Effects of temperature and irradiance on quantum yield of PSII photochemistry and xanthophyll cycle in a tropical and a temperate species

The effect of a wide range of temperatures (-15 and 60A degrees C) in darkness or under strong irradiation [1,600 mu mol(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 47.7A degrees C; 54.5 K) and of Musa sp. (from -2.2A degrees C to 49.5A degrees 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; LT10). The critical high temperature threshold for PSII (T-c) was 43.8A degrees C in A. ursinum and 44.7A degrees 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. Delta F/F-m' revealed distinctly different optimum temperature ranges: 27-36A degrees C for Musa sp. and 18-27A degrees C for A. ursinum exceeding maximum growth temperature by 2-7 K. In both species only at temperatures > 30A degrees C zeaxanthin increased and violaxanthin decreased significantly. At nonlethal low temperature relative amounts of xanthophylls remained unchanged. At temperatures > 40A degrees C beta-carotene increased significantly in both species. In Musa sp. lutein and neoxanthin were significantly increased at 45A degrees 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.