Plant growth under drought stress: Significance of mineral nutrients

Abstract

In natural and agroecosystems, plants are continuously exposed to several environmental stress factors. Besides having evolved several indigenous tolerance mechanisms, proper management and agricultural practices contribute to avert the deleterious impact of stressful conditions. Mineral nutrients have an irreplaceable role in plant metabolism. Deficiency of any mineral element impedes plant growth, which has direct associations with the yield potential of the plant. Mineral elements mediate several physiological and biochemical processes like enzyme activation, osmoregulation, protein synthesis and photosynthesis. Proper supplementation of mineral elements to crop plants can contribute to avoid drought stress through their active participation in several defence mechanisms like osmoregulation and antioxidant systems. The present review discusses role of mineral elements (macro- and microelements) in growth and defence mechanisms of plants in response to drought stress.

Full text

649
Water Stress and Crop Plants: A Sustainable Approach, Volume 2, First Edition. Edited by Parvaiz Ahmad.
© 2016 John Wiley & Sons, Ltd. Published 2016 by John Wiley & Sons, Ltd.
37.1 Introduction
Drought is an important environmental stress factor,
which affects plant growth and development and causes
significant productivity losses worldwide. It is very
important to understand the roles of physiological,
biochemical and nutritional networks under drought
stress (Valliyodan and Nguyen, 2006; Ramegowda and
Senthil‐Kumar, 2015). Plant biomass and height
decreases with increasing water stress (Ahmad et al.,
2009). In addition to its obvious impact on plant growth
and development, drought stress imparts imbalances in
mineral nutrition of plants leading to secondary effects.
Drought reduces transport of mineral nutrients from the
root to the shoot by decreasing transpiration rates and
changing function of membrane transporters. By under-
standing the impact of drought stress on plant mineral
nutrition, useful strategies can be adopted to reduce the
amount of damage caused by the drought and subsequent
nutrient deficiency (Marschner, 1995; Sanchez‐Bel etal.,
2008; Silva etal., 2011). The role of mineral nutrients in
increasing or decreasing drought stress tolerance of
plants has been studied by several researchers, however,
it is still inadequate and somewhat elusive. Among the
mineral nutrients, macronutrients form important struc-
tural components of plants and their deficiency‐induced
sensitivity in plants can be readily observed and targeted.
In contrast, micronutrients may directly or indirectly
affect the susceptibility of plants to stress factors via
changing enzyme activity, modulating the signal trans-
duction pathways and/or producing some metabolites
(Hajiboland, 2012). Plants possessing the ability to
acquire and retain more water have high water use
efficiency and can withstand drought conditions better.
Response of plants to drought stress largely depends on
the severity of drought as well as the growth stage of
plant (Taiz and Zeiger, 2010; Shanker and Venkateswarlu,
2011). Plants undergo several physiological and
biochemical changes in response to stress, such as
changes in relative water content, photosynthesis,
metabolism of carbohydrates, proteins, amino acids and
enzyme activity. Ample reports are available pertaining
to the effects of drought stress on photosynthesis, but
studies regarding the impact of drought on uptake of
mineral nutrients and their subsequent effects on plant
physiology still has many dark areas (Farooq etal., 2009).
Even if the plant grows on nutrient rich soils, drought
stress can cause deficiency of the nutrients because of its
direct influence on the physicochemical properties of
soils and hence reducing mobility and uptake of nutri-
ents by the plant. Mineral nutrients are usually taken
from soils as inorganic ions required for plant growth
and development. Under drought conditions, nutrient
uptake is impaired as a result of reduced soil moisture
that eventually leads to slow diffusion of mineral nutri-
ents from the soil to the root surface and hence
Plant growth under drought stress: Significance
ofmineral nutrients
Mohammad Abass Ahanger1, Narges Morad‐Talab2, Elsayed Fathi Abd-Allah3, Parvaiz Ahmad4,
andRoghiehHajiboland2
1 Stress Physiology Laboratory, Department of Botany, Jiwaji University, Gwalior, India
2 Plant Science Department, University of Tabriz, Tabriz, Iran
3 Plant Production Department, College of Food and Agricultural Sciences, King Saud University, Riyadh, Saudi Arabia
4 Department of Botany, S.P. College, Srinagar, Jammu and Kashmir, India
CHAPTER37

650 Water stress and crop plants: A sustainable approach
translocation speed to the leaves is also reduced. Drought
induces early closure of stomata, thus reduces transpira-
tion rate and also limits the transport of nutrients from
the root to the shoot. Thus drought stress reduces avail-
ability and transport of nutrients in the soil matrix and
plant tissues (Singh and Sale, 2000; Silva et al., 2011).
Despite various reports available on the effects of nutrient
supply on plant growth under drought conditions, it is
generally accepted that under drought conditions
increasing nutrient supply will not improve plant growth
if adequate amounts of nutrients already exist in the soil
(Hu and Schmidhalter, 2005).
37.2 Macronutrients andplant growth
37.2.1 Nitrogen
Nitrogen (N) is one of the main macroelements required
for proper plant growth and development (Smithson
and Sanchez, 2001). Availability of nitrogen is one of
the important growth limiting factors as nitrogen forms
the main structural component of proteins, coenzymes,
chlorophyll, pyrimidines, purines and nucleic acids.
Nitrogen is actively involved in many metabolic
processes making a direct contribution to increasing
productivity and improving crop yields (Barker and
Pilbeam, 2007). Since plants need nitrogen in large
amounts it should be supplemented to the soil via fertil-
izer application as few plants such as legumes get some
proportion of required nitrogen from the atmosphere
through the processes of nitrogen fixation. Nitrogen is a
mobile element and in soils it is available as nitrate
(NO3
‐) or ammonium (NH4
+) for plants and presence of
adequate soil moisture is essential for nitrogen mineral-
ization process. Though ammonium is toxic at higher
concentrations it is quickly converted into organic com-
pounds in the roots while nitrate is easily transported
and stored in the storage organelles of root and shoot
cells (Marschner, 1995). In cytoplasm, nitrate is con-
verted into ammonium by the sequential action of
enzymes nitrate reductase and nitrite reductase (Barker
and Pilbeam, 2007). Drought reduces the activity of
nitrate reductase and affects various aspects of nitrogen
metabolism and uptake. Drought induces reduction of
water level in plant tissues and water potential, and
subsequently reduces leaf elongation, leaf photosyn-
thesis rate, photosynthetic pigment contents and dam-
ages photosystem II, alters protein synthesis, nitrogen
metabolism and cell membrane properties leading to a
reduction in plant productivity (Saneoka et al., 2004).
Reduced soil water potential perturbs mobility of
nitrogen in the soil, thus plants exposed to/or facing
drought stress will also face nitrogen deficiency, which
also reduces plant growth. Nitrogen deficiency causes
leaf chlorosis especially in older leaves and if the deficit
continues or becomes more severe, then leaves will fall
off because nitrogen relocates from older leaves to
younger ones. Reduced leaf area and photosynthesis are
immediate effects of altered nitrogen uptake, which has
also its bearings with enzyme activity and chlorophyll
synthesis. In saline soils, even if the soil is watered well,
a condition of physiological drought arises resulting in
salt‐induced reduction of soil water potential (Taiz and
Zeiger, 2006). In leguminous plants drought reduces
nitrogen‐fixing processes by affecting nodule structure,
nodule membrane permeability and enzyme activities
(Ramos et al., 1999; Pimratch et al., 2010). A specific
concentration of nitrogen is crucial for enhancing pho-
tosynthetic activity and water use efficiency (WUE),
thereby showing a distinct trade‐off between water and
nitrogen use efficiencies but the mechanism remains
still unclear (Shi etal., 2014).
37.2.1.1 Nitrogen role inalleviating drought stress
Efficient water use and improved growth with both
restricted supply of water and nitrogen are desirable
characteristics for crops in dry environments (Shangguan
et al., 2000). Many studies under drought conditions
suggest that nitrogen fertilizer often causes less biomass,
increases sensitivity to stress, seedling mortality and
decreases plant growth, thus under such conditions
nutrient stress may enhance the tolerance of plants to
drought and some other stresses as well (Rahimi etal.,
2013). In addition, no significant interactions between
nitrogen supply and drought stress for root dry mass,
root/shoot ratio and WUE have been found (Song etal.,
2010; Rahimi et al., 2011; 2013). In contrast, many
other studies have found that drought tolerance in
plants can be improved by increased nitrogen applica-
tion (Halvorson and Reule, 1994). Nitrogen application
enhances WUE and alleviates drought stress effects on
plant growth (Saneoka et al., 2004). Applying higher
levels and increasing nitrogen nutrition may prevent
cell membrane damage and enhance osmoregulation.
Nitrogen application under drought stress conditions
increases N, K, Ca and glycine betaine concentrations in

