2. INTRODUCTION
Soil salinity is an issue of global importance causing many socio-
economic problems. It also results in losses of (806.4 billion rupees) per year.
3. Extent and distribution of salt affected soils in India
Sr. No. State Saline soils
(ha)
Alkali soils
(ha)
Coastal saline soil
(ha)
Total
(ha)
1 Andhra Pradesh 0 196609 77598 274207
2 A & N islands 0 0 77000 77000
3 Bihar 47301 105852 0 153153
4 Gujarat 1218255 541430 462315 2222000
5 Haryana 49157 183399 0 232556
6 J & K 0 17500 0 17500
7 Karnataka 1307 148136 586 150029
8 Kerala 0 0 20000 20000
9 Maharashtra 177093 422670 6996 606759
10 Madhya Pradesh 0 139720 0 139720
11 Orissa 0 0 147138 147138
12 Punjab 0 151717 0 151717
13 Rajasthan 195571 179371 0 374942
14 Tamil Nadu 0 354784 13231 368015
15 Uttar Pradesh 21989 1346971 0 1368960
16 West Bengal 0 0 441272 441272
Total 1710673 3788159 1246136 6744968
CSSRI, 20173
14. Percentage of senescent area of rice cv. (A) Fatmawati and
(B) IR64 in high salt stress
( Hairmansis et al., 2014 )
14
15. Variations of succulence contents of the selected plant species
Leaf succulence was estimated as the ratio of plant water content
and dry weight as followed by Dehan and Tal, 1978
Youssef (2009)
15
16. Correlation between salt tolerance index (ST) and the relative
water fraction (RWF) in Arabidopsis thaliana ecotypes
( Negrao et al., 2017 )16
17. Effect of salinity on Chlorophyll content in Catharanthus roseus
(L.) on 90 DAS
( Jaleel et al., 2008 ) 17
18. Stomatal response
• Stomata – main channel for water loss and
CO2 diffusion
• Stomatal density & Stomatal conductance – to
regulate water and gas circulation.
• Stomatal conductance – regulated by osmotic
concentrations of the guard cells.
18
20. Effect of salinity stress on the number of stomata and stomatal aperture
on the upper and lower leaf surfaces of amaranthus
(Omami et al., 2006 )
20
Genotype/Nacl
conc. (mM)
Upper leaf surface Lower leaf surface
Stomatal no.
per mm2
Length of
stomatal
aperture (µm)
Stomatal no.
per mm2
Length of
stomatal
aperture (µm)
A. Tricolor
0 267 9 272 12
100 213 5 221 7
A. Cruentus
0 152 6 163 8
100 80 4 89 5
42. The major transcription factor families that are induced by the
abiotic stresses are listed below:
a) DREB1A TF includes dehydration-responsive elements (DRE’s)
b) AP2/EREBP TF (Apetala 2/ethylene responsive element binding
factor) family
c) bZIP TF family
d) MYB TF family
e) bHLH TF family, such as AtMYC2 and ICE1
f) NAC TF family, such as RD26/ANAC072, ANAC019 and
ANAC055
g) Cys2His2 zinc-finger TF family, such as ZAT12
h) Homeodomain TF family, such as HOS9
i) Cys2Cys2 zinc-finger; WRKY; HB
j) HSF functioning in stress-inducible gene expression
(Jenks et al., 2007)
42
43. Species
Nacl concentration
(mM)
Gene name Gene function Reference
Brassica juncea
Brassica
campestris
25 and 50
SOS1
SOS2
SOS3
AtNHX1
(i) Plasma membrane
Na+/K+ antiporter.
(ii) Protein kinase.
(iii) Calcium-binding protein.
(iv) Vacuolar Na+/K+ antiporter.
Chakraborty et al., 2012
Oryza sativa 50
PRP
SAG
HSPC025
(i) Proline-rich proteins and cell-
wall protection.
(ii) Senescence associated genes,
regulatory processes,
and cellular signal transduction.
(iii) Heat-shock proteins, protein
stabilizing.
Roshandel and Flowers,
2009
Oryza sativa 100
OsHSP23.7
OsHSP71.1,
OsHSP80.2
Heat-shock proteins, molecular
chaperones, folding, assembling,
and transporting proteins.
Zou et al., 2009
Arabidopsis
thaliana
150 AtSKIP
Transcription factor,
transcriptional
pre-initiation, splicing, and
polyadenylation.
