Growth of Avena coleoptiles and pH drop of protoplast suspensions induced by chlorinated indoleacetic acids

Several indoleacetic acids, substituted in the benzene ring, were compared in the Avena straight growth bioassay. 4-Chloroindoleacetic acid, a naturally occurring plant hormone, is one of the strongest hormones in this bioassay. With an optimum at 10^-6 mol l^-1, it is more active than indoleacetic acid, 2,4-dichlorphenoxyacetic acid and naphthaleneacetic acid. 5-Chloro- and 6-chloroindoleacetic acids are very strong auxins as well. Other derivatives tested have a lower activity. 5,7-Dichloro- and 5-hydroxyindoleacetic acids have very low auxin activity at 10^-4 mol l^-1 and may be anti-auxins. Some of the derivatives were compared for their effect on pH decline in stem protoplast suspensions of Helianthus annuus L. and Pisum sativum L. The change of pH occurs without a lag period or with only a very short one. Derivatives which are very active in the Avena straight growth assay cause a larger pH decline than indoleacetic acid, while inactive derivatives cause effectively no pH decline.Abbreviations IAA Indoleacetic acid - 4-Cl-IAA 4-chloroindoleacetic acid - 5,7-Cl₂-IAA etc 5,7-dichloroindoleacetic acid     

A potent allelopathic substance in cucumber plants and allelopathy of cucumber

A potent growth inhibitory substance was isolated from an aqueous methanol extract of cucumber (Cucumis sativus L. cv. Phung Tuong) plants and determined as (2S)-2,3-dihydro-2??-(4-hydroxy-3-methoxyphenyl)-7-methoxy-5-(1,2,3-trihydroxypropyl)benzofuran-3??-methanol (sisymbrifolin) by spectral data. Sisymbrifolin inhibited the growth of cress (Lepidium sativum) and Echinochloa crus-galli seedlings at concentrations greater than 3???M. Concentration of sisymbrifolin in the cucumber plants was the greatest among four growth inhibitory substances, (S)-2-benzoyloxy-3-phenyl-1-propanol, 9-hydroxy-4,7-megastigmadien-9-one, (6S,7E,9S)-6,9,10-trihydroxy-4,7-megastigmadien-3-one, and sisymbrifolin found in the cucumber, whereas growth inhibitory activity of 9-hydroxy-4,7-megastigmadien-9-one against cress and E. crus-galli was the greatest. Total activities of these substances (concentration of the substance/concentration required 50?% growth inhibition) were 14.4, 13.2, 8.5 and 10.7 for (S)-2-benzoyloxy-3-phenyl-1-propanol, 9-hydroxy-4,7-megastigmadien-9-one, (6S,7E,9S)-6,9,10-trihydroxy-4,7-megastigmadien-3-one and sisymbrifolin, respectively. These total activities were about 100-fold greater than those of phenolic acids, which are often mentioned as putative allelochemicals of plants. Thus, these substances may play important roles in the allelopathy of cucumber plants through the growth inhibition of neighboring plant species.     

Some aspects of the control of root growth and georeaction: the involvement of indoleacetic Acid and abscisic Acid

Apical segments of roots of Zea mays L. cv. Orla and cv. Anjou show a strong georeaction during 7 hours geostimulation. This is abolished by detipping the segments and restored by replacing the tips upon the apical cut surfaces. After exodiffusion of endogenous indoleacetic acid (IAA) the retipped segments showed a significantly lower geocurvature. Application of low concentrations of IAA to the basal cut surface of root segments from which endogenous IAA had not been allowed to exodiffuse increased the geocurvature of retipped Orla segments but decreased geocurvature of Anjou segments. At appropriate concentration basally applied IAA restored the georeaction capacity of root segments from which the endogenous auxin had exodiffused. The implications of the interaction between exogenous and endogenous IAA in the control of root georeaction are discussed with special reference to the normal role of endogenous IAA in the regulation of root georeaction and the variation in endogenous IAA content of roots of different cultivars of maize. The probability that the normal control of root growth and georeaction involves concomitant actions in the elongation zone of IAA moving preferentially in the acropetal direction and basipetally transported growth inhibitors (such as abscisic acid) produced in the cap cells is stressed.     

