Impact of water stress on reproductive development in...

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H.Z. Jaafar, C.R. Black, J.G. Atherton and J.A. RobertsJ. Trop. Agric. and Fd. Sc. 30(1)(2002): 57–65

Impact of water stress on reproductive development in sweet pepper(Capsicum annuum L.) IV. Inhibition of CEPA-induced ethyleneproduction by silver thiosulphate[Kesan sesakan air terhadap perkembangan reproduktif cili sayur (Capsicum annuumL.) IV. Penghalangan pembebasan etilena yang teraruh oleh CEPA dengan argentumtiosulfat]

H.Z. Jaafar*, C.R. Black**, J.G. Atherton*** and J.A. Roberts**

Key words: sweet pepper (Capsicum annuum L.), flower abscission, ethylene production,ethylene action inhibitor, silver thiosulphate (STS), ethylene-releasing compound,2-chloroethylphosphonic acid (CEPA)

AbstrakPenyelidikan di dalam rumah kaca menggunakan pasu dijalankan untukmemperkukuh bahawa peningkatan pengeluaran etilena endogen akibat teraruholeh sesakan air memainkan peranan utama sebagai pengantara dalammerangsang pengguguran bunga. Kajian ini membabitkan penggunaan argentumtiosulfat (STS) untuk menyekat pengguguran akibat teraruh oleh etilena. Olehsebab bahan yang menjana etilena, asid 2-kloroetilfosfonik (CEPA), mimik kesanpeningkatan keguguran bunga akibat teraruh oleh sesakan air dan meningkatkanevolusi etilena sebelum pengguguran, maka menyembur kudup bunga denganSTS sebelum CEPA sepatutnya dapat menghalang keguguran bunga akibatteraruh oleh CEPA. Menyembur STS pada kepekatan 1.0 mM sehari sebelumCEPA sangat berkesan untuk menghalang keguguran bunga dari pokok yang telahdirawat dengan CEPA pada kepekatan 50 and 100 mg/liter. Penyemburan dengan1.0 mM STS juga memaparkan keberkesanan yang berterusan dengan menahanbunga daripada gugur walaupun setelah 40 hari disembur.

Pada masa yang sama, pokok yang disembur dengan CEPA dan tidakdilindungi dengan STS jelas mempamerkan peningkatan dalam keguguran bunga,yang meningkat mengikut masa dan kepekatan CEPA. Pokok yang dirawatdengan kepekatan 100 mg/liter CEPA dan tidak dilindungi oleh STSmempamerkan peratus keguguran bunga yang lebih tinggi (83% berbandingdengan 11%) dalam masa yang lebih pendek (iaitu 3 hari selepas disemburdengan STS). Kesemua bunga gugur selepas 2 hari kemudian. Pokok yangdisembur dengan 50 mg/liter CEPA pada mulanya mempamerkan peratuskeguguran bunga yang rendah (11%). Bagaimanapun, pokok akhirnya melaluipeningkatan keguguran daripada 50% hingga 94%, dalam masa 5–7 hari selepas

*Strategic Resources Research Centre, MARDI Headquarters, Serdang, P.O. Box 12301,50774 Kuala Lumpur, Malaysia

**Department of Plant Environmental Physiology, Sutton Bonington, Nr. Loughborough, Leicestershire,LE12 5RD, UK

***Department of Agriculture and Horticulture, Sutton Bonington, Nr. Loughborough, Leicestershire,LE12 5RD, UK

Authors’ full names: Hawa Z. Jaafar, Colin Black, Jeffery Atherton and Jeremy Robertse-mail: [email protected]©Malaysian Agricultural Research and Development Institute 2002

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Impact of water stress on reproductive development in sweet pepper

disembur dengan STS. Kesemua keputusan memberi bukti yang kukuh bahawaetilena menjadi pengantara dalam pengguguran bunga dan kudup bunga cili sayurakibat teraruh oleh sesakan air.

AbstractA glasshouse pot experiment was carried out to further establish that increasedwater stress-induced endogenous ethylene production has a major role inmediating the induction of flower abscission. The work involved the use of silverthiosulphate (STS) to arrest ethylene-induced abscission. Since ethylene-generating substance, 2-chloroethylphosphonic acid (CEPA), mimicked theeffects of increased water stress-induced flower abscission and increased ethyleneevolution preceding abscission, pre-treating flower buds with STS prior to CEPAapplication should provide protection against CEPA-induced flower abscission.Administering STS at 1.0 mM one day prior to CEPA application was markedlyeffective in preventing flower abscission in CEPA treated plants at bothconcentrations of 50 and 100 mg/litre. The STS treatment also demonstrated apersistent efficacy by retaining all flowers from the 100 mg/litre CEPA treatmenteven after 40 days of treatment with STS.

