Agron. Sustain. Dev. 29 (2009) 185212c INRA, EDP Sciences, 2008DOI: 10.1051/agro:2008021

Review article

Available online at:www.agronomy-journal.org

for Sustainable Development

Plant drought stress: eects, mechanisms and management

M. Farooq1 ,3*, A. Wahid2, N. Kobayashi3 D. Fujita3 S.M.A. Basra4

1 Department of Agronomy, University of Agriculture, Faisalabad-38040, Pakistan2 Department of Botany, University of Agriculture, Faisalabad-38040, Pakistan

3 International Rice Research Institute (IRRI), DAPO Box 7777, Metro Manila, Philippines4 Department of Crop Physiology, University of Agriculture, Faisalabad-38040, Pakistan

(Accepted 3 April 2008)

Abstract Scarcity of water is a severe environmental constraint to plant productivity. Drought-induced loss in crop yield probably exceedslosses from all other causes, since both the severity and duration of the stress are critical. Here, we have reviewed the eects of droughtstress on the growth, phenology, water and nutrient relations, photosynthesis, assimilate partitioning, and respiration in plants. This article alsodescribes the mechanism of drought resistance in plants on a morphological, physiological and molecular basis. Various management strategieshave been proposed to cope with drought stress. Drought stress reduces leaf size, stem extension and root proliferation, disturbs plant waterrelations and reduces water-use eciency. Plants display a variety of physiological and biochemical responses at cellular and whole-organismlevels towards prevailing drought stress, thus making it a complex phenomenon. CO2 assimilation by leaves is reduced mainly by stomatalclosure, membrane damage and disturbed activity of various enzymes, especially those of CO2 xation and adenosine triphosphate synthesis.Enhanced metabolite ux through the photorespiratory pathway increases the oxidative load on the tissues as both processes generate reactiveoxygen species. Injury caused by reactive oxygen species to biological macromolecules under drought stress is among the major deterrents togrowth. Plants display a range of mechanisms to withstand drought stress. The major mechanisms include curtailed water loss by increaseddiusive resistance, enhanced water uptake with prolic and deep root systems and its ecient use, and smaller and succulent leaves to reducethe transpirational loss. Among the nutrients, potassium ions help in osmotic adjustment; silicon increases root endodermal silicication andimproves the cell water balance. Low-molecular-weight osmolytes, including glycinebetaine, proline and other amino acids, organic acids, andpolyols, are crucial to sustain cellular functions under drought. Plant growth substances such as salicylic acid, auxins, gibberrellins, cytokininand abscisic acid modulate the plant responses towards drought. Polyamines, citrulline and several enzymes act as antioxidants and reduce theadverse eects of water decit. At molecular levels several drought-responsive genes and transcription factors have been identied, such as thedehydration-responsive element-binding gene, aquaporin, late embryogenesis abundant proteins and dehydrins. Plant drought tolerance can bemanaged by adopting strategies such as mass screening and breeding, marker-assisted selection and exogenous application of hormones andosmoprotectants to seed or growing plants, as well as engineering for drought resistance.

drought response / stomatal oscillation / osmoprotectants / hormones / stress proteins / drought management / CO2

1. INTRODUCTION

Faced with scarcity of water resources, drought is the singlemost critical threat to world food security. It was the catalystof the great famines of the past. Because the worlds watersupply is limiting, future food demand for rapidly increasingpopulation pressures is likely to further aggravate the eectsof drought (Somerville and Briscoe, 2001). The severity ofdrought is unpredictable as it depends on many factors such asoccurrence and distribution of rainfall, evaporative demandsand moisture storing capacity of soils (Wery et al., 1994).

Investigations carried out in the past provide consider-able insights into the mechanism of drought tolerance in

*Corresponding author: farooqcp@gmail.com, m.farooq@cgiar.org

plants at molecular level (Hasegawa et al., 2000). Three mainmechanisms reduce crop yield by soil water decit: (i) re-duced canopy absorption of photosynthetically active radia-tion, (ii) decreased radiation-use eciency and (iii) reducedharvest index (Earl and Davis, 2003). The reproducibility ofdrought stress treatments is very cumbersome, which signif-icantly impedes research on plant drought tolerance. A slowpace in revealing drought tolerance mechanisms has hamperedboth traditional breeding eorts and use of modern genet-ics approaches in the improvement of drought tolerance ofcrop plants (Xiong et al., 2006). Although plant responsesto drought are relatively well known, plant performance un-der a more complex environment where multiple stresses co-occur is fragmentary. That is why the plants have to respond

186 M. Farooq et al.

simultaneously to multiple stresses, e.g. drought, excessivelight and heat, which may coincide in the eld. These kindsof investigations are usually not predictable from single factorstudies (Zhou et al., 2007).

It is imperative to improve the drought tolerance of cropsunder the changing circumstances. Currently, there are no eco-nomically viable technological means to facilitate crop pro-duction under drought. However, development of crop plantstolerant to drought stress might be a promising approach,which helps in meeting the food demands. Development ofcrops for enhanced drought resistance, among other things,requires the knowledge of physiological mechanisms and ge-netic control of the contributing traits at dierent plant de-velopmental stages. Valuable work has been done on droughttolerance in plants. Ingram and Bartels (1996) more than adecade ago elegantly reviewed those appreciable eorts. Morerecent reviews deal with specic aspects of plant drought tol-erance (Penna, 2003; Reddy et al., 2004; Agarwal et al., 2006).This review encompasses an overview of the current work re-ported on some eects and mechanisms of drought tolerancein higher plants and important management strategies to over-come the drought eects, mainly on eld crops.

2. EFFECTS OF DROUGHT ON PLANTS

The eects of drought range from morphological to molec-ular levels and are evident at all phenological stages of plantgrowth at whatever stage the water decit takes place. An ac-count of various drought stress eects and their extent is elab-orated below.

2.1. Crop growth and yield

The rst and foremost eect of drought is impaired ger-mination and poor stand establishment (Harris et al., 2002).Drought stress has been reported to severely reduce germina-tion and seedling stand (Kaya et al., 2006). In a study on pea,drought stress impaired the germination and early seedlinggrowth of ve cultivars tested (Okcu et al., 2005). Moreover,in alfalfa (Medicago sativa), germination potential, hypocotyllength, and shoot and root fresh and dry weights were reducedby polyethylene glycol-induced water decit, while the rootlength was increased (Zeid and Shedeed, 2006). However, inrice, drought stress during the vegetative stage greatly reducedthe plant growth and development (Fig. 1; Tripathy et al.,2000; Manikavelu et al., 2006).

Growth is accomplished through cell division, cell enlarge-ment and dierentiation, and involves genetic, physiological,ecological and morphological events and their complex inter-actions. The quality and quantity of plant growth depend onthese events, which are aected by water decit (Fig. 2). Cellgrowth is one of the most drought-sensitive physiological pro-cesses due to the reduction in turgor pressure (Taiz and Zeiger,2006). Under severe water deciency, cell elongation of higherplants can be inhibited by interruption of water ow from thexylem to the surrounding elongating cells (Nonami, 1998).

Well-watered Drought-stress

Figure 1. Eect of drought stress on the vegetative growth of rice cv.IR64. Both the plants were grown under well-watered conditions upto 20 days following emergence. One pot was submitted to progres-sive soil drying (drought stress). The afternoon before the drought, allpots were fully watered (to saturation). After draining overnight, thepots were enclosed around the stem to prevent direct soil evaporation.A small tube was inserted for re-watering pots. The decrease in soilmoisture was controlled by partial re-watering of the stressed pots toavoid a quicker imposition of stress and to homogenize the develop-ment of drought stress. A well-watered control pot was maintainedat the initial target weight by adding the daily water loss back to thepot. This gure shows the plants 20 days after imposition of droughtstress.

Drought stress(Reduced water availability)

Loss of turgor Impaired mitosis

Obstructedcell elongation Limited

cell division

Diminished growth

Figure 2. Description of possible mechanisms of growth reductionunder drought stress. Under drought stress conditions, cell elongationin higher plants is inhibited by reduced turgor pressure. Reduced wa-ter uptake results in a decrease in tissue water contents. As a result,turgor is lost. Likewise, drought stress also trims down the photo-assimilation and metabolites required for cell division. As a conse-quence, impaired mitosis, cell elongation and expansion result in re-duced growth.

Plant drought stress: eects, mechanisms and management 187

Table I. Economic yield reduction by drought stress in some representative eld crops.

Crop Growth stage Yield reduction ReferencesBarley Seed lling 4957% Samarah (2005)Maize Grain lling 7981% Monneveux et al. (2005)Maize Reproductive 6387% Kamara et al. (2003)Maize Reproductive 7047% Chapman and Edmeades (1999)Maize Vegetative 2560% Atteya et al. (2003)Maize Reproductive 3292% Atteya et al. (2003)Rice Reproductive (mild stress) 5392% Latte et al. (2007)Rice Reproductive (severe stress) 4894% Latte et al. (2007)Rice Grain lling (mild stress) 3055% Basnayake et al. (2006)Rice Grain lling (severe stress) 60% Basnayake et al. (2006)Rice Reproductive 2484% Venuprasad et al. (2007)Chickpea Reproductive 4569% Nayyar et al. (2006)Pigeonpea Reproductive 4055% Nam et al. (2001)Common beans Reproductive 5887% Martnez et al. (2007)Soybean Reproductive 4671% Samarah et al. (2006)Cowpea Reproductive 6011% Ogbonnaya et al. (2003)Sunower Reproductive 60% Mazahery-Laghab et al. (2003)Canola Reproductive 30% Sinaki et al. (2007)Potato Flowering 13% Kawakami et al. (2006)

Impaired mitosis, cell elongation and expansion result in re-duced plant height, leaf area and crop growth under drought(Nonami, 1998; Kaya et al., 2006; Hussain et al., 2008).

