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

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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 photosynthesis occurring at 14°C 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 "winter 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 26°C whilst air temperature was ca. 9°C. Accordingly, under high radiation and even with the normal coastal windy conditions, thallus temperatures of 35°C are very likely to occur, with perhaps 45°C 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 35°C. Even after three weeks storage under 45°C, 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.