Plant growth under drought stress: Significance ofmineral nutrients 651
leaf tissues. Drought stress enhances malondialdehyde
(MDA) concentration in leaves, while nitrogen supple-
mentation reduces MDA in both control as well as
water‐stressed plants (Saneoka et al., 2004). Under
drought stress, nitrogen supply improves photosyn-
thetic pigment contents and photosynthetic capacity by
increasing leaf area (LA), therefore enhancing photo-
synthetic efficiency and alleviating photo‐damage under
water stress (Wu etal., 2008). Application of nitrogen
enhances plant tolerance to drought, affects the
efficiency of PSII and decreases the degree of photo‐
inhibition and injury under water stress by improving
Fv/Fm (Zhou and Oosterhuis, 2012). Application of
nitrogen fertilizer increases the yield of wheat (Triticum
aestivum L.), maize (Zea mays L.) and barley (Hordeum
vulgare L) and enhances WUE and soil quality (Halvorson
and Reule, 1994). In wheat, nitrogen fertilization
enhances biomass, grain production and WUE under
drought by increasing leaf area index and maintaining
leaf area duration (Latiri‐Souki etal., 1998). However,
drought as a limiting factor for root growth leads to a
lower nitrate mobility in the soil so under extreme
drought stress application of nitrogen would not cause a
significant enhancement in plant growth (Latiri‐Souki
etal., 1998). Compared to water supply level, nitrogen is
a greater determining factor in switch grass (Panicum vir-
gatum L.) yield and performance. Nitrogen has a far
greater effect on single‐leaf photosynthesis rates than
water. However, water stress does have a significant
effect on the specific leaf nitrogen content and plants
supplied with higher nitrogen show a greater reduction
in photosynthesis and leaf water potential under
drought stress (Stroup etal., 2003). The photosynthetic
gas exchange parameters of winter wheat are remark-
ably improved by sufficient water and nitrogen nutri-
tion and the regulative capability of nitrogen nutrition is
influenced by water status. The effects of nitrogen nutri-
tion on photosynthetic characteristics are not identical
under different water status. WUE of the plants supplied
with high nitrogen nutrition decreases to a large extent
compared to low‐N treatment. Higher nitrogen induces
a larger decrease in photosynthetic rate than in transpi-
ration rate (Shangguan et al., 2000). Xu et al. (2013)
reported that water is the primary limiting factor for
photosynthesis in Bothriochloa ischaemum L. and appro-
priate levels of nitrogen fertilization improves its photo-
synthetic capacity under water deficit conditions. In
drought‐stressed cotton seedlings, the resistance to
water stress is influenced by nitrogen and has a strong
association with the activities of antioxidative enzymes.
Antioxidative enzymes activities are altered by the
nitrogen supply level and MDA content increases signif-
icantly in water stress treatments. There is a dramatic
decrease of nitrogen accumulation under water stress.
Low‐nitrogen treatment increases C accumulation and
thus nitrogen application may cause drought tolerance
by enhancing the activity of antioxidative enzymes
resulting in reduction of lipid peroxidation and enhance-
ment of root vigour (Zhou and Oosterhuis, 2012).
37.2.2 Phosphorus
Phosphorus (P) is one of the main mineral elements for
plant growth and has an important role in preserving
and transferring of energy in plant metabolism (Wang
etal., 2015). Phosphorus is a component of important
molecules such as nucleic acids, phospholipids and ATP
(Amtmann and Blatt, 2009). In nature, phosphorus
exists in both organic and inorganic forms. Organic
phosphorus consists of undecomposed plant and animal
residues, microbes and organic matter in the soil.
Inorganic phosphorus is usually associated with
aluminium, iron and calcium compounds of varying
solubility and availability to plants. In many soils the
application of phosphorus is necessary for plant produc-
tivity since the recovery of applied phosphorus by crop
plants is very low and more than 80% of the phos-
phorus becomes immobile and unavailable for plant
uptake as a result of adsorption, precipitation or
conversion to the organic form (Schachtman et al.,
1998). Phosphorus in soil is in the form of orthophos-
phate; plants take up H2PO4
‐ or HPO4
2‐ ions depending
on the soil pH and the highest phosphorus uptake is
usually observed between pH 5.0 and 6.0, where H2PO4
‐
dominates (Furihata et al., 1992). Under normal
physiological conditions there is a requirement for
energized transport of inorganic phosphorus (Pi) across
the plasma membrane from the soil into the plant
because of the relatively high concentration of Pi in the
cytoplasm and the negative membrane potential that is
characteristic of plant cells.
In soil, phosphorus can be rapidly converted into forms
unavailable for plants. A part of inorganic phosphorus is
rapidly changed to organic form in root or shoot after
uptake by plant. The ability of plant to gain inorganic
phosphorus from the soil and to convert it to organic
phosphorus in the roots and plant phosphorus use

652 Water stress and crop plants: A sustainable approach
efficiency are important criteria in plant phosphorus
nutrition. Both efficiency and conversion of phosphorus
can be accelerated by efficient addition of phosphorus
fertilizer to the soil (Schachtman etal., 1998). Arbuscular
mycorrhizal fungi (AMF) can enhance all these parame-
ters (Karagiannidis and Hadjisavva‐Zinoviadi, 1998).
AMF hyphae are more efficient than plant roots and
enhance phosphorous efficiency of host plants (Bolan,
1991). In plants colonized by AMF, the majority of the
phosphorus is of hyphal origin (Smith et al., 2004).
Similar to the nitrogen, phosphorus deficiency inhibits
plant growth and causes a reddish colour in the leaves
and stem due to accumulation of anthocyanin.
Phosphorous deficiency causes a change in colour of
leaves from dark green to purple (Marschner, 1995). Low
phosphorous leads to inhibition of many metabolic
processes such as respiration, photosynthesis, cell divi-
sion and expansion. Moreover, phosphorus deficiency
also reduces the uptake and assimilation of nitrates in
plants (Pilbeam etal., 1993). Drought stress reduces the
transfer of phosphorus from the soil to the root and its
subsequent transport to the stem (Goicoechea etal., 1997;
Cramer etal., 2009). In dry soil phosphorus mobility is
reduced because it moves mainly through diffusion.
Therefore, availability of sufficient water in soil is an
important factor enhancing mobility and uptake of phos-
phorus. In maize plants lower soil water contents reduce
phosphorus uptake (through hampered diffusion) and
decreases plant biomass (Radersma et al., 2005). Low
transpiration rate under soil water deficit causes phos-
phorus losses in plants during a dry growing season
leading to leaf senescence. However, a constant nutrient
supply may alleviate the effects of drought under certain
circumstances (Wade etal., 1998; Ge etal., 2012).
37.2.2.1 Phosphorus role inalleviating
droughtstress
Ample studies indicate that drought tolerance and WUE
of many species can be improved by enhanced phos-
phorus nutrition (Dodd et al., 1992; Singh and Sale,
1998; Garg et al., 2004; Waraich et al., 2011b). Some
possible mechanisms have been proposed that explain
the positive effects of phosphorus on plant growth under
drought stress such as an increase in root growth (Singh
and Sale, 1998), stomatal conductance and nitrate
reductase activity (Bruck etal., 2000; Naeem and Khan,
2009; Oliveira etal., 2014), leaf area and photosynthesis
(Singh et al., 2006; 2013), higher cell‐membrane
stability and water relations (Faustino et al., 2013;
Sawwan etal., 2000; Kang etal., 2014; Singh etal., 2006).
AMF colonization enhances uptake of immobile soilions
such as phosphorus, potassium, calcium, magnesium,
sulfur, iron, zinc, copper and manganese (Marschner,
1995; Liu etal., 2007), and improves uptake and trans-
port of mobile nitrogen ions under drought conditions
(Liu etal., 2007). Singh and Sale (1998) reported that
phosphorus content in leaves was significantly greater
for plants growing in dry soil compared with wet soil. In
cotton, phosphorous‐induced increases in the yield
were closely related with leaf area. Plant height, leaf
fresh mass and leaf area per plant were positively related
to the phosphorus content of leaf. Water‐stressed
phosphorous deficient plants have reduced plant height,
leaf area and leaf water content. Individual leaf area and
water content of fresh leaf (ratio of dry mass to fresh
mass) are significantly dependent on leaf phosphorous
content (Singh etal., 2006). Kang etal. (2014) reported
and concluded that placing phosphorous fertilizer deep
into soil is a practical and feasible means of increasing
grain yield and WUE in winter wheat cultivated in semi‐
arid regions. Deep placement of fertilizers mediates
enhanced growth by promoting deep root development
(Kang et al., 2014). In addition to application of
phosphorous fertilizer, water stress tolerance is also
determined by the type of soil in which plants are
growing (Graciano et al., 2005). For example, in
Eucalyptus grandis seedlings it has been observed that
phosphorous fertilization was more effective in clay
soils, even if a moderate drought is likely to occur, as
compared to sandy soils where phosphorous application
proved effective only under well‐watered conditions
(Graciano etal., 2005).
37.2.3 Potassium
Potassium (K) is an important macroelement after
nitrogen and phosphorous. It plays a vital role in main-
taining plant water status, stomatal movements, enzyme
activity, osmoregulation and membrane stability
(Farooq et al., 2009; Marschner, 2012; Ahmad et al.,
2014; Jatav etal., 2014; Erel etal., 2015). Potassium has
high mobility in long distance transport through the
xylem and phloem (Marschner, 1995). As a most abun-
dant cation in the cytoplasm it has an important role in
osmotic potential (Shabala and Pottosin, 2010). Plant
water status strongly influences the accumulation of
potassium in leaves. Its role in the opening and closing

