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Articles

Limiting Factors in Photosynthesis

V. Photochemical Energy Supply Colimits Photosynthesis at Low Values of Intercellular CO₂ Concentration

Department of Plant and Soil Biology, University of California, Berkeley, California 94720

Although there is now some agreement with the view that the supply of photochemical energy may influence photosynthetic rate (P) at high CO₂ pressures, it is less clear whether this limitation extends to P at low CO₂. This was investigated by measuring P per area as a function of the intercellular CO₂ concentration (C_i) at different levels of photochemical energy supply. Changes in the latter were obtained experimentally by varying the level of irradiance to normal (Fe-sufficient) leaves of Beta vulgaris L. cv F58-554H1, and by varying photosynthetic electron transport capacity using leaves from Fe-deficient and Fe-sufficient plants. P and C_i were determined for attached sugar beet leaves using open flow gas exchange. The results suggest that P/area was colimited by the supply of photochemical energy at very low as well as high values of C_i. Using the procedure developed by Perchorowicz et al. (Plant Physiol 1982 69:1165-1168), we investigated the effect of irradiance on ribulose bisphosphate carboxylase (RuBPCase) activation. The ratio of initial extractable activity to total inducible RuBPCase activity increased from 0.25 to 0.90 as leaf irradiance increased from 100 to 1500 microeinsteins photosynthetically active radiation per square meter per second. These data suggest that colimitation by photochemical energy supply at low C_i may be mediated via effects on RuBPCase activation.

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