Empirical Models for Xylogenesis in Populus deltoides

Empirical quantitative models were constructed for Populus deltoidesdescribing temporal and spatial changes in vessel characteristicsof metaxylem, both within individual central leaf traces andwithin all central leaf traces considered as a morphologicalunit at a given transverse level in the stem (the central tracesympodia). Similar models were constructed for secondary vesselcharacteristics. The growth processes of the stem segment throughwhich the vasculature extended were incorporated in these modelsto illustrate how a functional vascular system is maintainedin the stem as a whole. The central trace sympodia representedthe integrals of the temporal and spatial functions for individualcentral leaf traces. Metaxylem vessel production ceased in individualleaf traces two plastochrons before the cessation was reflectedin the central trace sympodia because of the integrative natureof the sympodial complex. A functional continuum of developmentwas apparent between metaxylem vessels of the central tracesympodia and secondary vessels of the stem. The transition betweenmetaxylem and secondary xylem production in the central tracesympodia corresponded with cessation of leaf and internode elongation. Populus deltoides Bartr. ex Marsh., cottonwood, primary xylem, secondary xylem, primary-secondary vascular transition, leaf growth, xylogenesis     

The Effects of N-1-Naphthylphthalamic Acid and (2-Chloroethyl)-phosphonic Acid on the Gravity-induced Lateral Transport of 2, 4-Dichlorophenoxyacetic Acid

N-1-Naphthylphthalamic acid markedly reduced the polar lateralmovement of 2, 4-D-[1-¹⁴C] induced by gravity in horizontalsegments of Helianthus hypocotyl. The pattern of this inhibitionresembled the inhibition of basipetal polar transport, suggestingthat both polar movements are operated by essentially the samephysiological mechanisms though there are indications of sensitivitydifferences between the two systems. Ethrel pretreatment of segments, though inhibiting the basipetaltransport of 2, 4-D by 33 per cent over the concentration range0.2 to 2.0 per cent, has little significant effect on gravity-inducedlateral polarity of movement, suggesting that ethylene doesnot act directly on the auxin transport system.     

Identification and Characterization of Linoleic Acid as an Endogenous Modulator of in Vitro N-1-Naphthylphthalamic Acid Binding

An endogenous inhibitor of the in vitro binding of the phytotropin N-1-naphthylphthalamic acid to microsomal membranes was detected in extracts prepared from etiolated pea (Pisum sativum L.) epicotyls. Following extensive purification, the inhibitor was identified as linoleic acid. Authentic linoleic acid inhibited N-1-naphthylphthalamic acid binding noncompetitively in a dose-dependent manner, exhibiting a 50% inhibitory concentration of approximately 24 ([mu]M. Using a variety of fatty acids and their derivatives, this inhibition was found to exhibit strict structural requirements, with both linoleic and linolenic acids being the most inhibitory. A variety of membrane-solubilizing detergents elicited no such inhibitory activity when tested at equivalent concentrations. The possible physiological significance of this interaction is discussed and it is proposed that linoleic acid serves as an intracellular modulator of phytotropin binding and therefore polar auxin transport.     

Regulation of the Activity of N-1-Naphthylphthalamic Acid Binding Protein by ATP and Phosphatase

N-1-naphthylphthalamic acid (NPA), an auxin transport inhibitor, was found to bind specifically to a crude membrane preparation from sugar beet seedling leaf cell suspension cultures. The dissociation constant (Kd) and binding protein concentration were found to be 1.71 mol dm^–3 and 220 pmol g^–1(membrane protein), respectively. The amount of specific ³H-NPA binding was significantly increased by adding Mg²⁺ATP to the binding assay solution. Treatment of membrane preparations with acid phosphatase, prior to the NPA binding assay, resulted in lower specific binding. ATP activation and phosphatase inactivation were culture stage dependent. Although a considerable effect could be detected when using cells from day 8 (representing the linear phase), the same treatment did not alter the binding if cells from day 1 (representing lag phase) or day 14 (representing the stationary phase) were used. These observations have strongly highlighted the possible involvement of a phosphorylation and dephosphorylation mechanism in vivo in the regulation of the activity of the NPA binding protein. High phosphatase activity was found in the supernatant, but not in the membrane pellet) after 50 000 g centrifugation. Our present study has indicated that receptor activity could be regulated by a phosphorylation and dephosphorylation mechanism in plants.     

