Soil & Tillage Research 77 (2004) 179–187

Pot study on wheat growth in saline andwaterlogged compacted soil

II. Root growth and leaf ionic relations

Muhammad Saqib∗, Javaid Akhtar, Riaz Hussain QureshiDepartment of Soil Science, University of Agriculture, Faisalabad, Pakistan

Received 24 April 2003; received in revised form 14 November 2003; accepted 15 December 2003

Abstract

Soil compaction affects plant growth by causing increased resistance to root penetration and a decreased uptake of waterand ions. A pot experiment was conducted to study the effect of soil compaction in conjunction with the soil salinity andwaterlogging on root growth and leaf ionic composition of two wheat genotypes (Aqaab and MH-97). Compaction wasachieved at a 10% soil moisture content by dropping 5 kg weight, controlled by a tripod stand for 20 times from 0.6 m heighton a wooden block placed inside the soil-filled pots. Soil bulk density of non-compact and compact treatments was measured as1.21 and 1.65 Mg m−3, respectively. The desired salinity level (15 dS m−1) was developed by mixing required amount of NaClin the soil before filling the pots. Waterlogging was developed by flooding the pots for 21 days both at tillering and bootingstages. Compaction significantly reduced the root length density (RLD) of both the wheat genotypes while the combinedeffect of compaction× salinity was more drastic on root length density than compaction alone. Waterlogging however, didnot decrease the root length density, rather it mitigated the effect of compaction. Compaction decreased the concentrationof K+ and K+:Na+ ratio in leaves. Salinity also decreased the concentration of K+ and K+:Na+ ratio, but increased theconcentrations of Na+ and Cl− in the crop leaves. Salinity and compaction interacted to cause a greater reduction in K+concentration and the K+:Na+ ratio, while there was lesser increase in the concentrations of Na+ and Cl− compared withsalinity alone. Waterlogging also decreased the concentration of K+ and K+:Na+ ratio in leaves. It intensified the effectof salinity but decreased the effect of compaction on leaf ionic composition. Therefore, the effect of compaction on rootgrowth and ion uptake is more severe under salt-affected soil conditions than under normal soil conditions while occasionalwaterlogging of a compact soil for a few days makes the soil conditions favorable for root growth both under non-saline aswell as saline soil conditions. Also, the performance of a genotype in stressed environment is related to maintenance of higherroot length density, leaf K+ concentration and K+:Na+ ratio and lower leaf Na+ and Cl− concentrations.© 2004 Elsevier B.V. All rights reserved.

Keywords: Salinity; Waterlogging; Compaction; Ionic composition; Root length density; Wheat

∗ Corresponding author. Present address: Institute für Pflanzen-ernährung, Interdisziplinäres Forschungszentrum, UniversitätGießen, Heinrich Buff Ring 26-32, 35392 Gießen, Germany. Tel.:+49-641-9939164; fax:+49-641-9939169.E-mail address: drhmsab@yahoo.com (M. Saqib).

1. Introduction

Adverse effects of a compact soil horizon onroot growth and concomitant poor plant growth andyields have been recognized by many research work-ers (Oussible et al., 1992; Unger and Kaspar, 1994;Jorajuria et al., 1997). Root elongation in soils is

0167-1987/$ – see front matter © 2004 Elsevier B.V. All rights reserved.doi:10.1016/j.still.2003.12.005

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possible only when the root pressure exceeds themechanical impedance of the soil (Bennie, 1991).Oussible et al. (1992)found that compacting a clayloam soil to a density of 1.52 from 1.33 Mg m−3

significantly reduced the root elongation of wheat by19–36% and a correlation coefficient of−0.93 wasobtained between root length density (RLD) and me-chanical impedance. A significant decrease in rootlength density of wheat due to subsoil compactionhas also been reported byIshaq et al. (2001a).

Limited water and nutrient availability to plantsdue to compaction are also the major constraintsto plant growth and yields in compact soils.Ishaqet al. (2001a)reported that an increase in bulk den-sity (from 1.65 to 1.93 Mg m−3) and penetrationresistance (from 1.00 to 4.83 MPa) due to subsoilcompaction decreased the nutrient uptake by wheat.The reduction in nutrient concentration in wheat was12–35% for N, 17–27% for P and up to 24% for K.Water use efficiency and nutrient use efficiency bywheat was also decreased significantly by increasedsoil strength that resulted in decreased yield of wheatby 12–38% (Ishaq et al., 2001b). Other researchers(Gameda et al., 1994; Lal, 1996) have also reportedthat crop yields were reduced by soil compaction dueto increased resistance to root growth and decreasesin water and nutrient use efficiencies.

