Effect of Ethylene and Ethylene Precursors on Protein Phosphorylation and Xylogenesis in Tuber Explants of Helianthus tuberosus (L.)

Koritsas, V. M. 1988. Effect of ethylene and ethylene precursorson protein phosphorylation and xylogenesis in tuber explantsof Helianthus tuberosus (L.).J. exp. Bot. 39: 375–386. The role of ethylene in protein phosphorylation and xylem-celldifferentiation was studied in explant cultures of Jerusalemartichoke, Helianthus tuberosus (L.). Explants on xylem-inducingmedium growing at 25 °C developed more xylem elements andproduced more ethylene gas within 3 d of culture than controlcultures growing and developing at similar rates. L-methionineand 1-aminocdopropane-1-carboxylic acid (ACC) incorporated intoboth xylem-inducing and control media enhanced xylem differentiation,increased protein levels and stimulated protein kinase activityin the explants. Inhibitors of the ethylene biosynthetic pathway,cobalt nitrate and aminoethoxyinyl glycine (AVG), depressedxylem differentiation, protein phosphorylation and polypeptidesynthesis. Ethylene gas or ACC reversed the effects of the inhibitors.Ethylene production, protein phosphoryilation and xylem-celldifferentiation were all linked. Ethylene thus promotes xylem-celldifferentiation in Jerusalem artichoke explants, the processprobably being regulated by protein phosphorylation. Key words: Ethylene, phosphorylation, xylem differentiation, Jerusalem artichoke explants     

Stimulatory and Inhibitory Effects of Sucrose Concentration on Xylogenesis in Lettuce Pith Explants; Possible Mediation by Ethylene Biosynthesis

Numbers of tracheary elements differentiating in lettuce pithexplants rose with increase in concentration of sucrose in themedium up to an optimal concentration of 0·2%, and fellwith further increase in concentration to about one-tenth maximalat 3% sucrose. Although a few tracheary elements formed withoutexogenous sucrose, a very low concentration of sucrose (0·001%)was sufficient to stimulate additional xylogenesis. Pretreatmentof explants with 3% sucrose caused a persisting inhibition ofxylogenesis, especially in tissue that had been near the siteof sucrose application (sandwich technique). The requirementfor adequate, but not inhibitory, concentrations of sucrosefor xylogenesis may underlie the development of xylem alongsidethe sucrose-rich phloem in normal apical morphogenesis. For callus growth the response to sucrose was different: theoptimal concentration was 3%, with a broad plateau from 1 to4% sucrose. Sucrose concentrations of 2 to 3%, used in manytissue culture media, are thus roughly optimal for callus growth,but ten times the optimum for xylogenesis in lettuce pith explants. It is surprising that 0·001% (0·03 mM) sucrose,applied exogenously, can stimulate xylogenesis: endogenous sugarconcentrations are normally higher. Perhaps the stimulationis mediated by ethylene biosynthesis, which is known to be xylogenic.Rates of ethylene production per explant rose with increasingsucrose concentration from about 0·1 nl h^-1 at 0% sucroseto a slightly (significantly) higher level at 0·004%sucrose and to about 0·5 nl h^-1 at 3% sucrose. D -glucoseresembled sucrose in its effects on xylogenesis and ethyleneproduction, but L-glucose yielded no xylogenesis and littlestimulation of ethylene biosynthesis.Copyright 1994, 1999 AcademicPress Lactuca sativa, Coleus blumei, Nicotiana tabacum, lettuce pith explants, tracheary element differentiation, sucrose, glucose, ethylene     

Regulation of Tracheary Element Differentiation by Exogenous L-Methionine in Callus of Soya Bean Cultivars

The relationship between the induction of tracheary elementdifferentiation and exogenous L-methionine was examined in agar-growncultures of soya bean callus initiated from Glycine max L. ‘Wayne’and ‘Clark 63’. Although Wayne is a normal cultivarsoya bean, seedlings of Clark 63 exhibit abnormal growth at25 °C due to exessive ethylene biosynthesis at this temperature.Wayne callus showed increased xylogenesis in the presence ofexogenous L-methionine (3.7 µg 1^–1) in comparisonto IAA–KN controls at both 20 and 25 °C. Clark 63callus produced greater numbers of tracheary elements in responseto exogenous L-methionine only at 25 °C. The induction ofxylem differentiation was independent of the maintenance temperatureof the stock cultures of both cultivars. Xylogenesis initiatedbyan IAA–KN medium was inhibited by the addition of AgNO₃(20 mg 1^–1) to the extent of 76.5 per cent in cv. Wayneand 6 per cent in cv. Clark 63. The inhibitory effect was partiallyreversed by the addition of L-methionine (3.7 µg 1^–1)to the IAA–KN–AgNO₂ medium. These data support thehypothesis that xylogenesis in vitro involves auxin, cytokininand ethylene. differentiation, xylogenesis, L-methionine, ethylene, Glycine max L., soya bean, callus culture, auxin, kinetin     

