Comparative effects of gibberellic acid and N6-benzyladenine on dry matter partitioning and osmotic and water potentials in seedling organs of dwarf watermelon

Apical applications of 0.2 g N⁶-benzyladenine (BA), a synthetic cytokinin, or 5 g of gibberellic acid (GA₃) significantly enhanced hypocotyl elongation in intact dwarf watermelon seedlings over a 48-h period. Accompanying the increase in hypocotyl length was marked expansion of cotyledons in BA-treated seedlings and inhibition of root growth by both compounds. A study on dry matter partitioning indicated that both growth regulators caused a preferential accumulation of dry matter in hypocotyls at the expense of the roots; however, GA₃ elicited a more rapid and greater change than did BA. In comparison to untreated seedlings, BA decreased total translocation of metabolites out of the cotyledons. Water potentials of cotyledons and hypocotyls were determined by allowing organs to equilibrate for 2 h in serial concentrations of polyethylene glycol 4000. Osmotic potentials were determined by thermocouple psychrometry. During periods of rapid growth in cotyledons and hypocotyls of BA-treated seedlings and in hypocotyls of GA-treated seedlings, the osmotic potential increased and the turgor pressure decreased in relation to untreated seedlings, indicating that cell wall extensibility was being increased. Osmotic potentials were lower in hypocotyls of GA-treated than in those of BA-treated seedlings, even though growth rates were higher in GA-treated seedlings, indicating that the latter treatment was generating more osmotically active solutes in hypocotyls.Scientific Contribution No. 1219 from the New Hampshire Agricultural Experiment Station.     

Promotion of Hypocotyl Elongation in Watermelc Seedlings by 6-Benzyladenine

Hypocotyl elongation under white fluorescent light was aboutdoubled in dwarf watermelon (Citrullus lanatus0 (Thunb.) Matsu.and Nakai) seedlings treated with 0.1 to 0.3 µg apicaland 3 x 10–6 to 10.3 M root applications of 6-benzyladenine(BA). BA-enhancement of growth occurred primarily during thefirst 48 h after treatment. Increased hypocotyl length in BA-treatedseedlings was attributed more to longer cells than to an increasein cell number. Early hypocotyl growth of normal seedlings wasalso significantly enhanced by BA although final hypocotyl lengthwas not substantially affected. Benzyladenine caused expansion of cotyledons and, at higherdoses, lateral expansion of hypocotyls. BA-induced increasesin fresh weight of cotyledons and hypocotyls were accompaniedby an increase in dry weight of hypocotyls at the expense ofroots which had less dry matter than untreated seedlings.     

Relationship of IAA-oxidase Activity to Gibberellinand IAA-induced Elongation of Light-grown Cucumber Seedlings

The growth and IAA-oxidase activity of light-grown cucumber seedlings (cv. Aonagajibae) were investigated in response to GA₃ and IAA. Both GA₃ and IAA induced significant elongation of the hypocotyl. The fresh and dry weights of the hypocotyl increased due to GA₃ or IAA treatment, whereas no significant change was observed in the cotyledons of GA₃-treated seedlings as compared with the controls. The fresh and dry weights of IAA-treated cotyledons were both lower than those of controls. Treatment with GA₃ or IAA resulted in retardation of IAA-oxidase activity in the hypocotyl and cotyledons. The degree of retardation was less in the cotyledons than in the hypocotyl. An inverse relationship was recognized between GA₃- or IAA-induced elongation and IAA-oxidase activity in the hypocotyl. The auxin-mediated mechanism for gibberellin action was discussed.     

