Soil drought stress and high-temperature effects on photosystem II in different juvenile spruce provenances

With prolonged periods of drought and temperature extremes becoming more frequent, the stress on Norway spruce has increased, as its ecological optimum is in colder and more humid environments. However, it is a tree species with a good adaptability, due to its great geographic and intraspecific variability. In this study, we tested the sensitivity of chlorophyll a fluorescence tools for the early detection of drought and heat stress, as well as their combined effect, in four Slovak Carpathian provenances, under controlled laboratory conditions. Responses in photosynthetic parameters of fast and slow (light response curves) kinetics of chlorophyll a fluorescence were analysed in spruce seedlings under four treatments: control, drought, heat, and combined drought + heat. Heat and drought + heat stresses led to the decrease of photosynthetic efficiency to a greater extent than in the case of only individual drought stress. The less sensitive parameter impacted by the stresses was the maximum quantum yield of PSII photochemistry (Fv/Fm), while the more sensitive parameters were: the performance index based on absorption of light energy (PI_ABS), the effective quantum yield of the PSII (Φ_PSII), and the coefficients of non-photochemical fluorescence quenching (NPQ, q_N). The significant variability in responses of individual provenances was also observed. Seedlings from two studied provenances responded better to these abiotic stresses (Pa-680 and Pa-1500); however, Pa-1500 provenance from the highest altitude showed the best ability of resistance to soil drought stress, as well as to high temperature effect. This rapid screening of photosystems II efficiency showed the importance of the selection of more resistant populations to concurrent stresses as one of the measures to mitigate climate change impacts.