PARENT SESSION
Posters P1B Photo-oxidative stress, photoinhibition. Abstracts (394-443)

Partitioning and quenching of excitation energy in winter acclimated evergreen conifer Scots pine. Dmitry Sveshnikov^*,1, Ingo Ensminger², Alexander Ivanov³, Douglas Campbell⁴, Gunnar Öquist¹, ¹ Department of Plant Physiology, Umeå, Sweden² Max-Planck-Institut für Biogeochemie, Jena, Germany³ Department of Biology, London, Ontario, Canada⁴ Department of Biology, Sackville, New Brunswick, Canada

ABSTRACT- Quenching of energy absorbed in excess by winter acclimated green tissue of conifers undergoes different dissipative pathways. We tested experimentally how low temperatures and different light conditions determine acclimational responses of photosynthesis and how these translate into different strategies of energy quenching in seedlings of Scots pine (Pinus sylvestris L.). Photosynthetic activity in high light (350 E m^-2 s^-1) grown plants under cold temperatures suffered more than in shaded (50 E m^-2 s^-1) ones, as shown by room temperature fluorescence and oxygen evolution measurements. Thus, already at 5^oC, Fv/Fm decreased to 70% under high light, while shaded seedlings maintained almost initial levels of fluorescence. After further incubation at -5^oC, only 8% of the initial Fv/Fm value remained at high light, while shaded plants retained 40% of their initial Fv/Fm. The recovery of Fv/Fm in shaded plants upon subsequent incubation at 20^oC was also more complete than that of high light-grown seedlings (87% vs 65%). Estimation of energy partitioning in the needles revealed a higher fraction of energy being dissipated under low temperatures and high light as compared to low light, suggesting increased capacity of non-photochemical quenching in high light exposed seedlings. Concomitantly, the deepoxidation state of the xanthophyll cycle pigments noticeably increased in high light-grown seedlings at 5^oC, contributing to the non-photochemical quenching capacity, whereas significant deepoxidation in low light-grown plants was observed only at -5^oC. Additionally, as revealed by the characteristic shift of S₂Q_B band in thermoluminescence spectra, the level of reaction centre quenching in PSII under cold temperatures was more pronounced in the needles of high light grown plants. We conclude that both reaction centre-based and antenna-based quenching of excess energy represent important light-regulated mechanisms in the evergreen Scots pine serving to minimize the risk of photodestruction of green tissue during high excitation pressure under severe winter conditions.

KEY WORDS: Pinus (winter stress), Photosynthesis, Reaction centre quenching, Antenna quenching