Intensity effects on the fluorescence of in vivo chlorophyll

A technique for measuring relative quantum yields of fluorescence with a picosecond streak camera is described. We show that Chlorella pyrenoidosa exhibit an intensity dependent quantum yield when irradiated with single picosecond light pulses. This effect also occurs under conditions that inhibit the activity of the reaction centres, which can therefore be excluded as the cause. When a pulse train (pulse separation 6.9 ns) was used, the quantum yield was further reduced by the light absorbed from previous pulses, which indicates the formation of a quenching species having a relatively long lifetime. Absolute quantum yields calculated from the fluorescence decay show that single excitation pulses of 3 - 10(13) photons/cm2 give results comparable to those obtained by very low intensity methods.