During the same period, plants sprayed with CEPA and not protected withSTS exhibited a marked increase in percentage abscission, which increased withtime and with CEPA concentration. Plants treated with 100 mg/litre CEPA andnot protected with STS exhibited higher percentage of flower abscission (83% vs11%) within a shorter period of time (i.e. 3 days after STS treatment).Subsequently complete abscission occurred 2 days later. Plants sprayed with 50mg/litre CEPA initially exhibited a lower percentage of flower abscission (11%).However, with time the plants ultimately experienced an increased abscissionfrom 50% to 94%, which occurred within 5–7 days after STS application. Theresults provide strong evidence that ethylene mediate stress-induced abscission offlowers and buds in sweet pepper.

IntroductionProgressive imposition of severe andprolonged water stress on youngreproductive plants induced early andincreased flower abscission in sweet pepper(Jaafar et al. 1994; Jaafar 1995, 2001).Reductions in the availability ofphotoassimilates due to water stress hasbeen suggested as the cause of thisabscission (Peltonen-Sainio 1991). However,no consistent reduction in total shoot drymatter was observed by Jaafar et al. (1994,1998). The observed decline in turgorshortly before flower abscission also couldnot be related directly to flower abscissionor shoot dry matter production. Thissuggests that other changes occurred much

sooner and at lower stress levels than thoseassociated with turgor loss (Hsiao 1973).

A role for ethylene in stress-inducedabscission in other species is welldocumented (Halevy and Mayak 1981;Durieux et al. 1983; Sexton et al. 1985;Ohno 1991; Abeles et al. 1992). Whileethylene production has often been found toincrease following the imposition of waterstress or drought in many species, ultimatelyresulting in leaf abscission (Jordan et al.1972; McMichael et al. 1972; El-Beltagyand Hall 1974; Apelbaum and Yang 1981;Kimmerer and Kozlowski 1982; Kirkham1985), very few studies have examined therole of drought-induced ethylene productionin flower abscission in horticultural crops.

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H.Z. Jaafar, C.R. Black, J.G. Atherton and J.A. Roberts

Jaafar et al. (1998) in their work onsweet pepper observed a dramatic increasein tissue ethylene production precedingflower abscission induced by the impositionof water stress. Later, Jaafar et al. (1999)through the application of an ethylene-releasingcompound, 2-chloroethylphosphonic acid(CEPA), confirmed that exogenous ethylenealso increased ethylene evolution from theabscinding buds. Similar effects had beendescribed by others (Kays and Beaudry1987; Furutani et al. 1989; Mason andMiller 1991; Abeles et al. 1992). The workby Jaafar et al. (1998, 1999) so far, stronglysuggested that ethylene may mediate theimpact of water stress on flower abscission.Recent work by Jaafar et al. (2000)established that ethylene has a major role inpromoting flower or bud abscission in waterstressed sweet pepper, a suppositionsupported by research works involving theuse of silver thiosulphate (STS) to blockwater stress-induced ethylene production. Itwas observed then that foliar application ofSTS at 1 mM one day after imposing waterstress proved highly effective in blockingthe increase of endogenous ethyleneevolution and thus prevent water stress-induced flower abscission.

The involvement of endogenous andexogenous ethylene in flower abscissionfollowing the imposition of stress or theapplication of ethylene-generating sprays iswell established (Halevy and Mayak 1981;Sexton et al. 1985; Abeles et al. 1992; Jaafaret al.1998, 1999). Although ethyleneevolution increased significantly (p <0.05)in severely stressed plants (Apelbaum andYang 1981; Kimmerer and Kozlowski 1982;Kirkham 1985; Abeles et al. 1992), nosimilar promotion of ethylene evolution wasobserved in plants treated with STS(Cameron and Reid 1983; Dostal et al. 1991;Jaafar et al. 2000). Silver complexed withthiosulphate (STS) is known to inhibit theaction of ethylene competitively (Veen 1983,1986) by blocking binding sites for ethylene.It (STS) combined with the ethylenereceptor, thereby preventing the cells from

responding to ethylene (Veen 1986; Abeleset al. 1992). Unlike silver nitrate, anotherchemical known for its ethylene blockingcharacteristic (Beyer 1976), STS isextremely mobile within plant, is lessphytotoxic at effective concentrations (Veenand Van De Geijn 1978) and remains activewithin plant tissues for extended periods(Reid et al. 1980).