Plant growth under drought stress: Significance ofmineral nutrients 653
of stomata is crucial because potassium channels in sto-
matal guard cells are sensitive to plant water status (Taiz
and Zeiger, 2006). Under drought conditions the soil
potassium availability for plants is lowered and that
limits its uptake by the root ultimately affecting its root‐
shoot translocation (Wang etal., 2013). Erel etal. (2015)
observed a reduction in enzymatic pathways of photo-
synthesis and it was reported to slow the photoprotec-
tive mechanisms in olive trees.
37.2.3.1 The role ofpotassium inalleviating
drought stress
There are many studies that explain the relationship
between potassium and its involvement in physiological
and molecular mechanisms for enhancing plant drought
tolerance. Adequate potassium nutrition can improve
WUE and hence plant growth under drought conditions
(Eakes etal., 1991; Egilla etal., 2001; Jatav etal., 2014).
Potassium fertilization facilitates plant tolerance via dif-
ferent mechanisms such as osmotic adjustment, main-
taining the activity of aquaporins and hence water
uptake, cell elongation, promotion of root growth and
cell membrane stability, stomatal regulation as well as
detoxification of reactive oxygen species resulting in
improved drought stress tolerance (Wang etal., 2013).
In sorghum under stress conditions, application of
potassium enhances photosynthetic rate, plant growth
and yield (Sharma etal., 1996; Egilla etal., 2001). An
adequate supply of potassium and magnesium in the
leaves is essential in supplying photoassimilates to the
roots to cover the energy requirement for root growth
and development as well as ion uptake. In maize (Zea
mays L.), a strong relationship between soil moisture,
root growth and the rate of potassium diffusion in the
soil has been observed. Reduced soil moisture level
resulted in lower potassium acquisition capacity leading
to reduction of root length density and inhibition of root
growth in the dry soil (Li and Zheng, 1996; Römheld
and Kirkby, 2010). Supplementation of potassium to
water‐stressed maize plants caused greater adaptation of
plants to water deficit. Potassium nutrition increases cell
membrane stability, leaf water potential, turgor poten-
tial and reduces stomatal resistance. An improved
potassium nutritional status increases plant productivity
by maintaining osmotic balance and influencing directly
leaf characteristics like thickness and water content
(Premachandra etal., 1991). Kanai etal. (2010) reported
a close relationship between stem diameter expansion
and activities of aquaporins and potassium channel
transporters in tomato roots grown under greenhouse
conditions. Potassium deficiency decreases aquaporin
activity, root hydraulic conductance and water supply
level to plants (Kanai etal., 2010). Root hydraulic con-
ductivity and associated anatomical characteristics has
direct influence on crop growth and yield and increases
hydraulic conductivity that is believed to have an
important role in enhancing stress tolerance. In olive
trees, potassium starvation favoured stomatal conduc-
tance and transpiration, as well as shoot growth inhibi-
tion (Benlloch‐Gonzalez et al., 2008). Potassium
starvation increases the transcription of genes involved
in ethylene production, hence stimulating its overpro-
duction and mediates signalling. Increased ethylene can
also inhibit the action of abscisic acid (ABA) on stomata
and delay stomata closure. During drought stress, the
stomata cannot function properly in K‐deficient plants,
resulting in greater water loss. Adequate levels of
potassium nutrition enhance plant drought resistance
by affecting water relations and WUE leading to better
plant growth (Wang etal., 2013). Oxidative damage to
cells is induced by excessive formation of reactive
oxygen species (ROS), especially during photosynthesis.
Drought stress causes stomatal closure and reduces CO2
fixation. Potassium is required for maintenance of pho-
tosynthetic CO2 fixation. ROS production in drought‐
stressed plants is increased due to impairment of
photosynthesis and disturbances in carbohydrate
metabolism (Jiang and Zhang, 2002). Increasing
potassium nutritional status lowers greatly the ROS pro-
duction through its direct influence on the activity of
NAD(P)H oxidases. Lower activity of NAD(P)H oxidases
results in lower production of toxic ROS and hence pro-
tecting cells from ROS‐induced oxidative damages
leading to a maintained photosynthetic electron trans-
port (Cakmak, 2005).
37.2.4 Magnesium
One of the major irreplaceable functions of magnesium
(Mg) is its involvement in photosynthesis as it forms an
essential component of the chlorophyll molecule.
Magnesium also plays a role in energy conservations
and protein synthesis as a cofactor for many enzymes
associated with de‐phosphorylation, hydrolysis and in
stabilizing the structure of nucleotides and sugar
accumulation (Merhaut, 2007; Yang etal., 2012; Xiao
et al., 2014; Blasco et al., 2015). Magnesium is less

654 Water stress and crop plants: A sustainable approach
available, physically and physiologically, under drought
conditions and the plant roots cannot take up sufficient
magnesium for plant normal growth. Magnesium is
physiologically mobile within the plant and under defi-
ciency conditions it can be reallocated from older parts
and transported through the phloem to the actively
growing sinks, so its deficiency symptoms will first be
observed in the oldest leaves by the appearance of chlo-
rosis. Magnesium has functions in synthesis of proteins
that can affect the size, structure and function of chloro-
plasts. Hence degradation of proteins in chloroplasts of
magnesium‐deficient plants causes loss of chlorophyll as
much as the loss of magnesium for chlorophyll synthesis
(Merhaut, 2007). Magnesium has an important role in
activation of certain key enzymes like ATPases,
ribulose‐1,5‐bisphosphate (RuBP) carboxylase, RNA
polymerase and protein kinases (Marschner, 1995).
Magnesium deficiency also arises as a result of drought
stress and the presence of competing cations in soil
preventing magnesium uptake, such as calcium in
calcareous soils; H, NH4 and Al in acidic soils and Na in
saline soils (Shaul, 2002). Magnesium deficiency occurs
particularly in plants growing in highly leached acid
soils with low cation exchange capacity and in such soils
leaching removes magnesium into deep layers, limiting
supply of magnesium to plants (Bose etal., 2011).
37.2.4.1 Magnesium role inalleviating
droughtstress
Magnesium plays key role in partitioning of carbohy-
drates and dry matter production between roots and
shoots, photosynthetic CO2 fixation and ROS formation
and related photooxidative damage. Adequate amount
of magnesium is needed during the reproductive growth
stage to keep maintain the transport of important carbo-
hydrates from source organs. The efficiency of foliar
application of nutrients such as magnesium is directly
correlated with the mobility of the nutrient within the
plant (Ashraf et al., 2008). Nitrogen, potassium and
magnesium have fast mobility through the phloem and
thus their foliar applications would be beneficial in
enhancing plant stress tolerance, while calcium and iron
have relatively slow mobility and hence they may be
less effective (Marschner, 1995). An alternative method
is foliar application of ionic or chelated forms of nutri-
ents (Ashraf and Foolad, 2005). Foliar application of
magnesium to crop plants can improve yield under
drought stress conditions, which reduces uptake of
magnesium (Cakmak and Kirkby, 2008). Foliar applica-
tion of zinc, potassium or magnesium resulted in
improved growth and yield of mung bean (Vignaradiata L.)
plants grown under drought stress conditions. Foliar
application of magnesium sulfate increases net assimila-
tion rates, seed yield and crude protein content of plants
(Thalooth et al., 2006). Asada (2006) reported that in
magnesium‐deficient leaves, the reduction in transport
and accumulation of carbohydrates occurs as a result of
altered photosynthetic carbon metabolism and limited
CO2 fixation. Possible mechanisms of magnesium nutri-
tion in alleviating drought stress include its role in
improving the root growth and root surface area,
thereby leading to increased uptake of water and nutri-
ents by roots. Hence, synthesis, as well as transport of
photoassimilates, is enhanced and increased carbohy-
drate translocation leading to mitigation of drought‐
induced deleterious changes (Waraich etal., 2011a).
37.2.5 Calcium
Calcium (Ca) is a multifunctional mineral nutrient in
plants. Calcium deficiency results in stunted growth,
development of weak stems, abnormal foliage, prema-
ture shedding of blossoms and buds (Marschner, 1995).
The calcium ion (Ca2+) is an important signalling
messenger mediating the actions of many hormone and
environmental factors, including interaction with biotic
and abiotic stress factors. Ca2+ is involved in regulating
many processes such as cytoplasmic streaming, thigmot-
ropism, gravitropism, cell division, cell elongation and
differentiation, cell polarity, photomorphogenesis and
stress responses. Elevated cytoplasmic calcium is critical
for transduction of ABA signalling in guard cells. There
are two different pathways operating in plants that
include Ca2+‐dependent and Ca2+‐independent signalling
processes. These two pathways are actively involved in
plants for bringing coordination between initial guard cell
signalling and the initiation of defence responses as well
as phytochrome‐induced signalling (Shao etal., 2008).
Osmotic stress can cause rapid changes in cytosolic
free calcium in Arabidopsis seedlings and these changes
in calcium levels mediate increased expression of
drought responsive genes encoding proteins that act to
protect plants (Knight etal., 1998). Calcium is needed for
recovery of plants from dehydration and is involved in
activity of plasma membrane ATPase, which is required
to pump back the nutrients leaked as a result of stress‐
induced membrane injury (Palta, 2000). Reduction in