5'-Azido-N-1-Napthylphthalamic Acid, a Photolabile Analog of N-1-Naphthylphthalamic Acid : Synthesis and Binding Properties in Curcurbita pepo L

A photolabile analog of N-1-naphthylphthalamic acid (NPA), 5′-azido-N-1-naphthylphthalamic acid (Az-NPA), has been synthesized and characterized. This potential photoaffinity label for the plasma membrane NPA binding protein competes with [³H]NPA for binding sites on Curcurbita pepo L. (zucchini) hypocotyl cell membranes with K_0.5 = 2.8 × 10⁻⁷ molar. The K_0.5 for NPA under these conditions is 2 × 10⁻⁸ molar, indicating that the affinity of Az-NPA for the membranes is only 14-fold lower than NPA. While the binding of Az-NPA to NPA binding sites is reversible in the dark, exposure of the Az-NPA treated membranes to light results in a 30% loss in [³H]NPA binding ability. Pretreatment of the membranes with NPA protects the membranes against photodestruction of [³H]NPA binding sites by Az-NPA supporting the conclusion that Az-NPA destroys these sites by specific covalent attachment.     

The N-1-Naphthylphthalamic Acid-Binding Protein Is an Integral Membrane Protein

N-1-Naphthylphthalmic acid (NPA)-binding protein is a plasmalemma (PM) protein involved in the control of cellular auxin efflux. We re-evaluated the spatial relationship of this protein with the PM of zucchini (Cucurbita pepo L.) hypocotyls. First, Triton X-114 partitioning indicated that the NPA-binding protein was more hydrophobic than most PM proteins. Second, the NPA-binding activity was found to be resistant to proteolytic digestion in membranes. Maximum concentrations of binding sites for NPA were virtually identical in untreated and proteinase K-treated PMs: 19.2 and 20.6 pmol [3H]NPA bound/mg protein, respectively. The insensitivity of the NPA-binding protein was not due to its presence inside tightly sealed vesicles or due to lack of protease activity in the conditions tested. This protein could be made sensitive to proteolytic degradation upon solubilization by 3-[(3-cholamidopropyl)dimethylammonio]-1-propanesulfonate in the presence of sodium molybdate. Proteinase K treatment decreased the concentration of binding sites to 0.84 pmol [3H]NPA bound/mg protein from 9.2 for untreated, solubilized PM. Third, this activity could not be solubilized by chaotropic agents or sodium carbonate treatment of intact PM. This study indicates that the NPA-binding protein may be an integral membrane protein and contradicts previously reported findings that suggested that this protein was peripheral to the PM.     

Kinetic characterization of N-1-Naphthylphthalamic acid binding sites from maize coleoptile homogenates

When treated with blue light, intact cells of Euglena gracilis Klebs var. bacillaris Cori, bleached strain W₃BUL, show a series of positive peaks at 384, 411, and 440 nm in the blue-light-minus-dark difference spectrum; bleached strain 1224-5/24 shows a series of positive peaks at 386, 417, and 448 nm under the same conditions. The same changes are observed in a 27,000xg supernatant from darkgrown W₃. The absorption change appears to be a consequence of shifts in the absorption of carotenoids; it is not seen in cells of W₃BUL grown on SAN 9789 (4-chloro-5-(methylamino)-2-(,, -trifluoro-m-tolyl)-3(2H)pyridazinone) to deplete the carotenoids or in cells of W₁₀BSmL, a mutant lacking carotenoids. Inhibitors of flavin-mediated reactions, reductants and valinomycin had no effect on the activity of the system. The activity in the 27,000xg supernatant was associated with material of a molecular weight more than 2.5×10⁶ and was insensitive to heating for 2 min at 100° C but was reduced or eliminated on longer heat treatment or addition of Triton X-100, indicating a possible association with membrane material. Photoactivity is enriched in the lower density fractions of a flotation gradient, and correlates with the -carotene content in all fractions. Similar spectral changes can be obtained by comparing the iodine catalyzed cis-to-trans isomerization of -carotene in a CS₂-CHCl₃ solvent. The action spectrum for the absorbance change shows effectiveness peaks in the 370–390 and 420–448-nm regions, with no marked effectiveness past 500 nm. Thus the photosensitizer may not be a carotenoid (at least not a normally-occurring C₄₀ carotenoid). These blue-lightinduced absorption changes and their action spectra are discussed in relation to such blue-light-mediated responses as carotenogenesis, chloroplast development and phototaxis.Abraham and Etta Goodman Professor of Biology, to whom reprint requests should be directed     

