An analysis of the changes in growth rate occurring during the initial stages of geocurvature in shoots

Abstract A detailed study of the changes in growth rate on the two sides of a shoot prior to, and following geostimulation has been undertaken. Data from etiolated Zea seedlings, and cucumber hypocotyls, and from light grown sunflower hypocotyls are presented. In all cases, the differential growth which brings about geocurvature begins simultaneously along the length of the organ. This is contrary to previous reports in the literature which suggested that curvature begins first at the tip of an organ and progresses basipetally. The feature which is common to all species investigated is that growth of the upper side of the shoot ceases following geostimulation. In some cases there is also a marked acceleration in growth rate on the lower side of the shoot (sunflower) but other species show no such acceleration of growth (cucumber). It has been assumed for many years that the major factor causing upward curvature was an acceleration of growth on the lower side of the organ. The data presented here show that the cessation of growth on the upper side is a major, and in some cases the only factor bringing about geocurvature. The data are discussed in relation to the mechanisms which might control geotropic curvature.     

Time sequence of the effect of ethylene on transport, uptake and decarboxylation of auxin

The effect of ethylene on the uptake, decarboxylation and basipetaltransport of IAA-1-¹⁴C, IAA-2-¹⁴C and NAA-1-¹⁴C in cotton stemsections (Gossypium hirsutum L., var. Stoneville 213) was studied.A reduction in the capacity of cotton stem sections to transportauxin basipetally appears in sections excised from plants exposedto ethylene for only 3 hr and increases with fumigation time. In addition to reducing transport, increasing ethylene pretreatmentperiods from 3 to 15 hr also progressively reduced the uptakeof ¹⁴C and increased the release of ¹⁴C as ¹⁴CO₂ from IAA-1-¹⁴C.The effect of ethylene on the decarboxylation of IAA-1-¹⁴C wassignificant when expressed as either the cpm of ¹⁴C releasedper hr per mg dry weight or the cpm released per hr per mm²in contact with the IAA donor. Comparative experiments usingIAA-1-¹⁴C and IAA-2-¹⁴C demonstrated that the effect of ethyleneon the decarboxylation of IAA was primarily a cut surface effectwhich apparently contributes to the reduction of IAA uptakeby ethylene. Although ethylene significantly reduced the transport of NAA-1-¹⁴C,uptake was significantly increased rather than decreased aswith IAA-1-¹⁴C while decarboxylation was unaffected. Ethylene pretreatment caused no significant changes in the dryweight or the cross-sectional area of the absorbing surfaceof the transport tissue. ¹A contribution of the Texas Agricultural Experiment Station.Supported in part by Grant GB-5640, National Science Foundationand grants from the Cotton Producers Institute and the NationalCotton Council of America. ²Present address: Central Research Department, E. I. Du PontDe Nemours and Company, Wilmington, Delaware 19898, U. S. A. (Received May 29, 1969; )     

Geotropic response of cucumber hypocotyls

Exogenous auxin applied to the apical bud of intact cucumberseedlings promoted elongation of the hypocotyl, mostly of theapical zone (zone I) and to a lesser extent, the middle andbasal zones (zones II and III). Geotropic curvature was clearlyobserved 30 min after geostimulation. The curvature began atzone I and moved toward the basal zones with prolonged geostimulation.Decapitation (removal of apex and cotyledons) did not produceany appreciable effect on the geotropic response, but the curvaturewas reduced with the removal of zone I. Auxin application toseedlings without zone I restored their ability to respond togravity. The lower epidermis of horizontally oriented hypocotylsplayed a more important role than the upper in the geotropicresponse. The extensibility of the lower epidermal cell wallwas increased significantly by the geostimulus before the onsetof the curvature response. (Received August 24, 1973; )     

Stimulation of auxin-induced elongation of cucumber hypocotyl sections by dihydroconiferyl alcohol. Dihydroconiferyl alcohol inhibits indole-3-acetic acid degradation in vivo and in vitro

