Environmental Engineering Reference
In-Depth Information
Arabidopsis
revealed a link between stress and cell division illustrating the mechanism in
molecular level (Wang et al. 1998). ICK1 (cyclin-dependent-protein-kinase inhibitor) which
is induced by ABA can reduce the activities of cyclin-dependent protein kinase to restrain cell
division and ABA is a central plant hormone under abiotic stress, involved in cellular
signaling, regulation of plant growth and stomatal conductance (Zhu 2003; Davies et al.
2005). Yet studies in barley displayed that ABA concentration increased transiently under salt
stress and returned to normal after 24h with the leaf growth rate reduced (Fricke et al. 2004).
The nearest research shows that it is DELLA protein which is a common negative factor in
plant growth and a key component in GA signal transduction that integrate signals from
hormones and abiotic stress to plant growth (Achard et al. 2006).
The accumulated Na
+
within plants also can produce ionic toxicity. Without a few
exceptions, the glycophytes especially, a large proportion of crops cannot bear Na
+
concentration at 50 mM or higher (Munns and Tester 2008). The excess salt can inhibit
enzyme activity and lead to death to leaves (Munns 2005). Or salt may build up in the
chloroplast exerting a toxic effect on photosynthetic processes and photosynthetic
components directly (Munns and Tester 2008). The MPK6 has been demonstrated to play an
important role in sodium detoxification through phosphorylating the Na
+
/H
+
antiporters and
inducing the efflux of sodium (Davies et al. 2005).
Biomass is only a broad and morphological parameter, while photosynthesis is one of the
most direct and conclusive elements in deciding plant growth and crop yields. Photosynthetic
rate would decrease if any part of its process was affected (Yan et al. 2013). For instance, the
stresses can reduce stomatal and mesophyll conductance and decreased CO
2
availability
lowering photosynthesis in end (Davies et al. 2005). The CO
2
can be destroyed by salt stress
directly, for example, the Rubisco activity which is the key enzyme in photosynthesis
(Brugnoli and Björkman 1992; Yang et al. 2008; Tarchoune et al. 2011). Gas exchange and
chlorophyll fluorescence are used to monitor the photosynthesis to reflect the disadvantages
influences (Jedmowski et al. 2013). In order to resist the disadvantageous impact, some plants
choose to change photosynthetic pathway to cope with (Parida and Das 2005). The
M.
Crystallinum
adopted to transform C3 pathway to CAM to reduce water loss (Cushman
1989). The halophytic plants
Atriplex lentiformis
is able to shift from C3 to C4 to answer the
salt stress (Meinzer and Zhu 1999). Some transgenic C3 plants displayed higher
photosynthetic efficiency and better growth characteristics (Ruan et al. 2012). Moreover, the
salt stress can also weaken photosynthesis indirectly through reducing chlorophyll and total
carotenoid contents. It was reported that the contents of total chlorophyll and carotene was
decreased by NaCl stress in leaves of tomato and
B. Parviflora
(Khavari-Nejad and Mostofi
1998; Parida et al. 2002). All in all, salinity is accompanied by a significant reduction in
photosynthesis.
O
SMOTIC
S
TRESS
E
FFECTS AND
T
OLERANCE
As is mentioned above, there are two aspects of salt tolerance: osmotic stress and ionic
stress. Osmotic stress is not only confined to drought, it is also the result of salinity. The
abundant amount of ions in the saline soil lower the flow of water causing difficult for water
uptake or even loss of intracellular water (Yan et al. 2013). On this occasion, the resistant
