Photosynthesis and photorespiration in freshwater organisms: Amphibious plants

Abstract

Amphibious macrophytes are remarkable for their ability to photosynthesise in air and water, since these two media are very different. Specifically, water has a higher water potential, greater thermal stability, but reduced availability of light, CO 2 and O 2 than air. In heterophyllous amphibious plants, production of an appropriate leaf form is triggered by water potential, temperature, photoperiod, ratio of red to far-red light, or CO 2 concentration in some species. These effects may be mediated by endogenous growth regulators. High concentrations of CO 2 are necessary to saturate photosynthesis of submerged shoots of amphibious species underwater, largely because of boundary-layer resistance. The low rate of CO 2 supply underwater is reflected in low activities of carboxylating and other enzymes involved in photosynthesis in submerged shoots. Any limitation of photosynthesis by CO 2 is rarely overcome by HCO 3 use; rather, the aerial portion may allow access to an environment with greater availability of CO 2. Alternatively, submerged-CAM found in amphibious species such as Isoetes howellii Engelm. may reduce CO 2 limitation of photosynthesis by exploiting the higher CO 2 concentration often found at night. Photorespiration is present in submerged and aerial shoots, but incubation underwater at high temperature (30°C) and long days (14 h) reduces photorespiration in both shoot types in Myriophyllum brasiliense Cambess. and Proserpinaca palustris L. Higher relative activities of phosphoenolpyruvate carboxylase in submerged compared with emerged shoots of some amphibious species may be a response which reduces photorespiration. Submerged shoots require less light to saturate photosynthesis and have generally lower light compensation points at air levels of CO 2. It is unclear whether or not this is partly caused by the interacting effect of CO 2. Amphibious macrophytes are poorly understood, and future work should distinguish between the effects of air and water, environmental pretreatment, and linkage to changes in leaf form when photosynthetic and photorespiratory responses of submerged and aerial shoots are compared.