Effect of Gibberellic Acid on Growth, Gibberellin Content, and Chlorophyll Content of Leaves of Potato (Solanum tuberosum)

Spraying potato plants with a solution of gibberellic acid (GA)when the 15th leaf was emerging increased the area of this leafand its total gibberellin content, assayed by dwarf French beanleaf disks. GA, assayed by lettuce hypocotyls, was not detectedin leaves from untreated plants. The GA content of leaves fromGA-treated plants decreased after 2 weeks and none was detectableafter 5 weeks; apparently it was converted to another gibberellin,possibly the same as the natural gibberellin. GA increased chlorophyllcontent per leaf but increased leaf area more so that the chlorophyllper unit area decreased, and the leaves were paler than untreatedleaves.     

Effect of CCC and glycine betaine on growth and growth-substance content of primary leaves of dwarf Frenchbean (Phaseolus vulgaris L.)

CCC (2-chloroethyltrimethylammonium chloride) decreased growth of primary leaves and stem internodes above the hypocotyl of dwarf French bean, probably because it decreased the amount of auxin produced by interaction between tryptophan and phenolic esters in the primary leaves. Growth of leaves was less affected than that of stems, and the gibberellin content of the primary leaves, previously shown to be associated with their expansion, was unaffected by CCC. CCC delayed death of the primary leaves, the breakdown of chlorophyll, and the increase in auxin associated with death of bean leaves. CCC had less effect on the growth of leaf discs, probably because they are not sites of growth-substance production. Glycine betaine, a quaternary ammonium compound similar to CCC, did not affect growth of bean plants, their metabolism of growth substances or the longevity of their primary leaves, but inhibited growth of leaf discs, probably directly.     

Comparative Potency of Nine Gibberellins

Gibberellins A₁ to A₉ have been compared, each at several doselevels, in bioassays based on extension of stems of dwarf gardenpea (Pisum sativum), dwarf bean (Phaseolus vulgaris) and Lunariaannua, of hypocotyls of cucumber (Cucumis sativus) and lettuce(Lactuca sativa), and of leaf sheaths of three dwarf mutants(d–1, d–3, d–5) of maize (Zea mays). GibberellinsA₃ (gibberellic acid) and A₇ are of high potency in most bioassays.A₈ is of negligible potency in all and is probably not a functionalhormone. The other gibberellins show a more or less marked tendencyto specificity. The plants used as bioassay material also differin the specificity of their response. Some, for example, maizedwarfs d–3 and d–5 and lettuce, respond well tomost gibberellins; others, for example, cucumber, respond onlyto a few; extreme specificity is shown by Lunaria annua which,in the unvernalized condition, responds by stem elongation onlyto gibberellin A₇. Dose/response curves of the various gibberellinsare usually parallel, but certain exceptions to this have beenfound. Possible explanations of specificity are discussed inrelation to the results obtained, and it is concluded that insufficientevidence is available to make it possible to draw any validconclusions. Current definitions of gibberellins, whether basedon chemical structure or biological activity, are unsatisfactory.     

Solvent and Chemical Impurities as Sources of Gibberellin-like Growth-Promoting Activity

Experiments are described in which a standard procedure usedfor the extraction of endogenous gibberellin-like substancesfrom plants was carried out in the absence of plant materialto give ‘blank’ extracts. Acidic, basic, and neutralfractions of blank extracts all showed variable gibberellin-likeactivity on d-5 dwarf maize and lettuce hypocotyls, though oftenat a low level. Acidic fractions showed activity on d-2 dwarfmaize, but none of the fractions was active on dwarf peas oron tomato seedlings. One sample of sodium bicarbonate appearedto be contaminated by two gibberellin-like substances. Someactivity was observed in ethanol, chromatography paper, andammonium acetate, but none in methanol or redistilled ethylacetate. There were wide variations between the results of comparableexperiments. In the light of these results possible precautionsfor experiments involving extraction of endogenous gibberellin-likesubstances are discussed.     