Plant growth under drought stress: Significance ofmineral nutrients 655
soil water potential causes reduction in the uptake of
calcium (Nahar and Gretzmacher, 2002). This stress‐
induced reduction in calcium uptake is slightly less in
comparison to phosphorus and potassium. Oertli (1991)
reported that adequate Ca2+ should be present in arid
soils with high pHs and the Ca2+ status in more acidic
tropical soils of semi‐arid areas can be more critical (Hu
and Schmidhalter, 2005). In Dalbergia sissoo seedlings
grown under different levels of water regimes,
availability of soil nutrients decreased when compared
with the initial soil and mean data of all the treatments
indicated that decrease was 54% for calcium compared
to the initial values (Singh and Singh, 2004).
37.2.5.1 The role ofcalcium inalleviating
droughtstress
In many studies it has been reported that calcium can
alleviate and postpone oxidative damage resulting from
drought. Phosphorylation, calcium and pH are impor-
tant factors modulating aquaporin activity (Farooq etal.,
2009). In two contrasting wheat genotypes, application
of ABA and calcium chloride ameliorated the effects of
water stress and a combination of the two was more
effective (Nayyar and Kaushal, 2002). Experiments
show that calcium and auxin participate in signalling
mechanisms leading to drought‐induced proline
accumulation. Jaleel et al. (2007), reported that CaCl2
can lower the proline concentration by enhancing the
level of proline degradation enzyme and reducing the
proline synthesizing ones. Application of exogenous
calcium as CaCl2 solution to soil of Honeysuckle (Lonicera
japonica Thunb) under drought stress increased catalase
activity and chlorophyll content while it reduced the
cell membrane leakage and chlorophyll decomposition.
Moreover, exogenous calcium application decreased
proline and soluble sugar content (Qiang etal., 2012).
Foliar application of moderate calcium concentration
(CaCl2) to water‐stressed zoysia grass (Zoysia japonica)
plants improved drought tolerance by increasing mem-
brane integrity, chlorophyll content, balancing the
osmotic strength of the cytoplasm, the Fv/Fm ratio,
superoxide dismutase (SOD), peroxidase (POD) and cat-
alase (CAT) activity. Upadhyaya et al. (2011) also
reported that foliar spray of CaCl2 mitigates the drought‐
induced changes and calcium could increase the dry
mass of leaves in the recovery phase. Increasing Ca2+
availability may reduce drought damage by increasing
membrane integrity (Xu etal., 2013). Foliar application
of CaCl2 improves the photosynthetic rate of water
stressed soybean seedlings grown in pots (Genping etal.,
1995). In figs, Irget etal. (2008) observed that addition
of calcium up to a certain level to basic NPK fertilization
enhances fruit quality by increasing average fruit weight
and lowering cull ratio and can alleviate the negative
effects of drought conditions.
37.2.6 Sulfur
Sulfur (S) is another essential element required for
growth and physiological functioning of plants. It is
necessary for the formation of chlorophyll, vitamins,
enzymes and proteins. Uptake and assimilation of sulfur
and nitrogen by plants are strongly interrelated and
dependent upon each other. Sulfur mostly occurs in the
sulfate form and has low mobility in the soil. Sulfur also
is available through the atmosphere in different forms
such as sulfur dioxide, hydrogen sulfide, carbonyl sul-
fide, methyl mercaptan and carbon disulfide (Marschner,
1995). In plants, the major proportion of sulfur is present
in reduced form mostly cysteine and methionine amino
acids, and also in other organic sulfur compounds such as
thiols and sulfolipids. Sulfur concentrations in leaves
vary with the available sulfur in soils. An insufficient
sulfur supply decreases crop productivity and quality,
affects plant health and also impairs nitrogen use
efficiency (Haneklaus et al., 2007). Drought limits the
availability of sulfate to shoot, thereby causing the down-
regulation of the sulfur assimilating pathway in leaves.
37.2.6.1 Sulfur role inalleviating drought stress
Some reports are available regarding the role of sulfur in
alleviating effects of drought stress. Recent studies on the
chloroplast retrograde signal pathways show that more
attention should be given into the role and use of sulfur
in alleviation of drought stress. Application of sulfur alle-
viates the harmful effects of water stress on growth, yield
components and nutrient uptake of Sesamum indicum L.
(Heidari etal., 2011). During drought stress, an increased
content of glutathione (GSH) is required for the efficient
detoxification of ROS, thus the regulation of sulfur
assimilation during drought is vital due to the depen-
dency of GSH synthesis on the sulfur assimilation
pathway (Ahmad, 2013). Recent research points that
sulfur metabolism plays an important role in drought
stress signalling and responses. Primary sulfur uptake
and assimilation events, such as sulfate transport in the
vasculature, its assimilation in leaves and the subsequent

