3. Salicylic acid and abiotic stress tolerance

As a phytohormone, the role of SA in regulating plant growth and development is well known. The role of SA in mitigating abiotic stress has widely been studied since last few decades (Tables 1–4). A large volume of research reports indicate that both endogenous SA synthesis and exogenous application enhance plants tolerance to salinity [38–42], drought [43–45], extreme temperature [46–49], toxic metal and metalloids [50–53], and others [54–58]. Exogenous SA showed enhanced plant growth, photosynthesis, and decreased ROS production under various abiotic stresses (Tables 1–4 and Figure 2).

#### 3.1. Salinity

Among the prevailing catastrophic abiotic stresses, salinity or salt stress can be considered as the most devastating one. It shows enormous negative effects, both direct and indirect, on morphological, physiological and biochemical attributes of plants. When plants are exposed to

Figure 2. Some possible ways of SA-induced oxidative stress tolerance to plants.


Modifications of SA often render it inactive but these modifications are also related to accumulation, function, and/or mobility. Glucosylation inactivates SA and allows vacuolar storage. Methylation inactivates SA and increases its membrane permeability, volatility which is vital for long-distance transport of this defense signal. Amino acid conjugation of SA is involved in

34 Phytohormones - Signaling Mechanisms and Crosstalk in Plant Development and Stress Responses

As a phytohormone, the role of SA in regulating plant growth and development is well known. The role of SA in mitigating abiotic stress has widely been studied since last few decades (Tables 1–4). A large volume of research reports indicate that both endogenous SA synthesis and exogenous application enhance plants tolerance to salinity [38–42], drought [43–45], extreme temperature [46–49], toxic metal and metalloids [50–53], and others [54–58]. Exogenous SA showed enhanced plant growth, photosynthesis, and decreased ROS production under various

Among the prevailing catastrophic abiotic stresses, salinity or salt stress can be considered as the most devastating one. It shows enormous negative effects, both direct and indirect, on morphological, physiological and biochemical attributes of plants. When plants are exposed to

SA catabolism [37].

3.1. Salinity

3. Salicylic acid and abiotic stress tolerance

Figure 2. Some possible ways of SA-induced oxidative stress tolerance to plants.

abiotic stresses (Tables 1–4 and Figure 2).


and K<sup>+</sup> content increased


Plant species

G. jamesonii L. cv. Amaretto

B. juncea L. cv Pusa Jai Kisan

S. lycopersicum Mill. L. cv. Rio Fuego

Hordeum vulgare L. cv. Gustoe

100 mM NaCl

150 mM NaCl, 14 d

Salinity level

100 mM NaCl, 15 d

100 mM NaCl, 30 d

Effect of salinity SA

36 Phytohormones - Signaling Mechanisms and Crosstalk in Plant Development and Stress Responses

• EL and MDA content increased • Activities of SOD, POD, CAT and APX increased • Higher Pro content

• TBARS and H2O2 contents increased 2.5-times and 3.8-times respectively, compared to con-

• Increased Na+ and Cl contents in leaves • Increased activities of DHAR, APX and GR by 30, 217 and 79%, respectively compared to con-

• Activities of ATPS and Serine acetyl transferase (SAT) and cystein (Cys) contents increased by 30, 23, and 70%, respectively, but S content decreased by 32% compared to con-

• Increased DHA, GSH, and GSSG contents but, decreased AsA content • Reduced net photosynthesis, gs and Ci by 40.0, 26.4, and 41.3%, respectively, compared to con-

• Higher accumulation of ABA in both leaves and

• Shoot FW and height decreased by 30 and 36% respectively, compared to

trol

trol

trol

trol

root • Ethylene production increased • Reduced net CO2 fixation

rate

control • Photosynthetic pigment contents decreased by 57% compared to control • MDA content increased by 40% compared to con-

trol

application

0.5 mM SA pretreatment, 2 d

0.5 mM SA spray, 15 d

Pretreated with 0.01 μM and 100 μM SA, 21 d

50 μM SA, 14

d

Protective effects Reference

Kumara et al. [39]

Nazar et al. [41]

Horváth et al. [62]

Fayez and Bazaid [63]

• EL and MDA content decreased • Activities of SOD, POD, CAT and APX further

• TBARS and H2O2 contents were decreased sig-

increased • Lower Pro content

nificantly • Reduced Na+ content by 36% and Cl content by half compared to salt treated plants • Activities of DHAR, APX and GR further increased by 54, 248 and 111%, respectively compared to

control • Activities of ATPS and SAT, and contents of Cys and S increased by 87, 76, 128, and 63% respectively, compared to con-

trol • GSH content further increased while DHA and GSSG both were reduced, AsA content

increased • Limited the decreases in the above characteristics to 22, 19 and 25% respectively, compared to con-

trol

• Lower accumulation of ABA compared to stressed plants • Prevented higher production of ethylene • Net CO2 fixation rate enhanced

• Shoot FW and height increased compared to NaCl treated plants • Photosynthetic pigment contents decreased by only 39% compared to

• MDA content was lower compared to salt-stressed

control

plants • Na<sup>+</sup> content decreased and K<sup>+</sup> content increased


Table 1. Salicylic acid–mediated tolerance of different plant species to salinity stress.

salt, they not only face osmotic and ionic stresses, but also water stress and other subsequent stresses may emerge. These ultimately reduce the quality and quantity of the desired yield. However, good news is there are certain species that show some tolerance mechanisms and also some protectants that can help plants to develop tolerance against the salt stress. In the recent era where global warming and rising of sea level are the most alarming issues, these can be promising facts to be considered for further research. There are a number of studies that prove the protective roles of SA against salt stress in many plant species (Table 1).

Salicylic acid has been proved to have effective roles on enhancing the germination percentage, shoot and root length, fresh weight (FW) and dry weight (DW) of both shoot and root of plants, uptake of beneficial ions, and also some antioxidant enzyme activities. It also has been proved to reduce the toxic ions uptake and accumulation in plants, membrane damage and transpiration rate, etc. Photosynthesis, growth, and yield were improved and oxidative damages were ameliorated with the application of effective concentrations of SA in different plant species. To demonstrate the role of SA in alleviating the salt stress-induced damage, an experiment was conducted by Arfan et al. [59] with two Triticum aestivum varieties, of which one is salt-tolerant (S-24) and another one is salt-sensitive (MH-97). They applied different levels of SA starting from 0.25 to 1.00 mM and created salt stress with 150 mM NaCl in the


salt, they not only face osmotic and ionic stresses, but also water stress and other subsequent stresses may emerge. These ultimately reduce the quality and quantity of the desired yield. However, good news is there are certain species that show some tolerance mechanisms and also some protectants that can help plants to develop tolerance against the salt stress. In the recent era where global warming and rising of sea level are the most alarming issues, these can be promising facts to be considered for further research. There are a number of studies that

Salicylic acid has been proved to have effective roles on enhancing the germination percentage, shoot and root length, fresh weight (FW) and dry weight (DW) of both shoot and root of plants, uptake of beneficial ions, and also some antioxidant enzyme activities. It also has been proved to reduce the toxic ions uptake and accumulation in plants, membrane damage and transpiration rate, etc. Photosynthesis, growth, and yield were improved and oxidative damages were ameliorated with the application of effective concentrations of SA in different plant species. To demonstrate the role of SA in alleviating the salt stress-induced damage, an experiment was conducted by Arfan et al. [59] with two Triticum aestivum varieties, of which one is salt-tolerant (S-24) and another one is salt-sensitive (MH-97). They applied different levels of SA starting from 0.25 to 1.00 mM and created salt stress with 150 mM NaCl in the

prove the protective roles of SA against salt stress in many plant species (Table 1).

