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

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.