EFFECT OF CARBONIC-ANHYDRASE INHIBITION ON PHOTOSYNTHESIS BY LEAF PIECES OF C-3 AND C-4 PLANTS

The role of carbonic anhydrase (CA) in the photosynthesis of C-3 and C-4 plants was investigated by use of the effects of the CA inhibitor ethoxyzolamide on peeled and vacuum infiltrated leaf pieces. Two C-4 (Zea mays and Amaranthus edulis) and three C-3 (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 C-4 species compared with the C-3 species. However, the degree of inhibition in the C-4 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.