Plant species

Medicago sativa cv. Aragon

T. aestivum L. cv. Yumai 34

Z. mays L., Hamidiye F1

S. lycopersicum cv. Rio Fuego

Salinity level

200 mM, 12 d

250 mM NaCl, 3 d

40 mM NaCl, 56 d

100 mM NaCl, 7 d

Effect of salinity SA

38 Phytohormones - Signaling Mechanisms and Crosstalk in Plant Development and Stress Responses

• Reduced the FV/FM ratio

by 15% • Decreased the nodule mass about 60% • Activities of CAT, APX and GR reduced but SOD, POX, and DHAR

increased

• GSH content increased but AsA content decreased

• Membrane permeability and MDA content increased • Higher Na+ and Cl concentrations • Decreased N and

increased P concentra-

(AsA) and GSH contents increased in leaves and

Table 1. Salicylic acid–mediated tolerance of different plant species to salinity stress.

• Activities of APX, GR increased and SOD, CAT

decreased • Both total ascorbate

tions

roots

application

Pretreated with 0.1 and 0.5 mM, 2 d

0.5 mM SA, 3 d

Pretreated with 0.1 μM or 100 μM SA, 21 d

Soil incorporated with 0.5 mM SA, 56 d

Protective effects Reference

Palma et al. [20]

Li et al. [66]

Gunes et al. [60]

Tari et al. [42]

• Reduced the FV/FM ratio by only 2% • Reduced the decreasing of the nodule mass • Only CAT activity further reduced, but the activities of APX, GR, SOD, POX and DHAR further increased

• GSH content further

• Ameliorated the deterioration of membrane and reduced MDA content • Lower accumulation of Na<sup>+</sup> And Cl ions • Increased N and

decreased P concentra-

• Activities of APX, GR further increased along with SOD and CAT • Further increase of total AsA and GSH

content

tions

increased along with AsA


Table 2. Salicylic acid–mediated tolerance of different plant species to drought stress.

growing media. Both SA and NaCl were exposed to plants from the very beginning and data were taken at 30 days after sowing (DAS). Their results showed that at lower concentration


growing media. Both SA and NaCl were exposed to plants from the very beginning and data were taken at 30 days after sowing (DAS). Their results showed that at lower concentration

Plant species Drought

Z. mays Withholding

Musa acuminata cv. 'Berangan', AAA

H. vulgare L. cv Nosrat

O. sativa L. cv. Super-Basmati

T. aestivum L. cv. Hassawi

condition

water, 5 d

1, 2 and 3% of PEG in vitro, 60 d

Water was reduced to 50% of field capacity

Reduced water to 60 and 30% field capacity

T. aestivum L. 20 % PEG, 24 h

Effect of drought SA

40 Phytohormones - Signaling Mechanisms and Crosstalk in Plant Development and Stress Responses

• Decreased antioxidant enzyme activities

• Decreased RWC, leaf MSI, chl and K content

• Slight increase in proliferation rate,

> decreased the dry mass and net CO2 assimilation rate

• Increased H2O2, MDA, and relative membrane permeability

• Decreased MSI, increased total soluble sugar and soluble protein content but decreased yield

• Decreased content of photosynthetic pigments • Increased soluble carbohydrate, protein, and Pro • Decreased insoluble carbohydrates and proteins • Free amino acids were significantly increased in roots, while it was decreased in shoots

Table 2. Salicylic acid–mediated tolerance of different plant species to drought stress.

FW • Decreased RWC and chl content

40% FC • Drought stress

application

100, 150, 200 ppm

1.0, 2.0, and 3.0 mM SA

Spraying with 500 μM SA, 15 d

100 mg L<sup>1</sup> SA

10 μM SA, 24

h

Protective effects Reference

Rao et al. [79]

Bidabadi et al. [80]

Habibi [81]

Farooq et al. [82]

Khan et al. [73]

Azooz and Youssef [83]

• SA conferred drought tolerance mediated by

• Increased RWC, leaf MSI, chl, and K con-

• Increased Pro content • Reduction in H2O2 and MDA contents

• Increased the dry mass, net CO2 assimilation rate and gs

• Increased tissue water potential, increased synthesis of metabolites and enhanced capacity of the antioxidant system

• Increased MSI,

tein content • Increased yield

50 ppm SA • Stimulated growth,

increased total soluble sugar and soluble pro-

photosynthetic pigments and accumulation of soluble and insoluble carbohydrates and proteins

H2O2 • Reduced leaf rolling

tents



Plant species Temperature

Matricaria chamomila

Cucumis sativa

Musa acuminata and duration

HT, Min (10.1–28.2C), Max (21– 44C), 8 months

HT (40C, 36 h)

40C, 24 h)

Chilling (5C, 3 d)

C, 48–72 h)

0.5C, 12 h)

B. juncea HT (30 or

T. aestivum Chilling (3

V. vinifera HT (38

Damaging effects SA dose and

• Reduced plant

affected

ties

plant • Decreased Pn, gs, Ci, WUE and SPAD

value • Increased activities of CAT, POD SOD, and Pro accumula-

tion • Decreased N, P, K contents in leaves

• Reduced SOD, CAT, APX activities but improved POX activity • Increased accumulation of H2O2

• Reduced chl, CO2 assimilation and rate of respiration • RuBisCO activity decreased • Decreased SOD • Increased glycolate oxidase (GO) and CAT activities • Highest MDA content found

• Reduced H+ and Ca2


+

• Decreased root

length, shoot length, FW and DM of the

• Highly increased EL • Increased H2O2 and TBARS contents • Improved SOD, CAT, DHAR, GPX, APX and GR activi-

height, capitol diameter, fresh flower weight, dried flower weight etc. • Decreased total chl content • Essential oil content was not significantly

42 Phytohormones - Signaling Mechanisms and Crosstalk in Plant Development and Stress Responses

duration

foliar spraying

1 mM SA, foliar spraying, 12 h

10 μM, foliar spraying

0.5 mM, pretreatment, 1 d

500 μM, foliar spraying, 24 h

100 μM, pretreatment, 6 h

1, 10, 25 and 100 mg L<sup>1</sup> ,

Protective effects References

Ghasemi et al. [96]

Shi et al. [97]

Hayat et al. [98]

Kang et al. [99]

Yordanova and Popova [100]

Liu et al. [101]

• Improved plant height, capitol diameter, fresh flower weight, dried flower weight etc. • Increased total Chl con-

• Improved essential oil content

> DHAR, GPX, APX and GR activities but inhibited CAT activity

tent

• Decreased EL • Reduced H2O2 and TBARS contents • Improved SOD,

• Root length, shoot length, DW and FW of plant increased • Increased Pn, gs, Ci, WUE and SPAD value • Further enhancement of CAT, POD SOD, and Pro accumulation • Improved N P, K contents in leaves

• Increased SOD, CAT and APX activities but unaffected POX activity

overproduced H2O2

• Improved Chl content and rubisco activity • Enhanced CAT, APX, POX, and glycolate oxidase (GO) activities but GR activity found not

• Increased H+ and Ca2+- ATPase activities • Remained cerium phosphate grain

• Decreased

affected • Reduced MDA content


Table 3. Summary of the protective roles of exogenous SA in mitigating extreme temperature-induced damages in different crop plants.

