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PertanikaJ. Trop. Agric. Sci. 25(2): 131 -141 (2002) ISSN: 1511-3701© Universiti Putra Malaysia Press

The Exploitation of Root-sourced Signals to Reduce Irrigation and toRegulate Leaf Growth of Pepper Plants Capsicum annuum L.

MOHAMED HAMAD AWAD\ MOHD RAZI ISMAIL1, MARZIAH MAHMOOD2 &MAHMUD T.M MOHAMED1

department of Crop Science, 2Department of Biochemistry,Universiti Putra Malaysia, 43400 UPM Serdang, Selangor, Malaysia

Keywords: Water deficit, partial root drying, irrigation, pepper

ABSTRAK

Pengaruh pengeringan separa akar (PSA) terhadap pertumbuhan dan proses fisiologi tanaman cM, Capsicumannuum. L kultivar MCI2 telah dikaji dalam keadaan suhu dan kelembapan tinggi di iklim tropika. Potensiair daun, kadar fotosintesis, konduksi stomata dan kadar transpirasi Capsicum yang diberi rawatan PSA dalamjangka masa 10 hari adalah rendah dibandingkan dengan pokok yang diberikan air yang berterusan.Pembuangan akar yang terdedah kepada tegasan air menghasilkan pemulihan semula kesemua parameterpertukaran gas mengesahkan kehadiran signal dihasilkan terns dari akar yang mengalami tegasan air,Pertumbuhan daun dipengaruhi secara negatif melalui teknik PSA, juga menyebabkan pengurangan luas daunsepokok. Penghasilan pokok yang ditanam dalam keadaan PSA adalah 73 % dari jumlah hasil pokok yangdiberikan air berterusan manakala jumlah air yang diberikan kepada pokok PSA adalah separuh dari jumlahyang diberikan kepada pokok kawalan.

ABSTRACT

The effect of partial root drying (PRD) on growth and physiological responses of pepper plants Capsicumannuum L. cultivar MC12 was investigated under high temperatures and a humid tropical climate. The leafwater potential, photosynthetic rate, stomatal conductance and transpiration rate of Capsicum plants grownunder PRD over a period of 10 days were lower than those grown under well watered conditions. Removal of thedroughted roots resulted in resumption of all leaf gas exchange parameters confirming the presence of signalscoming directly from the droughted roots. Leaf growth rate was negatively affected by PRD techniques; as aconsequence leaf area per plant was reduced. Total fruit yield of PRD plants was about 73% of the control plants'fruit yield while the amount of water added to the PRD plants was half the quantity added to the controls.

INTRODUCTION

The control of the stomatal aperture is oftenparamount in the regulation of water balanceand it represents the first defence line againstadverse environmental stimuli such as water stress(Raschke 1975). A fall in water potential andturgor of the leaf tissue can result in stomatalclosure (Begg and Turner 1976). It is still notfully understood to what extent this closure isdue to a direct hydraulic effect on the guardcells or whether a signal within the leaf plays arole. Ample evidence indicates that shoot proc-esses such as growth rate and stomatal closureare controlled by signals from the root and thatthe root is capable of conveying this information

to the shoot in the form of a positive chemicalsignal such as an increase in ABA (Davies andZhang 1991; Davies et al 2000; Loveys et al2000). This suggests that the shoot growth andfunctioning may be modified as a function ofthe chemical signals synthesized in the root aftercontact with drying soil (Loveys et al 1984; 1991;1998; 2000).

Loveys and colleagues, who first exploitedthe use of PRD which generated from the splitroot of Gowing (1990), successfully reduced theneed for grapevine pruning without altering thebalance between leaf production and fruit yieldwhile the quality of the fruit produced wasgreatly increased. This experiment was designed

MOHAMED HAMAD AWAD, MOHD RAZI ISMAIL, MARZIAH MAHMOOD & MAHMUD T.M. MOHAMED

to study the effects of root to shoot signaling ofCapsicum plants when half of the root system isexposed to drying soil. It also attempts to deter-mine to what extent the technique of partialroot drying can be used to reduce the irrigationand to regulate containerized Capsicum plantgrowth under protective environmental agricul-ture.

