Further results on the photoactive chlorophyll aII in photosynthesis.

1. The flash-induced difference spectra of chlorophyll aII in untreated and in heat-treated spinach chloroplasts are presented. 2. Heat-treatment leads to an increase of the half-life of the chlorophyll aII absorbance changes. After the addition of hydroquinone plus sodium ascorbate the half-life of the chlorophyll aII reaction returns nearly to the value in untreated chloroplasts. 3. In digitonin-treated chloroplasts, or in the presence of histone, the chlorophyll aII activity is low when the electron acceptor is benzylviologen and high when ferricyanide is used. 4. The half-life of the chlorophyll aII reaction depends on the temperature of the reaction mixture. The activation energy of the chlorophyll aII reaction is about 7.5 kcal/mol. 5. From the experimental results we conclude that in heat-treated (and in Tris-washed and aged) chloroplasts the linear electron flow in Photosystem II is replaced by a cyclic one, which is sensitive to 3-(3,4-dichlorophenyl)-1,1-dimethylurea.