(0.25 mM) SA could reduce the negative effects of salt on the shoot and root FW and DW in tolerant variety, but this was true at higher concentration (0.75 mM) for the sensitive one. However, yield attributes such as grain yield and 100-grain weight were increased in both the varieties at lower concentrations (0.25 and 0.50 mM) SA under salt stress. Similarly, application of SA also increased the water use efficiency (WUE) of those varieties. Similarly, Nazar et al. [12] also chose two such varieties of Vigna radiata cvs. Pusa Vishal (salt-tolerant) and T44 (saltsensitive). They also used different concentrations of SA (0.1, 0.5, and 1.0 mM) against 50 mM NaCl stress and 0.5 mM was concluded as the most suitable concentration for both varieties irrespective of their tolerance ability. At this concentration, V. radiata seedlings could reduce the accumulation of toxic Na+ and Cl ions, and increase S and N uptake and nitrate reductase activity. Salicylic acid application also enhanced the water and osmotic potential which was higher in the tolerant one. Stomatal conductance (gs), intercellular CO2 concentration (Ci), and chlorophyll (chl) fluorescence were increased along with leaf ATP-sulfurylase activity. However, mainly the reduction of electrolyte leakage (EL), malondialdehyde (MDA), H2O2, oxidized glutathione (GSSG) contents, superoxide dismutase (SOD) activity and enhancement of reduced glutathione (GSH) content, and ascorbate peroxidase (APX) and glutathione reductase (GR) activities prove the role of SA in reducing salt-stress damages. Similar results were observed with V. radiata seedlings. These seedlings were exposed to 100 mM NaCl at 10 DAS and then was spraying SA (0.5 mM) at 15 DAS. At 30 DAS some parameters were monitored related to gas exchange e.g., net photosynthesis (Pn), gs, and Ci; and also carboxylation efficiency, WUE, and plant dry mass [17]. Application of SA increased Pn, gs, and Ci by 17.9, 19.2, and 23.5%, respectively, under salt stress condition. It also enhanced carboxylation


(0.25 mM) SA could reduce the negative effects of salt on the shoot and root FW and DW in tolerant variety, but this was true at higher concentration (0.75 mM) for the sensitive one. However, yield attributes such as grain yield and 100-grain weight were increased in both the varieties at lower concentrations (0.25 and 0.50 mM) SA under salt stress. Similarly, application of SA also increased the water use efficiency (WUE) of those varieties. Similarly, Nazar et al. [12] also chose two such varieties of Vigna radiata cvs. Pusa Vishal (salt-tolerant) and T44 (saltsensitive). They also used different concentrations of SA (0.1, 0.5, and 1.0 mM) against 50 mM NaCl stress and 0.5 mM was concluded as the most suitable concentration for both varieties irrespective of their tolerance ability. At this concentration, V. radiata seedlings could reduce the accumulation of toxic Na+ and Cl ions, and increase S and N uptake and nitrate reductase activity. Salicylic acid application also enhanced the water and osmotic potential which was higher in the tolerant one. Stomatal conductance (gs), intercellular CO2 concentration (Ci), and chlorophyll (chl) fluorescence were increased along with leaf ATP-sulfurylase activity. However, mainly the reduction of electrolyte leakage (EL), malondialdehyde (MDA), H2O2, oxidized glutathione (GSSG) contents, superoxide dismutase (SOD) activity and enhancement of reduced glutathione (GSH) content, and ascorbate peroxidase (APX) and glutathione reductase (GR) activities prove the role of SA in reducing salt-stress damages. Similar results were observed with V. radiata seedlings. These seedlings were exposed to 100 mM NaCl at 10 DAS and then was spraying SA (0.5 mM) at 15 DAS. At 30 DAS some parameters were monitored related to gas exchange e.g., net photosynthesis (Pn), gs, and Ci; and also carboxylation efficiency, WUE, and plant dry mass [17]. Application of SA increased Pn, gs, and Ci by 17.9, 19.2, and 23.5%, respectively, under salt stress condition. It also enhanced carboxylation

Table 3. Summary of the protective roles of exogenous SA in mitigating extreme temperature-induced damages in

Damaging effects SA dose and

44 Phytohormones - Signaling Mechanisms and Crosstalk in Plant Development and Stress Responses

• Increased endogenous SOD and SA activities

• Disturbed seedling emergence, root and shoot growth, FW, and DW • Lowered RWC and increased membrane permeability

• Hampered seedling growth • Increased EL • Increased CAT, POX enzyme activities

duration

50, 100, and 150 mg L<sup>1</sup> , seed priming, 24 h

10 and 20 μM, pretreatment, 2 h

Protective effects References

Farooq et al. [108]

Kaur et al. [3]

• Enhanced defense system and radical scavenging mechanism

• Improved seedling emergence, root and shoot growth, FW and

• Conferred protection of membrane • Reduced EL

• Increased FW, DW, and total soluble sugar • Increased CAT, POX, and appearance of heat shock proteins

DW • Increased RWC and decreased membrane permeability • Highly activated CAT, SOD and APX activities

Plant species Temperature

Z. mays Chilling

Brassica sp. HT (40–55C, 3 h)

different crop plants.

and duration

(15C)

45



Plant

Toxic metals/

Doses and

Toxic effects

•

Increased activities of SOD, CAT,

APX, and GR

SA doses and

Protective effects

References

duration

metalloids

duration

species

T. aestivum

Cd

100, 400 and 1000

•

Inhibited root growth and

enhanced Cd

roots

•

Decreased RWC, chl content, and

CO2 fixation

> •

Increased MDA, H

contents

•

Altered root and chloroplast

ultrastructure

Linum

Cd

50 and100 mM

•

Inhibited growth and nutrient

250 and1000 μM,

•

Alleviated growth inhibition

Belkhadi et al.

46 Phytohormones - Signaling Mechanisms and Crosstalk in Plant Development and Stress Responses

[119]

and nutrient absorption

presoaking

grains, 8 h

 of •

Ameliorated

MDA content and EL

> •

Alleviated the harmful effect

on total lipid and Chl contents

 the enhanced

absorption

• •

Reduced total lipid and chl con-

tents

Enhanced MDA content and EL

CdCl2,4d

usitatissimum

Phaseolus

Cd

50 and 100 μM

•

Enhanced H duction in the root

> •

Increased TBARS content and

relative EL rate

> •

Increased antioxidant such as SOD, CAT and APX

activities

T. aestivum

Z. mays

Cr

50 mg L1 K2Cr

7 d

O2 7,

•

Decreased growth, photosyn-

100 μM L1, foliar

•

A significant decline in MDA,

Islam et al. [52]

H

O2 2, Pro and Cr contents

application,

 15 d

thetic pigments, CHO metabo-

lism

Cd

500 and 1000 μM

•

Reduced growth, chl content,

500 μM, seed

•

Mitigated adverse effects of Cd

Agami and

Mohamed [121]

on chl, RWC and SOD, CAT

and POX activities

> •

Alleviated damaging effects on

EL

•

Improved leaf anatomy and

reduced uptake of Cd

soaking, 12 h

RWC and SOD, CAT, POX activi-

ties

•

Enhanced Pro, EL and Cd con-

tents

CdCl2,3d

 enzymes

O2 2 and O2

•

pro-

100 μM SA, seed

•

Significantly

O2

•

Enhanced TBARS content and

relative EL

> •

•

SOD, CAT and APX activities

Further increased

production

 in the root

•

 decreased H

O2 2,

Zhang et al.

[120]

soaking, 16 h

CdCl2, 3 or 6 d

aureus

O2 2 and Pro

accumulation

 in

pretreatment,

 20 h •

Ameliorated

effects on RWC, chl content,

and CO2 fixation

> •

Reduced MDA, H

contents

•

Recovered chloroplast

ultrastructures

 and root

O2 2 and Pro

 the adverse

500 μM,

•

Reversed root growth inhibi-

Moussa and El-

Gamal [118]

tion

μM CdCl2 . 2.5

H O, 30 d

2



Plant

Toxic metals/

Doses and

Toxic effects

•

Inhibited APX, CAT, MDA and

Pro contents

> •

POD, SOD, and soluble sugar

contents were affected

> P. pratensis

Cd

5,10, or 50 mM

•

Reduced growth, chl and nutri-

500 mM SA,

•

Increased growth, chl, and

Guo et al. [127]

> nutrient (K, Ca, Fe, Mg) ele-

ments

•

Decreased MDA and H

contents and Cd uptake

> •

Marked increase in SOD, APX,

and POD but decreased CAT

activity

O2 2

pretreatment,

 12 h

ent (K, Ca, Fe, Mg) elements

> •

Elevated MDA and H

tents

•

Increased Cd uptake and accu-

mulation

Spinacia

B

50 mg kg1, H3BO3 •

Enhanced B

plants

•

Decreased chl and

contents

•

Increased MDA, H

matal resistance Increased storage root diameter

0.5 mM kg1

,

• •

Increased carotenoid

•

Controlled

pro shoots

 metal toxicity and

accumulation

 in roots and

 contents

anthocyanin

 and

[129]

Enhanced storage root DW

Eraslan et al.

cotreatment

•

Increased oxidative damage as

indicated by increased MDA

content

•

Lowered chl content

O2 2 and sto-

anthocyanin

Accumulation

 in

0.5 mM kg1

,

• •

Improved chl and

contents

•

Influenced

enzymes activity and stomatal

resistance

 antioxidant

anthocyanin

[128]

Decreased B

accumulation

Eraslan et al.

