Plants constitute a source of novel phytotoxic compounds to be explored in searching for effective and environmentally safe herbicides. From a previous screening of plant extracts for their phytotoxicity, a dichloromethane extract of Ammi visnaga (L.) Lam. was selected for further study. Phytotoxicity-guided fractionation of this extract yielded two furanochromones, khellin and visnagin, for which herbicidal activity had not been described before. Khellin and visnagin were phytotoxic to model species lettuce (Lactuca sativa) and duckweed (Lemna paucicostata), with IC values ranging from 110 to 175 μM. These compounds also inhibited the growth and germination of a diverse group of weeds at 0.5 and 1 mM. These weeds included five grasses [ryegrass (Lolium multiflorum), barnyardgrass (Echinocloa crus-galli), crabgrass (Digitaria sanguinalis), foxtail (Setaria italica), and millet (Panicum sp.)] and two broadleaf species [morningglory (Ipomea sp.) and velvetleaf (Abutilon theophrasti)]. During greenhouse studies visnagin was the most active and showed significant contact postemergence herbicidal activity on velvetleaf and crabgrass at 2 kg active ingredient (ai) ha. Moreover, its effect at 4 kg ai ha was comparable to the bioherbicide pelargonic acid at the same rate. The mode of action of khellin and visnagin was not a light-dependent process. Both compounds caused membrane destabilization, photosynthetic efficiency reduction, inhibition of cell division, and cell death. These results support the potential of visnagin and, possibly, khellin as bioherbicides or lead molecules for the development of new herbicides.

Pre‐emergence herbicides are taken up by seeds before germination and by roots, hypocotyls, cotyledons, coleoptiles or leaves before emergence, whereas post‐emergence herbicides are taken up primarily by foliage and stems. Most modern pre‐emergence herbicides are lipophilic, but post‐emergence herbicides may be lipophilic or hydrophilic. The metabolic conversion of herbicides to inactive or active metabolites after plant uptake is of major importance for some compound classes. Several herbicides are proherbicides as for example some acetyl‐coenzyme A carboxylase (ACCase)‐inhibitors. The physicochemical characteristics of proherbicides and herbicides are usually unrelated. A major role can be attributed to the site of action at a cellular level. A great number of herbicides such as photosystem II (PS II)‐inhibitors, protoporphyrinogen oxidase (PPO)‐inhibitors or carotenoid biosynthesis inhibitors require light for activity. Others, such as cellulose‐biosynthesis and mitotic inhibitors seem to be primarily active in belowground organs. Several lipophilic barriers against the uptake of xenobiotics exist in aboveground and belowground plant parts. The relevance of these barriers needs, however, further clarification. Uptake and translocation models are valuable tools for the explanation of the potential movement of compounds. Many factors other than uptake and translocation have, however, to be considered for the design of herbicides. For post‐emergence herbicides, ultraviolet (UV) light stability, stability in formulations, and mixability with other agrochemicals have to be kept in mind while, in addition to the aforementioned factors soil interaction plays a major role for pre‐emergence herbicides. In our opinion, general physicochemical characteristics of pre‐ or post‐emergence herbicides do, unfortunately not exist yet. © 2021 The Authors. Pest Management Science published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.

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Starting from bulk chemicals, novel isoxazoledicarboxylic acid esters were prepared and converted to the corresponding dicarboxylic acid monoamides in a few reaction steps and in high overall yields. Key intermediates for the cycloaddition step were chlorooximinoacetates or amides as nitrile oxide equivalents, prepared in one-pot reactions from diketene or acetoacetic acid esters. Isoxazoledicarboxylic acid monoamides combined good herbicidal activity with chemical flexibility. They were characterized as inhibitors of photosynthesis. Particularly, they affected the photosynthetic electron transport in photosystem I1 and the rate of carbon dioxide assimilation. Applied postemergence, the new compounds were found to control a broad spectrum of key weeds in corn. Excellent activity was found on Abutilon theophrasti (L.) Medik., Amaranthus retrojlexus L., Chenopodium album L., Ipomoea spp., Polygonum persicaria L., Solanum nigrum L. and Xanthium strumarium L. with rates between 0.2 and 0.5 kg ha-'. As a side-effect, the compounds also showed activity against grass weeds. The compounds are excellent tank-mix partners, e.g. for sulfonylureas, to complete the weed spectrum (Chenopodium album L., Solanum nigrum L.) and/or to reduce the risk of developing herbicide-resistant weeds.