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

Abstract

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 (Pn) on warm, usually sunny, days (canopy air temperature [Tc]> 20°C), but had relatively high Pn on cool days (Tc<10°C). Laboratory measurements of light and temperature responses of Pn showed that high temperature, not visible irradiance, was responsible for depressions in Pn on warm sunny days. The optimal leaf temperatures (Tl) for Pn in C. quitensis and D. antarctica were 14 and 10°C, respectively. Both species had substantial positive Pn at 0°C Tl, which were 28 (C. quitensis) and 32% (D. antarctica) of their maximal Pn, and we estimate that their low‐temperature compensation points occurred at −2°C Tl (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 Pn 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 Pn 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 Pn/intercellular CO2 concentration (ci) response curve analyses. The breakpoint or critical temperature (Tcr) of minimal fluorescence (Fo) was ≈42°C in both species, which was well above the temperatures where reductions in Pn were evident, indicating that thylakoid membranes were structurally intact at supraoptimal temperatures for Pn. The optimal Tl for photochemical quenching (qp) and the quantum yield of photosystem II (PSII) electron transfer (φPSII) were 9 and 7°C in C. quitensis and D. antarctica, respectively. Supraoptimal temperatures resulted in lower qp and greater non‐photochemical quenching (qNP), but had little effect on Fo, maximal fluorescence (Fm) or the ratio of variable to maximal fluorescence (Fv/Fm) in both species. In addition, carboxylation efficiencies or initial slopes of their Pn/ci 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, Tc at our field site averaged 4.3°C and was below the temperature optima for Pn in these species for the majority of diurnal periods (86%) during the growing season, suggesting that continued warming will usually improve their rates of Pn.

Respuestas antioxidantes en dos ecotipos de Colobanthus quitensis (Caryophyllaceae) expuestos a alta radiación UV-B y baja temperatura

“…C. quitensis habita en regiones con episodios propicios para la generación de EAOs: alta radiación PAR, UV-B y baja temperatura. Una elevada capacidad antioxidante sería determinante en la capacidad de esta planta de sobrevivir en ambientes propicios para la generación de estrés oxidativo (Xiong et al 1999, Bravo et al 2001.…”

“…No obstante lo anterior, los niveles de Fv/Fm obser vados en ambos ecotipos no denotan un daño permanente al PSII, lo que queda demostrado por la recuperación de los valores iniciales de Fv/Fm pasadas 12 horas en ausencia de radiación UV-B, sugiriendo que el descenso de la eficiencia fotoquímica máxima se trata de una respuesta regulatoria que permite disminuir el exceso de energía absorbido, limitando el potencial daño al aparato fotosintético. Las variaciones de Fv/Fm obser vadas son consistentes con variaciones estacionales medidas en terreno en ambos ecotipos, lo que refuerza la idea que no se debiera atribuir a un daño importante del aparato fotosintético y que estaría dentro de los rangos normales de sobrevivencia de la especie en su hábitat natural (Casanova-Katny 2006, Xiong et al 1999. Asimismo, nuestros resultados muestran que el ecotipo Andino presenta menos daño a nivel de membranas y del aparato fotosintético.…”

“…Por ejemplo, Gianoli et al (2004), basándose en estudios de resistencia al congelamiento, mor fología y secuenciación de regiones ITS establecieron que existe una marcada diferenciación ecotípica entre las poblaciones de la Cordillera de los Andes en Chile central y las poblaciones antárticas. El ecotipo Antártico es catalogado como resistente al frío (Bravo et al 2001), puede mantener una tasa fotosintética positiva a 0 ºC y un aumento en la actividad antioxidante cuando es expuesta a baja temperatura y alta intensidad lumínica (Xiong et al 1999, Perez-Torres et al 2004. El ecotipo Antár tico también ha sido repor tado como resistente a los crecientes niveles de radiación UV-B presentes en su hábitat, provocados por el adelgazamiento de la capa de ozono, mostrando cambios en la anatomía de las hojas, aumento de la concentración de carotenos junto con compuestos solubles que absorben en la región UV-B (280-320 nm) (Xiong et al 1999a, Day et al 2001.…”

Adaptation and Acclimation of Photosynthetic Microorganisms to Permanently Cold Environments

“…This ability to increase growth rates correlates with an increase in photosynthetic energy utilization, and accounts for the lower excitation pressure exhibited in cultures of C. raudensis in comparison with cultures of the mesophilic species. Thus, on the basis of pigment composition and structural alteration of LHCII, C. raudensis may to respond to increased excitation pressure in a manner similar to wheat, rye, and Arabidopsis thaliana as well as Antarctic vascular plants (17,279). Despite the ability of the psychrophile to maintain lower excitation pressure under lower temperature and higher irradiance conditions by increasing utilization of photosynthetic energy products (by increasing growth rates), functional changes in the light energy distribution between LHCII and PSII were observed at the level of 77 K fluorescence emission.…”

Cold Season Physiology of Arctic Plants

“…One of the lowest temperature studies on Antarctic vascular plants was by Xiong et al (1999), who performed photosynthesis and respiration field studies on both C. quitensis and D.…”