The Elongation Response of Watermelon Hypocotyls to Indole-3-Acetic Acid: A Comparative Study of Excised Segments and Intact Plants

Carrington, C. M. S. and Esnard, J. 1988. The elongation responseof watermelon hypocotyls to indole-3-acetic acid: a comparativestudy of excised segments and intact plants.—J. exp. Bot39: 441–450. The auxin-growth response along the hypocotyl of Citrullus lanatus(Thumb.) Mansf. seedlings was studied. In excised segments,promotion of elongation was seen in all zones at the concentrationsof IAA used (10–4–10–2 mol m-3). In intactplants, only the most basal zone showed unequivocal IAA-extensionwhile in the most apical zone elongation was inhibited by auxin.This difference between segments and intact plants for apicalzones suggests a modifying effect of the apex and cotyledonson the growth response. Indeed, removal of the apex and colyledonsonly affected elongation in the zones adjacent to the excisionbut only in buffer-treated plants, not auxin-treated plants.Auxin supplied apically to the intact plant only resulted ina short-lived promotion of elongation whereas basally suppliedauxin gave a longer-lasting effect Zonal differences betweenauxin-promoted growth of excised segments suggests that sensitivityto auxin varies in the hypocotyl. The response of intact plantsto auxin was shown to be more complex than in segments. Thus,responses given by segments are poor indicators of auxin activityin intact plants. Key words: IAA, Citrullus lanatus, growth, plant hormone sensitivity     

On the interaction of growth retardants with IAA and kinetin

In the pea test a highly positive response to the treatment with IAA reversed to a negative one or became 5 to 6 times weaker when CCC was applied together with IAA. In cultivating pea seedlings, following their decapitation, for two days in a 0.25 per cent CCC solution and then in water, growth of their cotyledonous axillaries (cotylaries) were inhibited. This inhibitive action of CCC could be made ineffective when the seedlings, following two-days’ cultivation in the CCC solution, were grown further in kinetin solutions (0.37–3 mg per 1). Cotylaries of decapitated pea seedlings, when grown in kinetin solutions were inhibited. With kinetin solutions of 6–12 mg/l a strong inhibition also occured in the growth of roots at the apical parts of which spherical swellings were developing. The CCC supplied to the roots of intact etiolated pea seedlings is translocated acropetally into the stem at a rate of about 5 cm per hour. Decapitation of the plant causes retardation of this transport, yet a coat of 0.00001–1% IAA or kinetin paste produces acceleration of the stream. Existence of an antagonism between CCC and IAA, demonstrated earlier, was found holding true also for B-9 (N, N-dimethyl-aminesuccinamic acid) and IAA, as the inhibitive action of B-9, 0.06% solution on the growth of lettuce hypocotyls was reduced to a highly significant degree when the plants were supplied with B-9 together with IAA at a concentration of 10 mg/l.     

Developmental changes in the gibberellin-induced growth response in stem segments of light-grown pea genotypes

The effects of GA on stem elongation were studied using segments from one tall and three dwarf light-grown pea genotypes varying in endogenous hormone content. Stem segments were cut at two distinct ages: when the fourth internode was at about 6–13% of full expansion (early-expansion) or at 18–25% of full expansion (mid-expansion). Light microscopy and flow cytometry were used to demonstrate that GA does not induce cell division in excised pea stem segments. The growth studied here was strictly elongation. Measurement of final segment length after 48 hours and high resolution measurement of growth kinetics over 20 hours using an angular position transducer were done on segments treated with hormone solutions. Our data indicate that the action of GA on stem elongation can be classified into two distinct modes. The first, apparent in early-expansion stem segments, shows distinct growth kinetics and is independent of the endogenous IAA concentration of the segments. Quantitation of IAA by GC/MS in early-expansion segments of wild type pea incubated with gibberellin shows that an increase in IAA concentration is part of the GA response in such segments. The second mode of GA action is evinced in mid-expansion segments. Whereas there is no short term (<20 h) response to GA alone (as determined by growth kinetics), there is a long term (48 h) response whose magnitude decreases across the genotypes with decreasing endogenous hormone content. Growth responses indicate that in mid-expansion segments exogenous GA acts by enhancing IAA action but appears to be unable to augment endogenous IAA content. Contradictory reports of the response of excised stem segments to GA can be reconciled when tissue genotype and developmental stage are considered.     