MATERIALS AND METHODS

Experimental Treatment and DesignThe experimental materials used were pepperplants Capsicum annuum L., cultivar MC12. Thiscultivar was produced by Malaysian AgriculturalResearch Development Institute (MARDI). Pep-per plants were raised from seeds sown in peat-based compost under a shade house. Seedlingswere potted into seedling pots when their firsttrue leaf pair emerged and sown for a further 15days at 28-32°C daily temperatures without sup-plementary light. The seedlings were irrigateddaily to drip point with tap water and twice perweek with half strength Cooper solution (Cooper1976). Seedlings of similar size and vigor werethen selected and transferred to the shade housewith their root system washed free of peat. Theroot system was separated into approximatelytwo equal parts and the plants were re-pottedinto 2-liter pots which were refilled with a soilmixture of 3:2:3 (soil: sand: peat respectively). Acompound fertilizer (NPK) was added to theplants at a rate of 5 g per pot after transplanting.Plants were allowed 25 days to establish theirroot system in the new containers under well-watered conditions before treatments were ap-plied. The two pots were raised from the groundto prevent drained water from well-watered potsreaching the un-watered ones. The daily tem-perature of the shade house was 28-32°C with-out supplementary light. The relative humiditywas about 65% and 80% day and night. Afterseedlings established their roots in the new con-tainers, the seedlings were divided into twogroups: 1) control plants where the two potsreceived watering to drip point and 2) stressedplants in which both pots received watering todrip point at the beginning of the experimentand then roots in one pot remained withoutirrigation and allowed to dry the soil to about40% of the soil moisture content (Fig. 1). After10 days the previously dry pots (10 pots out of20) were watered to drip point and the previ-ously watered pots were allowed to dry the soil

moisture to about 40%. The cycle of the dryingperiod was 10 days. As for the other 10 plants,after the first partial root drying regime (10days), their roots in drying soil were severed andcut off the plants to investigate the physiologicalresponses of the plants. The rest of the plantscontinued the partial root drying cycle untilharvest. The experiment was arranged in a com-pletely randomized design and each treatmentwas replicated 20 times. Stomatal conductanceof fully expanded leaves was recorded with adiffusion prometer (AP-4, Delta-T Devices Ltd.,Cambridge). Leaf water potential was measureddaily on fully expanded leaves with a pressurechamber. Photosynthetic and transpiration ratesof plants' leaves were monitored using an LCA-3 portable infrared gas analyzer (It is an opensystem used with a Parkinson broad leaf cuwattewith leaf area of 6.2 cm2). The airflow rate was400 cnVmin1. Measurements were carried outon the same matured fully expanded leaves usedfor stomatal conductance. The expansion rateof newly expanding leaves was measured dailythroughout the experimental period on boththe well watered and PRD plants. A number ofthe flowers opened and flower abscise was re-corded daily. Abscission was confirmed whenflowers fell down or fell after a gentle touch.Mature fruits were harvested every 3 days and ateach harvest, the number of fruit, total yield offruit per plant and length of the fruit weredetermined.

RESULTS

Midday Leaf Water PotentialMidday leaf water potential was determined dailyin the 2 youngest fully expanded leaves of 5plants randomly selected from each treatmentfrom the onset of the PRD regime. Fig. 2 dis-plays the results of measurements over two weeksfrom the onset of the treatments in control andstressed plants (PRD) and after the removal ofthe droughted roots. The leaf water potential ofcontrol plants remained high throughout theperiod as compared to that of the PRD plants.The leaf water potential of PRD plants started todecline from the onset of the experimental treat-ment but no significant differences were ob-served during the first 5 days. The statisticallysignificant differences between the leaf waterpotential of the controls and the PRDs wereclearly observed from Day 6 to Day 10 of thetreatments. PRD plants exhibited a lower leaf

