SELENIUM (Se) IMPROVES DROUGHT TOLERANCE IN CROP PLANTS- A MYTH OR FACT?

Rashid Ahmada, Ejaz Ahmad Waraicha*, Fahim Nawaza,b, M.Y. Ashraf c and Muhammad Khalidd a

Department of Crop Physiology, University of Agriculture, Faisalabad. Pakistan

a,b

Department of Plant Sciences, University of Oxford, OX1 3RB, United Kingdom

c

Nuclear institute of Agriculture and Biology, Faisalabad, Pakistan.

d

Institute of Soil and Environmental Sciences, University of Agriculture, Faisalabad.

Pakistan. *

Corresponding author. Tel: +92 41 24098933, + 92 336 6553081; E-mail uaf_ewarraich @

yahoo.com (Ejaz Ahmad Waraich).Fax no.92-041-9200764 (Attn. Ejaz Ahmad Waraich, Crop Physiology)

This article has been accepted for publication and undergone full peer review but has not been through the copyediting, typesetting, pagination and proofreading process, which may lead to differences between this version and the Version of Record. Please cite this article as doi: 10.1002/jsfa.7231

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ABSTRACT Climate change has emerged as one of the most complex challenges of the 21st century and has become an area of interest in the past few decades. Many countries of the world have become extremely vulnerable to the impacts of climate change. The scarcity of water is the serious concern for food security of these countries and climate change has aggravated the risks of extreme events like drought. Oxidative stress caused by a variety of active oxygen species formed under drought stress, damage many cellular constituents such as, carbohydrates, lipids, nucleic acids and proteins, which ultimately reduces plant growth, respiration and photosynthesis. Se has become an element of interest to many biologists due to its physiological and toxicological importance. It plays beneficial role in plants by enhancing growth of plants, reducing damage caused by oxidative stress, enhancing chlorophyll contents under light stress, stimulating the senesce to produce antioxidants, and improving plant tolerance to drought stress by regulating water status. The researchers have adopted different strategies to evaluate the role of selenium in plants under drought stress. Some of the relevant work available regarding the role of Se in alleviating adverse effect of drought stress is discussed in this paper. __________________________________________________________________________ Keywords: Selenium; Drought tolerance; Oxidative stress; Antioxidants; Crop Plants

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1. Drought Stress and Plant growth Drought stress is one of the major limitations to agricultural productivity around the globe 1. The recurrent droughts and temperature change would alter the biophysical relationships of crops by modifying their growing periods, scheduling of cropping seasons, altering irrigation water requirements, changing soil characteristics, and increasing the risk of drought stress, thus severely limiting the agricultural productivity2. Under drought, plants usually face the soil and atmospheric water deficit at different growth stages of their life cycle 3. Unlike other stress factors, drought stress develops slowly and increases with time in intensity and causes damages 4. It results in impaired germination and seedling growth 5, causes premature plant death

6-7

thus reducing harvestable yield of plants 8, The severity,

duration and timing of water stress are extremely important for the better understanding of plant responses to drought stress 9. Several researchers have reported the adverse effect of drought stress on germination and seedling growth of various crops such as sunflower 12

13

14

10-11

,

15

sugar beet , maize , sorghum , and kochia . The limited or non-availability of water significantly reduces growth of crop plants through its effects on various physiological and biochemical processes in plants. A reliable and effective way to quantify plants response to drought stress is leaf water potential 16. The variation in water relation characteristics among species and lines indicate their potential as drought resistant or sensitive

17

. The leaf water potential decreases due to accumulation of

solutes under limited water conditions and variation exists among cultivars in their response to water potential under well watered as well as drought stress conditions 8. A decrease in water potential and yield of wheat at both tillering and anthesis stages was observed

18

however, the cultivars were found to be more sensitive to water stress at anthesis than tillering stage. Similar decrease in photosynthesis, water potential, osmotic potential and chlorophyll contents of two Brassica cultivars was also observed under drought stress conditions 19. Oxidative stress caused by a variety of active oxygen species formed under drought stress damages many cellular constituents such as, carbohydrates, lipids, nucleic acids and proteins which ultimately reduces plant growth, respiration and photosynthesis. The synthesis of starch is strongly inhibited even under moderate deficiency of water,

