Abstract: Curvularia eragrostidis strain QZ2000, a potential biocontrol candidate for large crabgrass [Digitaria sanguinalis (L.) Scop], produced a mycotoxin with herbicidal activity, which was identified as α,β-dehydrocurvularin. Inhibition of photosynthetic electron transport in isolated thylakoids from large crabgrass was investigated. The electron transport activities of the photosystem II (PS II) in isolated thylakoids was significantly inhibited by the mycotoxin. At a concentration of 0.688 mmol·L-1, the electron transport rate of PS II was reduced by 19.37%. The mycotoxin had a little effect on the electron transport rate of the photosystemⅠ (PSⅠ). When the detached leaves were treated with mycotoxin (0.516 mmol·L-1) for 24 h, Fv/Fo, Fv/Fm, ΦPSⅡ, qP, qN, Fm values of large crabgrass declined significantly, but Fo values increased considerably. In the experiment, the changes of chlorophyll fluorescence parameter suggested that the mycotoxin was similar as herbicides atrazine, simazine, and diuron with inhibitory activities on the PS II, namely, the mycotoxin may damage the PS II reaction center and blocks the reoxidation of the primary electron acceptor (QA). These results indicate that the mode of action of mycotoxin might be the damage of PS II reaction center and inhibition of the reoxidation of the primary electron acceptor (QA), which resulted in the inhibition in electron transport between PS II reaction center and QA or between QA and the second electron acceptor (QB). Sequentially photophosphorylation and carbon assimilation were inhibited and metabolism of large crabgrass was disturbed. The decrease of qN value suggested that mycotoxin caused the accumulation of reductive electron acceptor and increased the production of the free radicals. This could be the cause of the damage of PS II reaction center.

Key words: Curvularia eragrostidis, Digitaria sanguinalis, α, β-dehydrocurvularin, Photosystem II, Chlorophyll fluorescence, Herbicidal activity