SUMMARYCarbon uptake and allocation in plants that were largely dependent on microbial symbionts for N and P was compared to that in plants given inorganic fertilizer. Soybeans (Glycine max L. Merr.) were grown in sterilized soil and were either left uninoculated, or were inoculated with Rhizobiumjaponicum (Kirschner), or both R.japonicum and Glomus fasciculatum (Thaxter sensu Gerd.). Uninoculated plants were given N and/or P fertilizer at rates required to produce plants similar in size to inoculated plants. Carbon flows to plant parts, root nodules and vesiculararbuscular mycorrhizas were measured in six-and nine-week-old plants by determining the distributions of "C after pulse labelling with ^^COg.Root nodules in non-mycorrhizal plants utilized 9 % of total photosynthate; this was increased to 12% in nodulated, mycorrhizal plants. Mycorrhizas used 17% ofthe total photosynthate of six-week-old plants; this fell to 8% after nine weeks. Rates of "CO^ fixation in leaves of nodulated or nodulated plus mycorrhizal plants were up to St% higher than in plants without microbial symbionts. Part of the increase was due to higher specific leaf area in plants colonized by symbionts, but other factors such as source-sink relationships, starch mobilization and leaf P concentrations were also involved in the host-plant adaptations to the C demand of the microbial endophytes.

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Summary Soybean plants were grown in a soil very low in available P. Seedlings were inoculated with two vesicular-arbuscular mycorrhizal (VAM) fungi or were left non-inoculated and fertilized with P. Assimilation and allocation of mieronutdents (Fe, Mn, Zn, and Cu) were determined during host development, and the uptake of trace elements in VAM plants was compared to P-fertilized, non-VAM plants of similar weight, growth stage, and P status. Copper and zinc concentrations were always higher in VAM plants, while iron and manganese concentrations were lower than in the equivalent P-fertilized soybeans. Differences in the micronutrient content of fully-mature soybean pods reflected differences in the leaves and roots. Thus, for trace elements, seed quality can be altered by VAM colonization in a fashion not duplicated by P fertilizer.