Sensitivity of cell division and cell elongation to low water potentials in soybean hypocotyls

Summary The response of cell division and cell elongation to low cell water potentials was studied in etiolated, intact soybean hypocotyls desiccated either by withholding water from seedlings or by subjecting hypocotyls to pressure. Measurements of hypocotyl water potential and osmotic potential indicated that desiccation by withholding water resulted in osmotic adjustment of the hypocotyls so that turgor remained almost constant. The adjustment appeared to involve transport of solutes from the cotyledons to the hypocotyl and permitted growth of the seedlings at water potentials which would have been strongly inhibitory had adjustment not occurred. Growth was ultimately inhibited in hypocotyls due to inhibition of cell division and cell elongation to a similar degree. The inhibition of cell elongation appeared to result from a change in the minimum turgor necessary for growth. On the other hand, when intact hypocotyls were exposed to pressure for 3 h, osmotic adjustment did not occur, turgor decreased, and the sensitivity of growth to low cell water potentials increased, presumably due to inhibition of cell elongation. Thus, although cell division was sensitive to low cell water potentials in soybean hypocotyls, cell elongation had either the same sensitivity or was more sensitive, depending on whether the tissue adjusted osmotically. Osmotic adjustment of hypocotyls may represent a mechanism for preserving growth in seedlings germinating in desiccated soil.Supported by a grant from the Illinois Agricultural Experiment Station, University of Illinois and grant 1-T1-GM-1380 from the United States Public Health Service.     

Narrow-band light regulation of ethylene and gibberellin levels in hydroponically-grown <Emphasis Type="Italic">Helianthus annuus</Emphasis> hypocotyls and roots

Both hypocotyl and root growth of sunflower (Helianthus annuus) were examined in response to a range of narrow-band width light treatments. Changes in two growth-regulating hormones, ethylene and gibberellins (GAs) were followed in an attempt to better understand the interaction of light and hormonal signaling in the growth of these two important plant organs. Hydroponically-grown 6-day-old sunflower seedlings had significantly elongated hypocotyls and primary roots when grown under far-red (FR) light produced by light emitting diodes (LEDs), compared to narrow-band red (R) and blue (B) light. However, hypocotyl and primary root lengths of seedlings given FR light were still shorter than was seen for dark-grown seedlings. Light treatment in general (compared to dark) increased lateral root formation and FR light induced massive lateral root formation, relative to treatment with R or B light. Levels of ethylene evolution (roots and hypocotyls) and concentrations of endogenous GAs (hypocotyls) were assessed from both 6-day-old sunflower plants either grown in the dark, or treated with FR, R or B light. Both R and B light had similar effects on hypocotyl and root growth as well as on ethylene and on hypocotyl GA levels. Dark treatment resulted in the highest ethylene levels, whereas FR treatment significantly reduced ethylene evolution for both hypocotyls and roots. R- and B-light treatments elevated ethylene evolution relative to FR light. Endogenous GA₅₃ and GA₁₉ levels in hypocotyls were significantly higher and GA₄₄, GA₂₀ and GA₁ levels significantly lower, for dark and FR light treatments compared to R and B light-treatments. The patterns seen for changes in GA concentrations indicate FR-, R- and B-light-mediated effects [differences] in the metabolism of the early C₂₀ GAs, GA₅₃ → GA₄₄ → GA_19. Surprisingly, GA₂₀, GA₁ and GA₈ levels in hypocotyls were very much reduced by treatment of the plants with FR light, relative to B and R-light treatments, e.g. the increased hypocotyl elongation induced by FR light was correlated with reduced levels of all three of the downstream C₁₉ GAs. The best explanation, albeit speculative, is that a more rapid metabolism, i.e. GA₂₀ → GA₁ → GA₈ → GA₈ conjugates occurs under FR light. Although this study provided no evidence that elevated ethylene evolution by roots or hypocotyls of sunflower is controlling growth via endogenous GA biosynthesis, there are differences between soil-grown and hydroponically-grown sunflower seedlings with regard to trends seen for hypocotyl GA concentrations and both root and hypocotyl ethylene evolution in response to narrow band width R and FR light signaling.     