Growth and gibberellin a(1) metabolism in excised lettuce hypocotyls

Excised lettuce (Lactuca sativa L. cv. Arctic) hypocotyls retain the ability to elongate in response to exogenously supplied gibberellic acid and gibberellin A(1) (GA(1)). We have studied the relationship between metabolism of GA(1) and elongation in this tissue. In 24 hours at 28 C, hypocotyls treated with 3 mum GA(1) double in length while controls elongate less than 45%. After an exogenous hormone supply is removed, hypocotyls continue to grow faster than untreated controls, although as the hormone application time is decreased, the GA(1) concentration required to effect a given length change increases. [(3)H]GA(1) was used to determine rates of hormone uptake, efflux, and metabolism. In the presence of [(3)H]GA(1), hypocotyls accumulate and metabolize lable for at least 24 hours. When the exogenous label is removed, the amount of acidic GA in the hypocotyl declines rapidly to a constant level while ethyl acetate-insoluble metabolites increase rapidly to a constant level. Lable accumulation and metabolism at any time are proportional to the external GA(1) concentration below 50 mum GA(1). Chromatographic analysis of radioactive compounds present in tissue extracts suggests that unaltered GA(1) is the major component of the acidic ethyl acetate-soluble fraction, and gibberellin A(8) is a minor component. The ethyl acetate-insoluble fraction appears to contain an unidentified GA(1) metabolite with chromatographic properties similar to those of GA(1). The strong retention of accumulated GA(1) confirms the possibility of a continuing requirement for GA(1) during the sustained response to a GA(1) "pulse" but raises the question of accessibility of the stored hormone for growth promotion.     

Effect of Seed Treatment on Growth and Growth-substance Content of Dwarf French Beans (Phaseolus vulgaris)

The longer the seeds of dwarf French bean (Phaseolus vulgaris)were soaked in water before sowing, the smaller were the seedlingsthey produced. Soaking for 24 hours removed only little drymatter, but detectable amounts of a gibberellin and a betaine.The amounts of these two removed increased with increased timeof soaking. Long soaking of seeds decreased the amounts of gibberelinand auxin in the cotyledons, and of gibberellin, auxin, andbetaine in the primary leaves of seedlings. Aerating, cooling,or increasing the volume of water in which the seeds were soakedalso retarded the growth of the seedlings, but did not affectthe amount of growth substance removed from the seeds or containedin the cotyledons. The deleterious effect of soaking seeds wasnot alleviated by treating the seeds or seedlings with gibberellicacid or glycine betaine. Although larger seedlings were producedby heavy than by light seeds, their cotyledons and primary leavescontained similar concentrations of growth substances.     

Stem growth,flower formation,and endogenous gibberellins in a normal and a dwarf strain of Silene armeria

The physiological basis of dwarfism in a single-gene, recessive mutant of Silene armeria L. was investigated through comparison with a normal strain. Exposure of the normal strain to long days led to stem growth and flower formation while similar exposure of the dwarf strain led only to flowering, with very little stem growth. Application of gibberellin A₃ or A₄₊₇ in short days promoted stem elongation in the normal strain, but had a much lesser effect in the dwarf strain. Upon extraction and chromatographic fractionation of the endogenous gibberellins (GAs) in the normal strain of S. armeria, three zones of GA activity were found. An increase in one zone of activity was found in both strains after 1 long day. Neither the quality nor the quantity of the extractable GAs differed greatly between the dwarf and the normal strain. Vegetative dwarf scions, grafted onto fully induced, normal stocks formed flowers, but their growth habit was not changed. Thus, the lack of stem growth in response to long days in the dwarf strain appears to result from a lack of GA sensitivity in the stem tissue of these plants. However, during flower formation dwarf plants did exhibit elongation of the peduncles. This response was suppressed by the growth retardant 2-isopropyl-4-dimethylamino-5-methylphenyl-1-piperidine-carboxylate methyl chloride (AMO-1618), and applied GA₃ could partially overcome this inhibition. Thus, peduncle elongation in the dwarf strain appears to be regulated by endogenous GAs.Abbreviations AMO-1618 2-isopropyl-4-dimethylamino-5-methylphenyl-1-piperidine-carboxylate methyl chloride - GA(s) gibberellin(s) - LD long day(s) - SD short day(s)     

Influence of Gibberellic Acid on Growth and Endogenous Auxin Level in Epicotyl and Hypocotyl Tissues of Normal and Dwarf Bean Plants

