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Acanthospermum hispidum D.C. was particularly susceptible to fluazifop-P-butyl, an aryloxyphenoxypropionate herbicide, and the primary action site for the herbicide was shoot apical meristem, which is also the main site of indole-3-acetic acid (IAA) biosynthesis and action. Membrane lipid peroxidation caused by increasing levels of reactive oxygen species (ROS) was considered as an action mechanism of fluazifop-P-butyl in A. hispidum . To further clarify the ROS inducing mechanism of fluazifop-P-butyl in the plant, the interactions between fluazifop-P-butyl and auxin compounds IAA or 2,4-dichlorophenoxyacetic acid (2,4-D) were studied. Haloxyfop- P -methyl, an AOPP herbicide which is inactive on A. hispidum , was used for comparison. The results showed that the growth inhibition and malondialdehyde or H 2 O 2 increases induced by fluazifop-P-butyl on A. hispidum were reversed by IAA or 2,4-D. The IAA content was decreased but the contents of three IAA oxidation metabolites, indole-3-methanol, indole-3-aldehyde and indole-3-carboxylic acid were increased by fluazifop-P-butyl in A. hispidum , but not by haloxyfop- P -methyl. The growth of A. hispidum was not inhibited by three IAA oxidative compounds. Moreover, the activities of IAA oxidase and peroxidase were increased by fluazifop-P-butyl but not by haloxyfop- P -methyl, and the increase was reversed by IAA or 2,4-D. We suggest that there is an antagonistic effect between fluazifop-P-butyl and IAA or 2,4-D, and the IAA oxidation may be involved in the action mechanism of fluazifop-P-butyl in A. hispidum . [ABSTRACT FROM AUTHOR]

Acanthospermum hispidum DC, an Asteraceae weed species, was very susceptible to fluazifop-P-butyl, but tolerant to other aryloxyphenoxypropionate herbicides, such as haloxyfop- P -methyl. However, other Asteraceae weeds including Bidens pilosa were all tolerant to fluazifop-P-butyl. Membrane lipid peroxidation by increasing the levels of reactive oxygen species (ROS) was proposed as an action mechanism of fluazifop-P-butyl in A. hispidum . To further clarify the primordial action site of fluazifop-P-butyl in this species, the effects on chlorophyll fluorescence characteristics and cytohistology of apical meristems were studied. Chlorophyll fluorescence characteristics (CFC) in sensitive A. hispidum seedlings were markedly affected by 10 μM fluazifop-P-butyl, with the dark fluorescence yield (Fo), maximal fluorescence yield (Fm), maximal PS II quantum yield (Fv/Fm), effective photosystem II (PS II) quantum yield [Y(II)], and quantum yield of regulated energy dissipation [Y(NPQ)] declining, quantum yield of nonregulated energy dissipation [Y(NO)] rising, but these measures were not affected in Bidens pilosa . The effects of fluazifop-P-butyl on chlorophyll fluorescence properties were observed on the growing point before the mature leaves by about 4–6 h. Haloxyfop- P -methyl, a control herbicide, had no effects on CFC of either A. hispidum or B. pilosa . In addition, damage to apical meristem cells of A. hispidum was observed at 6 HAT prior to changes in chlorophyll fluorescence parameters suggesting that the primary action site of fluazifop-P-butyl in this species is in the apical meristem and the effects on CFC may be the results of secondary action. [ABSTRACT FROM AUTHOR]