Environmental Engineering Reference
In-Depth Information
et al. 1999; Hernandez et al. 2000; Sreenivasulu et al. 2000; Shalata et al. 2001; Bor et al.
2003; Mittova et al. 2003; Demiral and Turkan 2004; Demiral and Turkan 2005; Koca et al.
2006; Koca et al. 2007; Sekmen et al. 2007; Yazici et al. 2007; Chen et al. 2011). In addition,
transgenic plants overexpressing ROS-scavenging enzymes such as SOD (Badawi et al. 2004;
Tseng et al. 2007), CAT (Tseng et al. 2007), APX (Wang et al. 1999) and GR (Foyer et al.
1995) showed enhanced tolerance to the oxidative stresses in plants under adverse conditions.
It should be noticed that phenolics also can work as antioxidant because of their free-
radical trapping properties (Treutter 2006). The importance of phenolics accumulation in
resisting salt-induced oxidative stress has been demonstrated (Wahid and Ghazanfar 2006;
Ksouri et al. 2007). Recently, Abrol et al. (2012) reported that secondary metabolites contents
were initially increased compared with the later increase in antioxidant enzymes activities,
when
Swertia chirata
Buch.-Ham. was subjected to salinity stress. Biochemical relationship
between the induction of antioxidant enzymes and production of secondary metabolites in
plants exposed to salinity can be further discussed in the future.
Lipid peroxidation level which was determined in terms of malondialdehyde content is a
classic parameter to indicate the oxidative damage to membrane lipids in cells. Contents of
H
2
O
2
and O
2
.-
also can be easily measured by biochemical methods. Ordinary antioxidant
enzymes, SOD, CAT, GPXs and APX, as well as AsA and GSH contents can be conveniently
assayed by using spectrophotometer and high performance liquid chromatography. Phenolics
is composed of many types of components such as flavonoid, lutin, tannin and so on, and
their contents also can be detected by using high performance liquid chromatography.
K
+
and Na
+
Homeostasis under Salt Stress
The homeostasis of intracellular ion concentrations is critical for the metabolism of living
cells. Plant cells have to keep the concentrations of toxic ions low and accumulate essential
ions through proper regulation of ion flux. Na
+
is toxic to the plant cells because of the
similarity in physicochemical properties between Na
+
and K
+
. Na
+
competes with K
+
for
major binding sites in key metabolic processes in the cytoplasm, such as enzymatic reactions,
protein synthesis and ribosome functions (Munns and Tester 2008).
Active transport mediated by H
+
-ATPase and secondary transport with channels and co-
transporters have been evolved in plant cells to maintain high K
+
concentration and low Na
+
concentrations in the cytosol. Na
+
exclusion from cells and compartmentation of excessive
Na
+
in vacuole are important protective ways in response to ion toxicity induced by salt stress
at the cellular level. As a whole body to adapt to salt stress, plant can preserve Na
+
in the
roots and restrict Na
+
flux to the shoot or leaves, and in addition, more Na
+
may be loaded
into the organs or tissues with less physiological activity such as senescing leaves, leaf sheath
or epidermis. In some halophytes, leaves or stems can evolve to be succulent for diluting the
concentration of toxic ions, and salt glands or bladders may distribute in the shoot for the
secretion of excess ions. Recently, the molecular regulation and physiological mechanisms to
maintain normal K
+
and Na
+
homeostasis in plant cell have been reviewed in detail (Tester
and Davenport 2003; Zhu 2003; Apse and Blumwald 2007; Shabala and Cuin 2008), and it is
not required to repeat again. However, there is not a confessed way to indicate the capability
to coordinate K
+
and Na
+
homeostasis in plant cells.
