An inhibitor [3,3-dichloro-2-(dihydroxyphosphinoylmethyl)-propenoate (DCDP)] of phosphoenolpyruvate carboxylase (Pepcase) was used to investigate the participation of this carbon fixing-enzyme in the gas exchange characteristics of the low-photorespiratory phenotype of the submersed aquatic angiosperm Hydrilla verticillata (L.F.) Royle. Exposure of such low photorespiratory plants to DCDP resulted in increases in the CO2 compensation point (from 10 to 59 ul/l), O2 inhibition of photosynthesis (from 0.5 to 15% at 21% O2) and CO2 evolution in the dark by 51%. These DCDP-exposed individuals exhibited a 27% decrease in apparent photosynthesis. In contrast, DCDP had no negative effects upon high photorespiratory individuals. By directly inhibiting the enzyme, these data confirm that Pepcase paraticipates in the expression of low photorespiratory gas exchange characteristics in H. verticillata. Following the induction of the low photorespiratory phenotype, both old tissue originally in the high photorespiratory state (53 ul/l) and new tissue produced during induction exhibited values of <20 ul/l. Therefore, induction of the low photorespiratory state seems not to depend on shoot age. Alathough Pepcase activity was negatively correlated with during induction of the low photorespiratory phenotype, the relationship appears to be non-linear suggesting that expression of low photorespiratory gas exchange characteristics might require subsequent induction of C4 photosynthetic enzymes other than Pepcase.