Fig. 1. Seasonal dynamics of weather and photochemical performance of PSII measured by chlorophyll fluorescence in Scots pine needles.

Changes in temperature (°C) (Left Y-axis) and solar radiation (watt m⁻²) (Right Y-axis) during 2015–2016 (a), 2016–2017 (b), 2017–2018 (c) measuring seasons. Seasonal dynamics of PSII photochemistry: (d) Changes in maximal (Fm) and basic (Fo) fluorescence. (e) Maximal quantum efficiency of PSII measured as Fv/Fm (f) Effective quantum yield of PSII (Φ(II)). Energy dissipation measured as regulated and non-regulated non-photochemical quenching: (g) Changes in NPQ with increasing PAR. (h) Induction of NPQ with constant actinic light. (i) Quantum yield of non-regulated non-photochemical quenching. Recovery of pine needles under artificial conditions at 80 µmol photons m⁻² s⁻¹ of light for 48 h with 18/6 photoperiod: (j) Changes in Fo. Fm and Fv/Fm. (k) Changes in Φ(II). (l) Induction of NPQ with constant actinic light. Quantum yields were calculated at actinic light illumination of 300 μmol m⁻² s⁻¹ in the light response curve and NPQ induction was measured either at 300/800 μmol m⁻² s⁻¹ of actinic light illumination. All measurements were taken after 30 min of dark adaptation at 4 °C from winter to late spring and room temperature in summer. Error bars indicate the mean ± SD (n = 3) for (d–g), and (i); ± SD (n = 4) for (j, k). Error bars indicate the mean ± SE (n = 3) for (h, l). The statistically significant mean differences (t-test) are marked by asterisks indicating the following confidence intervals: *≤95%; **≤99%.; ***≤99.9%. Exact p values are provided in the source data file.