Salinity and waterlogging are also common stresseslimiting plant growth and productivity. In high sub-strate salinity, growth depression results from waterdeficit, ion imbalance and ion toxicity (Wyn Jones,1981). The uptake, transport and utilization in theplants of different nutrient ions is also disturbed(Marschner, 1995). Salt stress inhibits the uptake andtransport of K+ (Lynch and Lauchli, 1984) and causesa net efflux of K+ (Shabala, 2000) which can result ina toxic accumulation of Na+ and Cl− (Nawaz, 1993;Saqib et al., 1999, 2000). Oxygen deficiency underwaterlogged conditions also reduces the ability ofroots to supply water and nutrients to shoots (Jackson,1990). Roots become energy limited as the modeof respiration is changed from aerobic to anaerobic(Qureshi and Barrett-Lennard, 1998). Inhibited nutri-ent uptake leads to nutrient deficiency in shoots thatleads to leaf senescence and cessation of shoot growthunder waterlogged conditions (Trought and Drew,1980). The depressive effects of root asphyxiation onthe hydromineral uptake has been ordered byMorard

and Silvestre (1996)as: K+ > N > P > H2O>Mg2+∼ Ca2+. However, a negative correlation between soilwater contents and penetration resistance has beenobserved (Shafiq et al., 1994), therefore, waterloggingmay ease the penetration resistance for roots due tosoil compaction.

Salinity and waterlogging interact to cause damagein excess of that caused by salinity or waterloggingalone (Barrett-Lennard, 1986; Akhtar et al., 1994;Qureshi and Barrett-Lennard, 1998). Oxygen defi-ciency in the roots results in a breakdown of salttolerance mechanisms operating in roots, such as saltexclusion. Compaction is another stress that interfereswith salinity and waterlogging thus changing their ef-fects on plant growth. The information on this aspectis scanty in Pakistan. The overall goal of this studywas to quantify the interactive effects of compaction,salinity and waterlogging on wheat genotypes. Thespecific objective is to assess the effects of soil com-paction on root growth and leaf ionic relations at var-ious levels of salinity and waterlogging. The effectsof these stresses on grain yield and yield componentshave been reported in the companion paper.

2. Materials and methods

This pot study was conducted in a wire house at De-partment of Soil Science, University of Agriculture,Faisalabad, Pakistan. The seeds of wheat genotypes(Aqaab and MH-97) used in this study were collectedfrom Saline Agriculture Research Centre, Universityof Agriculture, Faisalabad, Pakistan. The experimentwas replicated five times in a completely randomizedfactorial arrangement. The soil used was non-salinenon-sodic in nature with sandy clay loam texture(sand: 53.5%; silt: 24%; clay: 22.5%). There weretwo sets of treatments, viz. non-compact (bulk density(Db): 1.21 Mg m−3) and compact (Db: 1.65 Mg m−3)soil. In each set four treatments, viz. control (elec-trical conductivity of the saturated soil extract (ECe)measured by EC meter: 2.6 dS m−1), waterlogging(flooding for 21 days at tillering and booting stages),salinity (ECe: 15 dS m−1) and salinity×waterloggingwere applied. These treatments were developed inearthen glazed pots of 12 kg capacity (height: 0.31 m;diameter: 0.23 m). Compaction levels were achievedat 10% moisture level (on mass basis) by dropping a

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5 kg weight, controlled by a tripod stand for 20 timesfrom a 0.6 m height on a wooden block (width/height:0.05 m; diameter: 0.18 m) placed inside the soil filledpot. The desired salinity levels were developed bymixing the required amount of NaCl in soil beforefilling the pots. After development of salinity andcompaction treatments, seeds were sown at a depth of0.01 m. Waterlogging was developed by flooding thepots for 21 days at the tillering as well as at bootingstage. The more detailed information on growth condi-tions, treatment development and seed sowing is givenin the companion paper. However, the methodology ofthe parameters discussed in this paper is given below.

2.1. Measurement of root length

Soil samples with the help of a soil core, were col-lected from each pot at the grain filling stage. Rootswere separated by washing the soil samples under agentle flow of water. The root length was determinedfollowing the technique ofTennant (1975)and theroot length density (mm cm−3) was calculated by di-viding the root length (mm) by the volume (cm3) ofthe sampling core.