The protective influence of STS againstflower abscission has been reportedpreviously (Cameron and Reid 1983; Dostalet al. 1991), but only a few studiesconcentrated on water stress. Recent workby Jaafar et al. (2000) has shown thatethylene was involved in mediating flower-buds abscission as a result of severe waterstress, and that STS was effective inpreventing this water stress-inducedabscission by blocking the act of increasedethylene evolution resulted from severewater stress. If ethylene is truly involved inthe water stress-induced abscission, thenpre-administering STS to plants prior to anapplication with ethylene-releasingcompound CEPA should result in similarprotective effect. The main aim of theexperiment was to further establish thatincreased endogenous ethylene production,resulted from the application of ethylene-generating substance, 2-chloroethylphosphonicacid (CEPA), has a major role in mediatingthe induction of flower abscission and thatSTS could provide partial or completeprotection against CEPA-induced flowerabscission in sweet pepper.

Materials and methodsPropagation and seedling managementThe seeds of Capsicum hybrid variety BlueStar (Know-You Seed Company, Taiwan)were sown thinly in flat trays containingLevington F2 compost (Fisons HorticultureLtd. Ipswich, UK). The trays were placed ona propagating bed, which provided a basaltemperature of 24 °C with a glasshouseambient temperature of 20–22 °C. Uniformseedlings of fully expanded cotyledons werepricked out into 9-cm pots containing

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Levington M2 potting compost. The potswere placed on benches receiving naturallighting, which were supplemented with400 W high-pressure sodium lamps (SON/T)when the third pair of true leaves of theseedlings measured about 1 cm long.Supplemented lighting was operatedbetween 0500 h and 2300 h to provide an18-h daylength and additional irradiance of2.5 MJ m2 of PAR (photosynthetic activeradiation: 400–700 �m). The mean dailytemperature of the glasshouse was alsoincreased to 26 ± 3 °C with ventilation set at29 °C. The seedlings were watered everymorning throughout the growing period tomaintain optimum growth.

Preparation of STS solutionThe STS solution was prepared as describedby Reid et al. (1980) and Jaafar et al.(2000). The STS was prepared on the day ofthe application by mixing known volumes ofstock solution of silver nitrate (0.1 M) andsodium thiosulphate (0.1 M) (FisonsScientific Apparatus, Loughborough, UK)with deionised water to produce aconcentration of 1.0 mM. Silver nitrate wasthen added slowly to sodium thiosulphatesolution in the ratio of 1:4 to avoid theformation of a black precipitate of silversulphide (Ag2S; Cameron et al. 1985). Themixing of these aqueous solutions shouldresult in essentially complete formation ofthe STS complex, implying stability withoutdissociation into silver sulphide (Ghosh1974).

Preparation of CEPA solutionTwo concentrations of CEPA solution, 50and 100 mg/litre, were prepared followingJaafar et al. (1999). The solution wasprepared on the day of the experiment bydiluting the substance with distilled waterand stored in a refrigerator (4 °C) betweensprayings. Approximately 10 mL of thechemical was applied daily directly onto thesecondary flower buds when they reachedthe stage that is most sensitive to waterstress (diameter 4 mm). This treatment has

been found to induce early flower abscissionin pepper (Tripp and Wien 1989; Jaafar1999).

Flower countsPeriodic flower counts were carried out onthe 3rd, 5th and 7th day after STSapplication to assess the effectiveness ofSTS in preventing flower abscission.Protocol for flower counts was described byJaafar (1995, 2001). The measurementsincluded total number of flowers producedand number of flowers abscinded.

TreatmentsThe treatments were imposed 61 days aftergermination when secondary flower buds(flowers borned at the second branchingposition) had reached a diameter of 3.5– 4.0mm. The treatment lasted for 10 days. Thebuds were first sprayed with 10 mL of STSand 1 day later they were covered with freshcotton wool balls and sprayed with 10 mLof CEPA or distilled water in the control.The flower buds were covered with cottonballs (each pre-weighed at 500 mg), whichremained on the flower buds throughout thetreatment period. This was to ensure goodcontact with the chemical, except whencarrying out flower counts. Both chemicalswere reapplied in the same way 3 days later.

