Morphological adaptations and photosynthetic rates of amphibious Veronica anagallis-aquatica L. (Scrophulariaceae) under different flow regimes

We investigated the influence of water velocity on the morphology of stem and leaves, and the photosynthetic rates of amphibious Veronica anagallis-aquatica L. (Scrophulariaceae), growing naturally in the Batise Springs, a shallow clear stream in the state of Idaho, USA. This amphibious species occurs with emerged and/or totally submerged leaves in low velocity areas and with totally submerged leaves at high velocity areas. Plants with emerged leaves have larger leaf area, thickness, density, dry mass, stem internode lengths, and internode diameters than plants with submerged leaves at high velocity. The stem internodes have similar anatomy, except for the pith region. In this region, a lumen is present in the apical and intermediate internodes from plants with emerged leaves. Emerged leaves presented the highest chlorophyll content and maximum photosynthetic oxygen evolution rates. Submerged leaves at low and high velocity had different chlorophyll content but similar photosynthetic rates under saturating CO2. The emerged leaves are similar in morphology to sun leaves while submerged leaves at low and high velocities are similar to shade leaves of terrestrial plants. The increase in stem diameter, length of internodes, and the presence of the lumen are apparently adaptations that allow the development of temporary aerial branches in the spring and summer, which develop flowers and fruits, allowing for sexual reproduction. The smaller internodes, leaf area, and thickness of submerged plants at high velocity apparently confer greater flexibility to the stem, allowing it to withstand the associated pulling forces of the water. (C) 2002 Elsevier Science B.V. All rights reserved.