The mechanism by which dihydroconiferyl alcohol (DCA) stimulatesindole-3-acetic acid (IAA)-induced elongation of cucumber hypocotylsections was studied. Although DCA did not affect the uptakeof IAA-5-³H by hypocotyl sections, the endogenous level of IAA-5-³Hin DCA-treated sections was much higher than in DCA untreatedones. IAA-5-³H in the incubation medium was degraded in thepresence of hypocotyl sections, and this degradation of IAAwas inhibited by DCA. An in vitro experiment with horseradishperoxidase revealed that DCA inhibited the IAA degrading activityof the oxidase, as did caffeic acid and ferulic acid. Theseresults suggested that DCA enhances IAA-induced cucumber hypocotylelongation by acting as an antioxidant of IAA. (Received June 4, 1975; )     

Comparison of electric and growth responses to excision in cucumber and pea seedlings. I. Short-distance effects are a result of wounding

The local electric response to stem excision in both pea epicotyls and cucumber hypocotyls is a depolarization of the cells in the wound area. If we define wound area as the region of local depolarization, we find that it extends for approximately 10 mm from the cut or wound site in pea epicotyls, whereas it can reach up to 40 mm in cucumber hypocotyls. The wound-induced depolarization in pea cells is transient, reaching its maximal amplitude within 1–2 min, whereas in cucumber cells this depolarization is more sustained. A third difference between wound responses in pea and cucumber is the intermittent appearance of spikes, i.e. very short, rapidly reverted depolarizations which frequently accompany the basic depolarization in cucumber but not in pea cells. These spikes can propagate in both directions along the hypocotyl axis. The cause of the different responses of pea and cucumber cells is unknown. A possible explanation might be found in different degrees of electrical cell coupling in the two species. This possibility was investigated in cucumber hypocotyls by measuring the cell input resistance (R_in) of epidermal cells at various axial distances from the cut. Shorter distances increase the likelihood of shunting the cell membrane resistance through the shortened symplastic path to the cut surface. With a series of cuts made at decreasing distances from the measured site, cell depolarization increased without comparable changes in R_in. Two conclusions were drawn. Firstly, wound-induced depolarizations are not brought about by shunting of the cell resistance in the wound area. Secondly, the depolarization is probably not carried by ion channels but may be caused by an inhibition of proton pump activity. Parallel to its depolarizing effect on the membrane potential, excision led to a severe and sustained decline in the cucumber hypocotyl growth rate only when carried out sufficiently close to the growing region (45 mm from the hook). Similar excision in pea epicotyls failed to change the growth rate. Both electrical and growth data support the concept that the high and sustained responsiveness of cucumber seedlings to wounding is caused by a particular sensitivity of their proton pump mechanism.     

Apical Dominance in Vicia faba

Apical dominance phenomena have been studied in seedlings ofVicia faba particularly in relation to the movement about theplant of uracil-2-¹⁴C applied to the cotyledons. Decapitationjust below the second node releases the growth of the lowermostlateral bud and inhibition is completely reimposed by applicationof indole-3-acetic acid (IAA) to the cut surface. Uracil-2-¹⁴Capplied in solution to the cotyledons is distributed in thestems of all experimental seedlings with no consistent differencesdue to decapitation or IAA application. On the other hand, decapitationresults in a rapid increase in uracil-2-¹⁴C content in the lateralbuds which far exceeds their promoted growth. This uptake iscompletely suppressed by IAA application. A ring of tri-iodobenzoicacid (TIBA)-lanolin paste around the stem above the bud suppressesIAA action both in bud growth and on uracil-2-¹⁴C uptake, andalso on the movement of IAA-1-¹⁴C down the stem. TIBA-application to the base of the bud does not prevent IAAaction on bud growth, but also does not prevent the movementof IAA-1-¹⁴C (or a water soluble product of its metabolism)into the bud. Direct application of kinetin to the lateral bud of intact plantscauses a short-lived release of growth. Gibberellic acid producesa smaller and scarcely significant increase which is additiveto the kinetin effect. Neither has any effect on uracil-2-¹⁴Cmovement into the bud. The implications of these findings are discussed in relationto various existing theories of the mode of auxin action inapical dominance and it is concluded that their strongest supportis for a mechanism involving the suppression of phloem differentiationin the vascular supply to the bud.     