48 Phytohormones - Signaling Mechanisms and Crosstalk in Plant Development and Stress Responses

cotreatment

oleracea

Daucus

B

25 mg kg1, H3BO3 •

carota

B. juncea

Mn

3.0, 6.0, or 9.0 mM

•

Decreased growth, photosyn-

10 μM, 14 d, foliar

•

Improved growth, photosyn-

Parashar et al.

[130]

thetic pigments, carbonic

anhydrase activity, and water

relations

•

Lowered MDA, H

in a

> •

Further accelerated

antioxidant

SOD, POD)

 enzymes (CAT,

dose-dependent

 manner

 activity of

O2 2, and EL

application

thetic pigments, carbonic

anhydrase activity, and water

relations

•

Increased MDA, H

mulation

•

Elevated antioxidant (CAT, SOD, POD) activity in a

dose-dependent

 manner

 enzymes

O2 2, Pro accu-

MnCl2,3d

O2 2 con-

CdCl2,7d

SA doses and

Protective effects

enzymes activities. Lowered

MDA and Pro contents

> •

Reduced the adverse effects of

Pb on POD, SOD, and soluble

sugar contents

References

duration

metalloids

duration

species



efficiency and WUE compared to salt-stressed plants. Plant dry mass was increased by 25.2% under salt condition compared to control plants. Meanwhile, Zea mays was tested with different levels of SA under salt stress, and positive roles of SA was demonstrated in ameliorating the membrane damage by reducing MDA content [60]. It can also decrease the accumulation of Na+ and Cl ions and increase uptake of N and P and thus render tolerance to plants against salt stress. Another experiment was conducted with Lens esculenta, which included only four with Lens esculenta, which included only four treatments: nonsaline control (I), 0.5 mM SA (II), 100 mM NaCl (III), and the combination of 100 mM NaCl + 0.5 mM SA (IV). The results showed that growth parameters: germination (%), shoot and root length, FW and DW were improved in treatment (IV) compared to treatment (III). In addition, SA increased the free Pro and GB content in shoot and also the activities of pyrroline-5-carboxylate reductase (P-5-CR) and γ-glutamyl kinase which are the enzymes related to Pro anabolism. But, in contrast, it reduced the activity of Pro oxidase [61]. In the case of pretreatment with SA, it also showed some positive results. Solanum lycopersicum seeds pretreated with 10 μM SA improved the chl content and reduced MDA content under salt stress (100 mM NaCl) [38]. Higher accumulation of abscisic acid (ABA) in shoot and enhancement of water potential of SA-treated seedlings compared to the seedlings exposed to salt alone were also observed [38]. These results were supported by Horváth et al. [62] in the same plants with the equal concentrations of SA. Similarly, when pretreatment with SA (0.5 and 1.0 mM) was done, Gerbera jamesonii seedlings also showed positive results in salt stress (100 mM). Salicylic acid application reduced the EL, MDA and Pro contents and increased the activities of SOD, catalase (CAT), peroxidase (POD), and APX compared to salt stressed seedlings [39]. But, these effects were more acceptable in case of lower (0.5 mM) concentration of SA. Recently, Nazar et al. [41] again used SA (0.5 mM) to demonstrate the preventive role of it in Brassica juncea seedlings exposed to 100 mM of NaCl stress for 30 consecutive days. Application of SA reduced thiobarbituric acid reactive substances (TBARS) and H2O2 contents, also dehydroascorbate (DHA) and GSSG contents. It was found to increase the activities of dehydroascorbate reductase (DHAR), APX, and GR to a remarkable content. And most importantly, it reduced the toxic Na+ and Cl uptake to almost half of the salt-stressed plants [41].

From the above-mentioned studies, the role of SA in alleviating salt stress can be considered as clear and concise. But, there are also some points to be considered as higher concentrations of SA may itself cause damage to plants [12, 59] and very lower concentrations may have a minimum effect [42, 62] against salt stress. So, the concentration of SA, application method and time, duration of salt stress and plant age are some of the important points to be considered while using SA against salt stress.

#### 3.2. Drought

Plant

Toxic metals/

Doses and

Toxic effects

•

Increased MDA, nonprotein

thiol, EL percentage

tents

•

Increased POD and SOD activi-

ties

Pisum

Cd

0.5, 1, 2, and 5 μM

•

Decreased FW, CO2 fixation, chl

500 μM, seed

•

Restored FW, CO2 fixation, chl

Popova et al.

[136]

content and RuBPC activity

pretreatment,

 6 h •

Alleviated the effects on MDA,

Pro contents, and EL percent-

age

content and RuBPC activity

> •

Increased MDA, Pro content, and

EL percentage

CdCl2

sativum

Cannabis

Cd

CdCl2. 2.5 H O at 0,

25, 50, and

100 mg kg1 sands

2

• •

Slightly reduced

capacity

•

Increased Cd uptake

photosynthetic

soaking, 6 h

500 μM, seed

•

Counteracted

tion

•

Improved

capacity

• •

Enhanced SOD and POD

activities

Reduced Cd uptake

photosynthetic

 growth inhibi-

Shi et al. [137]

50 Phytohormones - Signaling Mechanisms and Crosstalk in Plant Development and Stress Responses

Inhibited plant growth

sativa

H. vulgare

Zn2+, Cu2+,

0.1, 0.2, 0.5, and 1

•

Inhibitory effects on SOD and

2 mM,

7 d

cotreatment,

•

Alleviated the harmful effects

Song et al. [138]

> on antioxidant

SOD) activities

 enzymes (CAT,

CAT at higher dose

mM Zn2+, Cu2+, Mn2, Cd2+, Hg2+,

Mn2, Cd2+,

Hg2+, and

Pb2+,7d

Table 4.

Summary of the protective roles of SA in mitigating

 toxic

metal/metalloid–induced

 damages in different crop plants.

and Pb2+

 and Pro con-

SA doses and

Protective effects

References

duration

•

In the case of Cr, SA accelerated the toxic effects and the

plats died

metalloids

duration

species

Drought stress is one of the most devastating abiotic stresses adversely affecting growth and developmental processes of the plant. Drought stress affects the physiological processes, brings biochemical changes, leads to the formation of secondary metabolites, significantly accumulates endogenous reactive oxygen species (ROS) and increases toxins (such as methylglyoxal). Drought stress hampering the reproductive development drastically reduces yield or productivity of plants [67].

