Agriculture Reference
In-Depth Information
Calcium chloride (CaCl
2
) and/or SNP-acting mustard leaves improved the
activities of nitrate reductase (NR) and carbonic anhydrase (CA), and also leaf
chlorophyll (Chl) content, leaf ion concentration, and leaf relative water content
(LRWC), in comparison with leaves treated with only NaCl. Increased levels of
H
2
O
2
and damages to membranes are caused by salinity stress, which led to
lipoperoxidation.
Alleviation of salinity stress is achieved by enhanced antioxidant enzyme activ-
ities due to the presence of SNP and CaCl
2
. This effect was eliminated in the
presence of cPTIO, indicating that SNP in association with CaCl
2
improves plant
tolerance to salinity stress by increasing antioxidative defense system, ionic homeo-
stasis, and osmolyte accumulation (Khan et al.
2012
).
Exogenous administration of SNP enhanced growth in seedlings of cucumber
particularly leaves and roots under NaCl stress (e.g., increased plant height, stem
thickness, fresh weight, and increased dry matter accumulation). Since the level of
free polyamines and the activity of polyamine oxidase (PAO) in cucumber seedling
leaves and roots initially increased under salinity stress, after the treatment, values
of spermine + spermidine/putrescine (Spd + Spm)/Put also decreased under NaCl
stress, in comparison with the control group. Improvement of plant tolerance to
salinity was shown to be dependent to the high (Spd + Spm)/Put value and to the
accumulation of Spm. The authors suggested that the mechanism by which NO
enhanced the cucumber seedlings tolerance to salinity stress involves the regulation
of the content and proportions of different
types of free polyamines (Fan
et al.
2013
).
9.2.4 Exogenous NO Donors in Plants Tolerance to Metal
Toxicity
Studies of metal toxicity in plants indicate that the plants have developed various
extra- and intracellular defense mechanisms to fight against heavy metal toxicity,
such as preventing the entry of heavy metals to root cells (e.g., mycorrhizal
association). Or if enter the second line of defense is activated, which includes
binding of metal ions with cell wall or plasma membrane, use of phytochelatins and
metallothioneins or sequestration in vacuole. The multidisciplinary approach com-
bining plant physiology, soil microbiology and biochemistry, genetic engineering,
as well as agricultural and environmental engineering will help in optimization of
plants for heavy metal detoxification (Dalvi and Bhalerao
2013
; DalCorso
et al.
2013
).
Important metal ion chelators are phytochelatins, which can be found in plants
and also in some fungi and invertebrates. Phytochelatins have key role in metal
tolerance, and they received much attention in phytoremediation programs.
Phytochelatins are oligomers of glutathione (GSH, characterized by the general
structure (
-Glu-Cys)
n
-Gly and synthesized by phytochelatin synthase, that is
γ