Effect of malformin on adventitious root formation and metabolism of indoleacetic acid-2-14C by Phaseolus vulgaris

Malformin inhibited rooting on cuttings of Phaseolus vulgaris.IAA antagonized malformin-induced inhibition of rooting, butmalformin inhibited IAA-induced swelling on the base of thecuttings. It was suggested that IAA-induced swelling was mediatedby ethylene. Malformin did not inhibit transport of root-promotingsubstances from upper portions of the cuttings or polar transportof IAA-2-¹⁴C, nor did it alter the melting point of DNA or thebinding of DNA to histone. Although malformin appeared to alterthe metabolism of IAA-2-¹⁴C, the effect may have been the resultof a marked and selective stimulation of efflux of IAA-2-¹⁴Cmetabolites by malformin. Efflux of IAA or its metabolites maycontribute toward inhibition of rooting by malformin. ¹ Journal Paper No. 4688 of the Purdue Agricultural ExperimentStation. Supported in part by grant GB-7158 from the NationalScience Foundation. ² Present address: Botanisches Institut der Technischen UniversitätBraunschweig, 3300 Braunschweig, Humboldtstraße 1. (Received March 9, 1972; )     

Effects of Morphactin on Indol-3yl-acetic Acid Transport, Growth, and Geotropic Response in Cereal Coleoptiles

The effects of the morphactin 2-ehloro-9-hydroxyfluorene-9-carboxylicacid methyl ester [CFM] on growth, geotropic curvature and transportand metabolism of indol-3yl-acetic acid [IAA-5-³H] in the coleoptilesof Zea mays and A vena saliva have been investigated. A strongcorrelation has been found to exist between the inhibition ofthe geotropic response and the inhibition of auxin transport.CFM supplied at concentrations sufficient to abolish auxin transporthas been shown to promote the elongation of Zea, but not ofAvena, coleoptile segments. CFM does not change the patternof metabolism of IAA in Zea coleoptile segments. In these segmentsIAA is metabolized when its concentration is high, but the radioactivitytransported basipetally, or laterally in geotropically stimulatedcoleoptiles, is virtually confined to the IAA molecule. Radioactivityexported into the basal receiver blocks is wholly confined toIAA. It is concluded that CFM inhibits the geotropic responsein coleoptiles by suppression of the longitudinal and lateralauxin transport mechanisms. The growth-promoting propertiesof this substance cannot be linked with its effects on eitherauxin metabolism or transport.     

Uptake, transport and stability of [2-14C] abscisic acid in cucumber following foliar application

The uptake, transport and stability of ABA after foliar applicationto cucumber plants was studied using [2-¹⁴C] ABA. ABA uptakethrough uninjured epidermis of cucumber leaves appeared to bevery slow compared with the uptake through abraded leaves. Transportof [2-¹⁴C] ABA in the same system was also found to be veryslow. No metabolic changes in the absorbed [2-¹⁴C] ABA weredetected 24 hr after its application as checked by TLC and GLCtechniques. These results are compared and discussed in relationto previous reports. ¹The Irene and David Schwartz Professor of Plant Genetics (Received April 26, 1976; )     

Pathways of auxin transport in the intact pea seedling (Pisum sativum L.)

Summary When small colonies of the pea aphid [Acyrthosiphon pisum (Harris)] were established on the stem of Meteor Dwarf Pea seedlings (Pisum sativum L.), ¹⁴C was found in the honeydew 4.5 h after applying IAA-1-¹⁴C to a fully-expanded foliage leaf. In contrast, no activity was found in the honeydew or aphids 4.5 h after the application of IAA-1-¹⁴C to the intact apical bud even though the internode upon which the aphids were feeding contained high levels of ¹⁴C. The lack of radio-activity in aphids feeding on stems to which IAA-1-¹⁴C was applied via the apical bud was found not to be influenced by the internode position or by the transport interval allowed (up to 24 h).Radioactivity derived from either foliar or apical applications of IAA-1-¹⁴C was not transported through stem tissues killed by heat treatment. Xylem function was shown not to be impared by the heat treatment employed.It was concluded that the long-distance transport of IAA from the apical bud of intact pea seedlings does not take place in the phloem sieve tubes involved in the transport of metabolites from foliage leaves, or in the non-living tissues of the xylem.     