2.2. Extraction of leaf sap and ion analysis

The leaf second to the flag leaf was collected at thebooting stage. The detached leaves were quickly rinsedin distilled water, blotted with tissue paper and storedin the separate Eppendorf tubes at−10◦C. Frozenleaf samples were thawed and crushed using a stain-less steel rod (diameter: 0.65 cm) with a tapered end.The sap was collected in the other Eppendorf tubes bya Gilson pipette and centrifuged at 6500 revolutionsper minute (rpm) for 10 min (Clandon T-53 centrifugemachine; internal chamber: 0.35 m× 0.18 m; radiusof rotation: 0.15 m). The supernatant sap was takenin the new Eppendorf tubes and was used for ionicdeterminations. The centrifuged leaf sap was dilutedas required by adding distilled water and Na+ andK+ were determined using a Sherwood 410 flamephotometer. Chloride in the leaf sap was determinedusing a Sherwood 926 chloride analyzer.

2.3. Statistical analysis

Data were analyzed statistically for analysis of vari-ance (ANOVA) following the methods described by

Gomez and Gomez (1984). MSTATC computer soft-ware was used to carry out the statistical analysis(Russell and Eisensmith, 1983). The significance ofdifferences among means was compared by using thestandard error. The standard error was computed ass/

√n, wheres is the standard deviation andn shows

the number of observations.

3. Results

3.1. Root length density of wheat genotypes

Salinity and salinity× waterlogging treatments sig-nificantly reduced the root length density of both thewheat genotypes (Aqaab and MH-97) in non-compactsoil (Fig. 1). However, waterlogging alone did notaffect the RLD.

Soil compaction(compaction×control) also signif-icantly reduced RLD of both the wheat genotypes ascompared to non-compact control. The waterloggingtreatment under compact soil conditions decreasedthe RLD of Aqaab as well as MH-97 as comparedto non-compact control and non-compact waterlog-ging treatments, but the decrease was significant onlyin the case of MH-97. The RLD of Aqaab underthe compact waterlogged treatment was significantlyhigher than that in the compaction× control treat-ment, showing a decrease in the effect of compactiondue to the waterlogging treatment.

Soil compaction significantly enhanced the negativeimpact of salinity. However, the RLD of both the wheatgenotypes under compact saline-waterlogged condi-tions was significantly higher than under the com-pact saline treatment. This again showed a decreasein the negative effect of compaction due to waterlog-ging even under saline soil conditions (compact saline-waterlogged) as compared to compaction×salinity. Agenotypic comparison shows that significantly higherRLD was obtained for Aqaab than MH-97 in compactsaline and compact saline-waterlogged treatments.

3.2. Na+ concentration in leaf sap

In the non-compact soil, salinity significantlyincreased Na+ concentration in the leaf sap of boththe wheat genotypes compared to the non-compactcontrol (Fig. 2). A combination of salinity with

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Fig. 1. Effect of soil compaction on root length density (mm cm−3) of wheat under saline and waterlogged conditions.

waterlogging further increased the Na+ concentrationin leaf sap compared to salinity alone.

Compaction×control and compaction×waterloggingtreatments did not increase leaf Na+ concentrationcompared with the non-compact control. Compactionand salinity in combination, however, significantlydecreased the Na+ concentration in leaf sap of boththe wheat genotypes as compared to salinity in thenon-compact soil. Compaction did not effect the Na+concentration under saline waterlogged conditions ascompared to the salinity× waterlogging treatment innon-compact soil conditions.

Fig. 2. Effect of soil compaction on Na+ contents (mol m−3) of wheat leaves under saline and waterlogged conditions.

The Na+ concentration in leaf sap of MH-97was significantly greater than of Aqaab under thenon-compact saline, non-compact saline-waterloggedand compact saline-waterlogged treatments.

3.3. K+ concentration in leaf sap

The concentration of K+ in leaf sap of both thewheat genotypes was decreased significantly due tosalinity and waterlogging alone, or in combinationin the non-compact soil conditions (Fig. 3). The leafK+ concentration of Aqaab in the saline-waterlogged

M. Saqib et al. / Soil & Tillage Research 77 (2004) 179–187 183

Fig. 3. Effect of soil compaction on K+ contents (mol m−3) of wheat leaves under saline and waterlogged conditions.

treatment was significantly lower than in the wa-terlogged treatment but statistically similar to thatobserved in salinity alone. The K+ concentrationin the leaf sap of MH-97 was higher in the controltreatment followed by the waterlogging, salinity andsalinity × waterlogging treatments, with significanttreatment differences.