656 Water stress and crop plants: A sustainable approach
recycling of sulfur‐containing compounds, are related to
the drought stress response. In drought stress responses,
there are some sulfur‐related metabolites resulting from
drought‐dependent regulation of sulfur assimilation that
include glutathione and 3‐phosphoadenosine 5‐phosphate
(PAP) originating from two competing branches of the
sulfur assimilation pathways (Chan etal., 2013). PAP acts
as a chloroplast retrograde signal and activates stress
responsive genes during drought stress (Estavillo et al.,
2011). The regulation and activation of transport pro-
teins involved in the translocation of the retrograde sig-
nals, which may themselves be constituents of metabolic
pathways (such as PAP) and their mechanism of action
in the nucleus, are crucial missing links. There is a com-
plex balancing function to coordinate primary and
secondary sulfur metabolism during the drought stress
response in plants (Chan etal., 2013).
37.3 Micronutrients anddrought
stress
The role of micronutrients in improving WUE and in
alleviating water stress are not so well studied.
Micronutrients can activate certain physiological,
biochemical and metabolic processes under drought
stress. The roles of micronutrients in reducing drought
stress effects are discussed next.
37.3.1 Zinc anddrought stress
Zinc (zn) has several physiological roles in plants include
catalytic, cocatalytic and structural involvement in more
than 300 enzymes such as carbonic anhydrase, carboxy-
peptidase, alcohol dehydrogenase, CuZn superoxide
dismutase, alkaline phosphatase (microorganisms),
phospholipase, RNA polymerase, Zn‐PPiase (tonoplast),
fructose 1,6 bisphosphatase, aldolase, participating in
membrane integrity, Zn‐finger motif class of transcrip-
tion factors, integrity of ribosomes, protein, RNA, DNA
and carbohydrates, reactive oxygen species and indole
acetic acid (IAA) metabolism (Marschner, 1995;
Hajiboland, 2012; Blasco etal., 2015). In dry land condi-
tions, improving WUE is an important factor that affects
crop yield (Cakmak et al., 1996; Hajiboland and
Amirazad, 2010; Khan etal., 2004). Zinc uptake by the
plant root is decreased by low water availability in the
soil since under these conditions zinc mobility in the soil
is impeded (Marschner, 1995). In wheat plants, drought
stress during flowering and grain‐filling stages inhibits
micronutrient acquisition by roots resulting in yield
losses and low micronutrient concentrations in cereal
grains (Karim et al., 2012). WUE and photosynthesis
rate are far more reduced in plants under combinative
effect of zinc deficiency and drought stress since low
zinc supply leads to increased stomatal limitation and
ROS production (Hajiboland, 2012). Zinc deficiency is
critical in reproductive organs of plants, which ulti-
mately leads to lower productivity (Khurana and
Chatterjee, 2001). In Brassica rapa, Blasco etal. (2015)
have observed that zinc deficient plants maintained
lower activities of antioxidant enzymes compared to
plants supplied with sufficient zinc.
Prolonged duration and severity of the drought results
in severe oxidative damage since reactive oxygen species
levels produced surpass the capacity of plant for detoxifi-
cation (Carvalho, 2008). Zinc application can reduce
drought stress effects on plant growth by reducing the
activity of membrane‐bound NADPH oxidase, prevent-
ing photooxidative damage, reducing generation of reac-
tive oxygen species and increasing the activities of SOD,
POD and CAT involved in detoxifying ROS (Waraich
etal., 2011a; Hajiboland, 2012). SODs are classified into
three types based on their metal cofactor: manganese
(MnSOD), iron (FeSOD) or zinc and copper (CuZnSOD).
In general, MnSOD is located in the mitochondria,
FeSOD in the chloroplast and peroxisomes and CuZnSOD
in the chloroplast and cytosol. Measurements of these
isoforms of SOD are useful for assessing the micronu-
trient status of plants and have been used in the studies
on deficiencies of zinc, iron, manganese and copper
(Lopez‐Millan et al., 2005). In leaves of different wheat
and rice cultivars, zinc deficiency reduces total SOD
activity; among different types, the activity of CuZn‐SOD
is affected while Mn‐SOD is not related to the zinc nutri-
tional status of plants (Cakmak et al., 1997; Hajiboland,
2000). Zinc deficiency reduces carbonic anhydrase (CA)
enzyme activity in the chloroplast and cytoplasm in C3
plants, which causes a decline in photosynthesis. Zinc
deficiency can also affect auxin levels and application of
sufficient zinc results in maintained hormone levels to
tolerate drought stress (Waraich etal., 2011a).
37.3.2 Manganese anddrought stress
Manganese (Mn) is an important micronutrient playing
different roles in plants. It causes the activation of
several enzymes of the tricarboxylic acid cycle and

Plant growth under drought stress: Significance ofmineral nutrients 657
shikimic acid pathway leading to the biosynthetic
pathway of isoprenoids and other secondary metabo-
lites. Manganese is also involved in the photosynthetic
system related to photosystem II, ATP synthesis, RuBP
carboxylase reactions and the biosynthesis of fatty acids,
acyl lipids and proteins. PSII has several transition metal
cofactors such as haem, non‐haem iron and an oxygen‐
evolving manganese cluster containing four manganese
ions. Manganese has an important role in keeping chlo-
rophyll concentration and superoxide dismutase activity
well balanced (Upadhyaya etal., 2012). MnSOD is more
sensitive to UV light than CuZnSOD suggesting the
importance of the manganese cofactor in photoinhibi-
tion of MnSOD (Hakala et al., 2006). Manganese can act
as a scavenger of superoxide (O2
•–) and hydrogen per-
oxide (H2O2) radicals (Millaleo et al., 2010). Oxidative
stress caused by manganese deficiency indirectly causes
chlorophyll losses in leaves (Hajiboland, 2012). Drought
stress can cause deficiencies in manganese. Low
manganese availability in dry soil results from reduced
conversion to more soluble forms (Hu and Schmidhalter,
2005). In Hordeum vulgare, Hebbern (2009) reported
that manganese starved plants showed considerable
reduction in WUE. In manganese‐deficient barley
plants, lower WUE is related to higher stomatal con-
ductances during the daytime, imperfect nocturnal
closure of stomata and/or increased cuticular conduc-
tance due to degradation of epicuticular wax layer.
Hebbern et al. (2009) reported reduced transpiration
rate due to manganese deficiency in sugar beet, wheat
and spearmint plants. It was observed that supplemen-
tation of optimum level of manganese increased grain
yield and stress tolerance in wheat (Gholamin and
Khayatnezhad, 2012).
37.3.3 Iron anddrought stress
Iron (Fe) is involved in the production of chlorophyll
pigment molecule. It is a component of many enzymes
associated with energy transfer, nitrogen reduction and
fixation and lignin formation. In association with sulfur,
iron forms compounds that catalyse other reactions in
plants. Drought‐induced deficiency of iron causes chlo-
rosis of leaves due to low levels of chlorophyll. Leaf
chlorosis first appears on the younger upper leaves in
interveinal tissues. Severe iron deficiencies cause leaves
to turn completely yellow or almost white leading to
their death. Uptake of iron decreases with increased soil
pH and high levels of available phosphorus; manganese
and zinc in soils also have adverse effects on iron uptake
(Waraich etal., 2011a).
Soil water content affects both content and avail-
ability of iron to plants. In wet soils the Fe2+/Fe3+ ratio is
higher making the availability of iron for plants much
easier. But in drought conditions a decrease in the Fe2+/
Fe3+ ratio occurs possibly due to the increased O2 levels
in soil. Higher O2 reduces available iron for plant uptake
because Fe3+ is less soluble than Fe2+ (Sardans et al.,
2008). Because of the low solubility of iron bearing min-
erals, plants use two strategies to absorb sufficient iron.
Strategy I plants (dicotyledonous and non‐graminaceous
monocotyledonous plants) are able to respond to the
lack of iron in the soil by increasing the capacity of root
tissues to reduce apoplastic Fe, by the acidification of the
rhizosphere and accumulation and release of organic
acids (mainly citrate) to increase iron solubility in soil.
By the advent of these events, the iron uptake activities
in rhizodermal root cells and mobility within plants are
increased. In Strategy II plants, members of the mugin-
eic acid family of phytosiderophores are secreted into
the rhizosphere, which help to solubilize Fe3+ by chela-
tion to form the Fe3+‐mugineic acid complex. The Fe3+‐
mugineic acid complex is then taken up by root cells
through the action of Yellow Stripe 1 (YS1) protein (von
Wiren et al., 1994).
Abadia et al. (1999) demonstrated that iron‐defi-
ciency‐induced leaf chlorosis is due to decrease in the
leaf concentrations of photosynthetic pigments (chloro-
phylls and carotenoids). Iron‐deficient leaves showed
decreases in the actual PSII efficiency at steady‐state
photosynthesis, due to reduction of photochemical
quenching and intrinsic PSII efficiency (Abadia et al.,
1999). Iron nutrition has a critical role in the protection
of plants against oxidative stress resulting from drought.
Iron protects plants from oxidative stress‐induced
damage by affecting the activity POD, APX and CuZn‐
SOD enzymes. Iron deficiency educes activity of CAT
and PODs, the ubiquitous haem‐containing enzymes.
High levels of H2O2 in iron‐deficient plants (Marschner,
1995) indicate reduced capacity of iron‐deficient plants
for detoxification of peroxide radicals and therefore
increasing oxidative damage (Hajiboland, 2012).
Lombardi et al. (2003) indicated that the adequate
supply of iron is essential for the efficient function of
antioxidant enzymes. They also reported that iron‐
deficient onion has decreased activity of CAT, SOD and
POX. Reduced SOD activity in soybean and lemon due

