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RESEARCH PAPER OPEN ACCESS

Influence of drought stress on assimilation of carbon, activity

of photosynthetic enzymes, heat shock proteins, antioxidants,

proline accumulation and protein contents in crop plants

Somayeh Ghofran Maghsud1, Mehrnaz Zargari2, Mohammad Lakzayi1, Abbas

Keshtehgar1, Khashayar Rigi1*

1Department of Agronomy, Zahedan Branch, Islamic Azad University, Zahedan, Iran

2Department of Agronomy, University of Zabol, Iran

Key words: Chlorophyll content, water use efficiency, heat shock proteins, stomatal conductance.

http://dx.doi.org/10.12692/ijb/5.5.162-175

Article published on September 10, 2014

Abstract

Drought stress is one of the most important abiotic stress factors which are generally accompanied by heat stress

in dry season. Drought is perceived as the most significant environmental stress in agriculture worldwide, and

improving yield under drought is therefore a major goal of plant breeding. The data on water stress induced

regulation of the activity of photosynthetic enzymes other than rubisco are scarce. Recently, most researchers

have agreed that the stomatal closure is the main determinant for decreased photosynthesis under mild and

moderate stress. Severe drought stress also inhibits the photosynthesis of plants by causing changes in

chlorophyll content, by affecting cholorophyll components and by damaging the photosynthetic apparatus.

Water availability mostly affects accumulation of some organic compatible solutes such as sugars, betaines and

proline which adjusts the intercellular osmotic potential is also early reaction of plants to water stress.

* Corresponding Author: Khashayar Rigi krigi66@yahoo.com

International Journal of Biosciences | IJB |

ISSN: 2220-6655 (Print) 2222-5234 (Online)

http://www.innspub.net

Vol. 5, No. 5, p. 162-175, 2014

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Introduction

Water is essential at every stage of plant growth and

agricultural productivity is solely dependent upon

water and it is essential at every stage of plant growth,

from seed germination to plant maturation (Turner,

1991). Drought stress is one of the most important

abiotic stress factors which are generally

accompanied by heat stress in dry season (Dash and

Mohanty, 2001). Water deficit stress due to drought,

salinity or extremes in temperature is the main

limiting factors for plant growth and productivity

resulting in large economic losses in many regions of

the world (Borsani et al, 2001). Plants respond to

water stress through a number of biochemical,

physiological and developmental changes

(Pattanagul, 1999. Shinozaki, 1997). Drought is

perceived as the most significant environmental

stress in agriculture worldwide, and improving yield

under drought is therefore a major goal of plant

breeding (Cattivelli et al., 2008). With a projected

increase in drought with climate change, the breeding

for drought-tolerant crops is even more emphasised

(Witcombe et al., 2008). In addition to drought,

temperature-induced stress causes variability in

wheat yields (Semenov and Shewry, 2011; Asseng et

al., 2011), corn and soybean (Schlenkera and Roberts,

2009) and other crops (e.g. Tashiro and Wardlaw,

1989; Prasad et al., 2000; Challinor et al., 2005).

Drought and heat stress often occur simultaneously,

but they can have very different effects on various

physiological, growth, developmental and yield

forming processes (Rizhysky et al., 2004; Boote et al.,

2005). The majority of indices have been developed

to determine long-term (months) drought or excess

rain and are less suitable for short-term effects

(weeks). Examples are the Drought Severity Index

(PDSI; Palmer, 1965), the Reconnaissance Drought

Index (RDI; Tsakiris et al., 2007) and Standardised

Precipitation Index (SPI; McKee et al., 1993).

Deficit irrigation

Deficit irrigation was proposed long time ago as a

technique that irrigates the entire root zone with less

evapotranspiration and leads to reduce the irrigation

water use with maintaining farmers’ net profits

(Hoffman et al., 1990). The decline in water

availability for irrigation and the positive results

obtained in some fruit tree crops have renewed the

interest in developing information on deficit

irrigation for a variety of crops (FAO Report, 2002;

Dorji et al., 2005 and Fereres and Soriano, 2007).

