Sucrose Loading in Isolated Veins of Pisum sativum: Regulation by Abscisic Acid, Gibberellic Acid, and Cell Turgor

Enzymatically isolated vein networks from mature pea (Pisum sativum L. cv Alaska) leaves were employed to investigate the properties of sucrose loading and the effect of phytohormones and cell turgor on this process. The sucrose uptake showed two components: a saturable and a first-order kinetics system. The high affinity system (K_m, 3.3 millimolar) was located at the plasmalemma (p-chloromercuriphenylsulfonic acid and orthovanadate sensitivity). Further characterization of this system, including pH dependence and effects of energy metabolism inhibitors, supported the H⁺-sugar symport concept for sucrose loading. Within a physiological range (0.1-100 micromolar) and after 90 min, abscisic acid (ABA) inhibited and gibberellic acid (GA₃) promoted 1 millimolar sucrose uptake. These responses were partially (ABA) or totally (GA₃) turgor-dependent. In experiments of combined hormonal treatments, ABA counteracted the GA₃ positive effects on sucrose uptake. The abolishment of these responses by p-chloromercuriphenylsulfonic acid and experiments on proton flux suggest that both factors (cell turgor and hormones) are modulating the H⁺ ATPase plasmalemma activity. The results are discussed in terms of their physiological relevance.     

In vitro and in vivo modification of sugar transport and translocation in celery by phytohormones

In an attempt to address the controversy in the literature as to whether phytohormones have any direct effect on phloem loading of sucrose, we investigated the effect of gibberellic acid (GA₃) and indoleacetic acid (IAA) on sugar transport and translocation in celery (Apium graveolens L. cv. Utah 5270). Both hormones enhanced sucrose uptake into isolated vascular bundles and phloem tissue of celery and enhanced the export of ¹⁴C assimilates from leaves of intact plants in vivo. The hormone-induced increase of uptake into isolated vascular bundles or phloem was specific for sucrose and mannitol which are translocated in phloem. Furthermore, the hormone-induced increase in translocation was not due to an increase in sink demand, since neither glucose nor sucrose uptake rates were affected in the storage parenchyma tissue discs (sink region) in the presence of GA₃ or IAA. The evidence suggests that phytohormones may have a direct effect on phloem loading of sucrose. The possibility of short-term GA₃ and IAA effects on processes resulting in membrane transport of sugars in celery is discussed.     

Gibberellin-mediated synergism of xylogenesis in lettuce pith cultures

Major gibberellins (GAs) in lettuce (Lactuca sativa L. cv Romaine) pith explants have been identified by gas chromatography-mass spectrometry (GC-MS) or GC-selected ion monitoring (GC-SIM) as GA₁, 3-epi-GA₁, GA₈, GA₁₉, and GA₂₀. Treatment of pith explants with indole-3-acetic acid (IAA) (57 micromolar) plus kinetic (0.5 micromolar) induced xylogenesis. In this xylogenic treatment, the concentration of a biologically active, polar GA-like substance(s) increased during the first 2 days of culture, although all of the above GAs decreased (as measured by GC-SIM). In non-xylogenic treatments, where explants were cultured without exogenous hormones, or with IAA or kinetin alone, the concentration of the biologically active, polar GA-like substance(s) decreased during the first two days of culture, as did all of the above GAs (as measured by GC-SIM). Treatment of pith explants with exogenous GA₁ alone did not induce xylogenesis, but GA₁ at very low concentrations (0.0014 and 0.003 micromolar) synergized xylogenesis in the IAA plus kinetin-treated cultures. These results suggest that changes in the concentration of certain endogenous GAs may be involved in xylogenesis mediated by IAA plus kinetin in lettuce pith cultures.     