Enhancement of lateral bud growth in Coleus blumei BENTH. by (2-chloroethyl) trimethylammonium chloride (CCC)

The relationship of GA to apical dominance in Coleus was examinedby substituting 1 % IAA, in lanolin, for the shoot apex of CCC-treated,control and GA-treated plants containing, theoretically, hyponormal,normal and hypernormal GA levels, respectively. The greatestinhibition of lateral bud growth was obtained in the treatmentcombining 1 % IAA and 100 ppm GA, suggesting that GA may beimportant in the apical dominance of Coleus. CCC inhibited main axis growth, reduced the level of endogenousGA and caused a marked release of lateral buds from apical dominance. The significant stimulation of lateral bud growth by CCC couldnot be ascribed to reduced endogenous GA since it was not reversedby exogenous GA, or by GA plus IAA, whereas 100 ppm GA overcamethe inhibition of main axis growth by CCC. It was also shownthat the CCC stimulation was not a result of compensatory growth,that is, enhanced lateral bud growth resulting from reducedapical bud growth. The CCC effect on lateral buds was interpretedas involving a system independent of auxin and GA or else apossible immobilization of auxin in addition to inhibition ofGA biosynthesis. (Received December 5, 1967; )     

Effects of Chlorocholine Chloride on Phytohormones and Photosynthetic Characteristics in Potato (<Emphasis Type="Italic">Solanum tuberosum</Emphasis> L.)

Effects of chlorocholine chloride (CCC) on phytohormones and photosynthetic characteristics of Zhongshu 3, a potato (Solanum tuberosum L.) variety widely cultivated in south China, were studied by foliar CCC application on 24 and 28 days after emergence, that is, at the tuber initiation stage. It was found that on 42 days after emergence, that is, at the tuber bulking stage, spraying CCC increased indolacetic-3-acid (IAA) and zeatin (Z) contents but decreased abscisic acid (ABA) content in leaves. The content ratios of IAA/Z, IAA/ABA, Z/ABA, and (IAA + Z)/ABA in leaves treated with CCC were higher than those of the control. CCC plays a prominent regulating role in the photosynthesis of Zhongshu 3. The net photosynthetic rate (P _n), stomatal conductance (G _s), intercellular CO₂ concentration (C _i), and transpiration rate (T _r) of treated leaves were superior to those of controls at the tuber bulking stage. CCC markedly increased tuber yield and quality. The contents of sucrose and starch in tubers treated with CCC increased at the end of the vegetation period, whereas the contents of reducing sugars and solanine decreased. CCC at 2.0 g L⁻¹ was found to be the most effective concentration. Collectively, the results of this research identify phytohomonal metabolism and photosynthetic physiology of potato leaves as processes affected early after application of CCC resulting in significantly improved increases in tuber yield and quality.     

Dynamics of IAA and cytokinins in flower tissues of transgenic tobacco mutant plants with mutant phenotype

A comparative analysis of IAA and total cytokinin contents at various developmental stages of floral morphogenesis (I–XII) in transgenic tobacco (Nicotiana tabacum L.) plants with mutant phenotypes was performed. Disturbances in flower structure were correlated with changes in the content and dynamics of these phytohormones. In plants with the phenotype “wavy corolla”, IAA content increased by three times as compared with control at stage VIII; in plants with the phenotype “longostyly”, IAA content increased gradually from VIII to XII stage and sharply (by nine times) at stage XII. In plants with the phenotype “wavy corolla”, total content of cytokinins was lower than in control at all developmental stages (except for stage I), whereas their dynamics coincided with that in control flowers. In plants with the phenotype “longostyly”, cytokinin content was twice lower than in control at early developmental stages, whereas it oscillated at later stages, approaching to control value at stage XII. It is supposed that differences observed could result from T-DNA insertion into plant own genes, which led to changes in the content and dynamics of phytohormones in developing flowers and production of anomalies in the flower structure.     

