Photosynthetic carbon metabolism in an amphibious sedge, eleocharis baldwinii (torr.) chapman: Modified expression of C₄ characteristics under submerged aquatic conditions

The photosynthetic characteristics of Eleocharis baldwinii (Torr.) Chapman, an amphibious leafless plant in the Cyperaceae, were investigated in both the terrestrial form and the submerged form of the plant. Anatomical observation of the culm, which is the photosynthetic organ in this plant, revealed that the terrestrial form has the Kranz type of anatomy, whereas the submerged form has an inner structure that is similar to that of submerged aquatic plants, with a reduction in both the number and the size of bundle sheath cells and vascular bundles and relatively well developed mesophyll cells. In ¹⁴C-pulse ¹²C-chase experiments with the terrestrial form, 80% of the total fixed ¹⁴C was incorporated into C₄ dicarboxylic acids after a 10-s pulse. The radioactivity in the C₄ acids decreased rapidly, while that in sucrose increased to 36% during a 120-s chase. In the submerged form, 64% and 30% of the total fixed ¹⁴C was incorporated into C₄ acids and phosphate esters, respectively, after a 10-s pulse. The radioactivity of these compounds decreased relatively slowly during a 120-s chase. The specific activities on a chlorophyll basis of C₄ photosynthetic enzymes that are involved in the NAD-ME subtype were high in the terrestrial form, while they were intermediate between those of C₃ and C₄ plants in the submerged form. The activity of ribulose 1,5-bisphosphate carboxylase was 1.5 times higher in the submerged form than in the terrestrial form. By contrast, the activity of carbonic anhydrase exhibited the reverse tendency. Western blot analysis of soluble proteins extracted from the mesophyll cells and the bundle sheath strands of the terrestrial form demonstrated that ribulose 1,5-bisphosphate carboxylase/oxygenase protein was present in the mesophyll cells as well as in the bundle sheath cells, with a higher level in the latter, although phosphoenolpyruvate carboxylase and pyruvate, P_i dikinase proteins were restricted to the mesophyll cells. In the submerged form, diurnal fluctuations in levels of malate were observed with significant fixation of CO₂ at night. However, the diurnal changes of malate were smaller than those reported for CAM plants. These data indicate that the terrestrial form of Eleocharis baldwinii fixes atmospheric CO₂ essentially via the C₄ pathway, while the submerged form fixes inorganic carbon via a complex metabolic system that resembles an intermediate between C₃ and C₄ metabolism in association with a CAM-like profile.