Abstract
The effect of long-term (weeks to months) CO(2) enhancement on (a) the gas-exchange characteristics, (b) the content and activation state of ribulose-1,5-bisphosphate carboxylase (rubisco), and (c) leaf nitrogen, chlorophyll, and dry weight per area were studied in five C(3) species (Chenopodium album, Phaseolus vulgaris, Solanum tuberosum, Solanum melongena, and Brassica oleracea) grown at CO(2) partial pressures of 300 or 900 to 1000 microbars. Long-term exposure to elevated CO(2) affected the CO(2) response of photosynthesis in one of three ways: (a) the initial slope of the CO(2) response was unaffected, but the photosynthetic rate at high CO(2) increased (S. tuberosum); (b) the initial slope decreased but the CO(2)-saturated rate of photosynthesis was little affected (C. album, P. vulgaris); (c) both the initial slope and the CO(2)-saturated rate of photosynthesis decreased (B. oleracea, S. melongena). In all five species, growth at high CO(2) increased the extent to which photosynthesis was stimulated following a decrease in the partial pressure of O(2) or an increase in measurement CO(2) above 600 microbars. This stimulation indicates that a limitation on photosynthesis by the capacity to regenerate orthophosphate was reduced or absent after acclimation to high CO(2). Leaf nitrogen per area either increased (S. tuberosum, S. melongena) or was little changed by CO(2) enhancement. The content of rubisco was lower in only two of the five species, yet its activation state was 19% to 48% lower in all five species following long-term exposure to high CO(2). These results indicate that during growth in CO(2)-enriched air, leaf rubisco content remains in excess of that required to support the observed photosynthetic rates.