DOI: 10.1007/s11099-012-0070-2 PHOTOSYNTHETICA 51 (1): 13-21, 2013 13 Effects of temperature and irradiance on quantum yield of PSII photochemistry and xanthophyll cycle in a tropical and a temperate species A. DONGSANSUK, C. LÜTZ, and G. NEUNER + Institute of Botany, University of Innsbruck, Sternwartestrasse 15, A-6020, Innsbruck, Austria Abstract The effect of a wide range of temperatures (–15 and 60°C) in darkness or under strong irradiation [1,600 μmol(photon) m –2 s –1 ] on quantum yield of photosystem II photochemistry and xanthophyll cycle pigments was investigated in a tropical fruit crop (Musa sp.) and a temperate spring flowering plant (Allium ursinum L.). In darkness within the nonlethal thermal window of A. ursinum (from –6.7 to 47.7°C; 54.5 K) and of Musa sp. (from –2.2°C to 49.5°C; 51.7 K) maximal quantum yield of PSII photochemistry (F v /F m ) was fairly unaffected by temperature over more than 40 K. At low temperature F v /F m started to drop with ice nucleation but significantly only with initial frost injuries (temperature at 10% frost damage; LT 10 ). The critical high temperature threshold for PSII (T c ) was 43.8°C in A. ursinum and 44.7°C in Musa sp. Under strong irradiation, exposure to temperatures exceeding the growth ones but being still nonlethal caused photoinhibition in both species. Severity of photoinhibition increased with increasing distance to the growth temperature range. ΔF/F m ´ revealed distinctly different optimum temperature ranges: 27–36°C for Musa sp. and 18–27°C for A. ursinum exceeding maximum growth temperature by 2–7 K. In both species only at temperatures > 30°C zeaxanthin increased and violaxanthin decreased significantly. At nonlethal low temperature relative amounts of xanthophylls remained unchanged. At temperatures > 40°C β-carotene increased significantly in both species. In Musa sp. lutein and neoxanthin were significantly increased at 45°C, in A. ursinum lutein remained unchanged, neoxanthin levels decreased in the supraoptimal temperature range. In darkness, F v /F m was highly temperature-insensitive in both species. Under strong irradiation, whenever growth temperature was exceeded, photoinhibition occurred with xanthophylls being changed only under supraoptimal temperature conditions as an antiradical defence mechanism. Additional key words: Allium ursinum; β-carotene; freezing stress; heat; ice nucleation; lutein; Musa sp.; photosynthetic pigments. Introduction Chlorophyll (Chl) fluorescence studies have long been used as an indicator of functional changes of PSII photochemistry under temperature stress (Berry and Björkman 1980). Numerous studies have investigated the effect of certain temperatures in combination with various irradiation intensities (e.g. Lichtenthaler 1998, Adams and Demmig-Adams 2004). However, hardly any study except for Pospíšil et al. (1998) working with spring barley, has investigated the temperature response of quantum yield of PSII photochemistry in darkness within the growth temperature range and the whole nonlethal stress temperature range including both low- and high- temperature thermal limits of photosynthetic functions. Particularly in the low-temperature range, chilling- susceptible tropical species may show significant diffe- rences when compared to freezing-tolerant plants. While in the tropical species ice formation in the mesophyll exerts immediate damage, mesophyll cells of freezing- tolerant plants survive extracellular ice formation and consequent freeze dehydration down to a certain ——— Received 13 January 2012, accepted 5 September 2012. + Corresponding author; fax: +43–512–50751099, phone: +43–512–51026, e-mail: gilbert.neuner@uibk.ac.at Abbreviations: Chl a(b) – chlorophyll a(b); F 0 – minimal fluorescence in the dark-adapted state; F 0 /T – temperature-dependent change in minimal fluorescence; F m – maximal fluorescence in the dark-adapted state; F m ´ – maximal fluorescence in the light-adapted state; F s – steady state fluorescence; F v – variable fluorescence; F v /F m – maximal quantum yield of PSII photochemistry; ∆F/F m ´ – effective quantum yield of PSII photochemistry; LT 10 (LT 50 ) – temperature at 10% (50%) heat or frost damage, respectively; PPFD – photosynthetic photon flux density; PSII – photosystem II; ROS – reactive oxygen species; T c – critical high temperature threshold of the F 0 /T curve calculated as the intersection of a horizontal line at the level of F 0 at 20°C and of a line fitted visually to the fast rising phase; T p – peak temperature of the F 0 /T curve. Acknowledgements: We are thankful to Dr. D. Remias for useful advice concerning the HPLC analyses and S. Aigner for expert technical assistance in HPLC work. We would like to acknowledge the Botanical Garden of the University of Innsbruck for cultivation of plant material. In addition, we are grateful to all reviewers for their valuable comments.