134 S. Signorelli et al.
6.4 Waterlogging and Salinity: A Combined Stress
in Legumes
Salt stress is certainly one of the most serious environmental factors limiting the
productivity of crop plants (Ashraf and O’Leary 1999 ). Salinity reduces the ability
of plants to take up water, causing rapid reductions in growth rate, along with an
array of metabolic changes identical to those caused by water stress (Munns 2002 ).
High salt concentration in the external solution of plant cells produces several
deleterious consequences. First, salt stress causes an ionic imbalance (Niu et al.
1995 ). The homeostasis of not only Na+ and Cl− but also K+ and Ca+2 ions is dis-
turbed (Rodriguez-Navarro 2000 ; Hasegawa et al. 2000 ; Serrano et al. 1999 ). As a
result, plant survival and growth will depend on adaptations that re-establish ionic
homeostasis, thereby reducing the duration of cellular exposure to ionic imbalance.
Second, high concentrations of salt impose a hyperosmotic shock by decreasing
water and causing loss of cell turgor. This negative effect in the plant cell is thought
to be similar to the effects caused by drought. Third, reduction of chloroplast stro-
mal volume and generation of ROS, in salt-induced water stress, are also thought to
play important roles in inhibiting photosynthesis (Price and Hendry 1991 ). On the
molecular level, these responses are manifested as changes in the pattern of gene
expression (Maggio et al. 2002 ).
The process of salinization results from the interaction between climate, geo-
morphology, hydrology, land use and surface water properties and dynamics of the
salts. Regions with salinity are frequently associated with geographical localization
with inundation events; thus it is not infrequent that salt and flood stress occurs
simultaneously.
Salinity and waterlogging interact adversely to reduce production of crops and
pastures, as very few species used in agriculture can tolerate the combination of
both stresses (Barrett-Lennard 2003 ). Moreover, annual pasture legumes are par-
ticularly sensitive to combined salinity and waterlogging (Bennett et al. 2009 ).
One of the most important consequences of energy limitation under anoxia is
altered redox state of the cell. Under low oxygen pressure conditions, the interme-
diate electron carriers in electron transport chain become reduced, affecting redox-
active metabolic reactions. Therefore, for maintaining redox homeostasis cells need
to regulate NADH to NAD ratio under flooding (Chirkova et al. 1992 ). Saturated
electron transport components, the highly reduced intracellular environment and
low-energy supply are the factors favourable for ROS generation. The consequenc-
es of ROS formation depend on the intensity of the stress as well as on the physico-
chemical conditions in the cell (i.e. antioxidant status, redox state and pH). As was
mentioned for other stresses, ROS accumulation may cause damage to different cell
structures and biomolecules. H 2 O 2 production during O 2 deprivation was observed
in the plant cells (Blokhina et al. 2001 ), and its degradation was found to play an
important role in waterlogging tolerance in non-legume plants (Lin et al. 2004 ).
A trait that is essential for root survival during water logging or flooding is the
development of aerenchyma (Armstrong 1979 ). Aerenchymas are cortical airspaces