EFFECT OF FROST STRESS ON CHLOROPHYLL a FLUORESCENCE AND MODULATED 820 nm REFLECTION IN ARABIS ALPINA POPULATION FROM RILA MOUNTAIN

Summary: Cold stress is one of the most important abiotic factors which in the context of the global climatic changes strongly affects plant productivity. The perrenial plant Arabis alpina (Brassicaceae) was used as a model system to study the effect of cold stress on photosynthetic performance as measured by prompt chlorophyll a fluorescence (PSII activity), as well as by the modulated 820 nm reflection (PSI activity) . Intact wild plants collected from Rila Mountain, and plants grown in laboratory conditions from seeds were used in this study. Plants were grown at 22°C under control conditions and after that they were exposed to chilling stress (+4°C) for 4 days followed by freezing stress (-7°C) for 12h. Recovery from the frost stress was studied after return of the plants to +4°C (4 days) and further to 22°C (4 days). It was found that both the intact wild plants and the plants from seeds were tolerant to the frost stress. Net photosyntetic rate was gradually inhibited during chilling and freezing stresses with CO 2 uptake still active after the frost stress. Photosynthesis was restored during the period of recovery to values close to the control. After frost treatment both intact plants and plants grown in control conditions showed a very slight inhibition of PSII activity as revealed by the quantum yield of the primary photochemical reaction (φ Po ), whereas intersystem electron transport (φ Eo ), reduction of the end electron acceptors (φ Ro ) and the performance index of photosynthesis (PI abs ) were inhibited to a greater extent. All chlorophyll a fluorescence parameters of PSII were restored during the period of recovery. In contrast to PSII activity, the activity of PSI as probed by the modulated 820 nm reflection was much strongly inhibited after the frost stress especially in its re-reduction phase. Our results indicated that the higher tolerance of A. alpina to frost stress was accompanied with higher resistance of the overall photosynthetic performance, including both PSII and PSI.