| Abstract: | A comparative study is made, at 15 degrees C, of flash-induced absorption changes around 820 nm (attributed to the primary donors of Photosystems I and II) and 705 nm (Photosystem I only), in normal chloroplasts and in chloroplasts where O2 evolution was inhibited by low pH or by Tris-treatment. At pH 7.5, with untreated chloroplasts, the absorption changes around 820 nm are shown to be due to P-700 alone. Any contribution of the primary donor of Photosystem II should be in times shorter than 60 mus. When chloroplasts are inhibited at the donor side of Photosystem II by low pH, an additional absorption change at 820 nm appears with an amplitude which, at pH 4.0, is slightly higher than the signal due to oxidized P-700. This additional signal is attributed to the primary donor of Photosystem II. It decays (t 1/2 about 180 mus) mainly by back reaction with the primary acceptor and partly by reduction by another electron donor. Acid-washed chloroplasts resuspended at pH 7.5 still present the signal due to Photosystem II (t 1/2 about 120 mus). This shows that the acid inhibition of the first secondary donor of Photosystem II is irreversible. In Tris-treated chloroplasts, absorption changes at 820 nm due to the primary donor of Photosystem II are also observed, but to a lesser extent and only after some charge accumulation at the donor side. They decay with a half-time of 120 mus. |
