Searching for new natural allelochemicals as herbicides, in our laboratory, we isolated from the aerial parts of Croton ciliatoglanduliferus the labdanes 8a,15-dihydroxy-labdane (L1  ) and 15-O-acetyl,15a-hydroxylabdane (L2  ) and two flavonoids (retusin and paquipodol), all of them proved to act as Hill reaction inhibitors. Pachypodol, L1  and L2  were found to be the most active. The aim of this study was to prepare and evaluate several synthetic derivatives of compound L1 , and different flavonoids to increase the potential herbicide activity. From labdane L1  derivatives (L3-L6  ) were prepared, and the flavonoids, quercetin, apigenin, genistein and eupatorin, were purchased from Sigma-Aldrich Co. The derivative 15-O-benzoyl-8a-hydroxylabdane (L5  ) was seven times more active than paquipodol as Hill reaction inhibitor; the flavonoids, retusin, pachypodol, quercetin, apigenin, genistein and eupatorin acted as Hill reaction inhibitors with the exception of apigenin which was non-active. The complex L5   with the adjuvant 2-hydroxypropyl-�-cyclodextrin (L5:HPB  ) was sprayed on Physalis ixocarpa (tomato)plants in vivo, 48 h later the complex inhibited PSII by transforming the active reaction centers into silent reaction centers, after 72 h this effect disappeared, probably due to plant metabolism. The complex L5:HPB  showed another inhibition site on Trifolium alexandrinum (clover) leaves, at PQH2 reduction, decreasing the dry-biomass of tomato by 30 to 40 % and clover plants by 46 %. Equally treated Lolium perenne plantswere not affected. Further studies indicated that flavonoids retusin, pachypodol, quercetin, and genistein induced the appearance of K-and J-bands at 72 h after treatment of P. ixocarpa leaves, determined by the fluorescence of the chlorophyll a of PSII, indicating that these compounds need time to penetrate the plant. After 7 days of treatment the bands decreased. All tested flavonoids enhanced seed respiration in a concentration dependent manner, being quercetin the most active, probably by uncoupling the mitochondrial respiration from the oxidative phosphorylation. Flavonoids behaved as pre- and post-emergent herbicides, with selectivity for dicotyledonous plants. Quercetin and genistein were the most active.

Labdanes and flavonoids as photosynthesis light reaction inhibitors and herbicides

248th National Meeting (2014)

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