Soil compaction(compaction×control) significan-tly decreased the concentration of K+ in leaf sap ofboth the wheat genotypes as compared to the non-compact control. Compaction× salinity furtherdecreased the K+ concentration of both the geno-types compared with the non-compact salinity treat-ment. Aqaab accumulated significantly higher K+than MH-97 under all the treatments except in thenon-compact control, compaction× control andcompaction× waterlogging treatments.

3.4. K+:Na+ ratio in leaf sap

In non-compact soil, salinity and waterloggingalone as well as together, caused a significant reduc-tion in the K+:Na+ ratio in leaf sap of both the wheatgenotypes (Fig. 4). The K+:Na+ ratio was lower un-der saline-waterlogged conditions than in the salinitytreatment, which in turn, was lower than that of thewaterlogged treatment.

The compaction× control treatment also signifi-cantly decreased the K+:Na+ ratio in leaves of Aqaabas compared to the non-compact control treatment.Compaction did not significantly change the effectof waterlogging on the K+:Na+ ratio, but intensifiedthe effect of salinity and significantly lowered theK+:Na+ ratio in leaves of both the genotypes as com-pared to the non-compact saline treatment. The be-havior of the genotypes differed significantly under allthe treatments, except under the non-compact control,compaction× control and compaction× waterloggedtreatments, with Aqaab producing a higher K+:Na+ratio.

3.5. Cl− concentration in leaf sap

The salinity and salinity× waterlogging treatmentscaused a significant increase in leaf Cl− concentra-tions of both the wheat genotypes as compared to thenon-compact control (Fig. 5). Salinity× waterloggingcaused a higher increase in the leaf Cl− concentrationthat was also significant for MH-97.

The compaction of saline soil(compaction×salinity) decreased the concentration of Cl− in leaf sapas compared to the respective non-compact treatment.The genotypes differed significantly only in the case ofthe saline-waterlogged treatment in non-compact soil

184 M. Saqib et al. / Soil & Tillage Research 77 (2004) 179–187

Fig. 4. Effect of soil compaction on K+:Na+ ratio in wheat leaves under saline and waterlogged conditions.

Fig. 5. Effect of soil compaction on Cl− contents (mol m−3) of wheat leaves under saline and waterlogged conditions.

conditions and in the saline and saline-waterloggedtreatments in compact soil conditions, with a lowerCl− concentration in the leaves of Aqaab.

4. Discussion

Roots are directly exposed to soil stresses includ-ing compaction, salinity and low oxygen levels. Theresponse of plant roots may be different for different

types of stresses and for different combinations ofstresses.

In the present study combined stress of compactionand salinity interacted to cause the maximum reduc-tion in root length density. This reduction in RLDwas 29% greater than for soil salinity alone and34% greater than for compaction alone. However,compaction and waterlogging interacted oppositely,and the RLD in the compact waterlogging treatmentwas 10% greater than in the compaction× control

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treatment. Similarly, the RLD in the compact salinewaterlogged treatment was 11% greater than in thecompact saline treatment. In compact soil, the me-chanical impediment to root growth due to increasedbulk density may have rendered the root unable topenetrate deeply and hence the volume of soil ex-plored by roots is decreased. A reduced root elonga-tion and root length density due to compaction havealso been reported earlier byOussible et al. (1992)andIshaq et al. (2001a), respectively.

A decrease in root length density under saline con-ditions may be due to decreased availability of pho-tosynthates from the shoot, and water stress and iontoxicity due to salts around the root. A decrease inroot length of wheat under saline conditions has alsobeen reported earlier byNawaz (1993).

The maximum reduction in RLD in this study wasobserved in the compact saline treatment that resultedfrom reduced root growth due to salinity and a restric-tion to root penetration due to soil compaction. Theimposition of waterlogging in compact soil conditionsmay have decreased the root penetration resistancebecause there exists a negative correlation betweensoil water contents and penetration resistance (Shafiqet al., 1994).