Turgor regulation of sucrose transport in sugar beet taproot tissue

Sink tissues that store osmotically active compounds must osmoregulate to prevent excessively high turgor. The ability to regulate turgor may be related to membrane transport of solutes and thus sink strength. To study this possibility, the kinetics of sugar uptake were determined in sugar beet (Beta vulgaris L.) taproot tissue discs over a range of cell turgors. Sucrose uptake followed biphasic kinetics with a high affinity saturating component below 20 millimolar and a low affinity linear component at higher concentrations. Glucose uptake exhibited only simple saturation type kinetics. The high affinity saturating component of sucrose and glucose uptake was inhibited by increasing cell turgor (decreasing external mannitol concentrations). The inhibition was evident as a decrease in V_max but no effect on K_m. Sucrose uptake by tissue equilibrated in dilute buffer exhibited no saturating component. Ethylene glycol, a permeant osmoticum, had no effect on uptake kinetics, suggesting that the effect was due to changes in cell turgor and not due to decreased water potential per se. p-(Chloromercuri)benzene sulfonic acid (PCMBS) inhibited sucrose uptake at low but not high cell turgor. High cell turgor caused the tissue to become generally leaky to potassium, sucrose, amino acids, and reducing sugars. PCMBS had no effect on sucrose leakage, an indication that the turgor-induced leakage of sucrose was not via back flow through the carrier. The ability of the tissue to acidify the external media was turgor dependent with an optimum at 300 kilopascals. Acidification was sharply reduced at cell turgors above or below the optimum. The results suggest that the secondary transport of sucrose is reduced at high turgor as a result of inhibition of the plasma membrane ATPase. This inhibition of ATPase activity would explain the reduced V_max and leakiness to low molecular weight solutes. Cell turgor is an important regulator of sucrose uptake in this tissue and thus may be an important determinant of sink strength in tissues that store sucrose.     

Altered Growth Response to Exogenous Auxin and Gibberellic Acid by Gravistimulation in Pulvini of Avena sativa

Pulvini of excised segments from oats (Avena sativa L. cv Victory) were treated unilaterally with indoleacetic acid (IAA) or gibberellic acid (GA₃) with or without gravistimulation to assess the effect of gravistimulation on hormone action. Optimum pulvinus elongation growth (millimeters) and segment curvature (degrees) over 24 hours were produced by 100 micromolar IAA in vertical segments. The curvature response to IAA at levels greater than 100 micromolar, applied to the lower sides of gravistimulated (90°) pulvini, was significantly less than the response to identical levels in vertical segments. Furthermore, the bending response of pulvini to 100 micromolar IAA did not vary significantly over a range of presentation angles between 0 and 90°. In contrast, the response to IAA at levels less than 10 micromolar, with gravistimulation, was approximately the sum of the responses to gravistimulation alone and to IAA without gravistimulation. This was observed over a range of presentation angles. Also, GA₃ (0.3-30 micromolar) applied to the lower sides of horizontal segments significantly enhanced pulvinus growth and segment curvature, although exogenous GA₃ over a range of concentrations had no effect on pulvinus elongation growth or segment curvature in vertical segments. The response to GA₃ (10 micromolar) plus IAA (1.0 or 100 micromolar) was additive for either vertical or horizontal segments. These results indicate that gravistimulation produces changes in pulvinus responsiveness to both IAA and GA₃ and that the changes are unique for each growth regulator. It is suggested that the changes in responsiveness may result from processes at the cellular level other than changes in hormonal sensitivity.     

Role of auxin and gibberellin in differentiation of primary Phloem fibers

The hypothesis that auxin and gibberellic acid (GA₃) control the differentiation of primary phloem fibers is confirmed for the stem of Coleus blumei Benth. Indoleacetic acid (IAA) alone sufficed to cause the differentiation of a few primary phloem fibers. In long term experiments auxin induced a considerable number of fibers in mature internodes. GA₃ by itself did not exert any effect on fiber differentiation. Combinatiosn of IAA with GA₃ completely replaced the role of the leaves in primary phloem fiber differentiation qualitatively and quantitatively. Although the combined effect of the two growth hormones diminished considerably with increasing distance from the source of induction, auxin with GA₃ or IAA alone induced fibers in a few internodes below the application site. When various combinations of both hormones were applied, high concentrations of IAA stimulated rapid differentiation of fibers with thick secondary walls, while high levels of GA₃ resulted in long fibers with thin walls. The size of the primary phloem fibers correlated with the dimensions of the differentiating internode, thereby providing evidence that both growth regulators figure in the control of stem extension. High IAA/low GA₃ concentrations have an inhibitory effect on internode elongation, whereas low IAA/high GA₃ concentrations promote maximal stem elongation.     