Assimilation of carbon

Assimilation of carbon by plants incurs water costs.

According to Boyer (1982) the ratio of carbon fi xation

to water loss (water use effi ciency) is critical to plant

survival, crop yield and vegetation dynamics. At a leaf

level, water use effi ciency can be defi ned as a ratio of

photosynthetic rate to transpiration rate or to leaf

conductance for water vapour (WUEi - intrinsic water

use efficiency) (Escalona et al., 1999).

Activity of photosynthetic enzymes

The data on water stress induced regulation of the

activity of photosynthetic enzymes other than

Rubisco are scarce. Thimmanaik et al (2002) studied

the activity of several photosynthetic enzymes under

progressive water stress in two different cultivars of

Morus alba. Unlike Rubisco, which is highly stable

and resistant to water stress, the activity of some

enzymes involved in the regeneration of ribulose-1,5-

bisphosphate (RuBP) are progressively impaired from

very early stages of water stress. Thus, these results

present the possibility that some enzymes involved in

the regeneration of RuBP could play a key regulatory

role in photosynthesis under water stress. During

water stress induced by polyethilen glycole, Rubisco

activity significantly increased in young potato leaves,

while decreased in mature leaves (Bussis et al., 1998).

But NADP-GAPDH and PRK activities have been

decreased and this change became faster in the course

of drought. While decreased Rubisco activity may not

be the cause of photosynthetic reduction during water

stress, its down-regulation may still be important

because it could preclude a rapid recovery upon

rewatering (Ennahli and Earl, 2005). Similarly, some

reports have shown strong drought-induced

reductions of Rubisco activity per unit leaf area

(Maroco et al., 2002) and per mg showed that the

decrease of Rubisco activity in vivo was not connected

164 Maghsud et al.

Int. J. Biosci. 2014

with the protein content. It occurs because of CO2

concentration decrease in the carboxylation center in

consequence of the partly closing of stomata (Flexas

et al., 2006). But it is known, that enzyme regulation

occurs not only in transcription, but also in

posttranscriptional level. Activities of the tested

enzymes are regulated by light as well as by the

concentration of photosynthetic metabolites (Raines,

2006). Reductions of more than 50% in the levels of

NADP-GAPDH, FBP, PRK, and plastid aldolase were

also needed before photosynthetic capacity was

affected (Stitt and Schulze, 1994).

Photosynthesis

The question as to whether drought mainly limits

photosynthesis through stomatal closure or metabolic

impairments has not been finally answered (Cornic,

2000; Flexas et al., 2004). The relative part of

stomatal limitation of photosynthesis depends on

stress severity. Recently, most researchers have

agreed that the stomatal closure is the main

determinant for decreased photosynthesis under mild

and moderate stress. Changes in photosynthetic

reactions are considered as a prevailing factor which

led do photosynthesis depression under severe water

stress (Yordanov et al., 2003). According to Chaves et

al. (2003) discrepancies in results concerning the

contribution of stomatal and non-stomatal factors in

photosynthesis inhibition may be explained by

differences in the rate of imposition and severity of

stress, developmental stage and plant condition,

species studied and superimposition of other stresses.

Biochemical and physiological changes

Usually, water deficit stress has detrimental effects on

many processes in plants, which include reducing

photosynthesis, accumulation of dry matter, stomatal

exchanges, and protein synthesis that affect their

growth stages (Larcher, 2003; Ohashi et al., 2006).

Generally, plants respond to water deficit stress

through developmental, biochemical and

physiological changes and the type of the observed

response depends on several factors such as stress

intensity (SI), stress duration and genotype

(Moradshahi et al., 2004). Many physiological

processes associated with crop growth and

development is reported to be influenced by water

deficits (Turner and Begg 1978 ; Bousba et al, 2009).

Previous research has shown various physiological

and biochemical changes in durum wheat plants

(Ykhlef et al, 2011) when drought stressed, such as

the regulation of stomatal aperture, osmotic pressure

of cells, and protein synthesis (Lu et al., 1994; Cheng,

1995).

