SuniShade Differences as Adaptations
Several features ~f plant form, physiology, and resource allocation vary with the level
of irradiance to which plants are acclimated and/or ecologically restricted (Table 1).
Traditionally, three approaches have been used to identify variations in such features
as adaptations to a specific level of irradiance, based on (1) convergence, (2) correlation
with photosynthetic impact, and (3) detailed cost-benefit analysis. Convergence among
species from different families or orders in the expression of a given trait in plants
restricted to, or grown under, a specific level of irradiance is usually taken as prima facie
evidence that such behaviour is a result of natural selection. This approach can be
applied to either fixed or developmentally plastic traits; in the latter case, convergence
in the pattern of response to different irradiance levels would identify the pattern of
acclimation-presumably itself genetically determined-as adaptive. Arguments based
solely on convergence are, however, limited because they cannot identify how or why
variation in a given trait contributes to competitive ability.
A second, more mechanistic approach to identifying traits as adaptations to
irradiance level is based on a detailed study of the photosynthetic light response of leaves
acclimated to different light levels, together with an analysis of how various features of
their morphology and physiology contribute to photosynthetic performance under those
levels (e.g. Bjorkman 1968a, 1981; Bjdrkman et al. 1972a, 1972b; Boardman et al.
1972; Nobel 1976; Bjdrkman and Powles 1984; Ludlow and Bjorkman 1984). This
approach generally involves two tacit assumptions: (i) that the photosynthetic rates of
leaves acclimated (or ecologically restricted) to a specific irradiance level are greater at
that level than the photosynthetic rates of leaves grown under other irradiance levels;
and (ii) that if variation in a given trait enhances leaf photosynthesis-expressed almost
invariably per unit area-at a specific irradiance level, then it is an adaptation to that
level.
The first of these assumptions seems valid, at least in extreme cases: leaves of plants
grown under, or ecologically restricted to, high irradiance levels generally have higher