Legume-rhizobia symbiotic performance under abiotic stresses: factors influencing tolerance behaviour - Legumes Under Environmental Stress: Yield, Improvement and Adaptations - page 128

plant tissues, nodule ionic status, nodule-cortex struc-
ture and oxygen permeability (Aydi et al., 2004; Mhadhbi
et al., 2011a). In contrast, studies performed with legume
crops, such as chickpea ( Cicer arietinum ) and common
bean ( Phaseolus vulgaris ), did not discern a significant
effect of the cultivar. In these experiments the bacterial
partner was the key factor influencing symbiosis behav-
iour mainly under stressful conditions (Mhadhbi et al.,
2004, 2008, 2011b; Jebara et al., 2005). The contribution
of the rhizobial partner to the variance of symbiosis
effectiveness even under osmotic stress, suggests that
the selection of effective strains could enhance legume
crop production especially in nitrogen-poor soils (Serraj
et al., 1999; Zahran 1999; Mnasri et al., 2007a). Moreover,
to achieve integration in agricultural practice, the
adaptation of the selected rhizobial strains and their
ability to compete with the local soil microflora should
be considered (Mrabet et al., 2005). Our experience sug-
gests that in field conditions, native rhizobial strains
nodulating chickpea ( Mezorhizobium ciceri CMG6) and
common bean ( Rhizobium etli 12a3) were more efficient
than the commercial counterparts ( M. ciceri UPMCa7 T )
and R. tropici CIAT899) (Ben Rhomdhane et al., 2007;
Tajini et al., 2008). However, under laboratory condi-
tions the reference strains were more or equally effective
than the local ones (Mhadhbi et al., 2008).
This study provided evidence for a significant contribu-
tion of the rhizobial partner to the symbiotic effectiveness
of different chickpea cultivars under osmotic stress as
well as under non-stressed conditions. Field trials are
required to confirm our suggestion that tolerance of sym-
biosis to osmotic stress could be enhanced by selection of
effective strains (Mhadhbi et al., 2011b).

stress on nodule metabolism appears as a general
slowdown of nitrogenase activity concomitant with a
decrease in nodule protein content and leghaemoglobin
content along with an accumulation of lipid peroxida-
tion products (Mhadhbi et al., 2008, 2011a). Such
alterations indicate loss of integrity of cell membranes
and perturbation in the photosynthates required for
nodule performance. Nitrogenase activity is highly
sensitive to oxygen concentration (Aydi et al., 2004;
Kratsch & Graves, 2005; Mhadhbi et al., 2011a). The
transport of oxygen within nodule organelles is per-
formed by leghaemoglobin, which plays a critical role in
the process of bacteroid development and cellular respi-
ration. Leghaemoglobin oxidation (non-functional
oxy-leghaemoglobin) by ROS reduces the oxygen supply
to nodules (Mhadhbi et al., 2011a).
Oxidative damage is not the only factor responsible
for the decline of nodular activity. Indeed, as we have
already reported, nodule activity is closely dependent
on photosynthetic products provided by the plant. This
supply is reduced under stress due to diminution of
photosynthesis (decrease in quantity) as a consequence
of ionic disruption. Under such circumstances the bacte-
roids counteract ionic disruption by accumulating
osmoprotective substances such as glycine betaine
(Zahran, 1999).
To overcome stressful conditions, nodules invoke var-
ious mechanisms including morphological modifications
(Matamoros et al., 1999; Verdoy et al., 2004; Kratsch &
Graves, 2005; Luqueno et al., 2008) and physio-
biochemical adaptations (Mhadhbi et al., 2011a,b,c).
Adaptive responses involve the modulation of some
metabolic mechanisms to ameliorate the activity/energy
ratio, and stimulation of expression of some protective
molecules such as antioxidant compounds and enzymes
(Hernandez-Jimenez et al., 2002; Tejera et al., 2004;
Mhadhbi et al., 2011a,c).
Correlation between antioxidant capacity and salt tol-
erance is reported for several food crops and model
plants, including rice (Lee et al., 2001), common bean
(Jebara et al., 2005; Mhadhbi et al., 2011b), chickpea
(Mhadhbi et al., 2004, 2008), Medicago truncatula
(Mhadhbi & Aouani, 2008; Mhadhbi et al., 2011a,c),
Arabidopsis thaliana (Xu et al., 2008; Ellouzi et al., 2011)
and some halophyte species (Ellouzi et al., 2011, 2013).
Thus, the alleviation of oxidative damage by the use of
different antioxidants and ROS scavengers can enhance
plant resistance to salt stress (Mittler, 2002; Ellouzi et al.,

8.6 effect of osmotic stress on nodule
integrity and functioning

Salinity and drought imply secondary stresses, namely
osmotic and ionic, that both affect metabolic activities
(Tonon et al., 2004). The primary response to metabolic
disturbance is production and accumulation of ROS,
resulting in oxidative stress (Zhu, 2001; Ahmad et al.,
2008, 2010, 2011, 2012, 2014; Ahmad 2014). In nod-
ules, ROS are reported to have deleterious effects on
tissue integrity and nodule functioning (Becana et al.,
2000, Hernandez-Jimenez et al., 2002; Matamoros et al.,
2003; Mhadhbi et al., 2011a). The effect of oxidative