M. Hungria et al., NITROGEN-FIXATION CAPACITY AND NODULE OCCUPANCY BY BRADYRHIZOBIUM-JAPONICUM AND B-ELKANII STRAINS, Biology and fertility of soils, 27(4), 1998, pp. 393-399
In a previous study soybean Bradyrhizobium strains, used in Brazilian
studies and inoculants over the last 30 years, and strains adapted to
the Brazilian Cerrados, a region frequently submitted to environmental
and nutritional stresses, were analyzed for 32 morphological and phys
iological parameters in vivo and in vitro. A cluster analysis allowed
the subdivision of these strains into species Bradyrhizobium japonicum
Bradyrhizobium elkanii and a mixed genotype. In this study, the bacte
ria were analyzed for nodulation, N-2 fixation capacity, nodule occupa
ncy and the ability to increase yield. The goal was to find a relation
ship between the strain groups and the symbiotic performance. Two stra
ins of Brazilian B. japonicum showed higher rates of N-2 fixation and
nodule efficiency (mg of N mg(-1) of nodules) under axenic conditions.
These strains also showed greater yield increases in field experiment
s when compared to B. elkanii strains. However, no differences were de
tected between B. japonicum and B. elkanii strains when comparing nodu
le occupancy capacity. The adapted strains belonging to the serogroup
B. elkanii SEMIA 566, most clustered in a mixed genotype, were more co
mpetitive than the parental strain, and some showed a higher capacity
of N-2 fixation. Some of the adapted strains, such as S-370 and S-372,
have shown similar N-2 fixation rates and nodulation competitiveness
to two Brazilian strains of B. japonicum. This similarity demonstrates
the possibility of enhancing N-2 fixing ability, after local adaptati
on, even within B. elkanii species. Differences in the DNA profiles we
re also detected between the parental SEMIA 566 and the adapted strain
s by analyses with the ERIC and REP-PCR techniques. Consequently. gene
tic, morphological and physiological changes can be a result of adapta
tion of rhizobia to the soil. This variability can be used to select s
trains capable of increasing the contribution of N-2 fixation to soybe
an nutrition.