Seasonal Changes in Photosynthesis in the Lichen Peltigera Rufescens

Abstract

<p>The seasonal response of net photosynthesis to temperature, light and moisture has been examined in two populations of the lichen Peltigera rufescens. The sub-arctic population had a temperature optimum of 35°C and no seasonal changed in the photosynthetic response were observed. In contrast, the gas exchange rate did change on a seasonal basis in the second population studied (from a temperate climatic zone). In this population, the temperature optimum for the net photosynthesis shifted from 25°C in the winder to 35°C in the summer. This seasonal change can be induced in the laboratory storing the lichen under appropriate temperature and photoperiod conditions. This change was shown, with the use of photosynthetic-illumination curves, to be restricted to alterations in the light saturated rates of net photosynthesis only, indicating that the acclimation process resulted from a change in the dark reactions of photosynthesis. This hypothesis was further investigated with studies on an important rate controlling enzyme of the Calvin cycle, fructose-1, 6-bisohosphatase. This enzyme was purified from summer and winter collections of the temperate population of P. rufescens and the total activity, thermal stability, substrate affinity and activation energy was compared in these isolates. It was shown that the temperature response of the Km for this enzyme closely paralleled the net photosynthetic response and that the activation energy was lower in the summer form of the enzyme. These kinetic differences largely explain the seasonal change in net photosynthetic capacity observed in the temperate population and support the enzymatic theory of acclimation suggested by the photosynthetic-illumination curves.</p>