20

while the soluble

21

sugar contents remain constant or even increase under stress conditions . Various enzymatic and non-enzymatic systems are found in plants to cope with reactive oxygen species (ROS) 22

. The increase in invertase activity due to the accumulation of simple sugars such as glucose

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and frucctose has beeen observeed in the leaaves of the plants p exposed to drought stress 211, 23. The mechannism of osm motic adjustm ment for droought toleraance by plannts has been n well docuumented. It is faccilitated by compatible c solutes prod duced at higgher levels under limiteed water coonditions 24-26

. These T comppounds acccumulated in high aamounts acct as an osmoprotect o tants of

membraanes and prrevent proteein disintegration27. Thhe accumulaation of total sugars annd other compatible solutes such as pollyols is a chharacteristicc feature of most plantss under stresss 28-29.

A Abiotic Sttresses

ROS

Oxidattive damag ge

r Figuree 1. Schemaatic Diagraam of Reacttive Oxygen Species (ROS) geneeration in response to Abiootic stressees causing oxidative o daamage 2. Mechanism ms of Drought Toleran nce Plants aadapted to drought d connditions have evolved a number off mechanism ms to cope with w low soil watter such as structural s chhanges in leeaves, osmootic adjustm ment, reduceed growth, sshort life cycle, longer main n roots and production of ROS scavenging ennzymes 30. These mechanisms may varry in differeent plants reesulting in differential d tolerance against a drou ught 30. Som me of the mechannisms are suummarized bbelow. 2.1. Osmotic Adjustment A t Osmotic addjustment iss an importaant plant defense mechhanism proteecting enzym mes and d by accum mulation of o solutes produced in high membraane structuures. It is facilitated concenttration undeer stress conndition. Th he accumulaation of orgganic solutes acts as sccavenger of reacttive oxygen n species

31

. The accu umulation of o these orrganic soluttes prevent cellular

dehydraation and heelp cells to retain r waterr in cells 32, without deeteriorating macromoleecules 33. These ccompounds are accumuulated in hiigh amountss under streess and act as osmoprootectants of mem mbrane to prevent p prottein disintegration

27

. These are classified as a sugars, glycerol, g

amino aacid (prolinne and glycine betaine)), sugar alccohols (man nitol) and other low molecular m weight metabolitess 34. Severaal researcherrs have statted the impo ortance of osmotic o adjjustment d toleerance of pllants in imprroving the drought

24- 26

. The T total acccumulationn of solutes and the

nature of o accumulaated solutes may vary within w speciies and cultiivars 35.

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2.2. Production of ROS scavenging enzymes In general, a well-known adaptive mechanism in plants as response to drought conditions is the production of antioxidant enzymes such as peroxidase (POD), catalase (CAT), ascorbate peroxidase (APX), superoxide dismutase (SOD) and glutathione peroxidase (GPx)

36

. Both enzymatic and non-enzymatic antioxidants such as glutathione (GSH),

flavonoids, ascorbic acid (AsA), α-tocopherol, β-carotene and hydroquinones collaboratively serve as scavengers of ROS under stress. Ascorbate peroxidase (APX) is one of the most important antioxidant enzyme involved in the scavenging of ROS to protect cells of higher plants

37

. It is the constituent of water-water and ascorbate-glutathione (ASH-GSH) cycles

that scavenge H2O2 in the chloroplasts of plant cells under stress defense system is activated/modulated by plant water relations

38, 37

39-40

. The antioxidant

which results in the

synthesis of APX under drought stress. The non-availability of water found significantly increased the activity of APX in three cultivars of P. asperata (Yang, et al., 2008)41 and P. vulgaris (Zlatev, et al., 2006)42. The APX activity is reported to increase in chloroplast of rice seedlings subjected to mild drought stress but to decrease under high stress (Sharma and Dubey, 2005) 43. The sequential and simultaneous action of various antioxidant enzymes like catalase (CAT) and peroxidase (POD) is a key factor for survival of plants in changing environments. Both CAT and POD are major scavengers of H2O2 and play an important role in converting toxic levels of accumulated H2O2 into water and oxygen 44. Peroxidase uses several available reductants to reduce H2O2 into H2O in plant cells under stress

45

. Wang et al.

