Increased Synthesis of Photosystem II in Triticum vulgare when Grown in the Presence of BAS 13-338

Photosynthesis, Electron Transport, Chlorophyll Fluorescence, Photosystem II, BAS 13-338 Addition of BAS 13-338 (4-chloro-5-dim ethylam ino-2-phenyl-3(2H )-pyridazinone) to a sus­ pension of chloroplast thylakoids caused an increase in the / level o f chlorophyll fluorescence induction without affecting the F0 level and with a slight decrease in the Fmax level in a manner similar to the addition o f DCM U to a thylakoid suspension. Addition o f BAS 13-338 also inhibited the rate o f Hill reaction H20 -» dichlorophenol indophenol with 50% inhibition occurring at about 10 hm BAS 13-338. The inhibition was not reversed by diphenyl carbazide used as an artificial electron donor to photosystem II. These results suggest that the site o f inhibi­ tion by BAS 13-338 is between Q (next to the primary electron acceptor) and plastoquinone. When the plants were grown in the presence o f sublethal dose o f BAS 13-338, the following changes were noted in the thylakoids o f the treated plants as compared to the thylakoids isolated from the control plants: The F0 and the normalized variable fluorescence (4F/ F0) levels increased, chlorophyll a/b ratio decreased, chlorophyll/P700 ratio increased. Furthermore, the rate o f photosystem II electron transport both under saturated intensity and the limiting intensity of illumination increased, and the ratio o f plastoquinone to Q decreased. These observations have been interpreted as due to an increase in the ratio o f photosystem II to photosystem I in plants grown in the presence o f BAS 13-338.