Comparison of gibberellin-like substances from cotyledons and phloem exudate collected from cotyledons of Pharbitis nil in relation to photoperiodic flowering

Gibberellin (GA)-like substances were analyzed in extracts from cotyledons and phloem exudate collected from cotyledons in photoinduced and vegetative seedlings of the short-day plant Pharbitis nil Chois. var. Violet, using high performance liquid chromatography (HPLC) and the dwarf rice bioassay, to see whether any specific GA-like substances were transported from the photoinduced cotyledons via phloem. Cotyledon extracts exhibited five peaks of free GA-like activity in HPLC, whereas only one or two active peaks were detected in phloem exudate extracts. The level of free GA-like activity was considerably lower in phloem exudate than in the cotyledons. In five out of six analyses of cotyledons and phloem exudate, there were substantially higher levels of free GA-like substances in photoinduced plants. Conjugated GA-like substances were present in much higher levels than free GA-like substances in the cotyledon extracts but the levels were not influenced by daylength. In phloem exudate extracts there was no conjugated GA-like substances. The free GA-like substances that are transported via phloem cochromatographed with GA_5/20 and GA₁₉ on HPLC. These were significantly higher in photoinduced plants and thus could have some influence on the photoperiodically-induced flowering in P. nil.     

The Role of Gibberellins in Early Growth and Development of Sugar Beet

Two gibberellin(GA)-like compounds were found in both rootsand shoots of sugar beet plants using the barley endosperm bioassay.One had chromatographic properties similar to GA₃ and GA₁; theother was highly non-polar, relatively inactive in the endospermassay, and may be a new gibberellin. Presence of the GA_3/1-likecompound was confirmed with the dwarf rice bio-assay. The quantityof this GA was relatively high in the root compared with theshoot at the 3–4 leaf stage when the first supernumerarycambia are being formed in the root. As plants developed throughthe 8–9 leaf stage and the 15–16 leaf stage thequantity of GA per unit fresh weight of material decreased. Application of gibberellic acid (GA₃) to the roots of youngsugar beet plants caused a significant increase in root dryweight shortly after treatment and the rate at which supernumerarycambia were produced was increased. Application of GA₃ to asingle petiole caused a significant increase in both root andshoot dry weight. GA₃ applied to either root or shoot causeda reduction in the rate of leaf formation although total leafarea per plant and shoot dry weight were unaffected. The probablerole of GA-like substances in controlling the growth and developmentof young sugar beet plants is discussed.     

Changes of Endogenous Gibberellin-like Substances with Sex Reversal of the Apical Inflorescence of Corn

In developing apical meristems of corn, the level of acidic, ethyl acetate-soluble gibberellin (GA)-like substances increased to a maximum of 108 micrograms GA₃-equivalents per kilogram dry weight of tissue at inflorescence initiation, and then fell rapidly. At anthesis, only a trace (0.2 microgram per kilogram) of GA-like activity remained in the apical (male) inflorescences, whereas moderate activity (32 micrograms per kilogram), mostly of a nonpolar nature, was present in lateral, female, inflorescences.     

Photoperiod-induced changes in gibberellin metabolism in relation to apical growth and senescence in genetic lines of peas (Pisum sativum L.)

In an early-flowering line of pea (G2) apical senescence occurs only in long days (LD), while growth in short days (SD) is indeterminate. In SD, G2 plants are known to produce a graft-transmissible substance which delays apical senescence in related lines that are photoperiod-insensitive with regard to apical senescence. Gibberellic acid (GA₃) applied to the apical bud of G2 plants in LD delayed apical senescence indefinitely, while N⁶-benzyladenine and -naphthaleneacetic acid were ineffective. Of the gibberellins native to pea, GA₉ had no effect whereas GA₂₀ had a moderate senescence-delaying effect. [³H]GA₉ metabolism in intact leaves of G2 plants was inhibited by LD and was restored by placing the plants back in SD. Leaves of photoperiod-insensitive lines (I-types) metabolized GA₉ readily regardless of photoperiod, but the metabolites differed qualitatively from those in G2 leaves. A polar GA₉ metabolite, GA_E, was found only in G2 plants in SD. The level of GA-like substances in methanol extracts from G2 plants dropped about 10-fold after the plants were moved from SD to LD; it was restored by transferring the plants back to SD. A polar zone of these GA-like materials co-chromatographed with GA_E. It is suggested that a polar gibberellin is synthesized by G2 plants in SD; this gibberellin promotes shoot growth and meristematic activity in the shoot apex, preventing senescence.Abbreviations GA gibberellin - GA₃ gibberellic acid - SD short days - LD long days     

