PHOTOSYNTHETIC CARBON METABOLISM IN AN AMPHIBIOUS SEDGE, ELEOCHARIS-BALDWINII (TORR) CHAPMAN - MODIFIED EXPRESSION OF C-4 CHARACTERISTICS UNDER SUBMERGED AQUATIC CONDITIONS

The photosynthetic characteristics of Eleocharis baldwinii (Torr.) Cha pman, an amphibious leafless plant in the Cyperaceae, were investigate d in both the terrestrial form and the submerged form of the plant. An atomical 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 sim ilar to that of submerged aquatic plants, with a reduction in both the number and the size of bundle sheath cells and vascular bundles and r elatively well developed mesophyll cells. In C-14-pulse C-12-chase exp eriments with the terrestrial form, 80% of the total fixed C-14 was in corporated into C-4 dicarboxylic acids after a 10-s pulse. The radioac tivity in the C-4 acids decreased rapidly, while that in sucrose incre ased to 36% during a 120-s chase. In the submerged form, 64% and 30% o f the total fixed C-14 was incorporated into C-4 acids and phosphate e sters, respectively, after a 10-s pulse. The radioactivity of these co mpounds decreased relatively slowly during a 120-s chase. The specific activities on a chlorophyll basis of C-4 photosynthetic enzymes that are involved in the NAD-ME subtype were high in the terrestrial form, while they were intermediate between those of C-3 and C-4 plants in th e submerged form. The activity of ribulose 1,5-bisphosphate carboxylas e was 1.5 times higher in the submerged form than in the terrestrial f orm. By contrast, the activity of carbonic anhydrase exhibited the rev erse tendency. Western blot analysis of soluble proteins extracted fro m the mesophyll cells and the bundle sheath strands of the terrestrial form demonstrated that ribulose 1,5-bisphosphate carboxylase/oxygenas e protein was present in the mesophyll cells as well as in the bundle sheath cells, with a higher level in the latter, although phosphoenolp yruvate carboxylase and pyruvate,Pi 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 ni ght. However, the diurnal changes of malate were smaller than those re ported for CAM plants. These data indicate that the terrestrial form o f Eleocharis baldwinii fixes atmospheric CO2 essentially via the C-4 p athway, while the submerged form fixes inorganic carbon via a complex metabolic system that resembles an intermediate between C-3 and C-4 me tabolism in association with a CAM-like profile.