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)     

Osmoregulation,solute distribution,and growth in soybean seedlings having low water potentials

Soybean (Glycine max (L.) Merr.) seedlings osmoregulate when the supply of water is limited around the roots. The osmoregulation involves solute accumulation (osmotic adjustment) by the elongating region of the hypocotyls. We investigated the relationship between growth, solute accumulation, and the partitioning of solutes during osmoregulation. Darkgrown seedlings were transplanted to vermiculite containing 1/8 (0.13 x) the water of the controls. Within 12–15 h, the osmotic potential of the elongating region had decreased to-12 bar, but it was-7 bar in the controls. This osmoregulation involved a true solute accumulation by the hypocotyls, since cell volume and turgor were virtually the same regardless of the water regime. The hypocotyls having low water potentials elongated slowly but, when deprived of their cotyledons, did not elongate or accumulate solute. This result indicated a cotyledonary origin for the solutes and a dependence of slow growth on osmotic adjustment. The translocation of nonrespired dry matter from the cotyledons to the seedling axis was unaffected by the availability of water, but partitioning was altered. In the first 12 h, dry matter accumulated in the elongating region of the 0.13 x hypocotyls, and osmotic adjustment occurred. The solutes involved were mostly free amino acids, glucose, fructose, and sucrose, and these accounted for most of the increased dry weight. After osmotic adjustment was complete, dry matter ceased to accumulate in the hypocotyls and bypassed them to accumulate in the roots, which grew faster than the control roots. The proliferation of the roots resulted in an increased root/shoot ratio, a common response of plants to dry conditions.Osmotic adjustment occurred in the elongating region of the hypocotyls because solute utilization for growth decreased while solute uptake continued. Adjustment was completed when solute uptake subsequently decreased, and uptake then balanced utilization. The control of osmotic adjustment was therefore the rate of solute utilization and, secondarily, the rate of solute uptake. Elongation was inhibited by unknown factors(s) despite the turgor and substrates associated with osmotic adjustment. The remaining slow elongation depended on osmotic adjustment and represented some optimum between the necessary inhibition for solute accumulation and the necessary growth for seedling establishment.     

Anomalous Temperature Dependence of Seedling Development in Some Soybean (Glycine max [L.] Merr.) Cultivars: Role of Ethylene

In Clark and Shelby soybean (Glycine max [L.] Merr.) seedlings, hypocotyl elongation was inhibited and hypocotyl swelling and root dry weight were increased by a temperature of 25 C. At 20 and 30 C, development was normal, as was development of Hawkeye and Mandarin soybean seedlings at all three temperatures. Dry matter distribution at 9 days indicates that inhibition of hypocotyl elongation is not due to a lack of translocation from cotyledons, but to a diversion of dry matter from hypocotyl to root. Ethylene evolution by Clark seedlings at 25 C exceeds that at 20 and 30 C. At all three temperatures, Mandarin seedlings' ethylene evolution is at the same low rates as those of Clark at 20 and 30 C. Clark's enhanced rate at 25 C precedes the deceleration of hypocotyl elongation occurring at 5 days. The abnormal effects of a temperature of 25 C on Clark seedlings' development is partially reversed by CO₂ and GA₃ and can be duplicated in Mandarin by applying ethylene. In Clark, effects of the temperature can be further accentuated by indoleacetic acid, which stimulates ethylene evolution, and by applying ethylene to the seedlings. It is concluded that the temperature-induced symptoms, similar in most respects to the well known “triple response” of legume seedlings, are caused by abnormally high levels of ethylene in tissues of the anomalous cultivars.     