An improved disk bioassay for determining activities of plant growth regulators

An improved filter paper disk bioassay for determining activities of plant growth regulators was developed and evaluated. Indole-3-acetic acid (IAA), (±)-abscisic acid (ABA), and 6-furfurylamino-purine (kinetin) were dissolved in 95% ethanol at fixed dilutions. Specific concentrations of each growth regulator were then evenly dropped onto individual 6-cm paper disks and the solvent evaporated. The activities of the above three water-insoluble plant growth regulators in both solution and the disk assay were compared using the excised cucumber (Cucumis sativus L.) cotyledon root formation bioassay (IAA), wheat (Triticum aestivum L.) coleoptile straight growth bioassay (ABA), and cucumber cotyledon expansion bioassay (kinetin), and similar results were obtained at each concentration. The possible principle of this method has been studied using the cucumber cotyledon expansion bioassay. The results suggested that the disks, as carriers, had highly dispersed kinetin molecules on them and greatly accelerated the dissolution and diffusion of kinetin from disks to water.     

Hormonal regulation of iron-stress response in sunflower roots: a morphological and cytological investigation

Summary Sunflowers are known to respond to Fe deficiency (-Fe) with a typical root tip swelling and the formation of root hairs and transfer cells in the rhizodermis. The possible regulation of this process was examined by a comparative study of root morphology and cytology of intact seedlings (Helianthus annuus L. cv. Giganteus) under -Fe and hormonal treatment in nutrient solution. Longitudinal sections of -Fe roots showed root tip swelling is due to cessation of cell elongation and isodiarnetric volume increase of the cortical cells. Enhanced cell division in the pericycle leads to the formation of lateral root primordia in the swollen zone. Xylem vessel differentiation is markedly accelerated and accompanied by early differentiation of the casparian band in the endodermis. Exogenous application of IAA (10^–8-10^–7 M) via the nutrient solution to Fe sufficient plants causes symptoms which closely mimick the characteristics of Fe deficiency including root hair development. Moreover, rhizodermal cells produce peripheral protuberances reminiscent of -Fe transfer cells. Ethylene-releasing ethephon (10^–4M) also causes subapical swelling and root hair formation. However, wall protuberance development is less pronounced. ABA (10^–5 M) leads to similar root thickening and root hair formation but without any comparable transfer cell differentiation. From the striking similarities between -Fe and IAA treatment it is concluded that this hormone (possibly in cooperation with ethylene) is involved in the Fe stress response of sunflower roots. The importance of a continuous polar IAA transport for this process is discussed.Abbreviations ABA abscisic acid - ACC 1-aminocyclopropane-1-carboxylic acid - Ethephone 2-chloro-ethylphosphonic acid - Fe(III)-EDTA ethylenediaminetetraacetic ferric-sodium salt - IAA indole-acetic acid - TIBA triiodobenzoic acid     

Ontogenetic Changes in the Transport of Indol-3yl-acetic Acid into Maize Roots from the Shoot and Caryopsis

The quantities of endogenous indol-3yl-acetic acid (IAA) in endosperms and scutella of 6-day-old maize seedlings (Zea mays L. cv Giant White Horsetooth) were determined by a fluorimetric method. Endosperms were found to contain 33.4 nanograms IAA per plant, and scutella 7.5 nanograms IAA per plant. [5-³H]IAA applied to endosperms of 6-day-old seedlings moved into the roots and radioactivity accumulated at the apex of the primary root within 8 hours. Two to 7-day-old seedlings were treated simultaneously with [5-³H]IAA in the endosperm and [2-¹⁴C] IAA on the shoot apex. The patterns of transport into the root were found to change during ontogeny: in successively older plants, transport from the shoot into the roots increased relative to transport from the endosperm into the roots. The auxin required for the growth of maize roots could, therefore, partially be contributed by the shoot and endosperm. Ontogenetic changes in the relative importance of these two supplies could be of significance for the integration of growth and development between shoot and root.     

ffects of Some Inhibitors on Potassium-and IAA-Induced Adventitious Root Formation of Excised Cucumber Cotyledon

The effects of some inhibitors on potassium- and IAA-induced rooting were studied adopting the root-formation bioassay in the excised cucumber ( Cucumis sativus L. ) cotyledon. 5-fluomuracil at 7 Ï 10-4 – 7 Ï10-1 mmol/L and cycloheximide at 3.5 Ï 10-4 – 1.05 Ï10-2 mmol/L significantly inhibited potassium- and IAA-induced adventitious root formation of the excised cucumber cotyledons, respectively. Na3VO4 at 0.1 – 1.0 mmol/L obviously inhibited potassium and IAA-induced adventitious rooting of the excised cucumber cotyledons, and similar inhibitory effect was found with 2,3,5-triiodobenzoic acid (TIBA) at 2 Ï 10-4 – 2 Ï 10-l mmol/L.There is a close relationship between potassium and IAA-induced adventitious rooting and the promotive effect of potassium on rooting is possibly brought about via affecting the endogenous level of IAA.     

