C3and CAM Photosynthetic Characteristics of the Submerged Aquatic MacrophyteLittorella uniflora: Regulation of Leaf Internal CO2Supply in Response to Variation in Rooting Substrate Inorganic Carbon Concentration

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Abstract
The relationships between CO2 concentrating mechanisms, photosynthetic efficiency and inorganic carbon supply have been investigated for the aquatic macrophyte Littorella uniflora. Plants were obtained from Esthwaite Water or a local reservoir, with the latter plants transplanted into a range of sediment types to alter CO2 supply around the roots. Free CO2 in sediment-interstitial-water ranged from 1–01 mol m−3 (Esthwaite), 0.79 mol m−3 (peat), 0.32 mol m−3 (silt) and 0–17 mol m−3 (sand), with plants maintained under PAR of 40 μmol m−2 s−1. A comparison of gross morphology of plants maintained under these conditions showed that the peat-grown plants with high sediment CO2 had larger leaf fresh weight (0–69 g) and total surface area (223 cm2 g−1 fr. wt. including lacunal surface area) than the sand-grown plants (0.21 g and 196 cm2 g−1 fr. wt. respectively). Root fresh weights were similar for all treatments. In contrast, leaf internal CO2 concentration [CO2], was highest in the sand-grown plants (2–69 mol m−3, corresponding to ∼ 6.5% CO2 in air) and lowest in the Esthwaite plants (1–08 mol m−3). Expression of CAM in transplants was also greatest in the low CO2 regime, with ΔH+ (measured as dawn-dusk titratable acidity) of 50μmol g− fr. wt., similar to Esthwaite plants in natural sediment. Assuming typical CAM stoichiometry, decarboxylation of malate could account largely for the measured [CO2]1 and would make a major contribution to daytime CO2 fixation in vivo. A range of leaf sections (0–2, 1–0, 5–0 and 17–0 mm) was used to evaluate diffusion limitation and to select a suitable size for comparative studies of photosynthetic O2 evolution. The longer leaf sections (17.0 mm), which were sealed and included the leaf tip, were diffusion-limited with a linear response to incremental addition of CO2 and 1–0 mol m−3 exogenous CO2 was required to saturate photosynthesis. Shorter leaf sections were less diffusion-limited, with the greatest photosynthetic capacity (36 μmol O2 g−1 fr. wt. h−1) obtained from the 1.0 mm size and were not infiltrated by the incubating medium. Comparative studies with 1.0 mm sections from plants grown in the different sediment types revealed that the photosynthetic capacity of the sand-grown plants was greatest (45 μmol O2 g−1 fr. wt. h−1) with a K0.5 of 80 mmol m−3. In terms of light response, saturation of photosynthesis in tissue slices occurred at 850–1000 μmol m−2 s−1 although light compensation points (6–11 μmol m−2s−1) and chlorophyll a: b ratios (1.3) were low. While CO2 and PAR responses were obtained using varying numbers of sections with a constant fresh weight, the relationships between photosynthetic capacity and CO2 supply or PAR were maintained when the data were expressed on a chlorophyll basis. It is concluded that under low PAR, CO2 concentrating mechanisms interact in intact plants to maintain saturating CO2 levels within leaf lacunae, although the responses of the various components of CO2 supply to PAR require further investigation.

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