GERMINATION AND SEEDLING DEVELOPMENT OF SOYBEANS IN A CARBON DIOXIDE-DEFICIENT ATMOSPHERE

During the initial phase of germination and seedling development of soybeans, most of the increase in dry weight in the embryo axis occurred in the hypocotyl. The epicotyl did not undergo a rapid increase in size and dry weight until the 4th to 5th day of growth. From day 1 to 11 dry weights of the hypocotyls in the “CO₂-normal” and “CO₂-Iimiting” (less than 50 ppm [0.005%] atmospheric CO₂) treatments were similar. By day 13 the CO₂-normal hypocotyls had continued their rapid increase in dry weight while the dry weight of the hypocotyls grown under CO₂-limiting conditions had decreased. The parallel decrease in dry weight of the cotyledons that occurred for both CO₂ treatments (days 1 to 13) suggested that CO₂ content of the atmosphere is not a controlling factor in the digestion or hydrolysis of food reserves in the cotyledon during germination and seedling development. This is supported by the failure of the CO₂ treatments to produce significantly different patterns in utilization of lipids, total carbohydrates, or proteins in the cotyledons. The seedlings grown under either the limiting or normal CO₂ atmospheres utilized their cotyledonary carbohydrates and lipids before the proteins. Sufficient food reserves are present in the soybean cotyledon storage cells for seedling development until about day 9. Net CO₂ uptake by soybeans in a CO₂-normal environment first occurred between days 9 to 11.     

Interaction of Brassinosteroids with Light Quality and Plant Hormones in Regulating Shoot Growth of Young Sunflower and <Emphasis Type="Italic">Arabidopsis</Emphasis> Seedlings

Sunflower hypocotyls elongate as light quality changes from the normal red to far-red (R/FR) ratio of sunlight to a lower R/FR ratio. This low R/FR ratio-induced elongation significantly increases endogenous concentrations of indole-3-acetic acid (IAA) and also of three gibberellins (GAs): GA₂₀, GA₁, and GA₈. Of these, it is likely GA₁ that drives low R/FR-induced growth. Brassinosteroids are also involved in shoot growth. Here we tested three R/FR ratios: high, normal, and low. Significant hypocotyl elongation occurred with this stepwise reduction in R/FR ratio, but endogenous castasterone concentrations in the hypocotyls remained unchanged. Brassinolide was also applied to the seedlings and significantly increased hypocotyl growth, though one that was uniform across all three R/FR ratios. Applied brassinolide increased hypocotyl elongation while significantly reducing (usually) levels of IAA, GA₂₀, and GA₈, but not that of GA₁, which remained constant. Given the above, we conclude that endogenous castasterone does not mediate the hypocotyl growth that is induced by enriching FR light, relative to R light. Similarly, we conclude that the hypocotyl growth that is induced by applied brassinolide does not result from an interaction of brassinolide with changes in light quality. The ability of applied brassinolide to influence IAA, GA₂₀, and GA₈ content, yet have no significant effect on GA₁, is hard to explain. One speculative hypothesis, though, could involve the brassinolide-induced reductions that occurred for endogenous IAA, given IAA’s known ability to differentially influence the expression levels of GA20ox, GA3ox, and GA2ox, key genes in GA biosynthesis.     

Influence of Growth Regulators on Root and Shoot Initiation from Flax Meristem-Tips and Hypocotyls in vitro

Hypocotyl sections and isolated meristem-tips from 7- to 10-day-old seedlings of flax were cultured on a Murashige and Skoog (1962) medium. The optimum concentrations of naphthaleneacetic acid (NAA) and benzyladenine (BA) were determined for shoot and root initiation from hypocotyls. Meristem-tip cultures differentiated shoots and roots in response to these concentrations. The frequency and yield of shoots, promoted by BA, decreased if NAA or gibberellic acid (GA₃) was included in the medium. At high NAA concentration GA₃ and BA inhibited root initiation. Morphological development of cultures differed depending on the growth regulators in the medium.     