扁桃生理落果期不同组织激素浓度的动态变化及其对落果的影响

为探明扁桃幼果生理脱落与GA3、IAA和ABA等3种激素的关系,以新疆‘纸皮’扁桃为试材,分析新梢、结果枝组、幼果和果柄(包括正常发育幼果和果柄、即将脱落幼果和果柄)中3种内源激素浓度的动态变化规律,并分别涂抹3种外源激素调查其对坐果率的影响。结果表明:(1)‘纸皮’扁桃幼果脱落期和新梢生长期重合,扁桃生理落果期不同组织中3种内源激素浓度变化趋势与新梢生长期和幼果脱落期的动态特征基本一致。(2)扁桃生理落果期间,正常的和即将脱落的幼果及其果柄中内源激素浓度呈规律性变化,即:GA3和IAA浓度表现为正常果和正常果果柄始终大于相应的落果和落果果柄,而内源ABA浓度表现则与之相反,同时对应外源涂抹试验也印证了幼果和幼果果柄中高GA3和IAA浓度、低ABA浓度有利于扁桃坐果。(3)新梢和幼果中对应内源激素之间的浓度平衡关系也是调控扁桃幼果生理脱落的重要因素,即:新梢与幼果的GA3比值和IAA比值增大、而ABA比值减少将会促进幼果脱落,反之则减缓幼果脱落。     

Interaction of gibberellic and indole-3-acetic acid on root formation in pea (Pisum sativum L.) epicotyl cuttings

Indole-3-acetic acid (IAA) strongly enhanced rooting of etiolated pea epicotyl cuttings while gibberellic acid (GA₃) enhanced rooting only slightly. The promoting effects of the hormones appeared not until 14 d after the onset of treatment. When GA₃ and IAA were applied together, the initiation of rooting started already after 6 d after onset of treatment. It is suggested that gibberellin plays an important role, in combination with auxin, in the initiation of root formation in Pisum cuttings.Abbreviations IAA Indole-3-acetic acid - GA₃ Gibberellic acid     

Estimation of endogenous contents of phytohormones during internode development in <Emphasis Type="Italic">Merremia emarginata</Emphasis>

During the entire period of internode growth of Merremia emarginata contents of gibberellic acid (GA₃), phenyl-acetic acid (PAA), indole-3-acetic acid (IAA, free and conjugated) and abscisic acid (ABA, free and conjugated) were estimated by ELISA using polyclonal antibodies raised against each hormones. At the time of internode elongation free auxin content was low and increased with the decrease in the rate of elongation. In contrast, conjugated IAA showed declining trend where free IAA content was remarkably high, suggesting thereby that conjugated IAA might have mobilized during the later phase of internode development. The endogenous GA₃ contents were high as compared to other hormones; however, no significant role of GA₃ was discernible in elongation growth. Conjugated ABA contents remained very low during the elongation growth and increased thereafter.     

Enhancement of [C]Sucrose Export from Source Leaves of Vicia faba by Gibberellic Acid