Slow wave potentials in cucumber differ in form and growth effect from those in pea seedlings

A positive hydraulic signal in the form of a xylem pressure step was applied to the roots of intact seedlings of Cucumis sativus L. and Pisum sativum L. Surface electrodes at three positions along the epicotyl/hypocotyl recorded a propagating depolarization which appeared first in the basal, then the central and sometimes the apical electrode positions and fitted the characteristics of a slow wave potential (SWP). This depolarization differed between pea and cucumber. It was transient in cells of pea epicotyls but sustained in cucumber hypocotyls. It was not associated with a change in cell input resistance in pea epicotyls but preceded an increase in the input resistance of cucumber hypocotyl cells. With the increased xylem pressure the growth rate (GR) of cucumber hypocotyls and pea epicotyls underwent a transient increase, peaking after 5 min. If the depolarization reached the growing upper region, it preceded a sustained decrease in the GR of cucumber hypocotyls but only a transient decrease in the GR of pea epicotyls. A temperature jump in the root medium (heat treatment) induced a steep pressure spike in the xylem of the cucumber hypocotyl which showed similar electric and growth effects as the previously applied, non-injurious pressure steps. We suggest that the observed differences in the electric and growth responses between the species were caused by the closure of ion channels in depolarized cells of cucumber but not pea seedlings.     

The Redistribution of Radioactivity in Geotropically Stimulated Hypocotyls of Helianthus annuus Pretreated with Radioactive Calcium

The purpose of this research was to study the movement of ⁴⁵Cain geotropically stimulated hypocotyls of Helianthus annuus.Six-day old seedlings fed with ⁴⁵CaCl₂ were placed horizontalfor 1, 2, or 3 hours. The ⁴⁵Ca accumulated more in the upperhalf of the hypocotyl than in the lower half, particularly afterthree hours. Geotropic stimulation under nitrogen causes nosignificant difference between the two halves. The role of thisdifferential accumulation of Ca in geotropic curvature is discussed.     

Perception et réaction géotropiques de l'épicotyle d'Asparagus officinalis

Geoperception and georeaction in Asparagus officinalis epicotyls The location and the role of the statenchyma were investigated in Asparagus officinalis epicotyls 1, 2 or 3 cm in length. The statocytes represent a monocellular sheath which comprises the outer layer of the pericycle. Three phases of differentiation (S₁, S₂, S₃) of the statocytes may be distinguished by criteria of cell length, the state of vacuolisation and the sedimentation of amyloplasts and nuclei with respect to gravity. The statenchyma was found to be located only in the first 10 mm of the epicotyl 1, 2 or 3 cm in length; in effect the oldest statocytes (S₃) differentiate into sclerenchyma beneath this level. The geotropic curvature of the epicotyls begins with a positive (downward) curvature and the negative (upward) response is only realized after 15–30 min of geotropic stimulation. The upper limit of the bending is located in the zone containing the mature statocytes (S₃), while the lower limit is located much closer to the base of the epicotyl. Experiments on decapitation of the epicotyls have indicated that the S₁ and S₂ statocytes do not play an important role in the perception of gravity. However, with S₃ statocytes, it has been demonstrated that there is a statistical correlation between the initial rate of curvature of the epicotyls and the length of their sheaths. Thus, the longer the sheath, the more the epicotyls curve. From the results obtained it is proposed that the statocytes are responsible for the geotropic perception of Asparagus officinalis epicotyls.     