The STS was administered toindividual developing buds since theeffectiveness of STS to block the act ofethylene was found similar to that applied tothe whole plant system (Cameron and Reid1983). An application rate of STS at 1.0 mMwas found to be the most suitable andeffective concentration used without anyphytotoxic symptom (Jaafar 1995; Jaafar etal. 2000). At a higher concentration of 5.0mM, however, STS caused crinkling of theleaves and left black specks on the flowerbuds (Jaafar, H.Z., Inhibition of ethyleneaction by silver thiosulphate, University ofNottingham, UK, unpublished 1995).Phytotoxicity at a higher concentration ofSTS had also been reported by Cameron andReid (1981), Wang and Dunlap (1990) and

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Dostal et al. (1991). Thus, the fivetreatments applied comprised two levels ofCEPA (50 and 100 mg/litre applied in 10mL aliquots), with and without STS (0 and1 mM in 10 mL aliquots) and a controlsprayed with distilled water. Each treatmentcontained 12 plants.

Experimental design and statistical analysisThe treatments of factorial 2 x 2 and acontrol were arranged in a RandomisedComplete Block Design (RCBD) blockedthree times as in Jaafar (1995). Data wereanalysed using the Analysis of Variance(ANOVA).

ResultsFlower abscissionTable 1 clearly demonstrates the inhibitoryeffect of STS application on ethylene-induced flower abscission. Application ofSTS at 1.0 mM completely inhibited flowerbud abscission at both levels of CEPA(p <0.001) at all three sampling dates.During the same period, plants sprayed withCEPA and not protected with STS exhibiteda marked increase in percentage abscission,

which increased with time and with CEPAconcentration.

Plants treated with higher CEPAconcentration (100 mg/litre vs 50 mg/litre),and not protected with STS exhibited higherpercentage of flower abscission (83% vs11%) within 3 days after STS treatment.Subsequently, complete abscission occurred2 days later. Although plants sprayed withlower CEPA concentration initially exhibiteda lower percentage of flower abscission(11% on 3rd day after STS treatment), withtime they ultimately experienced increasedabscission to 94%, which occurred 7 daysafter STS application. The results providestrong evidence that ethylene may mediatestress-induced abscission of flowers andbuds.

Visual flower retentionAn application of STS at 1.0 mM waseffective in preventing CEPA-inducedflower abscission even at a higherconcentration of 100 mg/litre. Visual flowerretention over time as a result of STSapplication was dramatically featured inPlate 1 and Plate 2. At 40 days afterapplication of STS followed by CEPA, no

Table 1. Inhibitory effect of silver thiosulphate (STS) on flower abscission inducedby 2-chloroethylphosphonic acid (CEPA; n = 18)

CEPA (mg/litre) STS (mM) Flower abscission (%)

Days after STS application

3 5 7

Control 0 0 0

50 0 11 50 941.0 0 0 0

100 0 83 100 1001.0 0 0 0

SED(Control) ns 0.32*** 2.77***

SED(Control*CEPA) 6.09*** 0.36*** 3.03***

SED(Control*STS) 6.09*** 0.36*** 3.03***

SED(Control*CEPA*STS) 7.03*** 0.41*** 3.50***

*significant at p <0.05**significant at p <0.01***significant at p <0.001ns = not significantSED = Standard Error of the Difference between means

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flower or bud abscission was observed at all.This implies that STS at a concentration of1.0 mM is very effective in retaining allsweet pepper flowers and buds that havebeen applied with 100 mg/litre of ethylenereleasing compound, CEPA, and that theSTS efficacy persisted for more than amonth without any show of phytotoxicsympthom. Plants, which have not beenprotected with STS (Plate 3), however,featured a complete flower abscission afterCEPA application.

DiscussionThe involvement of endogenous andexogenous ethylene and their roles in flowerabscission following the imposition of stressor the application of ethylene-generating

Plate 1. Treatment with 1.0 mM STS preventsflower abscission in plants sprayed with100 mg/litre CEPA. At 28 days after STSadministration followed by CEPA application, theplant showed normal reproductive growth anddevelopment. Arrows show the position of well-retained secondary flowers which have beenprotected with STS spray

sprays are well established (Halevy andMayak 1981; Sexton et al. 1985; Abeles etal. 1992; Jaafar et al. 1998, 1999). In recentstudies, Jaafar et al. (1998) observed adramatic increase in tissue ethyleneproduction preceding flower abscissioninduced by the imposition of water stress.Later, Jaafar et al. (1999) confirmed thatexogenous ethylene through the applicationof CEPA, an ethylene-releasing compound,also increased ethylene evolution just priorto bud abscission and hence mediated theimpact of water stress on flower abscission.Jaafar et al. (2000) confirmed the role ofincreased endogenous ethylene production inmediating the induction of flower abscission

Plate 2. A CEPA treated plant at 100 mg/litrewhich had been pre-treated with STS at 1.0 mMshowing effective retention of flowers from theCEPA-induced flower abscission and normalreproductive and vegetative growth at 40 daysafter treatment. Arrows show the position of well-retained flowers and some that have developedinto fruits from a profusely flowering plant,implication of a good and persistent coveragefrom STS

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by water stress, from their work using STSto block ethylene-induced abscission. Foliarapplication of STS one day after imposingwater stress proved highly effective inblocking abscission without signs ofpromotion of ethylene evolution as inseverely stressed plants.