Application of gibberellic acid to the apex of dwarf bean plants (cv. Alabaster) stimulated the elongation growth of epicotyl and hypocotyl but showed no significant effect on elongation growth in a normal cultivar (‘Blue Lake’). Gibberellin-treatment of dwarf plants was characterized by about twofold increase in the level of endogenous auxin. Maximum increase in IAA level was observed after 48 h of GA treatment. There was less increase in IAA content in normal bean plants. — Gibberellin treatment to excised epicotyl and hypocotyl sections of either dwarf or normal cultivar showed no effect on elongation growth. However, a considerable increase in the auxin level was observed in the sections of the dwarf cultivar. The maximum effect occurred with only 1 h incubation in basal medium containing gibberellin. — The indolo-α-pyrone spectro-fluoremetric method for IAA determination was used.     

Control by Light of Hypocotyl Elongation and Levels of Endogenous Gibberellins in Seedlings of Lactuca sativa L.

Four 13-hydroxygibberellins, gibberellin A₁ (GA₁), 3-epi-GA₁,GA₁₉ and GA₂₀ were identified by full-scan GC/MS in extractsof lettuce seedlings (Lactuca sativa L. cv. Grand Rapids). Theresults suggest that the early-13-hydroxylation biosyntheticpathway to GA₁ functions in the lettuce seedlings. It was alsofound that GA₁ is active per se in the control of hypocotylelongation in lettuce seedlings. To investigate the relationshipbetween control by light of hypocotyl elongation and levelsof endogenous GAs in lettuce, endogenous levels of GAs werequantified by radioimmunoassay in seedlings that had been grownfor 5 days in the dark (5D) and in those that had been grownfor 4 days in the dark and then under white light for 1 day(4D1L). The endogenous level of GA₁ in the upper and lower partsof hypocotyls in 5D seedlings was about four times higher thanthat in 4D1L seedlings. The response of explants (hypocotylsegments with cotyledons) from dark-grown seedlings to GA₁ isknown to be similar in the dark and under white light when theexplants are treated with inhibitors of the biosynthesis ofGA. Therefore, the above information suggests that the highlevel of GA₁ in hypocotyls of dark-grown seedlings is responsiblefor the rapid elongation of hypocotyl, while irradiation bywhite light decreases the endogenous level of GA₁ in the hypocotylswith a resultant decrease in the rate of hypocotyl elongation. (Received March 13, 1992; Accepted May 21, 1992)     

Tuberization in Potato at High Temperatures: Gibberellin Content and Transport from Buds

Warm temperatures (35°C day/30°C night) which inhibittuberization in potato (Solanum tuberosum L., cv. Sebago) increasedgibberellin activity in crude extracts from buds, but not frommature leaves, as determined by the lettuce hypocotyl bioassay.Changes in the growth of tubers and stolons indicate the occurrenceof basipetal movement of GA₃ applied to the terminal bud ora mature leaf. ¹⁴C labelling from GA₃ or mevalonic acid injectedjust below the terminal bud was recovered in the lower shoot,stolons and tubers, but the amount transported was greater atcool temperatures (20/15°C). It is concluded that high temperaturespromote the synthesis of gibberellin in the buds rather thantransport to the stolons. Solanum tuberosum L., potato, tuberization, gibberellin     

Changes in Gibberellin Content during Seed Ripening in Grasses

Floret samples of Aberystwyth S. 321 perennial ryegrass andS. 352 timothy were analysed for gibberellin content at 2–5day intervals from inflorescence emergence to seed harvest.Both species had a high gibberellin content at emergence whichdiminished during anther formation. After anthesis there wasa sharp increase in gibberellin content with maxima of 256 and116 GA3 µ equivalents/Kg dry matter occurring in S. 321and S. 352 respectively. During the final stages of seed maturation,the level fell steadily to reach a stable content prior to harvest.Biological activity in ryegrass extracts was always identifiedchromatographically as gibberellin A₃, (GA₃), but in timothy,GA₃ was replaced after anthesis by a more mobile component,chromatographically similar to gibberellin A₃. The relationshipof these observations to seed formation and ripening is discussed.     