Growth Suppression of Canola through Wheat Stubble II. Investigating Impacts of Hypocotyl Elongation using Simulated Stubble

In south-eastern Australia, surface-retained wheat stubble can reduce the growth and yield of canola as a result of reductions to the quality and quantity of light under the stubble and the associated elongation of the hypocotyl. This paper reports a series of pot experiments that examined the effect of hypocotyl elongation on the leaf area development of canola, the allocation of dry mass, and the absolute and relative growth rates compared to non-etiolated seedlings. The primary aim was to determine the magnitude of the growth reductions caused by hypocotyl elongation in canola seedlings under controlled conditions. Seedling hypocotyl elongation was induced by growing canola seedlings in narrow poly-pipe tubing of different lengths placed over the seedlings as they began to emerge through the soil, and removing the shade cloth covering the top of the tube when the cotyledons had reached it, to mimic the plant reaching and overtopping a stubble layer. Plants with longer hypocotyls had smaller root systems, less leaf area and less leaf and root biomass. These plants had lower relative growth rates than plants that allocated fewer resources to hypocotyls and more to roots and leaves. The magnitude of the growth responses observed in these experiments was similar to those of plants with long hypocotyls growing through stubble layers in previously reported field studies. This suggests that a significant portion of the effect of stubble observed in the field under stubble retention is due to the re-allocation of resources to the production and growth of the hypocotyls, rather than other biochemical effects of the stubble.     

Some Morphogenic Differences between Monoecious and Gynoecious Cucumber Seedlings as Related to Ethylene Production

Exposure of gibberellic acid-treated seedlings of a monoecious cucumber cultivar `Chipper' (Cucumis sativus L.) to ethylene caused thickening of the hypocotyl, inhibited longitudinal growth, and had no effect on fresh weight. Downward curvature of cotyledons was increased by the presence of ethylene. A gynoecious breeding line, `Gy 3,' had thicker hypocotyls and displayed its cotyledons in a more downward position than `Chipper'. Excised hypocotyls of the gynoecious seedlings produced three times as much ethylene as did the monoecious Chipper hypocotyls. Thus, ethylene may play a role in the regulation of cucumber seedling morphology.     

Effects of colchicine and lumicolchicine on hypocotyl elongation,respiration rates and microtubules in gibberellic-acid-treated lettuce seedlings

Gibberellic-acid (GA₃)-induced hypocotyl elongation of intact lettuce (Lactuca sativa L.) seedlings was inhibited by colchicine (4×10^-4 M) but not by lumicolchicine (4×10^-4 M). In excised lettuce hypocotyls, GA₃ (10^-5 M) increased respiration over water controls, while both colchicine and lumicolchicine alone, or in combination with GA₃, reduced respiration. Microtubules were present in the hypocotyls of lumicolchicine-treated seedlings but absent in those treated with colchicine. It is suggested that lumicolchicine is a useful drug to discriminate between the metabolic and microtubule-mediated processes in cell morphogenesis.     

High frequency plant regeneration from cotyledon and hypocotyl explants of ornamental kale

A high frequency shoot regeneration system for ornamental kale [Brassica oleracea L. var. acephala (D.C.) Alef.] was firstly established from seedling cotyledon and hypocotyl explants. The ability of cotyledon and hypocotyl to produce adventitious shoots varied depending upon genotype, seedling age and culture medium. The maximum shoot regeneration frequency was obtained when the explants from cv. Nagoya 4-d-old seedlings were cultured on Murashige and Skoog (MS) medium supplemented with 3 mg dm⁻³ 6-benzylaminopurine (BA) and 0.1 mg dm⁻³ naphthaleneacetic acid (NAA). The frequency of shoot regeneration was 65.0 % for cotyledons, 76.1 % for hypocotyls; and the number of shoots per explant was 4.3 for cotyledons, 8.2 for hypocotyls. Hypocotyl explants were found to be more responsive for regeneration when compared with cotyledons. Among the 4 cultivars tested, Nagoya showed the best shoot regeneration response. The addition of 3.0 mg dm⁻³ AgNO₃ was beneficial to shoot regeneration. Roots were formed on the base of the shoots when cultured on half-strength MS medium.     

