Photosynthetic Characteristics of an Amphibious C4 Plant, Eleocharis retroflexa ssp. chaetaria

Eleocharis retroflexa (Poir.) Urban ssp. chaetaria (Roem. & Schult.) T. Koyama, an amphibious leafless sedge, grows not only under terrestrial conditions but also under completely submerged aquatic conditions. We investigated the photosynthetic traits and structural features of the culms, which are the photosynthetic organs, in the terrestrial and submerged forms of this species and compared them with those of other amphibious species of Eleocharis which are known to change the photosynthetic modes. The culms of the terrestrial form had Kranz anatomy with well-developed Kranz (bundle sheath) cells and high levels of C4 enzyme activity typical of the NAD-malic enzyme (NAD-ME) subtype of C4 metabolism. They also had a δ 13C value typical of C4 plants, indicating that the terrestrial form fixes carbon through the C4 pathway. The culms of the submerged form had not only a Kranz-like anatomy but also revealed anatomical traits typical of leaves of submerged aquatic plants. The activities of the C4 enzymes in the submerged form were lower than those in the terrestrial form, but were still in the range typical of G4 plants, 14C pulse-12C chase experiments with the submerged form indicated that almost all of the fixed 14C was incorporated into G4 compounds, and subsequently the raioactivity was transferred into C3 compounds and sucrose. The submerged form showed no diurnal fluctuation in malate level. These data demonstrate that a C4 metabolism is operative even in the submerged form. This unique amphibious C4 plant provides an intriguing example of the physiological and ecological adaptability of C4plants