Resistance to CO2Fixation in the Submerged Aquatic MacrophyteCallitriche stagnalisScop

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Abstract
The resistance to the photosynthetic carbon fixation in the freshwater angiospcrm Callitriche stagnalis Scop was investigated by gas exchange experiments in a closed water-flow system. An electrical analogue model was used to analyse uptake of carbon in terms of boundary layer resistance, cytoplasmic resistance and carboxylation resistance. The most important rate-limiting factor was the boundary layer resistance which was from 86% to 91 % of the total resistance of 852–1221 s cm−1. The cytoplasmic and carboxylation resistances during active photosynthesis were of minor importance being 89 s cm−1 and 24–30 s cm−1, respectively. The calculated thickness of the boundary layer surrounding the foliage of the shoots was 103–155 μm or 2–3 times the total thickness of the leaves. The physiological and morphological characteristics of submerged aquatic macrophytes are discussed as adaptations to the low availability of carbon due to the high boundary layer resistance.