Agriculture Reference
In-Depth Information
decreased growth of plant (Munns
2002
), whereas the ionic effect leads to ion
(mainly Na
+
) accumulation mainly in leaf tissues leading to necrosis. “Necrosis” is
death or degeneration of tissue, visible as yellowing or dark patches on plant leaves.
Due to excessive salt in soil, the required nutrient becomes unavailable for plants.
The salt ions (Na
+
) intervene the transporters of root plasma membrane and hamper
root growth, thus obstructing the nutrient uptake by plants (Yadav et al.
2011
). Salt
stress causes water deficit, which results in oxidative stress due to formation of
ROS, causing membrane dysfunction and cell death (Parida and Das
2005
). Lipids
also act as a target for oxidative reactions and, being structural constituent of
membranes and insulator for internal organs, damage the cellular structure aggra-
vating negative effects of the salt stress (Singh et al.
2002
). The high concentration
of solutes in root medium interferes with the water absorption by roots and reduces
root conductivity. These effects further lead to decreased plant growth and photo-
synthetic rate. The chlorophyll and carotenoid content in leaves decline under salt
stress. Symptoms of chlorosis appear on leaves due to the reduction of photosyn-
thetic pigments. Salt stress affects different physiological processes such as cessa-
tion of carbon assimilation in leaves, reduction in permeability due to dehydration,
closure of stomata affecting chloroplast activity, senescence, ionic leakage into the
cytosol leading to inactivation of photosynthetic and respiratory electron transport
(Allakhverdiev et al.
2000
; Parvaiz and Satyawati
2008
), and altered enzyme
activity due to change in cytoplasmic structure.
11.2.1.3 Metal Stress
Heavy metals (HM) can be defined as elements with metallic properties and higher
range of molecular weight and include transition elements. The industrial revolu-
tion and anthropogenic activities have dramatically raised the metal concentration
in soil (Yan-de et al.
2007
; Oves et al.
2012
). Among these metals, iron (Fe),
molybdenum (Mo), and manganese (Mn) are known as essential micronutrients
required by the plants, while a few, for example, cadmium (Cd), do not have any
biological activity. Other metals like chromium (Cr), copper (Cu), mercury (Hg),
lead (Pb), and nickel (Ni) are also common in soil. Based on the requirement, HM
can be divided into essential and nonessential elements, although the excessive
accumulation of both of these in soils adversely affects the plants (Wani et al.
2012
;
Morsy et al.
2013
) as well as soil microflora (Oliveira and Pampulha
2006
; Wani
and Khan
2010
). The plentiful HM in soil is absorbed and translocated to various
organs of plants and impairs plant metabolism and growth (Bingham et al.
1986
;
Cheng
2003
; Ahmad et al.
2012b
). The excessive metals in soil also affect soil
properties and fertility, making it unsuitable for agricultural activities.
The possible toxic impact of heavy metals on plant includes (1) disintegration of
cell organelles and (2) disruption of membranes and physiological processes like
(a) photosynthesis, (b) inactivation of protein synthesis, (c) inactivation of respira-
tion and carbohydrate metabolism, and (d) nutrient uptake (Jing et al.
2007
; Wani
et al.
2007
; Wani et al.
2008
; Khan et al.
2012
). Metal accumulation also results in