Exogenous Auxin Effects on Lateral Bud Outgrowth in Decapitated Shoots

In 1933 Thimann and Skoog demonstrated exogenous auxin repressionof lateral bud outgrowth in decapitated shoots ofVicia faba. This evidence has given strong support for a role of auxinin apical dominance. Most, but not all, investigators have confirmedThimann and Skoog's results. In the present study, auxin treatmentswere carried out on ten different species or plant types, manyof which were treated with auxin in different forms, media andunder different light conditions. The Thimann–Skoog experimentdid work for most species (i.e. exogenous auxin did repressbud outgrowth) including thedgt tomato mutant which is knownto be insensitive to auxin in certain responses. Toxic auxinsymptoms were observed in some but not all species. The Thimann–Skoogexperiment did not work for greenhouse-grownColeus or forArabidopsis. Light was shown to reduce apical dominance inColeus andIpomoeanil . apical dominance; lateral bud outgrowth; axillary bud; auxin; IAA; decapitation; Vicia faba ; Ipomoea nil ; Pisum sativum ; Phaseolus vulgaris ; Lycopersion exculentum ; dgt ; Coleus blumei ; Arabidopsis thaliana ; Helianthus annuus ; Thimann–Skoog     

1-萘氨甲酰苯甲酸、外源IAA和GA3对拟南芥根的伸长和向重力性反应的影响

拟南芥幼苗用1-萘氨甲酰苯甲酸(NPA)、IAA和GA3处理后测定根的伸长和向重力性弯曲的结果表明,低浓度(0.001μmol·L-1)IAA和(0.01~1μmol·L-1)GA3促进根的伸长和向重力性弯曲,高浓度(0.01~10μmol·L-1)IAA和(10~100μmol·L-1)GA3的则相反。NPA在总体上是抑制根的伸长和向重力性弯曲,但低浓度(0.4μmol·L-1)的NPA有促进根伸长的趋势。低浓度的IAA和GA3均拮抗NPA对根伸长的影响,且低浓度的GA3对根伸长的促进作用并不依赖IAA。     

Defoliation effects on isoprene emission from Populus deltoides

Isoprene emission from plants is one of the principal ways in which plant processes alter atmospheric chemistry. Despite the importance of this process, few long-term controls over basal emission rates have been identified. Stress-induced changes in carbon allocation within the entire plant, such as those produced by defoliation, have not been examined as potential mechanisms that may control isoprene production and emission. Eastern cottonwood (Populus deltoides) saplings were partially defoliated and physiological and growth responses were measured from undamaged and damaged leaves 7 days following damage. Defoliation reduced isoprene emission from undamaged and damaged leaves on partially defoliated plants. Photosynthetic rates and leaf carbon and nitrogen pools were unaffected by damage. Photosynthetic rate and isoprene emission were highly correlated in undamaged leaves on undamaged plants and damaged leaves on partially defoliated plants. There was no correlation between photosynthetic rate and isoprene emission in undamaged leaves on partially defoliated plants. Isoprene emission was also highly correlated with the number of source leaves on the apical shoot in damage treatments. Increased carbon export from source leaves in response to defoliation may have depleted the amount of carbon available for isoprene synthesis, decreasing isoprene emission. These results suggest that while isoprene emission is controlled at the leaf level in undamaged plants, emission from leaves on damaged plants is controlled by whole-branch allocation patterns. Received: 12 May 1998 / Accepted: 9 November 1998     

