Primary Events, Energy Transfer, and Reactions in Photosynthetic Units: Quantum Accumulation in Photosynthetic Oxygen Evolution

Three independent methods have been used to determine the size of the quantum accumulation unit in green plant photosynthesis. This unit is defined as that group of pigment molecules within which quantal absorption acts must take place leading to the evolution of a single O₂ molecule. All three methods take advantage of the nonlinearity of oxygen yield with light dose at very low dosages. The experimental values of this unit size, based on an assumed model for the charge cooperation in O₂ evolution, ranging from 800 to 1600, suggest that there is either limited energy transfer between energy-trapping units or chemical cooperation among oxygen precursors formed in several neighboring energy-trapping units. Widely diffusible essential precursors to molecular oxygen are ruled out by these results. Inhibition studies show that O₂ evolution is blocked when 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU) is added to chloroplasts after two preliminary flashes and before a third flash which would have yielded O₂ in the absence of DCMU. This experiment is interpreted as evidence that the site of DCMU inhibition is on the oxidizing side of system II. Pretreatment of chloroplasts with large concentrations of Tris, previously believed to destroy O₂ evolution by blocking an essential reaction in the electron chain between water and system II, may be alternately interpreted as promoting the dark reversal of the system II light-induced electron transfer.