126 S. Signorelli et al.
Legumes are typically subjected to a variety of different environmental stresses
such as water stress. At the cellular level, this stress induces overproduction of reac-
tive oxygen species (ROS; Fig. 6.1), such as hydrogen peroxide (H 2 O 2 ), superoxide
radical (O 2 ●−) and hydroxyl radical (●OH), which are responsible for oxidative dam-
age associated with stress (Dat et al. 2000 ). Plants respond to stress using differ-
ent enzymatic and non-enzymatic antioxidant systems. Oxidative stress responses
may involve increased activity of superoxide dismutase (SOD), catalase (CAT)
and ascorbate–glutathione cycle activities such as glutathione reductase (GR) or
ascorbate peroxidase (APX), which can confer greater tolerance against a specific
environmental stress (Sade et al. 2011 ). Increased levels of non-enzymatic soluble
antioxidants including glutathione (GSH), ascorbic acid and tocopherols are also
produced in response to water stress-induced oxidative stress (Feng et al. 2004 ).
Plant antioxidant defence systems normally provide adequate protection against
ROS damage under optimal growth conditions. The generation of higher levels of
ROS may overcome the defence provided by these systems and result in oxida-
tive stress (Mittler 2002 ; Noctor and Foyer 1998 ; Valderrama et al. 2006 ). Cellular
damage caused by oxidative stress includes lipid peroxidation, which increases in
various tissues during water stress and is also a common marker of oxidative stress
(Sade et al. 2011 ).
In response to water deficit, plant cells also accumulate low-molecular-mass
compounds termed compatible solutes, mainly proline, glycine betaine, sugars
and polyols, in the cytoplasm to control the ionic balance in the vacuoles (Parida
and Das 2005 ). Among these solutes, proline has been associated with different
Fig. 6.1 ROS production in the chloroplast. Chl chlorophyll, Chl* excited chlorophyll. PSI pho-
tosystem I, PSII photosystem II. Cyt cytochrome, PQ plastoquinone, PC plastocyanin. Superoxide
(O 2 ●−) can be produced by electron transfer to oxygen. Hydrogen peroxide (H 2 O 2 ) is produced
from superoxide by spontaneous dismutation or SOD activity. Hydroxyl radicals (●OH) are pro-
duced from hydrogen peroxide by homolysis or Fenton reaction in the presence of Fe3+. Singlet
oxygen is generated from oxygen by energy transfer from excited chlorophylls