Environmental regulation of photosynthetic metabolism in the amphibious sedge Eleocharis baldwinii and comparisons with related species

The amphibious leafless sedge, Eleocharis baldwinii, expresses C-4 characteristics in the terrestrial form and intermediate characteristics between C-3 and C-4 photosynthesis in the submerged form. This study examined the immunocytochemical localization of C-3 and C-4 enzymes in culms of the two forms to elucidate the regulatory mechanism of photosynthetic metabolism and compared the activities and amounts of C-3 and C-4 enzymes with those in other Eleocharis species, E. vivipara and E. retroflexa, which show C-4 characteristics on land but C-3 and C-4 characteristics under water. The terrestrial form of E. baldwinii exhibited a C-4-like pattern of enzyme localization. The submerged form exhibited a modified anatomy with well-developed mesophyll cells and small Kranz cells. The C-4 enzyme levels declined conspicuously in outer mesophyll cells adjacent to the epidermis, whereas Rubisco levels increased throughout the mesophyll in the submerged form. These results suggest that intermediate photosynthesis between C-3 and C-4 photosynthesis in the submerged form results from the predominant operation of the C-3 pathway in the outer mesophyll cells and the C-4 pathway in both the inner mesophyll and Kranz cells. Differences in the degree of C-4 expression in terrestrial forms of Eleocharis species may cause the differences in the expression of photosynthetic modes under water.