132 PERTANIKA J. TROP. AGRIC. SCI. VOL. 25 NO. 2, 2002

THE EXPLOITATION OF ROOT-SOURCED SIGNALS IN THE CULTIVATION OF PEPPER PLANTS

Fig. 1: Schematic representation of partial root drying technique, droplets represent watering. Dark color represents wellwatered and light color represents drying treatment

water potential compared to that of controls.This suggests that the PRD plants had under-gone a water deficit compared to the controls.After the removal of the roots growing in thedrying soil, the leaf water potential of PRDplants increased progressively and reached thevalues of the controls. Two days after the re-moval of the roots the leaf water potential ofPRD exceeded that of the controls.

Stomatal Conductance

The stomatal conductance of well-watered plantsand those of PRDs are presented in Fig. 3. Thestomatal conductance of well-watered controlsremained high throughout the experimentalperiod. The stomatal conductance of PRDsstarted to decline from day 4 of the treatment.On day 10 the value of the stomatal conduct-ance of PRD plants was about 44% of the con-trols. After the removal of the roots, stomatalconductance of the PRD plants increased and

reached a value of 84% of the control plants 2days later, but never reached the value of thecontrol plants.

Transpiration Rate

Transpiration rates of the Capsicum annuum Lcv. MCI 2 plants grown under control or PRDare presented in (Fig. 4). The transpiration rateof the control plants fluctuated during the pe-riod of the measurement but remained highthroughout the period as compared to that ofthe PRD plants. Three days after the onset ofthe treatment, the rate of transpiration startedto decline progressively and reached a value ofabout 37% of the control value on Day 9. Afterthe removal of the droughted roots transpira-tion rates of the PRD plants started to increasebut did not reach the value of the controlplants. The trend of the transpiration rate fol-lows that of the stomatal conductance (Fig. 3).

PERTANIKA J. TROP. AGRIC. SCI. VOL. 25 NO. 2, 2002 133


-0.4

-0.9

0 12 142 4 6 8 10

Days from start of treatment

Fig. 2: Midday leaf water potential of Capsicum plants cv. MCI2 grown under well-watered conditions (Control, closedcircle) or half of the root system grown under well-watered conditions and the other half allowed to deplete soil moisture

over weeks (PRD, open circle). Bars represent standard error of the means

250

removal of droughted roots


Fig. 3: Stomatal conductance of Capsicum cultivars MC12 with both halves of the root system grown under well-wateredconditions. Control plants (chsed circle) or plants grown with half of the root system in drying soil and the other half in

well-watered conditions. PRD (open circle). Bars represent standard error of the means

134 PERTANIKAJ. TROP. AGRIC. SCI. VOL. 25 NO. 2, 2002


4.5 -

1.0

0

Days from start of treatmentFig. 4: Transpiration rate of Capsicum cultivar MC12 grown under well-watered conditions (Control plants closed circle)

or plants grown with half of the root system in drying soil and the other half in well watered conditions (PBD plantsopen circle). Bars represent standard error

Photosynthetic Rate

The photosynthetic rate of the PRD plants re-mained similar to that of the controls up to Day5 of the treatment (Fig. 5). After Day 6 thephotosynthetic rate of the plants started to de-cline until it reached about 40% of the controlson Day 10. After the removal of the droughtedroots, the photosynthetic rate of the PRD plantsincreased successively but did not reach that ofthe controls.

