Photosynthetic Carbon Metabolism in an Amphibious Sedge, Eleocharis baldwinii(Torr.)chapman : Modified Expression of C_4 Characteristics under submerged Aquatic Conditions : GROWTH AND DEVELOPMENT : ENVIRONMENTAL AND STRESS RESPONSES

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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 ^<14>C-pulse ^<12>C-chase experiments with the terrestrial form, 80% of the total fixed ^<14>C was incorporated into C_4 dicarboxylic acids after a 10-s pulse. The radioactivity in the C_4 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 ^<14>C was incorporated into C_4 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 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 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 CO_2 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 CO_2 essentially via the C_4 pathway, while the sub-merged form fixes inorganic carbon via a complex metabolic system that resembles an intermediate between C_3 and C_4 metabolism in association with a CAM-like profile.

Photosynthetic Carbon Metabolism in an Amphibious Sedge, Eleocharis baldwinii(Torr.)chapman : Modified Expression of C_4 Characteristics under submerged Aquatic Conditions : GROWTH AND DEVELOPMENT : ENVIRONMENTAL AND STRESS RESPONSES

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