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Symbiotic nitrogen fixation and physiological performance of bean (Phaseolus vulgaris L.) plants as affected by Rhizobium inoculum position and bean rugose mosaic virus infection

Abstract:

Bean (Phaseolus vulgaris L. var. Tacarigua) plants were grown in sterilized Leonard jars under controlled conditions. Before sowing, 1 g of gamma irradiated peat containing the Rhizobium tropici strain CIAT899 was placed at either 2 or 10 cm below the sand surface. Mechanical infection of bean rugose mosaic virus (BRMV) was carried out in 3-d-old seedlings. Thus, the early events of nodulation occurred before the arrival of virus particles to roots. Rhizobium inoculation at 2 cm deep resulted in the formation of nodule clusters close to the crown, in contrast to the homogeneous nodulation along the roots observed in plants inoculated with Rhizobium at a depth of 10 cm. The uniform arrangement of nodules on the roots enhanced the plant shoot biomass, although the total nodule mass per plant did not differ between Rhizobium inoculation treatments. Nodules located on deeper roots resulted in higher ureide concentrations in shoots and leaves and in reduced carbohydrate concentrations in leaves. In healthy plants, nodules formed on deeper roots had higher allantoinase activity and a greater carbohydrate concentration when compared to that of nodules located close to the crown. Deeper nodules had ureide concentrations similar to those of upper nodules, probably as a consequence of increased translocation of N-compounds to aerial organs. A similar pattern of nodule formation and response to inoculum position was observed in BRMV-infected plants at all harvests. However, virus infection resulted in reduced total nodule mass, shoot biomass, total leaf area and induced transitory alterations in the ureide, α-amino-N and carbohydrate concentration in the different plant compartments. The effect of BRMV infection on plant parameters was more evident during the vegetative stages of growth. Nevertheless, the magnitude of the effect was always more pronounced in plants inoculated with Rhizobium at a depth of 2 cm compared to those Inoculated at 10 cm due to a greater activity of deeper nodules despite virus infection. Deeper nodules in BRMV-infected plants showed higher carbohydrate concentration as well as higher allantoinase and uricase activity than those developed close to the crown, at all harvests. This observation was further supported by ultrastructural analysis of virus-infected nodules, since virus replication took place in cells containing bacteroids of upper and lower nodules, but only in the interstitial cells of the latter. BRMV infection did not hinder the allantoinase activity and the chlorophyll content of uppermost mature leaves regardless of inoculum position. At the flowering and fruiting stages, healthy and BRMV-infected plants did not differ with regard to any of the tested parameters. Only inoculum position had an effect. The nearly normal functioning of the symbiotic process at these stages of growth was attributed to the formation of a new generation of nodules in BRMV-infected plants subjected to each of the Rhlzobium inoculation treatments. © 1994 Oxford University Press.