Photoinhibition of net photosynthesis in Stereocaulon virgatum and S. tomentosum, a tropical–temperate comparison

Abstract

Stereocaulon tomentosum Fr., collected in pine–lichen woodlands of southwestern Alberta, Canada, and S. virgatum Ach., collected from recent lahar flows on La Soufrière, Guadeloupe (French West Indies), were examined for their response of net photosynthesis and respiration to brief periods of high light exposure at temperatures ranging from 10 to 40 °C. In each species, a 30-min exposure period to photon flux densities of 1500 μmol∙m−2∙s−1 resulted in a significant reduction of subsequently measured rates of net photosynthetic CO2 uptake in those treatment groups which had been held at temperatures at or above 30 °C. These results suggest an altered stability of interactions between light and dark reactions of net photosynthesis at higher temperatures. They also point to the need for close monitoring of field microclimatic conditions during periods when hydrated lichen thalli are exposed to full sun conditions. Selective pressures imposed on hydrated lichen thalli during these periods of insolation shock may well prove of much greater importance in the shaping of net photosynthetic response patterns than has previously been recognized and may account for many of the previously observed disparities between those temperatures prevailing during typical periods of thallus hydration (i.e., under overcast conditions) and those at which maximal rates of net photosynthetic uptake are seen (often a full 10 to 15 °C higher). These interactions appear equally important to lichens of tropical origins and to those of north temperate habitats and suggest common evolutionary drives on thermal acclimation in both environments.