Silver thiosulphate can competitivelyinhibit the action of ethylene (STS; Veen1983, 1986) by blocking the binding sitesfor ethylene. The STS combined with theethylene receptor and thereby prevent thecells from responding to ethylene (Veen1986; Abeles et al. 1992). Many previousstudies have observed the protectiveinfluence of STS application against stress-induced flower abscission. The impact ofSTS in reducing, preventing or increasingabscission depends on the concentrationapplied (Cameron and Reid 1981, 1983;Dostal et al. 1991; Jaafar et al. 1999).Application of 4 mM STS on zygocactusplants (Schlumbergera truncata) was seen toproduce occasional phytotoxic symptoms ofblistering of the leaves which eventually

subsided to form dark depressions, whereasat concentrations lower than 2 mM providedonly partial protection against ethylene- andstress-induced abscission (Cameron andReid 1981).

Jaafar et al. (2000) showed that foliarapplication of STS at 1.0 mM was highlyeffective in blocking the increase ofendogenous ethylene evolution and thusprevent flower abscission in severelystressed plants from day 6 onwards. Theabscission of sweet pepper flowers exposedto gradual severe water stress for 10 daysincreased very markedly to 53.3% followedby a significant increase in endogenousethylene evolution of 0.89 ηL/g FW/h.Conversely, flowers from severely stressedplants treated with 1.0 mM STS sprayexhibited complete flower retention.

Cameron and Reid (1983)demonstrated a marked effect of STS inpreventing the abscission of floral organsand flowers in potted flowering plants. Petalabscission from geranium flowers(Pelargonium hortorum Bailey) exposed to

Plate 3. A CEPA treated plant at 100 mg/litre whichhad not been pre-treated with STS showingcomplete abscission of secondary and other flowersat 40 days after treatment. At 100 mg/litre, CEPAwas capable of promoting a continuous completeflower and bud abscission over a long period oftime after application. Arrows show positions of theabscinded flowers

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continuous light at 25 °C was completelysuppressed by a 0.5 mM foliar spray of STS,while a similar treatment reduced flowerdrop from 83% to 22% in Calceolariaherbeohybrida Voss plants exposed to 4days of drought in darkness at 25 °C. Earlierwork by Cameron and Reid (1981) showedthat foliar application of STS produced80–90% retention of flowers and buds inzygocactus plants stressed by exposure toethylene at 26 °C and in darkness for 4weeks after application. These treatmentswould otherwise have induced completeabscission of buds, flowers and leaflets.

Similar to water stressed, plants pre-treated with STS showed no abscissionresponse to CEPA (Cameron and Reid 1981;1983; Jaafar et al. 2000). Protectiveinfluence of STS against stress-inducedflower abscission was also observed whenEaster lily was sprayed with ethephonfollowing treatment with STS and thenexposed to a 92% reduction in irradiance for14 days (Mason and Miller 1991). Theeffectiveness of the STS spray persisted forabout 2 weeks after treatment. Theapplication of exogenous ethylene in theform of CEPA mimicked the effects of waterstress by inducing increased flowerabscission and increased ethylene evolutionpreceding abscission (Jaafar et al. 1999). Itwas clearly demonstrated in the presentstudy that CEPA-induced flower abscissioneven at a higher concentration (100 mg/litre)could be successfully arrested by theapplication of STS at 1.0 mM. The efficacyof the STS persisted for more than a monthwithout any symptoms of phytotoxic on theplants. These results suggest that the STSapplications have great potential forreducing stress-induced abscission in pottedflowering plants, including sweet pepper. Itacts by blocking the act of stress-inducedethylene production. STS was also shown toremain active within plant tissues forextended periods as demonstrated by itseffectiveness to retain flowers even at 40days from the start of the treatment asdepicted in Plate 2, as had been observed by

Reid et al. (1990). The effectiveness of STSagainst CEPA-induced flower abscissionthus further confirms the role of ethylene inmediating the impact of water stress onflower abscission in sweet pepper.

AcknowledgementThis research was sponsored by theMalaysian Agricultural Research andDevelopment Institute (MARDI).

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Accepted for publication on 17 January 2002

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