The biological activity of fluorogibberellins

Summary The biological activities of gibberellin A₉ (GA₉), gibberellin A₁₂ (GA₁₂) and monofluoro-analogues (F-GA₉ and F-GA₁₂), substituted in the 1 -methyl group, were compared in the barley endosperm, cucumber hypocotyl, lettuce hypocotyl, Meteor dwarf pea, dwarf-5 maize and Rumex leaf disc assays. In most cases the fluorosubstituted compounds had a potency similar to, or less than, the relevant unmodified gibberellin but, in the lettuce assay, F-GA₉ was approximately 5 times more active than GA₉ up to a dose rate of 10^-1 g.A 27–30% mixture of fluorogibberellin A₃ (F-GA₃) in GA₃ had a lower activity than 100% GA₃ in the barley endosperm, lettuce hypocotyl and dwarf maize assays. This suggested that pure F-GA₃ may be a competitive inhibitor of GA₃ action. The findings are discussed in the context of the structure/activity relationships of the gibberellins.     

Endogenous gibberellins and kauranoids identified from developing and germinating barley grain

Several gibberellins (GAs) and kauranoids were identified in extracts of barley (Hordeum vulgare) by combined capillary gas chromatography-mass spectrometry (GC-MS). A partially purified acidic ethyl acetate extract from 21-day postanthesis developing barley grain (cv. Proctor) contained GA₁ (trace), GA₄ (trace), GA₈ (trace), GA₁₂, GA₁₇, GA₂₀ (tentative) (trace), GA₂₅, GA₃₄, GA₄₈, 18-hydroxy-GA₄, 12-hydroxy-GA₉, and 18-hydroxy-GA₃₄ (tentative). A hydrolyzed butanol extract contained GA₁₇, GA₂₀, GA₄₈, and 18-hydroxy-GA₃₄ (tentative). An acidic ethyl acetate extract from 3-day-old germinating barley grain (cv. Maris Otter) contained GA₁, GA₃ (possibly a contaminant), GA₁₇, GA₁₉, GA₂₀, GA₃₄, GA₄₈, and 18-hydroxy-GA₃₄ (tentative). A hydrolyzed butanol extract contained GA₃₄, GA₄₈, and 18-hydroxy-GA₃₄ (tentative). In germinating grain, levels of all GAs were very low. Two hydroxylated kauranoic acids and a number of other kauranoids were also detected in the above extracts. 1-Hydroxylated GAs previously found in wheat were not found in barley in this study.This work has been reported in a poster demonstration (Gaskin et al. 1982).     

Endogenous Gibberellins and Shoot Growth and Development in Brassica napus

Greenhouse-grown oilseed rape (Brassica napus, annual Canola variety `Westar') plants were harvested at six dates from the vegetative phase until the early pod (silique)-fill/late flowering stage. Endogenous gibberellin (GA)-like substances were extracted from stems, purified, and chromatographed on silica gel partition columns prior to bioassay in serial dilution using the `Tan-ginbozu' dwarf rice microdrop assay. The concentrations of total endogenous GA-like substances were low during vegetative stages (1 nanogram GA₃ equivalents/gram dry weight), and rose 300-fold by the time of floral initiation. After floral initiation the concentration of GA-like substances fell, then rose again during bolting to maximal levels during the early pod-fill stage (940 nanograms per gram dry weight). The qualitative profiles of GA-like substances varied across harvests, with higher proportions of a GA₁-like substance at the early pod-fill stage. In a second study stems were similarly harvested at eight dates and the concentrations of endogenous GA₁, the principal bioactive native GA of oilseed rape, were determined by gas chromatography-selected ion monitoring using [17,17-²H]GA₁ as a quantitative internal standard. The concentration of GA₁ increased at about the time of floral initiation and then subsequently fell, thus confirming the pattern noted above for total GA-like substances. The exogenous application of paclobutrazol (PP333), a persistent triazole plant growth regulator (PGR) which blocks GA biosynthesis, or another triazole, triapenthenol (RSW0411), prevented flowering as well as bolting; plants remained at the vegetative rosette stage. These results imply a causal role for endogenous GA, in the control of bolting, which normally precedes anthesis. Further, the rise in the concentration of total endogenous GA-like substances, including GA₁, which was associated with floral initiation, and the prevention of visable floral development by the triazole PGRs, also indicates a role for endogenous GAs in the regulation of flowering in B. napus.     