658 Water stress and crop plants: A sustainable approach
to iron deficiency has also been observed. At the same
time, ROS production might be enhanced as a
consequence of the alterations in the electron transport
chain, the ultrastructural damages to the chloroplasts and
reduced carotenoid biosynthesis (Lombardi etal., 2003).
In soybean, Rotaru (2011) demonstrated that in soil
culture conditions application of adequate phosphorus
and iron not only enhances growth and nutritional status
but also alleviates negative effect of drought stress on its
performance. In legumes, iron deficiency alters the sym-
biotic association that is very sensitive to iron deficiency
and, in sand culture, iron application stimulated nitrogen
fixation of soybean plants (Rotaru, 2011).
37.3.4 Boron anddrought stress
Dry soils and soils with high pH are usually boron defi-
cient. As an immobile element, plants a need constant
boron (B) supply to avoid its deficiency (Taiz and
Zeiger, 2006). Drought stress causes a reduction in
boron uptake even if plants are supplied with adequate
boron. Root boron uptake is mostly a passive process
involving permeation across the membrane and is
mainly determined by the rate of water uptake by root
cells and the flow through water channels. Reduced
water potential of soil solution in combination with
reduced mass flow and diffusion rate reduces the avail-
ability of boron in drying soils (Hajiboland and
Farhanghi, 2011). Boron is a constituent of cell wall in
cross‐links of rhamnogalacturonan II, involved in cell
wall synthesis and cell extension, improves regulation
of lignin biosynthesis and xylem differentiation.
Moreover, it enhances photosynthetic rate and integ-
rity of membranes, improves sugar transport and IAA
synthesis, maintains carbohydrate, protein and RNA
metabolism, improves seed and pollen germination and
pollen tube growth as well. It affects metabolism of
phenolics and hence reduces production of ROS
(Waraich etal., 2011b; Hajiboland, 2012).
Mottonen etal. (2005) reported reduction of height,
growth, root biomass, formation of root tips, mycor-
rhizal inoculation and nutrient uptake in Norway
spruce seedlings, Picea abies, under low boron.
However, they found no direct effects of boron on
shoot water potential and gas exchange parameters.
Repeated drought stress caused visible symptoms of
damage to seedlings with low boron. They observed
more damage in the upper shoot of the boron‐deficient
seedlings, especially in those exposed to two periods of
drought. Boron deficiency may indirectly affect rate of
photosynthesis through impeded leaf area and leaf
constituents, stomatal aperture, chlorophyll and pro-
tein content.
In boron‐deficient plants, reduction in root hydraulic
conductance may be caused by a perturbation and inhi-
bition of growth of new roots formed. Boron is necessary
for function and activity of H+ ATPase and the gene
encoding for it. Boron deficiency has direct influence on
the uptake of other important mineral nutrients like K+
and solutes into cells required for maintenance of water
uptake, cell turgidity and expansion. Drought not only
affects transport of boron from root to shoot but also
retranslocation of boron. Lower transport of boron in
phloem under drought conditions is the mechanism of
significantly lowered ratio of young to old leaf boron
content under these conditions (Hajiboland, 2012).
In citrus leaves, Han et al. (2008) have reported that
boron deficiency triggers accumulation of starch,
glucose and fructose, but not sucrose. Assimilation of
CO2 may be feedback‐regulated by the excessive
accumulation of starch and hexoses via a direct inter-
ference with chloroplast function and/or indirect
repression of photosynthetic enzymes. Although
boron‐deficient leaves show high activity of antioxi-
dant enzymes, their antioxidant system as a whole does
not provide sufficient protection from oxidative
damage. In boron‐deficient citrus leaves, the activities
of ribulose‐1,5‐bisphosphate carboxylase/oxygenase,
NADP‐glyceraldehyde‐3‐phosphate dehydrogenase
and stromal fructose‐ 1,6‐bisphosphatase were reduced
(Han et al., 2008). Photochemical quenching decreased
significantly in plants subjected to combined effect of
boron deficiency and drought stress. Reduction of pho-
tochemical quenching could be related to photoinhibi-
tion rather than to a direct damage to PSII. One of the
causes of photoinhibition is lower Chl content in boron‐
deficient leaves that have an important role in suscepti-
bility to photoinhibition. Moreover, exposure to drought
stress intensifies inhibitory effects of boron deficiency
on the photochemistry of leaves resulting in reduced
photosynthesis (Hajiboland and Farhanghi, 2011).
37.3.5 Copper anddrought stress
Copper (Cu) is another essential metal for plants. It
plays key roles in photosynthetic and respiratory elec-
tron transport chains, ethylene sensing, cell wall
metabolism and protection from oxidative stress

Plant growth under drought stress: Significance ofmineral nutrients 659
(Yruela, 2005). Copper is also involved in pollen
formation and has an important role in maintaining its
viability, mediates pollination, biosynthesis of lignin,
quinones and carotenoids (Hajiboland, 2012). Several
enzymes bear copper ion as cofactor, for example Cu/
ZnSOD, cytochrome c oxidase, ascorbate oxidase,
amino oxidase, laccase, plastocyanin and polyphenol
oxidase. At cellular level, copper plays an essential role
in cell wall metabolism, signalling, protein trafficking
machinery, oxidative phosphorylation, iron mobiliza-
tion and the biogenesis of molybdenum cofactor. Thus,
an appropriate concentration of copper is essential for
normal growth and development, and its deficiency
develops specific symptoms in plants. Young leaves and
reproductive organs are the most sensitive organs to
copper deficiency. A deficiency in the copper supply
can alter essential functions in plant metabolism
(Yruela, 2005).
In peas, Ayala etal. (1992) reported that copper defi-
ciency reduces the content of thylakoids, PSII and
LHCII. Reduced PSI electron transport due to copper
stress is attributed to altered formation of plastocyanin,
a major target site of copper deficiency in photosyn-
thesis. Copper‐deficient plants show disintegration of
the thylakoid membranes as well as decreased pigment
(Chl and carotenoids) content, reduced plastoquinone
synthesis and lower unsaturated C18 fatty acid contents
(Ayala etal. 1992).
The moisture content of the soil is important for
copper retention. Dry soils have low bioavailable copper
(Tom‐Petersen etal., 2004). Drought stress increases lig-
nification in plants but copper deficiency reduces lignifi-
cation of cell walls because of its direct involvement in
the lignin biosynthesis via two copper‐enzymes, poly-
phenol oxidase (catalysing the oxidation of phenolics,
which act as precursors of lignin) and diamine oxidase
(provides the H2O2 required for oxidation by peroxi-
dases) (Hajiboland, 2012).
More study is needed to investigate role of copper
application in alleviating drought stress effects on plants.
In transgenic tobacco (Nicotiana tabacum cv. Xanthi)
plants’ over‐expression of cytosolic Cu/Zn‐superoxide
dismutase improves tolerance against drought stress. To
some extent over‐expression of Cu/ZnSOD alleviates
the water stress‐induced damage through maintaining
WUE and photosynthetic rates and reduction of peroxi-
dation of lipid, electrolyte leakage and H2O2 generation
(Faize etal., 2011).
37.3.6 Chloride anddrought stress
Chlorine (Cl) is classified as a micronutrient, but is often
taken up by plants at levels comparable to a macronu-
trient. There are plenty of sources of chlorine in nature
and its symptoms of deficiency are rarely observable.
Some plant species such as members of Palmaceae and
kiwifruit (Actinidia deliciosa) require higher chlorine
content, therefore in these species chlorine deficiency
can easily be observed. Chlorine is required for main-
taining optimal activity of asparagine synthethase, amy-
lase and ATPase. In photosynthesis, chloride is an
essential cofactor for the activation of the oxygen evolv-
ing enzyme associated with PSII (Hajiboland, 2012).
Chloride (Cl−) is a mobile anion in plants and indirectly
affects plant growth by stomatal regulation of water
loss. Chloride enhances WUE by reducing wilting and
restricting highly branched root systems in cereal crops
(Waraich etal., 2011b).
Chlorine is an essential cofactor for oxygen evolving
complex of photosystem II and is closely related to the
Mn4Ca cluster. Kawakami et al. (2009) demonstrated
that there are two anion‐binding sites on either side of
the Mn4Ca cluster at a similar distance from the nearest
manganese atom and calcium atom. They concluded
that these two sites bind the functional Cl− ions required
for oxygen evolution because substitution by I− com-
pletely abolishes oxygen evolution and the activity
could be recovered simply by replacing I− with Cl− or
Br− (Kawakami etal., 2009). Removal of Cl− from PSII of
chloroplast thylakoids causes an aberration and inter-
ruption of the normal progress of charge accumulation
in and around the Mn‐cluster of the water oxidizing
complex (Homann, 1988).
Cl− ions are involved in the mechanisms controlling
and regulating stomatal movements, particularly during
the dry seasons. Chloride (Cl−) has a certain physiological
role in coconut. There is a high concentration of chlo-
ride in coconut leaf tissues, which shows its osmotic role
in preserving tissue turgor during drought (Braconnier
et al., 1998).
37.3.7 Molybdenum anddrought stress
Molybdenum (Mo) is a part of the four main enzymes
catalysing diverse redox reactions in plants; (1) nitrate
reductase catalyses the key step in inorganic nitrogen
assimilation, (2) aldehyde oxidase (s) have been shown
to catalyse the last step in the biosynthesis of the phyto-
hormone ABA, (3) xanthine dehydrogenase, which is

