CO2 EXCHANGE IN SOYBEAN PLANTS AND SYMBIO TIC NITROGEN-FIXATION UPON JOINT INOCULATION WITH NODULE BACTERIA AND EITHER RHIZOSPHERE PSEUDOMONADS OR ENDOMYCORRHIZAL FUNGI

In pot experiments on growing soybean (Glycine max L.) plants in soil culture, an increase in the leaf area and significant stimulation (by 10 - 21%) of CO2 exchange were achieved upon joint inoculation of the plants with Bradyrhizobium japonicum 110 and either Pseudomonas specie s or the endomycorrhizal fungi Glomus mosseae, as compared with applic ation of the nodule bacteria only. Joint inoculation of the soybean wi th B. japonicum and either Pseudomonas or G. mosseae did not affect th e photosynthetic activity of plants as estimated per unit of leaf area . The amount of ''biological'' nitrogen in plants, as assayed by the N -15-dilution method, and the weight of soybean grain increased by 22 - 30% and 14 - 17%, respectively, as compared to the nodule bacteria tr eatment. In pot experiments with fertilization by 0.5, 10, 20, or 40 m g P2O5 per 100 g substrate (soil : sand = 1 : 1), the effect of pseudo monads on nodulation, symbiotic nitrogen fixation, and plant weight st rongly depended on the dose of phosphorus fertilizer. When the nodule bacteria and pseudomonads were applied in the range of 5 to 20 mg P2O5 per 10 g substrate, the weight and nitrogenase activity of the nodule s, as well as the plant weight, markedly exceeded those in the case of application of only the nodule bacteria. It was shown that rhizospher e pseudomonads primarily stimulated the growth of soybean plants, incl uding nodulation, probably as a result of the stimulation of phytohorm ones. The influence of phosphorus on soybean growth upon inoculation w ith B. japonicum was mainly due to improved nodulation, symbiotic nitr ogen fixation, and nitrogen supply of plants. This effect was more pro nounced at high phosphorus doses.