Comparison of effects of ABA and IAA on phospholipid bilayers

The plant hormones indole-3-acetic acid (IAA) and abscisic acid (ABA) affect the properties of phospholipid bilayers differently. IAA enhances permeability of bilayers composed of phosphatidylcholine to the non-electrolyte erythritol while ABA requires an additional phospholipid in the membrane to produce substantial enhancement. Similar conclusions are obtained by measuring hormone-induced permeability to chloride ions; IAA is effective with single component phosphatidylcholine membranes while ABA requires a second phospholipid. Erythritol permeability is shown to be pH dependent for both hormones. Although IAA is more effective at increasing erythritol permeability at pH 4 than at pH 7, both dissociated and undissociated IAA affect the process. In comparison ABA is almost totally ineffective in the dissociated form (at pH 7). Spin label electron spin resonance measurements demonstrated that neither hormone substantially disrupts acyl chain mobility within the membrane, indicating that the mechanism of permeability enhancement is not a general non-specific pertubation of membrane ordering and fluidity. Both hormones can also effect the stability of small unilamellar (sonicated) vesicles. Phosphatidylcholine vesicles are relatively stable and do not rapidly aggregate with either ABA or IAA. However, when phosphatidylethanolamine is incorporated as a minor component (10 mol%) into phosphatidylcholine vesicles ABA causes rapid aggregation while IAA has no effect. These experiments indicate that the two hormones may exhibit completely different behaviour on membranes without the requirement for specific proteinaceous receptors.     

Indoleacetic acid enhancement of the inhibition of Lemna growth caused by abscisic acid

Summary Indoleacetic acid substantially increased the inhibitory influence of abscisic acid on growth measured on fresh weight basis of Lemna gibba L. A similar synergistic action was obtained with indolebutyric acid while neither naphthylacetic acid nor 2,4-dichlorophenoxyacetic acid showed any synergism. The antiauxin para-chlorophenoxy-isobutyric acid did not counteract the synergistic action of IAA and ABA. The results indicate that the enhancing effect of IAA on the ABA action is not a typical auxin effect.Abbreviations ABA abscisic acid - IAA indoleacetic acid - IBA indolebutyric acid - NAA naphthylacetic acid - 2,4-D 2,4-dichlorophenoxyacetic acid - PCIB para-chlorophenoxy-iso-butyric acid     

Regulation of genes associated with auxin, ethylene and ABA pathways by 2,4-dichlorophenoxyacetic acid in Arabidopsis

The chemical 2,4-dichlorophenoxyacetic acid (2,4-D) regulates plant growth and development and mimics auxins in exhibiting a biphasic mode of action. Although gene regulation in response to the natural auxin indole acetic acid (IAA) has been examined, the molecular mode of action of 2,4-D is poorly understood. Data from biochemical studies, (Grossmann (2000) Mode of action of auxin herbicides: a new ending to a long, drawn out story. Trends Plant Sci 5:506–508) proposed that at high concentrations, auxins and auxinic herbicides induced the plant hormones ethylene and abscisic acid (ABA), leading to inhibited plant growth and senescence. Further, in a recent gene expression study (Raghavan et al. (2005) Effect of herbicidal application of 2,4-dichlorophenoxyacetic acid in Arabidopsis. Funct Integr Genomics 5:4–17), we have confirmed that at high concentrations, 2,4-D induced the expression of the gene NCED1, which encodes 9-cis-epoxycarotenoid dioxygenase, a key regulatory enzyme of ABA biosynthesis. To understand the concentration-dependent mode of action of 2,4-D, we further examined the regulation of whole genome of Arabidopsis in response to a range of 2,4-D concentrations from 0.001 to 1.0 mM, using the ATH1-121501 Arabidopsis whole genome microarray developed by Affymetrix. Results of this study indicated that 2,4-D induced the expression of auxin-response genes (IAA1, IAA13, IAA19) at both auxinic and herbicidal levels of application, whereas the TIR1 and ASK1 genes, which are associated with ubiquitin-mediated auxin signalling, were down-regulated in response to low concentrations of 2,4-D application. It was also observed that in response to low concentrations of 2,4-D, ethylene biosynthesis was induced, as suggested by the up-regulation of genes encoding 1-aminocyclopropane-1-carboxylic acid (ACC) synthase and ACC oxidase. Although genes involved in ethylene biosynthesis were not regulated in response to 0.1 and 1.0 mM 2,4-D, ethylene signalling was induced as indicated by the down-regulation of CTR1 and ERS, both of which play a key role in the ethylene signalling pathway. In response to 1.0 mM 2,4-D, both ABA biosynthesis and signalling were induced, in contrast to the response to lower concentrations of 2,4-D where ABA biosynthesis was suppressed. We present a comprehensive model indicating a molecular mode of action for 2,4-D in Arabidopsis and the effects of this growth regulator on the auxin, ethylene and abscisic acid pathways. Experiment station: Plant Biotechnology Centre, Primary Industries Research Victoria, Department of Primary Industries, La Trobe University, Bundoora, Victoria 3086, and the Victorian Microarray Technology Consortium (VMTC).     

