Environmental Engineering Reference
In-Depth Information
contents can be determined by using spectrophotometer and high performance liquid
chromatography.
Oxidative Stress Induced by Salt Stress
Similar with other environmental stress, salt stress can also induce the increase of
reactive oxygen species, and then lead to oxidative damage in plant cells. Reactive oxygen
species (ROS) are partially reduced forms of atmospheric oxygen. They typically result from
the excitation of O
2
to form singlet oxygen or from the transfer of one, two or three electrons
to O
2
to form a superoxide radical (O
2
−
), hydrogen peroxide (H
2
O
2
) or a hydroxyl radical (-
OH) respectively. In contrast to atmospheric oxygen, ROS are capable of unrestricted
oxidation of various cellular components and can lead to the oxidative destruction in plant
cells (Mittler 2002). ROS are generated in the normal metabolism in plant cells, and there are
many potential sources of ROS. Photosynthetic electron transport and respiration process are
the major origins of ROS production in plant cells. Under normal growth conditions, ROS
production is low, but it may increase under environmental stress. Plants have evolved a suite
of protective systems—antioxidant system—to keep ROS under control under normal growth
conditions. Consequently, the antioxidant system is considered to play a fundamental role in
mediating environmental stress resistance (Tausz et al. 2007; Jaleel et al. 2009). Antioxidant
system consists of small molecular antioxidants and antioxidant enzymes. Superoxide
dismutase (SOD, EC 1.15.1.1), ascorbate peroxidase (APX, EC 1.11.1.11), Glutathione
peroxidases (GPXs, EC 1.11.1.9) and catalase (CAT, EC 1.11.1.6) are crucial ROS-
scavenging enzymes in plant cells. SOD catalyzes the dismutation of O
2
-.
to molecular oxygen
and H
2
O
2
, which can be subsequently scavenged by APX and CAT. They work together to
control the concentrations of O
2
-.
and H
2
O
2
, which then limits the generation of -OH, the most
dangerous species of all the ROS (Mittler 2002). As the major antioxidants, Ascorbate (AsA)
and glutathione (GSH) can directly scavenge ROS and act as reducing substrate for APX and
GPXs to detoxify H
2
O
2
. Glutathione reductase (GR), dehydroascorbate reductase (DHAR)
and monoascorbate reductase (MDAR) act as antioxidant-regenerating enzymes, and they
cooperate to regenerate GSH and AsA from their oxidized forms at the expense of reducing
power (Mittler 2002).
If ROS production induced by environmental stress overwhelmed the scavenging
capability of antioxidant system, oxidative damage would occur. Oxidative stress often
occurred in plants under salinity condition. Under salinity stress, availability of atmospheric
CO
2
declined because of the increased stomatal closure, and subsequently, NADPH
consumption by the Calvin Cycle decreased. When ferrodoxine is over-reduced during
photosynthetic electron transfer, electrons may be transferred to oxygen to form O
2
.-
by the
process called Mehler Reaction, and subsequently, more harmful oxygen radicals may be
generated through chain reactions (Hsu and Kao 2003; Shao et al. 2008). Under oxidative
stress, plants can detoxify the excess ROS by accumulating the contents of antioxidants such
as AsA and GSH and by up-regulating the activities of antioxidant enzymes such as SOD,
APX and CAT. Consequently, the degree of cellular oxidative damage in plants resulting
from abiotic stress is mediated (Turkan et al. 2005). It has been reported that salinity
tolerance is closely related to antioxidant capacity in many plant species (Gossett et al. 1994;
Hernandez et al. 1995; Gueta Dahan et al. 1997; Dionisio-Sese and Tobita 1998; Meneguzzo