An efficient in vitro regeneration protocol for Tagetes minuta

Tagetes minuta is a source of secondary products which are used as pharmaceuticals, pesticides and as flavour components in the food industry. Cotyledons and hypocotyls of T. minuta were cultured on MS medium with combinations of IAA or NAA and BA. Hypocotyl-derived callus developed adventitious shoots which failed to develop further. Cotyledon-derived callus, cultured on medium with IAA, regenerated adventitious shoots which developed into plantlets on MS medium or half-strength MS with 2.85 μM IAA. Cotyledons cultured on medium with 5.71 μM IAA + 44.4 μM BA and transferred to MS medium for shoot growth yielded the highest number of shoots. Nodal segments from developing shoots were micropropagated on half-strength MS medium with 2.58 μM IAA and 95% of plantlets produced adapted successfully to greenhouse conditions. In vitro plants micropropagated from nodes had many shoots whereas plants regenerated from shoot tips had only a single main stem. This difference in morphology was retained after two months growth in a greenhouse. There were no significant differences in leaf and shoot fresh and dry weights among the regenerated plants after two months growth. After six subcultures of cotyledon-derived callus on medium with IAA and BA all explants lost their ability to regenerate except those cultured on medium with 17.23 μM IAA and 44.4 μM BA. The methods of regeneration developed will facilitate selection of T. minuta plants more tolerant of environmental stress, their micropropagation, and the in vitro production of secondary products. This revised version was published online in June 2006 with corrections to the Cover Date.     

Effect of CCC on the gibberellin content of excised sunflower organs

Summary CCC was shown to be effective in retarding stem growth of sunflower; this effect was overcome by gibberellic-acid application. Using an agar-diffusion technique, the gibberellin (GA) content of sunflower apices treated with CCC was found to be significantly reduced as compared to controls. Similarly, the GA content of agar diffusates obtained from 2-day-old sunflower root tips treated with CCC was also significantly reduced as compared to controls.Root exudate or bleeding sap obtained from mature CCC treated sunflower plants contained no measurable GA-like substance, although it could not be argued that this was due to suppression of GA synthesis in the root systems.     

Changes in the level of endogenous gibberellins and auxins in apical buds ofChenopodium rubrum L. after application of growth substances reversing the effect of (2-chlorethyl)-trimethylammonium chloride (CCC) on flowering

The application of CCC at concentrations inhibiting flowering ofChenopodium rubrum reduces the level of endogenous gibberellins in the apical buds of the plants. The effect of CCC may be reversed by appropriate concentrations of gibberellin (GA-), indole acetic acid (IAA) or kinetin. Kinetin applied to the apical bud during floral induction reduced the level of endogenous gibberellins similarly as CCC and if both CCC and kinetin were applied simultaneously their action was additive. On the other hand IAA applied under the same conditions increased the level of endogenous gibberellins and after joint application of CCC and IAA their level was the same as in untreated control plants. After application of CCC during floral induction the level of endogenous auxins did not change markedly but an active substance “x” appeared on the chromatograms of indole compounds. This substance was found also after simultaneous application of GA- and CCC but not after joint application of CCC and kinetin. If follows from our results that the same morphological phenomenon (flowering) can take place in plants considerably differing as to their level of endogenous growth substances. The ratio of different growth substances is obviously more important than the actual level of the single substances.     

Effect of auxins and plant oligosaccharides on root formation and elongation growth of mung bean hypocotyls

The interaction of auxins – IAA, IBA or NAA – with galactoglucomannan oligosaccharides (GGMOs) on adventitious root formation and elongation growth of mung bean hypocotyl cuttings was studied. GGMOs induced adventitious roots in the absence of auxins; however, their effect was lower compared with IBA or NAA. On the other hand, in the presence of auxins, GGMOs inhibited adventitious root induction. Their effect depended on the concentration of oligosaccharides and the type of auxin used. The highest inhibition effect of GGMOs at a concentration of 10⁻⁸ M in the presence of IBA and NAA was observed. In the presence of IAA their inhibition was non-significant in regard to the concentration. The interaction of auxins with GGMOs resulted in the formation of adventitious roots on a shorter part of hypocotyls compared with the effect of auxins alone. However, roots were induced more extensively along the hypocotyls treated with GGMOs compared with the control. GGMOs inhibited the length of induced adventitious roots in the presence of IAA, while in combination with IBA or NAA they were ineffective. The elongation of hypocotyls induced by IAA or IBA was inhibited by GGMOs, too. However, in the presence of NAA or by endogenous growth they were without any significant effect on elongation growth. These findings suggest that GGMOs in certain concentrations might inhibit rooting and the elongation process dependant on auxin used.     

