only one metabolic function. Each individual plant works out its own relationship with its habitat, so that the flows of energy between itself and its surroundings are maintained by the controls on its internal chemistry and the individual adaptations of its external form.