Photoinhibition Sites in Chloroplasts of Spinach
WU Ji-Tuo, SHEN Yun-Kang(Shanghai Institute of Plant Physiology, Academia Sinica, Shanghai 200032)
Isolated spinach chloroplasts in high light were much more easily inactivated than those in the dark. It has been argued for a long time what the photoinhibition sites are. Our results from measurements of partial electron transport activity and C_(550) suggested that, in the presence of oxygen, photoinhibition sites in thylakoid be lccated not only in P680 but also at many other points, which included oxidizing and reducing sites of P680, PSI and membrane permeability of thylakoids (Fig. 1, 2). By using SDS-PAGE and the technique of ~(14)C-atrazine binding, it was found that almost every thylakoid membrane polypeptide degraded (Fig. 3, 5). When thylakoids were illuminated in the absence of oxygen, PS Ⅰ reaction center and Q_B protein were destroyed while other thylakoid membrane polypeptides remained unchanged (Fig. 6, 7). Therefore, it is suggested that the effect on photoinhibition depends on whether there is oxygen in the isolated chloroplasts suspension during illumination. In the absence of oygen, photoinhibition occurs mainly around PSⅠ reaction center. In the presence of oxygen, nonspecificity of photoinhibition sites is perhaps due to the involvement of oxygen in the degradation of thylakoid membrane polypeptides in general.