Cytokinin and Ethylene Affect Auxin Transport-Dependent Rhizogenesis in Hypocotyls of Common Ice Plant (<Emphasis Type="Italic">Mesembryanthemum crystallinum</Emphasis> L<Emphasis Type="Italic">.</Emphasis>)

Hypocotyl explants of Mesembryanthemum crystallinum regenerated roots when cultured vertically with either the apical end (AE) or basal end (BE) in media containing indole-3-acetic acid (IAA). IAA alone induced roots regularly from the basal end of the explants, either from the cut surface immersed in the medium or from the opposite side. The inhibitors of auxin efflux carriers, α-naphthylphthalamic acid (NPA) and 2,3,5-triiodobenzoic acid (TIBA), inhibited rhizogenesis only from AE-cultured explants, indicating the role of polar auxin transport in root regeneration in this system. Cytokinin (zeatin, kinetin, BAP) added to auxin-containing medium reduced rhizogenesis from the explants maintained with BE and AE and additionally changed the IAA-induced pattern of rooting in AE-cultured explants by favoring rooting from the apical end and middle part of the hypocotyl with its concomitant reduction from the basal end. The addition of kinetin did not influence the content of IAA in the explants maintained with AE, suggesting that the cytokinin effect on root patterning was not dependent on auxin biosynthesis. Kinetin, however, strongly enhanced ethylene production. The importance of ethylene in regulating PAT-dependent rhizogenesis was tested by using an ethylene antagonist AgNO₃, an inhibitor of ethylene synthesis aminoethoxyvinylglycine (AVG), and a precursor of ethylene, 1-aminocyclopropane-1-carboxylic acid (ACC). AgNO₃ applied together with IAA or with IAA and kinetin strongly reduced the production of ethylene, inhibited rhizogenesis, and induced nonregenerative callus from BE, suggesting the need for ethylene signaling to elicit the rhizogenic action of auxin. A reduction of rhizogenesis and decrease of ethylene biosynthesis was also caused by AVG. In addition, AVG at 10 μM reversed the effect of cytokinin on root patterning, resulting in roots emerging only from BE on the medium with IAA and kinetin. Conversely, ACC at 200 μM markedly enhanced the production of ethylene and partly mimicked the effect of cytokinin when applied with IAA alone, thus confirming that in cultured hypocotyls of ice plant, cytokinin affects IAA-induced rhizogenesis through an ethylene-dependent pathway.     

The decrease in auxin polar transport down the lupin hypocotyl could produce the indole-3-acetic Acid distribution responsible for the elongation growth pattern

The variation of indole-3-acetic acid (IAA) transport along Lupinus albus L. hypocotyls was studied using decapitated seedlings and excised sections. To confirm that the mobile species was IAA and not IAA metabolites, dual isotope-labeled IAAs, [5-³H]IAA + [1-¹⁴C]IAA, were used. After apical application to decapitated seedlings, the longitudinal distribution of both isotopes at different transport periods showed that the velocity of IAA transport was higher in the apical, elongating region than in the basal, non-growing region. This variation in velocity was not a traumatic consequence of decapitation because after application of IAA to the basal region of decapitated seedlings, both the velocity and intensity of IAA transport were lower than in the apical treatment. The variation in IAA transport down the hypocotyl was confirmed when it was measured in excised sections located at different positions along the hypocotyl. The velocity and, to a greater extent, the intensity of IAA transport decreased from the apical to the basal sections. Consequently, if the amount of IAA reaching the apical zones of lupin hypocotyl were higher than the IAA transport capacity in the basal zones, accumulation of mobile IAA might be expected in zones located above the basal region. In fact, an IAA accumulation occurred in the elongating region during the first 4-h period of transport after apical treatment with IAA. It is proposed that the fall in IAA transport along the hypocotyl might be responsible for the IAA distribution and, consequently, for the growth distribution reported in this organ. An indirect proof of this was obtained from experiments that showed that the excision of the slowly transporting basal zones strongly reduced the growth in the remaining part of the organ, whereas excision of the root caused no significant modification in growth during a 20-h period.     