Heat shock proteins (Hsps)

Chen and Wang, 2003; Zhu and Zhang, 2003; Xie et

al., 2005 founded that the synthesis of some original

proteins (namely stress-induced proteins) may be

induced or up regulated to adjust osmotic potential of

cells in order to keep a certain turgor and thus to

ensure the normal proceeding of physiological

processes such as cell growth, stomatal opening and

photosynthesis it can concluded that to cope with

environmental stress, plants activate a large set of

genes leading to the accumulation of specific stress-

associated proteins (Vierling 1991; Ingram and

Bartels 1996; Bohnert and Sheveleva 1998;

Thomashow 1999; Hoekstra et al. 2001).

Heat-shock proteins (Hsps) and late embryogenesis

abundant (LEA)-type proteins are two major types of

stress-induced proteins that accumulate upon water,

salinity, and extreme temperature stress. They have

been shown to play a role in cellular protection during

the stress (Bakalova et al. 2008 ; Thomashow 1998).

Irrigation methods and management

Irrigation methods and management are of

importance to soil water status, and thus, to plant

water status. Inappropriate irrigation could result in

water stress. Drip irrigation provides more efficient

water use for crops than furrow irrigation because

drip irrigation applies frequent small amounts of

water to the root zone and reduces adverse effects of

cyclic over irrigated and water stress commonly

caused by furrow irrigation. Many studies and reports

have addressed that yield and quality of hop

(Humulus Lupulus L.), potato (Solanum tuberosum)

tuber, tomato, cotton, and cantaloupe could be

165 Maghsud et al.

Int. J. Biosci. 2014

improved with drip irrigation (Bernstein and

Francois, 1973; Sammis, 1980; Wample and Farrar,

1983; Wood, 1988). Reports about the effect of

irrigation on chile pepper yields are few (Wierenga

and Hendrickx, 1985).

Antioxidation strategies

Drought stress is accompanied by the formation of

ROS such as O2, H2O2, and OH (Moran et al. 1994 ;

Mittler 2002), which damage membranes and

macromolecules. Plants have developed several

antioxidation strategies to scavenge these toxic

compounds. Enhancement of antioxidant defense in

plants can thus increase tolerance to different stress

factors. Antioxidants (ROS scavengers) include

enzymes such as catalase, superoxide dismutase

(SOD), ascorbate peroxidase (APX) and glutathione

reductase, as well as non-enzyme molecules such as

ascorbate, glutathione, carotenoids, and

anthocyanins. Additional compounds, such as

osmolytes, proteins can also function as ROS

scavengers (Bowler et al. 1992; Noctor and Foyer

1998). The antioxidant defenses appear to provide

crucial protection against oxidative damage in cellular

membranes and organelles in plants grown under

unfavorable conditions (Al- Ghamdi, 2009; Kocsy et

al., 1996). Plant cells synthesize a variety of

antioxidants to cope with ROS produced under

normal and stress conditions (Noctor and Foyer,

1998).

Environmental stresses exert their effects on plant

growth and development indirectly through

formation of ROS (Arora et al., 2002). The

antioxidant defenses appear to provide crucial

protection against oxidative damage in cellular

membranes and organelles in plants grown under un-

favorable conditions (Kocsy et al., 1996). Cellular

antioxidative defense system, which keeps ROS under

control and functions as a reductant for many free

radicals, minimizes the damage caused by oxidative

stress (Al-Ghamdi, 2009; Noctor and Foyer, 1998).

Plants possess a complex antioxidant system, which

consists of ascorbic acid, glutathione and enzymes

that protect the plant against oxidative damage

induced by environmental stresses. ASC (ascorbate),

a ubiquitous soluble antioxidant in photosynthetic

organisms, is the most important reducing substrate

for H2O2 detoxification (Chen et al., 2007). An

antioxidant system consists of low molecular weight

antioxidants such as ascorbate, glutathione, α-

tocopherol and carotenoids, as well as several

enzymes such as SOD (superoxide dismutase), CAT

(catalase), POD (peroxidase), APX (ascorbate

peroxidase), and GR (glutathione reductase) (Feng et

al., 2004). SOD is a group of metalloenzymes that

catalyse the disproportionation of O2˙ (superoxide)

to H2O2 and O2. Thus, SOD constitutes the first line

of defense against O2˙ derived oxidative stress in the

plant cell. Cellular H2O2 is detoxified by CAT and in

chloroplasts by ascorbate-glutathione cycle with its

key enzymes APX and GR (Arora et al., 2002).

