Photosynthetic temperature response of the Antarctic vascular plants Colobanthus quitensis and Deschampsia antarctica

The photosynthetic temperature response of the Antarctic vascular plants Colobanthus quitensis and Deschampsia antarctica was examined by measuring whole-canopy CO₂ gas exchange and chlorophyll (Chl) a fluorescence of plants growing near Palmer Station along the Antarctic Peninsula. Both species had negligible midday net photosynthetic rates (P(n)) on warm, usually sunny, days (canopy air temperature [T(c)] 20°C), but had relatively high P(n) on cool days (T(c) < 10°C). Laboratory measurements of light and temperature responses of P(n) showed that high temperature, not visible irradiance, was responsible for depressions in P(n) on warm sunny days. The optimal leaf temperatures (T(l)) for P(n) in C. quitensis and D. antarctica were 14 and 10°C, respectively. Both species had substantial positive P(n) at 0°C T(l), which were 28 (C. quitensis) and 32% (D. antarctica) of their maximal P(n), and we estimate that their low-temperature compensation points occurred at -2°C T(l) (C. quitensis) and -3°C (D. antarctica). Because of the strong warming trend along the peninsula over recent decades and predictions that this will continue, we were particularly interested in the mechanisms responsible for their negligible rates of P(n) on warm days and their unusually low high-temperature compensation points (i.e., 26°C in C. quitensis and 22°C in D. antarctica). Low P(n) at supraoptimal temperature (25°C) appeared to be largely due to high rates of temperature-enhanced respiration. However, there was also evidence for direct impairment of the photosynthetic apparatus at supraoptimal temperature, based on Chl fluorescence and P(n)/intercellular CO₂ concentration (c(i)) response curve analyses. The breakpoint or critical temperature (T(cr)) of minimal fluorescence (F(o)) was ≃ 42°C in both species, which was well above the temperatures where reductions in P(n) were evident, indicating that thylakoid membranes were structurally intact at supraoptimal temperatures for P(n). The optimal T(l) for photochemical quenching (q(p)) and the quantum yield of photosystem II (PSII) electron transfer (Φ(PSII)) were 9 and 7°C in C. quitensis and D. antarctica, respectively. Supraoptimal temperatures resulted in lower q(p) and greater non-photochemical quenching (q(NP)), but had little effect on F(o), maximal fluorescence (F(m)) or the ratio of variable to maximal fluorescence (F(v)/F(m)) in both species. In addition, carboxylation efficiencies or initial slopes of their P(n)/c(i) response were lower at supraoptimal temperatures, suggesting reduced activity of ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco). Although continued warming along the peninsula will increase the frequency of supraoptimal temperatures, T(c) at our field site averaged 4.3°C and was below the temperature optima for P(n) in these species for the majority of diurnal periods (86%) during the growing season, suggesting that continued warming will usually improve their rates of P(n).