The effect of gibberellic acid (GA₃) on sucrose export from source leaves was studied in broad bean (Vicia faba L.) plants trimmed of all but one source and one sink leaf. GA₃ (10 micromolar) applied to the source leaf, enhanced export of [¹⁴C]sucrose (generated by ¹⁴CO₂ fixation) to the root and to the sink leaf. Enhanced export was observed with GA treatments as short as 35 minutes. When GA₃ was applied 24 hours prior to the ¹⁴CO₂ pulse, the enhancement of sucrose transport toward the root was abolished but transport toward the upper sink leaf was unchanged. The enhanced sucrose export was not due to increased photosynthetic rate or to changes in the starch/sucrose ratio within the source leaf; rather, GA₃ increased the proportion of sucrose exported. After a 10-min exposure to [¹⁴C]GA₃, radioactivity was found only in the source leaf. Following a 2 hour exposure to [¹⁴C]GA₃, radioactivity was distributed along the entire stem and was present in both the roots and sink leaf. Extraction and partitioning of GA metabolites by thin layer chromatography indicated that there was a decline in [¹⁴C]GA₃ in the lower stem and root, but not in the upper stem. This pattern of metabolism is consistent with the disappearance of the GA₃ effect in the lower stem with time after treatment. We conclude that in the short term, GA₃ enhances assimilate export from source leaves by increasing phloem loading. In the long term (24 hours), the effect of GA₃ is outside the source leaf. GA₃ accumulates in the apical region resulting in enhanced growth and thus greater sink strength. Conversely, GA₃ is rapidly metabolized in the lower stem thus attenuating any GA effect.     

Evidence on the mechanism of enhanced sucrose uptake at low cell turgor in leaf discs of Phaseolus coccinius

Plants often tolerate water deficits by lowering the osmotic potential of their cell sap. This may be achieved by accumulation of solutes which results in the maintenance of a positive turgor potential. In this study, the effect of water deficit on sugar uptake was investigated in leaf discs of Phaseolus coccinius L. (cv. Scarlet). Evidence is presented that cell turgor affects the kinetics of sugar transport at the membrane level. Uptake kinetics of sucrose, glucose and 3-O-methyl glucose by tissues equilibrated in solutions of relatively high (200–400 mOsm) osmotic concentration consisted of a sat-urable and a linear component. Low external osmotic concentration i.e., high cellular turgor inhibited the saturating component of sucrose uptake, resulting in a linear uptake profile. However, high cell turgor had no effect on glucose or 3-O-methyl glucose uptake kinetics. The effect of turgor versus osmotic component of water potential was differentiated by comparing responses to non-penetrating (manmtol) or polyethylene glycol, (3350) and penetrating (ethylene glycal) osmotica. Changes in sucrose uptake rates and kinetics were due to changes in cellular turgor and not osmotic potential. Furthermore, at low cellular turgor, a net increase in sucrose uptake occurred as a consequence of enhanced influx rates and not as a result of reduced efflux rates. The data are consistent with previous findings that sugar uptake rates are enhanced under low turgor. We present first evidence indicating that the mechanism by which higher rates of sucrose uptake are maintained underwater deficit conditions is by the activation of the saturable transport system. This mechanism supports previous suggestions that changes in cell turgor are sensed and manifested at the membrane level.     

Gibberellic Acid stimulation of cucumber hypocotyl elongation : effects on growth, turgor, osmotic pressure, and cell wall properties

Recently developed techniques have been used to reinvestigate the mechanism by which gibberellic acid (GA₃) stimulates elongation of light-grown cucumber (Cucumis sativus L.) seedlings. Osmotic pressure and turgor pressure were slightly reduced in GA₃-treated seedlings, which elongated 3.5 times faster than control seedlings. This indicated that GA₃ enhancement of growth was not controlled by changes in the osmotic properties of the tissues. Stress/strain (Instron) analysis revealed that plastic extension of the cell walls of GA₃-treated seedlings increased by up to 35% above the control values. Stress-relaxation measurements on frozen-thawed tissue showed that T₀ the minimum relaxation time, was reduced following application of GA₃. In vivo wall relaxation (measured by the pressure block technique) showed that the wall yield coefficient was increased, and the yield threshold was slightly reduced. Thus GA₃ affected both the mechanical (viscoelastic) and biochemical (chemorheological) properties of the cell walls of light-grown cucumber. The previous hypothesis, that GA₃ stimulates cucumber hypocotyl growth by increasing osmotic pressure and cell turgor, is contradicted by our results.     

