Effects of an inhibitor of phosphoenolpyruvate carboxylase on photosynthesis of the terrestrial forms of amphibious Eleocharis species

The leafless amphibious sedge Eleocharis vivipara develops culms with C-4 traits and Kranz anatomy under terrestrial conditions, but develops culms with C-3 traits and non-Kranz anatomy under submerged conditions. The culms of the terrestrial form have high C-4 enzyme activities, while those of the submerged form have decreased C-4 enzyme activities. The culms accumulate ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) in the mesophyll cells (MC) and the bundle sheath cells. The Rubisco in the MC may be responsible for the operation of the C-3 pathway in the submerged form. To verify the presence of the C-3 cycle in the MC, we examined the effects of 3,3-dichloro-2-(dihydroxyphosphinoylmethyl) -propenoate (DCDP), an inhibitor of phosphoenolpyruvate carboxylase (PEPC), on photosynthesis in culms of the terrestrial forms of E. vivipara and related amphibious species, E. baldwinii and E. retroflexa ssp. chaetaria. When 1 mM DCDP was fed via the transpiration stream to excised leaves, photosynthesis was inhibited completely in Fimbristylis dichotoma (C-4 control), but by only 20% in potato (C-3 control). In the terrestrial Eleocharis plants, the degree of inhibition of photosynthesis by DCDP was intermediate between those of the C-4 and C-3 plants, at 58-81%. These results suggest that photosynthesis under DCDP treatment in the terrestrial Eleocharis plants is due mainly to fixation of atmospheric CO2 by Rubisco and probably the C-3 cycle in the MC. These features are reminiscent of those in C-4-like plants. Differential effects of DCDP on photosynthesis of the 3 Eleocharis species are discussed in relation to differences in the degree of Rubisco accumulation and C-3 activity in the MC.