1989. The Glycirte-Glorrtus-Bradyrhizobium symbiosis. IX. Nutritional, morphological and physiological responses of nodulated soybean to geographic isolates of the mycorrhizal fungus Glomus mosseae. -Physiol. Plant. 76: 226-232.The objective of the work was to determine differences in plant response to geographic isolates of a vesicular-arbuscular mycorrhizal (VAM) fungus, and to demonstrate the need for such determinations in the selection of desirable host-endophyte combinations for practical applications. Soybean {Glycine max (L.) Merr.) plants were inoculated with Bradyrhizoblum japonlcum and isolates of the VAM-fungal morphospecies Glomus mosseae (Nicol. & Gerd.) Gerd. and Trappe, collected from an arid (AR), semiarid (SA) or mesic (ME) area. Inoculum potentials of the VAMfungal isolates were determined and the inocula equalized, achieving the same level of root colonization (41%, /'>0.05) at harvest (50 days). Plants of the three VAM treatments (AR, SA and ME) were evaluated against nonVAM controls. Significant differences in plant response to colonization were found in dry mass, leaf K, N and P concentrations, and in root/shoot, nodule/root, root length/leaf area and root length/ root mass ratios. The differences were most pronounced and consistent between the AR and all other treatments. Photosynthesis and nodule activity were higher (P<0.05) in all VAM treatments, but only the AR plants had higher (P<0.05) photosynthetic water-use efficiency than the controls. Nodule activity, evaluated by HT evolution and CjH, reduction, differed significantly between treatments. The results are discussed in terms of nutritional and non-nutritional effects of VAM colonization on the development and physiology of the tripartite soybean association in the light of intraspecific variability within the fungal endophyte.

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A method is presented for quantifying infectivity of vesicular‐arbuscular mycorrhizal (VAM) fungal inocula by directly counting infection units in roots. Inocula of five isolates of VAM fungi [Glomus etunicatum Becker and Gerd., Glomus pallidum Hall, and Glomus mosseae (Nicol. & Gerd.) Gerd. and Trappe from California, Florida and Nevada] were assessed. Sorghum bicolor L. was grown for 14 d under controlled conditions in 100 mL growth tubes containing 48, 12, 3, or 0.75 g of each inoculum (96, 24, 6, or 1.5 for G. etuicatum) mixed with sterile soil to provide a total growth medium of 96 g. Discrete infection units were found for all isolates at all inoculum density levels, except for G. etunicatum (no infection at lowest density) and G. mosseae‐California (coalescing infection units at highest density). The relationship between inoculum density and discrete infection units per gram root fresh weight was linear (P < 0.01). This relationship is particularly suitable for determining amounts of inocula of equivalent potential for use in short‐term experiments.

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