The effect of cell turgor on sugar uptake in strawberry fruit cortex tissue

A reduction in cell turgor has been shown to stimulate sugar uptake in several plant sink tissues and it may regulate the import of assimilate into the sink apoplast, as well as maintain cell turgor. To determine whether cell turgor influences sugar uptake by strawberry (Fragaria x ananassa Duch. cv. Brighton) fruit cortex tissue, disks were cut from greenhouse-grown primary fruit at the green-white stage of development and placed in buffered incubation solutions containing either mannitol or ethylene glycol as an osmoticum. Cell turgor of fruit disks was calculated from the difference between the water potential of bathing solution and tissue solute potential after incubation at various osmolarities. Cell turgor increased when tissue disks were placed into mannitol incubation solutions more dilute than the water potential of fresh tissue (about 415 mOsmol kg^?1). The rate of uptake of [¹⁴C]-sucrose or [¹⁴C]-glucose decreased as osmolarity of the incubation solution increased, i.e. as cell turgor declined. Cell turgor and the rate of [¹⁴C]-sucrose uptake were unaffected when rapidly permeating ethylene glycol was used as an osmoticum. A decrease in cell turgor reduced both the V_max of the saturable (carrier mediated) kinetic component of sucrose uptake, and the slope of the linear (diffusional) component. The sulfhydryl binding reagent p-chloromercuibenzenesulfonic acid, an inhibitor of the plasma membrane sucrose carrier, strongly inhibited only the saturable component of sucrose uptake. Increased uptake of the nonmetabolizable sugar, O-methyl-glucose, at high turgor was similar to that of glucose, indicating that carrier activity was influenced by cell turgor, not cell metabolism. Turgor did not influence efflux of [¹⁴C]-sucrose from disks and had no effect on cell viability. Strawberry fruit cells do not possess a sugar uptake system that is stimulated by a reduction in turgor.     

Gibberellin and auxin signals control scape cell elongation and proliferation in <Emphasis Type="Italic">Agapanthus praecox</Emphasis> ssp. <Emphasis Type="Italic">orientalis</Emphasis>

Plant height is determined by the processes of cell proliferation and elongation. Plant hormones play key roles in a species-dependent manner in these processes. We used paclobutrazol (PAC) at 400 mg·L^-1 in this study to spray Agapanthus praecox ssp. orientalis plants in order to induce dwarf scape (inflorescence stem). Morphological examination showed that PAC reduced scape height by inhibiting the cell elongation by 54.56% and reducing cell proliferation by 10.45% compared to the control. Quantification and immunolocalization of endogenous gibberellins (GAs) and indole-3-acetic acid (IAA) showed that the GA₁, GA₃, and GA₄ levels and the IAA gradient were reduced. Among these hormones, GA4 was the key component of GAs, which decreased 59.51-92.01% compared to the control in scape. The expression of cell wall synthesis related genes cellulose synthase (CESA) and UDP-glucuronic acid decarboxylase (UXS) were upregulated significantly, whereas cell wall loosening gene xyloglucan endotransglucosylase 2 (XET2) was downregulated by 99.99% surprisingly. Correlation analysis suggested GA regulated cell elongation and auxin modulated cell proliferation in Agapanthus scape. Additionally, the accumulation of sugars played roles in cell wall synthesis and cell expansion. These results indicated GA and IAA signals triggered a downstream signaling cascade, controlled cell expansion and proliferation during scape elongation.     

Higher Production of Forskolin in Genetically Transformed Cultures of Coleus forskohlii Briq Induced by Growth Regulators

