| Abstract: | Bean (Phaseolus vulgaris L. var. Tacarigua) plants were grownin sterilized Leonard jars under controlled conditions. Beforesowing, 1 g of gamma irradiated peat containing the Rhizobiumtropici strain CIAT899 was placed at either 2 or 10 cm belowthe sand surface. Mechanical infection of bean rugose mosaicvirus (BRMV) was carried out in 3-d-old seedlings. Thus, theearly events of nodulation occurred before the arrival of virusparticles to roots. Rhizobium inoculation at 2 cm deep resultedin the formation of nodule clusters close to the crown, in contrastto the homogeneous nodulation along the roots observed in plantsinoculated with Rhizobium at a depth of 10 cm. The uniform arrangementof nodules on the roots enhanced the plant shoot biomass, althoughthe total nodule mass per plant did not differ between Rhizobiuminoculation treatments. Nodules located on deeper roots resultedin higher ureide concentrations in shoots and leaves and inreduced carbohydrate concentrations in leaves. In healthy plants,nodules formed on deeper roots had higher allantoinase activityand a greater carbohydrate concentration when compared to thatof nodules located close to the crown. Deeper nodules had ureideconcentrations similar to those of upper nodules, probably asa consequence of increased translocation of N-compounds to aerialorgans. A similar pattern of nodule formation and response toinoculum position was observed in BRMV-infected plants at allharvests. However, virus infection resulted in reduced totalnodule mass, shoot biomass, total leaf area and induced transitoryalterations in the ureide,  -amino-N and carbohydrate concentrationin the different plant compartments. The effect of BRMV infectionon plant parameters was more evident during the vegetative stagesof growth. Nevertheless, the magnitude of the effect was alwaysmore pronounced in plants inoculated with Rhizobium at a depthof 2 cm compared to those Inoculated at 10 cm due to a greateractivity of deeper nodules despite virus infection. Deeper nodulesin BRMV-infected plants showed higher carbohydrate concentrationas well as higher allantoinase and uricase activity than thosedeveloped close to the crown, at all harvests. This observationwas further supported by ultrastructural analysis of virus-infectednodules, since virus replication took place in cells containingbacteroids of upper and lower nodules, but only in the interstitialcells of the latter. BRMV infection did not hinder the allantoinaseactivity and the chlorophyll content of uppermost mature leavesregardless of inoculum position. At the flowering and fruitingstages, healthy and BRMV-infected plants did not differ withregard to any of the tested parameters. Only inoculum positionhad an effect. The nearly normal functioning of the symbioticprocess at these stages of growth was attributed to the formationof a new generation of nodules in BRMV-infected plants subjectedto each of the Rhlzobium inoculation treatments. Key words: Bean rugose mosaic virus, symbiotic nitrogen fixation, bean, Rhizobium inoculum position, nodule ultrastructure |