Auxin transport in intact pea seedlings (Pisum sativum L.): The inhibition of transport by 2,3,5-triiodobenzoic acid

Summary The application of 2,3,5-triiodobenzoic acid (TIBA, 10 mg·g^-1 in lanolin) to the stem of intact pea seedlings (Pisum sativum L.) inhibited the basipetal transport of ¹⁴C from indoleacetic acid-1-¹⁴C (IAA-1-¹⁴C) applied to the apical bud, but not the transport of ¹⁴C in the phloem following the application of IAA-1-¹⁴C or sucrose-¹⁴C to mature foliage leaves. It was concluded that fundamentally different mechanisms of auxin transport operate in these two pathways.When TIBA was applied at the same time as, or 3.0 h after, the application of IAA-1-¹⁴C to the apical bud, ¹⁴C accumulated in the TIBA-treated and higher internodes; when TIBA was applied 24.0 h before the IAA-1-¹⁴C, transport in the stem above the TIBA-treated internode was considerably reduced. TIBA treatments did not consistently influence the total recovery of ¹⁴C, or the conversion of free IAA to indoleaspartic acid (IAAsp). These results are discussed in relation to the possible mechanism by which TIBA inhibits auxin transport,.Attention is drawn to the need for more detailed studies of the role of the phloem in the transport of endogenous auxin in the intact plant.Abbreviations TIBA 2,3,5-triiodobenzoic acid - IAAsp indoleaspartic acid     

Effect of Xyloglucan Nonasaccharide on Cell Elongation Induced by 2,4-Dichlorophenoxyacetic Acid and Indole-3-acetic Acid

Xyloglucan nonasaccharide (XG9) is recognized as an inhibitorof 2,4-D-induced long-term growth of segments of pea stems.In the presence of 10^–5 M 2,4-D, inhibition by 10^–9M XG9 of elongation of third internode segments of pea seedlingswas detected within 2 h after the start of incubation, in someexperiments. Analysis by double-reciprocal (Lineweaver-Burk)plots of elongation in the presence of various concentrationsof 2,4-D, with or without XG9, gave parallel lines, indicatingthat XG9 inhibited 2,4-D-induced elongation in an uncompetitivemanner. XG9 did not influence the 2,4-D-induced cell wall loosening.Thus, XG9 does not fulfill the proposed definition of an "antiauxin". XG9 at 10^–11 to 10^–6 M did not influence IAA-inducedelongation of segments from pea third internodes, azuki beanepicotyls, cucumber hypocotyls, or oat coleoptiles. Inhibitionof IAA-induced elongation by XG9 was not observed even whenthe segments from pea or azuki bean were abraded. Furthermore,fucosyl-lactose at 10^–11 to 10^–4 M did not affectthe IAA-induced elongation of segments of pea internodes orof azuki bean epicotyls. XG9 may be incapable of inhibitingthe IAA-induced cell elongation (especially in oat) or, alternatively,the endogenous levels of XG9 may be so high that exogenouslyapplied XG9 has no inhibitory effect on IAA-induced elongation. (Received February 28, 1991; Accepted May 25, 1991)     

The Effect of Morphactin (Methyl 2-Chloro-9-hydroxyfluorene-9-carboxylate) on Basipetal Transport of Indol-3-ylacetic Acid in Hypocotyl Sections of Phaseolus vulgaris L.

The effect of morphactin (methyl 2-chloro-9-hydroxyfluorene-9-carboxylate)on basipetal transport of auxin (Indol-3-ylacetic acid-2-¹⁴C)was studied in bean (Phaseolus vulgaris) hypocotyl with thedonor-receiver block method. Morphactin (5 x 10^–6m) reduced IAA (5 x 10^–6m) transportintensity by an average of 83 per cent and auxin transport capacityby 90 per cent, but transport velocity was not affected. Morphactin did not inhibit uptake of IAA into hypocotyl tissue,but it did prevent transfer of IAA from the tissue into receiverblocks. Chromatographic analysis of the tissue after 4 h IAA-2-¹⁴Ctransport showed that 54 per cent of the total activity wasin the form of IAA in the control and 42 per cent in the morphactintreated tissue. No difference was found in the rate of decarboxylationof IAA-1-¹⁴C between control and morphactin treated tissue sections.Nor could any difference between control and morphactin be shownin the radioactivity associated with a TCA ppt fraction. Ina study of the transportable auxin pool, morphactin decreasedthe size of the pool and increased the half-life of decay ofauxin transport from 1•22 h to 3•85 h. In a kineticanalysis of the reversal of morphactin (5 x 10^–6m) inhibitionby increasing concentration of IAA-2-14C (5 x 10^–6m to2 x 10^–5m), it was shown that IAA transport resemblesMichaelis-Menten enzyme reaction kinetics, and that inhibitionby morphactin fitted a ‘mixed type’ model. IAA hada dissociation constant of 8•5 x 10^–6m and morphactinthat of 4•3 x 10^–7m with a K_m for the transport processof 8•5 x 10^–6m.     