Enhanced activities of antioxidants are associated

with resistance to environmental stresses. In

sunflower seedlings, it was reported that there was an

induction of defense enzyme activities and an

increase in glutathione content when plants reached a

moderate level of water deficit stress. Wheat plants

subjected to water stress showed that ascorbate-

glutathione cycle allowed the plants to maintain

H2O2 at the control level, despite a greater capacity

of the thylakoid membranes to leak electrons towards

O2 (Szechynska et al., 2007; Loschiavo et al., 1989;

Vergara et al., 1990).

Stomata control

Stomata control gas exchange between the interior of

a leaf and the atmosphere. Therefore they mainly

contribute to the ability of plants to control their

water relations and to gain carbon (Hetherington and

Woodward, 2003).

It has been shown that environmental signals such as

light intensity, carbon dioxide concentration and

water availability may affect stomatal development by

modifying their size and frequency (Knapp et al.,

1994; Nautiyal et al., 1994; Dyki et al., 1998). The

ability of a plant to live under stress has been shown

to be related to stomatal density and size, since these

were found to be related to the plant gas exchange

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Int. J. Biosci. 2014

(Aguirre et al., 1999). Therefore, it is possible that

variations in stomatal characteristics may influence

plant growth and productivity (Kundu and Tigerstedt,

1998).

Fig. 1. Plant survival against on abiotic stress.

Chlorophyll content

Severe drought stress also inhibits the photosynthesis

of plants by causing changes in chlorophyll content,

by affecting cholorophyll components and by

damaging the photosynthetic apparatus

(IturbeOrmaetxe et al., 1998). Ommen et al. (1999)

reported that leaf chlorophyll content decreases as a

result of drought stress. Drought stress caused a large

decline in the chlorophyll a content, the chlorophyll b

content, and the total chlorophyll content in all

sunflower varieties investigated (Manivannan et al.,

2007). The decrease in chlorophyll under drought

stress is mainly the result of damage to chloroplasts

caused by active oxygen species (Smirnoff 1995). The

decrease in chlorophyll under stress condition can be

attributed to the sensitivity of this pigment to

increasing environmental stresses, especially to

salinity and drought, which has been reported by

several researchers (Mekliche et al., 2003; Younis et

al., 2000) ; many studies indicated that stay-green is

associated with improved yield and transpiration

efficiency under water-limited conditions in sorghum,

maize and wheat (Borrell et al. 2000 ; Verma et al.

2004). The results are agreement with Nyachiro et al.

(2001), who described a significant decrease of

chlorophyll a and b caused by water deficit in six

Triticum aestivum cultivars. Decreased or unchanged

chlorophyll level during drought stress has been

reported in other species, depending on the duration

and severity of drought (Kpyoarissis et al., 1995). A

decrease of total chlorophyll with drought stress

implies a lowered capacity for light harvesting. Since

the production of reactive oxygen species is mainly

driven by excess energy absorption in the

photosynthetic apparatus, this might be avoided by

degrading the absorbing pigments (Herbinger et al.,

2002).

Drought tolerance

Drought tolerance consist the ability of crop against

the growth and production under water deficit

conditions. A long term drought stress effects on

plant metabolic reactions associates with, plant

growth stage, water storage capacity of soil and

physiological aspects of plant. Drought tolerance in

crop plants is different from wild plants. In practice,

planting of crop encounter severe water deficit, it dies

or seriously loses yield while in wild plants their

surviving under this conditions but no yield loss, is

taken into consideration. However, water deficit has

always been considered as a main problem and is of

great importance, so has taken into account as one of

the breeding factors (Talebi, 2009). Nowadays many

physiological, biochemical and molecular biology

studies on the mechanisms of drought tolerance of

agriculturally important crops have been performed

(Simova-Stoilova et al., 2006).