Several studies demonstrated and proved the pivotal roles of SA in alleviating drought damage and improving drought stress tolerance in plants (Table 2). Salicylic acid pretreatment (0.5 mM) alleviated substantial water loss and its damaging effects on wheat seedlings that enhanced drought tolerance [43]. Pretreatment with SA upregulated 37 protein spots under drought stress which has been investigated through proteomics. Glutathione S-transferases, APX, and 2-cysteine peroxiredoxin were enhanced under drought stress. Enhancement of antioxidant defense system worked against the oxidative damage [43]. Proteins involved in ATP synthesis are also upregulated by SA under drought stress. Salicylic acid supplementation with drought also upregulated 21 protein spots, including RuBisCo and related enzymes [43]. In their other experiment, influential role of SA was also demonstrated on AsA-GSH cycle [68]. Exogenous SA supplementation enhanced the transcription of GST1, GST2, GR, and MDHAR genes during almost the entire drought period. The increase of DHAR was noticed at 12 h, GPX1 at 48 h, phospholipid hydroperoxide glutathione peroxidase (GPX2) at 12 and 24 h, and glutathione synthetase (GSHS) at 12, 24, and 48 h of drought stress. Upregulation of transcription level of AsA-GSH cycle enzymes contributed to drought tolerance [68]. SA-accumulating (siz1 and cpr5) genes were highly expressed in guard cells of drought which modulated movement of stomatal aperture in Arabidopsis plants. The generation of ROS was also modulated in this plant [69]. In tomato (Lycopersicon esculentum), SA treatment with drought has been demonstrated to protect the activity of nitrate reductase which helps to maintain the protein and nitrogen contents of the leaves, compared to the drought affected plant without SA addition. Photosynthetic parameters, membrane stability, water potential and activity of carbonic anhydrase were maintained by SA which also contributed to drought stress tolerance [70]. In sunflower, water stress-induced decrease in the yield and oil content. Salicylic acid (0.724 mM) application increased the Pro content, head diameter, number of achene, 1000 achene weight, achene yield, and oil yield of sunflower, compared to drought treatment alone [71]. The addition of acetyl SA in (0.1–1.0 mM) also improved drought tolerance of muskmelon seedlings [72]. Two wheat varieties viz. Wafaq-2001 and Punjab-96 were subjected to drought stress. Drought stress significantly decreased membrane stability index (MSI) and yield. Salicylic acid supplementation caused 37% increase in soluble sugars in Wafaq-2001 cultivar which was higher, compared to Punjab-96 cultivar. Salicylic acid also increased protein content and MSI in both cultivars with a higher increase in Wafaq-2001. The overall drought tolerance was higher in Wafaq-2001 after SA application which is evident from higher yield [73]. Exogenous addition of SA increased the activity of antioxidant enzymes which helped to alleviate the drought stress damage in Ctenanthe setosa [74]. Mustard (B. juncea L. cv. BARI Sharisha 11) seedlings were subjected to two different levels of drought with 10 and 20% polyethylene glycol (PEG) for 48 h. Leaf relative water content (RWC), chl b and chl (a + b) decreased but Pro content increased. Disrupting the antioxidant defense system, drought stress increased oxidative damage which was indicated by high MDA and H2O2 levels. Supplementation of SA in drought-stressed seedlings increased the leaf RWC and chl content, increased the AsA and GSH, decreased the GSSG content, and maintained a higher ratio of GSH/GSSG. Salicylic acid increased the activities of monodehydroascorbate reductase (MDHAR), DHAR, GR, glutathione peroxidase (GPX), CAT, glyoxalase I (Gly I), and glyoxalase II (Gly II) in drought affected seedlings as compared to the drought-stressed plants without SA supplementation, with a concomitant decrease in H2O2 and lipid peroxidation level [44]. Methyl-SA (at 0.1 mM) spray promoted drought-induced leaf senescence in Salvia officinalis [75]. Drought stress adversely affected growth performance of winter wheat, Cheyenne. Application of SA analogue 4-hydroxybenzoic acid (4-HBA) increased drought tolerance of winter wheat Cheyenne [16]. Foliar application of SA (10 μM) protects lemongrass (Cymbopogon flexuosus Steud. Wats.) varieties (Neema and Krishna) from drought stress by improving growth parameters, modulating the activities of nitrate reductase, carbonic anhydrase, and EL, Pro content, free amino acid, and in PEP carboxylase activity [76].

#### 3.3. Extreme temperatures

Several studies demonstrated and proved the pivotal roles of SA in alleviating drought damage and improving drought stress tolerance in plants (Table 2). Salicylic acid pretreatment (0.5 mM) alleviated substantial water loss and its damaging effects on wheat seedlings that enhanced drought tolerance [43]. Pretreatment with SA upregulated 37 protein spots under drought stress which has been investigated through proteomics. Glutathione S-transferases, APX, and 2-cysteine peroxiredoxin were enhanced under drought stress. Enhancement of antioxidant defense system worked against the oxidative damage [43]. Proteins involved in ATP synthesis are also upregulated by SA under drought stress. Salicylic acid supplementation with drought also upregulated 21 protein spots, including RuBisCo and related enzymes [43]. In their other experiment, influential role of SA was also demonstrated on AsA-GSH cycle [68]. Exogenous SA supplementation enhanced the transcription of GST1, GST2, GR, and MDHAR genes during almost the entire drought period. The increase of DHAR was noticed at 12 h, GPX1 at 48 h, phospholipid hydroperoxide glutathione peroxidase (GPX2) at 12 and 24 h, and glutathione synthetase (GSHS) at 12, 24, and 48 h of drought stress. Upregulation of transcription level of AsA-GSH cycle enzymes contributed to drought tolerance [68]. SA-accumulating (siz1 and cpr5) genes were highly expressed in guard cells of drought which modulated movement of stomatal aperture in Arabidopsis plants. The generation of ROS was also modulated in this plant [69]. In tomato (Lycopersicon esculentum), SA treatment with drought has been demonstrated to protect the activity of nitrate reductase which helps to maintain the protein and nitrogen contents of the leaves, compared to the drought affected plant without SA addition. Photosynthetic parameters, membrane stability, water potential and activity of carbonic anhydrase were maintained by SA which also contributed to drought stress tolerance [70]. In sunflower, water stress-induced decrease in the yield and oil content. Salicylic acid (0.724 mM) application increased the Pro content, head diameter, number of achene, 1000 achene weight, achene yield, and oil yield of sunflower, compared to drought treatment alone [71]. The addition of acetyl SA in (0.1–1.0 mM) also improved drought tolerance of muskmelon seedlings [72]. Two wheat varieties viz. Wafaq-2001 and Punjab-96 were subjected to drought stress. Drought stress significantly decreased membrane stability index (MSI) and yield. Salicylic acid supplementation caused 37% increase in soluble sugars in Wafaq-2001 cultivar which was higher, compared to Punjab-96 cultivar. Salicylic acid also increased protein content and MSI in both cultivars with a higher increase in Wafaq-2001. The overall drought tolerance was higher in Wafaq-2001 after SA application which is evident from higher yield [73]. Exogenous addition of SA increased the activity of antioxidant enzymes which helped to alleviate the drought stress damage in Ctenanthe setosa [74]. Mustard (B. juncea L. cv. BARI Sharisha 11) seedlings were subjected to two different levels of drought with 10 and 20% polyethylene glycol (PEG) for 48 h. Leaf relative water content (RWC), chl b and chl (a + b) decreased but Pro content increased. Disrupting the antioxidant defense system, drought stress increased oxidative damage which was indicated by high MDA and H2O2 levels. Supplementation of SA in drought-stressed seedlings increased the leaf RWC and chl content, increased the AsA and GSH, decreased the GSSG content, and maintained a higher ratio of GSH/GSSG. Salicylic acid increased the activities of monodehydroascorbate reductase (MDHAR), DHAR, GR, glutathione peroxidase (GPX), CAT, glyoxalase I (Gly I), and glyoxalase II (Gly II) in drought affected seedlings as compared to the drought-stressed plants without SA supplementation, with a concomitant decrease in H2O2 and lipid peroxidation

52 Phytohormones - Signaling Mechanisms and Crosstalk in Plant Development and Stress Responses

Temperature is one of the vital factors that determine plants establishment, growth, development, and productivity. Due to climate change, global average temperature is fluctuating very rapidly and threatening the survival of living beings. Thus, among the various abiotic stresses, extreme temperature has become the talk of the topic in recent decades because of its devastating and damaging effects on plants [84]. Extreme temperature includes both high temperature (HT) and low temperature (LT) that can injure plants. The high temperature is the increasing of temperature beyond the critical threshold level that can deplete plant growth and metabolism depending on the sufficient time period [85]. Heat stress often becomes worse because of its combination with other stresses including drought [86]. High temperature severely alters the plant physiological processes including germination, photosynthesis, respiration, transpiration, partitioning of dry matter, etc. [87]. In addition, HT results in enzyme inactivation, protein denaturation, disruption of proteins and membranes which ultimately affects plant growth [88, 89]. Low-temperature consists of both freezing (<0C) and chilling (0– 15C) temperatures. In chilling stress plant faces injury without formation of ice whereas, in freezing stress, the formation of ice occurs in plant tissues. Chilling and freezing stresses are together called cold stress or LT stress. Low-temperature stress shows various damaging symptoms in plants including faster senescence and decay [90, 91], interference with germination, cell membrane disruption, photosynthesis, water and nutrients uptake, reproductive development as well as growth and yield [92]. Either HT or LT conditions, at molecular level, leads to the overproduction of ROS which ultimately gives rise to the oxidative stress [84, 93]. Nowadays, to develop temperature-stress tolerance, the use of exogenous SA is one of the common approaches. Salicylic acid being the endogenous growth regulator or phytohormone acts as an important signaling molecule and develops abiotic stress tolerance in plants [94]. Recent advances on SA-mediated temperature stress tolerance have been listed in Table 3.

