The variable chlorophyll (Chl) a fluorescence yield is related to the photochemical activity of photosystem II (PS II) of the oxygen evolving organisms. The kinetics of the fluorescence rise from the minimal yield Fo to the maximal yield Fm is a monitor of the accumulation of net reduced Qa with time in both active (Qb-containing) and inactive (non-Qb) PS II centers. The measurements of true Fo and that of the complete fluorescence transient from true Fo to Fm are useful in obtaining a kinetic picture of PS II activity. Using a shutter-less system (Plant Efficiency Analyzer, Hansatech, UK) that is capable of providing the first measured point at about 20 microseconds and that allows data accumulation over several orders of magnitude of time, we have measured the complete fluorescence transient in low and moderate (up to 700 W m^-2 ) light intensities in several photosynthetic systems (higher plant leaves and chloroplasts; and the cell suspensions of green alga Chlamydomonas reinhardtii and several of its herbicide-resistant mutants, altered in single amino acids in its Dl protein). In all cases, the fluorescence transient follows a regular pattern of O-J-I-P—T, where two intermediate inflections J (at about 2 ms) and I (at about 20 ms) appear between Fo and Fm levels. Furthermore, the ratio of Fm to Fo is about 5 in all cases and the lowered published ratio in several cases is suggested to be due to the J level being mistaken for Fo. We also present data on the effects of varying the dark times between preillumination and measurements of the transient, on the intensity dependence, and on the effect of the addition of diuron. The relationship of the 0-J rise to the fast fluorescence rise observed by other investigators will also be discussed.

Chlorophyll (Chl) a fluorescence transients provide information on the photochemical efficiency of photosystem I I (PS I I) of the oxygen evolving organisms ^1,2,3,4. Fo ( or the 0 level) level is the “Instantaneous” low fluorescence when photochemical efficiency is maximal; here, the concentration of the electron acceptor Qa is maximal. Measurement of true Fo level is not a trivial problem in instruments that use camera shutters. Most mechanical shutters have a full opening time of one or more ms. On the other hand, light emitting diode has the advantage that no shutter is needed and the time resolution can be as short as 500 ns.