Fluorescence and delayed light emission in Tris-washed chloroplasts.

Yamashita and Butler [I] have shown that extraction of chloroplasts with a high concentration of Tris (0.8 M, pH 8.0) eliminates flow of electrons from water to nicotinamide adenine dinucleotide phosphate (NADP+)* or other Hill oxidants. They suggest that washing with Tris causes a block on the water side of the electron transport chain. Addition of electron donors such as reduced phenylene diamine (PDA) restores the electron flow to NADP’. This donation of electrons by PDA is sensitive to the herbicide diuron (DCMU), a well known inhibitor of photosystem II (PS II) reactions. Measurements of chlorophyll a (Chl) fluorescence in Tris-washed chloroplasts (without any added donor) show a very low level of fluorescence, but the addition of DCMU increases the yield of fluorescence [I]. One could suggest that the low yield of Chl fluorescence in Tris-washed chloroplasts is due to “trickling” of electron flow from some endogenous donor (cf. [ 11). However, if this were so, we should expect the fluorescence yield to slowly rise and reach the maximum yield at longer times. But, this never happens. No satisfactory explanation was available for this peculiar behavior of fluorescence