Plant growth and photosynthesis in the transition zone between land and stream

Many plant species grow in the transition between land and water in unshaded lowland streams. Our objective was to determine the importance of exposure to air or water on photosynthesis, CO2 conductance and relative growth rate in four common amphibious species in streams. Photosynthesis was 2-3 times higher in air than in stream water six-fold supersaturated with CO2, because diffusive boundary layers constrained CO2 fluxes much more in water than in air. Leaves acclimated to life in air or water by increasing CO2 affinity during photosynthesis at rate-limiting concentrations due to changes in leaf surface conductance and stomatal density. Leaves grown in water had a 1.8-4.6 times higher CO2 conductance than leaves grown in air, when both the leaf types were measured under water. Aerial leaves had a 1.3-1.6 times higher CO2 conductance than aquatic leaves, when both were measured in air. Growth of amphibious plants was severely constrained by low CO2 availability in air-equilibrated water, while CO2 enrichment in water to 40-fold supersaturation stimulated plant growth without fully reaching the growth rates in air. The widespread CO2 supersaturation of streams facilitates the successful development of submerged populations of amphibious plants, but terrestrial populations continue to support faster growth and denser plant stands. (C) 1999 Elsevier Science B.V. All rights reserved.