High temperature (30C) resulted in a significant reduction in FW and soluble starch synthase activity of T. aestivum [46]. Foliar application of SA improved the FW, total RNA, and soluble starch synthase activity. In Z. mays, HT (40 1C) induced oxidative damage and reductions in dry biomass were reversed by exogenous SA treatment. SA developed HT tolerance by improving CAT, SOD, and POX activities [48]. The effects of SA on seed germination and physiological attributes of heat stressed (32/26C, 12/12 h, day/night) L. lycopersicum were investigated. It has been revealed that SA reduced the germination time and increased the germination percentage together with the increased vitamin, lycopene, total soluble solid (TSS) and titratable acidity (TA) contents [49]. Temperature above 40C, increased TBARS and H2O2 contents but decreased the net photosynthesis, RuBisCo activity, chl, and WUE of T. aestivum

plant. Negative HT effects were counteracted significantly by exogenous SA supplementation [13]. Heat stress (45C) in Digitalis trojana Ivanina, compared to its normal temperature, lowered the important antioxidant enzymes (CAT and SOD) activities. Pretreatment with SA significantly increased CAT and SOD activities with increased Pro, phenolic, and flavonoid contents [47]. The sharp decline of photosynthetic apparatus was found in Vitis vinifera in response to heat (43C) stress. Alleviation of photosynthetic rate and RuBisCo activity were found when pretreated with SA [95]. Cannabis sativa induced thermotolerance against (40C) temperature, when supplied with exogenous SA, were studied. Improved activities of antioxidant enzymes (SOD, DHAR, GPX, APX, and GR) were documented with decreased CAT activity. Decreased EL percentage with reduced H2O2 and TBARS contents were also evident [97]. Salicylic acid involved in various protective functions in B. juncea after HT (30 or 40C) exposure. However, increased growth, gs and CO2 fixation along with improved defense system were found with SA treatment [98]. Chilling (5C) stressed Musa acuminate when treated with exogenous SA increased SOD, CAT, and APX activities with decreased H2O2 accumulation [99]. Low temperature (3C) disrupted the RuBPC and PEPC activities with decreased rate of CO2 assimilation and respiration. Treatment with SA improved the activity and ameliorated the chilling effects [100]. Performance of V. vinifera was investigated upon HT (38 0.5C) [101]. Pretreatment with SA increased H+ and Ca2+-ATPase activities with cerium phosphate grain appearance and thus gave higher stress tolerance. Wang and Li [102] noted that upon freezing (3C) stress in V. vinifera, besides upregulating antioxidants, increased maintenance of AsA-GSH pool and cytosolic Ca2+ homeostasis caused improved heat stress tolerance. High-temperature stress mediated increased ROS generation and oxidative stresses have been reported in several other plant species. Exogenous SA treatment resulted in the reduced ROS generation and oxidative stress in Pratylenchus pratensis [103], Arabidopsis thaliana [109]. Night ambient temperature ranges from 27 to 32C in Oryza sativa caused the significant reduction in spikelet fertility and grain size. Exogenous SA treatment improved the rice grain fertility and hence, increased yield [104]. Prunus persica fruits were pretreated with SA before imposition of chilling injury (0C). Reduced chilling injury was observed due to higher activities of antioxidants and heat shock protein 101 (HSP101) expression [105]. Leaves of heat (38/30C, day/night) stressed Rhododendron became withered, defoliated, and brown. Total soluble protein and Chl contents were also reduced. Lower damage rate of leaves with higher chl and soluble protein were observed when supplemented with SA [106]. In a study with HT stressed (50C) V. radiata, SA treatment increased the CAT, APX, POD, and GSH contents with enhanced defense system and radical scavenging mechanism [107].

#### 3.4. Toxic metal/metalloids

In the industrial era, the most important and potential threat for crop production is the abiotic stress. Among them, toxic metal stress is one of the major concerns. Growing population and fast industrialization coincide together, results in the generation and dissemination of huge amount of toxic metals in the environment [110]. Toxic metal consists of a set of harmful elements having no biological role in organisms such as Cd, Pb, Hg, St, Al, etc. Although toxic metals and heavy metals (HMs) are often thought to be synonymous, some lighter metals such as Al may also cause toxicity. Toxic and HMs are differed in the case of their biological role. Some HMs having a biological role in plants also considered toxic when they are used in high concentrations, viz. Ni, Cu, Zn, etc. On the other hand, metalloid includes those elements that show behavior both like metals and nonmetals including B, Si, Ge, St, As, Sb, etc. The underlying parent material and atmosphere are the two main sources of toxic metals. Metals are uptaken and accumulated easily by plants and causes toxicity within the plant tissue. They directly interact with the proteins, enzymes, and causes phytotoxicity. The inhibition of growth rate is the most certain consequences of metal toxicity [111]. Leaf rolling, chlorosis, necrosis, stunted growth, stomatal dysfunctioning, cation efflux, reduced water potential, alterations in the membrane, photosynthesis, metabolism, and various key enzymes are some other toxic metal effects in plants [111, 112]. Toxic metals also manipulate the nutrient homeostasis, water uptake, transport, transpiration, respiration, and ultimately may lead to plant death [113, 114]. Metal toxicity at the cellular level results in the overproduction of ROS [110]. To mitigate metal induced stresses in plants, plant biologists are trying to develop new strategies. Salicylic acid is a very important molecule that induces defense responses against various toxic metal/metalloids stresses (Table 4).

plant. Negative HT effects were counteracted significantly by exogenous SA supplementation [13]. Heat stress (45C) in Digitalis trojana Ivanina, compared to its normal temperature, lowered the important antioxidant enzymes (CAT and SOD) activities. Pretreatment with SA significantly increased CAT and SOD activities with increased Pro, phenolic, and flavonoid contents [47]. The sharp decline of photosynthetic apparatus was found in Vitis vinifera in response to heat (43C) stress. Alleviation of photosynthetic rate and RuBisCo activity were found when pretreated with SA [95]. Cannabis sativa induced thermotolerance against (40C) temperature, when supplied with exogenous SA, were studied. Improved activities of antioxidant enzymes (SOD, DHAR, GPX, APX, and GR) were documented with decreased CAT activity. Decreased EL percentage with reduced H2O2 and TBARS contents were also evident [97]. Salicylic acid involved in various protective functions in B. juncea after HT (30 or 40C) exposure. However, increased growth, gs and CO2 fixation along with improved defense system were found with SA treatment [98]. Chilling (5C) stressed Musa acuminate when treated with exogenous SA increased SOD, CAT, and APX activities with decreased H2O2 accumulation [99]. Low temperature (3C) disrupted the RuBPC and PEPC activities with decreased rate of CO2 assimilation and respiration. Treatment with SA improved the activity and ameliorated the chilling effects [100]. Performance of V. vinifera was investigated upon HT (38 0.5C) [101]. Pretreatment with SA increased H+ and Ca2+-ATPase activities with cerium phosphate grain appearance and thus gave higher stress tolerance. Wang and Li [102] noted that upon freezing (3C) stress in V. vinifera, besides upregulating antioxidants, increased maintenance of AsA-GSH pool and cytosolic Ca2+ homeostasis caused improved heat stress tolerance. High-temperature stress mediated increased ROS generation and oxidative stresses have been reported in several other plant species. Exogenous SA treatment resulted in the reduced ROS generation and oxidative stress in Pratylenchus pratensis [103], Arabidopsis thaliana [109]. Night ambient temperature ranges from 27 to 32C in Oryza sativa caused the significant reduction in spikelet fertility and grain size. Exogenous SA treatment improved the rice grain fertility and hence, increased yield [104]. Prunus persica fruits were pretreated with SA before imposition of chilling injury (0C). Reduced chilling injury was observed due to higher activities of antioxidants and heat shock protein 101 (HSP101) expression [105]. Leaves of heat (38/30C, day/night) stressed Rhododendron became withered, defoliated, and brown. Total soluble protein and Chl contents were also reduced. Lower damage rate of leaves with higher chl and soluble protein were observed when supplemented with SA [106]. In a study with HT stressed (50C) V. radiata, SA treatment increased the CAT, APX, POD, and GSH contents with

