The technique of (γ-ray) radiation inactivation has been used to estimate the functional sizes of the PS II centers (PS IIα and PS IIβ) responsible for the biphasicity of the fluorescence induction curve from DCMU-inhibited spinach chloroplasts. It is found that the size of PS IIα (266 ± 5 kDa) is almost twice as large as that of PS IIβ (135 ± 5 kDa). The result clearly indicates that the two types of PS II are structurally distinct and furthermore, the difference exists in the reaction center per se. Implications on the organization of PS II reaction centers are discussed.