Peroxidase activity and lignification in soybean root growth-inhibition by juglone

The changes in activities of soluble and cell wall-bound peroxidases and lignin contents in juglone-stressed soybean (Glycine max) seedlings and their relationships with root growth were investigated. Soybean seedlings (3-d-old) were cultivated in nutrient solution supplemented with 0.5 to 25 μM juglone for 24 h. Length and dry mass of roots decreased after 5 to 25 μM juglone treatments. Low juglone concentrations (≤ 1 μM) increased soluble peroxidase activity, while high concentrations (≥ 10 μM) inhibited activities of soluble and cell wall-bound peroxidases. Juglone (≤ 1 μM) did not affect lignin content but highly increased lignification after 5 to 25 μM treatments. Results indicate that lignification may be an important step in root growth reduction of juglone-stressed soybean.     

Ethylene Biosynthesis and Xylogenesis in Lactuca Pith Explants Cultured In Vitro in the Presence of Auxin and Cytokinin: The Effect of Ethylene Precursors and Inhibitors

The possible involvement of ethylene in the induction of xylemdifferentiation was studied in lettuce (Lactuca saliva L. cv.Romaine) pith parenchyma explants. The addition of the ethyleneprecursors L-methionine (0.25 µM), S-adenosylmethionine(25 µM) and 1-aminocyclopropane-l-carboxylic acid (0.01µM), or the ethylene-releasing agent 2-chloroethylphosphonicacid (1.0 µM), to a standard IAA-kinetin-containing mediumenhanced xylogenesis compared to control explants cultured inthe absence of these compounds. In the presence of the ethyleneinhibitors aminoethoxyvinylglycine, Co(NO₃)₂ and AgNO₃, xylogenesiswas inhibited. Inhibition of xylogenesis by aminoethoxyvinylglycine(75 µM), Co(NO₃)₂ (50 µM) and AgNO₃ (6.0 µM)was reversed by exogenous 1-aminocyclopropane-l-carboxylic acid(0.01 µM), 2-chloroethylphosphonic acid (5.0 µM)and L-methionine (0.25 µM), respectively. Ethylene productionby explants cultured on media containing L-methionine or 1-aminocyclopropane-l-carboxylicacid was greater than the biosynthesis of ethylene by explantscultured in the absence of these compounds. The incorporationof 2-chloroethylphosphonic acid into the culture medium resultedin higher rates of ethylene production compared to explantscultured on the IAA-kinetin medium. The presence of either aminoethoxyvinylglycineor Co(NO₃)₂ inhibited ethylene production by explants culturedon the IAA-kinetin medium. The data support the hypothesis thatethylene plays a positive role in the initiation of xylem differentiation. Key words: Xylogenesis, Differentiation, Ethylene, IAA, Kinetin, Lactuca sativa     

Effects of Growth Regulators on Polyamine Content and Peroxidase Activity in Hevea brasiliensis Callus

3, 4-dichlorophenoxyacetic acid (3,4-D) and benzylaminopurine(BAP) at 9 µM (control medium) was compared with 4.5,2.25, and 0.45 µM for ability to induce callogenesis andembryogenesis from seed explants of Hevea brasiliensis. Supplyingthese growth regulators at 4.5 µM for 20 d improved embryogenicpotential compared with the control medium (El Hadrami, Carronand d'Auzac, 1991, Annals of Botany 67, 511–515), sustainedputrescine, spermidine and spermine at a higher level throughoutof much of the culture period (40–70 d), and maintainedlow levels of peroxidase activity. In the control medium, poorcallus embryogenesis is considered a consequence of rapid ageingof tissues characterized by (i) acceleration of an early buttransient production of polyamines, which promoted embryogeniccapacity, and (ii) an early peak in peroxidase activity thatwas positively correlated with callus browning, one of the factorslimiting embryogenesis. Somatic embryogenesis, polyamines, peroxidase, Hevea brasiliensis, rubber-tree     

