CARBON ECONOMY OF SOYBEAN-RHIZOBIUM-GLOMUS ASSOCIATIONS

Carbon 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 Rhizobium japonicum (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 vesicular-arbuscular mycorrhizas were measured in six- and nine-week-old plants by determining the distributions of ¹⁴ C after pulse labelling with ¹⁴ CO₂ . Root nodules in non-mycorrhizal plants utilized 9% of total photosynthate; this was increased to 12% in nodulated, mycorrhizal plants. Mycorrhizas used 17% of the total photosynthate of six-week-old plants; this fell to 8% after nine weeks. Rates of ¹⁴ CO₂ fixation in leaves of nodulated or nodulated plus mycorrhizal plants were up to 52% 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.