Excitation transfer and quenching in photosystem II, enlightened by carotenoid triplet state in leaves

• Published in: Photosynthesis research Vol. 160; no. 1; pp. 31 - 44
• Main Authors: Laisk, Agu, Peterson, Richard B., Oja, Vello
• Format: Journal Article
• Language: English
• Published: Dordrecht Springer Netherlands 01-04-2024; Springer Nature B.V
• Subjects: Biochemistry; Biomedical and Life Sciences; Carotenoids; Chlorophyll; Fluorescence; Leaves; Life Sciences; Light-Harvesting Protein Complexes; Pair bond; Photons; Photosystem II; Photosystem II Protein Complex; Plant Genetics and Genomics; Plant Physiology; Plant Sciences; Triplet state; Leaves; Photoprotection; Non-photochemical quenching; Triplet states
• ISSN: 0166-8595; 1573-5079
• DOI: 10.1007/s11120-024-01086-6

Accumulation of carotenoid (Car) triplet states was investigated by singlet–triplet annihilation, measured as chlorophyll (Chl) fluorescence quenching in sunflower and lettuce leaves. The leaves were illuminated by Xe flashes of 4 μs length at half-height and 525–565 or 410–490 nm spectral band, maximum intensity 2 mol quanta m
−2
 s
−1
, flash photon dose up to 10 μmol m
−2
or 4–10 PSII excitations. Superimposed upon the non-photochemically unquenched
F
md
state, fluorescence was strongly quenched near the flash maximum (minimum yield
F
e
), but returned to the
F
md
level after 30–50 μs. The fraction of PSII containing a
3
Car in equilibrium with singlet excitation was calculated as
T
e
 = (
F
md
—
F
e
)/
F
md
. Light dependence of
T
e
was a rectangular hyperbola, whose initial slope and plateau were determined by the quantum yields of triplet formation and annihilation and by the triplet lifetime. The intrinsic lifetime was 9 μs, but it was strongly shortened by the presence of O
2
. The triplet yield was 0.66 without nonphotochemical quenching (NPQ) but approached zero when NP-Quenched fluorescence approached 0.2
F
md
. The results show that in the
F
md
state a light-adapted charge-separated PSII
L
state is formed (Sipka et al., The Plant Cell 33:1286–1302, 2021) in which Pheo
−
P680
+
radical pair formation is hindered, and excitation is terminated in the antenna by
3
Car formation. The results confirm that there is no excitonic connectivity between PSII units. In the PSII
L
state each PSII is individually turned into the NPQ state, where excess excitation is quenched in the antenna without
3
Car formation.