Many yield-determining physiological processes in plantsrespond to water stress. Yield integrates many of these phys-iological processes in a complex way. Thus, it is dicultto interpret how plants accumulate, combine and display theever-changing and indenite physiological processes over theentire life cycle of crops. For water stress, severity, durationand timing of stress, as well as responses of plants after stressremoval, and interaction between stress and other factors areextremely important (Plaut, 2003). For instance, water stressapplied at pre-anthesis reduced time to anthesis, while at post-anthesis it shortened the grain-lling period in triticale geno-types (Estrada-Campuzano et al., 2008). In barley (Hordeumvulgare), drought stress reduced grain yield by reducing thenumber of tillers, spikes and grains per plant and individualgrain weight. Post-anthesis drought stress was detrimental tograin yield regardless of the stress severity (Samarah, 2005).

Drought-induced yield reduction has been reported in manycrop species, which depends upon the severity and duration ofthe stress period (Tab. I). In maize, water stress reduced yieldby delaying silking, thus increasing the anthesis-to-silking in-terval. This trait was highly correlated with grain yield, specif-ically ear and kernel number per plant (Cattivelli et al., 2008).Following heading, drought had little eect on the rate ofkernel lling in wheat, but its duration (time from fertiliza-tion to maturity) was shortened and dry weight reduced atmaturity (Wardlaw and Willenbrink, 2000). Drought stress insoybean reduced total seed yield and the branch seed yield(Frederick et al., 2001). In pearl millet (Pennisetum glaucum),co-mapping of the harvest index and panicle harvest indexwith grain yield revealed that greater drought tolerance wasachieved by greater partitioning of dry matter from stover tograins (Yadav et al., 2004).

Drought at owering commonly results in barrenness. Amajor cause of this, though not the only one, was a reductionin assimilate ux to the developing ear below some thresholdlevel necessary to sustain optimal grain growth (Yadav et al.,2004). Moisture decit reduced cotton (Gossypium hirsutum)lint yield, although the timing, duration, severity and speedof development undoubtedly had pivotal roles in determininghow the plant responded to moisture decit. Lint yield wasgenerally reduced due to reduced boll production because offewer owers and greater boll abortions when the stress inten-sity was greater during reproductive growth (Pettigrew, 2004).

Grain lling in cereals is a process of starch biosynthesisfrom simple carbohydrates. It is believed that four enzymesplay key roles in this process: sucrose synthase, adenosinediphosphate-glucose-pyrophosphorylase, starch synthase andstarch branching enzyme (Taiz and Zeiger, 2006). Decline inthe rate of grain growth resulted from reduced sucrose syn-thase activity, while cessation of growth resulted from inac-tivation of adenosine diphosphate-glucose-pyrophosphorylasein the water-stressed wheat (Ahmadi and Baker, 2001). Wa-ter decit during pollination increased the frequency of kernelabortion in maize (Zea mays). Under water stress, diminishedgrain set and kernel growth in wheat and a decreased rate ofendosperm cell division was associated with elevated levels ofabscisic acid in maize (Morgan, 1990; Ober et al., 1991). Inpigeonpea, drought stress coinciding with the owering stagereduced seed yield by 4055% (Nam et al., 2001). In rice, onthe other hand, water stress imposed during the grain-llingperiod enhanced remobilization of pre-stored carbon reservesto grains and accelerated grain lling (Yang et al., 2001). Insummary, prevailing drought reduces plant growth and devel-opment, leading to hampered ower production and grain ll-ing, and thus smaller and fewer grains. A reduction in grainlling occurs due to a reduction in the assimilate partitioningand activities of sucrose and starch synthesis enzymes.

188 M. Farooq et al.

2.2. Water relations

Relative water content, leaf water potential, stomatal resis-tance, rate of transpiration, leaf temperature and canopy tem-perature are important characteristics that inuence plant wa-ter relations. Relative water content of wheat leaves was higherinitially during leaf development and decreased as the dry mat-ter accumulated and leaf matured (Siddique et al., 2001). Ob-viously, water-stressed wheat and rice plants had lower relativewater content than non-stressed ones. Exposure of these plantsto drought stress substantially decreased the leaf water poten-tial, relative water content and transpiration rate, with a con-comitant increase in leaf temperature (Siddique et al., 2001).A conservative inuence of decreased stomatal conductancein non-irrigated plants was negated by a leaf-to-air vapor pres-sure dierence caused by the associated higher leaf temper-ature. Transpiration rates were similar in both treatments andthe lower total water use of the non-irrigated stand resulted en-tirely from a smaller leaf area index (Craufurad et al., 2000).

Nerd and Nobel (1991) reported that during drought stress,total water contents of Opuntia cus-indica cladode were de-creased by 57%. The water-storage parenchyma of the clado-des lost a greater fraction of water than the chlorenchyma, andthus showed a lower turgor potential. In another study on Hi-biscus rosa-sinensis, relative water content, turgor potential,transpiration, stomatal conductance and water-use eciencywere decreased under drought stress (Egilla et al., 2005).

The ratio between dry matter produced and water con-sumed is termed as water-use eciency at the whole-plantlevel (Monclus et al., 2005). Abbate et al. (2004) concludedthat under limited supply, water-use eciency of wheat wasgreater than in well-watered conditions. They correlated thishigher water-use eciency with stomatal closure to reduce thetranspiration. In another study on clover (Trifolium alexan-drinum), water-use eciency was increased due to loweredwater loss under drought stress, primarily by decreased tran-spiration rate and leaf area, and relatively lesser reduction inyield (Lazaridou and Koutroubas, 2004). Also, in Pinus pon-derosa and Artemisia tridentata, drought stress did not reducethe water-use eciency; rather, it was increased, mainly dueto a rapid decrease in stomatal conductance with increasingwater decit (DeLucia et al., 1989). Lazaridou et al. (2003)further reported that leucern (Medicago sativa) grown underdrought had greater water-use eciency than that under irri-gated conditions, for the same leaf water potential. However,in potato, early season drought stress signicantly minimizedthe water-use eciency, leading to greatly decreased growthand biomass accumulation (Costa et al., 1997).

In fact, although components of plant water relations are af-fected by reduced availability of water, stomatal opening andclosing is more strongly aected. Moreover, change in leaftemperature may be an important factor in controlling leaf wa-ter status under drought stress. Drought-tolerant species main-tain water-use eciency by reducing the water loss. However,in the events where plant growth was hindered to a greater ex-tent, water-use eciency was also reduced signicantly.

2.3. Nutrient relations

Decreasing water availability under drought generally re-sults in limited total nutrient uptake and their diminished tis-sue concentrations in crop plants. An important eect of waterdecit is on the acquisition of nutrients by the root and theirtransport to shoots. Lowered absorption of the inorganic nu-trients can result from interference in nutrient uptake and theunloadingmechanism, and reduced transpirational ow (Garg,2003; McWilliams, 2003). However, plant species and geno-types of a species may vary in their response to mineral up-take under water stress. In general, moisture stress induces anincrease in N, a denitive decline in P and no denitive eectson K (Garg, 2003).

Transpiration is inhibited by drought, as shown for beech(Peuke et al., 2002), but this may not necessarily aect nutri-ent uptake in a similar manner. Inuence of drought on plantnutrition may also be related to limited availability of energyfor assimilation of NO3 /NH

+4 , PO

34 and SO

24 : they must be

converted in energy-dependent processes before these ions canbe used for growth and development of plants (Grossman andTakahashi, 2001).

As nutrient and water requirements are closely related, fer-tilizer application is likely to increase the eciency of cropsin utilizing available water. This indicates a signicant inter-action between soil moisture decits and nutrient acquisition.Studies show a positive response of crops to improved soilfertility under arid and semi-arid conditions. Currently, it isevident that crop yields can be substantially improved by en-hancing the plant nutrient eciency under limited moisturesupply (Garg, 2003). It was shown that N and K uptake washampered under drought stress in cotton (McWilliams, 2003).Likewise, P and PO34 contents in the plant tissues diminishedunder drought, possibly because of lowered PO34 mobility asa result of low moisture availability (Peuke and Rennenberg,2004). In drought-treated sunower, the degree of stomatalopening of K+-applied plants initially indicated quicker de-cline. However, at equally low soil water potential, diusiveresistance in the leaves of K+-applied plants remained lowerthan those receiving no K+ (Lindhauer et al., 2007). In sum-mary, drought stress reduces the availability, uptake, translo-cation and metabolism of nutrients. A reduced transpirationrate due to water decit reduces the nutrient absorption andeciency of their utilization.

2.4. Photosynthesis

A major eect of drought is reduction in photosynthesis,which arises by a decrease in leaf expansion, impaired pho-tosynthetic machinery, premature leaf senescence and associ-ated reduction in food production (Wahid and Rasul, 2005).When stomatal and non-stomatal limitations to photosynthesisare compared, the former can be quite small. This implies thatother processes besides CO2 uptake are being damaged. Therole of drought-induced stomatal closure, which limits CO2uptake by leaves, is very important. In such events, restricted

Plant drought stress: eects, mechanisms and management 189

Stomatal closure

Diminished CO2 influx

Drought stress(Reduced water availability)

ABA-signalling

Limited carboxylation

Lower tissue water potential

Rubisco binding inhibitors

Diminished activities of PEPcase,NADP-ME, FBPase, PPDK

Lower Rubiscoactivity

Down-regulation of

non-cyclic e-transport Obstructed ATPsynthesis

Declinedphotosynthesis

ROS production

Attack onmembranes

Figure 3. Photosynthesis under drought stress. Possible mechanismsin which photosynthesis is reduced under stress. Drought stress dis-turbs the balance between the production of reactive oxygen speciesand the antioxidant defense, causing accumulation of reactive oxy-gen species, which induces oxidative stress. Upon reduction in theamount of available water, plants close their stomata (plausibly viaABA signaling), which decreases the CO2 inux. Reduction in CO2not only reduces the carboxylation directly but also directs moreelectrons to form reactive oxygen species. Severe drought conditionslimit photosynthesis due to a decrease in the activities of ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco), phosphoenolpyru-vate carboxylase (PEPCase), NADP-malic enzyme (NADP-ME),fructose-1, 6-bisphosphatase (FBPase) and pyruvate orthophosphatedikinase (PPDK). Reduced tissue water contents also increase the ac-tivity of Rubisco binding inhibitors. Moreover, non-cyclic electrontransport is down-regulated to match the reduced requirements ofNADPH production and thus reduces the ATP synthesis. ROS: re-active oxygen species.