An alternative explanation for plant growth promotion by bacteria of the genus Azospirillum

Experiments were performed to identify the substances that are excreted by the soil bacterium Azospirillum brasilense Sp7 and that were reported to stimulate the formation of lateral roots and of root hairs of grasses. Azospirillum forms indole-3 acetic acid (IAA) but only in the late stationary growth phase or when tryptophan is present in the medium, but not in continuous cultures or in the logarithmic growth phase of batch cultures. Formation of IAA by Azospirillum requires aerobic conditions. Nitrite can replace IAA in several phytohormone assay, and is even more active than IAA in a test with wheat root segments in which the increase of wet weight is determined. Higher amounts of nitrite are necessary for activity in other classical auxin assays. Nitrite shows 40–60% of the activity of IAA in the straight-growth test of Avena coleoptiles and in the formation of C₂H₄ by pea epicotyl segments. Like IAA, nitrite is inactive in promoting C₂H₄ formation by ripe apple tissues. Since nitrite alone can hardly exert phytohormonal effects, it is postulated that nitrite reacts with a substance in the cells and that a product formed by this reaction functions as auxin. Such a substance could be ascorbate. Exogenously added ascorbate enhances the rate of nitrite-dependent C₂H₄ formation by pea epicotyl sections and the nitrite-dependent increase in the wet weight of wheat root segments. Nitrite is formed by nitrate respiration of Azospirillum. The findings that nitrite can have phytohormonal effects offers an alternative explanation of the promotion of the growth of roots and the enhancement of mineral uptake of grasses by Azospirillum. Indole-acetic acid completely and nitrite partly substitute for an inoculation with Azospirillum in an assay where the increase of the dry weight of intact wheat roots is determined after an incubation for 10 d. Nitrite and IAA are, therefore, possibly the only factors causing an enhancement of the growth of roots of grasses.Abbreviations HPLC high-performance liquid chromatography - IAA indole-3-acetic acid     

On the Physiological Effects of Phloridzin, Phloretin and some Related Substances upon Higher Plants

Some dihydrochalcones and related flavonoids were tested Cor their effects upon indoleacetic acid oxidase (mainly from pea roots) and upon oxidative phosphorylation in mitochondria from cucumber hypocotyls. Also their influence upon the growth of wheat roots and upon the absorption of ¹⁴C-labelled sugars and 2, 4-dichloro-phenoxyacetic acid in wlieat roots was studied. The dihydrochalcone glucoside phloridzin known for its strong inhibitory effects upon sugar transport in some animal tissues gives no corresponding specific effects in higher plants. Phloridzin and some related dihvdrochalcone glycosides as well as the structurally similar compounds naringenin and 2′, 4, 4′-trihydroxychalcone are potent stimulators of indoleacetic acid oxidase. Sieboldin, which compared to phloridzin has an additional hydroxyl group (giving an ortho-diphenolic substitution in the B-ring) is a strong inhibitor of indoleacetic acid oxidase (10^?6M gives about 50 per cent Inhibition). Phloridzin and sieboldin replace each other in different groups of species in the genus Malus. The aglycones phloretin, naringenin and 2′, 4, 4′-trihydroxychalcone stimulate wheat root growth (especially in the presence of auxins or growth-inhibilory sugars) and inhibit the absorption of sugars and of 2, 4-dichlorophenoxyacetic acid. They also inhibit oxidative phosphorylalion and give a distinct uncoupling effect. The glycosides are as a rule less active in all these respects and sieboldin is almost inactive. There is a rather good correlation between uncoupling of oxidative phosphorylation and effects upon root growth, whereas there is no agreement in the details between the effects upon root growth and upon IAA oxidase.     

