Conversely, in strong light, photons are abundant and require greater capacity for energy processing by leaves (hence the higher Chl a/ b ratio). As a result the relative abundance of Chl b will increase and the Chl a/ b ratio will be lower compared with that in strong light. In weak light, optimisation of leaf function calls for greater investment of leaf resources in light harvesting rather than energy processing. Both forms of chlorophyll are involved in light harvesting, whereas special forms of only Chl a are linked into energy-processing centres of photosystems. Such variation is easily reconciled with contrasting functional roles for both Chl a and Chl b. Chl a/ b ratios commonly range from 3.3 to 4.2 in well-nourished sun-adapted species, but can be as low as 2.2 or thereabouts in shade-adapted species grown at low light. Additional chlorophylls have been discovered that exist in cyanobacteria which extends their absorption spectrum into the infrared (Figure 1.9).Ĭhl a and Chl b differ with respect to both role and relative abundance in higher plants. Absorption maxima at 659 and 642 for Chl a and Chl b respectively would thus serve for assay in diethylether, but these peaks will shift slightly according to solvent system, and such shifts must be taken into account for precise measurement (see Porra et al. Both chlorophylls show absorption maxima at wavelengths corresponding to blue and red, but chlorophyll assay in crude extracts, which inevitably contain carotenoids as well, is routinely based on absorption maxima in red light to avoid overlap with these accessory pigments that show strong absorption below 500 nm. These two chemical variants of chlorophyll are universal constituents of wild vascular plants and express highly characteristic absorption spectra (Figure 1.8, upper curves). Anderson, BBA 892: 75-82, 1987)Ĭhlorophylls are readily extracted from (soft) leaves into organic solvent and separated chromatographically into constituent types, most notably chlorophyll a (Chl a) and chlorophyll b (Chl b). A secondary peak at 472 nm and a shoulder at 653 nm indicate contributions from Chl b to these broadened absorption spectra which have been normalised to 10 µM Chl solutions in a 1 cm path length cuvette. Lower curves: In situ absorption spectra (eluted from gel slices) for pigment-protein complexes corresponding to photosystem II reaction centre (PSII RC) and light-harvesting chlorophyll (a,b)-protein complexes (LHC). Fluorescence emission spectra (inset, redrawn from Lichtenthaler 1986) show peaks only in red, and at wavelengths characteristically longer than corresponding absorption peaks, namely 648 cf. Figure 1.8 Upper curves: Diethylether solutions of chlorophyll a (Chl a, solid line) and chlorophyll b (Chl b, dotted line) show distinct absorption peaks inīlue and in red regions of the visible spectrum (redrawn from Zscheile and Comar’s (1941) original data).
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