Changes in the Activities of Various Glycosidases during Carrot Cell Elongation in a 2,4-D-Free Medium

Changes in the activities of some glycosidases were studiedin carrot suspension cultures with and without 2,4-D. Remarkablecell elongation occurs in a medium without 2,4-D, while fewcells elongate in a medium containing it. Glycosidases werefractionated into soluble, ionically wall-bound, tightly wall-boundand extracellular enzymes. The optimum pHs of all the ionicallybound glycosidases were in an acidic range, 4.4–5.0. The activities of the ionically and tightly bound ß-xylosidasesand ß-galactosidases were higher in elongating thanin non-elongating cells. Furthermore, the activities of theseenzymes increased with cell elongation during culture, suggestingthat they may play important roles in cell elongation. Higheractivities of soluble and cell wall-bound ß-glucosidaseand -mannosidase were found in non-elongating rather than inelongating cells. The activities of all soluble glycosidasesexcept ß-xylosidase were also higher in non-elongatingcells. Only ß-xylosidase and ß-galactosidaseactivities were detectable in the medium of the elongation culture. ¹ Present address: Department of Agricultural Chemistry, ObihiroUniversity of Agriculture and Veterinary Medicine, Obihiro,Hokkaido 080, Japan.     

Polyamines in Relation to Cell Division and Xylogenesis in Cultured Explants of Helianthus tuberosus: Lack of Evidence for Growth-Regulatory Action

Phillips, R., Press, M. C. and Eason, A. 1987. Polyamines inrelation to cell division and xylogenesis in cultured explantsof Helianthus tuberosus: lack of evidence for growth-regulatoryaction.—J. exp. Bot. 38: 164–172. The polyamines spermidine, diaminopropane, and cadaverine werefound to accumulate in cultured tuber explants of H. tuberosus(Jerusalem artichoke). Rapid increases in all amines occurredduring the initial 24 h corresponding to the period of activationand the onset of mitosis. Levels then declined during the followingphases of rapid cell proliferation and xylem differentiation.The type and distribution of polyamines was not markedly affectedby changes in medium or culture conditions, and the inhibitorMGBG did not alter cell division rates or polyamine contentmarkedly although xylem differentiation was substantially depressed.Exogenously supplied spermidine and putrescine did not substantiallyalter the cellular responses of explants cultured in the presenceof auxin. In the absence of supplied auxin, spermidine at 1?0mol m^–3 produced an increase in cell division, althoughthis was small in comparison with auxin-stimulated responses.The implications of these findings on the possibility that polyaminesact as growth regulators in plants is discussed. Key words: Polyamines, Jerusalem artichoke, cultured explants, cell division, xylem differentiation     

Xylem Development in the Gynoecium of the Apple (Malus pumila L.) Cv, Cox's Orange Pippin

Lignification of the xylem within the carpellary bundles ofapple flowers spreads acropetally from a point 900–1400µm below the base of the locules. At the same time, anotherwave of lignification spreads basipetally from a point justbelow the stigma. The acropetal spread at first progresses morequickly, but at later stages the number of lignified xylem elementsjust below the stigma increases rapidly, reaching a peak justas the flower opens. This increase is very localized and thenumber declines greatly within only 25% of the stylar distancebelow the stigma. Lignification of the xylem in the bundlesserving other flower parts precedes that serving the gynoecium,and spreads basipetally from a point above the base of the locules. Malus pumila L, anatomy, apple, carpel, Cox's Orange Pippin, development, flower, gynoecium, pedicel, pistil, stigma, style, vasculature, xylem     

Xylem Development in the Gynoecium of the Apple (Malm pumila L.) Cv. Cox's Orange Pippin K. A. D. *J. COSTA TURA{dagger}

Lignification of the xylem within the carpellary bundles ofapple flowers spreads acropetally from a point 900–1400µm below the base of the locules. At the same time, anotherwave of lignification spreads basipetally from a point justbelow the stigma. The acropetal spread at first progresses morequickly, but at later stages the number of lignified xylem elementsjust below the stigma increases rapidly, reaching a peak justas the flower opens. This increase is very localized and thenumber declines greatly within only 25 % of the stylar distancebelow the stigma. Lignification of the xylem in the bundlesserving other flower parts precedes that serving the gynoecium,and spreads basipetally from a point above the base of the locules Malus pumila, L., anatomy, apple, carpel, Cox's Orange Pippin, development, flower, gynoecium, pedicel, pistil, stigma, style, vasculature, xylem regenreation     

