Bacterial growth rate and growth pouch nodulation profile differences as possible ways of Bradyrhizobium japonicum strain screening for low P soils

This work studied the effects of P fertilization on nodulation of field-grown soybean by two Bradyrhizobium strains (SMGS₁ and THA₇), and checked if differences between strains were consistent with bacterial growth and growth pouch nodulation ability in response to P availability. In the field, nodule dry weight and nitrogen fixation activity of inoculated soybean were studied on typical acid soils of Thaïland at the flowering (R₁) stage and at the end of grain filling. Grain yield, growth and phosphorus content were recorded. The bradyrhizobial strains were cultivated in culture medium, and growth parameters recorded. Nodulation patterns were observed during growth pouch experiments: infective root cells were inoculated with strains cultivated at two P concentrations in their culture media, namely 1 M and 1 mM. Ten days after inoculation, the position of each nodule was measured relative to the root tip (RT) mark, expressed relative to the smallest emerging root hairs-RT distance in the nodulation frequency profile, and the consistency of responses was tested. In the field, on P deficient soils, dry weight of nodules was higher with Bradyrhizobium japonicum strain SMGS₁ than with strain THA₇. P supply increased the number and dry weight of nodules for both strains, with a higher dry weight response for THA₇ than for SMGS₁. It also had a positive effect on tissue phosphorus status and grain yield at R₈ stage. In growth media, significant differences were recorded between strains under P-limiting conditions: The growth rate was higher for strain SMGS₁, as well as the maximal number of bacterial cells supported. With growth pouch, inoculating plants with bacteria grown in P-deficient medium resulted in a less intense nodulation of roots by THA₇, and with nodules appearing earlier on roots than in the case of SMGS₁. At 1 mM P, there was no significant difference between strains. Thus, strain THA₇ is more affected by P deficiency than strain SMGS₁. Although P was not supplied in the same way in the soil and in the growth pouch experiments, this consistency of behaviour between work scales indicates that phosphorus availability is a key component for a successful inoculation. Furthermore, the study of bacterial growth rates and nodulation profile represents an interesting step for bacterial screening for low P soils. [-11pt]