Fig. 2. Some physiological and biochemical

perturbations in plants caused by fluctuations in

abiotic environments.

Plant breeding programs

Achieving a genetic increase in yield under these

environments has been recognized to be a difficult

167 Maghsud et al.

Int. J. Biosci. 2014

challenge for plant breeders while progress in yield

grain has been much higher in favorable

environments (Richards et al., 2002). Improving

drought resistance is, therefore, a major objective in

plant breeding programs for reined agriculture in

these regions. Knowledge of genetic behavior and

type of gene action controlling target traits is a basic

principle for designing an appropriate breeding

procedure for the purpose of genetic improvement.

Hence, the success of any selection or hybridization

breeding program for developing drought-tolerant

varieties depends on precise estimates of genetic

variation components for traits of interest consisting

of additive, dominant and non-allelic interaction

effects (Farshadfar et al., 2011; Nouri et al., 2011). In

plant breeding program, several characters are

simultaneously considered that make it feasible to

approximate the genetic divergence by using

multivariate techniques. These multivariate

techniques include principal component and cluster

analysis which have analogous efficacy to establish

the most suitable cross combinations (Amjad-Ali et

al., 2011).

Fig. 3. Physiological and molecular basis of drought stress tolerance.

C4 concentrating mechanism

Some controlled environment studies of well watered

plants suggest that growth at elevated [CO2] can

directly impact C4 photosynthesis by a number of

mechanisms (for review, see Ghannoum et al., 2000).

As examples, intercellular [CO2] (ci) below the

saturation point of the photosynthetic intercellular

CO2 response curve has been reported under ambient

[CO2], allowing direct stimulation of photosynthesis

under elevated [CO2] (Wong, 1979; Watling and

Press, 1997; Ziska and Bunce, 1997). Bundle sheath

leakiness increased under elevated [CO2], reducing

the initial slope and CO2-saturated photosynthetic

rate of the curve in sorghum (Sorghum bicolor;

Watling et al., 2000). In developing Flaveria trinervia

leaves, 10% of CO2 fixation occurred directly in the

bundle sheath, without involvement of the C4

concentrating mechanism, allowing the possibility

that elevated [CO2] could directly stimulate

photosynthesis (Moore et al., 1986). Some immature

C4 leaves have C3-like photosynthesis and are

therefore more sensitive to enhanced photosynthesis

under elevated [CO2] (Dai et al., 1995; Ziska et al.,

1999). Enzymes of both the C4 cycle and Calvin cycle

in maize were consistently lower under elevated

[CO2], withmalate dehydrogenase (237%) and

glyceraldehyde-3-phosphate dehydrogenase activities

(229%) declining to the greatest extent in young

leaves (Maroco et al., 1999).

Protein contents

Plants can partly protect themselves against mild

drought stress by accumulating osmolytes. Proline is

one of the most common compatible osmolytes in

drought stressed plants. For example, the proline

content increased under drought stress in pea

(Sanchez et al., 1998; Alexieva et al., 2001). Proline

accumulation can also be observed with other stresses

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Int. J. Biosci. 2014

such as high temperature and under starvation

(Sairam et al., 2002). Proline metabolism in plants,

however, has mainly been studied in response to

osmotic stress (Verbruggen and Hermans 2008).

Proline does not interfere with normal biochemical

reactions but allows the plants to survive under stress

(Stewart, 1981). The accumulation of proline in plant

tissues is also a clear marker for environmental stress,

particularly in plants under drought stress (Routley,

1966). Proline accumulation may also be part of the

stress signal influencing adaptive responses (Maggio

et al. 2002). According to ROSE (1988) water stress

decreased protein contents in plants. The results of

present investigations were inconsistent with the

finding, which implies that soluble protein did not

contribute to osmotic adjustment. The increase in

proline content due to drought stress was more severe

at flowering stage than at the vegetative stage. The

proline content depends on plant age, leaf age, leaf

position or leaf part (Chiang and Dandekar, 1995).

