Studies of the phytotoxicity of saponins on weed and crop plants

Several saponins or sapogenins including beta-escin, betulin, beta-glycyrrhetinic acid, hecogenin, oleandrin, and oleanolic acid were tested in the laboratory, growth chamber, and greenhouse on various weed and crop species. Seed germination, root and shoot growth after root, foliar, or soil application, electrolyte leakage from leaf discs, and greening of etiolated plant tissues were monitored. Esterase activity using fluorescein diacetate (FDA) and p-nitrophenyl butyrate (PNPB) was also assayed. The compounds had differential effects on these parameters, depending on the species. The effects of these compounds on electrolyte leakage ranged from no effect to a 10-fold increase above untreated tissue levels after 72 h. Escin increased FDA activity up to 35% above untreated tissue, but other compounds caused no effect or reduced FDA activity. PNPB activity was generally not affected. In greening studies of excised tissue, escin reduced chlorophyll production by 90-100% in several species, with other compounds giving intermediate or no effects. Foliar application (1.0 mM) in the greenhouse had no substantial effect (visible or shoot biomass) on 10 species. However, in short-term (8 to 13 day) tests, beta-escin, applied to soil at 88 and 350 mu mol/kg soil, drastically reduced emergence in barnyardgrass (Echinochloa crus-galli L. Beauv.), hemp sesbania [Sesbania exaltata (Raf.) Rydb. ex A.W. Hill], wheat (Triticum aestivum L.), and soybean [Glycine mar (L.) Merr.]. beta-Escin also reduced growth in all species but soybean, and barnyardgrass was the most sensitive species tested. Results are discussed in relation to the role of these compounds as plant growth-regulating natural products.