Tracheary Element Differentiation Induced in Isolated Cylinders of Lettuce Pith: a Bipolar Gradient Technique

To study the influence of morphogenetic gradients on vasculardifferentiation patterns, a new technique was developed whichallows different substances to be applied at opposite ends ofa tissue block. It yielded information on the mobility of particularmorphogens and on the dependence of callus formation and trachearyelement differentiation on their presence. Application of indol-3ylacetic acid (1AA) (10 mg l^–1), zeatin (0.1 mg l^–1)and sucrose (3 per cent, w/v) in various combinations to theends of cylindrical explants of lettuce pith (Lactuca sativaL.) showed that (a) callus formation was stimulated by IAA,whereas induction of tracheary elements required both IAA andzeatin; (b) callus was confined to a few millimetres at theends of the explants, and tracheary elements occurred mainlywithin the callus; (c) sucrose or its metabolic products diffusedthe 10 mm length of the explants, while IAA and zeatin wereeffective only close to the application site; and (d) some callusand tracheary elements formed when no sucrose was applied, butboth increased with sucrose application, though inhibition oftracheary elements formation occurred with high sucrose concentrations. differentiation, pith explant, tissue culture, xylogenesis, indol-3yl acetic acid, sucrose, zeatin, lettuce, Lactuca sativa     

The Influence of Apoplastic Ascorbate on the Activities of Cell Wall-Associated Peroxidase and NADH Oxidase in Needles of Norway Spruce (Picea abies L.)

Cell wall-associated peroxidases (EC 1.11.1.7 [EC] ) were extractedfrom the current year's needles of Norway spruce trees (Piceaabies L.) in two fractions, namely soluble apoplastic peroxidasesand covalently wall-bound peroxidases. Peroxidase activitieswere determined with two substrates: coniferyl alcohol, whichis important for lignification, and NADH, which is necessaryfor the production of H₂O₂. Coniferyl alcohol peroxidase activitywas detected in both the soluble apoplastic fraction and thewall-bound fraction, whereas NADH oxidase activity was foundonly in the soluble apoplastic fraction. Net oxidation of coniferylalcohol and NADH was inhibited by ascorbate, which reduced theoxidized intermediates of the peroxidase- and oxidase-catalyzedreactions. Since ascorbate itself was oxidized in these reactions,the inhibition was not persistent and it was released once theascorbate present in the assay mixture had been oxidized. Ascorbatedelayed the oxidation of NADH 10-fold more efficiently thanthe oxidation of coniferyl alcohol. Although the level and theredox state of apoplastic ascorbate were lower in lignifyingneedles than in mature needles, the concentration, which was1.17 mM in apoplastic washing fluids, was sufficiently highto inhibit peroxidase activity in vitro. These results suggestthat peroxidases can catalyze lignification only if local differencesexist in the concentration of reduced ascorbate between lignifyingand non-lignifying tissues. (Received April 21, 1994; Accepted September 26, 1994)     

The Association of Ascorbate and Ascorbate Oxidase in the Apoplast with IAA-Enhanced Elongation of Epicotyls from Vigna angularis

The level of (ascorbic acid (AA) plus dehydroascorbic acid (DHA))and the ratio of the level of AA to that of AA plus DHA in intercellularwashing fluid (IWF) of epicotyl segments from Vigna angularisdecreased from 2.8±0.3 to 1.2±0.5nmol (g fr wt)^–1and from 0.23±0.03 to 0.13±0.01, respectively,after incubation of the segments without IAA for 20 h at 27°C.However, these values changed to 5.3±1.7 nmol (g fr wt)^–1and 0.07±0.05 after incubation with 0.1 mM IAA. The activityof cell wall-bound ascorbate oxidase increased by about 20%and 70% after incubation without IAA and with IAA, respectively.However, the activity of cell wall-bound peroxidase was notaffected by incubation with or without IAA. The activities ofascorbate oxidase and peroxidase in IWF decreased by about 85and 75% after incubation without IAA. IAA did not affect thesedecreases to any great extent. A lignin-like compound was formedduring the incubation of epicotyl segments in the absence ofIAA. Formation of this compound was inhibited by IAA. The resultssuggest that one of the causes of the enhancement of elongationgrowth by IAA is the inhibition of peroxidase-dependent lignificationas a result of increases in levels of AA and DHA and in ascorbateoxidase activity. (Received August 16, 1993; Accepted December 6, 1993)     

Effects of Sequential Exposure to Auxin and Cytokinin on Xylogenesis in Cultured Explants of Jerusalem Artichoke (Helianthus tuberosus L.)