Leaf Length Increment

The daily leaf length increment of control plantsand PRD plants are presented in Fig. 6. Thecontrol plants' daily increment in leaf growth isstatistically higher than that of the PRD plantson Day 5. Leaf length expansion rate of the PRDplants started to decline from Day 3, but thestatistically significant difference was observedon Day 5. The reduction in length growth rateof the PRD plants' leaves amounted to 12% ofthat of the control plants' leaves. The controlplants' leaves reached the highest growth rateon Day 7, after which the elongation rate startedto decline progressively, and reached a level of

growth plateauing between Day 12 and 13 fromthe start of treatments. On the other hand, theexpansion rate of the PRD plants' leaves startedto decline from Day 3 compared to that of thecontrols. On Days 8 and 9 no length incrementwas observed on the PRD plants' leaves. Afterthe removal of droughted roots of PRD plants,the leaves resumed the expansion growth and itwas significantly higher than that of the controlplants on Day 13.

Growth and Reproductive ResponsesTable 1 represents the responses of Capsicumannuum L. cv. MCI2 grown under well-wateredconditions and under partial root cyclic drying.Controls exhibited a higher total plant leaf areaas compared to those grown under PRD and thedifference between them after the final harvestwas statistically significant at a 0.05 level. Totalplant leaf area of the PRD plants was about 74%of the control plants suggesting that the PRDtechnique significantly affects the leaf area ofthe plants or that leaf growth of the PRD plantsis reduced which is reflected in the resultant leafarea.




Fig. 5: Photosynthetic rate of Capsicum cultivar MCI 2 grown either under well-watered conditions (control )closed symbols or plants grown with half of their root system in drying soil and the other half in well-watered

conditions (open symbols). Bars represent standard error

removal of droughted roots

-0.5


Fig. 6: Leaf elongation rate of Capsicum cultivar MC12 grown under well-watered conditions (control) closed symbols orhalf of the root system grown under soil drying and the other half under well-watered conditions (open symbols).

Bars represent standard error



TABLE 1Growth responses of Capsicum annuum L. cultivar. MCI 2 plants grown under well-watered (control) or half of the

root system under well-watered conditions and the other half allowed to dry soilmoisture over 10 days

Treatment

Control

PRD

Leaf area(cmVplant)

5350.45a(150)

4359.78b(138)

Plant height(cm)

83.70a(4.2, D.S)

76.52b(3.1)

No. offlowers /

plant

165.4a(16)

166.6a(13)

No. of flow-ers abscised/

plant

68.1a(8)

89.7b(6)

No. of fruitsper plant

46.1a(3.2)37.0b(2.6)

Total fruityield g/plant

143.9a(H.2)104.9b(8.7)

Fruit lengthcm

9.84a(0.5)8.94b(0.2)

Fruit set (%)

27.87a(3.2)

22.20b(4.1)

Data are means± standard error which are between two bracts (n=20).Means in the same column followed by similar letter are not significantly different at P<0.05 by least significantdifference (Lsd).

The higher total flower count produced perplant was produced by the PRD plants as com-pared to that of the controls but the differencebetween them was not significantly different at a0.05 level.

DISCUSSION

Shoot water status can regulate leaf expansionduring light periods, particularly when a low soilwater potential is coupled with a high evaporativedemand. The most likely control of leaf expan-sion in water-stressed plants would be exerted bysignals arising from root or mature leaves. How-ever, the regulation is complex and is not likelyto be due to the action of a single hormonealone (Stoll et al 2000). The results obtained inthis experiment revealed that PRD caused asignificant reduction in the leaf expansion ratewhich started to decline early after the applica-tion of the partial root drying technique, with-out any appreciable decline in leaf water poten-tial. This is probably due to the signals comingfrom the dried roots via the transpirationalstream. Those signals affect many physiologicaland biochemical processes in the shoot system(Awad et al. 2000) which associates peroxidaseactivity together with the elevated values of pHin the xylem sap of stressed pepper plants andplays a role in reducing the rate of leaf expan-sion during the time course of a water deficit.Similar findings were reported (Bacon et al1997; Wilkinson et al. 1997, 1998; Davies et al2000). The plant function that is most likely tobe affected by a water deficit is stomatal closurewhich can lead to a decrease in transpiration,leaf gas exchange and probably an increase inwater use efficiency, that is the dry matter pro-duced per unit of water transpired (During et al

1997). This may explain the reduced values ofphotosynthetic rates and transpiration of thePRD plants as compared to those of the con-trols. These results are in agreement with thefindings observed by Hussain et al (1999) andMullholland et al (1999) who suggested that arise in xylem sap pH plays a role on ABA whichin turn has a central role in the regulation of gasexchange of transgenics and mutant plants.

