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Hygrophila polysperma  is a heterophyllous amphibious plant. The growth of  H. polysperma  in submerged conditions is challenging due to the low CO 2  environment, increased resistance to gas diffusion, and bicarbonate ion (HCO 3  – ) being the dominant dissolved inorganic carbon source. The submerged leaves of  H. polysperma  have significantly higher rates of underwater photosynthesis compared with the terrestrial leaves. 4,4′-Diisothiocyanatostilbene-2,2′-disulfonate (DIDS), an anion exchanger protein inhibitor, and ethoxyzolamide (EZ), an inhibitor of internal carbonic anhydrase, repressed underwater photosynthesis by the submerged leaves. These results suggested that  H. polysperma  acclimates to the submerged condition by using HCO 3   –   for photosynthesis.  H. polysperma  transports HCO 3   –   into the leaf by a DIDS-sensitive HCO 3   –   transporter and converted to CO 2  by carbonic anhydrase. Additionally, proteome analysis revealed that submerged leaves accumulated fewer proteins associated with C4 photosynthesis compared with terrestrial leaves. This finding suggested that  H. polysperma  is capable of C4 and C3 photosynthesis in the terrestrial and submerged leaves, respectively. The ratio of phosphoenol pyruvate carboxylase to ribulose 1,5-bisphosphate carboxylase/oxygenase (Rubisco) in the submerged leaves was less than that in the terrestrial leaves. Upon anatomical observation, the terrestrial leaves exhibited a phenotype similar to the Kranz anatomy found among C4 plants; however, chloroplasts in the bundle sheath cells were not located adjacent to the vascular bundles, and the typical Kranz anatomy was absent in submerged leaves. These results suggest that  H. polysperma  performs proto-Kranz type photosynthesis in a terrestrial environment and shifts from a proto-Kranz type in terrestrial leaves to a HCO 3   –   use photosynthesis in the submerged environments.

Transition From Proto-Kranz-Type Photosynthesis to HCO3– Use Photosynthesis in the Amphibious Plant Hygrophila polysperma