EARLY NODULE DEVELOPMENT OF SOYBEAN CV BRAGG AND SOME NODULATION MUTANTS IN RESPONSE TO DIFFERENT LEVELS OF INOCULATION

Soybeans (Glycine max [L.] Merr.) cv. Bragg, the intermediate supernodulating genotype nts 1116, a supernodulating genotype nts 1007 and the non-nodulating mutant nod49 were examined for the number of curled root hairs and subepidermal cell divisions in dependence of time and inoculum dose. The non-nodulating genotype was shown to produce some curled root hairs, but did not form any nodules. At 3 days after inoculation, only a small proportion of root hairs of Bragg and nts 1116 were found to be curled (> 0.3 %), which was essentially unaltered by the dose of inoculation, whereas nts 1007 produced a substantially higher proportion of curled root hairs under medium and high inoculation (up to 4 %). After 8 days, a higher proportion (up to 8 %) of curled root hairs was foun with smaller differences between genotypes. Early steps of nodule ontogeny were faster in nts 1007, more frequent and a higher proportion of initiated nodules continued to develop than on the other nodulating genotypes. The data suggest that the early expression of autoregulation is facilitated by decreasing the speed of cortical cell development, leading to subsequent termination of less far developed nodules. Further, there is evidence for the capability of autoregulation to regulate nodule formation throughout various early stages of nodule ontogeny within the root.