Diffusible and Extractable Gibberellins in Bean Cotyledons in Relation to Dwarfisni

Dwarfism in bean plants (Phaseolus vulgaris L.) is investigated in relation to the diffusible and extractable gibberel-lins in cotyledonary tissues. These gibberellins were partitioned into four parts prior to thin layer chromatograplty: non-acidic and acidic ethyl acetate fractions, and non-acidic and acidic butanol fractions. Cotyledonary segments from a tall plant (cv. Kentucky Wonder) seem to diffuse, preferentially acropetally, more gibberellins in each fraction than those from a dwarf plant (cv. Masterpiece). The diffusion increases with the length of the segments and decreases with period after sowing. From experiments on extraetable gibberellins, however, it is concluded that these phenomena actually result from differences in the gibbereliin contents of the tall and the dwarf plants, from differences in the gibberellin contents of the distal and proximal parts of cotyledons, and from the gibberellin contents before and after imbibition. Ten kinds of gibberellin-Iike substances are detectable in the diffusates, exudates and extracts from the cotyledons of both the plants; two in the non-acidic ethyl acetate, three in the acidic ethyl acetate (GA₁, GA₆ and another), two in the non-acidic butanol, and three in the acidic butanol frac—tion. They are almost identical in quality in the dwarf and tall plants, but in the latter they are more abundant in the cotyledons, particularly in their distal part. With respect to the change in content during the period after sowing, the gibberellin-Iike substances are classified in three groups; unchanging, decreasing (GA₁, GA₆, others) and increasing (butanol soluble glucosyl esters and glucosides of gibberellins)-. The increase of glucose-bound gibberellins and the decrease of free gibberellins during the sowing period suggest the occurrence of conversion. This is obvious only in the tall plants. The gibberellin content in cotyledons is higher in the tall plant than in the dwarf plant. Thus, the marked hypocotyl growth in the tall plants may be dependent on the higher content of gibberellins in their cotyledons and on the higner rate of conversion from free to bound forms.     

Variations in Endogenous Gibberellins in Developing Bean Seeds II. Changes Induced in Acidic and Neutral Fractions by GA(1)

Immature (8-mm), medium mature (11-mm), and mature green (16- and 17-mm) bean seeds (Phaseolus vulgaris L. cv. Kentucky Wonder and Bountiful) were incubated in gibberellin A₁ solutions for 24 hours at 20°. Extracts from the seeds were separated into nonacidic, acidic ethyl acetate, and acidic butanol fractions. These were chromatographed. The eluates of the chromatograms were tested on Progress No. 9 dwarf peas grown under red light. The level of neutral gibberellin-like substances remained unchanged in immature seed, but they increased markedly in mature green seeds. Coincident with increased levels of the neutral substances, there were significant decreases in acidic ethyl acetate-soluble gibberellin-like substances, including applied GA₁, and in 1 acidic butanol-soluble gibberellin-like substance. Seed incubation in GA₁ brought about increased activity of substance B-II in immature and medium mature seeds. The level of butanol-soluble gibberellin-like substance B-I in seeds of any size was not affected by incubation in GA₁. Considering the marked increases in activity induced in the neutral fraction and the decreases in activity of certain eluates from the chromatograms of the acidic fractions, it was concluded that the neutral fraction may serve as a reserve form of gibberellins in the dry seed. The acidic ethyl acetate substances and substance B-II may be required for normal development of the bean seed.     

