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A number of the general points above are illustrated by a study of the evergreen shrub Heteromeles arbutifolia. This plant grows both in chaparral habitats in California, where shoots in the upper crown are consistently exposed to full sunlight and high temperatures, and also in shaded woodland habitats, where it receives around one‐seventh as much radiation (Figure 3.10a). The leaves of sun plants have a lower specific leaf area – they are thicker and have a greater photosynthetic capacity (more chlorophyll and nitrogen) per unit leaf area than those of shade plants (Figure 3.10b). Sun‐plant leaves are inclined at a much steeper angle to the horizontal, and therefore absorb the direct rays of the overhead summer sun over a wider leaf area than the more horizontal shade‐plant leaves. The more angled leaves of sun plants, though, are also less likely than shade‐plant leaves to shade other leaves of the same plant from the overhead rays of the summer sun (Figure 3.10c). But in winter, when the sun is much lower in the sky, it is the shade plants that are much less subject to this ‘self‐shading’. As a result, the proportion of incident radiation intercepted per unit area of leaf is higher in shade than in sun plants year round: in summer because of the more horizontal leaves, and in winter because of the relative absence of self‐shading.

Figure 3.10 Variations in the behaviour and properties of sun and shade leaves of an evergreen shrub. (a) Computer reconstructions of stems of typical sun (A, C) and shade (B, D) plants of the evergreen shrub Heteromeles arbutifolia, viewed along the path of the sun’s rays in the early morning (A, B) and at midday (C, D). Darker tones represent parts of leaves shaded by other leaves of the same plant. Scale bars = 4 cm. (b) Observed differences in the leaves of sun and shade plants. Standard deviations are given in parentheses; the significance of differences is given following the analysis of variance. (c) Consequent whole‐plant properties of sun and shade plants. Letter codes indicate groups that differed significantly in analyses of variance (P < 0.05).

Source: After Valladares & Pearcy (1998).

The properties of whole plants of H. arbutifolia, then, reflect both plant architecture and the morphologies and physiologies of individual leaves. The efficiency of light absorption per unit of biomass is massively greater for shade than for sun plants (Figure 3.10c). Despite receiving only one‐seventh of the radiation of sun plants, shade plants reduce the differential in their daily rate of carbon gain from photosynthesis to only a half. They successfully counterbalance their reduced photosynthetic capacity at the leaf level with enhanced light‐harvesting ability at the whole‐plant level. The sun plants, on the other hand, can be seen as striking a compromise between maximising whole‐plant photosynthesis while avoiding photoinhibition and overheating of individual leaves.