Endogenous hormone levels in explants and in embryogenic and non-embryogenic cultures of carrot

Carrot ( Daucus carota L. F1 hybrid Starca) excised hypocotyls were cultured on Murashige and Skoog medium with and without 2,4-dichlorophenoxy acetic acid (2,4-D) to determine the effect of this plant growth regulator on their further development and their endogenous hormone levels. Culture in the absence of 2,4-D stimulated root development at one end of the hypocotyl segments and increased the endogenous levels of free indole-3-acetic acid (IAA), zeatin/zeatin riboside and N ⁶(Δ²-isopentenyl) adenine/ N ⁶(Δ²-isopentenyl) adenosine, as determined by radio-immunoassay. On the other hand, the presence of 2,4-D in the culture medium promoted callus induction and proliferation, together with abscisic acid (ABA) accumulation, in the hypocotyl segments during the first weeks of culture. When the callus segments generated in the hypocotyl sections cultured in the presence of 2,4-D were cultivated further, the development of two callus types was observed, one composed of preglobular and globular embryos and the other translucent, watery and lacking any sign of organisation. The embryos of the first type germinated when callus segments were transferred to regeneration conditions, while no change was observed when the second type was induced to regenerate. Higher levels of free IAA and ABA were obtained in the embryogenic calli when compared to the non-embryogenic, while no differences were observed among callus types in the other hormones evaluated. The possible role of the different plant hormones during induction of somatic embryogenesis is discussed.     

Tissue cultures of Artemisia pallens: Organogenesis,terpenoid production

Germinated seedlings of Artemisia pallens gave three types of cultures on MS medium supplemented with different plant growth hormones. Medium containing BA+2,4-D stimulated unorganized callus; BA+IAA medium, semi-organized tissues interspersed with shoot buds; and BA+NAA+IAA medium, multiple shoot cultures. The in vitro shoots developed roots in medium devoid of growth hormones. TLC and GLC analysis of the tissue extracts showed that linalool was present in the cultured tissues, with maximum concentration in the unorganized tissue. Although the TLC profiles of the three culture extracts were similar, the extracts did not contain the major polar compounds of the plant. The plant extracts contained more polar compounds and gave the characteristic fragrance of davana.Abbreviations MS Murashige & Skoog's basal medium - BA benzyladenine - Kn kinetin - NAA naphthaleneacetic acid - IAA indoleacetic acid - 2,4-D 2,4-dichlorophenoxyacetic acid - PCV packed cell volume     

Unidirectional sodium fluxes in red beet storage tissue (Beta vulgaris L. cv. Platronde Egyptische): effects of the phytohormones indol-3yl-acetic acid and abscisic acid

Abstract. The influence of indol-3yl-acetic acid (IAA) and abscisic acid (ABA) on the capacities of the cytoplasm and vacuole and their effects on unidirectional sodium fluxes across the plasmalemma and the tonoplast of aged red beet storage tissue was investigated. After loading the tissue in a labelled NaCl solution the efflux of radio-activity was measured in unlabelled NaCl. By means of compartmental analysis the capacities and fluxes were determined and compared with those obtained after loading and elution in the presence of IAA or ABA.
It was established that both IAA and ABA affect sodium transport across the principal cell membranes. Both hormones inhibited the efflux across the plasma-lemma, possibly by affecting a Na⁺ for H⁺ exchanging system. Efflux across the tonoplast was stimulated by IAA and influx across the same membrane was enhanced by ABA. It was suggested that IAA stimulated a proton pump at this level while the influence of ABA remained difficult to explain.     

