Dependence of promiscuous soybean and herbaceous legumes on arbuscular mycorrhizal fungi and their response to bradyrhizobial inoculation in low P soils.

As the production of grain and herbaceous legumes is often limited by low l evels of available P in moist savanna soils, the potential for managing arb uscular mycorrhizal fungi (AMF) by selecting lines or accessions dependent on AMF as a strategy to improve plant P nutrition and productivity is requi red. The interactions between AMF and Bradyrhizobia sp. and their effects o n growth and mycorrhizal colonization of ten recent selections of promiscuo us soybean breeding lines and two herbaceous legumes (Lablab purpureus and Mucuna pruriens) were investigated. The pots contained soil (available P = 5.33 mu g P soil(-1), Bray 1) collected at Fashola from a derived savanna i n Nigeria. Mycorrhizal colonization differed among promiscuous soybean line s (ranging from 16 to 33%) and was on average 20% for mucuna and lablab. Sh oot weight of plants single or dually inoculated with AMF and Bradyrhizobia sp. were higher than those of uninoculated plants and the differences betw een lines and species were significant. Three groups of plants were obtaine d according to their mycorrhizal dependency (MD): (i) the highly dependent plants with (MD) >30%, e.g.,soybean line 1039 and mucuna; (ii) the intermed iate group, with MD between 10 and 30%, e.g., soybean line 1576 and lablab, and (iii) the majority of soybean lines (five lines out of 10) that were n ot mycorrhizal dependent. This great variability in MD and response to P ap plication among promiscuous soybean and herbaceous legumes offers a potenti al for the selection of plant germplasm able to grow in P deficient soil. ( C) 1999 Elsevier Science B.V. All rights reserved.