54 Phytohormones - Signaling Mechanisms and Crosstalk in Plant Development and Stress Responses

enhanced defense system and radical scavenging mechanism [107].

In the industrial era, the most important and potential threat for crop production is the abiotic stress. Among them, toxic metal stress is one of the major concerns. Growing population and fast industrialization coincide together, results in the generation and dissemination of huge amount of toxic metals in the environment [110]. Toxic metal consists of a set of harmful elements having no biological role in organisms such as Cd, Pb, Hg, St, Al, etc. Although toxic metals and heavy metals (HMs) are often thought to be synonymous, some lighter metals such as Al may also cause toxicity. Toxic and HMs are differed in the case of their biological role.

3.4. Toxic metal/metalloids

Several research findings demonstrated that exogenously applied SA improved the growth and photosynthetic traits in different plants by reducing the damaging effects of toxic metals. O. sativa exposed to As (25 and 50 μM) [50], A. thaliana exposed to As (100 μM) [115], T. aestivum exposed to Cd (500 and 1000 μM) [121], and Z. mays exposed to Cr (500 ppm) [52] grown well under SA supplementation. In a recent study, SA pretreatment reduced the oxidative stress in T. aestivum after Cd (0.01, 0.1, and 1 mM) exposure. Cd stress increased the lipid peroxidation and EL percentage. But the exogenous application of SA significantly declined the MDA content and EL percentage [53]. Salicylic acid also evidenced to alleviate the oxidative stress induced by metal toxicity in several other plant species by decreasing the toxic effects of overproduced ROS and lipid peroxidation. In O. sativa, enhanced MDA and H2O2 contents induced by As (25 and 50 μM) were reduced by the exogenous SA pretreatment [50]. Similarly, adverse oxidative stress was also demonstrated in Lolium perenne as induced by Cd (100 μM). Results showed that increased accumulation of O2 •, H2O2, and higher MDA were decreased by SA application. Some other research findings also supported that the SA mitigates metal-induced oxidative damage in Pisum sativum [136], Brassica oleracea var. botrytis [135], Medicago sativa [134], etc. It was reported that treatment with SA alleviated the Cd (15 μM) induced root growth inhibition and improved the antioxidant activities thus reduced the Cd-induced oxidative stress in Hordeum vulgare [51]. Salicylic acid supplementation in As (100 μM) stressed A. thaliana showed improved performance in terms of antioxidant enzymes (APX, CAT, SOD) activities and enhanced tolerance to metal stress. In other experiment it was demonstrated that when SA was exogenously applied against Cd (0.25, or 0.50 mM) stress, increased amelioration of metal stress was observed with increasing activities of defense responsive genes and upregulating the antioxidant (SOD, CAT, APX, and GR) enzymes [117]. Decreased root growth, RWC, and increased oxidative stress were decreased by seed priming with SA [118]. Under Cd (50 and 100 mM) stress, plant growth, chl, total lipid contents, and nutrient absorption became decreased which were further increased by soaking seeds with SA. Increased stress tolerance with reduction of oxidative damage was also evident [119]. Increased ROS production, TBARS content, and EL with increased SOD, CAT, and APX activities were found after Cd (50 and 100 μM) exposure.

Seed priming with SA significantly decreased the oxidative damage and increased the antioxidant enzymes activities. Agami and Mohamed [121] reported that SA efficiently alleviated the adverse Cd (500 and 1000 μM) stress by restoring the growth parameters and increasing the antioxidant defense system. Exogenous SA developed Cr (mg L<sup>1</sup> ) stress tolerance by improving growth, photosynthetic pigments and oxidative stress reduction by upregulating antioxidant defense system [52]. Inhibited antioxidant enzymes (APX, CAT, POX, and SOD), soluble sugars and chl contents were showed when T. aestivum exposed to Pb (10, 50, 100, 200 mg L<sup>1</sup> ). Alleviated inhibitory effects were found after SA supplementation as cotreatment [126]. In Zygophyllum fabago, increased Pb accumulation after Pb (0.75 mM) exposure was reduced by SA pretreatment. Upregulated antioxidants and downregulated oxidative damage together induced stress tolerance [125]. Salicylic acid treatment against Cd (5, 10 or 50 mM) stress increased tolerance in P. pratensis by controlling uncontrolled absorption of Cd and maintaining nutrients (K, Ca, Fe, Mg) homeostasis [127]. Effects of B was investigated (25 and 50 mg kg<sup>1</sup> ) toxity in Daucus carota and S. oleracea. In both plants, the growth, physiology, and antioxidant enzymes activity were affected by B toxicity. But exogenous SA application showed some protective effects in the alleviation of the metal toxicity [128, 129]. A similar finding was also demonstrated by Shi and Zhu [131] in Crocus sativus plant. Exposure to a toxic level of Mn (600 μM) with SA in C. sativus plant, maintained nutrient (Ca, Mg, Zn) homeostasis, reduced metal stress, and improved tolerance. Recent findings in B. juncea against Mn (3, 6, or 9 mM) toxicity revealed that SA is an important regulator of photosynthetic enzymes including carbonic anhydrase (CA). It together with the upregulated defense system and reduced oxidative damage-regulated the photosynthesis in a concentration-dependent manner [130]. Besides upregulation of antioxidant enzymes, SA involved in the activation of CaM-like protein genes and cytosolic Ca2+ in Al (30 μM) stressed Glycine max. It has been showed that increased metal stress tolerance resulted from exogenous SA treatment [132]. Effect of exogenous SA was investigated upon Pb (50 μM) stressed Vallisneria natans. Decreased chl, carotenoid, NADPH oxidase, nonprotein thiols, AsA but increased CAT, DHAR, and POD activities were observed. Pretreatment with SA increased the activities of NADH oxidase, AsA, and GSH in Hg (10 μM) stressed M. sativa and thus developed better tolerance to stress [133]. Recent evidences suggested that SA develops stress tolerance by involving in the regulation of photosynthetic pigments, activities of CA, NR, and anticancer alkaloids (vincristine and vinblastine) upon Ni (50, 100, and 150 mg kg<sup>1</sup> ) exposure in Catharanth roseus [122]. Toxic level of Ni (0.5 mM) impacts on Brasicca napus also suggested that application of SA decreased the leaves' toxicity symptoms (chlorosis, necrosis, etc.) and improved growth and survival [123]. Recently a combined effect of metals (Co, Ni, Cd, Cr, and Pb) (0.25 M) on B. oleracea var. botrytis has been investigated. It has been found that SA alleviated all the toxic effects of metals except for Cr. In the case of Cr, SA accelerated the toxic effects and the plant died [135]. Improved CO2 fixation, RuBPC activity, and chl content were found in Cd (0.5, 1, 2, and 5 μM) stressed P. sativum when supplied with exogenous SA.