Effects of the Indole-3-Acetic Acid (IAA) Transport Inhibitors N-1-Naphthylphthalamic Acid and Morphactin on Endogenous IAA Dynamics in Relation to Compression Wood Formation in 1-Year-Old Pinus sylvestris (L.) Shoots

Both N-1-naphthylphthalamic acid (NPA) and methyl-2-chloro-9-hydroxyfluorene-9-carboxylic acid (CF) inhibit the polar transport of indole-3-acetic acid (IAA) and, therefore, are attractive tools for investigating IAA's role in the regulation of plant growth. Ringing an intact conifer shoot with lanolin containing NPA or CF induces the formation of compression wood above the ring. This induction has been attributed to a postulated accumulation of IAA above the application site of the IAA transport inhibitor, but the validity of this postulation has never been confirmed. Using gas chromatography-selected ion monitoring-mass spectroscopy with [13C6]IAA as an internal standard, we measured the levels of endogenous free and conjugated IAA in 1-year-old Pinus sylvestris (L.) shoots ringed with NPA or CF. The level of free IAA was dramatically decreased below the ring, indicating that the polar transport of endogenous IAA was inhibited by the treatment. However, the free IAA level above the ring, where compression wood was formed, was also slightly lower than in control shoots. The lack of IAA accumulation above the site of the IAA transport inhibitor could not be explained by an increase in IAA conjugation. Furthermore, the turnover of [2-14C]IAA, measured using high-performance liquid chromatography with on-line radioactivity monitoring, was the same in NPA-treated and control shoots. The decrease in IAA level above a NPA or CF ring is attributed to these substances being transported acropetally and interfering with polar IAA transport along the shoot. It is concluded that compression wood formation above a NPA or CF ring is not associated with an overall increase in cambial region IAA level or increased IAA turnover. Instead, we suggest that acropetally transported NPA and CF induce compression wood formation by interacting with the NPA receptor in differentiating tracheids, thereby locally increasing IAA in these cells.     

Effects of Sequential Exposure to Auxin and Cytokinin on Xylogenesis in Cultured Explants of Jerusalem Artichoke (Helianthus tuberosus L.)

During the course of a 4-d culture period, explants of Jerusalemartichoke tuber were exposed to auxin (0.2 mg 1^–1 2, 4-dichlorophenoxyaceticacid), and cytokinin (5.0 mg 1^–1 benzyl-amino purine),under a range of sequential regimes, to study the influenceof each hormone on tracheary element formation. The resultsindicate that auxin was necessary early in the culture periodand was primarily involved in cell proliferation. Cytokininstimulated xylogenesis when present late in the culture period,concomitant with the phase of cytodifferentiation, but not whenrestricted to the early period. The implications for a sustainedperiod of commitment to differentiation are discussed. Xylem differentiation, Jerusalem artichoke, auxin, cytokinin, tissue culture     

The Auxin Transport Inhibitor N-(1-Naphthyl)phthalamic Acid Elicits Pseudonodules on Nonnodulating Mutants of White Sweetclover

The collection of symbiotic (sym) mutants of white sweetclover (Melilotus alba Desr.) provides a developmental sequence of mutants blocked early in infection or nodule organogenesis. Mutant phenotypes include non-nodulating mutants that exhibit root-hair deformations in response to Rhizobium meliloti, mutants that form ineffective nodules lacking infection threads, and mutants that form infection threads and ineffective nodules. Mutant alleles from both the sym-1 and the sym-3 loci exhibited a non-nodulating phenotype in response to R. meliloti, although one allele in the sym-1 locus formed ineffective nodules at a low frequency. Spot-inoculation experiments on a non-nodulating allele in the sym-3 locus indicated that this mutant lacked cortical cell divisions following inoculation with R. meliloti. The auxin transport inhibitor N-(1-naphthyl)phthalamic acid elicited development of pseudonodules at a high frequency on all of the sweetclover sym mutants, including the non-nodulating mutants, in which the early nodulin ENOD2 was expressed. This suggests that N-(1-naphthyl)phthalamic acid activates cortical cell divisions by circumventing a secondary signal transduction event that is lacking in the non-nodulating sweetclover mutants. The sym-3 locus and possibly the sym-1 locus appear to be essential to early host plant responses essential to nodule organogenesis.     