Lim et al., 2010
Oryza sativa 200
OsHsp17.0,
OsHsp23.7
Heat-shock proteins, molecular
chaperones, and folding,
assembling, and transporting
proteins.
Zou et al., 2012
Carrot 300 DcHsp17.7
Cell viability and membrane
stability under heat stress.
Song and Ahn, 2011
Arabidopsis
thaliana
300 JcDREB
Transcription factor
Tang et al., 2011
Genes in response to salinity stress.
43
47. • Molecular processes that control Na+ compartmentalization in
vacuoles have received much attention, but other essential
processes in tissue tolerance of Na+ and Cl- and osmotic
adjustment remain relatively unknown.
• Significant breakthroughs have been made on the mechanisms
and control of Na+ . Nevertheless, large gaps remain in our
knowledge of Na+ transport, notably the control of phloem
transport, the identity of the genes encoding nonselective
cation channels responsible for the initial entry of Na+ into
plants, and the role of other solutes in salinity tolerance,
including K+ and Cl-.
47
FUTURE ISSUES
48. 48
No. of Research article collected : 20
No. of Abstracts collected : 16
No. of Reviews collected : 7
Editor's Notes
• Excess salt in the soil, reduces the water potential of the soil and making the soil solution unavailable to the plants (physiological drought).
30 % of irrigated crops and 7 % of dryland agriculture worldwide is limited by salinity stress.
In addition, approximately 831 M ha of land is estimated to be salt affected across different countries (Martinez-Beltran and Manzur, 2005), while about a third of the irrigated land in United States was reported to be salinized (Wichelns, 1999). Also, In California, approximately half of the total cultivated land (~ 4.5 million acres) is salinized.
The primary effects of salt on plants osmotic stress, ion toxicity, nutrient imbalance and deficiencies, resulting in membrane damage, decreased cell expansion and division, changes in metabolic processes, oxidative stress and genotoxicity.
Plant salt tolerance is a highly complex phenomenon
Two types of plants – glycophytes and halophytes
Salt tolerant mechanisms in halophytes
The effect of changes in salinity of soil solution on elongation rate of a barley leaf (a) with no control of leaf water status, and (b) with the plant maintained at balancing pressure, i.e. at constant leaf water status, throughout the changes. The vertical broken lines mark the times at which the light was turned on or off, and the broken horizontal line in (a) marks zero elongation rate. At the changes, full strength nutrient solution was exchanged with the same solution containing 75 mM NaCl, or vice versa. Passioura & Munns (2000) in experiments using a pressurization technique in which plants were
maintained at maximum water status while the soil was salinized; the transient growth reduction when the salt was applied was prevented, and so was the transient surge when the salt was removed (Fig. 1b). In leaves there are rapid, essentially instantaneous, changes
in growth rates with a sudden change in salinity (Fig. 1a). Rapid and transient reductions in leaf expansion rates after a sudden increase in salinity have been recorded in maize (Cramer & Bowman 1991; Neumann 1993), rice (Yeo e t al. 1991) and wheat and barley (Passioura & Munns 2000).
wheat
Negrao et al., 2017 conducted an experiment with two hypothetical genotypes and the growth was recorded (Fig.1). Growth of two hypothetical genotypes is shown, before (T0 to T1) and after (T1 to T2) imposition of salinity stress. genotype A grows faster than genotype B under control conditions, but its growth is inhibited more by salinity. If growth were measured by biomass increase from T0 to T2, genotype A would appear to be more salt tolerant. However, if growth were measured only from T1 to T2, then genotype B would appear to be more salt tolerant.
(MKW, mean kernel weight; K/spike, kernel number per spike; spike no, spike number per area of wheat)
The salt levels were 0 mM, 100 mM, 150 mM and 200 mM NaCl, imposed two weeks after transplantation. Senescent area was determined through colour classification from the top view fluorescent images. The results are presented as the percentage of senescent pixels area to the total shoot area of the top view image. 23% of secescence area in the cultivar Fatmawadi whereas the rice cv. IR64 shows only < 5% which indicates that the rice cv. IR64 has high salt tolerance than the cv. Fatmawati.
TWC(%) = 100 × (FW-DW)/FW.
The relative water content (RWC) was measured as the following equation: RWC(%) = 100 ×(FW - DW)/(TW - DW), where TW stands for the
turgid fresh weight.
