Light‐intensity Dependence of the in Vivo Fluorescence Lifetime of Chlorophyll*

Phase‐fluorometer measurements of the fluorescence lifetime, τ, from chlorophyll in Chlorella , Bishop's 8 and 11 Scenedesmus mutants , sugarbeet leaf and chloroplast fragments demonstrate that: τ is independent of modulation frequency at 27 and 14 mc. in the experimental‐wavelength range from 650 to 735 nm (with blue or blue‐green excitation); with Chlorelfa and chloroplast fragments τ rises hyperbolically with intensity to τ max about 2 nsec and 0·7 nsec respectively; DCMU poisoned Chlorella and sugarbeet leaf as well as the mutants have τ values near 2 nsec; the lifetime‐incident intensity relationship for Chlorella and chloroplast fragments is quantitatively similar to the incident‐intensity dependence of fluorescence yield and oxygen evolution and thus supports the hypothesis that these three measuring variables are controlled by the concentration of ‘open’ trapping systems; τ is independent of emission wave‐length to suggest that fluorescence is dominated by a single chlorophyll species. The reaction velocity‐lifetime correlation indicates that fluorescence behavior is directly controlled by system II.