CO2 availability could possibly lead to increased susceptibilityto photo-damage (Cornic and Massacci, 1996).

Drought stress produced changes in photosynthetic pig-ments and components (Anjum et al., 2003), damaged pho-tosynthetic apparatus (Fu J. and Huang, 2001) and diminishedactivities of Calvin cycle enzymes, which are important causesof reduced crop yield (Monakhova and Chernyadv, 2002).Another important eect that inhibits the growth and photo-synthetic abilities of plants is the loss of balance between theproduction of reactive oxygen species and the antioxidant de-fense (Fu J. and Huang, 2001; Reddy et al., 2004), causingaccumulation of reactive oxygen species which induces ox-idative stress in proteins, membrane lipids and other cellularcomponents (Fig. 3). Some important components of photo-synthesis aected by drought are discussed below.

2.4.1. Stomatal oscillations

The rst response of virtually all plants to acute waterdecit is the closure of their stomata to prevent the tran-spirational water loss (Manseld and Atkinson, 1990). This

may result in response to either a decrease in leaf turgorand/or water potential (Ludlow and Muchow, 1990) or to alow-humidity atmosphere (Maroco et al., 1997). The debateas to whether drought mainly limits photosynthesis throughstomatal closure or metabolic impairment has continued fora long time (Sharkey, 1990; Tezara et al., 1999). During thelast decade, stomatal closure was generally accepted to be themain determinant for decreased photosynthesis under mild tomoderate drought (Cornic and Massacci, 1996; Yokota et al.,2002).

When the amount of available soil water is moderately orseverely limiting, the rst option for plants is to close stomata(Cornic and Massacci, 1996). This decreases the inow of CO2into the leaves and spares more electrons for the formation ofactive oxygen species (Fig. 3). As the rate of transpiration de-creases, the amount of heat that can be dissipated increases(Yokota et al., 2002). Various experiments have shown thatstomatal responses are often more closely linked to soil mois-ture content than to leaf water status. This suggested that stom-ata respond to chemical signals, e.g. abcissic acid, produced bydehydrating roots (Fig. 3), whilst leaf water status is kept con-stant (Morgan, 1990; Taylor, 1991; Turner et al., 2001). En-vironmental conditions that enhance the rate of transpirationalso increase the pH of leaf sap, which can promote abscisicacid accumulation and concomitantly diminish stomatal con-ductance. Increased cytokinin concentration in the xylem sappromotes stomatal opening directly and aects the sensitiv-ity of stomata towards abscisic acid (Wilkinson and Davies,2002).

Comparing results from dierent studies is complex due tointerspecic dierences in the response of stomatal conduc-tance and photosynthesis to leaf water potential and/or relativewater content; the parameters most often used to assess the de-gree of drought (Cornic and Massacci, 1996). It is clear thatstomata close progressively as drought progresses, followedby a parallel decline in net photosynthesis. However, stomatalconductance is not controlled by soil water availability alone,but by a complex interaction of intrinsic and extrinsic factors.

2.4.2. Photosynthetic enzymes

Very severe drought conditions limit photosynthesis due toa decline in Rubisco activity (Bota et al., 2004). The activityof the photosynthetic electron transport chain is nely tuned tothe availability of CO2 in the chloroplast and change in photo-system II under drought conditions (Loreto et al., 1995). De-hydration results in cell shrinkage, and consequently a declinein cellular volume. This makes cellular contents more viscous.Therefore, an increase in the probability of protein-protein in-teraction leads to their aggregation and denaturation (Hoekstraet al., 2001). Increased concentration of solutes, leading to in-creased viscosity of the cytoplasm,may become toxic and maybe deleterious to the functioning of enzymes, including thoseof the photosynthetic machinery (Hoekstra et al., 2001).

The level of Rubisco in leaves is controlled by the rateof synthesis and degradation. Even under drought stress theRubisco holoenzyme is relatively stable with a half-life of

190 M. Farooq et al.

several days (Hoekstra et al., 2001). However, drought stressshowed a rapid diminution in the abundance of Rubisco smallsubunit transcripts, which indicated its decreased synthesis(Vu et al., 1999). Rubisco activity is modulated in vivo eitherby reactionwith CO2 and Mg2+ to carbamylate a lysine residuein the catalytic site, or by binding inhibitors within the cat-alytic site (Fig. 3). Such a binding either blocks activity or thecarbamylation of the lysine residue, which is essential for ac-tivity. At night, 2-carboxyarabinitol-1-phosphate is formed inmany species, which binds tightly to Rubisco, inhibiting cat-alytic activity. It is reported that tight-binding inhibitors candecrease Rubisco activity in the light. In tobacco (Nicotianatabacum), decrease in Rubisco activity under drought stresswas not a primary result of changes in activation by CO2 andMg2+, and was rather due to the presence of tight-binding in-hibitors (Parry et al., 2002). A rapid decline in photosynthesisunder drought was accompanied by decreased maximum ve-locity of ribulose-1, 5-bisphosphate carboxylation by Rubisco,speed of ribulose-1, 5-bisphosphate regeneration, Rubisco andstromal fructose bis-phosphatase activities, and the quantumeciency of photosystem II in higher plants (Reddy et al.,2004; Zhou et al., 2007). Moreover, under severe drought, car-boxylation eciency by Rubisco was greatly declined, and itacted more as oxygenase than carboxylase (Fig. 3).

During water stress, activities of the phosphoenolpyruvatecarboxylase, nicotinamide adenine dinucleotide phosphate-malic enzyme, Rubisco, fructose-1, 6-bisphosphatase andpyruvate orthophosphate dikinase decreased linearly with low-ered leaf water potential (Fig. 3). Pyruvate orthophosphatedikinase activities were decreased 9.1 times during waterstress; a much greater reduction than other enzymes, whichwere from 2 to 4 times, suggesting that pyruvate orthophos-phate dikinase is very likely to be the limiting enzyme to pho-tosynthesis under water stress (Du et al., 1996).

2.4.3. Adenosine triphosphate synthesis

There is a long-standing controversy as to whether droughtmainly limits photosynthesis through stomatal closure (Cornicand Massacci, 1996) or by metabolic impairment (Tezaraet al., 1999). Evidence that impaired adenosine triphosphatesynthesis is the main factor limiting photosynthesis even un-der mild drought has further stimulated the debate (Lawlor andCornic, 2002). It is reported that impaired photophosphoryla-tion and adenosine triphosphate synthesis are the main factorslimiting photosynthesis even under mild drought (Tezara et al.,1999).

Under drought stress, production of limited nicotinamideadenine dinucleotide phosphate maintains the continuation ofelectron transport, although the status of the reductant may behigh even when the uxes are small, leading to a more in-creased demand than supply. Under drought stress, non-cyclicelectron transport is down-regulated to match the require-ments of decreased nicotinamide adenine dinucleotide phos-phate production and cyclic electron transport is activated.This generates a proton gradient that induces the protectiveprocess of high-energy-state quenching (Golding and Johnson,

2003). Support for this model came from the isolation of a mu-tant decient in high-energy-state quenching that lacked cyclicelectron transport (Munekage et al., 2002). Support for cyclicelectron transport under drought also came from non-steady-state measurements (Cornic et al., 2000).

Dissipation mechanisms of excess photon energy underwater stress were studied in ndhB-inactivated tobacco (cv.Xanthi) mutants, impaired in reduced nicotinamide adeninedinucleotide phosphate dehydrogenase-dependent cyclic elec-tron ow around photosystem I. The relative water contentand net CO2 assimilation was reduced to 30% and almostzero after an 11-day water stress regime in the mutant andwild-type plants, respectively. A decline in photosystem II ac-tivity (75%), and an increase in malondialdehyde (45%),an estimate of lipid peroxidation, were found in both theplant groups when subjected to water stress. Thus, a de-ciency in reduced nicotinamide adenine dinucleotide phos-phate dehydrogenase-dependent cyclic electron ow aroundphotosystem I did not lead to oxidative damage because themutant compensated for this deciency by activating alterna-tive dissipating routes of excess photon energy such as up-regulation of ferredoxin-dependentcyclic electron ow aroundphotosystem I and enhanced accumulation of -tocopherol(-toc) quinine (Munn-Bosch et al., 2005).

In fact, the activities of the enzymes of carbon assimilationand those involved in adenosine triphosphate synthesis are re-tarded and sometimes inhibited depending upon the extent ofavailable moisture. Of these, Rubisco, which shows dual func-tions, acts as oxygenase under water-limiting conditions; andtherefore limited CO2 xation is noticed.

2.5. Assimilate partitioning

Assimilate translocation to reproductive sinks is vital forseed development. Seed set and lling can be limited byavailability or utilization, i.e., assimilate source or sink lim-itation, respectively (Asch et al., 2005). Drought stress fre-quently enhances allocation of dry matter to the roots, whichcan enhance water uptake (Leport et al., 2006). De Souza andDa Silv (1987), while analyzing the partitioning and distribu-tion of photo-assimilates in annual and perennial cotton underdrought stress, reported that the root-to-shoot dry matter ratiowas high in perennial cotton, thereby showing a preferentialaccumulation of starch and dry matter in roots as an adaptationto drought. Thus, perennial cotton apparently owed its droughtresistance to the partitioning of assimilates that favored starchaccumulation and growth of the root system. The export rateof sucrose from source to sink organs depends upon the cur-rent photosynthetic rate and the concentration of sucrose in theleaves (Komor, 2000). Drought stress decreases the photosyn-thetic rate, and disrupts the carbohydratemetabolism and levelof sucrose in leaves that spills over to a decreased export rate.This is presumably due to drought stress-induced increased ac-tivity of acid invertase (Kim et al., 2000). Limited photosyn-thesis and sucrose accumulation in the leaves may hamper therate of sucrose export to the sink organs and ultimately aectthe reproductive development.

