Autoregulation of soybean nodulation: Delayed inoculation increases nodule number

The influence of seedling age at the time of inoculation on the regulation of nodule number in soybean (Glycine max [L.] Merr.) was examined in cv. Williams 82 and its hypernodulating mutant NOD1-3. Nodulation was evaluated on plants grown in plastic growth pouches or in vermiculite in 50- or 500-ml glass containers in growth chamber studies. Seeds or seedlings were inoculated once with Bradyrhizobium japonicum strain USDA 110 (10 8 cells seedling −1 ) between 0 and 15 days after sowing at 3- or 5-day intervals and were grown for 21 days after inoculation. Nodule number plant −1 was similar across inoculation times in plants grown in growth pouches, but was significantly greater when inoculation was delayed and plants were grown in vermiculite in 500-ml containers. Plant culture in vermiculite in 50- or 500-ml containers confirmed the suppressive effect of restricted space for root growth on nodulation. Inoculation with 10 5 or 10 9 USDA 110 cells revealed that nodulation was inhibited by a high inoculum dose. There was a large increase in nodule number plant −1 when plants were transferred from a restricted rooting environment (growth pouch culture) to a nonrestricted rooting environment (2-1 hydroponic pots). Autoregulation was also examined in split-root assemblies of plants in 500-ml containers of vermiculite. Controls involved concurrent inoculation of both root halves at 0, 4 or 8 days after transplant. Treatments involved time-separated inoculations of root halves with the primary and secondary inoculations being separated by 4 days. Plants were harvested at 21 days after inoculation. Williams 82 exhibited autoregulation of nodule number on the root half receiving delayed inoculation, regardless of plant age at the time of primary inoculation. Total nodule number plant −1 invariably increased with later inoculation times. In contrast, NOD1-3 exhibited little, if any, autoregulation of nodule number. It was concluded that although Williams 82 exhibits autoregulation of nodule number and NOD1-3 does not, there was no finite limit to nodule number in either line since any delay in inoculation resulted information of a greater nodule number on both lines if root growth was not restricted. Nodule number in Williams 82 and NOD1-3 appears to be a function of infection sites (root size) at the time of inoculation and of subsequent plant growth