Environmental Engineering Reference
In-Depth Information
Under salt stress, increase in Na
+
concentration and decrease in K
+
concentration are
often observed in plant leaves. Islam et al. (2007) and Luo et al. (2005) respectively
demonstrated salt-tolerant wheat and wild soybean can maintain lower level of Na
+
concentration in leaves by withholding of Na
+
in the roots and stems. Thus, the level of Na
+
and K
+
in leaves can be accepted as an indicator to evaluate the capability to resist ion
toxicity in plant under salt stress. However, Shabala et al. (2008) did not fully agree with
above viewpoint, and they believed that it was not the absolute quantity of Na
+
but rather the
cytosolic K
+
/Na
+
ratio determining cell metabolic competence and the ability of a plant to
survive in saline environments. Indeed, the cytosolic K
+
/Na
+
ratio has been repeatedly
referred as an important indicator for plant salt tolerance (Gorham et al. 1991; Gaxiola et al.
1992; Dvorak et al. 1994; Maathuis and Amtmann 1999; Cuin et al. 2003; Colmer et al.
2006), however, less direct experimental evidence can be found to support it due to no proper
measuring protocol. The major hurdle is the lack of appropriate and convenient techniques to
enable such compartmentation analysis for differentiating K
+
and Na
+
in vacuole from those
in the cytoplasm. So far, although non-invasive micro-test technique, energy-dispersive X-ray
microanalysis and multi-barrelled microelectrodes have been employed (Shabala et al. 2006),
it is hard to carry out these methods in ordinary labs due to the expensive instruments.
C
ONCLUSION AND
P
ROSPECTS
The harmful effects of salt stress on plants are related to osmotic stress, Na
+
toxicity,
nutritional imbalance and oxidative stress. Fortunately, plants can ameliorate these adverse
effects by physiological regulation. Plants accumulate compatible organic solutes such as
glycinebetaine and proline to reduce cell water potential in response to osmotic stress. An
antioxidant system can be activated to protect themselves against the attack from reactive
oxygen species under salt stress. In addition, active transport and secondary transport with
channels and co-transporters have been evolved in plant cells to maintain characteristically
high concentration K
+
and low concentrations of Na
+
in the cytosol. Thus, the procedure for
investigating physiological status in plants grown in saline soil can be concluded to two steps.
First, negative effects are evaluated by the salt stress on biomass, crop yield and
photosynthesis. Second, the underlying reasons are analyzed from three aspects including
osmotic regulation, antioxidant response and ion homeostasis.
To make out the genetic and physiological responses in plants under salt tolerance is only
the first step in developing a more salt tolerant crop. Enhancing plant salt tolerance is a
crucial step for culturing crops in saline land, and there are three methods — conventional
breeding program, gene engineering method and ordinary physiological methods. Due to the
complexity of functional genes in response to salt stress, conventional breeding program is
relatively hard to be executed effectively (Flowers 2004; Shabala and Cuin 2008; Shao et al.
2009; 2010). Although many studies have announced that salt tolerance was enhanced in
transgenic plants (Begcy et al. 2011; Hao et al. 2011; Jacobs et al. 2011; Rahnama et al. 2011;
Wei et al. 2011), the results are obtained under abnormal growth condition and not tested in
the field. Stress resistance training and exogenous application of growth and osmotic
regulators have been demonstrated to ameliorate plant salt stress (Amzallag et al. 1990;
Umezawa et al. 2000; Chattopadhayay et al. 2002; Demiral and Turkan 2006; Chen et al.