美洲黑杨不同无性系对分月扇舟蛾幼虫生长和食物利用的影响

本文研究了不同的美洲黑杨无性系对分月扇舟蛾Clostera anastomosis L.幼虫生长的影响。结果表明,抗性水平不同的无性系影响分月扇舟蛾幼虫体重、相对生长率、食物利用率和转化率。抗性较强的89-3号无性系对幼虫的生长有抑制作用,幼虫取食后,其食物利用率和转化率在第2天有显著下降,但随后表现不明显;而感性较强的74-4号无性系有一定的促进作用,幼虫取食后,其食物利用率和转化率都显著提高。     

Auxin uptake and action of N-1-naphthylphthalamic acid in corn coleoptiles

The validity of a chemiosmotic hypothesis for uptake of weak acids as an explanation for the accumulation of auxin by cells has been explored further by comparing the uptake of indole-3-acetic acid (IAA) by 1-mm segments of corn (Zea mays L.) coleoptiles with that of benzoic acid and two neutral indoles, indoleethanol and indoleacetonitrile, which do not ionize. These substances, while structurally related to IAA lack both auxin activity and polar transport. Uptake of IAA and benzoic acid increase with decreasing external pH, whereas the uptake of the two neutral indoles is independent of external pH.Although metabolism of IAA, during 90 min or less, is minimal and without significant effect on its uptake, metabolism of benzoic acid appears responsible for the apparent saturation of benzoic acid uptake at high concentrations. An inhibitor of auxin transport, N-1-naphthylphathalamic acid (NPA), stimulates uptake of IAA but has no effect on uptake of either benzoic acid or the two neutral indoles. Thus, NPA does not affect the driving forces for accumulation of weak acids but probably specifically decreases the flux of the auxin anions relative to undissociated auxin. Since the electrochemical potential of auxin anions is usually higher in than outside cells, blocking the anion flux with NPA would enhance auxin uptake. Azide, which abolishes accumulation of both IAA and benzoic acid, may simply collapse the pH gradient across the plasma membrane.In the absence of NPA, increasing concentrations of auxins or the analogoue -naphthaleneacetic acid (-NAA) exert two opposing effects on the uptake of IAA-depression and stimulation. Stimulation results from saturating the anion flux. With uptake fully stimulated by NPA, however, increasing concentrations of auxins or analogues only depress uptake of [³H]IAA. These results are consistent with more than one path for auxin transport each with a different dependence on concentration. In depressing NPA-stimulated IAA uptake, the effectiveness of -NAAIAA-NAA benzoic acid, a specificity similar to that of an auxin binding site in vitro that has been implicated by others in auxin transport. The results support the general hypothesis that cellular auxin uptake and polar transport through tissues are chemiosmotically coupled to the electrochemical potential of auxin and protons.Abbreviations IAA indole-3-acetic acid - -NAA -naphthaleneacetic acid - -NAA -naphthaleneacetic acid - NPA N-1-naphthylphthalamic acid     

Effect of Ethylene Treatment on Polar IAA Transport, Net IAA Uptake and Specific Binding of N-1-Naphthylphthalamic Acid in Tissues and Microsomes Isolated from Etiolated Pea Epicotyls