Biochemical characterization of enoyl reductase involved in Type II fatty acid synthesis in the intestinal coccidium Eimeria tenella (Phylum Apicomplexa)

Moniliophthora perniciosa is the causative agent of witches' broom disease in Theobroma cacao. Exogenously provided abscisic acid (ABA), indole-3-acetic acid (IAA), jasmonic acid (JA), and salicylic acid (SA) promoted mycelial growth, suggesting the ability of the pathogen to metabolize plant hormones. ABA, IAA, JA, and SA were found endogenously in the mycelium and in the fruiting body of the pathogen. The pathogen contained high amounts of SA in the mycelium (0.5+/-0.04 microg g(-1) DW) and IAA (2+/-0.6 microg g(-1) DW) in the basidiocarps. Growth of the mycelium in the presence of host leaves for 10 days did not affect ABA or JA content of the leaves but IAA and SA increased 2.5- and 11-fold, respectively. The amounts of IAA and SA in infected leaves increased beyond the levels of the uninfected leaves and suggest a synergistic response to host-pathogen interaction. The ability of M. perniciosa to produce and sustain growth in the presence of elevated endogenous IAA and SA levels during colonization indicates that these phytohormones contribute to its pathogenicity.     

Study on the extraction, purification and quantification of jasmonic acid, abscisic acid and indole-3-acetic acid in plants

Introduction  – Jasmonic acid (JA), abscisic acid (ABA) and indole‐3‐acetic acid (IAA) are important plant hormones. Plant hormones are difficult to analyse because they occur in small concentrations and other substances in the plant interfere with their detection. Objective  – To develop a new, inexpensive procedure for the rapid extraction and purification of IAA, ABA and JA from various plant species. Methodology  – Samples were prepared by extraction of plant tissues with methanol and ethyl acetate. Then the extracts were further purified and enriched with C₁₈ cartridges. The final extracts were derivatised with diazomethane and then measured by GC‐MS. The results of the new methodology were compared with those of the Creelman and Mullet procedure. Results  – Sequential elution of the assimilates from the C₁₈ cartridges revealed that IAA and ABA eluted in 40% methanol, while JA subsequently eluted in 60% methanol. The new plant hormone extraction and purification procedure produced results that were comparable to those obtained with the Creelman and Mullet's procedure. This new procedure requires only 0.5 g leaf samples to quantify these compounds with high reliability and can simultaneously determine the concentrations of the three plant hormones. Conclusion  – A simple, inexpensive method was developed for determining endogenous IAA, ABA and JA concentrations in plant tissue. Copyright © 2008 John Wiley & Sons, Ltd.     

Somatic embryogenesis from integument (perisperm) cultures of coffee

Somatic embryogenesis was induced in integument (perisperm) cultures of C x R hybrid cultivar of coffee, after a culture period of 15 months, using a sequence of 3 modifications of MS medium. Vigorously growing soft, white, watery crystalline calli were obtained on MS + TIBA (1 mg/l) + L-cysteine HCl (50 mg/l) + PVP (100 mg/l). After 45 d, the calli were subcultured to MS + IAA (0.5 mg/l) + 2,4-D (0.05 mg/l) + Kn (8.6 mg/l) and maintained for the next 9 months without any transfer. On this medium, the callus proliferation was initially vigorous which slowed down after 5–6 months, and then the calli turned light brown and somewhat compact. Later, when the calli were transferred to MS + thiamine HCl (10 mg/l) + pyridoxine HCl (3 mg/l) + nicotinic acid (2 mg/l) + 2,4-D (0.2 mg/l) + 2ip (2.5 mg/l) and cultured for 2 months, they turned darker, more compact and the proliferation almost stopped. These calli were subcultured onto fresh medium of the same composition. After another 2 months of culture cream-coloured, highly friable, embryogenic calli appeared, which in turn produced a few clearly identifiable SEs in another 1 month. Further proliferation and maturation of SEs was achieved by culturing the embryogenic calli on MS + ABA (1 mg/l) for 3 months. The SEs were germinated into 2 cm tall plantlets after 2–3 subcultures, each of 2 months duration on 1/2-MS + Kn (0.1 mg/l).Abbreviations MS Murashige and Skoog (1962) basal medium - ABA Abscisic acid - TIBA 2,3,5 -Triiodobenzoic acid - IAA Indole-3-acetic acid - 2,4-D 2,4-dichloro-phenoxyacetic acid - Kn Kinetin - 2ip N⁶-(2-isopentenyl) adenine - PVP Polyvinylpyrrolidone; - SEs Somatic embryos     

Comparative Effects of lndole-3-acetic acid, Abscisic acid, Gibberellic acid and 6-Benzylaminopurine on the Dark- and Light-induced Pulvinar Movements in Cassia fasciculata Michx