Effect of 2-chloroethyltrimethyl ammonium chloride on tuberization and endogenous GA3 in roots of potato cuttings

Cuttings of potato shoots treated with the plant growth retardant 2-chloroethyltrimethyl ammonium chloride (CCC) form tubers earlier and have less biologically-active gibberellin (GA)-like substances in the roots than control cuttings. The major GA-like substance in roots of potato cuttings was identified as GA₃ by gas-chromatography-mass spectrometry (GC-MS). The content of GA₃ in roots of control cuttings, estimated by GC-MS-selected ion monitoring (SIM) using [17, 17-²H]GA₃ as a quantitative internal standard, was 38.8 ng per g fresh weight (fw), and in roots of CCC-treated cuttings, in which tuberization was promoted, was 0.6 ng per g fw. Gibberellin A₁, GA₈ and GA₂₀ were also indicated as minor components of roots from both control and CCC-treated cuttings. The comparatively high GA₃ content in roots of control cuttings might be the root factor responsible for delaying tuberization in potato.Abbreviations CCC 2-chloroethyltrimethyl ammonium chloride - dw dry weight - EtOAc ethyl acetate - GA gibberellin - GC-MS-SIM gas chromatography-mass spectrometry-selected ion monitoring - HPLC high performance liquid chromatography - IAA indole-3-acetic acid - KRI Kovats' retention index - MeOH methanol - MeTMSi methyl ester trimethylsilyl ether - NAA naphthalene acetic acid - SD short day(s) - 2,4-D 2,4-dichlorophenoxy acetic acid     

Metabolism of Tritiated Gibberellins A(4) and A(9) in Norway Spruce, Picea abies (L.) Karst. : Effects of a Cultural Treatment Known to Enhance Flowering

Shoots of mature grafted propagules of Picea abies (L.) Karst. metabolized [³H]gibberellin A₄ (GA₄) to at least 14 acidic substances, two of which were tentatively identified by gas-liquid radiochromatography as GA₂ (possibly an artifact) and GA₃₄. [³H]GA₉ was converted into a number of metabolites, one of which was chromatographically similar to, but not identical with, GA₄. Metabolism was maximally 61 and 57% over 48 hours for GA₄ and GA₉, respectively, and was correlated with the rate of change (i.e. increase followed by decrease) in endogenous GA-like substances as shoot elongation progressed. Propagules covered with a clear plastic film, a treatment which promotes flowering, metabolized [³H]GA₄ more slowly than did control plants in the open. Inasmuch as a GA_4/7 mixture can also promote flowering in P. abies, the retarded metabolism of [³H]GA₄ may reflect the manner in which trees under plastic metabolize endogenous GA-like substances. If so, then the stimulating effect of this cultural treatment on flowering may come about through an increased level of endogenous, less polar GA-like substances.     

Alterations in Endogenous Levels of Gibberellin-like Substances during Germination of Phaseolus vulgaris Seeds

The endogenous gibberellin-like substances were determined in mature dry and germinating bean seeds, Phaseolus vulgaris L. cv. Alabaster. Methanol extracts were partitioned against ethyl acetate and butanol at neutral and acid pH. Each phase was individually chromatographed on a silica gel column. The gibberellin activity was measured with the Tan-ginbozu dwarf rice micro-drop bioassay. Each extract was tested in two dilutions. Extracts from dry seeds showed the highest gibberellin activity, largely attributable to ethyl acetate-soluble substances. The activity was considerably reduced in extracts from seeds imbibed for 1 day. Gibberellin-like substances soluble in butanol appeared in extracts from seeds soaked for 1 or more days.     

Metabolism of Tritiated Gibberellin A(9) by Shoots of Dark-grown Dwarf Pea, cv. Meteor

Tritium-labeled gibberellin A₉ (³H-GA₉) was metabolized by etiolated shoots of dwarf pea (Pisum sativum cv. Meteor) to GA₂₀, GA₁₀, 2,3-dihydro-GA₃₁, and a number of highly polar, acidic GA-like substances. Identifications were made by gasliquid radiochromatography and combined gas chromatography-mass spectrometry. Kinetic studies showed that GA₃₀ and 2,3-dihydro-GA₃₁ were produced within 5 hours following ³H-GA₉ application to pea shoots. The polar GA-like substances were produced between 5 and 10 hours after ³H-GA₉ application. Levels of GA₁₀ increased with time, and since no GA₁₀ was produced during the purification procedures, GA₁₀ was, in all probability, produced from ³H-GA₉ within the plant tissue. The radioactive interconversion products produced by pea from ³H-GA₉ have chromatographic properties similar to biologically active GA-like substances present in etiolated shoots of dwarf pea. Large scale applications of ³H-GA₉ with very low specific activity to etiolated pea shoots showed that the radioactivity of the interconversion products was correlated exactly with biological activity as assayed by dwarf rice (Oryza sativa cv. Tan-ginbozu).     