BA、GA3和IAA对富士苹果果形形成的影响

以9年生富士苹果为材料,研究富士苹果形成过程中BA、GA,和IAA对果形、品质、内源激素含量的影响。结果表明：在花期喷GA,对富士苹果的坐果率影响效果最好,分别比清水对照和无处理对照提高49％和79．45％。BA100mg·L^-1GA3300mg·L^-1＋IAA20mg·L^-1对降低富士苹果偏斜率效果最好。在花期、花期＋幼果期和幼果期分别喷BA和在花期喷GA3可以提高果实中可溶性固形物的含量。花期喷IAA、BA、GA,和BA＋GA,＋IAA对内源激素IAA和GA3的含量均有促进作用,在整个生长季内不同处理下IAA、GA3含量均高于清水对照且差异明显;花期喷BA、GA3和BA＋GA3＋IAA对内源激素ZT含量有促进作用,尤其在果实幼果期和膨大期与清水对照相比效果明显;花期喷GA3和BA＋GA3＋IAA与清水对照相比内源激素ABA含量有明显降低,其中幼果期变化显著。     

Kinetics of growth retardant and hormone interactions in affecting cucumber hypocotyl elongation

The capacities of indole-3-acetic acid (IAA) and gibberellin A₃ (GA₃) to counteract the inhibitory effects of (2-chloroethyl) trimethylammonium chloride (CCC), 2-isopropyl-4-dimethylamino-5-methylphenyl-1-piperidinecarboxylate methyl chloride (Amo-1618), and N,N-dimethylaminosuccinamic acid (B-995) on hypocotyl elongation in light-grown cucumber (Cucumis sativus L.) seedlings were investigated. One μg of GA₃ applied to the shoot tip was sufficient to completely nullify the effect of 10 μg of Amo-1618 or 25 μg of B-995 applied simultaneously to the shoot tip, and 10 μg of GA₃ completely counteracted the effect of 10⁻³ m CCC added to the root medium. One μg of IAA counteracted the effect of 10⁻³ m CCC in the root medium, but IAA did not nullify the action of either Amo-1618 or B-995. Experiments were conducted using 2 growth retardants simultaneously, which indicated that Amo-1618 and CCC inhibit a common process, namely GA biosynthesis, essential to hypocotyl elongation. However, since the effect of CCC was overcome by applications of both GA and IAA, growth retardation resulting from treatment with CCC apparently is not due solely to inhibition of GA biosynthesis. B-995 did not interact additively with either Amo-1618 or CCC, which suggests that B-995 affects a process different from those affected by the other 2 retardants. Thus, while inhibition evoked by B-995 is reversible by applied GA, the action of B-995 does not appear to be inhibition of GA biosynthesis.     

Melatonin: a growth-stimulating compound present in lupin tissues

Melatonin (N-acetyl-5-methoxi-tryptamine), a well-known animal hormone synthetised by the pineal gland, plays a key role in the circadian rhythm of vertebrates. An exhaustive bibliographical revision of studies on melatonin in plants published since 1990 points to very few studies (around 20), of which only 8 have a clear plant physiological focus. The data presented in this study demonstrate that melatonin plays a physiological role in plant tissues. Melatonin is seen to be a molecule that promotes vegetative growth in etiolated Lupinus albus L. hypocotyls, in a similar way to IAA. The measurements of melatonin and IAA in lupin hypocotyls by high-performance liquid chromatography with electrochemical detection, and their identification by tandem mass spectrometry, point to a different distribution of these molecules in etiolated hypocotyls.Abbreviations IAA Indole-3-acetic acid - MEL Melatonin - HPLC–EC High-performance liquid chromatography with electrochemical detection - MS/ESI+ Tandem mass spectrometry in positive electrospray ionization mode     

The role of GA,IAA and BAP in the regulation of <Emphasis Type="Italic">in vitro</Emphasis> shoot growth and microtuberization in potato

The effect of auxin, GA and BAP on potato shoot growth and tuberization was investigated under in vitro condition. The shoot length of potato explants increased with the increasing of concentrations (0.5 – 10 mg dm⁻³) of IAA treatment especially with the addition of GA₃ (0.5 mg dm⁻³), but was inhibited by BAP (5 mg dm⁻³). The root number and root fresh weight of potato explants increased with the increasing of IAA levels either in the presence of GA₃ (treatment IAA+GA) or not (IAA alone). However, no root was observed in the treatment IAA+BAP, instead there were brown swollen calli formed around the basal cut surface of the explants. The addition of GA₃ remarkably increased the fresh weight and diameter of calli. Microtubers were formed in the treatments of IAA+BAP and IAA + GA + BAP but not observed in the treatments of IAA alone or IAA + GA. IAA of higher concentrations (2.5 – 10 mg dm⁻³) was helpful to form sessile tubers. With the increasing of IAA levels, the fresh weight and diameter of microtubers increased progressively. At 10 mg/L IAA, the fresh weight and diameter of microtubers in the treatment of IAA + GA + BAP were 409.6 % and 184.4 % of that in the treatment of IAA + BAP respectively, indicating the interaction effect of GA and IAA in potato microtuberization.     