During the course of a 4-d culture period, explants of Jerusalemartichoke tuber were exposed to auxin (0.2 mg 1^–1 2, 4-dichlorophenoxyaceticacid), and cytokinin (5.0 mg 1^–1 benzyl-amino purine),under a range of sequential regimes, to study the influenceof each hormone on tracheary element formation. The resultsindicate that auxin was necessary early in the culture periodand was primarily involved in cell proliferation. Cytokininstimulated xylogenesis when present late in the culture period,concomitant with the phase of cytodifferentiation, but not whenrestricted to the early period. The implications for a sustainedperiod of commitment to differentiation are discussed. Xylem differentiation, Jerusalem artichoke, auxin, cytokinin, tissue culture     

Nitric oxide affecting root growth,lignification and related enzymes in soybean seedlings

This study analyzed the involvement of nitric oxide (NO) in the root lignification of soybean seedlings. To this end, changes in root cell viability; phenylalanine ammonia-lyase (PAL) and soluble and cell wall bound peroxidase (POD) activities and lignin and hydrogen peroxide (H₂O₂) contents of soybean roots treated with the NO-donor sodium nitroprusside (SNP) and its relationships with root growth were evaluated. Seedlings were cultivated in a nutrient solution supplemented with 5 to 1,000 μM SNP for 24 h. At an extremely low concentration (5 μM), SNP induced root growth and increased lignification and activities of related enzymes (PAL and cell wall-bound POD). At a high concentration (1,000 μM), SNP reduced root growth and lignification (PAL activity and H₂O₂ and lignin contents) and caused a loss of cell viability. Application of potassium ferrocyanide (an analog of SNP that cannot release NO) and PTIO (2-phenyl-4,4,5,5,-tetramethylimidazoleline-1-oxyl-3-oxide, a scavenger of NO) revealed that the inhibitory/stimulatory effects on root lignification may be due to NO itself. These results indicate that NO, depending on its concentration, may act as a stress factor, due to its toxic action, or as a signal molecule, inducing soybean root growth and lignification.     

Regulation of Stem Abscission and Callus Growth in Shoot Explants of Sweet Orange [Citrus sinensis (L.) Osbeck]

Two-node explants from Sweet Orange cv. St Ives Valencia orangeshoots produced prolific callus and formed secondary abscissionzones within internodes when cultured in vitro with abscisicacid (ABA, 5 µM) or -naphthaleneacetic acid (NAA, 5 µM).Benzyladenine (BA, 1 µm) induced callus but had littleeffect on abscission. Secondary abscission zone formation wasassociated with ABA-induced and auxin-induced ethylene formation.Treatment of explants with inhibitors of ethylene synthesis[aminoethoxyvinyl glycine (AVG), Co²⁺, PO₄^3–] preventedformation of secondary abscission zones but had variable effectson callus formation. Newly made explants contained high concentrationsof endogenous ABA (up to 6000 ng g^–1 f.wt), as measuredby GC/MS/SIM. Long-term subculture of explants (two years) inmedia containing BA (1 µm) led to a reduction in endogenousABA level (40 ng g^–1 f. wt) and to loss of capacity toform extensive callus and secondary abscission zones. Citrus sinensis (L.) Osbeck cv. St Ives Valencia, sweet orange, secondary abscission zones, in vitro, ethylene, endogenous ABA, endogenous IAA     

Soluble and cell wall peroxidases in reed canarygrass in relation to disease resistance and localized lignin formation

The relationship of peroxidases to an inducible disease-resistance mechanism involving lignification of leaf epidermal cell walls was studied. Reed canarygrass (Phalaris arundinacea L.) leaf discs were inoculated with Helminthosporium avenae Eidam and floated on water. In inoculated discs, the activity of soluble, ionic wall-bound and covalent wall-bound peroxidases was about twice the level of activity in noninoculated discs. The increase was attributable to increases in activity of three cathodic isoperoxidases and to the appearance of a new cathodic isoperoxidase. Peroxidase activity in cryostat microtome sections of inoculated discs was histochemically localized in the wall near the site of attempted penetration. When inoculated discs were floated on solutions of cycloheximide (25 μg/ml), increases in peroxidase activity were inhibited, and the fungus penetrated the tissue. The inhibition of peroxidase activity was related to inhibition of cathodic isoperoxidase activity. Anodic isoperoxidase activity did not show changes in response to inoculation or cycloheximide treatment.     

