Fig. 2. Seasonal changes of PSI photochemistry in Scots pine needles.

Energy partitioning in PSI considering Y(I) + Y(ND) + Y(NA) = 1, where Y(I) (a), Y(NA) (b), Y(ND) (c) are the photochemical quantum yield of PSI (when P700 is reduced and A is oxidized), energy dissipation in PSI (a measure of acceptor side limitation, when P700 and A both are reduced) and energy dissipation in PSI (a measure of donor side limitation, when P700 and A both are oxidized), respectively. Recovery of PSI photochemistry under artificial conditions: (d) Y(I), (e) Y(NA), (f) Y(ND) of PSI during the recovery period. Quantum yields were calculated from 300 μmol m⁻² s⁻¹ light illumination period of a light response curve. 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 (a–c); ± SD (n = 4) for (d–f). 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.