46

found that

POD activity increased in apple leaves under drought stress. Furthermore, the POD activity in root stock was higher in drought tolerant Malus hupehensis (141.97%) than in drought sensitive Malus hupehensis (84.19%). Similar results were reported by Turkan et al. bean cultivars. Simova-Stoilova et al.

48

47

in

examined CAT activity in wheat leaves under severe

soil drought and were of the view that drought stress increases CAT activity especially in sensitive cultivars. Nazarli et al.

49

reported no significant difference among different

irrigation regimes on CAT activity in wheat leaves. CAT activity is enhanced in high light conditions under drought stress 41. Pan et al. 50 evaluated the combined effect of drought and salt stress and reported a decrease in CAT activity in Glycyrrhiza uralensis seedlings. The identification of an effective stress ameliorant is necessary for obtaining optimum yield under environmental stress conditions 51. Se has become an element of interest to many

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biologists due to its physiological and toxicological importance. It plays beneficial role in plants by enhancing growth of plants stress

51, 53-54

52

reducing damage caused by UV-induced oxidative

enhancing chlorophyll contents under light stress 55, stimulating the senesce to

produce antioxidants, and improving plant tolerance to drought stress by regulating water status

56-58

. It is known to increase superoxide dismutase (SOD) and glutathione peroxidase

(GPX) activities thus activating the protective mechanisms against oxidative stress. It is also reported to exert an antioxidant effect directed towards a decreased concentration of intracellular active oxygen species by inducing the biosynthesis of proline and peroxidase production 56. The specific physiological mechanisms that underlie the positive effects of Se in plants have not been clearly elucidated 59. It is mostly used for the bio-fortification of crops but has also been reported to play an important role in the adjustment of plants water status under drought stress 57, 60-61. Selenium supply favors to increase the plant biomass by increasing root and antioxidant activity

62-63

. There are reports that Se application to plants under drought

stress improved drought tolerance and mitigated the adverse effects of drought on total dry weight, leaf area index (LAI), relative growth rate (RGR) and crop growth rate (CGR), yield and water use efficiency in maize

64-65

and rapeseed. It was found to improve plant biomass

through accumulation of osmoprotectants in wheat seedlings raised from Se primed seeds

66

and increased chlorophyll contents. This paper, therefore, is an effort to review the literature regarding the possible role of Se in improving drought tolerance in crop plants. 3. Selenium Se is similar to sulphur (S) in many ways like electronegativity, atomic size and have similar major oxidation states 67. As an elemental Se or cadmium selenide, Se has wide range of applications in many industries. It is used in electronics and photography, semiconductors, medical imaging equipment, photocopiers and solar cells. It also finds its application in glass industry to give beautiful ruby red color to the glass 68. Earlier Se was regarded as an undesirable and toxic element for higher organisms but in the second half of the 20th century, Schwarz and Foltz

69

reported that low concentrations

of Se are essential for dietary intake and interchangeable with vitamin E. Later in 1973, it was discovered that being an integral component of glutathione peroxidase (GPX) enzyme, Se protects cells against intracellular oxidative damage

70

. In 1979, the association of Keshan

disease with Se deficiency in the Keshan region in China provided the direct evidence for Se

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requirement in human nutrition 71. Further studies revealed that selenoproteins are involved in various physiological processes in mammals and today more than 30 selenoproteins have been reported to have a significant role in human nutrition

72-73

. Selenium toxicity causes

distraction of nervous and digestive systems in humans and animals and results in hair and nail loss, poor reproduction rate and growth in animals

74-75

. On the other hand, the

inadequate dietary intake of Se results in many disorders and diseases related to free radical damage such as greater risk of tumor formation, immunity disorders and cardiovascular diseases like hypertension and atherosclerosis. A very narrow range between Se deficiency (< 40 μg d-1) and toxicity (> 400 μg d-1) has been reported 75. Organic forms of Se (selenomethionine, selenocysteine and Se-methylselenocysteine) are mostly found in foods while it rarely occurs in inorganic forms (selenite or selenate). In most Se rich diets, selenomethionine is the major organic form however both organic and inorganic forms are involved in the formation of selenoproteins in the body 76. The biological role of selenocompounds in humans has been well documented