Effects of Repetitive Acid Immersion, Red Light, and Gibberellin A3 Treatments on Phytochrome-mediated Germination Control in Skotodormant Lettuce Seeds

Dry lettuce seeds (Lactuca sativa L. cv. Grand Rapids), whichreceived 5 min far-red light (FR) 0.5 h after the onset of waterimbibition, showed 17% and 50% germination without and withacid immersion treatment (pH 0.1) for 1 h and rinsing with water,respectively. The acid treatment caused only 6% germinationor less in FR-treated seeds held for 10 to 30 d in dark storage.The 10 to 30 d skotodormant seeds did not respond to red light(R) or gibberellin A₃ (GA₃) singly, but showed 84% or higherpercentage germination if 1 h acid immersion was given beforeR or GA₃. The 20 d skotodormant seeds, which received R treatmentat day 10 but remained dormant showed 89% germination with onlyacid treatment. Similar values were obtained with 30 d skotodormantseeds which received one or two R treatments at day 10 or 20,i.e. the only requirement for these R-treated dormant seedswas an acid immersion. This releases the skotodormancy and rendersthe seeds more sensitive to R or GA₃, but the skotodormancywas initiated again if no light or hormone treatments were givenimmediately. The repetitive R or GA₃ treatments, which did notcause skotodormant seeds to germinate, lessened the degree ofskotodormancy. The germination of these skotodormant seeds canonly be induced by the synergistic action of R and GA₃. In thisstudy, GA₃ caused higher germination percentages in R-treatedskotodormant seeds than R stimulated in GA3-treated seeds. Itis suggested that (i) repetitive R or Ga₃ treatments maintaina high endogenous level of the far-red-absorbing form of phytochrome(P_fr) and GA activity, respectively, (ii) the accumulated stableintermediates of phytochrome persist in fully-imbibed skotodormantseeds for up to 20 d, without phytochrome expressing its functionuntil the seeds are acidified and (iii) a model is formulatedto interpret the results of acidification, growth promotersand R effects on germination of light-sensitive lettuce seeds. Key words: Phytochrome, Latuca saliva, seed germination, dark reversion of phytochrome, gibberellin A₃, acidification, skotodormancy     

The Control of Flowering in the Strawberry Fragaria ananassa Duch: II. The Role of Gibberellins

Activity of gibberellin-like substances was lacking in leavesof strawberry plants at time of flower initiation, but peduncleelongation was accompanied by significant activity of two substancesprobably related to GA₃ and GA₇. Exogenous gibberellin (GAS),applied to strawberry plants at the flower-differentiation stage,strongly promoted peduncle elongation, while similar applicationof the growth-retardant (2 chloro-ethyl) trimethylammonium chloride(CCC), an inhibitor of gibberellin biosynthesis, inhibited peduncleelongation. These treatments also affected subsequent runnering,GA increasing the number of runners, as well as their mean individuallength, while CCC caused the opposite effect. Application of gibberellin immediately after flower-bud initiation,but before peduncle emergence, affected the distribution offruit yield. GA-treated plants produced more than untreatedones in the first half of the fruiting season, but the situationwas reversed in the second half of the season—so thattotal yields were unaffected. It was suggested that the exogenousgibberellin hastened yield by anticipating the endogenous gibberellinwhich is normally formed at a somewhat later stage in the plant'sdevelopment. The parallel responses with regard to elongationof vegetative runners and flower peduncles together with thedata of the other experiments seem to indicate that peduncleelongation is a typical vegetative growth response enhancedby gibberellin and checked by substances which inhibit biosynthesisof endogenous gibberellin.     

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.     