The effect of ethylene treatment on polar indole-3-acetic acid (IAA) transport, net IAA uptake in the presence and absence of N-1-naphthylphthalamic acid (NPA) and [³H]NPA binding characteristics was investigated in tissue segments or microsomes isolated from etiolated pea (Pisum sativum L. cv Alaska) epicotyls. Basipetal IAA transport in 5 millimeter segments isolated from ethylene-treated seedlings was inhibited by ethylene in a dose-dependent manner. Threshold, half-maximal and saturating concentrations of ethylene were 0.01, 0.55, 10.0 microliters per liter, respectively. This inhibition became apparent after 6 to 8 hours of ethylene treatment. Transport velocity in both control and ethylene-treated tissues was estimated to be 5 millimeters per hour. Net IAA uptake was stimulated in ethylene-treated tissues and the relative ability of the phytotropin NPA to enhance net IAA uptake was reduced in treated tissues. Specific binding of [³H]NPA to microsomes prepared from both control and ethylene-treated tissues was saturable and consistent with the existence of a single class of binding sites with an apparent affinity (K_d) toward NPA of 8 to 9 nanomolar. The density of these binding sites (per milligram protein) was lower (36% of control) in ethylene-treated tissues. Direct application of ethylene to microsomal preparations isolated from untreated seedlings had no effect on the level of specific [³H]NPA binding.     

Axillary shoot proliferation and growth of Populus deltoides shoot cultures

Shoot cultures of four genotypes of Populus deltoides Bartr. ex Marsh. were established from adventitious shoots regenerated from internodal stem explants. Stable shoot cultures for all four genotypes were maintained in a continuous culture regime for over one year. The stable shoot cultures were used as explants to investigate the effects of zeatin concentration and genotype on axillary shoot production and growth. The concentration of zeatin significantly affected the production of axillary shoots, with 1.0 mgL^–1 zeatin producing the greatest number of shoots (31.0 shoots per culture vessel) while 0.25 mgL^–1 zeatin produced the greatest growth (5.9 mg per axillary shoot) when measured by dry weight accumulation per shoot. Genotypic differences were significant in the production and growth of axillary shoots.Abbreviations DKW Driver and Kuniyuki Walnut medium - PAR Photosynthetically Active Radiation Journal Series No. 9111, Agricultural Research Division, University of Nebraska     

Impact of drought on productivity and water use efficiency in 29 genotypes of Populus deltoides x Populus nigra

We examined the relationships among productivity, water use efficiency (WUE) and drought tolerance in 29 genotypes of Populus x euramericana (Populus deltoides x Populus nigra), and investigated whether some leaf traits could be used as predictors for productivity, WUE and drought tolerance. At Orléans, France, drought was induced on one field plot by withholding water, while a second plot remained irrigated and was used as a control. Recorded variables included stem traits (e.g. biomass) and leaf structural (e.g. leaf area) and functional traits [e.g. intrinsic water use efficiency (Wi) and carbon isotope discrimination (Delta)]. Productivity and Delta displayed large genotypic variability and were not correlated. Delta scaled negatively with Wi and positively with stomatal conductance under moderate drought, suggesting that the diversity for Delta was mainly driven by stomatal conductance. Most of the productive genotypes displayed a low level of drought tolerance (i.e. a large reduction of biomass), while the less productive genotypes presented a large range of drought tolerance. The ability to increase WUE in response to water deficit was necessary but not sufficient to explain the genotypic diversity of drought tolerance.     

外源生长素对烟草(Nicotiana tabacum L.)小孢子早期胚胎发生的影响

烟草（Nicotlana tabacum L．）小孢子胚胎发生系统,不仅可以提供大量可供处理的小孢子胚,还由于小孢子胚胎发生的不同步性,可同时提供从2-3细胞原胚到分化胚一系列胚胎以供研究。利用这一便利系统,探讨了外源生长素处理对小孢子胚胎发育的影响。使用3种浓度的IAA：1、3、10μmol／L,分别对不同发育时期烟草小孢子胚进行了处理,结果发现,对不同发育时期的小孢子胚,生长素处理的效果明显不同。外源生长素对胚胎发生有促进作用,表现为2-3细胞比例与非处理组相比升高,而当小孢子发育到小球形胚后,加入外源生长素对小孢子胚的进一步发育却表现出明显抑制作用。这说明在小孢子胚胎发育过程中早期和晚期发育对生长素的需求是不同的,且对生长素的敏感程度亦不同。反映了生长素调控机制在两个不同发育时期的差异。     

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

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