Effects of plant hormones were examined on the dark- and light-inducedmovements of Cassia fasciculata. Indole-3-acetic acid (IAA),gibberellic acid (GA₃) and 6-benzylaminopurine (6-BAP) inhibitedthe scotonastic movement whereas abscisic acid (ABA) enhancedit. After brief treatments (5 to 30 min), the ABA effect wasinhibitory rather than promotional. Hormonal treatment in theacidic range gave the best physiological response for ABA, butthe greatest efficiency of IAA, GA₃ and 6-BAP was obtained withpH values close to neutrality. Three to 5 h were needed beforeexpression of the physiological effect triggered by GA₃ and6-BAP, while 5 min treatments were sufficient for IAA and ABA.Light-induced movements were largely enhanced by IAA and slightlyby GA₃ but inhibited by 6-BAP and ABA. The results are discussedin relation to the ionic changes in the pulvinar motor cells,regulating leaflet movements. Key words: Abscisic acid, auxins, cytokinins, gibberellic acid, pulvinar movements     

On the mutagenic and recombinogenic activity of certain herbicides in Salmonella typhimurium and in Aspergillus nidulans

The plant growth-regulating hormones indole-3-acetic acid (IAA) and indole-3-butyric acid (IBA), both strong recombinogens in Aspergillus nidulans, were tested in Salmonella typhimurium strains for his revertants at a range of concentrations from 1 to 2000 micrograms/plate with and without metabolic activation and were found negative. Also 3 herbicides of the chlorophenoxy group, 2,4-(dichlorophenoxy)acetic acid (2,4-D), 2,4-(dichlorophenoxy)butyric acid (2,4-DB) and 4-chloro-2-methylphenoxyacetic acid (MCPA), which show a plant growth hormone-like activity, and 2 of the triazine group, 2-ethylamino-4-chloro-6-isopropylamino-1,3,5-triazine (atrazine) and 2,4-bis(isopropylamino)6-chloro-1,3,5-triazine (propazine) were tested in S. typhimurium for point mutations and in A. nidulans for mitotic recombination. 2,4-D and MCPA were found to be weakly mutagenic at concentrations between 250 and 750 micrograms/plate in strain TA97a and only after metabolic activation and were recombinogens by inducing mainly mitotic crossing-over in A. nidulans at concentrations of 4-48 microM and 1500-3000 microM, respectively. 2,4-DB, atrazine and propazine were negative in both the Ames and the Aspergillus tests.     

Radioimmunoassay for pmol-quantities of indole-3-acetic acid for use with highly stable [125I]- and [3H]IAA derivatives as radiotracers

A radioimmunoassay for the detection of as little as 0.5–1 pmol indole-3-acetic acid (IAA) in unpurified or partially purified plant extracts is described. The assay makes use of either IAA[¹²⁵I]tyrosine methyl ester or [³H]IAA methyl ester as radioactive antigens and IAA methyl ester as the assay standard (measuring range: 1–200 pmol). Levels of extractable IAA in a number of biological samples have been estimated.Abbreviations BSA bovine serum albumin - 2,4-D 2,4-dichlorophenoxy acetic acid - DMF dimethyl formamide - GC-MS gas chromatography-mass spectroscopy - IAA indole-3-acetic acid - RIA radioimmunoassay - SICM selected ion current monitoring - TLC thin layer chromatography - TME tyrosine methyl ester Part 18 in the series: Use of immunoassay in plant science     

Inhibition of the indole-3-acetic acid-induced epinastic curvature in tobacco leaf strips by 2,4-dichlorophenoxyacetic acid

It has been reported that auxin induces an epinastic growth response in plant leaf tissues. Leaf strips of tobacco (Nicotiana tabacum L. 'Bright Yellow 2') were used to study the effects of indole-3-acetic acid (IAA), the principal form of auxin in higher plants, and a synthetic auxin, 2,4-dichlorophenoxyacetic acid (2,4-D), on epinastic leaf curvature. Incubation of leaf strips with 10 micro M IAA resulted in a marked epinastic curvature response. Unexpectedly, 2,4-D showed only a weak IAA-like activity in inducing epinasty. Interestingly, the presence of 2,4-D resulted in inhibition of the IAA-dependent epinastic curvature. In vivo Lineweaver-Burk kinetic analysis clearly indicated that the interaction between IAA and 2,4-D reported here is not a result of competitive inhibition. Using kinetic analysis, it was not possible to determine whether the mode of interaction between IAA and 2,4-D was non-competitive or uncompetitive. 2,4-D inhibits the IAA-dependent epinasty via complex and as yet unidentified mechanisms.     

