Mycorrhizae, biocides, and biocontrol, .1. Herbicide-mycorrhiza interactions in soybean and cocklebur treated with bentazon

Interactions between herbicides and vesicular-arbuscular mycorrhizal (VAM) fungi are little-known but may differentially affect the development of tolerant and susceptible plants. We conducted this greenhouse study to determine if VAM fungi modify the effects of the herbicide bentazon (3-isopropyl-1H-2,1,3-benzothidiazin-(4)3H-one 2,2-dioxide) on soybean (Glycine mar (L.) Merr.) and common cocklebur (Xanthium strumarium L.). The experiment was designed as an eight-treatment, 2 x 2 x 2 factorial, Individual potted soybean or cocklebur plants were grown in a high (28 mg kg(-1)) P-content soil. The plants were colonized by VAM fungi (a mixture of Glomus etunicatum Becker and Gerdemann, Glomus leptotichum Schenck and Smith, and Glomus mosseae (Nicol. and Gerd.) Gerd. and Trappe) and sprayed with bentazon. Alternatively, they were exposed only to VAM fungi or to the herbicide, or to neither of these factors. The effects of VAM fungi on plant dry mass were small, but larger in cocklebur than in soybean, especially for the herbicide-treated plants. Root/shoot ratios in cocklebur were larger and shoot dry matter content smaller than those in soybean. The VAM effect in the herbicide-treated plants was greater than in the nontreated ones for both parameters. Colonized root length in cocklebur was drastically reduced by the herbicide (43%), as were leaf dry weight and N, P, and K concentrations. Changes were small, and generally not significant in soybean. Gradual necrosis of treated cocklebur shoots was related to an accelerated loss of shoot dry matter, especially in VAM plants. The data suggest that changes in source-sink relations following herbicide treatment favor enhanced export of shoot dry matter to the roots in susceptible relative to tolerant plants. This biomass may be preferentially available for export from susceptible-plant roots to the soil system through the VAM mycelium.