Gibberellic Acid stimulation of cucumber hypocotyl elongation : effects on growth, turgor, osmotic pressure, and cell wall properties

Recently developed techniques have been used to reinvestigate the mechanism by which gibberellic acid (GA₃) stimulates elongation of light-grown cucumber (Cucumis sativus L.) seedlings. Osmotic pressure and turgor pressure were slightly reduced in GA₃-treated seedlings, which elongated 3.5 times faster than control seedlings. This indicated that GA₃ enhancement of growth was not controlled by changes in the osmotic properties of the tissues. Stress/strain (Instron) analysis revealed that plastic extension of the cell walls of GA₃-treated seedlings increased by up to 35% above the control values. Stress-relaxation measurements on frozen-thawed tissue showed that T₀ the minimum relaxation time, was reduced following application of GA₃. In vivo wall relaxation (measured by the pressure block technique) showed that the wall yield coefficient was increased, and the yield threshold was slightly reduced. Thus GA₃ affected both the mechanical (viscoelastic) and biochemical (chemorheological) properties of the cell walls of light-grown cucumber. The previous hypothesis, that GA₃ stimulates cucumber hypocotyl growth by increasing osmotic pressure and cell turgor, is contradicted by our results.     

ACTION OF GIBBERELLIC ACID ON CELL PROLIFERATION IN THE SUBAPICAL SHOOT MERISTEM OF WATERMELON SEEDLINGS

Rates of cell proliferation were about 2.5 fold greater in gibberellic acid-treated seedlings of dwarf watermelon than in untreated dwarf seedlings as determined by direct counts of pith cells in elongating hypocotyls of watermelon. The higher rates of cell proliferation observed in normal and GA-treated dwarf seedlings were due both to shorter cell cycle times and increased growth fractions of cells than in dwarf seedlings. The shorter duration of the cell cycle in GA-treated seedlings was primarily due to a reduced S period.     

Effect of benzyladenine on development of derooted watermelon seedlings

We have examined the effects of benzyladenine (BA) on derooted watermelon ( Citrullus vulgaris Schrad., cv. Fairfax) seedlings with special attention to the cotyledons. The growth regulator was supplied either as a droplet of solution between the cotyledons (application from above) or through the hypocotyl stump (application from below). Application of BA from above stimulates several developmental parameters of cotyledons [growth, lipid breakdown, chlorophyll and carotenoid accumulation, hydroxypyruvate reductase (EC 1.1.1.81.) activity]. The stimulation is much smaller with application from below. Also the distribution of labelled BA changes according to the method of application. The bulk of the label remains in the cotyledons when BA is supplied from above and in the hypocotyl when it is supplied from below. The absolute amount of radioactivity found in the cotyledons after 24 h of treatment is approximately the same in both cases. This seems to indicate that the different effects of the two treatments depend on concentration ratios of BA (or metabolic derivates) between hypocotyl and cotyledons rather than on absolute growth regulator levels.     

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; )     

Effects of Brassin-Complex on Auxin and Gibberellin Mediated Events in the Morphogenesis of the Etiolated Bean Hypocotyl

The excised, hooked bean hypocotyl was the system used to determine wheiher the ‘auxin- and gibberellin like’ effect of the lipoidal pollen extract, Brass in-complex (Br), were mediated through, or independent of, auxin and gibberellin. The morphogenetic events of hook opening and hypocotyl elongation in this system are regulated by auxin and gibberellin, respectively. Brassin complex, like IAA, elicited a book closure in (he dark and retarded its opening in red light. This effect was synergized by T1BA, IAA and the presence of the auxin-producing organs, the epicotyl and cotyledons. Br-elicited hook closure was inhibited by the antiauxin. PCIB. Both GA₃ and Br totally reversed the light inhibition of hypocotyl elongation. The GA₃-effect, but nol the Br elicited elongation, was overcome by Ancymidol. Hypocotyl elongation was partially inhibited by TIBA and PCIB. suggesting a possible auxin involvement also in this effect of Br. Br may elicit its growth responses through an effect on endogenous auxin levels, In this way it is different from other lipoidat growth regulators, such as the oleanimins which require the presence of exogenous growth regulators for activity.