DISSOLVED INORGANIC CARBON INFLUENCES THE PHOTOSYNTHETIC RESPONSES OFHYDRILLA 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 sur face where its canopy is exposed to solar irradiance of 1800 mu mol ph otons m(-2) s(-1). Leaves from plants grown at a moderate irradiance o f 300 mu mol m(-2) s(-1) had low light compensation (I-c), onset of li ght saturation (I-k), and light saturation (LSP) points of 10 mu mol p hotons m(-2) s(-1), 47 mu mol photons m(-2) s(-1), and 280 mu mol phot ons m(-2) s(-1), respectively. Plants exposed to 1800 mu mol 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 dism utase, an enzyme which scavenges . O-2(-) radicals produced under ligh t 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 H2O2, did not increase in activity. Two other enzymes of the ascorbate/glutathione cycle changed activity unde r high light and low DIC: monodehydroascorbate reductase which increas ed, and dehydroascorbate reductase which decreased. These changes pres umably enhanced the scavenging of toxic . O-2(-) radicals, and the rec ycling of NADP(+) to photosystem I under a high light/low DIC regime w hich reduced the capacity of the Calvin cycle to utilize NADPH.