Plant regeneration in vitro from embryogenic cultures of spring- and winter-type barley (Hordeum vulgare L.) varieties

Summary Immature embryos of 41 lines of barley were screened in vitro for callus induction and somatic embryogenesis on different media to establish totipotent cultures. The use of modified MS and CC media, both supplemented with 1 g/l casein hydrolysate, and the substitution of agarose for agar resulted in the highest frequencies of somatic embryo induction. Embryogenic callus was induced and plants regenerated from 23 of the lines tested. The auxins 2,4-D, dicamba, picloram and 2,4,5-T were suitable for embryogenic callus induction. High frequencies of somatic embryo germination occurred on CC medium supplemented with 1 mg/l IAA and 0.05 mg/l zeatin. A strong genotypic effect on the capacity and frequency of embryogenic callus formation was found. Cultivar Golden Promise always gave the best results. Experiments with field grown material in 3 consecutive years showed that environmental factors also strongly influenced the induction of somatic embryogenesis and plant regeneration.Abbreviations 2,4-D 2,4-dichlorophenoxyacetic acid - 2,4,5-T 2,4,5-trichlorophenoxyacetic acid - dicamba 3,6-dichloro-o-anisic acid - picloram 4-amino-3,6,6-trichloropicolinic acid - NAA naphtaleneacetic acid - IAA indole-3-acetic acid - ABA abscisic acid - BAP 6-benzyl amino purine - 2iP 6-(3-methyl-2 butenyl 1-amino)purine - GA₃ gibberellic acid     

Levels of endogenous abscisic acid and indole-3-acetic acid influence shoot organogenesis in callus cultures of rice subjected to osmotic stress

Osmotic stress and endogenous hormone levels may have a role in shoot organogenesis, but a systematic study has not yet to investigate the links. We evaluated the changes of the endogenous indole-3-acetic acid (IAA) and abscisic acid (ABA) levels in rice (Oryza sativa L. cv. Tainan 5) callus during shoot organogenesis induced by exogenous plant growth regulator treatments or under osmotic stress. Non-regenerable callus showed low levels of endogenous ABA and IAA, with no fluctuation in level during the period evaluated. The addition of 100 μM ABA or 2 mM anthranilic acid (IAA precursor) into Murashige and Skoog basal induction medium containing 10 μM 2,4-D enhanced the regeneration frequency slightly, to 5 and 35%, respectively, and their total cellular ABA or IAA levels were increased significantly, correspondingly to the treatments. However, the regeneration frequency was greatly increased to 80% after treatment with 0.6 M sorbitol or 100 μM ABA and 2 mM anthranilic acid combined. Both treatments produced high levels of total cellular ABA and IAA at the callus stage, which was quickly decreased on the first day after transfer to regeneration medium. Thus, osmotic stress-induced simultaneous accumulation of endogenous ABA and IAA is involved in shoot regeneration in rice callus.     

Effect of 2,4-dichlorophenoxyacetic acid on the dark- and light-induced pulvinar movements in Cassia fasciculata Michx

2,4-dichlorophenoxyacetic (2,4-D) applied to excised leaves of Cassia fasciculata modified the dark-induced (scotonasty) and light-induced (photonasty) leaflet movements, showing that this compound acts on rapid turgor variation and the concomitant ion migrations, in particular K⁺. 2,4-D inhibited the scotonastic closure in a dose-dependent manner from 10^–8 M to 10^–5 M and promoted the photonastic opening in the same range of concentrations. The compound acted rapidly since a treatment as short as 5 min gave an obvious effect on the motile reaction; however, a lag period of 45–60 min was needed to observe its effect. Although 2,4-D is a weak acid, its greatest physiological efficiency was obtained with pH values close to neutrality. The physiological results are discussed in relation to the chemical properties and the characteristics of transport of the molecule.Abbreviations ABA abscisic acid - 6-BAP 6-benzylaminopurine - 2,4-D 2,4 dichlorophenoxyacetic acid - GA₃ gibberellic acid - HEPES N-[2-hydroxyethyl] piperazine-N-[2-ethanesulphonic acid] - IAA indole-3-acetic acid - NAA 1-naphthaleneacetic acid - MES 2-(N-morpholino)-ethanesulphonic acid     

