Photosynthesis of Littorella uniflora grown under two PAR regimes: C3 and CAM gas exchange and the regulation of internal CO2 and O2 concentrations.

The submersed aquatic macrophyte Littorella uniflora was grown under 50 and 300 μmol m-2 s-1 photosynthetically active radiation (PAR) (low and high PAR regimes) but identical sediment CO2 supply (1.0 mol m-3). The interactions between plant morphology, whole plant CO2 and O2 exchange, CAM activity, [CO2] i and [O2] i have been investigated in comparison with in vitro CO2 and 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 CO2 uptake via the roots. Photosynthetic O2 evolution from intact plants was greater in high-PAR-grown L. uniflora (2.18 compared with 1.49 μmol O2g-1 fresh weight h-1 for the low PAR regime). Although net daytime CO2 uptake was similar for both PAR regimes (0.79 and 0.75 μmol g-1 fwt h-1), net dark CO2 uptake was at a higher rate (0.92 compared with 0.52 μmol CO2 g-1 fwt h-1), and dark fixation (as malic acid) was threefold greater in high PAR plants (ΔH+ 117 compared with 42 μmol H+ g-1 fwt). Comparison of dark CO2 uptake with dark fixation suggested that much of the CO2 fixed at night and regenerated during the day may be respiratory in origin (60% low PAR plants, 71% high PAR plants). Regeneration of CO2 from CAM could account for 62% of daytime CO2 supply in low PAR plants and 81% in high PAR plants. [CO2] i values (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-2 s-1 for both treatments). Thus PAR rather than CO2 supply appeared to limit photosynthesis even in high PAR grown plants, and CAM appears to have an important role in the regulation of CO2 supply for photosynthesis in response to variation in light regime.