TheParasponia parviflora—Rhizobium symbiosis. Host specificity, growth and nitrogen fixation under various conditions

Summary

Cross-inoculation experiments withParasponia parviflora plants and a large number of Rhizobium strains isolated from legumes, mainly of temperate origin, showed that strains ofRhizobium trifolii andR. lupini could produce root nodulation in Parasponia. Root nodulation was also obtained with some tropical Rhizobium isolates such as those from Arachis and Albizzia, but not withRhizobium japonicum strains. In addition, strains ofR. leguminosarum, R. phaseoli andR. meliloti produced abnormal root nodules or pseudo-root nodules in Parasponia seedlings. All root nodules induced in Parasponia by foreign Rhizobium species lacked, however, nitrogenase activity. Testing the reverse combination Parasponia-Rhizobium on legumes yielded with regard to root nodulation negative results, except withVigna sinensis and twoMacroptilium species (M. atropurpureum andM. lathyroides). The root nodules produced in the latter legumes showed nitrogenase activity.Trema cannabina seedlings inoculated with Parasponia-Rhizobium invariably did not produce root nodules. The feasibility of Parasponia to accept foreign rhizobia bacteria or to produce pseudo-nodulation is probably genetically determined. In a karyological study it was shown thatParasponia parviflora had the chromosome number 2n=20.

Growth experiments revealed thatP. parviflora can be cultivated in water culture, perlite and in soil (pot) culture. It can also be grown in agar tubes, although growth under these conditions is sometimes rather variable. Nitrogenase (C₂H₂) tests showed that water-culture root nodules were 5–10 times less active than those from perlite culture, soil or agar tubes. On the whole, nitrogenase activity of root nodules grown on the various substrates and receiving different treatments, was rather variable.Vigna sinensis plants inoculated with Parasponia-Rhizobium showed on basis of fresh nodule weight a 3–4 times higher nitrogenase activity as compared with the same strain on Parasponia. However, as shown by acetylene reduction tests of nodule slices, the main activity of mature Parasponia root nodules is situated in the apical part of the root nodule. On fresh weight basis the nitrogenase activity of this nodule section is of the same order as that of leguminous root nodules.

Intact Parasponia plants showed higher nitrogenase activities than excised root nodules. The maximal nitrogen fixation rate ofParasponia parviflora plants was 20–25 μmol. C₂H₄·g⁻¹ fresh weight nodule tissue.h⁻¹, which value is comparable to that of legumes and higher than estimates obtained by Parasponia in the field. No decrease of the nitrogen-fixation rate was observed in Parasponia plants tested in a diurnal cycle during the 12-hours dark period. Amino-acid analyses of Parasponia root nodules showed large quantities of aspartic acid, asparagine, glutamic acid and glutamine suggesting a role of these amino acids in the nitrogen fixation or transport processes. In contrast to actinorhizal root nodules Parasponia root nodules do not contain citrulline. The latter amino acid was, however, found—although in rather low concentration—inVigna sinensis andMacroptilium atropurpureum nodules obtained withParasponia-Rhizobium.