Robert Hill Institute, Department of Molecular Biology and Biotechnology, University of Sheffield, Firth Court, Western Bank, Sheffield S10 2TN, UK
It is believed that this response has adaptive benefit because of the differing absorption characteristics of the two photosynthetic units, PSI absorbing more strongly in the red and short-wavelength blue regions of the spectrum (due to its high levels of Chl a), PSII absorbing strongly at wavelengths absorbed by Chl b and carotenoids, particularly xanthophylls. However, there is only limited evidence for any such adaptive benefit: Chow et al. (1990) showed that the quantum efficiency of photosynthesis for peas is highest when they are exposed to a light source with similar spectral characteristics to that in which they are grown. We have probed photosynthesis in leaves and thylakoids of Arabidopsis plants grown under two different light sources to investigate how changes in the composition of the photosynthetic apparatus affect its function, and to shed light on how photosystem stoichiometry affects photosynthetic performance.
The figure shows measurements of oxygen evolution from leaves of plants grown in white or FR-enriched lights when exposed to two experimental light sources with markedly different spectral qualities: a white light regime similar to the white growth lights; and a regime enriched in red wavelengths preferentially absorbed by PSI. There was no significant effect of growth conditions or experimental light source on the maximum photosynthetic rate, but there were significant differences between the apparent quantum yield of oxygen evolution, depending on the light source used . When measured as the slope after linear regression of the plot oxygen evolution rate (micro mol O2 (mg Chl)-1 hr-1) versus irradiance (micro mol quanta m-2 s-1), quantum yield was very similar in W (1.34) and FRE (1.32) plants when they were exposed to "white" light. However, both W and FRE plants showed an increase in the apparent quantum yield when illuminated using the red- enriched light; the increase in the apparent quantum yield was appreciably greater in plants grown in FRE light (1.32 to 1.85), compared to W plants (1.34 to 1.57), indicating that the photosynthetic apparatus of FRE plants was able to make more effective use of longer wavelength light than that of W plants. Simultaneous measurements of chlorophyll fluorescence and absorption changes at 820 nm (not shown) indicated that differences between plants grown under alternative light regimes can be ascribed to altered excitation rates for PSI and PSII; growth in an altered light environment therefore leads to imbalances in photosystem excitation, leading to reduced photochemical efficiency.
Investigation of PSI and PSII function by measuring chlorophyll fluorescence emission and excitation spectra at 77 K (not shown) provides strong evidence that there is little or no difference in the composition or function of PSI or PSII between the two sets of plants. This implies that changes in photosynthetic stoichiometry are primarily responsible for the observed differences in photosynthetic function - i.e. the changes in photosystem stoichiometry have a direct adaptive benefit.
Chow WS, Melis A, Anderson JM (1990) Adjustments of photosystem stoichiometry in chloroplasts improve the quantum efficiency of photosynthesis. Proc Natl Acad Sci USA 87, 7502-7506
Walters RG, Horton P (1994) Acclimation of Arabidopsis thaliana to the light environment: Changes in composition of the photosynthetic apparatus. Planta 195, 248-256