#### 3.5. Ozone and ultraviolet radiation

Due to the gradual increase in atmospheric ozone (O3) concentration, it has become a major threat for plant species mainly because of its pollutant and photochemical oxidant affects [139]. Significant crop losses due to O3 damage is predicted to be increased by 25% in background O3 concentration over the next 30–50 years [140]. High concentrations of ozone induce oxidative stress, which activates programmed cell death and significantly inhibits plant growth, causing plant death and loss of quality [141]. It is the most noteworthy atmospheric pollutant in terms of phytotoxicity. It is to be noted that in the concentration of O3 has been decreased by 5% in the past 50 years due to the release of anthropogenic pollutants and, a larger proportion of the UV radiation (especially (UV-B) spectrum reaches the Earth's surface [142]. Although sunlight plays an integral role in harvesting light energy through photosynthesis, high light, especially ultraviolet (UV) radiation, resulted in stress to plants, which include damage to DNA, proteins, and other cellular components [143]. This episode is unavoidable as 7% of the electromagnetic radiation emitted from the sun is in the UV range (200–400 nm). UV radiation also leads to oxidative stress by photooxidation and excess generation of ROS [84]. To cope up the adverse effects of both O3 and UV radiation needs some adaptive mechanisms. In few plant species, SA was found to take part in enhancing the tolerance to O3 and UV radiation mainly by enhancing antioxidant defense and improving plant growth.

Seed priming with SA significantly decreased the oxidative damage and increased the antioxidant enzymes activities. Agami and Mohamed [121] reported that SA efficiently alleviated the adverse Cd (500 and 1000 μM) stress by restoring the growth parameters and increasing the

growth, photosynthetic pigments and oxidative stress reduction by upregulating antioxidant defense system [52]. Inhibited antioxidant enzymes (APX, CAT, POX, and SOD), soluble sugars and chl contents were showed when T. aestivum exposed to Pb (10, 50, 100, 200 mg L<sup>1</sup>

Alleviated inhibitory effects were found after SA supplementation as cotreatment [126]. In Zygophyllum fabago, increased Pb accumulation after Pb (0.75 mM) exposure was reduced by SA pretreatment. Upregulated antioxidants and downregulated oxidative damage together induced stress tolerance [125]. Salicylic acid treatment against Cd (5, 10 or 50 mM) stress increased tolerance in P. pratensis by controlling uncontrolled absorption of Cd and maintaining nutrients

Daucus carota and S. oleracea. In both plants, the growth, physiology, and antioxidant enzymes activity were affected by B toxicity. But exogenous SA application showed some protective effects in the alleviation of the metal toxicity [128, 129]. A similar finding was also demonstrated by Shi and Zhu [131] in Crocus sativus plant. Exposure to a toxic level of Mn (600 μM) with SA in C. sativus plant, maintained nutrient (Ca, Mg, Zn) homeostasis, reduced metal stress, and improved tolerance. Recent findings in B. juncea against Mn (3, 6, or 9 mM) toxicity revealed that SA is an important regulator of photosynthetic enzymes including carbonic anhydrase (CA). It together with the upregulated defense system and reduced oxidative damage-regulated the photosynthesis in a concentration-dependent manner [130]. Besides upregulation of antioxidant enzymes, SA involved in the activation of CaM-like protein genes and cytosolic Ca2+ in Al (30 μM) stressed Glycine max. It has been showed that increased metal stress tolerance resulted from exogenous SA treatment [132]. Effect of exogenous SA was investigated upon Pb (50 μM) stressed Vallisneria natans. Decreased chl, carotenoid, NADPH oxidase, nonprotein thiols, AsA but increased CAT, DHAR, and POD activities were observed. Pretreatment with SA increased the activities of NADH oxidase, AsA, and GSH in Hg (10 μM) stressed M. sativa and thus developed better tolerance to stress [133]. Recent evidences suggested that SA develops stress tolerance by involving in the regulation of photosynthetic pigments, activities of CA, NR, and

(K, Ca, Fe, Mg) homeostasis [127]. Effects of B was investigated (25 and 50 mg kg<sup>1</sup>

anticancer alkaloids (vincristine and vinblastine) upon Ni (50, 100, and 150 mg kg<sup>1</sup>

(0.5, 1, 2, and 5 μM) stressed P. sativum when supplied with exogenous SA.

3.5. Ozone and ultraviolet radiation

Catharanth roseus [122]. Toxic level of Ni (0.5 mM) impacts on Brasicca napus also suggested that application of SA decreased the leaves' toxicity symptoms (chlorosis, necrosis, etc.) and improved growth and survival [123]. Recently a combined effect of metals (Co, Ni, Cd, Cr, and Pb) (0.25 M) on B. oleracea var. botrytis has been investigated. It has been found that SA alleviated all the toxic effects of metals except for Cr. In the case of Cr, SA accelerated the toxic effects and the plant died [135]. Improved CO2 fixation, RuBPC activity, and chl content were found in Cd

Due to the gradual increase in atmospheric ozone (O3) concentration, it has become a major threat for plant species mainly because of its pollutant and photochemical oxidant affects [139]. Significant crop losses due to O3 damage is predicted to be increased by 25% in background O3

) stress tolerance by improving

).

) toxity in

) exposure in

antioxidant defense system. Exogenous SA developed Cr (mg L<sup>1</sup>

56 Phytohormones - Signaling Mechanisms and Crosstalk in Plant Development and Stress Responses

In A. thaliana, UV-C light stress activated the transition to flowering through SA. SA could regulate the time of flowering by inducing photoperiod and autonomous pathways which are evident by late flowering in SA-deficient plants. While investigating the genes responsible for flowering induction viz. constans (CO), flowering locus T (FT), suppressor of overexpression of constans 1 (SOC1), and flowering locus C (FLC) it was observed that the expression of CO, FT, and SOC1 transcripts decreased to around 50% in long day-grown SA-deficient plants when compared to contron plants [54]. In short day plants, only the levels of FT transcripts were reduced compared to CO. Thus, it indicated that SA might play important role in flowering under UV radiation [54]. The effect of UV was investigated (UV-A: 320–390 nm, UV-B: 312 nm, and UV-C: 254 nm radiation with a density of 6.1, 5.8, and 5.7 W m<sup>2</sup> ) on Capsicum annuum plants and found that activities of antioxidant enzymes were enhanced in leaves in response to UV-B and UV-C radiation. Moreover, SA treatment showed further enhancement in the activities of POD, APX, CAT, and GR while some other enzymes were modulated [55]. In another report, a clear decline was reported in photosynthetic pigments (chl a, chl b, and carotenoid) under UV-A, UV-B, and UV-C, while a foliar spray of SA recovered this decline. The level of anthocyanins, flavonoids, and rutin in SA-treated plants was also higher than in a UV-exposed plant grown without SA [56]. V. radiata, exposed to UV-B radiation (ambient+4–8kJ m<sup>2</sup> ) showed declined growth, photosynthetic pigments and photosynthesis (Fv/Fm and qP except NPQ) which were accompanied by significant decrease in SA level [57]. UV radiation also causes overproduction of ROS and concomitantly damaging effects on lipids, proteins, and membrane stability. However, SA pretreatment significantly alleviated the adverse effects. They also revealed that UV-B altered SA biosynthesis and SA-pretreatment might act as a signal that reduces oxidative stress by triggering upregulation of antioxidant defense and subsequent improvement of growth and photosynthesis [57]. In Satureja hortensis, both UV-B and UV-C exhibited decreased plant growth (plant height, root length, shoot DW, and leaf area), node number, internode distance and chl content, while stem diameter, leaf thickness, flavonoid content, phenolic content, and antioxidant activity were increased [58]. The increase in secondary metabolite such as flavonoid content, phenolics might be able to protect cells against free radicals but this level was not well enough under severe stress. On the other hand, plants treated with 1 mM SA exhibited higher growth and improved physiology compared to nontreated one and subsequently showed better appearance under UV radiation [58].