Plant drought stress: eects, mechanisms and management 191

Apart from source limitation, the capacity of the repro-ductive sinks to utilize the incoming assimilates is also af-fected under drought stress and may also play a role inregulating reproductive abortion (Zinselmeier et al., 1999).Drought-induced carbohydrate deprivation, enhanced endoge-nous abscisic acid concentration, and an impaired ability toutilize the incoming sucrose by the reproductive sinks arepotential factors contributing to seed abortion in grain crops(Setter et al., 2001). A reduced acid invertase activity can ar-rest the development of reproductive tissues due to improperphloem unloading (Goetz et al., 2001). In addition, droughtstress may inhibit important functions of vacuolar invertase-mediated sucrose hydrolysis and osmotic potential modula-tion. In drought-stressed maize, a low invertase activity in theyoung ovaries lowers the ratio of hexoses to sucrose. This mayinhibit cell division in the developing embryo/endosperm, re-sulting in weak sink intensity, and may ultimately lead to fruitabortion (Andersen et al., 2002).

In summary, drought stress not only limits the size of thesource and sink tissues but the phloem loading, assimilatetranslocation and dry matter portioning are also impaired.However, the extent of eects varies with the plant species,stage, duration and severity of drought.

2.6. Respiration

Drought tolerance is a cost-intensive phenomenon, as a con-siderable quantity of energy is spent to cope with it. The frac-tion of carbohydrate that is lost through respiration determinesthe overall metabolic eciency of the plant (Davidson et al.,2000). The root is a major consumer of carbon xed in pho-tosynthesis and uses it for growth and maintenance, as wellas dry matter production (Lambers et al., 1996). Plant growthand developmental processes as well as environmental condi-tions aect the size of this fraction (i.e. utilized in respiration).However, the rate of photosynthesis often limits plant growthwhen soil water availability is reduced (Huang and Fu, 2000).A negative carbon balance can occur as a result of diminishedphotosynthetic capacity during drought, unless simultaneousand proportionate reductions in growth and carbon consump-tion take place.

In wheat, depending on the growth stage, cultivar and nu-tritional status, more than 50% of the daily accumulated pho-tosynthates were transported to the root, and around 60% ofthis fraction was respired (Lambers et al., 1996). Drought-sensitive spring wheat (Longchun, 81392) used a relativelygreater amount of glucose to absorb water, especially in se-vere drought stress (Liu et al., 2004). Severe drought reducedthe shoot and root biomass, photosynthesis and root respira-tion rate. Limited root respiration and root biomass under se-vere soil drying can improve growth and physiological activ-ity of drought-tolerant wheat, which is advantageous over adrought-sensitive cultivar in arid regions (Liu and Li, 2005).

There are two mitochondrial electron transport pathwaysfrom ubiquinone to oxygen in plants. The alternative pathwaybranches from the cytochrome pathway and donates electronsto oxygen directly by alternative oxidase (Moore and Siedow,

O2-e- e-

O22-e-

O23- O-

H2OWater

2H+ H+

OHHydroxylradical

e-3O2 O2-

H2OWater

2H+

Oxide ion

Oxeneion

Peroxideion

Superoxideradical ionDioxygen

2H+H+

H2O2Hydrogen peroxide

H2OPerhydroxylradical

1O2Singlet oxygen

Figure 4. Generation of reactive oxygen species by energy transferor sequential univalent reduction of ground state triplet oxygen (Apeland Hirt, 2004; reproduced with permission).

1991). The alternative pathway is not coupled with adenosinetriphosphate synthesis, but can be induced in response to stressor inhibition of the main electron transfer pathway (Wagnerand Moore, 1997). When plants are exposed to drought stress,they produce reactive oxygen species, which damage mem-brane components (Blokhina et al., 2003). In this regard, al-ternative oxidase activity could be useful in maintaining nor-mal levels of metabolites and reduce reactive oxygen speciesproduction during stress. Oxygen uptake by sugar beet wascharacterized by a high rate, distinct cytochrome oxidase-dependent terminal oxidation and up to 80% inhibition of res-piration in the presence of 0.5 mM potassium cyanide. At anearly drought stage (10 days), a decrease in the activity of thecytochrome-mediated oxidation pathway was largely counter-balanced by the activation of mitochondrial alternative oxi-dase, whereas long-term dehydration of plants was accompa-nied by activation of additional oxidative systems insensitiveto both potassium cyanide and salicylhydroxamate (Shugaevaet al., 2007). In summary, water decit in the rhizosphere leadsto an increased rate of root respiration, leading to an imbal-ance in the utilization of carbon resources, reduced productionof adenosine triphosphate and enhanced generation of reactiveoxygen species.

2.7. Oxidative damage

Exposure of plants to certain environmental stresses quiteoften leads to the generation of reactive oxygen species, in-cluding superoxide anion radicals (O2 ), hydroxyl radicals(OH), hydrogen peroxide (H2O2), alkoxy radicals (RO) andsinglet oxygen (O12) (Munn-Bosch and Penuelas, 2003). Re-active oxygen species may react with proteins, lipids and de-oxyribonucleic acid, causing oxidative damage and impairingthe normal functions of cells (Foyer and Fletcher, 2001). Manycell compartments produce reactive oxygen species; of these,chloroplasts are a potentially important source because excitedpigments in thylakoid membranes may interact with O2 toform strong oxidants such as O2 or O12 (Niyogi, 1999; Reddyet al., 2004). Further downstream reactions produce other reac-tive oxygen species such as H2O2 and OH (Fig. 4). The inter-action of O2 with reduced components of the electron transportchain in mitochondria can lead to reactive oxygen species for-mation (Mller, 2001), and peroxisomes produce H2O2 when

192 M. Farooq et al.

glycolate is oxidized into glyoxylic acid during photorespira-tion (Fazeli et al., 2007).

Mechanisms for the generation of reactive oxygen speciesin biological systems are represented by both non-enzymaticand enzymatic reactions. The partition between these twopathways under oxygen deprivation stress can be regulatedby the oxygen concentration in the system. In non-enzymaticreactions, electron O2 reduction can occur at higher oxygenconcentrations (Apel and Hirt, 2004). At very low O2 concen-trations, plant terminal oxidases and the formation of reactiveoxygen species via the mitochondrial electron transport chainstill remain functional. Among enzymatic sources of reactiveoxygen species, xanthine oxidase, an enzyme responsible forthe initial activation of O2, should be mentioned. The elec-tron donor xanthine oxidase can use xanthine, hypoxanthineor acetaldehyde, while the latter has been shown to accumu-late under oxygen deprivation (Pster-Sieber and Braendle,1994; Apel and Hirt, 2004). This can represent a possiblesource for hypoxia-stimulated reactive oxygen species produc-tion (Fig. 4). The next enzymatic step is the dismutation ofthe superoxide anion by superoxide dismutase to yield H2O2(Lamb and Dixon, 1997). Peroxidases and catalases also playan important role in the ne regulation of reactive oxygenspecies in the cell through activation and deactivation of H2O2(Sairam et al., 2005). Several apoplastic enzymes may alsogenerate reactive oxygen species under normal and stressfulconditions. Other oxidases, responsible for the two-electrontransfer to dioxygen (amino acid oxidases and glucose oxi-dase) can contribute to H2O2 accumulation (Apel and Hirt,2004).

Reactive oxygen species are formed as by-products in theelectron transport chains of chloroplasts (Apel and Hirt, 2004),mitochondria and the plasma membrane (Sairam et al., 2005).The plant mitochondrial electron transport chain, with itsredox-active electron carriers, is considered as the most prob-able candidate for intracellular reactive oxygen species for-mation. Mitochondria can produce reactive oxygen speciesdue to the electron leakage at the ubiquinone site theubiquinone:cytochrome b region (Gille and Nohl, 2001) and at the matrix side of complex I (NADH dehydrogenase)(Mller, 2001).

Superoxide radical and its reduction product H2O2 arepotentially toxic compounds, and can also combine by theHaber-Weiss reaction to form the highly toxic OH (Sairamet al., 1998). Many reports show the deleterious eects of re-active oxygen species, whose production is stimulated underwater stress (Blokhina et al., 2003). Reactive oxygen speciescause lipid peroxidation, and consequentlymembrane injuries,protein degradation and enzyme inactivation (Sairam et al.,2005). Oxidative stress may also cause protein oxidation,with a loss of enzyme activity and the formation of protease-resistant cross-linked aggregates (Berlett and Stadtman, 1997).Oxidatively-damaged proteins accumulate in pea leaves sub-jected to moderate water stress (Moran et al., 1994).

Overall, the production of reactive oxygen species is lin-ear with the severity of drought stress, which leads to en-hanced peroxidation of membrane lipids and degradation ofnucleic acids, and both structural and functional proteins.

Various organelles including chloroplasts, mitochondria andperoxisomes are the seats as well as rst target of reactive oxy-gen species produced under drought stress.

3. DROUGHT RESISTANCE MECHANISMS

Plants respond and adapt to and survive under droughtstress by the induction of various morphological, biochemi-cal and physiological responses. Drought tolerance is denedas the ability to grow, ower and display economic yield un-der suboptimal water supply. Drought stress aects the waterrelations of plants at cellular, tissue and organ levels, causingspecic as well as unspecic reactions, damage and adaptationreactions (Beck et al., 2007). To cope with the drought, tol-erant plants initiate defense mechanisms against water decit(Chaves and Oliveira, 2004), which need to be investigated infurther detail (Zhou et al., 2007). In the following sections,mechanisms of drought tolerance at dierent levels are pre-sented.

3.1. Morphological mechanisms

Plant drought tolerance involves changes at whole-plant,tissue, physiological and molecular levels. Manifestation of asingle or a combination of inherent changes determines theability of the plant to sustain itself under limited moisture sup-ply. An account of various morphological mechanisms opera-tive under drought conditions is given below.