Ethylene Production and Plantlet Formation by Nicotiana Anthers Cultured in the Presence and Absence of Charcoal

Anthers of Nicotiana tabacum produce ethylene when culturedfor plantlet production. The rate is at a maximum 1–2weeks after the onset of culture. Charcoal in the medium increasesthe proportion of androgenic anthers in N. tabacum and severalother Nicotiana species. The level of ethylene in culture vesselsis reduced by charcoal. However, complete removal of ethylenedoes not significantly alter the incidence of androgenesis,nor does continuous flushing of cultures with air. It is concludedthat although charcoal reduces ethylene in the gas phase ofthe cultures its effect on androgenesis is exerted through someother mechanism.     

Effect of Phosphorus Deficiency on Phosphatase Activity of Cell Walls from Roots of Subterranean Clover

Clover (Trifolium subterraneum L. cv. Mt. Barker) was grownin solution culture with adequate (+P) or no phosphate (–P).Cell walls were extracted from roots in such a way that theywere uncontaminated by other cellular materials. Phosphataseactivity was assayed using p-nitro-phenylphosphate (NPP). Phosphatasebound to cell walls had a pH optimum between 5.0 and 6.0, irrespectiveof the P supply to the plants. Activity of phosphatase boundto cell walls increased with electrolyte concentration of theassay medium at pH 6.5 but not at pH 5.5. This increase in activitywas probably due to a higher degree of ionization of the cellwall at pH 6.5 than at pH 5.5, and to effects of high ionicstrength in decreasing the mutual repulsion of negatively chargedNPP from negative charges on the cell walls. Cell wall-boundphosphatase did not exhibit Michaelis-Menten kinetics: the concentrationof NPP at which activity was half the maximum rate (S_0.5) was0.7 mM for cell walls extracted from roots of both +P and –Pplants. Up to 30% of the phosphatase activity bound to cellwalls could be removed using buffer solutions of high pH andhigh ionic strength which contained Triton X100. Both soluble and cell wall-bound phosphatase(s) of roots increasedin activity with P deficiency. The phosphatase activity of cellwalls increased 1.5 fold as the P concentration in the rootsfell from 0.4–0.2% dry weight. Experiments with sterileroots of clover showed that increases in cell wall-bound phosphataseactivity associated with P deficiency were not due to microbialcontamination. It is argued that phosphatase(s) in cell wallsof roots could make a substantial contribution to the P nutritionof clover in soils deficient in inorganic phosphate by hydrolysingorganic phosphate compounds in the soil. Key words: Phosphatase, Clover, Roots, Phosphorus deficiency, Cell walls     

The Effect of Paclobutrazol on Physiological and Biochemical Changes in the Primary Roots of Pea

Primary roots of pea (Pisum sativum L. cv. Taichung No. 11)were treated with 0, 10 and 50 mg dm^–3 paclobutrazol [(2RS,3RS)-1-(4-chlorophenyl)-4, 4-dimethyl-2-(l,2,4-triazol-l-yl)pentan-3-ol]for 1 h at 48 h after germination. Paclobutrazol treatment inhibitedroot extension, promoted swelling (cell expansion was radialrather than longitudinal), and increased cell volume and theactivity of catalase and peroxidase enzymes. Paclobutrazol alsodecreased root respiration and ethylene production. However,under non-stressed conditions, paclobutrazol treatment did notaffect soluble carbohydrate content, water potential, osmoticpotential or water loss. Under osmotic stress with polyethyleneglycol (PEG), paclobutrazol diminished the increase of waterpotential and decreased the rate of water loss caused by theimposed stress, but had no effect on osmotic potential. Catalaseand peroxidase activity were increased in osmotically-stressedroots of treated plants. Key words: Root growth, paclobutrazol, pea, Pisum sativum, water shortage