Auxin effects on rooting in pea cuttings

Light-grown stem cuttingss of Pisum sativum L. cv. Weibull's Marma were rooted in a nutrient solution. The presence of 10 μ M indolylacetic acid (IAA) in the solution for 24 h or longer periods decreased the number of roots subsequently formed to about 50% of control, provided IAA was present in the solution during any of the 4 first 24 h periods. Treatment for 6 h or shorter periods caused no or small response. IAA did not appreciably change the time needed for root formation, the time course of root appearance or the pattern of root distribution along the basal internode. IAA at 100 μ M usually increased the number of roots although variable results were obtained with this IAA concentration.
The number of roots was strongly increased by treatment with indolylbutyric acid (IBA) or 2,4-dichlorophenoxyacetic acid (2,4-D). None of these or other synthetic auxins decreased the number of roots in suboptimal concentrations. Experiments with 10 μ M IBA showed that stimulation of rooting was obtained only if the auxin was present in the rooting solution for several days. Simultaneous treatment with IAA decreased the stimulating effect of IBA to some extent, whereas no such response was obtained if IAA was combined with 2,4-D.
IAA applied in lanolin to the stem of intact cuttings decreased the number of roots formed. Decapitation and debudding of the cuttings decreased the number of roots formed. If at least 2 leaves were left this decrease was efficiently counteracted by an optimal IAA dose applied to the upper part of the stem. A five times higher dose was less effective, indicating a negative effect on rooting also by IAA applied to the shoots.     

小麦根愈伤组织胚胎发育过程研究

实验通过对６个人工合成小麦品系和对照品种“中国春”种子根愈伤组织分化形成再生植株的过程进行形态和组织切片观察,发现分化初期有２种途径,一种是从愈伤组织先形成不定胚,然后再发育成不定芽和不定根,另一种途径是直接从愈伤组织中分化发育成不定根和不定芽;分化后期不定芽和不定根生长发育有３种类型：一种是不定芽发育先于不定根,一种是不定芽与不定期不定芽和不定根生长发育有种类型：一种是不一定芽发育先于不定根,一     

Cytokinins as inhibitors of root growth

The elongation of roots of wheat ( Triticum aestivum L. cv. Diamant II), flax ( Linum usitatissimum L. cv. Concurrent) and cucumber ( Cucumis sativus L. cv. Favör) seedlings in the dark was strongly inhibited by various native and synthetic cytokinins (kinetin, benzyladenine, isopentenyladenine, zeatin and their corresponding 9-ribosides). An inhibition of 50% was obtained for wheat roots with 3 · 10⁻⁹ M zeatin and for flax roots with 6 · 10⁻⁹ M isopentenyladenine. The ribosides were in all cases less inhibitory. The inhibition was reversed by various types of 'antiauxins' and 'antiethylenes' (such as structural auxin analogues, uncouplers, specific inhibitors of ethylene synthesis, free radical scavengers, inhibitors of ethylene action). These substances as a rule counteract also inhibitions caused by auxins. Auxins and cytokinins stimulate ethylene production synergistically, and the similar inhibitory effects of these two types of hormone can be understood if it is assumed that their effect is at least partly mediated through ethylene. The cytokinins must be considered as possible natural inhibitors and regulators of root growth.     

Phenolics and the activities of phenylalanine ammonia-lyase, phenol-β-glucosyltransferase and β-glucosidase in cucumber roots as affected by phenolic allelochemicals

Seven-day-old seedlings of cucumber (Cucumis sativus L.) cv. Wisconsin were treated with 0.1 mM solutions of cinnamic acid (ferulic and p-coumaric acids) and benzoic acid (p-hydroxybenzoic and vanillic acids) derivatives as stressors. The content of free and glucosylated soluble phenols and the activity of phenylalanine ammonia-lyase (E.C.4.3.1.5), phenol-β-glucosyltransferase (E.C.2.4.1.35.), and β-glucosidase (E.C.3.2.1.21.) in seedling roots as well as their length and fresh weight were examined. Changes in glucosylated phenolic content and phenol-β-glucosyltranspherase activity were observed under the influence of all phenolics applied. Treatment with ferulic and p-coumaric acids stimulated the increase of phenylalanine ammonia-lyase and β-glucosidase activity and slightly inhibited cucumber root growth.     