3.1.1. Escape

Escape from drought is attained through a shortened life cy-cle or growing season, allowing plants to reproduce before theenvironment becomes dry. Flowering time is an important traitrelated to drought adaptation, where a short life cycle can leadto drought escape (Araus et al., 2002). Crop duration is inter-actively determined by genotype and the environment and de-termines the ability of the crop to escape from climatic stressesincluding drought (Dingkuhn and Asch, Dingkuhn). Matchinggrowth duration of plants to soil moisture availability is criti-cal to realize high seed yield (Siddique et al., 2003). Droughtescape occurs when phenological development is successfullymatched with periods of soil moisture availability, where thegrowing season is shorter and terminal drought stress predom-inates (Araus et al., 2002). In eld-grown clones of robustacoee, leaf shedding in response to drought stress occurredsequentially from older to younger leaves, suggesting that themore drought-sensitive the clone, the greater the extent of leafshedding (DaMatta, 2004).

Time of owering is a major trait of a crop adaptation tothe environment, particularly when the growing season is re-stricted by terminal drought and high temperatures. Develop-ing short-duration varieties has been an eective strategy forminimizing yield loss from terminal drought, as early maturityhelps the crop to avoid the period of stress (Kumar and Abbo,

Plant drought stress: eects, mechanisms and management 193

2001). However, yield is generally correlated with the lengthof crop duration under favorable growing conditions, and anydecline in crop duration below the optimum would tax yield(Turner et al., 2001).

3.1.2. Avoidance

Drought avoidance consists of mechanisms that reduce wa-ter loss from plants, due to stomatal control of transpiration,and also maintain water uptake through an extensive and pro-lic root system (Turner et al., 2001; Kavar et al., 2007). Theroot characters such as biomass, length, density and depth arethe main drought avoidance traits that contribute to nal yieldunder terminal drought environments (Subbarao et al., 1995;Turner et al., 2001). A deep and thick root system is helpfulfor extracting water from considerable depths (Kavar et al.,2007).

Glaucousness or waxy bloom on leaves helps with mainte-nance of high tissue water potential, and is therefore consid-ered as a desirable trait for drought tolerance (Richards et al.,1986; Ludlow and Muchow, 1990). Varying degrees of glau-cousness in wheat led to increased water-use eciency, but didnot aect total water use or harvest index. Determination ofleaf temperature indicated that, compared with non-glaucousleaves, glaucous leaves were 0.7 C cooler and had a lowerrate of leaf senescence (Richards et al., 1986). These authorssuggested that a 0.5 C reduction in leaf temperature for sixhours per day was sucient to extend the grain-lling periodby more than three days. However, yield advantages are likelyto be small as many varieties already show some degree ofglaucousness.

3.1.3. Phenotypic exibility

Plant growth is greatly aected by water decit. At a mor-phological level, the shoot and root are the most aected andboth are the key components of plant adaptation to drought.Plants generally limit the number and area of leaves in re-sponse to drought stress just to cut down the water budget atthe cost of yield loss (Schuppler et al., 1998). Since roots arethe only source to acquire water from soil, the root growth, itsdensity, proliferation and size are key responses of plants todrought stress (Kavar et al., 2007).

It has long been established that plants bearing smallleaves are typical of xeric environments. Such plants withstanddrought very well, albeit their growth rate and biomass arerelatively low (Ball et al., 1994). Leaf pubescence is a xero-morphic trait that helps protect the leaves from excessive heatload. Hairy leaves have reduced leaf temperatures and tran-spiration (Sandquist and Ehleringer, 2003) whilst inter- andintra-specic variation exists for the presence of this trait. Un-der high temperature and radiation stress, hairiness increasesthe light reectance and minimizes water loss by increasingthe boundary layer resistance to water vapor movement awayfrom the leaf surface. Although drought stress also induces the

production of trichomes on both sides of wheat leaves, theyhad no signicant inuence on boundary layer resistance.

The water content in drought-treatedmature stems declinedby 4% and water potential by 0.25 MPa. It is shown thatactive phloem supply of assimilates and associated water re-serves from mature stems was the mechanism that alloweddeveloping stems of Hylocereus undatus to maintain growthunder drought conditions (Nerd and Neumann, 2004). More-over, girdling the phloem of growing stems rapidly inhibitedstem elongation, but secretion of sucrose-containing nectarwas maintained during drought. The water potential gradientwas in the wrong direction for xylem transport from matureto young growing stems and axial hydraulic conductivity waslow to negligible (Nerd and Neumann, 2004).

Roots are the key plant organ for adaptation to drought. Iftolerance is dened as the ability to maintain leaf area andgrowth under prolonged vegetative stage stress, the main basisof variation appears to be constitutive root system architecturethat allows the maintenance of more favorable plant water sta-tus (Nguyen et al., 1997). The possession of a deep and thickroot system allowed access to water deep in the soil, whichwas considered important in determining drought resistance inupland rice (Kavar et al., 2007). Evidence suggests that it isquality, i.e. the distribution and structure, and not quantity ofroots that determines the most ecient strategy for extractingwater during the crop-growing season (Fig. 5). The droughttolerance of tea, onion and cotton was increased by improvedroot growth and root functioning. Selection for a deep and ex-tensive root system has been advocated to increase productiv-ity of food legumes under moisture-decit conditions as it canoptimize the capacity to acquire water (Subbarao et al., 1995).

Studies carried out on the eects of alleles of the wheatshoot dwarng genes on root-shoot dry matter partitioningand drought resistance revealed that cultivars possessing thereduced height gene 1 and reduced height gene 2 gibberellin-insensitive dwarng genes were more susceptible to droughtstress than reduced height gene 1 and reduced height gene2 tall cultivars (Miralles et al., 1997). The semi-dwarng genesreduced height gene 1 and reduced height gene 2 resulted ingreater root biomass at anthesis due to increased thickeningof existing roots using surplus assimilates arising from the re-stricted stem growth. Thus, the benet of greater assimilatesavailable for root growth was not expressed as more extensiveor deeper root growth. Dierences have also been observed inthe adaptive response of root distribution to soil drying (Liuet al., 2004).

To summarize, plants may escape drought stress by cuttingshort their growth duration, and avoid the stress with the main-tenance of high tissue water potential either by reducing wa-ter loss from plants or improved water uptake, or both. Someplants may reduce their surface area either by leaf shedding orproduction of smaller leaves.

3.2. Physiological mechanisms

Osmotic adjustment, osmoprotection, antioxidation and ascavenging defense system have been the most important

194 M. Farooq et al.

Nip sl 13 sl 34 sl 45 sl 50

Well-watered

Drought stress

Figure 5. Root growth and proliferation under well-watered and drought stress conditions in various rice genotypes. Dierent rice genotypes(Nip, sl 13, sl 34, sl 45, sl 50) were grown under continuous ooded conditions (well-watered) and 15% soil moisture contents (drought stress).The study was conducted in root boxes. The gure shows root proliferation 38 days after seeding. (courtesy Ms. Mana Kano).

bases responsible for drought tolerance. The physiological ba-sis of genetic variation in drought response is not clear; inpart, because more intricate mechanisms have been suggested.Some of these mechanisms are described below.

3.2.1. Cell and tissue water conservation

Under drought stress, sensitive pea genotypeswere more af-fected by a decline in relative water content than tolerant ones(Upreti et al., 2000). In faba bean, determination of leaf waterpotential was useful for describing the drought eect, but wasnot suitable for discriminating tolerant from sensitive geno-types. This suggested that water potential was not the deningfeature of the tolerance (Riccardi et al., 2001). Nevertheless,other studies opined that determination of leaf water status inthe morning and water content in leaves in the afternoon werepotentially useful for screening drought tolerance in chickpea(Pannu et al., 1993).

Osmotic adjustment allows the cell to decrease osmotic po-tential and, as a consequence, increases the gradient for waterinux and maintenance of turgor. Improved tissue water statusmay be achieved through osmotic adjustment and/or changesin cell wall elasticity. This is essential for maintaining physi-ological activity for extended periods of drought (Kramer andBoyer, 1995). Wild melon plant survived drought by main-taining its water content without wilting of leaves even un-der severe drought. Drought stress in combination with strong

light led to an accumulation of high concentrations of cit-rulline, glutamate and arginine in leaves of wild watermelon.The accumulation of citrulline and arginine may be related tothe induction of dopamine receptor interacting protein gene 1,a homologue of the acetylornithine deacetylase gene in Es-cherichia coli, where it functions to incorporate the carbonskeleton of glutamate into the urea cycle (Yokota et al., 2002).

It has been identied that among various mechanisms, os-motic adjustment, abscisic acid and induction of dehydrinsmay confer tolerance against drought injuries by maintaininghigh tissue water potential (Turner et al., 2001). With the ac-cumulation of solutes, the osmotic potential of the cell is low-ered, which attracts water into the cell and helps with turgormaintenance. The maintenance of turgor despite a decrease inleaf water volume is consistent with other studies of specieswith elastic cell walls. Osmotic adjustment helps to maintainthe cell water balance with the active accumulation of solutesin the cytoplasm, thereby minimizing the harmful eects ofdrought (Morgan, 1990). Osmotic adjustment is an importanttrait in delaying dehydrative damage in water-limited environ-ments by continuedmaintenance of cell turgor and physiologi-cal processes (Taiz and Zeiger, 2006). The osmotic adjustmentalso facilitates a better translocation of pre-anthesis carbohy-drate partitioning during grain lling (Subbarao et al., 2000),while high turgor maintenance leads to higher photosyntheticrate and growth (Ludlow and Muchow, 1990; Subbarao et al.,2000).

Plant drought stress: eects, mechanisms and management 195

Abiotic stresses(Drought, salinity, heat, chilling)

Proteins, Lipids, DNA

ROS production(1O2, H2O, O22-, H2O2)

CAT, SODAPX

POD,GR, AA

Figure 6.Role of antioxidant enzymes in the ROS scavenging mecha-nism. Exposure to abiotic stresses (including drought, chilling, salin-ity, etc.) leads to the generation of ROS, including singlet oxygen(1O2), perhydroxyl radical (H2O), hydroxyl radicals (O22 ), hydro-gen peroxide (H2O2) and alkoxy radical (RO). The ROS may reactwith proteins, lipids and DNA, causing oxidative damage and im-pairing the normal functions of cells. The antioxidant defense sys-tem in the plant cell includes both enzymatic and non-enzymaticconstituents. Amongst the enzymatic components are superoxide dis-mutase, catalase, peroxidase, ascorbate peroxidase and glutathionereductase. Upon exposure to abiotic stresses, tolerant cells activatetheir enzymatic antioxidant system, which then starts quenching theROS and protecting the cell. ROS: reactive oxygen species.