Hydrogen ion-induced curvature in cucumber hypocotyls

A strip of filter paper saturated with a pH 3.0 buffer solutionand placed on the upper side of a light-grown cucumber hypocotylseedling can eliminate the negative geotropic curvature of horizontallyplaced seedlings, by promoting elongation of the upper side.These filter paper strips also caused strong curvature of verticallyplaced seedlings and induced cell wall loosening of the epidermis. (Received February 16, 1973; )     

Relation of Phytochrome-enhanced Geotropic Sensitivity to Ethylene Production

Brief exposure of etiolated pea (Pisum sativum cv. Alaska) seedlings to red light enhances subsequent development of geotropic curvature of the stem. Both this response and inhibition of ethylene production by red light become maximal 8 hours after illumination. Very low concentrations of applied ethylene inhibit development of geotropic curvature, whereas hypobaric treatment enhances geotropic sensitivity by removing endogenous ethylene. Increased geotropic sensitivity after illumination is accompanied by increased lateral migration of ³H-indoleacetic acid in response to gravity, and ethylene inhibits this lateral migration. It is suggested, therefore, that red light-enhanced geotropic sensitivity is caused by increased lateral auxin transport resulting from a reduction in ethylene production after illumination.     

Comparison of electric and growth responses to excision in cucumber and pea seedlings. II. Long-distance effects are caused by the release of xylem pressure

Excision of a growing stem causes local wound responses, such as membrane depolarization and growth inhibition, as well as effects at larger distances from the cut. In this study, cucumber hypocotyls were excised 100mm below the hook, so that the growing region was beyond the reach of the wound-induced depolarization (up to 40mm). Even at such a distance, the cut still caused a considerable and rapid drop in the hypocotyl growth rate. This growth response is not a direct wound response because it does not result from the cut-induced depolarization and because it can be simulated by root pressure manipulation (using a pressure chamber). The results indicate that the growth response resulted from the rapid release of the xylem pressure upon excision. To test this conclusion we measured the xylem pressure by connecting a pressure probe to the cut surface of the stem. Xylem pressure (P_x) was found to be +10 to +40kPa in cucumber hypocotyls and -5 to -10 kPa or lower in pea epicotyls. Excision of the cucumber hypocotyl base led to a rapid drop in P_x to negative values, whereas excision in pea led to a rapid rise in P_x to ambient (zero) pressure. These fast and opposite p_x changes parallel the excision-induced changes in growth rate (GR): a decrease in cucumber and a rise in pea. The sign of the endogenous xylem pressure also determined whether excision induced a propagating depolarization in the form of a slow wave potential (SWP). Under normal circumstances pea seedlings generated an SWP upon excision whereas cucumber seedlings failed to do so. When the P_x in cucumber hypocotyls was experimentally inverted to negative values by incubating the cumber roots in solutions of NaCN or n-ethylmaleimide, excision caused a propagating depolarization (SWP). The experiment shows that only hydraulic signals in the form of positive P_x steps are converted into propagating electric SWP signals. These propagating depolarizations might be causally linked to systemic ‘wound’ responses, which occur independently of the short-distance or direct wound responses.     

The function of the pentose phosphate pathway in Phaseolus mungo hypocotyls

The metabolism of d-gluconate-[1-¹⁴C] and -[6-¹⁴C] by segments from etiolated hypocotyls of Phaseolus mungo has been studied. The release of ¹⁴CO₂ from gluconate-[1-¹⁴C] was greater than that from gluconate-[6-¹⁴C] in all parts of hypocotyls examined. Incorporation of the radioactivity from gluconate-[6-¹⁴C] into RNA, lignin and aromatic amino acid fractions was greater in the upper (younger) part of the hypocotyls. Incorporation into sugars was greater in the lower (more mature) parts.