Shade leaves minimize light limitation through increases in capacity for light capture and decreased carboxylation capacity and mesophyll conductance, but this does not invariably lead to higher chlorophyll concentrations per unit leaf area which determines their absorptance (Terashima et al. 2001, Warren et al. 2007). Some highly shade-adapted species [e.g., Hedera helix (ivy) in the juvenile stage], however, may have substantially higher chlorophyll levels per unit leaf area in shade. This might be due to the fact that their leaves do not get much thinner in the shade; however, there may also be some photodestruction of chlorophyll in high light in such species. In most species, however, higher levels of chlorophyll per unit fresh mass and per chloroplast in shade leaves are compensated for by the smaller number of chloroplasts and a lower fresh mass per area. This results in a rather constant chlorophyll level per unit area in sun- and shade leaves.
The ratio between chlorophyll a and chlorophyll b (chl a/chl b) is lower in shade-acclimated leaves. These leaves have relatively more chlorophyll in the light-harvesting complexes, which contain large amounts of chl b (Lichtenthaler & Babani 2004). The decreased chl a/chl b ratio is therefore a reflection of the greater investment in LHCs (Evans 1988). The larger proportion of LHC is located in the larger grana of the shade-acclimated chloroplast (Fig. 12). Sun leaves also contain more xanthophyll carotenoids, relative to chlorophyll (Box 2A.3; Lichtenthaler 2007).
Sun leaves have larger amounts of Calvin-cycle enzymes per unit leaf area as compared with shade leaves, due to more cell layers, a larger number of chloroplasts per cell, and a larger volume of stroma, where these enzymes are located, per chloroplast, compared with shade leaves. Sun leaves also have more stroma-exposed thylakoid membranes, which contain the b6f cytochromes and ATPase (Fig. 13). All these components enhance the photosynthetic capacity of sun leaves. Since the amount of chlorophyll per unit area is more or less equal among leaf types, sun leaves also have a higher photosynthetic capacity per unit chlorophyll. The biochemical gradients for Rubisco/chlorophyll across a leaf are similar to those observed within a canopy, with adaxial (upper) cells having more Rubisco, but less chlorophyll than abaxial (lower) cells (Terashima & Hikosaka 1995).
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