Agriculture Reference
In-Depth Information
transpiration, and CO
2
assimilation during photosynthesis (Cornic
2000
). Also,
water stress results in reduced chlorophyll content, inhibits chloroplast activity
and disorganizes thylakoid membranes, decreases the activity of ribulose-1,5-
bisphosphate carboxylase/oxygenase and other enzymes in carbon reduction cycle
(Reddy et al.
2004
), impairs electron transport, and increases the concentration of
ROS. The imbalance in scavenging and formation of ROS and increased O
2
photoreduction in chloroplast results in ROS accumulation (Robinson and Bunce
2000
). The ROS damages photosynthetic apparatus, cell membrane, and macro-
molecules. DNA nicking, denaturation of structural and functional macromole-
cules, lipid peroxidation, oxidation of amino acids and proteins, and
photosynthetic pigments are some of the effects of ROS accumulation (Lisar
et al.
2012
). Stomata closure under drought stress is also found to be related to
altered nutritional status, xylem sap pH, and hydraulic conductivity as well as
declines water content in leaf (Oren et al.
1999
). Summarily, drought stress
interrupts the enzymatic reactions mainly involved in CO
2
fixation and ATP
synthesis and thus affects the plant by altering (1) photosynthesis, (2) transpiration,
(3) nutrient uptake, (4) hormone production, (5) homeostasis, and (6) other meta-
bolic processes.
11.2.1.2 Saline Stress
In agricultural terms, salinity can be defined as salt level exceeding the plant
requirements (Yadav et al.
2011
). In other words, it can also be defined in terms
of dissolved mineral salt concentration, i.e., electrolytes of cations and anions
where major cations involve Na
+
,Ca
2+
,Mg
2+
, and K
+
and anions involve Cl
,
SO
4
2
,CO
3
2
, HCO
3
, and NO
3
. According to USDA Salinity Laboratory
(Seidahmed et al.
2013
), saline soil is defined as soil having electrical conductivity
4dSm
1
or higher. The excessive concentrations of salt change the physico-
chemical properties of soil and affect the nutrient uptake from soil, making nutri-
ents inaccessible for plants. Some of the effects of salt stress on plants include
deteriorated growth, nitrogen content, photosynthetic capacity, and metabolic pro-
cesses including protein and lipid metabolism (Upadhyay et al.
2011
). Salt stress
has been reported to drastically affect the growth and yield of several crops (Parida
and Das
2005
; Ondrasek et al.
2011
). Broadly, effects of salt stress can be catego-
rized as (1) unavailability of water causing drought-like conditions; (2) high salt
content in plants, i.e., Na
+
and Cl
, leading to disrupted physiological and bio-
logical processes; and (3) high salt content affecting availability of other soil
nutrients. One of the most dominant symptoms of salt stress involves stunted
growth. Cessation of leaf expansion and reduction in dry weight and fresh weights
of stem, roots, and leaves are some other effects of the salt stress (Hernandez
et al.
1999
; Wang and Nil
2000
). Salt stress affects largely the shoot growth
compared to root growth and hence influences both vegetative and reproductive
stages of plants. It creates osmotic and ionic stress due to less water content and
high salt concentration, respectively. The osmolarity of external tissues results in
