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Greenhouse and growth chamber investigations were undertaken using selected Leucaena and Sesbania species to determine the extent to which root morphological characteristics and rhizosphere acid production could explain differences in mycorrhizal dependency of host plants. Compared with the moderately to very highly mycorrhizal-dependent Leucaena species, the marginally to moderately dependent Sesbania species were characterized by higher root mass, higher root density, higher root surface area, higher root length, smaller root diameter, higher percentage of root hair incidence, higher shoot to root ratio, and higher total P uptake. The two groups of species were not consistently different from each other with respect to mycorrhizal colonization level, root hair diameter, root hair length, P uptake per unit root surface area, and acid production in agar media. A stepwise regression model in which mycorrhizal dependency (MD) was used as the dependent variable and root characteristics as independent variables suggested that root mass, root hair length, root diameter, root density, and root hair incidence were important determinants of MD, with root mass accounting for 65.5% of the variability. The results suggest that differences in the mycorrhizal dependency of host species can be largely predicted from root characteristics data. Key words: Brassica, Leucaena, Sesbania, P uptake, root hair, root mass.

SUMMARYInteraction between Glomus fasciailatiis and Rhizobium japonicum and their effects on soybean in the field was studied in a phosphorus deficient sandy loam soil with pH 5 6. The number, dry weight and nitrogen content of the root nodules in plants inoculated with Glomus plus RJiizobium were significantly more compared to plants inoculated with only Rhizobium. Rhizobium inoculation did not have any significant eiTect on sporulation of CJ. fasciculatus in the rhizosphere. Although soybean plants inoculated witb G. fasciculatus recorded increased phosphorus content, dry weight and grain yield than uninoculated plants the differences were not statistically significant.In RJiizobium only inoculation markedly increased the nitrogen content of the plant and grain yield. Dual inoculation with both the symbionts increased significantly the dry weight of the shoot and its nitrogen content over single inoculation with either Glomus or RJiizobium. These results suggest that vesicular-arbuscular (VA) mycorrliiza can greatly assist nodulation and nitrogen fixation in field growia soybean inoculated with rhizobia.

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The level of vesicular-arbuscular mycorrhizal (VAM) infection in the roots of Leucaena grown in a sand-soil mixture in the greenhouse increased rapidly with time and reached a peak value of 84 % at 30 days from planting. The pattern of immobile nutrient uptake and accumulation closely paralleled that of the development of infection, particularly during the first 10-30 days after planting. Significant changes in dry matter yield were also observed only after a significant portion of the root length was colonized by Glomus aggregatum. The development of VAM infection was not accompanied by growth depression at any of the sampling periods. However, VAM roots had very high levels of Cu which was not translocated to shoots. It is hypothesized that such a diversion of Cu by the endophyte from the host could cause growth depression under conditions where the soil volume is supplied with sub-optimal levels of Cu.