Analysis of 3-indolylacetic acid and abscisic acid by high-performance liquid chromatography and gas-liquid chromatography

A method of analysis of 3-indolylacetic acid (IAA) and abscisic acid (ABA), allowing the simultaneous extraction of both regulators from plant material, has been developed. The method involves extraction with methanol, isolation of the acid fraction, diazomethane methylation, separation of the hormones through reverse-phase preparative high-performance liquid chromatography, and quantification of both compounds by gas-liquid chromatography. The recovery percentage at each step was monitored with radioactive compounds added at the beginning of the process. The final recovery was 70% for IAA and 96% for ABA. The method was applied to the analysis of the IAA and ABA content of stems of hazel (Corylus avellana L.).     

Tissue culture of the desert plant Anastatica hiërochuntica

Summary Callus derived from the winter annual desert plant Anastatica hiërochuntica was grown on different media, Murashige and Skoog (1962) medium giving the best results. Large amounts of lignified xylem elements were formed resulting in an extremely hard tissue. The growth responses to different auxins, cytokinins and abscisic acid were investigated. When salts (high Na⁺, Ca²⁺ and Cl^--contents) as they can be found in aqueous extracts of desert soils from a natural A. hiëerochuntica habitat were added to Abou-Mandour (1977) or MS-media, growth of callus was inhibited drastically. In the presence of abscisic acid, however, original growth was completely restored. In salt free control media on the other hand, ABA proved to be inhibitory. Drought stress caused a decrease of both cytokinins and indoleacetic acid in the callus while ABA levels were increased, but by far not as distinct as in intact plants. Proline level was not affected by stress.Abbreviations ABA abscisic acid - AM Abou-Mandour-medium - BAP benzylaminopurine - 2,4-D 2,4-dichlorophenoxyacetic acid - DHZR dihydroxyzeatinriboside - DW dry weight - ELISA enzyme linked immuno sorbent assay - FW freshweight - IAA indole-3-acetic acid - NAA 1-naphthaleneacetic acid - IBA indole-3-butyric acid - IPA isopentenyladenosine - Kin kinetin - MS Murashige and Skoog-medium     

核素铯胁迫下接种摩西球囊霉对宿根高粱内源激素和光合的影响

通过盆栽试验研究了宿根高粱接种摩西球囊霉形成共生体后在核素铯污染胁迫下内源激素和光合生理的响应.结果表明： 铯胁迫促进了宿根高粱叶片脱落酸(ABA)的合成,降低了生长素(IAA)、赤霉素(GA)以及玉米素核苷(ZR)的积累,从而导致了ABA/IAA和ABA/GA明显升高,接种摩西球囊霉减少了铯胁迫下IAA、GA以及ZR的降幅以及ABA的增幅,维持了ABA/IAA、ABA/GA和ABA/（IAA+GA+ZR）的稳定性;铯胁迫显著降低了植物净光合速率(P_n)、气孔导度(g_s)、胞间CO₂浓度(C_i)、蒸腾速率(T_r)、光呼吸(P_r)以及暗呼吸(D_r)等光合指标和呼吸指标,造成植物光合效率降低,接种摩西球囊霉缓解了铯胁迫给植物光合效率造成的负效应.说明在利用植物修复核素污染土壤时可引入摩西球囊霉等丛枝菌根真菌,以提高植物光合效率及同化产物,增强植物耐性,提高修复效率.     

Investigation of Effects of Plant Growth Regulators on Liposome Fluidity and Permeability

Naturally occurring plant growth regulators gibberellic acid (GA), indole-3-acetic acid (IAA), abscisic acid (ABA), ethylene and other growth regulating compounds such as 5-methyl-7-chloro-4-ethoxycarbonylmethoxy-2,1,3-benzothiadiazole (TH) and 2,4 dichlorophenoxyacetic acid (2,4-D), had no effect on the partition behavior of a piperidine based spin label in liposomes composed of pure or mixed phospholipids or a phospholipid-sterol mixture. Although no effect on fluidity was observed, TH significantly increased the initial rate of swelling of soybean lecithin-sitosterol liposomes in isotonic glycerol. IAA and ethylene did not influence this rate but ABA, GA and 2,4-D inhibited the initial rate of swelling. Lipid composition of liposomes determined the extent and direction of the effects on swelling rates. The observed swelling behavior was, therefore, not related to fluidity of the bulk membrane lipids but was due, instead, to modification of the access of glycerol to the phospholipid bilayer surface or, alternatively, to the creation of polar channels into the liposomes.