3.2.2. Antioxidant defense

The antioxidant defense system in the plant cell consti-tutes both enzymatic and non-enzymatic components. En-zymatic components include superoxide dismutase, catalase,peroxidase, ascorbate peroxidase and glutathione reductase.Non-enzymatic components contain cystein, reduced glu-tathione and ascorbic acid (Gong et al., 2005). In environmen-tal stress tolerance, such as drought, high activities of antioxi-dant enzymes and high contents of non-enzymatic constituentsare important.

The reactive oxygen species in plants are removed by a va-riety of antioxidant enzymes and/or lipid-soluble and water-soluble scavenging molecules (Hasegawa et al., 2000); theantioxidant enzymes being the most ecient mechanismsagainst oxidative stress (Farooq et al., 2008). Apart from cata-lase, various peroxidases and peroxiredoxins, four enzymesare involved in the ascorbate-glutathione cycle, a pathwaythat allows the scavenging of superoxide radicals and H2O2(Fig. 6). These include ascorbate peroxidase, dehydroascor-bate reductase, monodehydroascorbate reductase and glu-tathione reductase (Fazeli et al., 2007). Most of the ascorbate-glutathione cycle enzymes are located in the cytosol, stroma ofchloroplasts, mitochondria and peroxisomes (Jimnez et al.,1998). Ascorbate peroxidase is a key antioxidant enzyme in

plants (Orvar and Ellis, 1997) whilst glutathione reductasehas a central role in maintaining the reduced glutathione poolduring stress (Pastori et al., 2000). Two glutathione reduc-tase complementary deoxyribonucleic acids have been iso-lated; one type encoding the cytosolic isoforms (Stevens et al.,2000) and the other encoding glutathione reductase proteinsdual-targeted to both chloroplasts and mitochondria in dier-ent plants (Chew et al., 2003).

Among enzymatic mechanisms, superoxide dismutaseplays an important role, and catalyzes the dismutation of twomolecules of superoxide into O2 and H2O2; the rst step in re-active oxygen species scavenging systems. Lima et al. (2002),from a study utilizing two rapidly drought-stressed clones ofCoea canephora, proposed that drought tolerance might, orat least in part, be associated with enhanced activity of antiox-idant enzymes. In contrast, Pinheiro et al. (2004) did not nda link between protection against oxidative stress and droughttolerance when four clones of C. canephora were subjected tolong-term drought.

Carotenoids and other compounds, such as abietane diter-penes, have received little attention despite their capacity toscavenge singlet oxygen and lipid peroxy-radicals, as well asto inhibit lipid peroxidation and superoxide generation un-der dehydrative forces (Deltoro et al., 1998). The transcript ofsome of the antioxidant genes such as glutathione reductase orascorbate peroxidase was higher during recovery from a waterdecit period and appeared to play a role in the protection ofcellular machinery against damage by reactive oxygen species(Ratnayaka et al., 2003). A superoxide radical has a half-life ofless than 1 sec and is rapidly dismutated by superoxide dismu-tase into H2O2, a product that is relatively stable and can bedetoxied by catalase and peroxidase (Apel and Hirt, 2004).These metalloenzymes constitute an important primary line ofdefense of cells against superoxide free radicals generated un-der stress conditions. Therefore, increased superoxide dismu-tase activity is known to confer oxidative stress tolerance (Panet al., 2006).

Oxidative damage in the plant tissue is alleviated by aconcerted action of both enzymatic and non-enzymatic an-tioxidant systems. These include -carotenes, ascorbic acid,-tocopherol, reduced glutathione and enzymes including su-peroxide dismutase, peroxidase, ascorbate peroxidase, cata-lase, polyphenol oxidase and glutathione reductase (Hasegawaet al., 2000; Prochazkova et al., 2001). Carotenes form a keypart of the plant antioxidant defense system (Havaux, 1998;Wahid, 2007), but they are very susceptible to oxidative de-struction. The -carotene present in the chloroplasts of allgreen plants is exclusively bound to the core complexes ofphotosystem I and photosystem II. Protection against damag-ing eects of reactive oxygen species at this site is essentialfor chloroplast functioning. Here, -carotene, in addition tofunctioning as an accessory pigment, acts as an eective an-tioxidant and plays a unique role in protecting photochemi-cal processes and sustaining them (Havaux, 1998). A majorprotective role of -carotene in photosynthetic tissue may bethrough direct quenching of triplet chlorophyll, which pre-vents the generation of singlet oxygen and protects from ox-idative damage.

196 M. Farooq et al.

3.2.3. Cell membrane stability

Biological membranes are the rst target of many abioticstresses. It is generally accepted that the maintenance of in-tegrity and stability of membranes under water stress is amajor component of drought tolerance in plants (Bajji et al.,2002). Cell membrane stability, reciprocal to cell membraneinjury, is a physiological index widely used for the evalua-tion of drought tolerance (Premachandra et al., 1991). More-over, it is a genetically related phenomenon since quantita-tive trait loci for this have been mapped in drought-stressedrice at dierent growth stages (Tripathy et al., 2000). Dhandaet al. (2004) showed that membrane stability of the leaf seg-ment was the most important trait to screen the germplasm fordrought tolerance.

Cell membrane stability declined rapidly in Kentucky blue-grass exposed to drought and heat stress simultaneously(Wang and Huang, 2004). In a study on maize, K nutri-tion improved the drought tolerance, mainly due to improvedcell membrane stability (Gnanasiri et al., 1991). Tolerance todrought evaluated as increase in cell membrane stability underwater decit conditions was dierentiated between cultivarsand correlated well with a reduction in relative growth rateunder stress (Premachandra et al., 1991). In holm oak (Quer-cus ilex) seedlings, hardening increased drought tolerance pri-marily by reducing osmotic potential and stomatal regulation,improved new root growth capacity and enhanced cell mem-brane stability. Among treated seedlings, the greatest responseoccurred in seedlings subjected to moderate hardening. Vari-ation in cell membrane stability, stomatal regulation and rootgrowth capacity was negatively related to osmotic adjustment(Villar-Salvador et al., 2004).

The causes of membrane disruption are unknown; notwith-standing, a decrease in cellular volume causes crowding andincreases the viscosity of cytoplasmic components. This in-creases the chances of molecular interactions that can causeprotein denaturation and membrane fusion. For model mem-brane and protein systems, a broad range of compounds havebeen identied that can prevent such adverse molecular inter-actions. Some of these are proline, glutamate, glycinebetaine,carnitine, mannitol, sorbitol, fructans, polyols, trehalose, su-crose and oligosaccharides (Folkert et al., 2001). Another pos-sibility of ion leakage from the cell may be due to thermal-induced inhibition of membrane-bound enzymes responsiblefor maintaining chemical gradients in the cell (Reynolds et al.,2001). Arabidopsis leaf membranes appeared to be very resis-tant to water decit, as shown by their capacity to maintainpolar lipid contents and the stability of their composition un-der severe drought (Gigon et al., 2004).

3.2.4. Plant growth regulators

Plant growth regulators, when applied externally, and phy-tohormones, when produced internally, are substances that in-uence physiological processes of plants at very low concen-trations (Morgan, 1990). Both these terms have been used

interchangeably, particularly when referring to auxins, gib-berellins, cytokinins, ethylene and abscisic acid (Taiz andZeiger, 2006). Under drought, endogenous contents of auxins,gibberellins and cytokinin usually decrease, while those of ab-scisic acid and ethylene increase (Nilsen and Orcutte, 1996).Nevertheless, phytohormones play vital roles in drought toler-ance of plants.

Auxins induce new root formation by breaking root api-cal dominance induced by cytokinins. As a prolic root sys-tem is vital for drought tolerance, auxins have an indirectbut key role in this regard. Drought stress limits the pro-duction of endogenous auxins, usually when contents of ab-scisic acid and ethylene increase (Nilsen and Orcutte, 1996).Nevertheless, exogenous application of indole-3-yl-acetic acidenhanced net photosynthesis and stomatal conductance in cot-ton (Kumar et al., 2001). Indole-3-butyric acid is a naturallyoccurring auxin. Drought stress and abscisic acid applicationenhance indole-3-butyric acid synthesis in maize. Recently,it was revealed that Indole-3-butyric acid synthetase fromArabidopsis is also drought-inducible (Ludwig-Mller, 2007).Experiments with indole-3-yl-acetic acid and ethylene glycoltetra-acetic acid suggested that calcium and auxin participatein signaling mechanisms of drought-induced proline accumu-lation (Sadiqov et al., 2002).

Drought rhizogenesis is an adaptive strategy that occursduring progressive drought stress and is reported from Bras-sicaceae and related families by the formation of short, tuber-ized, hairless roots. These roots are capable of withstandinga prolonged drought period and give rise to a new functionalroot system upon rehydration. The drought rhizogenesis washighly increased in the gibberrelic acid biosynthetic mutantga5, suggesting that some gibberrelic acids might also partic-ipate in this process (Vartanian et al., 1994).

