Physiological aspects of the ecology of Dicranum fuscescens in the subarctic. I. Acclimation and acclimation potential of CO2 exchange in relation to habitat, light, and temperature

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Abstract
Laboratory measurements of net CO2 exchange in relation to light and temperature were made on D. fuscescens Turn. at Schefferville, Quebec [Canada] (latitude 55.degree. N), during the summer of 1974. Net CO2 exchange was measured using an open-flow IR gas analysis system. Moss samples were collected from 2 field sites (a lowland lichen woodland and highland semitundra region) immediately before the experiments. Temperature optima for photosynthesis in plants from both sites showed acclimation to higher temperatures in the middle of the season. Measured maximum rates of photosynthesis, attained in early July, equalled 2.1 mg CO2 .cntdot. g dry weight-1 .cntdot. h-1 in plants from the highland site and 0.74 mg CO2 .cntdot. g dry weight-1 .cntdot. h-1 in those from the lowland lichen woodland. Dark respiration rates showed no seasonal temperature acclimation. Radiation levels required for saturation of photosynthesis at optimum temperatures showed an increase from early season through midseason in samples from both populations. A reverse trend towards lower light requirements for saturation was detectable in the late season. Field-collected plants were exposed to different temperature regimes for 1 1/2 mo. in growth chambers. During this period, temperature acclimation of photosynthesis to ambient temperature conditions elicited a rapid shift in optimum temperatures for photosynthesis over periods as short as 48 h. All results are discussed in relation to measured environmental parameters in the 2 study sites throughout the 1974 growing season.

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