C mechanisms in aquatic angiosperms: comparisons with terrestrial C systems

Aquatic C 4 photosynthesis probably arose in response to dissolved CO 2 limitations, possibly before its advent in terrestrial plants. Of over 7600 C 4 species, only about a dozen aquatic species are identified. Amphibious Eleocharis species (sedges) have C 3 –C 4 photosynthesis and Kranz anatomy in aerial, but not submersed, leaves. Aquatic grasses have aerial and submersed leaves with C 4 or C 3 –C 4 photosynthesis and Kranz anatomy, but some lack Kranz anatomy in the submersed leaves. Two freshwater submersed monocots, Hydrilla verticillata and possibly Egeria densa , are C 4 NADP-malic enzyme (NADP-ME) species. A marine macroalga, Udotea flabellum (Chlorophyta), and possibly a diatom, are C 4 , so it is not confined to angiosperms. Submersed C 4 species differ from terrestrial in that β-carboxylation is cytosolic with chloroplastic decarboxylation and Rubisco carboxylation, so the C 4 and Calvin cycles operate in the same cell without Kranz anatomy. Unlike terrestrial plants, Hydrilla is a facultative C 4 that shifts from C 3 to C 4 in low [CO 2 ]. It is well documented, with C 4 gas exchange and pulse-chase characteristics, enzyme kinetics and localization, high internal [CO 2 ], relative growth rate, and quantum yield studies. It has multiple phospho enol pyruvate carboxylase isoforms with C 3 -like sequences. Hvpepc4 appears to be the photosynthetic form induced in C 4 leaves, but it differs from terrestrial C 4 isoforms in lacking a C 4 signature Serine. The molecular mass of NADP-ME (72 kDa) also resembles a C 3 isoform. Hydrilla belongs to the ancient Hydrocharitaceae family, and gives insight to early C 4 development. Hydrilla is an excellent ‘minimalist’ system to study C4 photosynthesis regulation without anatomical complexities. Keywords: C 3 – C 4 intermediate, C 4 photosynthesis, CO 2 concentrating mechanism, Egeria , Eleocharis , Hydrilla , Kranz anatomy, NADP-ME, Neostapfia , Orcuttia , phospho enol pyruvate carboxylase, Rubisco, Sagittaria , Thalassiosira , Tuctoria, Udotea . Functional Plant Biology 29(3) 379 - 392 Full text doi:10.1071/PP01219 © CSIRO 2002