660 Water stress and crop plants: A sustainable approach
involved in purine catabolism and stress reactions and
(4) sulfite oxidase is probably involved in detoxifying
excess sulfite (Mendel and Hansch, 2002).
Molybdenum is bound to a unique tricyclic pterin
compound named molybdenum cofactor (MoCo) assist-
ing in improving nitrogen metabolism (Mendel and
Bittner, 2006). Molybdenum can decrease the adverse
effects of water stress and improve WUE indirectly due
to its involvement in enzymes of nitrogen fixation/
metabolism, protein synthesis and sulfur metabolism. It
is also involved in pollen formation, so fruit and grain
formation are affected by molybdenum nutrition in
plants (Waraich etal., 2011a).
Low soil moisture induces deficiencies of
molybdenum and increases susceptibility of plants to
drought stress (Hu and Schmidhalter, 2005). LOS5 is
responsible in converting desulfo/dioxyo form of MoCo
to the sulfide form, a cofactor of aldehyde oxidase that
catalyses the last step of ABA biosynthesis. The los5
mutant (Low expression of Osmotically Responsive 5)
is highly sensitive to drought due to the lack of ability
for ABA biosynthesis. Drought increases the biosyn-
thesis and accumulation of ABA mainly by the
induction of genes coding for ABA biosynthetic
enzymes. Briefly, gene encoding zeaxanthinepoxidase
(ZEP), mediates conversion of zeaxanthin to epoxyca-
rotenoid and is defective in Arabidopsis mutants aba1
and los6. Conversion of epoxycarotenoids to xanthoxin
is done by cleaving enzyme 9‐cis‐epoxycarotenoid
dioxygenase (NCED). Finally, abscisic aldehyde oxidase
(AAO) converts abscisic aldehyde to ABA and is defec-
tive in the Arabidopsisaao3 mutant. Aldehyde oxidase as
a MoCo enzyme requires sulfuration for activation.
This step is catalysed by a MoCo sulfurase (MCSU)
encoded by the ABA3/LOS5 locus in Arabidopsis.
Mutations in the aldehyde oxidase apoprotein and
MoCo biosynthetic enzymes lead to ABA deficiency in
plants. The ABA‐deficient mutants flacca and aba3/los5,
show a defective MoCo sulfuration step, develop wilty
phenotypes and increased transpirational water loss
(Hajiboland, 2012).
37.3.8 Nickel anddrought stress
Nickel (Ni) is a component of urease and is required for
urea and arginine metabolism. In legumes and other
dicotyledonous plants, nickel deficiency decreases
activity of urease and subsequently results in urea tox-
icity which is usually observable as necrosis of leaf tip.
In bacterial systems, several families of nickel permeases
and ATP‐dependent nickel carriers have been character-
ized. In plant systems, most studies have been conducted
at unrealistically high soil‐nickel concentrations and as
such may be relevant for nickel toxicity, but are not
relevant for nickel uptake under normal conditions
(Brown, 2007). Higher nickel mobility was reported in
the soils with higher moisture content (Seregin and
Kozhevnikova, 2006).
Wood et al. (2004) observed that nickel deficiency
exists in certain crop and culture conditions and pecan
appears to be the first crop in which nickel deficiency
under field conditions is documented (Wood et al.,
2004). In barley, nickel deficiency decreased the capacity
to develop viable seeds because of hindered embryo
growth. The embryonic root developed poorly or even
stayed undeveloped; in addition, several anomalies
were reported in endosperm development together
with declined dehydrogenase activities. The critical
nickel concentration in barley tissues that reduced the
yield by 15% was 90 ng/g dry weight (Seregin and
Kozhevnikova, 2006).
There are two types of ureases; a tissue‐specific
enzyme found in the vegetative tissues of most plant
species and an embryonic enzyme, which is a
characteristic seed protein in the soybean, Arabidopsis,
Canavalia and so on. In soybeans, tissue and embryonic
ureases are encoded by different structural genes Eu1
and Eu4, respectively. Affinity and attachment of Ni2+ to
enzyme depends on the activities of two genes, Eu2 and
Eu3, encoding the auxiliary proteins that activate
urease. The mutations in these genes resulted in the loss
of urease activity. Nickel deficiency in the medium and
low activity of urease alter nitrogen metabolism and
lead to the accumulation of toxic urea levels in shoots
(Seregin and Kozhevnikova, 2006).
Nickel deficiency results in distinct biochemical
symptoms even before development of visible mor-
phological symptoms and disruption of vegetative
growth. A huge spectrum of metabolic alterations
caused by nickel deficiency in plants serve as an
evidence for the existence of unidentified physiological
roles of nickel in plants. Research and improvement
of knowledge about understanding the role of nickel
in plants may bring new insights into how nickel
nutrition affects plants stress responses by mediating
biochemical and physiological responses (Hajiboland,
2012).

Plant growth under drought stress: Significance ofmineral nutrients 661
37.3.9 Silicon anddrought stress
Although silicon (Si) has not been yet considered to be
an essential element for higher plants (Epstein, 1999),
but its beneficial effects have been demonstrated for
many plants, especially when they are exposed to biotic
or abiotic stresses (Ma and Yamaji, 2006; Liang et al.,
2007). It is essential nutrient for species from Equisetaceae
and wetland Poaceae. It prevents toxicity of P, manganese
and iron and reduces heavy metal stress, causes stability
of plants, cell wall rigidity and elasticity, increases leaf
erectness and the volume, rigidity of aerenchyma and
root oxidizing power of wetland plants, reduces cuticular
transpiration and effects of mutual shading and suscepti-
bility to lodging. Silicon also increases plant resistance
against fungi and pest attacks (Hajiboland, 2012). Under
drought stress conditions silicon increases retention of
water in plants and antioxidant production, improves
stomatal regulation, enhances photosynthesis, maintains
chloroplast and membrane integrity and decreases tran-
spiration, ROS generation and photooxidative damage
(Waraich etal., 2011a).
Gong et al. (2008) reported that the improvement of
drought tolerance of wheat by silicon was associated
with the increase of antioxidant defence abilities and
the alleviation of oxidative damage (Gong etal., 2008;
Pei etal., 2009). In wheat, at the filling stage, application
of silicon increased the SOD activity and decreased the
POD activity of drought‐stressed plants. It decreased
the content of H2O2 and protein carbonyl and increased
the content of total soluble protein. The content of thio-
barbituric acid reactive substances (TBARS) and the
activities of acid phospholipase (AP) and lipoxygenase
(LOX) in drought‐stressed plants were also decreased by
application of silicon (Gong etal., 2008).
Hattori etal. (2005; 2007) demonstrated that silicon
improved drought tolerance of sorghum via enhance-
ment of water uptake ability. Under dry conditions,
silicon‐treated sorghum had a lower shoot to root (S/R)
ratio, indicating the facilitation of root growth and the
maintenance of the photosynthetic rate and stomatal
conductance at a higher level compared with plants
grown without silicon application, which means silicon‐
applied sorghum could extract a larger amount of water
from drier soil (Hattori et al., 2005; 2007). Silicon
enhances osmotic adjustment through changes in the
accumulation of proline, inorganic ions and other
osmotic solutes in sorghum (Sonobe et al., 2010) and
rice (Ming etal., 2012; Liu etal., 2014).
Li et al. (2007) showed that under mild and severe
drought stress, silicon application could increase the
plant biomass by 31–33% and 24–41%, respectively,
compared with the control. Silicon improved the net
photosynthetic rate by 11–29%, enhanced the chloro-
phyll content and POD, SOD and CAT activities by
4–12%, 6–26%, 18–27% and 3–34%, respectively, and
limited leaf plasma membrane permeability and MDA
content (Li etal., 2007).
Gunes et al. (2008) demonstrated that silicon applica-
tion alleviates drought stress in sunflower cultivars by
preventing membrane damage. The CAT activity was
significantly decreased by drought stress, but silicon
application increased it. In general, SOD and APX activ-
ities of the cultivars were increased by drought and
decreased by application of Si. The non‐enzymatic anti-
oxidant activity in sunflower was significantly increased
by silicon under drought stress (Gunes et al., 2008).
Silicon increased WUE in corn, wheat, soybean and rice.
Under drought stress, silicon increased wheat dry mass
by 17%, soybean and rice dry mass by 20 to 30%.
Silicon in oldest corn leaves increased from 0.4 to 3% as
silicon increased from less than 0.01 to 0.8 mM in the
hydroponic solution (Janislampi and Bugbee, 2012).
Habibi and Hajiboland (2013) indicated that silicon
application significantly improved dry matter produc-
tion of pistachio plants only under drought conditions.
Silicon supplementation improved plant growth and
increased relative water content, maximum quantum
yield of PSII and net assimilation rate. Silicon applica-
tion increased the stomatal conductance of leaves,
which caused an improved CO2 fixation capacity under
water stress. Silicon reduced malondialdehyde content
resulting from activating antioxidant enzymes in the
presence of silicon (Habibi and Hajiboland, 2013).
In strawberry (Fragaria × ananassavar. Parus) plants
application of silicon increased significantly shoot and
root dry weight, relative water content, photosynthesis
rate and soluble protein content. SOD and CAT activity
were significantly higher in drought‐stressed plants sup-
plemented with silicon compared with plants without
silicon addition. Concentrations of H2O2, MDA and pro-
line were significantly lowered by silicon application
under drought stress compared with non‐supplemented
plants (Hajiboland and Morad‐Talab, unpublished data).
Liu et al. (2014) demonstrated that silicon treatment
can alleviate water deficit stress via different mecha-
nisms, which mostly remain unclear. In their study,