Chromosaponin I specifically interacts with AUX1 protein in regulating the gravitropic response of Arabidopsis roots

We have found that chromosaponin I (CSI), a gamma-pyronyl-triterpenoid saponin isolated from pea (Pisum sativum L. cv Alaska), specifically interacts with AUX1 protein in regulating the gravitropic response of Arabidopsis roots. Application of 60 microM CSI disrupts the vertically oriented elongation of wild-type roots grown on agar plates but orients the elongation of agravitropic mutant aux1-7 roots toward the gravity. The CSI-induced restoration of gravitropic response in aux1-7 roots was not observed in other agravitropic mutants, axr2 and eir1-1. Because the aux1-7 mutant is reduced in sensitivity to auxin and ethylene, we examined the effects of CSI on another auxin-resistant mutant, axr1-3, and ethylene-insensitive mutant ein2-1. In aux1-7 roots, CSI stimulated the uptake of [(3)H]indole-3-acetic acid (IAA) and induced gravitropic bending. In contrast, in wild-type, axr1-3, and ein2-1 roots, CSI slowed down the rates of gravitropic bending and inhibited IAA uptake. In the null allele of aux1, aux1-22, the agravitropic nature of the roots and IAA uptake were not affected by CSI. This close correlation between auxin uptake and gravitropic bending suggests that CSI may regulate gravitropic response by inhibiting or stimulating the uptake of endogenous auxin in root cells. CSI exhibits selective influence toward IAA versus 1-naphthaleneacetic acid as to auxin-induced inhibition in root growth and auxin uptake. The selective action of CSI toward IAA along with the complete insensitivity of the null mutant aux1-22 toward CSI strongly suggest that CSI specifically interacts with AUX1 protein.     

嫁接黄瓜地上部的南瓜根系分泌物对种子萌发的影响

经嫁接黄瓜接穗的南瓜根系分泌物对黄瓜和南瓜的发芽率和胚根、胚轴的伸长均具有明显的抑制作用.分析表明:嫁接黄瓜根系分泌物可以促进黄瓜和南瓜体内吲哚乙酸氧化酶的活性,抑制淀粉酶的活性,从而降低其吲哚乙酸(IAA)水平,影响子叶中贮藏物质的转化和利用,抑制其萌发和生长.     

Levels of endogenous indole-3-acetic acid and indole-3-acetylaspartic acid during adventitious root formation in pea cuttings

Levels of endogenous indole-3-acetic acid (IAA) and indole-3-acetylaspartic acid (IAAsp) were monitored in various parts of leafy cuttings of pea ( Pisum sativum L. cv. Marma) during the course of adventitious root formation. IAA and IAAsp were identified by combined gas chromatography—mass spectrometry, and the quantitations were performed by means of high performance liquid chromatography with spectrofluorometric detection. IAA levels in the root forming tissue of the stem base, the upper part of the stem base (where no roots were formed), and the shoot apex remained constant during the period studied and were similar to levels occurring in the intact seedling. A reduction of the IAA level in the root regenerating zone, achieved by removing the shoot apex, resulted in almost complete inhibition of root formation. The IAAsp level in the shoot apex also remained constant, whereas in the stem base it increased 6-fold during the first 3 days. These results show that root initiation may occur without increased IAA levels in the root regenerating zone. It is concluded that the steady-state concentration is maintained by basipetal IAA transport from the shoot apex and by conjugation of excessive IAA with aspartic acid, thereby preventing accumulation of IAA in the tissue.     

Saturated humidity accelerates lateral root development in rice (Oryza sativa L.) seedlings by increasing phloem-based auxin transport

Auxin transport plays a significant role modifying plant growth and development in response to environmental signals such as light and gravity. However, the effect of humidity on auxin transport is rarely documented. It is shown here that the transport of labelled indole-3-acetic acid (IAA) from the shoot to the root is accelerated in rice (Oryza sativa L. ssp. indica cv. IR8) seedlings grown under saturated humidity (SH-seedlings) compared with plants grown under normal humidity (NH-seedlings). The development of lateral roots in SH-seedlings was greatly enhanced compared with NH-seedlings. Removal of the shoot from SH-seedlings reduced the density of lateral roots, and the application of IAA to the cut stem restored the lateral root density, while the decapitation of NH-seedlings did not alter lateral root development. Phloem-based auxin transport appeared responsible for enhanced lateral root formation in SH-seedlings since (i) the rate of IAA transport from the shoot to the root tip was greater than 3.5 cm h-1 and (ii) naphthylphthalamic acid (NPA)-induced reduction of polar auxin transport in the shoot did not influence the number of lateral roots in SH-seedlings. It is proposed that high humidity conditions accelerate the phloem-based transport of IAA from the leaf to the root, resulting in an increase in the number of lateral roots.     

