Norway spruce, Non-radiative dissipation, Xanthophyll cycle

Zeaxanthin (Z) accumulation, occurring after light (?pH) activation of the violaxanthin de-epoxidase (VDE) catalysing deepoxidation of violaxanthin (V) to Z via an antheraxanthin (A), contributes to mitigation of oxidative damage of the thylakoid membranes. Particularly, Z modulated ?pH dependent non-radiative dissipation of excitation energy within photosystem II (NRD) is essential for photosystem II resistance against high light stress [1]. Especially, in Norway spruce acclimated to high-light a fast induction of maximal NRD and accumulation of Z was observed [2]. Recently, it has been shown that elevated temperatures can facilitate activation of VDE and accelerate V deepoxidation upon illumination [3]. Whereas in spruce seedlings the rapid phase of V deepoxidation (induced by either 10s illumination or 10 light-pulses of 1s duration at 1 min interval, 10x1s) was gradually stimulated upon increasing temperatures, in A. thaliana leaves considerable acceleration of V deepoxidation occurred just at 40°C. Moreover, only in spruce seedlings Z accumulation was induced by 1s light pulses at 32°C and higher temperatures, but neither in WT nor in L17 A. thaliana leaves. In agreement with these results, elevated temperatures stimulated formation of Z-dependent NRD induced by 1s light pulses only in spruce seedlings. Analysis of initial phase of V deepoxidation during illuminations shorter than 10s indicated a saturation of VDE capacity in spruce needles at elevated temperatures. These new findings about extremely rapid light-induced V deepoxidation and NRD induction observed in spruce seedlings at elevated temperatures are discussed with respect to specific fatty acid composition of spruce thylakoid membrane lipids and organization of photosystem II.