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, 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 dcreased 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 chlorphyll 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 phopshoenolpyruvate carboxylase and pyruvate, Pi dikinase proteins wee restricted to the mesophyll cells