Quantitative analysis of State 1–State 2 transitions in intact leaves using modulated fluorimetry — evidence for changes in the absorption cross-section of the two photosystems during state transitions

Abstract

Using a combination of modulated and non-modulated light with synchronized detection it has been possible to monitor State 1–State 2 transitions in intact leaves as changes in the yield of modulated chlorophyll fluorescence. In the presence of excess far-red non-modulated light (713 nm) absorbed mainly by Photosystem I (PS I), the modulated fluorescence intensity was taken to represent F o — the emission yield which occurs when the reaction centres of Photosystem II (PS II) are all open. On the other hand, superimposing saturating non-modulated wide-band, blue-green light resulted in a transitory maximum yield of modulated chlorophyll fluorescence, F m, due to the total closure of the PS II reaction centres. In the absence of these additional lights the fluorescence level assumed a steady-state value, F s, between F o and F m. All these parameters changed as the leaf slowly adapted to light of a given spectral composition. It was found that both F o and F m increased reversibly (by about 15–20%) during the transition from State 2 to State 1 such that the ratio of F m to F o remained constant, indicative of changes in absorption cross-section of PS II and PS I rather than alterations in ‘spillover’ which would cause preferential changes in F m. It was also possible to estimate the fractions of light, β and α, channeled to PS II and PS I, respectively, from the values of F o, F m and F s. In one approach, β was estimated in State 1, using the assumption that α + β = 1, and its variation during the subsequent state transition was assumed to follow proportional changes in F o (or F m). It was found that in State 2 there is a small loss (about 4%) of the total utilization of light in both photosystems. However, if such loss is neglected, assuming α + β is always unity, the calculated β was found to vary in the same direction and almost with the same magnitude as F o (or F m), indicating independently that a change in absorption cross-section in PS II (and PS I) had occurred. Consistent with these data were the light-saturation curves for the non-modulated far-red light-quenching effect in bringing the fluorescence from F s to F o in States 1 and 2. The ratio of the initial slopes of these curves indicates quantitatively both redistribution of light between PS I and PS II during the State 1–State 2 transitions and a partial loss of excitation energy in State 2.