Increased forskolin yield was obtained in transformed root, rhizogenic calli and cell suspension cultures of Coleus forskohlii when treated alone with various concentrations of auxins (IAA, IBA, NAA, 2,4-0), auxin conjugates ( IAA-ala, IAA-gly, IAA-phe, IAA-asp), cytokinins (Kn, BAP) and gibberellic acid (GA₃). An 8.9-fold stimulation in forskolin production was achieved in transformed rhizogenic line GCO-RCH-10 in presence of 1.0 mg I^-1 GA₃, 6-fold in cell suspension line GSO-5/7-k in presence of 2 mg I^-1 BAP and 4.3-fold in root line RC-ST -2/16 in presence of 0.5 mg I^-1 GA₃ at the end of a culture period of 4 weeks. Growth and morphology was found to be influenced by the growth regulators studied. A seven fold increase in biomass was obtained in rhizogenic line GCO-RCH-10 with 0.5 mg I^-1 GA₃ In root line RC-ST-2/16, different concentrations of IAA, IAA conjugates and GA3 stimulated growth while cytokinins inhibited growth of roots. The shoot culture line ST -2/51/d, showed prolific growth in the presence of all cytokinins but no forskolin was detected in the shoot cultures treated with any of these hormones.     

The effect of plant hormones on phosphate uptake and translocation in maize roots

In short-term (1 h) uptake experiments GA₃(10^-5M) stimulated P_i uptake into maize root cortex cells by 28.7 %, Ethrel (10^-3M) inhibited it by 18.5 % and BA, IAA, and ABA were inactive. In long-term (5 h) experiments ABA remained inactive, GA₃ lost its stimulatory effect, and BA (5. 10^-6M), IAA (10^-4 -10^-5M), and Ethrel (10^-3 -5. 10^-4M) decreased P_i uptake. When the hormones were present only during 3 h preincubation (“augmentation”) period ABA was inactive, GA₃ slightly raised and BA, IAA, and Ethrel slowed down subsequent P_i uptake. BA(10^-7 –10^-5M) decreased xylem sap volume flow and P_i translocation. ABA in all tested concentrations (10^-8 –10^-5M) reduced exudation rate and P_i translocation, its effect declining with time. IAA effect strongly depended on concentration used and on application time and varied from strong inhibition to moderate stimulation of both volume flow and P_i translocation. GA₃ (10^-7M) slightly stimulated xylem volume flow but inhibited phosphate translocation. Ethrel (10^-4 and 10^-5M) increased both parameters, but P_i transloeation much more than volume flow. IAA, BA, and ABA influenced volume flow and P transloeation to the same extent leaving P_i concentration in the xylem sap unchanged. GA₃ and Ethrel influence P_i concentration in the xylem sap and it is thus probable that these hormones regulate release of phosphate ions into the xylem sap.     

Effect of plant hormones on sucrose uptake by sugar beet root tissue discs

Abscisic acid (ABA), auxins, cytokinins, gibberellic acid, alone or in combination were tested for their effects on short-term sucrose uptake in sugar beet (Beta vulgaris cv USH-20) roots. The effect of ABA on active sucrose uptake varied from no effect to the more generally observed 1.4-to 3.0-fold stimulation. A racemic mixture of ABA and its trans isomer were more stimulatory than ABA alone. Pretreating and/or simultaneously treating the tissue with K⁺ or IAA prevented the ABA response while cytokinins and gibberellic acid did not. While the variable sensitivities of beet root to ABA may somehow be related to the auxin and alkali cation status of the tissue, tissue sensitivity to ABA was not correlated with ABA uptake, accumulation, or metabolic patterns. In contrast to ABA, indoleacetic acid (IAA) and other auxins strongly inhibited active sucrose uptake in beet roots. Cytokinins enhanced the auxin-induced inhibition of sucrose uptake but ABA and gibberellic acid did not modify or counteract the auxin effect. Trans-zeatin, benzyladenine, kinetin, and gibberellins had no effect on active sucrose uptake. None of the hormones or hormone mixtures tested had any significant effect on passive sucrose uptake. The effects of IAA and ABA on sucrose uptake were detectable within 1 h suggesting a rather close relationship between the physiological activities of IAA and ABA and the operation of the active transport system.     

