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The transformation of quantum energy to chemical energy in photosynthesis at some point must involve the separation of charges. To attain high quantum yields at low exciting-light intensities, these charges must be efficiently stabilized to prevent their back-reaction and mutual annihilation. Using energy considerations and evidence from delayed light and fluorescence, Arnold' has proposed an electron-hole model of the photosynthetic unit in which charge stabilization is realized by the chemical use of only one of the charges from any absorption act. The opposite, unused charges reunite with a probability of emitting a light quantum which may appear as fluorescence or delayed light. In his model, two quanta are required to produce both the oxidizing and reducing moieties at their respective reaction centers. This study shows that long-term delayed light emission by dark-adapted algae at room temperature is a two quantum-hit process-delayed light being emitted only after the second hit. Delayed light emission varies as the square of intensity of flashes of low-energy exciting light. Materials and Methods.-The delayed light apparatus is shown schematically in Figure 1. Except for the additional light sources, it is similar to the pumping system of Strehler and Arnold.2 Algae are pumped through several elements connected by black rubber tubing (RT in Fig. 1). The algal suspension was first equilibrated with 5 per cent CO2 in air and the desired continuous preillumination light (P) for at least four minutes in a 2-liter lollypop (L). When the pump was started, the algae were plunged into darkness as they were pulled into the rubber tubing. At given distances down the tubing the algae passed through short lengths of glass tubing into which the preillumination (FP) and/or assay (FA) lights could be introduced. These continuous lights through the glass tubing appeared to the algae as flashes whose duration depended upon the length of the glass tube exposed and the rate of flow. The algae then passed through additional black tubing into a 20-cm coil of glass tubing (S) which was taped to the face of a photomultiplier

Two quantum-hit requirement for delayed light emission from photosynthetic green algae.