Contents and recovery of gibberellins in monoecious and gynoecious cucumber plants

Diffusates from seedlings and root exudates from 6-week-old plants of a monoecious line of cucumber, Cucumis sativus L., contained considerably higher levels of gibberellin-(GA-) like substances than did those from plants of an isogenic gynoecious line. Most of the GA-like activity was found in a chromatogram region typical of GA₁ and GA₃; some activity, particularly in root exudates, appeared also at an R_F similar to that of GA₄ and GA₇.

When seedlings were treated with ³H-labeled GA₁, more radioactivity was found in the diffusates from monoecious seedlings than from gynoecious ones. The same was true of biological activity in root diffusates from older plants which had been treated with gibberellin A₄₊₇.

In conjunction with evidence present in literature, these results support the idea that endogenous GAs play a part in the regulation of sex expression in cucumber, relatively high levels favoring the formation of staminate flowers.

     

Slender barley: A constitutive gibberellin-response mutant

In barley (Hordeum vulgare L. cv. Herta), slender (sln1) is a single-locus recessive mutation which causes a plant to appear as if it had been grown in sturating concentrations of gibberellin (GA). We have investigated two of the GA-mediated processes in slender barley, shoot elongation and the induction of hydrolytic enzymes in aleurone layers. Shoot elongation is severely retarded in normal (wild-type) barley if the biosynthesis of GA is blocked by an inhibitor, ancymidol (-cyclopropyl--(p-methoxyphenyl)-5-pyrimidinemethanol). However, the slender mutant continues to elongate in the presence of ancymidol. In isolated normal aleurone layers, the synthesis and secretion of -amylase (EC 3.2.1.1), protease (EC 3.4) and nuclease (EC 3.1.30.2) are induced by exogenously applied GA₃. However, in the aleurone layers of the slender mutant these enzymes are produced even in the absence of GA but their synthesis is still susceptible to inhibition by abscisic acid. Bioassays of half-seeds of the slender mutant and their normal siblings show no detectable differences in endogenous levels of GA-like substances. We suggest that the slender mutation allows competent tissues to express fully, or over-express, appropriate GA-induced processes independent of GA. We also conclude that shoot elongation, and hydrolytic-enzyme secretion in aleurone layers, share a common regulatory element.Abbreviations ABA abscisic acid - GA gibberellin - GA₃ gibberellic acid     

Gibberellins and Heterosis in Maize : II. Response to Gibberellic Acid and Metabolism of [H]Gibberellin A(20)

Two maize inbreds, CM7 and CM49, and CM7 × CM49, their F₁ hybrid (which displayed significant heterosis), were examined with regard to response to exogenous gibberellin A₃ (GA₃), and in their ability to metabolize GA₂₀, a native GA of maize. The leaf sheath elongation response to GA₃ was far greater for the imbreds than for their hybrid. The inbreds also displayed significant elongation of the leaf blades in response to GA₃, whereas the hybrid was unaffected. Promotion of cell division in the leaf sheath of CM7 and the hybrid was effected by GA₃, but no promotion of cell elongation was observed in CM49, even though significant leaf sheath elongation occurred. Shoot dry weight of both inbreds was significantly increased by GA₃, but response by the hybrid in this parameter was slight and variable. Root dry weight of CM7 was significantly increased by GA₃, but was unchanged in CM49 and the hybrid. Thus, inbred shoot dry weight increases effected by GA₃ were not at the expense of the root system. Rapid metabolism of [2,3-³H]GA₂₀ occurred in all genotypes, although genotypic differences were observed. The hybrid had the highest rates of metabolism to GA glucosyl conjugate-like substances. Oxidative metabolism was also fastest in the hybrid, followed by CM7, and slowest in CM49, the slowest-growing inbred. Thus, rate of GA₂₀ metabolism is under genetic control in normal (i.e. not dwarfed) maize genotypes. These results, taken together with previous reports that the hybrid has significantly enhanced levels of endogenous GA-like substances, suggest that GA play a role in the expression of heterosis in maize.     