77

. The first identified

selenoprotein in mammals, glutathione peroxidase (GPX) protects cells from oxidative damage and is an important part of antioxidative defense system of the body. Thioredoxin reductase (TR), a recently discovered selenoprotein is an integral part of the thioredoxin system, well known for its antioxidant and redox regulatory roles in cells

78, 79

. Another

important selenoenzyme, selenophosphate synthethase (SePsyn) is thought to be involved in the production of other selenoenzymes and also catalyzes the synthesis of selenophosphate which is essential for the production of selenocysteinylated tRNASec 80. The protective role of Se against the toxicity of other heavy metals such as lead, silver and mercury has also been well reported

81-83

. Selenium levels in fish are high enough to prevent Hg toxicity; however

the exact mechanism of their interaction is not well known 82. The major sources of Se in most diets are meat, fish and cereals

84

. The level of

available Se in the soil used for growing food stuffs and dietary composition determines the Se intake by humans. Selenium usually occurs within the range of 0.10 – 0.60 mg kg-1 in fish, 0.05 – 0.60 in cereals, 0.05 – 0.30 in red meat, and 0.002 – 0.08 in fruit and vegetables 84. It depicts that cereals largely contribute to the total dietary intake of Se and nearly 70% dietary Se requirements are met with cereals and cereal products in Se deficient areas in China while they contribute about 40–54% to the Se dietary intake of low-income population in India (FAO, WHO, 2001). In developed countries like UK, 18-24% of the total Se intake comes from cereals and cereal products

85

. Among cereals, wheat is one of the most important Se

sources for humans as it is the most efficient Se accumulator 86. This article is protected by copyright. All rights reserved

4. Selenium and plant Growth Se due to its physiological and toxicological importance has become an element of interest to many plant scientists. There are reports in literature of selenium beneficial role in plants. It enhances growth of plants stress

52-54

52

reduces damage caused by UV-induced oxidative

enhances chlorophyll contents under light stress

55

stimulates senescing plants to

produce antioxidants, and improves plant tolerance to drought stress by regulating water status

56, 58, 87

. Selenium is known to increase superoxide dismutase and glutathione

peroxidase (GPX) activities thus activating the protective mechanisms against oxidative stress. It is also reported to exert an antioxidant effect directed towards a decreased concentration of intracellular active oxygen species by inducing the biosynthesis of proline and peroxidase production 56. In plants, selenium uptake and translocation takes place through sulphate transporters (STs) and phosphate transporters (PiTs). Once absorbed by the plants, the inorganic Se is converted into more bioavailable organic forms like selenomethionine (SeMet). It is presumed that STs located in root cell membranes are involved in the uptake of selenate while PiTs are generally accepted to be involved in selenite uptake

88

. A large number of

enzymes such as APS reductase (APSR), ATP sulphurylase (ATPS), sulphite reductase (SiR) etc. take part in sulphate assimilatory pathway to incorporate selenate or selenite into selenocysteine (SeCys) and selenomethionine (SeMet) 89. Selenium uptake may be influenced by genetic factors and cultivars may differ in their Se content

90

. Rate and method of Se

application are reported to affect grain Se levels while other factors like the yield of the crop may also affect grain Se concentration through dilution effects, i.e., a low yield crop of dry areas may have higher grain Se than a high yielding crop grown on an irrigated site 91. The soil and/or foliar application of Se fertilizers (agronomic biofortification) are often used to improve the Se concentration of food. Moreover, plants act as an effective buffer that can protect humans from toxic Se intakes that may take place with direct Se supplementation 74. The application of selenate fertilizers (soil and/or foliar application) has proved to be more effective to increase plant Se concentrations than selenite fertilization 92, 93 and hence selenate is the predominant form of Se in Se fertilization of plants 94. The chemical form of Se, soil characteristics, time of foliar and method of basal application affects the relative effectiveness of soil or foliar applied Se fertilizers

86

. Stephen et al.

