High internal resistance to CO2 uptake by submerged macrophytes that use HCO3−: measurements in air, nitrogen and helium
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Previous studies have shown that in water the affinity of submerged macrophytes for CO2 is higher for species restricted to CO2 than for species with an additional ability to use bicarbonate. We measured slopes of CO2 uptake versus CO2 concentration in the gas phase in air, nitrogen and helium for pairs of species, having or lacking the ability to use bicarbonate, but with similar leaf morphology. For species restricted to CO2, the slope in nitrogen and helium was 1.5 times and 3.2 times greater than in air. The increased slope in nitrogen results from a suppression of photorespiration. The further increase in helium reflects the increased rate of diffusion of CO2 and shows that, even in gas, external diffusion through the boundary layer is a significant hindrance to CO2 uptake. In contrast, in species able to use bicarbonate, the uptake slope was not affected by gas composition, suggesting that photorespiration is absent or photorespired CO2 is efficiently trapped and that internal resistance is high relative to external resistance. Elodea canadensis specimens grown under high concentrations of CO2 de-regulated their ability to use bicarbonate, and slopes of CO2 uptake in helium were significantly greater than in air or nitrogen. Overall, these results are consistent with the notion that while a high affinity for CO2 will maximise carbon uptake in species restricted to CO2, for species able to use bicarbonate, a high internal resistance would reduce loss of CO2 and help maintain high concentrations of CO2 at the site of fixation.
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