Agriculture Reference
In-Depth Information
capacity in January, 27.8% field capacity in June). AS soil was collected in late Spring (0.36
g Na
+
Kg
-1
dry soil) while AWA and AWB soils were collected in early Winter (0.32 g Na
+
Kg
-1
dry soil).
Considerable genetic variability in salt tolerance among legumes species and cultivars
has been reported. Some tree legumes such as
Prosopis
and
Acacia
spp. are highly tolerant to
salinity, while grain legumes have long been recognized as either sensitive or only moderately
tolerant to salinity. However, according to Subbarao and Johansen (1994)
Pisum sativum
is
one of the most halotolerant-cultivated legumes. For this reason, in this work, pea plants were
used as hosts to obtain
Rhizobium
isolates from soils.
To evaluate the influence of salt stress in the soil microflora, thirty-nine isolates of
Rhizobium leguminosarum
bv.
viciae
were obtained from root nodules of pea (
Pisum sativum
L.) plants grown in SB, V, CN, AWA, AWB and AS soils.
According to Hirsch et al. (1993) and Ibekwe et al. (1997) the distribution of
Rhizobium
strains in agricultural soils is affected both by current and previous cultivation: the presence
of the host plant generally leads to a population increase which persists for some years. The
difficulties of establishment and persistence of these bacteria in soils in the absence of their
specific leguminous hosts have emphasised the need for legume inoculation and have
prompted studies of their survival in soil under adverse conditions. In this chapter,
Pisum
sativum
plants were
grown in all soils in the moment of sampling or in the last three years.
USDA strains were provided by Peter vanBerkum (United States Department of
Agriculture), and GRA19 strain was a kind gift by Carmen Lluch (Granada University).
2.1. Characterization of NaCl Tolerance
Salt tolerance of
Rhizobium
isolates and strains were screened in YEM medium
supplemented with increasing NaCl concentrations (0 to 1800 mM). Salt tolerance was
defined experimentally as the maximum NaCl concentration where cell growth was inhibited
less than 75%. Isolates showed distinct responses to salt, being classified in three groups,
according to their salt tolerance: sensitive (0-50 mM NaCl); tolerant (100-500 mM NaCl);
and extremely tolerant (600-1800 mM NaCl). Figure 1 shows the percentage of sensitive,
tolerant and extremely tolerant isolates in each location, and Table 1 shows the growth
response of
Rhizobium
isolates and strains to salinity. Isolates from Vagos and S. Bernardo
evidenced high vulnerability to NaCl, being all sensitive. Massive growth inhibitions were
observed at concentrations of 25 mM for Vagos and 50 mM for S. Bernardo (Table 1). These
responses can probably be related to the large availability of water present in those locations
throughout the year.
Costa Nova isolates showed high salt susceptibility since 86% of them were sensitive
(Figure 1). The halotolerance of
Rhizobium
CN isolates (Table 1 and Figure 1) did not
correlate to salt levels normally present near the sea. It might be expected that salt stress
would be a constant condition at this site, especially in late Spring and Summer, when the soil
is influenced by reduced water content and concomitant increase in ion concentration.
According to Bååth (1992) and Bååth et al. (1998) environmental stresses exert a selective
pressure in soil microflora. The soil microbial population presents variability, which in a non-
stressed environment would not bring any advantage.
