Low-temperature acclimation of net photosynthesis in the crustaceous lichen Caloplaca trachyphylla

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Abstract
The seasonal net photosynthetic and respiratory response matrix is presented for Caloplaca trachyphylla (Tuck.) A. Zahlbr. at 7, 14, 21, 28 and 35 °C, under 0, 300, 600, 900, and 1200 μE m−2 s−1 illumination, over a full range of thallus hydration, in January, May, and July. The maximum rates of net photosynthesis recorded during the summer period, at 14–21 °C, under 1200 μE m−2 s−1 illumination are ca. 3.0 mg CO2 h−1 g−1. They are interpreted in terms of the thallus temperatures documented during early-morning dew events and during sporadic thunderstorm activity. During winter, net photosynthetic rates at 7 °C increase significantly from the summer rates of ca. 2.0 mg CO2 h−1 g−1 to ca. 3.2 mg CO2 h−1 g−1. This low-temperature photosynthetic-capacity change is shown to be temperature acclimation and it is suggested that the mechanism involves a change in the affinity constant (Km) of ribulose bisphosphate carboxylase. This acclimatory response can be readily induced, or reversed in winter, by air-dry storage at an appropriately high or low ambient temperature. In midsummer, however, comparable storage conditions fail to elicit an acclimatory response. The significance of low-temperature photosynthetic acclimation in C. trachyphylla is discussed in relation to winter snowmelt sequences during chinook conditions in Alberta.