Abstract
SUMMARYThe control of the net photosynthetic rate in Peltigera canina var. praetextata and P. polydactyla by temperature, light and moisture has been examined on a seasonal basis. Mean optimum values of 6 mg CO2 g−1·h−1 are achieved by young lobes of either species during late spring, summer and early winter. However, under full winter conditions optimum net photo‐synthetic rates fall to 3 mg CO2 g−1·h−1. Light saturation is approximately 500 μE m−2 s−1 at all times of the year, but the temperature at which maximum rates of net photosynthesis are achieved changes markedly throughout the year in both species. In winter, maximum rates occur at an experimental thallus temperature of 5°C but by mid‐summer, the optimum temperature for net photosynthesis is c. 30°C, which corresponds to equivalent ambient temperature levels in the field. This temperature acclimation of net photosynthetic optima is achieved without any significant change in respiration rate and shows a remarkable degree of homeostasis during spring, summer and early winter. The results strongly emphasize the unsatisfactory nature of the univariate experimental design which has often been used in lichen physiological ecology. The extreme importance of a controlled storage temperature for experimental lichen material is also clearly evident. The low net photosynthetic rates in winter show the same pattern in both Peltigera species during these low temperature and short day conditions.
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