ИСТИНА

As high-intensity solar radiation can lead to extensive damaging of photosynthetic apparatus, higher plants and algae have developed a mechanism of non-photochemical quenching (NPQ) of chlorophyll (Chl) fluorescence. A so called xanthophyll cycle was found to play a key role in stimulating energy dissipation within light-harvesting antenna proteins by NPQ. Under high photon flux densities dark adapted sample Chl fluorescence intensity undergoes significant changes due to complex relations between rate constant of photochemical and non-photochemical quenching resulting a nonlinear time-course, which is known as induction curve. Analysis of the induction curve allows to determinate many parameters of photosynthetic apparatus. In this work we present an approach allowing the estimation of Chl fluorescence life-time during different stages of induction curve. Using time correlated single photon counting technique we studied NPQ of chloroplasts Chl fluorescence from Spinach. Picosecond laser was operated in 50 MHz regime, fluorescence signal was recorded as cycles (f(t,T) mode of Becker & Hickl SPC) with a duration of signal accumulation up to one second for each cycle. It was shown that during adaptation to actinic light Chl fluorescence life-time gradually reduces up to 30 %. This stage corresponds to NPQ observed via conventional OJIP fluorimeter (Handy-PEA, Hansatech). Treatment of chloroplasts by nigericin almost completely removed NPQ resulting high values of Chl fluorescence life-times. This fact confirms that transmembrane proton gradient plays essential role in regulation of xanthophyll cycle enzymes and NPQ of Chl fluorescence. Thus time-resolved measurements allow us to estimate relations between the rate constants of photochemical and non-photochemical quenching as fluorescence life-time is directly related to fluorescence quantum yield.