Mycorrhizae, biocides, and biocontrol. Mycorrhizal fungi enhance weed control and crop growth in a soybean-cocklebur association treated with the herbicide bentazon

The mediation of herbicide effects on weed-crop combinations by vesicular-arbuscular mycorrhizal (VAM) fungi is essentially unknown. We conducted a greenhouse study to determine if VAM fungi, whose hyphae interconnect the roots of adjacent plants, modify herbicide effects by enhancing nutrient fluxes between associated plants. Soybean (Glyc ine max (L.) Merr.) and common cocklebur ( X a n t h i u m s t rumar ium L.) plants were grown together in pots (1.5 L) in a high-P (28 mg kg -1) soil. They were sprayed with the herbicide, bentazon (BEN, 3-isopropyl-lH-2,1,3-benzothidiazine-(4)3H-one 2,2-dioxide), at dose rates of 0, 1/3, 2/3, 3/3, and 4 /3 of field recommendation (FR, 1.12 kg a.i. ha1) while in the 7to 8-leaf stage. Labelled N (1 mL of 100 mM tSNHaNO3, 98 atom percent 15N) was applied to the cocklebur leaves 4 days before spraying and then assayed in the soybean leaves at harvest. Growth and nutrient contents of + VAM soybean shoots were enhanced only at the intermediate FR levels, while shoot growth in adjacent cocklebur was inhibited beyond the extent measured in VAM plants. Labelled N was at natural abundance in both + VAM and VAM soybean leaves at 4 /3-FR, but at 1 /3-FR to 3 /3 -FR 15N abundance was significantly higher in + VAM than in V A M plants. These results suggest that shifts in source-sink relations occurred both within each plant and between plants as a result of the selective stress imposed on cocklebur. Moreover, this shift in competitiveness permitted a VAM-mediated flux of nutrients from weed to crop.