Thermal adaptation and acclimation of ecotypic populations of Spirodela polyrhiza (Lemnaceae): thermostability and apparent activation energy of NAD malate dehydrogenase

Eleven ecotypes of Spirodela polyrhiza (L.) Schleid., an aquatic plant possessing an extensive geographic distribution, were studied to detect adaptive and acclimatory metabolic changes through a study of the thermostability and activation energy of malate dehydrogenase. Colonies were grown under controlled conditions with temperature (18, 23, and 28 °C) as the only variable. Thermostability is found to be affected by experimental temperatures (acclimation) but not by origin temperatures; there is genetic differentiation but related to some other environmental conditions than average temperature at the site of origin. Activation energy is unaffected by experimental temperatures or origin. It is suggested that, as S. polyrhiza naturally grows in bodies of water, it is less exposed to temperature variations than terrestrial plants, but is more affected by other physicochemical environmental factors; its main metabolic regulatory systems do not appear to be associated with thermal controls.