The concept of using PRD as a technique tocontrol water deficit responses originated fromthe observation that root-derived abscisic acidwas an important factor in regulating grapevinestomatal conductance (Loveys 1984; Gowing etal 1990) and in regulating shoot growth undercircumstances where vigour may be a disadvan-tage (Loveys 1991). The results obtained in thisexperiment showed that the shoot growth of thePRD plants could be significantly reduced byPRD. The reduction in leaf area of PRD plantsamounted to 20% of that of controls. Similarreductions were observed with the PRD plants(Davies et al 2000).

In this experiment the number of flowersproduced per plant in both controls and PRDwas similar, which means the flowering processwas sustained, but there were significant differ-ences in the total fruit yield per plant as PRDcaused a significant increment of abscised flow-ers. The PRD plants yield was 73% of that of thecontrols. When looking to the yield as a merefigure the differences are significant, but if werelate the yield obtained to the amount of wateradded during the growing season until harvest,the significant differences between the two treat-ments would diminish.

Large differences between leaf water poten-tial values of control plants and PRD plants were



observed in this experiment after 6 days. Thisreduction is probably attributed to the availabil-ity of water in the growing media; therefore thedifferences in the amount of irrigation water ledto the differences of media moisture which con-sequendy altered leaf water status. As a result ofan increase in the water deficit intensity in thedrying part, the leaf water potential of the PRDplants experiencing a water deficit was lower atmidday on Day 6 than during the first 5 days.After Day 6, the severity of the reduction inmedia water content of the PRD treatment plantscontainer was significantly higher compared tothat of control plants as PRD plants receivedhalf the amount of water added to the controlplant. Similar variations of leaf water potentialwere reported in field-grown sunflowers sub-jected to mild or severe water deficits (Dry et al1996; Tardieu et al 1996; Davies et al 2000) andpepper plants grown under partial root dryingtechniques (Gowing et al 1990).

The PRD technique resulted in a gradualreduction in stomatal conductance values ofstressed plants during the first 4 days. This re-duction was increased during the following daysuntil the dried roots were severed from theplants. The detachment of the dried roots re-sulted in an appreciable increment of the sto-matal conductance values of the PRD plants,which confirmed the existence of a chemicalsignal coming from the root system which al-tered the stomatal behaviour of the PRD plants.Similar findings were found to exist in previousresults (Dry et al 1996, 1999; Loveys et al 2000;Stoll et al 2000). This reduction may be attrib-uted to the ABA coming from the dried parts ofthe root system via the stream of the xylem sap(Gowing et al 1990; Dry and Loveys 2000).Although the stomatal conductance of the PRDplants after the removal of the dried roots didnot reach that of the control plants, this wasprobably due to the persistence of the signals'effects at the site of action. This reduction in thestomatal conductance value may give a goodexplanation for the rise in leaf water potentialfollowing the removal of the dried roots; thatstomatal conductance effectively governs the leafwater potential of the plants since it resulted insignificantly higher values of leaf water potentialof the PRD plants on Days 12, 13 and 14 com-pared to that measured in control plants. Thepresent results confirm previous observationswhich demonstrate that the photosynthetic rate

