Photosynthetic phenotype plasticity and the role of phosphoenolpyruvate carboxylase in Hydrilla verticillata

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 (Gamma, from 10 to 59 mu l l(-1)), O-2 inhibition of photosynthesis (from 0.5 to 15% at 21% O-2) 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 participates 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 (Gamma = 53 mu l l(-1)) and new tissue produced during induction exhibited Gamma values of <20 mu l l(-1). Therefore, induction of the low photorespiratory state seems not to depend on shoot age. Although Pepcase activity was negatively correlated with Gamma 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 C-4 photosynthetic enzymes other than Pepcase.