A Split‐Root Growth System for Evaluating the Effect of Salinity on Components of the Soybean Rhizobium japonicum Symbiosis1

Abstract

Study of the effects of salinity on symbiotic nitrogen fixation is complicated by the high degree of integration between shoot and nodules. Shoot photosynthetic capacity determines not only the amount of photosynthate available to the nodule, but the strength of the sink for nodule products. Salinity may also directly affect functional processes within the nodule. Two experiments were undertaken which employed a split‐root solution culture assembly that confined N2 fixation to one‐half of soybean [Glycine max (L.) Merr.] root systems. In one experiment, one‐half of the root system was inoculated with non‐N2 fixing strain (ineffective) of Rhizobium japonicum, SM‐5, 48 hours prior to inoculating the other side with effective strain USDA 110. Nodulation and N2 fixation by USDA 110 when in combination with SM‐5 was significantly suppressed compared to a second treatment where one‐half of the root system was inoculated with USDA 110 and the other half remained uninoculated. Root development was significantly greater on the side producing effective nodules than that of the uninoculated or SM‐5 nodulated half‐root systems. In another experiment, one‐half of the root system was nodulated with USDA 110 and the opposite side with SM‐5. After nodule development, NaCl (120 mM) was added to the nutrient solutions of: 1) neither half, effective nodule (USDA 110) half, 3) ineffective nodule (SM‐5) or 4) both halves of the root systems. Osmotic potentials ;(ψπ) of roots, nodules, and leaves were measured. Leaves of plants with only half their roots salinized had ψπ(−10.3 bar and −11.1 bar treatments 2 and 3 respectively) comparable to the non‐salinized control (−9.3 bar). Leaf ψπ of plants with both root halves in salt solutions was −17.9 bar. Nodules exposed to salt solutions had ψπ values ranging from −9.3 to −12.2 bar while ψπ of nodules in normal solutions ranged from −6.1 to −7.8 bar. Ineffective nodules always had a lower ψπ than similarly treated effective nodules. These results demonstrate that functional components of the soybean Rhizobium japonicum symbiosis can be independently subjected to salinity stress. This methodology can be used to evaluate the relative sensitivity of the components of the symbiosis.