Box 2A3 Continued

Figure 1. Profiles of chlorophyll and light absorption (A), and photosynthesis (B) in a leaf of Spinacia oleracea (spinach). The distribution of chlorophyll was derived from measurements of chlorophyll fluorescence, using a light source to illuminate the cut surface of a transversal section of the leaf. The absorption of green and blue light was also measured with chlorophyll fluorescence, but with light striking the upper leaf surface. The light-saturated photo-synthetic electron transport rate (Amax) was derived from 14C-fixation profiles and photosynthetic activity at and irra-diance of 500 and 50 mmol m-2 s-1 in green and blue light were calculated using Equation (1) (Vogelmann & Evans 2002; Evans & Vogelmann 2003).

has a lower absorptance, A in that spectral region is more homogeneously distributed across the leaf profile, whereas blue light causes a sharp peak closer to the upper surface. Calculated profiles of A show a close match with the experimental data of the 14C-fixation profile.

chloroplasts have a smaller volume of stroma, where the Calvin-cycle enzymes are located, but larger grana, which contain the major part of the chlorophyll. Such differences are found both between plants grown under different light conditions and between sun and shade leaves on a single plant, as well as when comparing chloroplasts from the upper and lower side of one, relatively thick, leaf of Schefflera arboricola (dwarf umbrella plant) (Fig. 12). The adaxial (upper) regions have a chlor-oplast ultrastructure like sun leaves, whereas shade acclimation is found in the abaxial (lower) regions of the leaf (Box 2A.3).

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