Time-resolved chlorophyll fluorescence spectra of intact leaves

Room temperature single photon counting measurements on intact leaves at low excitation energies have been analyzed using a four exponential kinetic model. The observed lifetimes and relative yields of the components have been compared to those obtained on isolated chloroplasts of the same species. At steady state conditions of fluorescence (Fs-level, reached after 5 min of preillumination with 625 nm pulsed laser light), the overall decay is characterized by lifetimes of approx. 20-30 ps (τ₁), 80-100 ps (τ₂), 400-450 ps (τ₃), and 800-900 ps (τ₄). By closing the reaction centers of PS II (application of the herbicide DCMU) the lifetimes of the two slowest components τ₃ and τ₄ increase by a factor of 4. The lifetimes of the two fastest components (τ₁, and τ₂) were found to be independent of PS II trap closure. A comparison of literature lifetime data of isolated pigment proteins (Hodges and Moya, 1986; Wittmershaus et al., 1987) with our results, obtained for the in vivo plant system suggsts that two fast decays can be attributed to PS I whereas three lifetime components are necessary to describe the fluorescence decay of PS II.