In saline waterlogged conditions, the root’s abilityto exclude Na+ and Cl− is reduced because of rela-tively low energy production due to reduced O2 sup-ply that can also lead to decreased selectivity for K+compared with Na+ (Barrett-Lennard, 1986; Qureshiand Barrett-Lennard, 1998). These changes result inhigher Na+ and Cl− concentrations and lower K+concentration in leaf sap and more growth reduction

Table 1Relationship between different yield components, root length density and ionic components

Grainyield

100 grainweight

Spikelength

Spikes perplant

Tillersper plant

Strawweight

Na+ K+ K+:Na+ Cl−

100 grain weight 0.91∗∗Spike length 0.88∗∗ 0.85∗∗Spikes per plant 0.82∗∗ 0.82∗∗ 0.65∗∗Tillers per plant 0.87∗∗ 0.81∗∗ 0.78∗∗ 0.79∗∗Straw weight 0.84∗∗ 0.79∗∗ 0.78∗∗ 0.76∗∗ 0.87∗∗Na+ −0.61∗∗ −0.68∗∗ −0.57∗∗ −0.61∗∗ −0.67∗∗ −0.67∗∗K+ 0.93∗∗ 0.87∗∗ 0.86∗∗ 0.74∗∗ 0.80∗∗ 0.78∗∗ −0.56∗∗K+:Na+ 0.83∗∗ 0.84∗∗ 0.74∗∗ 0.72∗∗ 0.81∗∗ 0.77∗∗ −0.85∗∗ 0.83∗∗Cl− −0.70∗∗ −0.77∗∗ −0.68∗∗ −0.66∗∗ −0.74∗∗ −0.74∗∗ 0.96∗∗ −0.67∗∗ −0.89∗∗RLDa 0.87∗∗ 0.78∗∗ 0.76∗∗ 0.69∗∗ 0.84∗∗ 0.79∗∗ −0.87∗∗ 0.84∗∗ 0.82∗∗ −0.81∗∗

a Root length density.∗∗ Significant atP = 0.01%.

under saline waterlogged conditions than under salineconditions. Similar results have also been reported byAkhtar et al. (1994)andSaqib et al. (1999).

Soil compaction decreased the concentrations ofNa+, K+ and Cl− in all the treatments comparedwith respective non-compact treatments but the effectwas most prominent in the case of saline treatment.It shows that restricted root proliferation due to com-paction resulted in a less uptake of all ions undercompact saline conditions. However, the lower con-centrations of Na+ and Cl− in leaf were masked bythe adverse effect of soil compaction. Similar, effectsof soil compaction on uptake of some nutrient ionslike N, P and K have also been reported byIshaq et al.(2001a).

There was a significant increase in concentrationsof Na+, K+ and Cl− in leaves of both the wheat geno-types under compact saline-waterlogged treatment ascompared to compact saline treatment that may bedue to a relatively more root growth (Fig. 1) undersaline waterlogged conditions. However, K+:Na+ratio was decreased. The K+:Na+ ratio is a good in-dicator of salt tolerance of a plant but the decreasedK+:Na+ ratio in compact saline-waterlogged treat-ment in this study (Fig. 4) does not match with bettergrowth in this treatment as compared to compactsaline treatment. It shows that in a very complicatedstress environment K+:Na+ ratio cannot be a goodmarker of salt tolerance.

Root length density, and the concentration of K+and K+:Na+ ratio showed a highly significant positivecorrelation with grain yield and different yield com-ponents (Table 1; the original data of grain yield and

186 M. Saqib et al. / Soil & Tillage Research 77 (2004) 179–187

yield components has been reported in the compan-ion paper). However, Na+ and Cl− showed a highlysignificant negative correlation with grain yield, yieldcomponents and RLD. The RLD showed a highly sig-nificant positive correlation with the concentration ofK+ and K+:Na+ ratio.

Genotypic comparison under various treatmentsshowed a relatively better performance in terms ofRLD by Aqaab as compared to MH-97 under all thestresses particularly in saline and saline waterloggedtreatments under compact soil conditions, where thedifferences were significant. Also, Aqaab have highK+ and K+:Na+ and low Na+ and Cl−concentrationin its leaf sap. However, the treatment trends werevery similar in case of both the genotypes.

5. Conclusions

The effect of compaction on root growth and ionuptake was more severe under salt-affected soil con-ditions than under normal soil conditions. The effectwas also irrespective of the ions studied. On the otherhand, occasional waterlogging of a compact soil fora few days made the conditions of the study soilmore favorable for root growth both under non-salineas well as saline conditions. The comparison amongdata for root length density, ionic concentrations andgrain yield showed that reduced RLD is more re-sponsible for reduced yields under compact salineconditions than toxic leaf ion concentrations whileunder non-compact saline conditions ion toxicity ismore important. Also, the better performance of onegenotype over the other in stressed environments isrelated to maintenance of higher RLD and K+ con-centration and K+:Na+ ratio and low Na+ and Cl−concentration. Therefore, for a good crop stand onsalt affected soils, salt tolerant genotypes as well as,the cultivation machinery and methodology causingless compaction, should be adopted.

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