Changes in endogenous growth regulators in nasturtium leaves during senescence

Summary By use of lettuce-hypocotyl and wheat-coleoptile bioassay, the presence of both gibberellin (GA)-like and abscisic-acid(ABA)-like components in acidic ethyl-acetate extracts of fully expanded nasturtium (Tropaeolum majus) leaves has been shown. During senescence of detached leaves there was a progressive decline in GA-like components and an increase in ABA-like components. Pretreatment of detached leaves with GA₃ or kinetin prevented changes in the levels of endogenous growth regulators and delayed senescence. The observations provide experimental verification for the concept that senescence is associated with changes in endogenous growth regulators.     

Gibberellins and heterosis in maize : I. Endogenous gibberellin-like substances

Under controlled environment and/or field conditions, vegetative growth (height, internode length, leaf area, shoot dry weight, grain yield) was greater in an F₁ maize hybrid than in either parental inbred. Endogenous gibberellin (GA)-like substances in apical meristem cylinders were also higher in the hybrid than in either inbred, both on a per plant and per gram dry weight basis. There were no apparent qualitative differences in GA-like substances, however. Levels of GA-like substances in all genotypes were highest prior to tassel initiation. Chromatographic comparisons of the GA-like substances and authentic standards of GA native to maize on gradient-eluted SiO₂ partition and reverse-phase C₁₈ high-pressure liquid chromatography columns are described. No consistent differences in abscisic acid levels of the three genotypes were observed. This correlation of heterosis for endogenous GA-like substances with heterosis for growth suggests that amounts of endogenous GA may be related to hybrid vigor in maize.     

Gibberellins and Gravitropism in Maize Shoots : Endogenous Gibberellin-Like Substances and Movement and Metabolism of [3H]Gibberellin A20

[³H]Gibberellin A₂₀ (GA₂₀) of high specific radioactivity (49.9 gigabecquerel per millimole) was applied equilaterally in a ring of microdrops to the internodal pulvinus of shoots of 3-week-old gravistimulated and vertical normal maize (Zea mays L.), and to a pleiogravitropic (prostrate) maize mutant, lazy (la). All plants converted the [³H]GA₂₀ to [³H]GA₁⁻ and [³H]GA₂₉-like metabolites as well as to several metabolites with the partitioning and chromatographic behavior of glucosyl conjugates of [³H]GA₁, [³H]GA₂₉, and [³H]GA₈. The tentative identification of these putative [³H]GA glucosyl conjugates was further supported by the release of the free [³H]GA moiety after cleavage with cellulase. Within 12 hours of the [³H]GA₂₀ feed, there was a significantly higher proportion of total radioactivity in lower than in upper halves of internode and leaf sheath pulvini in gravistimulated normal maize. Further, there was a significantly higher proportion of putative free GA metabolites of [³H]GA₂₀, especially [³H]GA₁, in the lower halves of normal maize relative to upper halves. The differential localization of the metabolites between upper and lower halves was not apparent in the pleiogravitropic mutant, la. Endogenous GA-like substances were also examined in gravistimulated maize shoots. Forty-eight hours after gravistimulation of 3-week-old maize seedlings, endogenous free GA-like substances in upper and lower leaf sheath and internode pulvini halves were extracted, chromatographed, and bioassayed using the `Tanginbozu' dwarf rice microdrop assay. Lower halves contained consistently higher total levels of GA-like activity. The qualitative elution profile of GA-like substances differed consistently, upper halves containing principally a GA₂₀-like substance and lower halves containing mainly GA₁-like and GA₁₉-like substances. Gibberellins A₁ (10 nanograms per gram) and A₂₀ (5 nanograms per gram) were identified from these lower leaf sheath pulvini by capillary gas chromatography-selected ion monitoring. Results from all of these experiments are consistent with a role for GAs in the differential shoot growth that follows gravitropism, although the results do not eliminate the possibility that the redistribution of GAs results from the gravitropic response.     

Gibberellins of sugarcane

In our hands a phosphate buffered celite column has given an adequate separation of GA₁ and GA₃. These 2 gibberellins are normally very difficult to separate. Young sugarcane growing under moisture stress contains at least 2 gibberellin-like substances. One is suspected to be GA₅. The other is unknown but has high activity in the barley endosperm assay and is neither GA₁ nor GA₃. Four-month-old cane contains 2 major growth promoters. From their chromatographic, fluorimetric and biological properties these are thought to be GA₁ and GA₃. Rapidly growing 6-month-old cane has a surprisingly low level of gibberellin-like substances.