Fig. 4: Large electron sinks downstream of PSI protect Scots pine and Norway spruce under illumination mimicking fluctuating growth light condition.
From: Two dominant boreal conifers use contrasting mechanisms to reactivate photosynthesis in the spring
a, b The relative yield of AEF (Y(AEF)), representing the Δ flow in conifers between PSI and PSII mainly contributed by CET and pseudo-CET. Y(AEF) is calculated as Y(AEF) = Y(I) – Y(II). The yield of PSII and PSI were measured simultaneously with rapid light curves, and the values under moderate high light (536 μmol photons m−2 s−1) presented here. Needles of Scots pine (open circle) and Norway spruce (closed triangle) were collected during March to June 2017 in (a). Samples in (b) were collected from the field in April, and measured immediately (black) and following recovery in room temperature for 24 h (gray). c–h Photosynthetic fluorescence measurement of Scots pine and Norway spruce under illumination mimicking fluctuating growth light condition. In vivo fluorescence and P700 signals monitored under 2 min dark, followed by four cycles of 5 min low light (58 μmol photons m−2 s−1, gray bar) and 1 min high light (1599 μmol photons m−2 s−1, yellow bar). Fluorescence parameters: c Y(II), operating efficiency of PSII; (d) Y(I), operating efficiency of PSI; e Y(ND), quantum yield of non-photochemical energy dissipation in PSI reaction centers that are limited due to a shortage of electrons (donor-side limitation); f Y(NA), quantum yield of non-photochemical energy dissipation in PSI reaction center s that are limited due to shortage of electron acceptors (acceptor-side limitation); g ETR(I)/ETR(II), the ratio of the electron transport rate of PSI to PSII; h ETR(I), the electron transport rate of PSI. Sun-acclimated needles from Scots pine (closed circle) and Norway spruce (closed triangle) were collected from the field in April. Seedlings from Scots pine (open circle) and Norway spruce (open triangle) grown in growth chamber (22 °C, 150 μmol photons m−2 s−1 and 8/16 h light/dark cycle) were used as controls. Significant differences are indicated with different letters above the bars (one-way ANOVA, P < 0.05). Each data point represents the mean of 4–6 biological replicates (mean ± SE, n = 4–6).