Under vegetative stage, drought stress increased

proline content about tenfold, this increasing roles as

an osmotic compatible and adjust osmotic potential

which resulted in drought stress avoidance in

chickpea. Prolin accumulation is believed to play

adaptive roles in plant stress tolerance (Verbruggen

and Hermans 2008). Accumulation of proline has

been advocated as a parameter of selection for stress

tolerance (Yancy et al., 1982. Jaleel et al., 2007).

Fig. 4. Visual aspect of shoot in Phaseolus vulgaris

plants exposed to drought by four days.

Water use efficiency (WUE)

Limited of irrigation means that the soil water deficit

is controlled at certain stages of crop growth, a

practice that has become more important in recent

years in areas where water resources are limited.

Water use efficiency (WUE) is defined here as the

ratio between grain yield and total evapotranspiration

during the growing season. Studies on the effects of

limited irrigation show that crop yield can be largely

maintained and product quality can sometimes be

improved while substantially reducing irrigation

volume (Li 1982; Shan 1983; Fapohunda et al. 1984;

Sharma et al. 1986; Singh et al. 1991; Zhang et al.

1999). Aggarwal et al. (1986) reported that WUE

decreased with increasing evapotranspiration,

whereas Musick et al. (1994) found that WUE did not

change with seasonal evapotranspiration. Under

limited irrigation, reductions in grain yield due to

restricted water availability depend on the degree,

duration and timing of the imposed soil moisture

deficit. The impact of soil moisture deficit on crop

yield depends on the particular phenological stage of

the crop, and the most sensitive stage can vary

regionally (Singh et al. 1991).

Fig. 5. The basic ROS cycle. This cycle modulates the

cellular levels of ROS during normal metabolism.

Some of the key ROS scavenging enzymes of plants,

ascorbate peroxidase(APX), and catalase(CAT) are

indicated.

Oxidative stress and proline accumulation

Water availability mostly affects accumulation of

some organic compatible solutes such as sugars,

betaines and proline which adjusts the intercellular

osmotic potential is also early reaction of plants to

water stress. Sairam and Saxena (2000) reported that

oxidative stress which caused metabolic damage in

water stress, increases lipid per oxidation, resulting in

greater membrane injury and pigment bleaching.

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Int. J. Biosci. 2014

Zlatev and Stoyanov (2005) suggested that proline

accumulation of plants could be only useful as a

possible drought injury sensor instead of its role in

stress tolerance mechanism. Vendruscolo et al.

(2007) found that proline is involved in to lerance

mechanisms against oxidative stress and this was the

main strategy of plants to avoid determental effects of

water stress.

Fig. 6. Sites of production of reactive oxygen species (ROS) in plants.

Fig. 7. Physiological mechanisms induced by water stress.

Stomatal conductance

Plants grown under drought condition have a lower

stomatal conductance in order to conserve water.

Consequently, CO2 fixation is reduced and

photosynthetic rate decreases, resulting in less

assimilate production for growth and yield of plants.

Diffusive resistance of the stomata to CO2 entry

probably is the main factor limiting photosynthesis

under drought (Boyer, 1970). Certainly under mild or

moderate drought stress stomatal closure (causing

reducted leaf internal CO2 concentration (Ci)) is the

major reason for reduced rates of leaf photosynthetic

(Chaves, 1991; Cornic, 2000; Flexas et al., 2004).

Varieties significantly differed in photosynthetic

activities, but these differences could only be

expressed under the control conditions. In many

experiments it has been shown that A decreases when

gs decreases (e.g., Tenhunen et al., 1987; Nilsen and

Orcutt, 1996). Chaves and Oliviera (2004) concluded

that gs only affect A at severe drought stress. The

decrease in photosynthesis in drought stressed plants

can be attributed both to stomatal (stomatal closure)

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Int. J. Biosci. 2014

and non-stomatal (impairments of metabolic

processes) factors. Under control treatment, the yield

of cultivars followed the same trend of A, under this

condition ‘Bivaniej’ showed highest A and seed yield.

