Photosynthetic pathways of some aquatic plants

Abstract

Over 40 species of aquatic angiosperms, including submersed, floating and emergent types, have been examined for photosynthetic status as part of a search for possible aquatic C 4 species. The C 4 system is viewed as potentially of adaptive value in certain aquatic situations, although evidence for its occurrence there is not conclusive. Emphasis was on plants from North-temperate softwater and hardwater lakes to explore both possibilities of CO 2 limitation, i.e. low total inorganic carbon in softwater vs. low free CO 2 in hardwater lakes. On the basis of leaf cross-section anatomy, all plants examined, with one exception, clearly did not show evidence of C 4 “Krantz anatomy”. In the submersed plant Potamogeton praelongus Wulf. large starch-producing chloroplasts were concentrated in cells surrounding vascular bundles and in a narrow band of cells between vascular bundles. The in situ photosynthetic rate of this plant was twice that of a related species, but other evidence including PEP carboxylase content and photorespiratory response to high O 2 did not confirm the presence of the C 4 photosynthesis.