The aquatic angiosperm Hydrilla verticillata lacks Kranz anatomy, but has an inducible, C(4)-based, CO(2) concentrating mechanism (CCM) that concentrates CO(2) in the chloroplasts. Both C(3) and C(4) Hydrilla leaves showed light-dependent pH polarity that was suppressed by high dissolved inorganic carbon (DIC). At low DIC (0.25 mol m(-3)), pH values in the unstirred water layer on the abaxial and adaxial sides of the leaf were 4.2 and10.3, respectively. Abaxial apoplastic acidification served as a CO(2) flux mechanism (CFM), making HCO (3) (-) available for photosynthesis by conversion to CO(2). DIC at 10 mol m(-3) completely suppressed acidification and alkalization. The data, along with previous results, indicated that inhibition was specific to DIC, and not a buffer effect. Acidification and alkalization did not necessarily show 1:1 stoichiometry; their kinetics for the apolar induction phase differed, and alkalization was less inhibited by 2.5 mol m(-3) DIC. At low irradiance (50 mumol photons m(-2) s(-1)), where CCM activity in C(4) leaves is minimized, both leaf types had similar DIC inhibition of pH polarity. However, as irradiance increased, DIC inhibition of C(3) leaves decreased. In C(4) leaves the CFM and CCM seemed to compete for photosynthetic ATP and/or reducing power. The CFM may require less, as at low irradiance it still operated maximally, if [DIC] was low. Iodoacetamide (IA), which inhibits CO(2) fixation in Hydrilla, also suppressed acidification and alkalization, especially in C(4) leaves. IA does not inhibit the C(4) CCM, which suggests that the CFM and CCM can operate independently. It has been hypothesized that irradiance and DIC regulate pH polarity by altering the chloroplastic [DIC], which effects the chloroplast redox state and subsequently redox regulation of a plasma-membrane H(+)-ATPase. The results lend partial support to a down-regulatory role for high chloroplastic [DIC], but do not exclude other sites of DIC action. IA inhibition of pH polarity seems inconsistent with the chloroplast NADPH/NADP(+) ratio being the redox transducer. The possibility that malate and oxaloacetate shuttling plays a role in CFM regulation requires further investigation.