The effects of temperature and the Rht3 dwarfing gene on growth, cell extension, and gibberellin content and responsiveness in the wheat leaf

The effects of low temperature and the Rht3 dwarfing gene onthe dynamics of cell extension in leaf 2 of wheat were examinedin relation to gibberellin (GA) content and GA-responsivenessof the extension zone. Leaf 2 of wild-type (rht3) wheat closelyresembled that of the Rht3 dwarf mutant when seedlings weregrown at 10C. The maximum relative elemental growth rate (REGR)within the extension zone in both genotypes was lower at 10Cthan at 20C, but the position with respect to the leaf basewas unaffected by temperature. The size of the extension zoneand epidermal cell lengths were similar in both genotypes at10C. Growth at 20C, instead of 10C, increased the lengthof the extension zone beyond the point of maximum REGR in thewild type, but not in the Rht3 mutant. Increasing temperatureresulted in longer epidermal cells in the wild type. Treatingwild-type plants at 10C with gibberellic acid (GA₃) also increasedthe length of the extension zone, but the Rht3 mutant was GA-non-responsive.However, the concentrations of endogenous GA₁ and GA₃ remainedsimilar across the extension zone of wild-type plants grownat both temperatures, despite large differences in leaf growthrates. The period of accelerating REGR as cells enter the extensionzone, and the maximum REGR attained, are apparently not affectedby GA. It is proposed that GA functions as a stimulus for continuedcell extension by preventing cell maturation in the region beyondmaximum REGR and that low temperature increases the sensitivitythreshold for GA action. Key words: Cell extension, gibberellin, Rht3 dwarfing gene, temperature, wheat leaf     

The Gibberellins of Developing Bean Seeds

Properties of gibberellin-like substances extracted from Frenchbean seeds during their development were studied. Two zonesof gibberellin-like activity were detected on paper chromatograms. Changes in activity of one of the zones correlated with changesin growth-rate of the seed. From considerations of Rf on paperchromatograms, and differential activity on dwarf peas, dwarf-1and dwarf-5 corn, it was deduced that activity of this zonewas due to substances resembling gibberellin A₁ and gibberellinA₅. Gibberellin A₅-like activity was highest in young seedsand disappeared after cell division in the cotyledons had ceased.Gibberellin A₁-like activity rose to its highest level duringthe period of rapid cell expansion in the cotyledons, afterthe disappearance of gibberellin A₅-like activity. The second zone of gibberellin-like activity was due mainlyto a non-acidic substance, which disappeared at the time GAj-likeactivity was rising to its highest level. A non-acidic substance that stimulated lateral bud growth ofdwarf peas also was detected in the extracts. It is presumedto be a cytokinin.     

The effect of cotyledons on hypocotyl elongation in light-grown bean seedlings: Comparison between dwarf and tall varieties

Hypocotyl sections with and without the cotyledons were cutfrom bean seedlings and incubated under white light of 6000lux. The cotyledons had an inhibitory effect as well as a promotiveeffect on hypocotyl growth. The former effect was more strikingin the dwarf variety, and the latter in the tall variety. Whenthe hypocotyl units were exposed to light for shorter times(6 hr or less) or incubated under weaker light (1600 and 50lux), the inhibitory effect of the cotyledons decreased greatly,and in the tall variety the presence of cotyledons producedno inhibition, but a promotion of hypocotyl growth. GA treatmentenhanced hypocotyl growth and counteracted the growth inhibitioncaused by the cotyledons. On the whole, the GA effect was moremarked in the tall variety than in the dwarf. The elongation of bean hypocotyls may be controlled by a balancebetween the inhibitory and promotive effects of cotyledons,and the predominance of the former over the latter may be oneof the causes for expressing dwarfing. (Received November 13, 1976; )     

The Role of Roots in Control of Bean Shoot Growth

Restriction of root growth by growing bean plants (Phaseolusvulgaris L.) in very small pots led to the development of dwarfplants. The leaves of those plants were smaller and their internodesshorter than those of control plants which were grown in largerpots and had developed a more extensive root system. A largequantity of starch—much more than in control plants —accumulated in the leaves and shoots of the dwarf plants. Increasingthe amount of minerals which was supplied to the roots, enhancedleaf growth of the control plants but failed to affect the dwarfones, in spite of the fact that in both cases the treatmentincreased the content of N, P and K in all the plant organs.The leaf water content was similar in both treatments, but theleaf water potential was higher in the dwarf plants. Exogenousapplication of gibberellic acid (GA₃) to the dwarf plants overcamethe reduction of stem growth completely, and that of the leavespartially. Application of the cytokinin, benzyladenine (BA)did not affect stem growth, but increased that of the primaryleaves. A combined supply of GA₂ + BA restored completely thegrowth of the stem and the primary leaves, and partially thatof the trifoliate leaves. It is concluded that a limited rootsystem restricts shoot growth through an hormonal system inwhich at least gibberellins and cytokinins are involved, andthat the dwarfing is not a consequence of mineral or assimilatedeficiency, or due to water stress. Phaseolus vulgaris L., leaf growth, stem growth, root restriction, gibberellic acid, benzyladenine, cytokinin     

