MYCORRHIZAL FUNGI INFLUENCE COMPETITION IN A WHEAT-RYEGRASS ASSOCIATION TREATED WITH THE HERBICIDE DICLOFOP

Vesicular-arbuscular mycorrhizal (VAM) fungi interconnect the root sys tems of adjacent plants, and mediate the transfer of nutrients between them. The objective of this study was to determine if the VAM fungus Glomus deserticola Trappe, Bless and Menge enhanced such a transfer in a crop-weed association when one associated plant was selectively wea kened by an herbicide. Wheat (Triticum aestivum L.) or perennial ryegr ass (Lolium perenne L.) plants were grown with or without VAM fungi ei ther in monocultures, or together as an intercrop for 7 weeks, when th e herbicide diclofop was applied as a soil drench at dose rates of 0%, 10%, 50%, 100% and 1000% of the field recommendation (0.9 kg ha(-1)). Seed yield at harvest (13 weeks) was significantly greater for the +V AM than for the -VAM intercropped wheat plants at all dose rates. When grown in monoculture, wheat yield was greater in the -VAM plants. Sho ot growth of intercropped +VAM wheat was enhanced, while that of ryegr ass was inhibited. In monoculture, the plant dry masses of both -VAM w heat and ryegrass were greater than those of the +VAM plants, while in intercrop the +VAM wheat plants fared better. The data showed that (1 ) the herbicide inhibited root colonization by C. deserticola in wheat but not in ryegrass, and (2) when grown together, wheat growth and yi eld were enhanced and ryegrass growth was inhibited in +VAM herbicide- treated associations. We interpret the findings as a change in interpl ant source-sink relations upon treatment with diclofop: ryegrass roots became sources of nutrients to the tolerant wheat roots (stronger sin ks), and the transfer process was enhanced by the VAM mycelium common to both. (C) 1997 Elsevier Science B.V.