CO 2 uptake patterns depend on water current velocity and shoot morphology in submerged stream macrophytes

Summary 1. The influence of current velocity on the pattern of photosynthetic CO 2 uptake in three species of submerged stream macrophytes was described by analysing the grain density in autoradiographs of leaves exposed to 14 CO 2 . 2. In Elodea canadensis , the CO 2 uptake was approximately two‐fold higher near the leaf periphery compared with the midrib section at high current velocity, whereas at low current velocity the area of relatively high CO 2 uptake expanded from the leaf periphery towards the midrib and basal sections of the leaves. 3. In Potamogeton crispus and Callitriche stagnalis the CO 2 uptake was uniform throughout the leaves at low current velocity, whereas at high current velocity the CO 2 uptake appeared to increase randomly in some areas of the leaves. 4. The relationship between the photosynthetic CO 2 uptake pattern and the dynamics of flow surrounding submerged shoots at low and high current velocity is discussed in relation to shoot morphology. In E. canadensis , thick diffusive boundary layers may develop between leaves because of screening effects at high current velocity. Increased diffusion path for CO 2 may contribute to inhibitory effects on photosynthesis in this species.