Abscisic acid is a growth inhibitor and produced under awide variety of environmental stresses, including drought. Allplants respond to drought and many other stresses by accumu-lating abscisic acid. Abscisic acid is ubiquitous in all oweringplants and is generally recognized as a stress hormone that reg-ulates gene expression and acts as a signal for the initiation ofprocesses involved in adaptation to drought and other environ-mental stresses (Fig. 7). It has been proposed that abscisic acidand cytokinin have opposite roles in drought stress. Increasein abscisic acid and decline in cytokinins levels favor stomatalclosure and limit water loss through transpiration under waterstress (Morgan, 1990). When plants wilt, abscisic acid levelstypically rise as a result of increased synthesis (Taylor, 1991).Increased abscisic acid concentration leads to many changesin development, physiology and growth. Abscisic acid altersthe relative growth rates of various plant parts such as increasein the root-to-shoot dry weight ratio, inhibition of leaf area de-velopment and production of prolic and deeper roots (Sharpet al., 1994). It triggers the occurrence of a complex seriesof events leading to stomatal closure, which is an importantwater-conservation response (Turner et al., 2001). In a studyon genetic variation for abscisic acid accumulation in rice,a consistent negative relationship between the ability of de-tached and partially dehydrated leaves to accumulate abscisicacid and leaf weight was established (Ball et al., 1994). By its

Plant drought stress: eects, mechanisms and management 197

Drought stress

ReceptorH2O2ABACa+2

} Protein Kinases

Salicylic acid

Mitochondria/Chloroplast

Changes in gene expression, protein/

enzyme abundance and regulation

Antioxidant activation/de novo synthesis

Proline/ Glycinebetaine

accumulation

Stomatal closure

Transcription factors

Drought tolerance

Figure 7. Proposed cellular events and signaling cascades in a plantcell responding to drought stress. Drought stress is perceived by anunknown mechanism, which then activates the signaling cascades,plausibly by abcissic acid (ABA), hydrogen peroxide (H2O2) and cal-cium (Ca+2). These cascades then activate the synthesis of specicprotein kinases which activate more downstream responses such aschanges in gene expression. The response to these signaling cascadesalso results in changes in plant metabolism including activation andsynthesis of antioxidants, synthesis and accumulation of osmoprotec-tants and solutes, and stomatal closure under acute drought stress.

eect in closing stomata, abscisic acid can control the rateof transpiration and, to some extent, may be involved in themechanism conferring drought tolerance in plants.

Abscisic acid induces expression of various water stress-related genes. In a recent study, Zhang et al. (2005) reporteda regulatory role of telomeric repeat binding factor gene 1 inabscisic acid sensitivity and drought response during seedlingdevelopment. Bray (1997) suggested the existence of abscisicacid-dependent and abscisic acid-independent transductioncascades and pathways to act as a signal of drought stress andthe expression of specic water stress-induced genes. Abscisicacid produces such changes that confer an ability to maintaincellular turgor to withstand dehydrative forces (Fig. 7).

Ethylene has long been considered a growth inhibitoryhormone, although it is involved in environmentally drivengrowth inhibition and stimulation (Taiz and Zeiger, 2006). Theresponse of cereals to drought includes loss of leaf functionand premature onset of senescence in older leaves. Ethylenemay serve to regulate leaf performance throughout its lifespanas well as to determine the onset of natural senescence and me-diate drought-induced senescence (Young et al., 2004). Recentstudies suggest that growth promotion is a common feature inethylene responses. To escape this adversity, plants can opti-mize growth and tolerate abiotic stresses such as drought, andthis response also involves ethylene synthesis (Pierik et al.,2007).

Among the other endogenously produced growth regulatingfactors, the role of salicylic acid in the induction of toleranceagainst several abiotic stresses has been emphasized recently.In the case of drought tolerance, the role of endogenously

produced salicylic acid is still enigmatic. Salicylic acid po-tentiates the generation of reactive oxygen species in photo-synthetic tissues of Arabidopsis thaliana during osmotic stress(Borsani et al., 2001).

Polyamines are known to have profound inuence on plantgrowth and development. Being cationic, polyamines can as-sociate with anionic components of the membrane, such asphospholipids, thereby protecting the lipid bilayer from dete-riorating eects of stress (Bouchereau et al., 1999). There hasbeen a growing interest in the study of polyamine participationin the defense reaction of plants against environmental stressesand extensive research eorts have been made in the last twodecades (Bouchereau et al., 1999; Kasukabe et al., 2004).Many genes for enzymes involved in polyamine metabolismhave been cloned from several species, and their expressionunder several stress conditions has been analyzed. For exam-ple, the apple spermidine synthase gene when overexpressedencodes high levels of spermidine synthase, which substan-tially improves abiotic stress tolerance including drought (Wenet al., 2007).

Among various polyamines, a rise in the putrescence levelis generally due to an enhanced arginine decarboxylase activ-ity (Bouchereau et al., 1999). Compared with sensitive plants,stress-tolerant plants generally have a greater capacity to syn-thesize polyamines in response to stress, resulting in a two-to three fold rise in endogenous polyamine levels over theunstressed ones (Kasukabe et al., 2004). Recent studies sug-gested that rice has a great capacity to enhance polyaminebiosynthesis, particularly spermidine and spermine in freeform and putrescence in insoluble-conjugated form, in leavesearlier in response to drought stress. This was considered as animportant physiological trait of drought tolerance in rice (Yanget al., 2007).

3.2.5. Compatible solutes and osmotic adjustment

One of the most common stress tolerance strategies inplants is the overproduction of dierent types of compatibleorganic solutes (Serraj and Sinclair, 2002). Compatible solutesare low-molecular-weight, highly soluble compounds that areusually nontoxic even at high cytosolic concentrations. Gen-erally they protect plants from stress through dierent meanssuch as contribution towards osmotic adjustment, detoxica-tion of reactive oxygen species, stabilization of membranes,and native structures of enzymes and proteins (Fig. 8).

Osmotic adjustment is a mechanism to maintain water re-lations under osmotic stress. It involves the accumulation of arange of osmotically active molecules/ions including solublesugars, sugar alcohols, proline, glycinebetaine, organic acids,calcium, potassium, chloride ions, etc. Under water decit andas a result of solute accumulation, the osmotic potential of thecell is lowered, which attracts water into the cell and helpswith the maintenance of turgor. By means of osmotic adjust-ment, the organelles and cytoplasmic activities take place atabout a normal pace and help plants to perform better in termsof growth, photosynthesis and assimilate partitioning to grainlling (Ludlow and Muchow, 1990; Subbarao et al., 2000). As

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Hydrated

De-hydrated

(a)

(c)(b)

Protection Degraded

Protein

Compatible solute

Destabilising molecule

Figure 8. Role of compatible solutes in drought tolerance. In the hy-drated state, the presence of water reduces the interaction of desta-bilizing molecules (a), in tolerant cells the synthesis of compatiblesolutes preferentially excludes the binding of destabilizing moleculesand stabilizes native protein conformation (b) and in sensitive cellsthe lack of compatible solutes results in the preferential binding ofdestabilizing molecules to the protein surface, leading to degradation(c). (Adapted from Hoekstra et al., 2001).

a mechanism, osmotic adjustment has been suggested as animportant trait in postponing the dehydration stress in water-scarce environments (Morgan, 1990). Variation in osmotic ad-justment among chickpea cultivars in response to soil droughthas been observed, and seed yield of chickpea was corre-lated with the degree of osmotic adjustment when grown un-der a line-source irrigation system in the eld (Moinuddin andKhannu-Chopra, 2004). Contrarily, Serraj and Sinclair (2002)found no yield advantage from osmotic adjustment in anycrop. Nevertheless, further investigations are imperative to es-tablish this controversy.

As mentioned above, osmotic adjustment is accomplishedwith the accumulation of compatible solutes. Of these, prolineis one amongst the most important cytosolutes and its free ac-cumulation is a widespread response of higher plants, algae,animals and bacteria to low water potential (Zhu, 2002; Wahidand Close, 2007). Its synthesis in leaves at low water potentialis caused by a combination of increased biosynthesis and slowoxidation in mitochondria. Despite some controversy, manyphysiological roles have been assigned to free proline includ-ing stabilization of macromolecules, a sink for excess reduc-tant and a store of carbon and nitrogen for use after reliefof water decit (Zhu, 2002). Proline contents were increasedunder drought stress in pea cultivars (Alexieva et al., 2001).Drought-tolerant petunia (Petunia hybrida) varieties were re-ported to accumulate free proline under drought that actedas an osmoprotectant and induced drought tolerance (Yamadaet al., 2005).

Glycinebetaine (N, N, N-trimethyl glycine) is one of themost extensively studied quaternary ammonium compoundsand compatible solutes in plants, animals and bacteria (Wahidet al., 2007). Many studies demonstrate that glycinebetaineplays an important role in enhancing plant tolerance under a

range of abiotic stresses including drought (Quan et al., 2004).The introduction of genes synthesizing glycinebetaine intonon-accumulators of glycinebetaine proved to be eective inincreasing tolerance to various abiotic stresses (Sakamoto andMurata, 2002). Naidu et al. (1998) reported that cotton cul-tivars adapted to water stress conditions accumulated higherglycinebetaine than the non-adapted ones under drought. Inaddition to direct protective roles of glycinebetaine eitherthrough positive eects on enzyme and membrane integrityor as an osmoprotectant, glycinebetaine may also protect cellsfrom environmental stresses indirectly by participating in sig-nal transduction pathways (Subbarao et al., 2000).

Citrulline, named after Citrullus; a Latin name of water-melon, from which it was isolated, is an amino acid. Althoughnot built into proteins during their synthesis, and not encodedby a nuclear gene, several proteins are known to contain cit-rulline (Kawasaki et al., 2000). Wild watermelon (Citrulluslanatus) has the ability to adapt to severe drought stress despitecarrying out normal C3-type photosynthesis, which seem to becorrelated with citrulline accumulation (Akashi et al., 2001).Wild watermelon primarily accumulated citrulline followed byglutamate and arginine, in place of proline and glycinebetaine(Kawasaki et al., 2000). Yokota et al. (2002) reported a highercitrulline accumulation in the wild watermelon leaves assum-ing that citrulline is located only in the cytosol and constitutes5% of the total volume of the mesophyll cells. Citrulline is anovel and the most eective OH scavenger among compati-ble solutes examined so far. Moreover, it can eectively pro-tect DNA and enzymes from oxidative injuries (Akashi et al.,2001; Bektasoglu et al., 2006).

