Eleocharis contains many amphibious species, and displays diversity of photosynthetic mechanism (C_3, C₄ or C₃-C₄ intermediates). A unique feature of Eleocharis is the plasticity in the photosynthetic mechanism of some species in response to the environment. In this study, we have examined the culm anatomy and photosynthetic property of several Eleocharis species grown terrestrially and the changes in the newly produced culms over a short period time frame after switching from terrestrial to submerged condition. Eleocharis baldwinii (Torrey) Chapman is C₄-like in terrestrial habitat, exhibiting O₂ inhibition of photosynthesis with Rubisco expressed in both mesophyll and bundle sheath cells and PEPC strictly in the mesophyll cells, but switches to C₃-C₄ intermediacy when submerged. In addition to Eleocharis vivipara Link type 1 (which switches from C₄-like to C₃), two other photosynthetic types examined in this study were shown to have different responses to growth in either terrestrial or submerged conditions. E. vivipara type 2 is a typical C₄ plant in the terrestrial habitat, but becomes a C₃-C₄ intermediate under submerged conditions. Further, terrestrially, E. vivipara type 3 is a C₃-C₄ intermediate, but when submerged the δ¹³C value increases to –6.7‰, indicating its use of bicarbonate as a major carbon source. The submerged form of this plant exhibited about three times higher photosynthetic O₂ evolution rate, compared to the C₃ species Eleocharis erythropoda Steudel. These Eleocharis species possess different molecular switches for regulating C₄ gene expression in response to environmental stimuli both between different species, and in E. vivipara among different populations. The apparent expression of a bicarbonate transport system by E. vivipara type 3 while submerged represents a unique adaptation to low CO₂ availability.