| The potential allelopathic activity of plant residues from sunflower (Helianthus annus L.) was evaluated in cotton (Gossypium hirsutum L.), soybean (Glycine max (L.) Merr.), and with associated weeds. Field experiments were conducted at Fayetteville AR, from 1989 to 1991 on a Pembroke silt loam. Fresh residue of immature sunflower incorporated just before cotton and soybean planting provided reduction in the emergence of entireleaf morningglory (Ipomoea hederacea var. integriuscula Gray), pitted morningglory (I. lacunosa L.), prickly sida (Sida spinosa L.), velvetleaf (Abutilon theoprasti Medicus.), and total weeds combined. Inhibitory effects to these weed species were also evident under greenhouse conditions. Control of entireleaf morningglory was improved when a preemergence application of metribuzin (4-amino-6-(1,1-dimethyl)-3-(methylthio)-1,2,4-triazin-5(4H)-one) was combined with residues of immature sunflower in soybean. Seedcotton yields were higher with 16000 kg ha;Residue of mature sunflower incorporated after mid-August provided early reduction the next year of the emergence of prickly sida, large crabgrass (Digitaria sanguinalis (L.) Scop.), tumble pigweed (Amaranthus albus L.), and total weeds combined. Among nonweeded plots, seedcotton yields were higher with 8000, 16000, and 32000 kg ha;Analysis of leaf tissues of sunflower by high pressure liquid chromatography indicated that concentrations of scopoletin and chlorogenic acid (two potential allelochemicals in sunflower) tend to increase during reproductive growth stages compared to early vegetative stages. The results of a soil bioassay indicated that microbial activity may not be involved in the inhibitory effect of sunflower during early stages of residue decomposition. In the greenhouse, emergence of entireleaf morningglory, pitted morningglory, velvetleaf, and prickly sida was mostly affected when seeds were sown immediately after incorporation of sunflower residue, indicating that dissipation of toxic chemicals may occur in a short period of time. |
