Effect of Carbonic Anhydrase Inhibition on Photosynthesis by Leaf Pieces of C3 and C4 Plants

The role of carbonic anhydrase (CA) in the photosynthesis of C3 and C4 plants was investigated by use of the effects of the CA inhibitor ethoxyzolamide on peeled and vacuum infiltrated leaf pieces. Two C4(Zea mays and Amaranthus edulis) and three C3 (Spinacea oleracea, Nicotiana rustica and Hordeum vulgare) species were studied. The results clearly showed that photosynthesis at limiting inorganic carbon was more severely inhibited in the C4 species compared with the C3 species. However, the degree of inhibition in the C4 species was less than might have been expected based on the rate limitation which the conversion of CO2 to HCO3- would impose on the provision of substrate to PEP carboxylase. The reasons for this lower than expected inhibition is discussed in terms of the lack of penetration of ethoxyzolamide, the diffusion of HCO3- through plasmodesmata and the possible existence of a plasma membrane CA activity.