Marginal habitats frequently limit growing season length and reduce metabolic activity due to low temperatures. For many species these limitations can be overcome by the ability to increase metabolic rates at low temperatures by means of increased amounts of active enzymes, an adaptation which is referred to as
metabolic or capacity adaptation. As a means of adaptation it is found in cold-water fish (Hochachka & Somero, 1973) as well as in plants. Arctic plants are notable not only for having high levels of the carbon dioxide fixing ribulose bisphosphate carboxylase (RuBisCO) but also for having high densities of mitochondria. The maintenance costs of large amounts of enzymatic proteins in high-latitude plants will be minimized as long as they remain within the low temperature environment and can exploit the long days of the short arctic summer. If as a result of climatic warming temperatures increase, particularly where nights are longer at lower latitudes, high levels of RuBisCO could prove maladaptive due to increased maintenance costs. Physiologically, the 24-hour light regime makes it economic to have higher levels of RuBisCO. This enzyme has a dual function as a car-boxylase and an oxygenase and can therefore fix or liberate carbon dioxide. In warm climates this functional duality can cause a significant loss of fixed carbon through light respiration. However, in the Arctic, temperatures are so low that the oxygenase activity is not significant and the 24-h usage of the high investment in carbon dioxide fixing capacity is amply repaid.
Alpine species are noted for their thicker leaves as compared with related species from lower altitudes (Korner et al., 1989). This is also a form of capacity adaptation, as thicker leaves, with a greater development of palisade tissues and a higher internal volume, will facilitate greater assimilation of carbon dioxide in regions where atmospheric carbon dioxide concentrations are reduced, as at high altitudes (Fig. 3.7). This adaptation is similar to the manner in which human beings living at high altitudes are partially adapted to low atmospheric oxygen concentrations, as is evident in Quechua Indians born and raised at high altitudes in the Andes who possess a greater vital capacity (air expired after inspiration; see also Chapter 10).
Functional adjustment can take a number of forms in flowering plants but is dependent in all cases on phenotypic plasticity (the ability of an individual to change during its lifetime) as opposed to genetic variation (heritable variation) which varies both between species and within populations of the same species. The crucial difference in terms of plant responses to the environment is that phenotypic plasticity provides an immediate reaction to environmental stress, while genetic variation only allows change from one generation to another as a response to selection. Such an immediate phenotypic adaptation is found in the ability of plants to change the density of stomata in their leaves. Plants of
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