The Polyphasic Rise of Chlorophyll Fluorescence upon Onset of Strong Continuous Illumination: II. Partial Control by the Photosystem II Donor Side and Possible Ways of Interpretation

The fluorescence rise kinetics in saturating light display two well separated com ponents with largely different properties. The rapid rise from F„ to a first intermediate level, I ,, is photochemically controlled, while the following phases leading to a secondary intermediate level, I2, and to a peak level, P , are limited by thermal reactions. Treatm ents which primarily affect components at the photosystem II donor side are shown to increase quenching at I t and/or to suppress the secondary fluorescence rise to I2. Preillumination by single turnover saturating flashes causes I,quenching oscillating with period-4 in dependence of flash number. It is suggested that this quenching correlates with (S; + S3) states of the watersplitting enzyme system. Suppression of the secondary, I , —1; rise component is invariably found with treatments which lower electron donation rate by the watersplitting system and are known to favor the low potential form of cyt b 559. Three different mechanisms are discussed on the basis of which donor-side dependent quench­ ing could be interpreted: 1) Non-photochemical quenching by accumulation of the P 6 8 0 + radical cation. 2) Dissipative photochemical quenching at a special population of PS II centers (ßor nonB centers) displaying low donor capacity and high rates of charge recombination. 3) Dissipative photochem ical quenching via cyclic electron flow around PS II, involving alternate donors to P 68 0 + (like cyt b 559 or carotenoid in their low potential forms), which can compete when donation rate from the w ater splitting system is slowed down. The possibility of donor-side limitation also being involved in “energy dependent” quenching is discussed.