title: Photosynthesis of Littorella uniflora grown under two PAR regimes: C3and CAM gas exchange and the regulation of internal CO2and O2concentrations.
abstract: The submersed aquatic macrophyte Littorella uniflora was grown under 50 and 300 μmol m-2s-1photosynthetically active radiation (PAR) (low and high PAR regimes) but identical sediment CO2supply (1.0 mol m-3). The interactions between plant morphology, whole plant CO2and O2exchange, CAM activity, [CO2]iand [O2]ihave been investigated in comparison with in vitro CO2and PAR response characteristics (using 1 mm leaf sections). In terms of morphology, high-PAR-grown plants were smaller and leaves contained less chlorophyll, although root growth was proportionally larger. Gas exchange fluxes over roots and shoots of intact plants were similar in direction under the two PAR regimes, with the majority of CO2uptake via the roots. Photosynthetic O2evolution from intact plants was greater in high-PAR-grown L. uniflora (2.18 compared with 1.49 μmol O2g-1fresh weight h-1for the low PAR regime). Although net daytime CO2uptake was similar for both PAR regimes (0.79 and 0.75 μmol g-1fwt h-1), net dark CO2uptake was at a higher rate (0.92 compared with 0.52 μmol CO2g-1fwt h-1), and dark fixation (as malic acid) was threefold greater in high PAR plants (ΔH+117 compared with 42 μmol H+g-1fwt). Comparison of dark CO2uptake with dark fixation suggested that much of the CO2fixed at night and regenerated during the day may be respiratory in origin (60% low PAR plants, 71% high PAR plants). Regeneration of CO2from CAM could account for 62% of daytime CO2supply in low PAR plants and 81% in high PAR plants. [CO2]ivalues (ranging from 0.42 to 1.03 mol m-3) were close to or above the concentration required to saturate photosynthesis in vitro (0.5 mol m-3) under both PAR regimes, and combined with the low [O2]i(2.6-4.3 mol m-3) should have suppressed photorespiration. However, PAR inside leaves would have been well below the in vitro light saturation requirement (850-1000 μmol m-2s-1for both treatments). Thus PAR rather than CO2supply appeared to limit photosynthesis even in high PAR grown plants, and CAM appears to have an important role in the regulation of CO2supply for photosynthesis in response to variation in light regime.
authors.affiliation: Department of Biology, The University, NE1 7RU, Newcastle upon Tyne, UK.|Windermere Laboratory, Institute of Freshwater Ecology, The Ferry House, Far Sawrey, LA22 OLP, Ambleside, Cumbria, UK.
authors.forename: W E
authors.initials: WE
authors.lastname: Robe
authors.affiliation: Department of Biology, The University, NE1 7RU, Newcastle upon Tyne, UK.
authors.forename: H
authors.initials: H
authors.lastname: Griffiths
keywords: Crassulacean acid metabolism; Gas exchange; Lacunal CO2 and O2 concentrations; Littorella uniflora; PAR acclimation