662 Water stress and crop plants: A sustainable approach
silicon effects on drought stress in sorghum seedlings
(Sorghum bicolor L.) under polyethylene glycol‐simulated
osmotic stress in hydroponic culture and water deficit
stress in sand culture were investigated. Osmotic stress
significantly decreased dry weight, photosynthetic rate,
transpiration rate, stomatal conductance and leaf water
content, but silicon application alleviated these effects.
Whole‐plant hydraulic conductance and root hydraulic
conductance (Lp) were higher in silicon‐supplemented
seedlings than in those without silicon supply under
osmotic stress. The contribution of aquaporins to Lp was
characterized using mercury (Hg) as an aquaporin
inhibitor. Under osmotic stress, the exogenous applica-
tion of HgCl2 reduced the transpiration rates of seedlings
with or without application of Si. After recovery induced
by dithiothreitol (DTT), however, the transpiration rate
was higher in silicon‐applied seedlings compared with
non‐applied ones. Transcription levels of several root
aquaporin genes were increased by silicon application
under osmotic stress. These results imply that the
silicon‐induced upregulation of aquaporins, which
caused increased Lp, improves root water uptake under
osmotic stress. Thus silicon plays a modulating role in
improving plant resistance to osmotic stress in addition
to its role as a physical barrier (Liu etal., 2014).
37.3.10 Selenium anddrought stress
Like other mineral nutrients selenium (Se) also has
different roles in plants including activation of the anti-
oxidant defence system, salicylic acid and jasmonic acid
pathways, mitigation and protection of plants against
UV stress, low and high temperatures, heavy metal
toxicity, pathogens and herbivory and it delays senes-
cence (Hajiboland, 2012).
In addition to animals, Kuznetsov et al. (2003) sug-
gested the presence of selenium‐containing proteins (e.g.
selenoperoxidases) in plant cells. This suggestion was
supported by the facts that the glutathione‐dependent
peroxidase activity was found in the extracts of many
plant species, for example glutathione‐containing perox-
idase was isolated from Aloe vera. In addition, it was
shown that plant cells contain individual specific compo-
nents for biosynthesis of selenium containing proteins,
for example the UGA‐decoding selenocysteine RNA
(tRNAsec). But the activity of the hypothetical seleno‐
peroxidase in plants is thought to be very low, and no
selenium‐containing amino acids have been found in its
active centre. In spring wheat, selenium averts water
stress effects by increasing water uptake capacity of the
root system rather than enhancing the economical use of
water in the process of transpiration. Selenium inhibits
the stress‐induced accumulation of proline and causes a
decrease in the peroxidase activity, which can also be
regarded as indirect evidence for the antioxidant function
of selenium (Kuznetsov etal., 2003).
Studies on common and tartary buckwheat, pump-
kins, ryegrass and lettuce show that, although selenium
is harmful for plants in high concentrations (reduction
of biomass), it can exert beneficial effects when present
in sufficiently low concentrations. Selenium can
increase the tolerance of plants to UV‐induced oxidative
stress, delays senescence and promotes the growth of
seedlings. Studies by Pennanen etal. (2002) have indi-
cated that plant growth promoted by selenium is the
result of increased starch accumulation in chloroplasts.
It has been shown that selenium has positive effects on
carbohydrate accumulation in potato. The positive
effects of selenium on the recovery of potato from pho-
tooxidative and paraquat‐generated oxidative stress
point to unknown mechanisms that protect chloroplasts
during stress (Germ etal., 2007).
In stressed plants selenium regulates ROS levels
through: (1) stimulation of spontaneous dismutation of
O2
•− into H2O2; (2) direct reaction between Se‐containing
compounds and ROS and (3) regulating antioxidant
enzymes. Another possible mechanism is that selenium
may affect the assembly of photosynthetic complexes to
regulate the levels of ROS. Selenium restores cellular
structure and function; the decreases in O2
•− and H2O2
levels upon selenium addition suggests reduced ROS
producing chain reactions, mitigation of damage to the
lipids of cell membranes. Optimal selenium supplemen-
tation has been shown to inhibit the accumulation of
malonaldehyde (MDA) in drought‐stressed rapeseed
seedlings. Application of appropriate levels of selenium
can reduce damage to the chloroplasts and increase the
chlorophyll contents. In sorghum, selenium application
significantly increased the photosynthetic rate, stomatal
conductance and transpiration rate. The regulation of
the uptake and redistribution of some essential elements
by selenium is believed to be an important mechanism
to reactivate associated antioxidants resulting in ROS
scavenging and improved stress tolerance. However,
available information about the effects of selenium on
the mineral uptake under stress conditions is insuffi-
cient (Feng etal., 2013).

Plant growth under drought stress: Significance ofmineral nutrients 663
For improving drought tolerance in wheat seedlings,
Nawaz et al. (2014) suggested that foliar spray of
selenium is more effective than selenium fertilization.
The uptake and accumulation of selenium within a
narrow range are beneficial for plants and are deter-
mined by the plants ability to absorb and metabolize Se.
It is well documented that increase in acidity, iron
oxides/hydroxides, and organic matter and high clay
content of soil decrease the bioavailability of selenium
to plants. The soil moisture also affects the availability of
selenium to plants as it is more available under low
precipitation conditions. Moreover, actively growing
tissues usually contain large amounts of selenium and
accumulation is higher in shoot and leaf than in root
tissues (Nawaz etal., 2014).
Hajiboland et al. (2014) demonstrated that selenium
application in two durum wheat (Triticum durum L.)
genotypes under drought conditions slightly increased the
biomass of both genotypes. Selenium application amelio-
rated the impact of drought on photosynthetic rate.
Selenium application increased soluble proteins and free
amino acids under drought conditions, indicating role of
selenium application in improving some physiological
parameters such as photosynthesis, accumulation of
osmolytes and WUE (Hajiboland etal., 2014).
37.4 Conclusion andfuture prospects
Mineral elements affect growth and physiology of plants
by mediating variety of ubiquitous functions including
photosynthesis, respiration, protein synthesis and so on.
Under normal conditions, the role of macroelements in
plant growth regulation has been well studied in differ-
ent crop plants; however, their role under stressful envi-
ronmental conditions has not been studied extensively.
Inadequate supply of nutrients leads to stunted growth
through altered plant metabolism. Restricted nutrient
supply alters processes like nitrogen metabolism, photo-
synthesis and respiration. Drought stress causes hin-
drance in nutrient acquisition even if nutrients are
available in sufficient quantities. Reduced mobility as
well as uptake of essential mineral nutrients under
drought conditions reduce growth performance of
plants and hence yield. Application of adequate nutri-
ents to crop help them to grow well, but for better utili-
zation of mineral nutrients an adequate amount of
water is also essential. Proper and adequate management
practices in addition of the proper nutrient and water
application should be given due consideration. Reducing
deleterious impacts of drought stress can better be
achieved by better nutrient acquisition and usage poten-
tial. So in context to this, developing plant cultivars
showing efficient uptake and utilization of available
mineral elements is one of the better strategies that
needs to be focused on.
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