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, wereinvestigated in both the terrestrial form and the submergedform of the plant. Anatomical observation of the culm, whichis the photosynthetic organ in this plant, revealed that theterrestrial form has the Kranz type of anatomy, whereas thesubmerged form has an inner structure that is similar to thatof submerged aquatic plants, with a reduction in both the numberand the size of bundle sheath cells and vascular bundles andrelatively well developed mesophyll cells. In ¹⁴C-pulse ¹²C-chaseexperiments with the terrestrial form, 80% of the total fixed¹⁴C was incorporated into C₄ dicarboxylic acids after a 10-spulse. 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 wasincorporated into C₄ acids and phosphate esters, respectively,after a 10-s pulse. The radioactivity of these compounds decreasedrelatively slowly during a 120-s chase. The specific activitieson a chlorophyll basis of C₄ photosynthetic enzymes that areinvolved in the NAD-ME subtype were high in the terrestrialform, while they were intermediate between those of C₃ and C₄plants in the submerged form. The activity of ribulose 1,5-bisphosphatecarboxylase was 1.5 times higher in the submerged form thanin the terrestrial form. By contrast, the activity of carbonicanhydrase exhibited the reverse tendency. Western blot analysisof soluble proteins extracted from the mesophyll cells and thebundle sheath strands of the terrestrial form demonstrated thatribulose 1,5-bisphosphate carboxylase/oxygenase protein waspresent in the mesophyll cells as well as in the bundle sheathcells, with a higher level in the latter, although phosphoenolpyruvatecarboxylase and pyruvate, P_i dikinase proteins were restrictedto the mesophyll cells. In the submerged form, diurnal fluctuationsin levels of malate were observed with significant fixationof CO₂ at night. However, the diurnal changes of malate weresmaller than those reported for CAM plants. These data indicatethat the terrestrial form of Eleocharis baldwinii fixes atmosphericCO₂ essentially via the C₄ pathway, while the submerged formfixes inorganic carbon via a complex metabolic system that resemblesan intermediate between C₃ and C₄ metabolism in associationwith a CAM-like profile. (Received September 12, 1994; Accepted November 21, 1994)