Three major conclusions were made in this work (1) From 100 μs to 20 ms (0-I phase of the fluorescent transient), there is parallel formation of oxidized P700, P700⁺, and reduced Q_A, Q_A^-; here, the two photosystems operate independent of each other, whereas from 20 ms to 200 ms (I-P phase of the fluorescence transient), there is a parallel relationship between the formation of re-reduced P700 and of Q_A^- when plastoquinol (formed by PSII) has begun to donate electrons towards PSl. This conclusion is confirmed by experiments where DCMU is used to block electron flow between PSII and PSI. (2) During dark-adaptation, the availability of intersystem electrons is in itself not enough to prevent the transient accumulation of P700⁺ during a second light pulse (3) In the first seconds after a 1 s pulse of light, the trapping rate of electrons is higher than in dark-adapted leaves.