The Control of Seasonal Rates of Net Photosynthesis by Moisture, Light and Temperature in Parmelia disjuncta Erichs.

Abstract

The seasonal response matrix of gas exchange in Parmelia disjuncta is presented for a number of factorial combinations of light, thallus temperature, and thallus water content throughout the year. The results show a general level of adaptation to a xeric, low-arctic environment with maximum rates of net photo- synthesis occurring at 140C and 100-150% water content by weight. The data show a significant increase in respiration in the winter collection, presumably signalling a cold-hardened state. In addition there is a clear response to summer high-light conditions with net photosynthetic rates responding markedly to the maximum levels of illumination used. These capacity changes are discussed in terms of win- ter hardening, photosynthetic acclimation (which infers homeostatic control), as well as capacity changes that offer an alternative strategy allowing maximum carbon gain with a minimum maintenance cost. Field measurements at Churchill, Manitoba show that thallus temperatures, under moderate radiation conditions in summer, reach 260C whilst air temperature was ca. 90C. Accordingly, under high radiation and even with the normal coastal windy conditions, thallus temperatures of 350C are very likely to occur, with per- haps 450C being reached around solar noon. These high thallus temperatures are induced by both the dark colour and the virtually crustaceous habit of the thallus. The response of air-dry replicates to thermal stress is remarkably uniform with no evidence of stress effects after 21 days storage with a maximum daily temperature of 350C. Even after three weeks storage under 450C, there is only slight evidence of reduced photosynthetic capacity. This finding emphasises that although there may be an overall correlation between latitude and the ability of a lichen to tolerate thermal stress, there will be numerous exceptions. Each species may have a very specific niche with an equally specific set of strategies allowing optimum carbon assimilation and growth.