High Frequency of Shoot Regeneration from Hypocotyls and Stem Segments of Antirrhinum majus (Snapdragon)

A broadly applicable direct shoot regeneration method from hypocotyls and stem explants has been developed for six cultivars of Antirrhinum majus L. In order to establish a stable and high frequency of shoot regeneration system, leaves, hypocotyls and stem explants of six cultivars were tested with 72 combinations of auxin (naphthaleneacetic acid (NAA) or 3-indoleacetic acid (IAA)) and cytokinin (6-benzylaminopurine (BA) or zeatin (Z)). A few adventitious shoots were directly regenerated from hypocotyl segments of cv. Orchid on MS medium with NAA + BA, IAA + BA, NAA + Z and IAA + Z. High frequency of direct shoot regeneration was obtained from hypocotyl segments on MS medium with 0.05, 0.1 or 0.25 mg l⁻¹ NAA + 2 mg l⁻¹ Z and 0.5 mg l⁻¹ IAA + 2 mg l⁻¹ Z. Finally, stable and high frequency (92–100%) of shoot regeneration with more than 10 adventitious shoots per explant was achieved from the hypocotyls and stem explants of all six cultivars on MS medium with 0.25 mg l⁻¹ NAA + 2 mg l⁻¹ Z. The shoots emerged directly from the hypocotyls and stem segments 4 weeks after culture initiation.     

Gibberellin Induced Changes in Diffusible Auxins from Savoy Cabbage

Diffusates from apices of young plants of savoy cabbage treated with gibberellie acid (GA) and apices of control plants have been examined with respect to their content of Indole auxins. Three indole Compounds were detected and identified on the basis of their chromatographic characteristics in several systems. These compounds were: glucoubrassicin, indole-3-acetic acid (IAA) and indole-3-acetonitrile (IAN). An effect of GA on the total auxin activity of the diffusate was noted 90 hours after treatment, while an increase in stem height occurred 48 hours later. This increase in auxin effect of the entire diffusates was shown bv chromogenic development and bioassay of chromatograms of diffusates to be a result of an increase in level of the IAA content. A concomitant decrease in I the glucobrassicin content was indicated. Since GA was found to have no effect on the enzymatic conversion of tryptophan or tryptamine to IAA, it is proposed that the effect of GA is on the conversion of glucobrassicin to IAA.     

气调和乙烯对梅果叶绿素和内源激素含量的影响

本文研究了在（25±1）℃下,气调包装和乙烯吸收剂处理对采后青梅果实叶绿素含量、内源激素IAA,GA3,ABA含量和乙烯释放量的影响及它们之间的关系。结果表明：气调包装果实叶绿素含量最高,其次是乙烯吸收剂处理的;各处理中气调包装果实的乙烯释放量始终很低,GA3含量较高,IAA和ABA含量则较低;对照果实的则相反,乙烯释放量很高,IAA和ABA含量较高,而GA3含量较低。乙烯吸收剂处理的处于二者之间。气调包装可以维持果实较高的GA3水平,降低ABA含量,保持较高的GA3／ABA值,抑制IAA和乙烯的生成,延缓梅果叶绿素的降解。     

Distribution of Indole-3-Acetic Acid in the Apoplast and Symplast of Squash (Cucurbita maxima) Hypocotyls

The concentration of endogenous IAA was higher in an apoplasticsolution (2.3xl0^–7M) than in a symplastic solution (0.5x 10^–7 M) obtained from segments of etiolated squash (Cucurbitamaxima Duch.) hypocotyls. Exogenously applied IAA (10^–5M) increased the level of IAA in both the apoplastic and thesymplastic solution. The concentration of IAA in the apoplasticsolution increased to 75% of the concentration of exogenousIAA in 4 h, but that in the symplastic solution increased onlyto 23% of the concentration of exogenous IAA. These resultsdemonstrate that the concentration of endogenous IAA is higherin the apoplast than in the symplast of squash hypocotyls, andthey suggest that IAA exerts its physiological effects, at leastto some extent, from the outside of cells. (Received September 20, 1996; Accepted January 10, 1997)