95

reported an

increase of 32% in Se concentrations of wheat plants by sodium selenate fertilization on silt loam soil. They supplied sodium selenate at varying rates of 5, 10, 15 and 20 g Se ha-1 as prills drilled with the wheat seed, Se seed coating or a foliar spray at mid-tillering and/or ear This article is protected by copyright. All rights reserved

emergence and found foliar application at ear emergence stage to be more effective than other methods of Se application. In Finland, Se is added at a rate of 10 mg kg-1 as a soil amendment in NPK fertilizer 96 and has proved to be a safe, easy, effective and cost efficient approach to increase Se levels in a human population. Treatment of soil with selenite and selenate (0-10 mg kg-1) increased the Se concentration in ryegrass seedlings and a significant positive correlation was recorded between shoot Se concentration and glutathione peroxidase (GSH-Px) activity 97. The higher shoot Se concentration in selenate treated plants suggested that activity of this enzyme was related with the chemical form of applied Se rather than with the concentration of Se in plant tissues. More recently, Ducsay et al.

98

reported that soil Se application significantly

increased the Se content in the dry matter of roots, straw and grains of wheat. The application of 0.2 mg Se kg-1 of soil gave highest Se content in grain (0.732 mg kg-1), straw (0.227 mg kg-1) and roots (1.375 mg kg-1) dry matter whereas the lowest dose of Se (0.05 mg Se kg-1 of soil) gave 0.155 mg Se kg-1 in grain. The results of the study showed that Se concentration was highest in grain and lowest in the straw dry matter. The foliar application of Se on wheat significantly increased the levels of plasma Se (53% increase after 6 weeks) in Serbia. It caused an increase in the activity of glutathione peroxidase in blood and decreased oxidative stress parameters 99. An increase in the levels of copper, iron and zinc by Se-enriched wheat was observed in erythrocytes, compared to consumption of low-Se wheat 100. Germ et al. 101 found that foliar application of 1 mg sodium selenate per liter doubled the concentration of Se (43-46 ng Se g-1 DM) compared to the control (21-24 ng Se g-1 DM) and it also increased the respiratory potential in young plants without any visible toxic effects. Xu and Hu

102

reported that the foliar Se application

elevated the Se concentration as well as the antioxidant activity in rice. The low rate of selenate (10 g ha-1) was found to be equally effective in both basal and foliar application while foliar gave better results at 50 g ha-1 and no significant difference was observed at the high rate of 500 g ha-1 (Yläranta,1983b) 103. In the follow-up trials, foliar applied selenate at the 3-4 leaf stage was found to be more effective than basal application in variety of soils. The wheat grain Se level increased from 16 to 168 μg kg-1 by the foliar application of selenate at the rate of 10 g ha-1 while basal application of 9g raised it to just 77 μg kg-1. Thus it was found that foliar application was the more effective method with the exception from low rainfall areas (Yläranta,1984) 104. The variation among cereal crop cultivars interms of uptake of nutrient like zinc and iron

105

suggests that the same applies to Se. In an experiment in controlled field trial by

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Lyons et al.

106

, it was detected that significant genetic variation does not exist among

modern wheat genotypes however, larger variation and higher grain Se concentration were observed in diploid wheat (Aegilops taushii) and rye. The soil physical and chemical properties significantly affect variation in Se accumulation in wheat and even within few meters of field; large variation in grain Se concentration might occur

106

. Therefore the

homogeneity of the field site for available soil Se is very important for the assessment of genotypic variation in grain Se concentration and content. The hydroponic conditions provide a better medium to study the uptake efficiency for Se. It not only removes the limitations of heterogeneous soil conditions but also provide more reliable information about Se accumulation by different genotypes

107

. The experiments

carried out in water cultures on lettuce revealed that sodium selenite (Na2SeO3.5H2O) applied at the rate of 5 µM in nickel (Ni) contaminated medium prevented the toxic effects of Ni and stimulated plant growth by increasing the concentration of assimilation pigments 108.