Rapid accumulation of the non-protein amino acid-aminobutyric acid was identied in plant tissues upon ex-posure to stress many years ago. -aminobutyric acid acts asa zwitterion, exists in free form, and has a exible moleculethat can assume several conformations in solution, includinga cyclic structure that is similar to proline. At physiologicalpH, -aminobutyric acid is highly water-soluble (Shelp et al.,1999), and may function as a signaling molecule in higherplants under stress (Serraj et al., 1998). The physiological rolesof -aminobutyric acid in drought tolerance entail osmotic reg-ulation (Shelp et al., 1999), detoxication of reactive oxygenradicals, conversion of putrescine into proline and intracellu-lar signal transduction (Kinnersley and Turano, 2000).

Drought stress initiates a signal transduction pathway, inwhich increased cytosolic Ca2+ activates Ca2+/calmodulin-dependent glutamate decarboxylase activity, leading to-aminobutyric acid synthesis (Shelp et al., 1999). ElevatedH+ and substrate levels can also stimulate glutamate decar-boxylase activity, leading primarily to -aminobutyric acidaccumulation. Experimental evidence supports the involve-ment of -aminobutyric acid in pH regulation, nitrogen stor-age, plant development and defense, as well as a compatibleosmolyte and an alternative pathway for glutamate utiliza-tion (Shelp et al., 1999; Wahid et al., 2007). After droughtstress the content of proline was more than 50% and at theend of recovery the -aminobutyric acid content reached 27%(Simon-sarkadi et al., 2006).

Plant drought stress: eects, mechanisms and management 199

Trehalose is a non-reducing disaccharide of glucose thatfunctions as a compatible solute in the stabilization of bio-logical structures under abiotic stress (Goddijn et al., 1997). Innature, trehalose is biosynthesized as a stress response by a va-riety of organisms including bacteria, fungi, algae, insects, in-vertebrates and lower plants (Wingler, 2002). Capacity to pro-duce trehalose, earlier thought to be absent from higher plants,has now been reported to accumulate in high amounts in somedrought-tolerant ferns, the resurrection plant Selaginella lep-idophylla (Penna, 2003) and desiccation-tolerant angiospermMyrothamnus abellifolia (Drennan et al., 1993). The pres-ence of low amounts of trehalose was demonstrated even in to-bacco (Goddijn et al., 1997) and many higher plants (Kosmaset al., 2006). Its metabolism may be channelized to enhancedrought tolerance in plants (Pilon-Smits et al., 1998; Penna,2003). Physiological roles of trehalose include ecient stabi-lization of dehydrated enzymes, proteins and lipid membranes,as well as protection of biological structures under desicca-tion stress (Wingler, 2002) rather than regulating water poten-tial (Lee et al., 2004). Karim et al. (2007) reported that en-hanced drought tolerance by trehalose depends on improvedwater status and expression of heterologous trehalose biosyn-thesis genes during Arabidopsis root development.

At a molecular level, exogenously applied trehalose maytrigger the abscisic acid-insensitive 4 gene expression butdecrease sucrose induction, providing a possible molecularmechanism for the trehalose eect on plant gene expressionand growth (Ramon et al., 2007). Trehalose-accumulatingorganisms produce this sugar in a two-step process by theaction of the enzymes trehalose-6-phosphate synthase andtrehalose-6-phosphate phosphatase when exposed to stress.Improved drought tolerance has been reported in the trans-genic plants overproducing trehalose-6-phosphate synthase inspite of minute accumulation of trehalose (Karim et al., 2007).

In fact, plants can withstand drought stress by conservingcell and tissue water principally by osmatic adjustment, main-tenance of the antioxidant defense system for the scaveng-ing of reactive oxygen species, and keeping the cell mem-branes stabilized. Plant growth regulators and polyamines,-aminobutyric acid, free amino acids and sugars also play avital role in drought tolerance by scavenging the reactive oxy-gen species, stomatal regulation and protection of vital macro-molecules, and maintenance of the cell water balance.

3.3. Molecular mechanisms

Plant cellular water decit may occur under conditions ofreduced soil water content. Under these conditions, changes ingene expression (up- and down-regulation) take place. Variousgenes are induced in response to drought at the transcriptionallevel, and these gene products are thought to function in toler-ance to drought (Kavar et al., 2007). Gene expression may betriggered directly by the stress conditions or result from sec-ondary stresses and/or injury responses. Nonetheless, it is wellestablished that drought tolerance is a complex phenomenoninvolving the concerted action of many genes (Agarwal et al.,2006; Cattivelli et al., 2008).

3.3.1. Aquaporins

Aquaporins have the ability to facilitate and regulate pas-sive exchange of water across membranes. They belong to ahighly conserved family of major intrinsic membrane proteins(Tyerman et al., 2002). In plants, aquaporins are present abun-dantly in the plasma membrane and in the vacuolar membrane.The structural analysis of aquaporins has revealed the gen-eral mechanism of protein-mediated membrane water trans-port. Although the discovery of aquaporins in plants has re-sulted in a prototype shift in the understanding of plant waterrelations (Maurel and Chrispeels, 2001), the relation betweenaquaporins and plant drought resistance is still elusive (Aharonet al., 2003). Nevertheless, it is believed that they can regu-late the hydraulic conductivity of membranes and potentiate aten- to twenty-fold increase in water permeability (Maurel andChrispeels, 2001).

Studies on aquaporins and plant water relations have beencarried out for many years. Mercury is a potential inhibitorof aquaporins. This was evident from a number of reports onmercury-induced decline in root hydraulic conductivity, whichsubstantiated that aquaporins play a major role in overall rootwater uptake (Javot and Maurel, 2002), and play a role incellular osmoregulation of highly compartmented root cells(Maurel et al., 2002; Javot et al., 2003). Reverse genetics pro-vides an elegant approach to explore aquaporin roles in plantwater relations (Kaldenho et al., 1998). The overexpressionof the plasma membrane aquaporin in transgenic tobacco pro-gressively improved plant vigor under favorable growth condi-tions, but the prolactin-inducible protein 1b gene overexpres-sion had retrogressive inuence under salinity, and caused fastwilting under water stress (Aharon et al., 2003). Phosphory-lation (Johansson et al., 1998), calcium and pH (Tournaire-Roux et al., 2003) are important factors modulating aquaporinactivity.

Recently, eorts have been concentrated on investigatingthe function and regulation of plasma membrane intrinsic pro-tein aquaporins. The aquaporins play a specic role in con-trolling transcellular water transport. For instance, they areabundantly expressed in roots where they mediate soil wateruptake (Javot and Maurel, 2002) and transgenic plants down-regulating one or more prolactin-inducible protein genes hadlower root water uptake capacity (Javot et al., 2003).

3.3.2. Stress proteins

Synthesis of stress proteins is a ubiquitous response to copewith prevailing stressful conditions including water decit.Most of the stress proteins are soluble in water and thereforecontribute towards the stress tolerance phenomena by hydra-tion of cellular structures (Wahid et al., 2007). Synthesis ofa variety of transcription factors and stress proteins is exclu-sively implicated in drought tolerance (Taiz and Zeiger, 2006).

Dehydration-responsive element-binding genes belongto the v-ets erythroblastosis virus repressor factor genefamily of transcription factors consisting of three sub-classes, dehydration-responsive element-binding gene1 and

200 M. Farooq et al.

dehydration-responsive element-binding gene2, which are in-duced by cold and dehydration, respectively (Choi et al.,2002). The dehydration-responsive element-binding genes areinvolved in the abiotic stress signaling pathway. It was pos-sible to engineer stress tolerance in transgenic plants by ma-nipulating the expression of dehydration-responsive element-binding genes (Agarwal et al., 2006). Introduction of anovel dehydration-responsive element-binding gene transcrip-tional factor eectively improved the drought tolerance abil-ity of groundnut (Mathur et al., 2004) and rice (Yamaguchi-Shinozaki and Shinozaki, 2004). After successful cloningof dehydration-responsive element-binding gene1 (Liu et al.,1998), many capsella bursa-pastoris-like genes have been re-ported to be synthesized in response to drought stress in var-ious plant species including rye and tomato (Jaglo et al.,2001), rice (Dubouzet et al., 2003), wheat (Shen et al., 2003),cotton (Huang and Liu, 2006), brassica (Zhao et al., 2006)and soybean (Chen et al., 2007). Introduction of dehydration-responsive element-binding gene1A genes in transgenic tallfescue (Festuca arundinacea) showed increased drought re-sistance with the accumulation of a high level of proline. Thisindicated the ability of capsella bursa-pastoris 3 to inducedrought tolerance (Zhao et al., 2007). Drought stress causesmany changes in the expression levels of late embryogenesisabundant/dehydrin-type genes and molecular chaperones thatprotect the cellular proteins from denaturation (Mahajan andTuteja, 2005).

Heat shock proteins belong to a larger group of moleculescalled chaperones. They have a role in stabilizing other pro-teins structure. Low-molecular-weight heat shock proteinsare generally produced only in response to environmentalstress, particularly high temperature (Wahid et al., 2007).But many heat shock proteins have been found to be in-duced by dierent stresses such as drought, anaerobic con-ditions and low temperatures (Coca et al., 1994). They arereported to serve as molecular chaperones that participate inadenosine triphosphate-dependentprotein unfolding or assem-bly/disassembly reactions and prevent protein denaturationduring stress (Gorantla et al., 2006).

Membrane-stabilizing proteins and late embryogenic abun-dant proteins are another important protein group responsiblefor conferring drought tolerance. These increase the water-binding capacity by creating a protective environment forother proteins or structures, referred to as dehydrins. They alsoplay a major role in the sequestration of ions that are con-centrated during cellular dehydration (Gorantla et al., 2006).These proteins help to protect the partner protein from degra-dation and proteinases that function to remove denatured anddamaged proteins. Dehydrins, also known as a group of lateembryogenesis abundant proteins, accumulate in response toboth dehydration and low temperature (Close, 1997). In addi-tion to their synthesis at the desiccating stage of seed, they alsoaccumulate during periods of water decit in vegetative tis-sues. These proteins are easily identiable from their particu-lar structur