6 Combined Abiotic Stress in Legumes 135
that provide a low-resistance internal pathway for the movement of O 2 from the
shoots to the roots, where it is consumed in respiration and may also reoxidize
the rhizosphere (Armstrong 1970 ; Armstrong 1971 , 1979 ). In legumes, aerenchyma
may also be important for supplying O 2 and N 2 to root nodules (Walker et al. 1983 ;
James et al. 1992 ; Zook et al. 1986 ; Pugh et al. 1995 ). Tolerance of Melilotus siculus
to waterlogging is associated with the production of a highly porous phellem, a type
of secondary aerenchyma, on taproots and upper lateral roots (Verboven et al. 2011 ).
Studies with plant species sensitive or tolerant to flooding–salt stress combina-
tion have shown that the rate of transport of Na+ and Cl− to the shoot is critical to
define the response. The ions transport rate increases significantly under combined
stress in comparison with salinity alone (Barrett-Lennard 2003 ). For more tolerant
species, there is only small or even no increase in shoot Na+ and Cl− in response to
combined salinity and waterlogging (Colmer and Flowers 2008 ), presumably due
to better root aeration. Moreover, in perennial legumes such as Trifolium repens L.
(Rogers and West 1993 ) and Liolaemus tenuis (Teakle et al. 2007 ), high root po-
rosity was associated with better shoot ion regulation under combined salinity and
waterlogging. Comparisons of annual pasture legumes in growth, ion regulation and
root porosity demonstrate that M. siculus has exceptional tolerance to combinations
of salinity and waterlogging (Teakle et al. 2012 ). Enhanced root aeration would
avoid energy deficits that could impair ion transport processes in roots, which de-
termines delivery of Na+ and Cl− to shoots via the xylem (Barrett-Lennard 2003 ;
Teakle et al. 2007 ; Colmer and Flowers 2008 ). Thus, traits of importance for toler-
ance to combined salinity and waterlogging are likely to include high root porosity,
leading to decreased shoot Na+ and Cl− concentrations.
6.5 Metabolic Changes in Responses to Stress
Combination
It is well known that the effect of a combination of different stresses on plants
can be quite different from those generated when plants are subjected to individual
types of stress (Rizhsky et al. 2002 ). Table 6.1 represents a summary of how the
combination of different stresses affects some parameters in legumes.
With reference to antioxidant responses, different patterns are observed when
more than one stress is imposed. However, it seems that in most cases the addition
of other stress did not alter the response. It implies that the signal molecules that
induce the expression of antioxidant enzymes probably are the same in different
stresses and so the imposition of both stresses is redundant. In other cases, the effect
of simultaneous stresses produces deleterious effects. For example, for APX and
CAT, one stress produces the induction of the activity (or at least a normal level of
activity), but the imposition of two stresses could produce a more nitrosative condi-
tion in the cell leading to the nitration of the enzyme, which is known to decrease
the activity of these enzymes.