5. Selenium and Plant Drought Tolerance The physiological and antioxidant properties of Se have increased the curiosity of many biologists in recent past. Although it does not take part in various vital metabolic processes in plants but may help to reduce the damage under physiological stresses

109, 55

.

Several studies have confirmed the positive role of Se against various environmental stresses such as cold

110

, high temperature

87

, salt

111

, heavy metals

112

, senescence

53

, UV-B

113-114

,

excess water 115 and desiccation 116. However, reports on the role of Se in plants under water stress conditions are scanty. It may regulate water status 56 and increases biomass production 66, 51

by the activation of antioxidant apparatus of water stressed plants 62, 111. Pittarelo (2012)

observed significant increase of 22.3% and 30% in biomass of Stanleya pinnata and Brassica juncea seedlings, respectively, by exogenous Se supply under well watered and mild drought stress conditions but found no significant differences under severe water deficit conditions. Moreover, Se-treated plants showed lower fv/fm ratio as compared to control which suggested a possible negative effect of Se on photosynthesis under drought stress. However, more recently Nawaz et al.

60

observed significant increase in photosynthetic rate and

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stomatal conductance of Se-treated wheat plants and were of the view that it was due to positive effect of Se on turgor maintenance and activation of antioxidant machinery. The antioxidative effect of Se is closely associated with the increased activity of glutathione peroxidase (GPX)

59

. The use of Se on a spring wheat cultivar showed that it

exerts an antioxidant effect directed towards a decreased concentration of intracellular active oxygen species by inducing the biosynthesis of proline and peroxidase production

56

. The

-1

addition of Se up to 1.0 mg kg significantly increased GPX activity in rye grass (Lolium perenne) that led to a simultaneous decrease in lipid peroxidation. The Se induced GPX activity suggests the presence of Se-dependent GPX and indicate positive relationship 53

between the Se concentration and GPX activity

. Ramos et al.

117

observed an increase in

lipid peroxidation at higher Se concentrations that decreased the yield of lettuce plants. They suggested low doses of Se optimal for increasing antioxidants activity and were of the view that Se application as selenate is more favorable for increasing Se translocation and Se levels in the shoot biomass than selenite. This high selenate uptake and translocation is due to high affinity of sulphate transporters for selenate

118

. In contrast, Cartes et al.

97

reported that

selenite was more efficient than selenate in increasing enzymatic activity. A schematic representation of role of selenium in improving tolerance against abiotic stresses is given in Figure.2. Numerous strategies viz. seed dressing/coating 119, seed soaking 66, soil application 120 and foliar spray

121, 60-61

have been used to supply Se in plants. However, the simplicity and

practicability of soil and foliar application make them widely accepted among these methods. Several studies confirmed the positive role of soil Se fertilization in various crops/plants such as rice

46, 121

, maize

122

, wild barley

123

and soybean

124

. The foliar Se application has been

reported to significantly promote growth in vegetables such as onion bulbs and leaves carrot roots and leaves cereals like wheat

91, 51

126

, radish flowers and leaves

127

as well as garlic bulbs

128

125

,

and in

.

The uptake and accumulation of Se within a narrow range is beneficial for plants

129

and is determined by the plants ability to absorb and metabolize Se. It is well documented that increase in acidity, iron oxides/hydroxides, organic matter and high clay content of soil decreases the bioavailability of Se to plants

130, 131

. The soil moisture also affects the

availability of Se to plants as it is more available under low precipitation conditions

132

.

Moreover, actively growing tissues usually contain large amounts of Se 133 and accumulation

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is higher in shoot and leaf than in root tissues

134

. Therefore, Se fertigation and foliar spray

are much more viable and effective approaches than soil application to increase Se translocation within plants.