Intro: 470, 652.4 and 665.2 nm, chlorophyll a, chlorophyll b and carotenoids, concentrations were calculated according to Linchtenthaler and Wellburn
Fig : A decrease in photosynthetic pigment content of Catharanthus plants under salt stress was observed. There was a decrease of 11% and 38% of chlorophyll a. chlorophyll b, the decrease was 16% and 33% in response to the 50 and 100-mM NaCl treatment, respectively, compared to the control. Total chlorophyll was reduced by 14% and 34% under low and high salinity, respectively
If stomatal conductance is high then it will be suitable as salinity tolerance.
Stomatal conductance of four durum wheat cultivars 1 day after the sequential addition of 50, 100 and 150 NaCl (in increments of 25 mM, twice a day).
The interactive effects of genotype and NaCl level on the number of stomata and length of stomatal aperture on the upper and lower leaf surfaces of two amaranth genotypes. In both genotypes, NaCl treatment resulted in a decrease in the number of stomata. A. tricolor had a greater number of stomata
than A cruentus. Salinity stress reduced the number of stomata on the upper and lower leaf surface by 47% and 45% in A. cruentus compared with 20% and 19% in A. tricolor. The length of stomatal aperture on the upper leaf surface was not affected by salinity in A. cruentus, whereas that of A. tricolor was reduced by 45%. On the lower leaf surface the length of stomatal aperture was reduced by 38% in A. cruentus and 42% in A. tricolor
Intro: As the NaCl solution is excreted through these glands, the water evaporates and salt crystals are formed. This poses a serious threat of leaf dehydration as thermodynamically, water moves from more dilute sites to those enriched in osmolites. Thus, the mechanism of salt excretion through glands must be accompanied by efficient osmotic adjust-ment in leaves, predominantly by mean of de novo synthe-sis of compatible solutes.
Pic. explanation: (A) Three different strategies to cope with saline environments evolved in land plants. (i) Glycophytes (all traditional crops) pump most of the taken up Na+ back to the environment to reduce the toxic salt load in their tissues. This can result in a progressive build-up of salinity in the rhizosphere and comes with a high energy cost due to futile Na+ cycling. (ii, iii) Halophytes, naturally salt-adapted plants capable of tolerating NaCl levels comparable with (or even higher than) seawater, deposit high (molar) amounts of NaCl in either swollen vacuoles in succulent tissues (ii), or in specialised external structures (iii) called epidermal bladder cells. From this point of view, salt bladders can be considered as ‘inverted vacuoles’. (B, C) Salt bladders in two halophyte species from the Chenopodiaceae family, Atriplex lentiformis (B) and Chenopodium quinoa (C). Such salt bladders are present on leaf (B) and stem (C) surfaces and provide plants with an opportunity to sequester large quantities of salt away from metabolically active parts. (D) Cross-section along the succulent leaf of a halophyte species Carpobratus rossii. Size bar in each panel is 50 mm.
In this figure, SOS pathway, H+ATPase and H+PPiase and Abscisic acid signaling pathway were shown.
Maintaining ion homeostasis by ion uptake and compartmentalization is not only crucial for normal plant growth but is also an essential process for growth during salt stress.
Transport mechanism of Na+ ion and its compartmentation (Fig. 8). The Na+ ion that enters the cytoplasm is then transported to the vacuole via Na+/H+ antiporter. Two types of H+ pumps are present in the vacuolar membrane: vacuolar type H+-ATPase (V-ATPase) and the vacuolar pyrophosphatase (V-PPase). Of these, V-ATPase is the most dominant H+ pump present within the plant cell. During non-stress conditions it plays an important role inmaintaining solute homeostasis, energizing secondary transport and facilitating vesicle fusion. Under stressed condition the survivability of the plant depends upon the activity of V-ATPase.
(A) Scanning electron microscopy image of quinoa leaf surface showing several (deflated) epidermal bladder cells. (B) A fully turgid EBC on the surface of a young quinoa leaf. (C) Close-up image of the EC–SC complex shown in (A). Both EBCs and SCs can be easily removed by a gentle brushing, suggesting an apoplastic connection within the EC–SC–EBC complex. (D) Given the pronounced concentration gradient for NaCl between bladder cells and the common epidermal cells (numbers in red), questions about the molecular mechanism of cellular Na+ and Cl transport and its regulation within the functional epidermis–bladder complex arise .
