COMPARATIVE KINETICS OF PHOTOSYNTHESIS IN FLOATING AND SUBMERGED POTAMOGETON LEAVES

We examined the morphology and photosynthetic performance of floating and submerged leaves in five Potamogeton species to evaluate the adaptations associated with air contact or full submergence. The floating leaves had stomata on the upper surface, were thicker and contained more chlorophyll per surface area than the submerged leaves. The light absorptance was higher and independent of areal chlorophyll concentration for floating leaves, but lower and strongly chlorophyll dependent for submerged leaves. The photosynthetic rate of both leaf forms was relatively high in their natural growth environment and quantum yields resembled the values attained for terrestrial leaves. Carbon utilization of floating leaves in air showed CO2 compensation points (32-49 mu 11(-1), corresponding to 1.2-1.8 mu M), typical of C-3 plants, and photosynthesis was not saturated at ambient atmospheric CO2 levels. During pH-drift experiments in water, submerged leaves were able to extract the majority of the dissolved inorganic carbon pool (82-89%) and attain high pH values (9.95-10.60) and low residual CO2 concentrations (6-80 nM), presumably as a result of HCO3- utilization. Photosynthesis of floating leaf forms declined only four-fold when placed in CO2 enriched water (approximately 200 mu M) Photosynthesis of submerged leaf forms declined two-fold when transferred to water-saturated air, owing to a highly increased respiration. The floating leaves of Potamogeton can support higher areal rates of photosynthesis than submerged leaves and are believed to be particularly important in habitats with turbid waters, and as floating platforms and carbon sources for the aerial flowers. The cost-efficiency of the two leaf forms will depend, however, on the environmental light and carbon conditions because floating leaves are also much thicker and expensive to produce, and their confined horizontal location at the water surface restricts the efficiency of light utilization in plant populations.