The characteristics of phenotypic plasticity
Plasticity is the degree to which an organism can be changed in response to environmental signals and is, as indicated earlier, a clear example of plant intelligence. Plasticity can be expressed in both physiology and morphology. Guard cell plasticity or, more exactly, plasticity in transpiration is clearly physiological plasticity. Other physiological examples are to be found in carbon assimilation (photosynthesis rates) and dry matter partitioning (Bloom et al., 1985; Korner, 1991; Bell and Sultan, 1999). Karban and Baldwin (1997) indicate that herbivory and pest defence mechanisms can generate enormous numbers of physiologically distinguishable individuals arising from the moving target model. This model suggests that pest attack results in effectively random resistance responses in identical tissues such as leaves. Indeed, data provided by these authors indicate that on a single tree every leaf was observed to be at a different stage of pest resistance.
Morphological or phenotypic plasticity has been studied for many years, largely by population geneticists because of its relevance to evolutionary studies (see Box 2). Phenotypic plasticity generated by environmental variation is commonly expressed in growth habit and size, morphology and anatomy of vegetative and reproductive structures, in absolute and relative biomass accumulation, growth rates, functional cleistogamy, variable sex expression and offspring developmental patterns (Bradshaw, 1965; Diggle, 1994; Bazzaz, 1996; Pigliucci, 1997; Schlichting and Pigliucci, 1998; Ackerley et al., 2000; Sultan, 2000). Variations are also common in stomatal frequency, hairiness of leaves, palisade vs. spongy mesophyll, modifications in vascular tissues, cuticular thickness and sclerenchyma. Even the number of petals on a flower can change after leaf removal (Tooke and Battey, 2000). Maryland Mammoth tobacco (Taiz and Zeiger, 1998), and the ability of gardeners to grow outsize giant vegetables indicate the extent to which variation is possible if the right growth conditions are provided. For example, the record pumpkin is 481 kg (Guinness Book of Records, 1998). How giant fruits and vegetables can be grown without the apparent selection of particular genotypes in the first place is indicative of the extent to which epigenetic phenomena must contribute to the final phenotype. It is generally accepted that genotype determines whether the individual phenotype or character can be plastic in the first place; expression and extent of that plasticity is environmentally regulated.
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