is reduced by water stress (Hsiao 1973). Therelationship between the photosynthetic rate andmidday leaf water potential is presented in Fig.7. It shows a considerable scatter in the photo-synthetic rate- midday leaf water potential. Highphotosynthetic rates (e^mmolmV) were re-corded at -0.75 MPa in the PRD plants beforeremoving roots from contact with drying soil,but it can be observed that even high - atrelatively low water potential (- 0.8 MPa) photo-synthetic rate can be reduced by as much as67% from the maximum value recorded. Thedecrease in the photosynthetic rate observed inwater-stressed plants has long been explained bythe reduction in the flow of CO2 from theexternal atmosphere, which results from partialor complete stomatal closure (Munns andCramer 1996). The close relationship obtainedwhen plotting photosynthetic rate against sto-matal conductance (Fig. 7) is in agreement withthe explanation and suggests that stomatal regu-lation effectively controls the water balance ofthe leaf at the expense of a lower photosyntheticrate. Differences in photosynthetic rates betweencontrol and PRD plants' leaves were associatedwith differences in leaf water potential suggest-ing that leaf water potential can be used as aparameter of plant water status in photosyn-thetic studies of pepper leaves. However, thepattern of scatters observed when plotting pho-tosynthetic rates against water potential suggestthat spot measurement of water potential andphotosynthetic rates should be handled withcare. Since stomata are the way by which carbondioxide enters the leaf and water vapour tran-spires, changes that a water deficit induces onstomatal opening affect CO2 and assimilation,and therefore plant growth. Under water deficitconditions, internal CO2 drops in the stomaticchamber thereby decreasing CO2 assimilation,Mansfield et al (1990). The partial closure ofthe stomata of PRD plants may be attributed tothe transient nature of ABA accumulation to-gether with pH gradients in the leaf tissueswhich control the ABA distribution in the leaf,and the ABA concentration at the primary site(s)of action at guard cell complexes, which theninfluences stomatal aperture and transpirationalwater loss (Bacon et al 1997; Dry and Loveys1996; Loveys et al 2000). ABA carriers at epider-mal plasma membranes, and the metabolism ofABA in the cytosol of mesophyll and epidermalcells are important for keeping the apoplastic



io H

6 ~

2

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6 H

£ 2

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-0.5 -0.6 -0.7

Midday leaf water potential (MPa)

-0.8

1Z ~

10 -

8 -

A 9 &

<>o

A D a'A

V i

80 100 i 20 140 160 180 200 220 240 260Stomatal conductance (mmolnrV1

Fig. 7: Relationship between photosynthesis, leaf water potential and stomatal conductance for young fullyexpanded leaves of Capsicum plants. Points represent measurements on different days over stress period.

Closed symbols represent control plants and Open symbols represent PRD plants.

ABA concentration in unstressed leaves low, andfor speeding up ABA redistribution under stress.As a result stomatal conductance value reducedand the transpiration rates of the plants under-going water deficit conditions declined. Thiscan probably explain the low values of stomatalconductance and transpiration observed in thePRD plants. It is worth stating that ABA is notthe unique candidate that can bring about thisaction, it is probable that other factors such asreduction in cytokinin concentration may alsoplay a role. Auer (1996) and Medford et al(1989) found that the ratio of ABA to cytokininthat occurs in roots of grape vine during thePRD cycle is altered and the cytokinin derivedfrom dried roots have great impact on stomatalconductance (Stoll et al. 2000).

CONCLUSIONWe concluded from this experiment that partialroot drying resulted in the transference of asignal from the root to the shoot system. As aconsequence, this signal modifies shoot growthand reduces stomatal conductance and transpi-ration rate. The amount of irrigation water addedto the partial root drying was half of that addedto the control plants, and this resulted in areduction in total yield obtained. The partialroot technique could be used in areas wherewater is a limited factor in yield production.

ACKNOWLEDGEMENTThis research was supported by the Ministry ofScience and Environment and Universiti PutraMalaysia with research grant IRPA 01-02-04-0507;

PERTANIKAJ. TROP. AGRIC. SCI. VOL. 25 NO. 2, 2002 139


IRPA 01-02-04-0082-EA001. Research work at theUniversity of Lancaster, United Kingdom waspartly funded by the British Council; P551 Envi-ronmental Stress Impact on Tropical Crops.

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(Received: 16 October 2001)(Accepted: 28 May 2002)

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