At present most researchers agree that the stomatal

closure and the resulting CO2 deficit in the

chloroplasts is the main cause of decreased

photosynthesis under mild and moderate stresses

(Flexas and Medrano, 2002). However, some authors

claim that impaired ATP is a likely explanation for

decreased photosynthesis under water stress (Lawlor,

2002; Tang et al., 2002).

Fig. 8.Water vapor moves out and carbon dioxide

move into the leaf through the stomata.

Plants anatomy, morphology and physiology

Water stress may affect on the plants anatomy,

morphology, physiology and biochemistry and

influence on almost all of their growth and

development aspects. Plants respond to drought

through morphologic, physiologic and metabolic

changes in all theirs organs (Heidary et al, 2007).

Drought stress, by disturbing the equilibrium

relations between water and plant and the structure

of cell biologic membrance m creates basic disorder in

rice growth and decreases the performance. Most

crops particularly during flowering phase to seed

development are sensitive to water shortage stress.

Even plants cultivating in dry and semidry regions are

influenced by drought stress (Mitra, 2001) .Plants

survival and production power of crops is greatly

influenced by water accessibility and decreased due to

decrease of this vital factor (Ozhur et al , 2009).

Time–domain reflectometry (TDR)

New methods such as time–domain reflectometry

(TDR) are now available for use; however, these

methods are limited because of difficulties in

conducting field operations when multiple cables are

installed in large fields for long periods. Moreover,

measuring multiple depths within the soil profile

requires installation of multiple TDR probes at a

location. This procedure makes water content

profiling impractical for monitoring soil water

available over the depth of the soil profile at a large

number of locations within a field. Sadler et al.

(2000) used TDR probes at eight sites in their field to

investigate water use and stress. Their results

demonstrated the need for within–season

observations of crop water use and stress to augment

interpretation of site–specific yield maps. Nijbroek

(1999) used conventional TDR probes in an irrigated

soybean field in Georgia to estimate drained upper

limit values based on nighttime drainage rates

measured at 2–hour intervals. Their system required

a long cable from the probes to the cable tester, so

data collection was limited to certain areas in the

field.

Fig. 9. Leaf rolling in corn.

Sensitive to water stress

It has been observed that vegetative and reproductive

growth in trees is differentially sensitive to water

stress. Additionally, reproductive growth is

differentially sensitive to water stress at different

times of the season. It has been reported that mild

water stress applied during the intermediate

developmental period of slow fruit growth has no

effect on crop yields but reduces vegetative growth in

peach (Mitchell and Chalmers 1982) and pear

(Mitchell et al. 1984). However, the final period of

very rapid fruit growth has been reported to be

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Int. J. Biosci. 2014

relatively sensitive to water stress in peach (Li et al.

1989, Crisosto et al. 1994), apple (Lotter et al. 1985)

and Asian pear (Caspari et al. 1994). Deficit irrigation

(DI) and partial root drying (PRD) are water-saving

irrigation strategies (Kang and Zhang, 2004). DI

irrigates the entire root zone with an amount of water

less than the potential evapotranspiration (ETfull)

and the minor stress that develops has minimal

effects on the yield (English et al., 1990). DI has

proved successfully with a number of crops; however

it has been difficult to manage in potatoes (Lynch et

al., 1995). PRD involves alternate watering to each

side of the plant root system, by which it allows the

plant to explore rootsourced ABA signaling to

regulate plant growth and water use thereby

increasing WUE (Dry et al., 2000). PRD has been

found to be promising in several crops (Kang and

Zhang, 2004). However, until now PRD has not been

studied in potatoes. It is suggested that plants under

PRD performed better than under DI when the same

amount of water was applied. Davies and Hartung

(2004) proposed that PRD could stimulate root

growth and maintain a constant ABA signaling to

regulate shoot physiology; whereas plants under DI,

some of the roots in dry soils for long period may die

and signaling may diminish and shoot water deficits

may occur.

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