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at levels that assure their survival, as suggested by Sprent and Zahran (1988) and Zerhari et
al. (2000).
Alentejo isolates showed higher growth rates under NaCl stress, tolerating salinity levels
up to 1600 mM (Table 1). Interestingly, Winter (AWA and AWB) isolates showed lower salt
tolerance than those isolated in Spring (AS). Actually, 57% of AS isolates were extremely
tolerant and only 14% of them were sensitive (Figure 1). The high salt tolerance observed in
the isolates from non-saline soils (AS) of Alentejo could be explained by the climatic
conditions observed in their habitat, considering the shortage of water experienced by
Rhizobium
populations in the Alentejo region, at least during part of the year. Drought and
salinity tolerance are often associated (Mohammad et al., 1989; Yeo, 1994) probably because
some of the mechanisms involved in water stress tolerance, as osmotic adjustment, can also
contribute for their tolerance to salinity. Bordeleau and Prévost (1994) corroborate these
findings, reporting that
Rhizobium
isolated from arid soils were capable of effective legume
nodulation under drought and saline conditions. Cordovilla et al. (1999) and Mohammad et
al.
(1989) have also used strains of salt tolerant
Rhizobium
to nodulate plants of
Medicago
sativa, Vicia faba
and
Pisum sativum
under water stress. However, there were differences in
salt tolerance of strains isolated in different seasons of the same location. These results may
be explained by the different water availability during the year. In June, the
Rhizobium
population was experiencing low water availability and high osmotic stress, since in this
season, extreme water shortage is frequent in the Alentejo region. The drought conditions
could select water stress tolerant strains, which would be more abundant in this time of the
year. In winter, water stress is a feature not so selective as in late spring. The high soil
moisture would therefore provide an increase in the number of individuals more adapted to
the new environmental conditions, which do not include water shortage, thus yielding a less
NaCl tolerant population.
In this chapter a marked variation in
Rhizobium
salt tolerance was observed, which is in
agreement with other reports (Zahran et al., 1994). Our results also suggest that salt tolerance
of
Rhizobium
populations was strongly influenced by their origin, which may represent
evidence of the population's adaptation to the climate conditions experienced in the habitat
they colonize. Hence, Alentejo populations seem to be well adjusted to extreme conditions of
water shortage and high temperatures, which yielded high salt tolerance. On the other hand,
SB and V populations are much more sensitive to salt stress because they come from a mild
environment.
2.2. Screening Rhizobia for Nitrogen Fixation Potential with
Pisum Sativum
Nodulation and nitrogen fixation in legume and rhizobial associations are commonly
limited by soil infertility conditions, including salinity and drought. According to
Somasegaran and Hoben (1994) the N content correlates well with the shoot dry weight, thus
presenting an acceptable basis for the comparison of the strains nodulation efficiency.
Relative nodulation and effectiveness of symbiotic dinitrogen fixation of
Rhizobium
isolates
was evaluated by shoot dry weight comparison between plants under 0 and 90 mM NaCl,
inoculated with different
Rhizobium
genotypes and controls (plants not inoculated and not
receiving inorganic nitrogen, (-N) control; and plants not inoculated and supplied with 5 mM
NO
3
-
, (+N) control), as described by Somasegaran and Hoben (1994).