Abiotic Stresses

Cold Stress

High temperature stress

Salt Stress

Heavy metals Stress

Excess water stress

Selenium (Se) Application

Increase Activity of Glutathione peroxidase (GPX)

Decrease intercellular active oxygen species (ROS)

Increase Proline and Peroxidase

Increase biomass production and plant growth

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Drought Stress

Figure 2. Schematic representation of role of selenium in improving tolerance against abiotic stresses Selenium increases the tolerance of plants to drought stress by regulating their water status. The foliar application of Se 20 g ha-1 in maize under drought stress increased yield and water use efficiency as compared to no Se application under water deficit conditions 64. In a similar study, Valadabadi et al. 65 evaluated the role of Se in improving drought tolerance of rapeseed cultivars and found that Se application significantly mitigated the adverse effects of drought on total dry weight, leaf area index (LAI), relative growth rate (RGR) and crop growth rate (CGR) of the cultivars. The combined treatment of re-watering and Se helped to improve biomass in wheat seedlings as compared to drought treatment and re-watering alone and promoted the recovery of malondialdehyde (MDA) content, superoxide radical (O2.-) production, soluble protein content and catalase (CAT) activity to the control values Zahedi et al.

136

135

.

reported foliar application of Se helpful in reducing electrolyte leakage in

canola cultivars subjected to drought stress. Proiettia et al. 137 observed that foliar spray of Se significantly improved photosynthesis, fruit yield and leaf water content of olive trees under water deficit conditions. An increase in GPX, CAT and APX activity was noted that suggests the protective role of Se against oxidative damage. Nawaz et al.

60

suggested that Se foliar

application influences nutrients uptake and improves wheat yield through turgor maintenance and enhancement in gas exchange characteristics and efficiency of antioxidant machinery. They reported marked reduction in osmotic potential by Se foliar spray that significantly improved turgor, transpiration rate and accumulation of total soluble sugars, free amino acids and activity of antioxidant system which improved the grain yield by 24%. Moreover, it also increased Se contents and improved iron (Fe) and sodium (Na) uptake and did not affect Calcium (Ca) contents in wheat straw. In a similar study, exogenous Se supply by different methods was found to be effective in increasing grain yield and quality of wheat

61

.

Supplemental Se improved the yield and quality of water stressed plants due to enhancement in the production of osmoprotectants and increased activity of antioxidant enzymes. Among different methods, Se foliar spray was observed to be more effective than Se seed priming and fertigation. However, Germ 138 observed an evident decrease in respiratory potential and mass of Se treated potato tubers exposed to water deficit conditions, whereas non-significant effect of Se treatment was recorded on number and size of leaf stomata. Selenium also

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promotes the growth of ageing seedlings

101

. Djanaguiraman et al.

87

reported that foliar Se

application to soybean promoted plant growth during senescence.

6. Conclusions Drought results in productions of the reactive oxygen species (ROS) in stressed plants which causes oxidative damage to the cellular constituents such as, carbohydrates, lipids, nucleic acids and proteins which ultimately reduces plant growth, respiration and photosynthesis. Application of Selenium decreases the oxidative damage by production of compatible solutes. The accumulation of these organic solutes prevents cellular dehydration and helps the plants to retain water in cells. Another role of selenium to improve drought tolerance in crop plants and protect them from oxidative stress by stimulating the senesce to produce ROS scavenging enzymes. Selenium increases superoxide dismutase (SOD) and glutathione peroxidase (GPX) activities thus activating the protective mechanisms against oxidative stress. It also exerts an antioxidant effect directed towards a decreased concentration of intracellular active oxygen species by inducing the biosynthesis of proline and peroxidase production. Se application can improve drought tolerance by increasing antioxidant enzyme activity and decrease membrane damage by ROS. The literature available on this aspect is insufficient to fully understand the role of Se to minimize the detrimental effects of drought in crop plants. Therefore, more future research is required for better understanding of interactions between drought and Se in soil-plant systems. Acknowledgement: Higher Education Commission is highly acknowledged for funding

References 1. Waraich EA, Ahmad R, Saifullah, Ashraf MY and Ehsanullah, Role of mineral nutrition in alleviation of drought stress in plants. Aust J Crop Sci 5:764-777 (2011). 2. GOP (Government of Pakistan), Economic Survey of Pakistan. 15-20 (2010-11). Islamabad, Pakistan. 3. Chaves

MM, Pereira JS, Maroco J, Rodrigues ML, Ricardo CPP, Oso´rio ML,

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