| Abstract
| - Effects of the alleochemical sorgoleone on photosynthetic electrontransport by oxygen-evolvingchloroplast thylakoids and Triton X-100-prepared Photosystem II (PSII)membranes were analyzed.The Hill activity of the thylakoids proved to be at least assensitive to inhibition by sorgoleone asit was to DCMU [3-(3,4-dichlorophenyl)-1,1-dimethylurea], a potentherbicidal inhibitor of PSII.However, a Photosystem I (PSI) partial reaction was not affectedby a 10-fold greater concentrationof sorgoleone than is required for complete inhibition of Hillactivity. Measurements of flash-inducedchlorophyll a variable fluorescence showed that sorgoleoneneither dissipated excitation energy nordiminished the amplitude of chlorophyll a variablefluorescence. However, it inhibited the decay ofvariable fluorescence as effectively as DCMU, which blocks theoxidation of the PSII-reduced primaryelectron acceptor, Q-A, by the PSII secondaryelectron acceptor, QB, by displacing QB fromthe D1protein. Additionally, sorgoleone competitively inhibited thebinding of [14C]atrazine to the QBlocus.Increasing durations of trypsin proteolysis of the PSII membranesor thylakoids and of the QB-binding niche itself caused parallel losses of inhibition ofO2 evolution from sorgoleone and DCMU,as well as from bromoxynil, a phenol-type herbicide also binding to theQB locus. Keywords: Allelopathy; sorgoleone; photosystem II; electron transfer;herbicide
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