The Role of Auxin and Gibberellin in Controlling Lignin Formation in Primary Phloem Fibers and in Xylem of Coleus blumei Stems

The hypothesis that auxin (IAA) and gibberellic acid (GA₃) control the formation of lignin is confirmed for the primary phloem fibers and for the secondary xylem in the stem of Coleus blumel Benth. Indoleacetic acid alone, or a combination of high IAA/low GA₃ (w/w), induced short phloem fibers with thick secondary walls, that contained lignin rich in syringyl units (high ratio of syringyl/guaiacyl). On the other hand, a combination of high GA₃/low IAA (w/w), which promoted the differentiation of long phloem fibers with thin walls, decreased the relative content of the syringyl units (low syringyl/guaiacyl ratio). In the secondary xylem, these hormonal treatments yielded only slight changes in the noncondensed monomeric guaiacyl units, confirming the relative stability of the guaiacyl lignification pattern in this tissue. In the xylem, indoleacetic acid alone, or a combination of high IAA/low GA₃ induced lignin poor in syringyl units (low syringyl/guaiacyl ratio). A combination of high GA₃/low IAA promoted a relatively slight increase in syringyl yield, indicating greater responsiveness of the syringyl lignification pattern to growth regulators. The possible functional and technological significance of our results is discussed.     

Mechanisms Underlying the Regulation of Root Formation in <Emphasis Type="Italic">Malus hupehensis</Emphasis> Stem Cuttings by Using Exogenous Hormones

Malus hupehensis is one of the most important Malus ornamental and rootstock species in the south China Yellow River Basin. In the present study, we treated the stem cuttings of M. hupehensis with one of three exogenous hormones, indole acetic acid (IAA), naphthalene acetic acid (NAA), or a compound plant growth regulator (GGR) to investigate the mechanisms underlying root formation in stem cuttings and to optimize stem cutting propagation techniques. The results showed that immersing the stem cuttings in 100 mg/L of IAA for 2 h before planting was most effective, which reduced the time to root formation by 21 days and increased rooting percentage by 129.4 %, compared to that in the control group. In addition, the levels of endogenous substances (endogenous hormones, soluble proteins, and carbohydrates) dynamically changed, with the time to peak value or time to valley value of each parameter synchronized well with the initiation of adventitious roots. The synchronized change suggested that root formation was coordinated with physiological metabolism. However, exogenous hormone treatment significantly accelerated the catabolism of the root inhibiting hormone, abscisic acid. On the other hand, exogenous hormone treatment significantly enhanced the accumulation of root promoting hormones [IAA, gibberellic acid (GA₃), and zeatin riboside (ZR)] and soluble proteins. Moreover, exogenous hormone treatments accelerated the consumption of starch and soluble sugars. Overall, the results indicated that exogenous hormone treatment (IAA) accelerated the synthesis of endogenous hormones (IAA, GA₃, and ZR), therefore, sped up the metabolism of carbohydrates and soluble proteins, and consequently quickened the root formation process.     

REGULATION OF GROWTH AND CELLULAR DIFFERENTIATION IN DEVELOPING AVENA INTERNODES BY GIBBERELLIC ACID AND INDOLE-3-ACETIC ACID

Auxin (IAA) at physiological concentrations causes significant reduction of GA₃-promoted growth in excised Avena stem segments. IAA is thus considered to be a gibberellin antagonist in this system. It was found to act non-competitively in repressing GA₃-augmented growth in these segments. In intercalary meristem cells at the base of the elongating internode, GA₃ blocks cell division activity and causes a marked increase in cell lengthening. IAA substantially promotes lateral expansion in comparable intercalary meristem cells, particularly in the vicinity of vascular bundles underlying the epidermis. It also alters the plane of cell division in differentiating stomata. IAA at high concentrations (10⁻³, 10⁻⁴ m ), in combination with GA₃, overrides the effects of GA₃ on cell lengthening, while with low concentrations of IAA (10⁻⁹, 10⁻¹⁰m ), the effects of GA₃ are clearly dominant. At intermediate concentrations of IAA (10⁻⁶, 10⁻⁷m ), in the presence of GA₃, the effects of this treatment on cell differentiation closely parallel the pattern of differentiation in untreated tissue. It is postulated that a lateral gradient of auxin and gibberellin could control cell expansion in long epidermal cells during intercalary growth of the internode.