A CO2-Flux Mechanism Operating via pH-Polarity in Hydrilla Verticillata Leaves With C3 and C4 Photosynthesis

摘要

The aquatic angiosperm Hydrilla verticillata lacks Kranz anatomy, but has an inducible, C4-based, CO2 concentrating mechanism (CCM) that concentrates CO2 in the chloroplasts. Both C3 and C4 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 CO2 flux mechanism (CFM), making HCO3 − available for photosynthesis by conversion to CO2. 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 μmol photons m−2 s−1), where CCM activity in C4 leaves is minimized, both leaf types had similar DIC inhibition of pH polarity. However, as irradiance increased, DIC inhibition of C3 leaves decreased. In C4 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 CO2 fixation in Hydrilla, also suppressed acidification and alkalization, especially in C4 leaves. IA does not inhibit the C4 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.