Root excretion of carboxylic acids and protons in phosphorus-deficient plants

Phosphorus deficiency-induced metabolic changes related to exudation of carboxylic acids and protons were compared in roots of wheat (Triticum aestivum L. cv Haro), tomato (Lycopersicon esculentum L., cv. Moneymaker), chickpea (Cicer arietinum) and white lupin (Lupinus albus L. cv. Amiga), grown in a hydroponic culture system. P deficiency strongly increased the net release of protons from roots of tomato, chickpea and white lupin, but only small effects were observed in wheat. Release of protons coincided with increased exudation of carboxylic acids in roots of chickpea and white lupin, but not in those of tomato and wheat. P deficiency-induced exudation of carboxylic acids in chickpea and white lupin was associated with a larger increase of carboxylic acid concentrations in the roots and lower accumulation of carboxylates in the shoot tissue compared to that in wheat and tomato. - Citric acid was one of the major organic acids accumulated in the roots of all investigated species in response to P deficiency, and this was associated with increased activity and enzyme protein levels of PEP carboxylase, which is required for biosynthesis of citrate. Accumulation of citric acid was most pronounced in the roots of P-deficient white lupin, chickpea and tomato. Increased PEP carboxylase activity in the roots of these plants coincided with decreased activity of aconitase, which is involved in the breakdown of citric acid in the TCA cycle. In the roots of P-deficient wheat plants, however, the activities of both PEP carboxylase and aconitase were enhanced, which was associated with little accumulation of citric acid. The results suggest that P deficiency-induced exudation of carboxylic acids depends on the ability to accumulate carboxylic acids in the root tissue, which in turn is determined by biosynthesis, degradation and partitioning of carboxylic acids or related precursors between roots and shoot. In some plant species such as white lupin, there are indications for a specific transport mechanism (anion channel), involved in root exudation of extraordinary high amounts of citric acid. This revised version was published online in June 2006 with corrections to the Cover Date.     

Growth inhibitors in oat grains. II. Bioassays for characterization of a new substance in leachate of oat hulls (Avena sativa) regulating root growth

Oat hull leachate, auxin and ethylene cause similar effects on oat ( Avena sativa L. cv. Risto) seedlings: inhibited root elongation, swollen roots and extensive root hair formation. Micrographs reveal that the leachate caused a reorientation of the cell expansion in the root from a longitudinal to a radial direction. Dry weights showed that the distribution of nutrients from the endosperm into the seedling was not inhibited, but regulated by the leachate. Tests with Ag⁺, inhibiting the effects of ethylene, as well as biotests with seeds of Amaranthus caudatus L. cv Mörk Purpur excluded ethylene from being responsible for the morphological changes. Results of growth tests and Avena coleoptile bioassuys indicated that oat hull leachate contains a substance that is not identical with IAA, but mediates the effects of endogenous IAA of the seedling. In a parallell investigation (Häggqquist, M.-L. et al. 1988. - Physiol. Plant. 72: 423–127) the active substance of an high performance liquid chromatography (HPLC) fraction of the leachate was identified as tryptophan.     

Control of root and shoot-bud formation from potato tuber tissue cultured in vitro

Comparative studies on the control of root and shoot-bud formation and plant regeneration have been undertaken in discs (1 × 6 mm in diameter) excised from tubers of potato ( Solanum tuberosum L. cv. Irish Cobbler) cultured in vitro. The results clarified that the optimal culture conditions for shoot-bud formation were quite different from those for root formation and, in conclusion, that (1) shoot-buds were produced when cultured in modified White's medium containing 0.25 M mannitol with 2.3 μ M zeatin and 0.57 μ M indole-3-acetic acid (IAA) at 20°C under relatively high light irradiation, while (2) roots were readily formed when cultured in modified White's medium containing 29 m M sucrose with 4.7 μ M kinetin plus 1.7 μ M IAA at 30°C in darkness.