CHANGES IN THE AMOUNT OF GIBBERELLIN-LIKE SUBSTANCES IN THE SEEDLING OF PHARBITIS NIL WITH SPECIAL REFERENCE TO EXPANSION OF COTYLEDON

Two gibberellin-like substances were separated chromatographicallyfrom the ethanol-extract of germinating cotyledons of Pharbitisnil. In the dark-grown seedling, these substances in the cotyledonincreased in amount gradually with the slow increase in freshweight and area of the tissue, and attained, 4 days after sowing,a maximum level of 0.03 µg gibberellin A₃ equivalent pera couple of cotyledon; in the light-grown seedling, they increasedrapidly with the rapid increase in the fresh weight and area,and attained a maximum content of 0.07 µg gibberellinA₃ equivalent per cotyledon on the 6th day. When the etiolatedseedling was exposed to light, the amount of the gibberellin-likesubstances in the cotyledon increased in parallel with openingof the hypccotyl hook and expansion of the cotyledon. The openingof the hook and increase of the substances occurred in red lightas well as in far-red. Relatively small amounts of the gibberellin-like substanceswere also contained in the germinating hypocotyl and root. The growth of cotyledonary disks was promoted by gibberellinA₃ and the gibberellin-like substances from the expanding cotyledon. The dwarf rice seedling and dwarfs 1, 3 and 5 of maize respondedto the gibberellin-like substances, but dwarf Pharbitis didnot. (Received August 20, 1963; )     

Effects of Fluorogibberellins on Plant Growth and Gibberellin 3r{beta}-Hydroxylases

3rß-Fluorogibberellin A₉ (3rß-fluoro-GA₉),3rßfluoro-GA₂₀, 3rß-fluorodeoxygibberellinC (3rß-fluoro-DGC) and 13-fluoro-GA₉ were prepared,and their effects on plant growth and gibberellin (GA) 3rß-hydroxyIaseswere examined. 3rß-Fluoro-GA₉ and 3rß-fluoro-GA₂₀promoted the growth of dwarf rice (Oryza sativa L. cv. Tan-ginbozu)seedlings to three times higher than the control seedlings ata dosage of 3 µ plant^–1, and 3rßfluoro-DGCto twice higher at the same dosage. 3rßg-Fluoro-GA₉was active in cucumber (Cucumis sativus L.) hypocotyl assay,its activity being about one-thirtieth as much as that of GA₄.3rß-Fluoro-GAs were active per se in promoting theshoot elongation of rice. 3rß-Fluoro-DGC inhibitedthe 3rß-hydroxylation of [³H₂]GA₉ to [³H]GA₄ by GArß-hydroxylase from bean (Phaseolus vulgaris L.),but 3rß-fluoro-GA₉ and 3rß-fluoro-GA₂₀ didnot show any effects on the enzyme activity. These 3rß-fluoro-GAsalso showed no or only a weak inhibitory effect on the rß-hydroxylasefrom pumpkin (Cucurbita maxima L.). 13-Fluoro-GA₉ promoted growthof rice and cucumber seedlings, and inhibited the 3rß-hydroxylasesfrom both bean and cucumber. 13-Fluoro-GA₉was converted into13-fluoro-GA₄ and 2,3-didehydro-13-fluoro-GA₉, in a cell-freesystem from bean, and conversion of 13-fluoro-GA₉ into 13-fluoro-GA₄was also observed in a cell-free system from pumpkin. Theseresults suggest that 13-fluoro-GA₉ is one of the substratesof GA 3rß-hydroxy-lases, and that 13-fluoro-GA₉ isactive as a result of the conversion to 13-fluoro-GA₄ in riceand cucumber seedlings. (Received October 27, 1997; Accepted March 13, 1998)