C4 acid fixation in photosynthesis of the submerged aquatic Eleocharis acicularis (L.) R. & S

Eleocharis acicularis (L.) R. & S. is a widely distributed submerged aquatic macrophyte. In southern California, it codominates shallow seasonal pools with the aquatic CAM plant Isoetes howellii Engelm. Like the latter species, E. acicularis is apparently restricted to uptake of free-CO₂; bicarbonate uptake is negligible, as indicated by poor carbon fixation at pH 7 and higher oxygen evolution at pH 5 than at pH 8 across a range of free-CO₂ concentrations. Unlike Isoetes, Eleocharis does not have CAM photosynthesis, however it does have the capacity for a low level of carbon fixation in the dark. ¹⁴C tracer studies in the light reveal C₄ acids constitute 40–50% of the initial labeled products, however the leaves lack Kranz anatomy. It is suggested that in seasonal pools, photosynthesis by E. acicularis is highest in the early morning when ambient free-CO₂ levels are maximal. Such sites are typically overcast in the early morning and like I. howellii, E. acicularis is light saturated at relatively low irradiance levels. Throughout much of the day, free-CO₂ concentrations are limited and oxygen concentrations are high. PEP carboxylase mediated carbon fixation may be of selective value under such daytime conditions.