Dissolved inorganic carbon influences the photosynthetic responses of Hydrilla to photoinhibitory conditions

Hydrilla verticillata (L.f.) Royle, like other submersed angiosperms, exhibits shade plant characteristics. However, Hydrilla not only grows well at low irradiance in benthic habitats, but also at the water surface where its canopy is exposed to solar irradiance of 1800 μmol photons m −2 s −1. Leaves from plants grown at a moderate irradiance of 300 μmol m −2 s −1 had low light compensation ( I c), onset of light saturation ( I k), and light saturation (LSP) points of 10 μmol photons m −2 s −1, 47 μmol photons m −2 s −1, and 280 μmol photons m −2 s −1, respectively. Plants exposed to 1800 μmol photons m −2 s −1 for 15 min at only 0.6 mM dissolved inorganic carbon (DIC) exhibited about 50% and 30% photoinhibition of the photosynthetic rate, and apparent quantum yield, respectively. However, the presence of 2 mM DIC protected the leaves from photoinhibition, and allowed them to acclimate, with a higher LSP and photosynthetic rate. Superoxide dismutase, an enzyme which scavenges ·O − 2 radicals produced under light and O 2 stress, increased over three-fold in activity within only a 15 min exposure to high irradiance, irrespective of the DIC. Ascorbate peroxidase, which detoxifies H 2O 2, did not increase in activity. Two other enzymes of the ascorbate/glutathione cycle changed activity under high light and low DIC: monodehydroascorbate reductase which increased, and dehydroascorbate reductase which decreased. These changes presumably enhanced the scavenging of toxic ·O − 2 radicals, and the recycling of NADP + to photosystem I under a high light/low DIC regime which reduced the capacity of the Calvin cycle to utilize NADPH.