After 35 days after sowing
The principal features of photosynthetic electron transport under high light stress that lead to the production of ROS in chloroplasts and peroxisomes. Two electron sinks can be used to alleviate the negative consequences of overreduction of the photosynthetic electron chain: (a) the reduction of oxygen by PSI that generates superoxide and H2O2, and (b) the Rubisco oxygenase reaction and the photorespiratory pathway that lead to H2O2 generation within the peroxisome. Under light stress, increasing amounts of singlet oxygen are produced within PSII. Bold arrows show the main routes of electron transport.Key enzymes discussed in the text are shown in encircled numbers: 1) superoxide dismutase, 2) Rubisco, 3) glycolate oxidase, 4) catalase, and 5) ascorbate peroxidase.
The principal modes of enzymatic ROS scavenging by superoxide dismutase (SOD), catalase (CAT), the ascorbate-glutathione cycle, and the glutathione
peroxidase (GPX) cycle. SOD converts hydrogen superoxide into hydrogen peroxide. CAT converts hydrogen peroxide into water. Hydrogen peroxide is also converted into water by the ascorbate-glutathione cycle. The reducing agent in the first reaction catalyzed by ascorbate peroxidase (APX) is ascorbate, which oxidizes into monodehydroascorbate (MDA). MDA reductase (MDAR) reduces MDA into ascorbate with the help of NAD(P)H. Dehydroascorbate (DHA) is produced spontaneously by MDA and can be reduced to ascorbate by DHA reductase (DHAR) with the help of GSH that is oxidized to GSSG. The cycle closes with glutathione reductase (GR) converting GSSG back into GSH with the reducing agent NAD(P)H. The GPX cycle converts hydrogen peroxide into water using reducing equivalents from GSH. Oxidized GSSG is again converted into GSH by GR and the reducing agent NAD(P)H.
Generation and scavenging of superoxide radical and hydrogen peroxide, and hydroxyl radical-induced lipid peroxidation and glutathione
peroxidase-mediated lipid (fatty acid) stabilization. APX, Ascorbate peroxidase; ASC, Ascorbate; DHA, Dehydroascorbate; DHAR, Dehydroascorbate
reductase; Fd, Ferredoxin; GR, Glutathione reductase; GSH, Red glutathione; GSSG, Oxi-glutathione; HO . , Hydroxyl radical; LH, Lipid; L . , LOO . ; LOOH, Unstable lipid radicals and hydroperoxides; LOH, Stable lipid (fatty acid); MDHA, Monodehydro-ascorbate; MDHAR, Mono dehydro-ascorbate reductase; NE, Non-enzymatic reaction; PHGPX, Phospholipid-hydroperoxide glutathione peroxidase; SOD, Superoxide dismutase.
Gama et al. (2009) observed that the activity of APX decreased (10%) in both cultivars at 100mMNaCl after 96 hours. On the other hand, the GR activity assay also showed a slight decline (5%) and an increase (28%) in ‘HRS 516’ and ‘RO21,’ respectively, at 100mMNaCl after 96 hours exposure to salinity treatment. For CAT, there was a significant decline (P < 0.05) of about 30% in the salt treatment, although there was a slight increase in both cultivars. However, the SOD activity showed marked increase of three folds in both cultivars under salinity treatment but was not significantly higher (P < 0.01) in cultivar ‘RO21’ (Fig. 7D).
ABA is synthesized from β-carotene via the oxidative cleavage of neoxanthin and conversion of xanthoxin to ABA via ABA-aldehyde. Stresses, including salinity stress, stimulate ABA biosynthesis and accumulation by activating genes involved in the ABA biosynthetic pathway, which itself could be mediated by a calcium-dependent phosphorylation cascade. ZEP, zeaxanthin epoxidase; NCED, 9-cis-epoxycarotenoid dioxygenase; AAO, ABA-aldehyde oxidase; MCSU, molybdenumcofactor sulfurase.
Transcription – the process of making messenger RNA (mRNA) from a DNAtemplate by RNA polymerase
Transcription factor – a protein that binds to DNA and regulates gene expression by promoting or suppressing transcription
Productivity - greatly affected by various environmental stresses.
Soil salinity affects plant growth and development by way of osmotic stress, injurious effects of toxic Na+ and Cl– ions and to some extent Cl– and SO4 2– of Mg2+ and nutrient imbalance caused by excess of Na+ and Cl– ions.
