The absorption changes at 254 nm which are coupled with the formation of plastohydroquinone (PQ=) depend strongly on the intensity and duration of the actinic light (fig. 2). At an intensity of 106 erg·sec-1·cm-2 and a duration of 0,1 sec, maximal reduction is obtained. The reduction of chlorophyll-ai ° was followed by absorption changes at 703 nm. From the changes at 254 and 703 nm (fig. 3 and tab. 1) follows: PQ=: Chl-aI = 5:1 (with Δε = 1,3·104l · Mol-1· cm-1 for PQ at 254 nm and Δε = 3,6 · 104 l · Mol-1 · cm-1 for Chl-aI at 703 nm). This value corresponds to a pool of plastoquinone with a dynamic capacity of ten electrons. This value is exactly the same as that which was derived in another way in the preceding paper 3. The time course of the reduction of plastoquinone is given by an exponential function (figs. 4 and 5) with a rate constant for the reduction k1′= 35 sec-1 at 25 °C. Together with the rate constant for the reoxidation (k2′ = 11 sec-1, see next paser 6), it follows for the total photochemically engaged plastoquinone PQ0: Chl-aI = 6.5: 1.