Effects of Auxin Transport Inhibitors on Gibberellins in Pea

The effects of the auxin transport inhibitors 2,3,5-triiodobenzoic acid (TIBA), 9-hydroxyfluorene-9-carboxylic acid (HFCA), and 1-N-naphthylphthalamic acid (NPA) on gibberellins (GAs) in the garden pea (Pisum sativum L.) were studied. Application of these compounds to elongating internodes of intact wild type plants reduced markedly the endogenous level of the bioactive gibberellin A₁ (GA₁) below the application site. Indole-3-acetic acid (IAA) levels were also reduced, as was internode elongation. The auxin transport inhibitors did not affect the level of endogenous GA₁ above the application site markedly, nor that of GA₁ precursors above or below it. When plants were treated with [¹³C,³H]GA₂₀, TIBA reduced dramatically the level of [¹³C,³H]GA₁ recovered below the TIBA application site. The internodes treated with auxin transport inhibitors appeared to be still in the phase where endogenous GA₁ affects elongation, as indicated by the strong response to applied GA₁ by internodes of a GA₁-deficient line at the same stage of expansion. On the basis of the present results it is suggested that caution be exercised when attributing the developmental effects of auxin transport inhibitors to changes in IAA level alone. Received April 13, 1998; accepted April 14, 1998     

Molecular identification of phytoplasmas in fasciated cacti and succulent species and associated hormonal perturbation

Fasciation, a frequent phenomenon in Cactaceae, has been attributed to various causes. The present study reports on phytoplasma-induced fasciation in Euphorbia coerulescens (Euphorbiaceae), Orbea gigantea (Asclepiadaceae), Opuntia cylindrica (Cactaceae), and Senecio stapeliiformis (Asteraceae). DNA was extracted from symptomless and fasciated tissues and amplified by nested PCR using universal primers P1/P7 followed by R16F2n/R16R2 produced amplicons of 1.2 Kb. The nucleotide sequence analyses of the amplicons indicated that fasciated plants were infected by phytoplasma. Phylogenetic analysis placed the cacti fasciation phytoplasmas in 16SrII group. The hormonal content of symptomless and fasciated tissues including indole-3-acetic acid (IAA), kinetin (Kin), N⁶-benzyladenine (BA), abscisic acid (ABA), and gibberellic acid (GA₃) was quantified by high performance liquid chromatography (HPLC). The results indicated that fasciation in O. gigantea was correlated with the accumulation of Kin and IAA increasing five and two times, respectively, as compared to symptomless tissue. However, there was no consistent pattern of hormones in other fasciated species (E. coerulescens, O. cylindrica, and S. stapeliiformis), suggesting that different plant species might have different mechanism to develop fasciation associated with phytoplasma infection.     

Effect of Gibberellin Biosynthesis Inhibitors on Native Gibberellin Content, Growth and Floral Initiation in Sorghum bicolor

CCC, uniconazol, ancymidol, prohexadione-calcium (BX-112), and CGA 163′935, which represent three groups of gibberellin (GA) biosynthesis inhibitors, were applied as a soil drench to Sorghum bicolor cultivars 58M (phyB-1, phytochrome B-deficient mutant) and 90M (phyB-2, equivalent phenotypically to wild type, PHYB, except for small differences in flowering dates). The inhibitors that block steps before GA₁₂ (CCC, uniconazol, and ancymidol) lowered the concentrations of all endogenous early-C13α-hydroxylation pathway GAs found in sorghum: GA₁₂, GA₅₃, GA₄₄, GA₁₉, GA₂₀, GA₁, and GA₈. In contrast, the inhibitors that block the conversion of GA₂₀→ GA₁, (CGA 163′935 and BX-112) drastically reduced GA₁ and GA₈ levels, but they either did not change or caused accumulation of intermediates from GA₁₂ to GA₂₀. Combinations of pre-GA₁₂ inhibitors and GA₃ plus GA₁ strongly reduced GAs other than GA₁ and GA₃. Each of these compounds inhibited shoot growth in both cultivars and delayed floral initiation in 90M. Floral initiation of 58M was also delayed by CCC, uniconazol, and ancymidol but not by CGA 163`935 and BX-112. This separation of shoot elongation from floral initiation in sorghum is novel. Both inhibition of shoot growth and delayed floral initiation were almost completely relieved by a mixture of GA₃ and GA₁ in both 58M and 90M. This observation, plus the much lower levels of endogenous GA₃ than of GA₁ observed in these experiments, implies that GA₁ is the major endogenous GA active in shoot elongation. CGA 163′935 and BX-112 also failed to promote tillering in 58M, whereas inhibitors active before GA₁₂ did so. The possibility that the GA₂₀→ GA₁ inhibitors fail to block flowering and promote tillering in 58M because biosynthetic intermediates between GA₁₂ and GA₂₀ accumulate and/or because 58M is altered in GA metabolism in this same region of the biosynthetic pathway is discussed. Received April 7, 1998; accepted July 31, 1998     

GA4 Does Not Require Conversion into GA1 to Delay Senescence of Alstroemeria hybrida Leaves

The biological activity and metabolism of applied GA₁ and GA₄ were studied in leaves of alstroemeria (Alstroemeria hybrida). It appeared that GA₄ was 2 orders of magnitude more active in delaying leaf senescence than GA₁. GA₃-13-OMe, a GA analog that cannot be hydroxylated on the 13-C position, also retarded chlorophyll loss, although less efficiently. Tritiated and deuterated GA₁, GA₄, and GA₉ were applied to leaves, and their metabolites were analyzed. According to high performance liquid chromatography and gas chromatography-mass spectrometry analyses, GA₉ was converted into GA₄ and GA₃₄, and GA₄ was converted into GA₃₄ and more polar components. No evidence was found for the conversion of both GA₉ and GA₄ into GA₁, even at the relatively high concentrations that were taken up by the leaf. The results strongly suggest that GA₄ is recognized directly by a receptor involved in regulation of leaf senescence in alstroemeria. Received November 24, 1997; accepted February 17, 1998     

Effects of Ring D-Modified Gibberellins on Gibberellin Levels and Development in Selected Sorghum bicolor Maturity Genotypes

Plants of early flowering mutant and wild type genotypes of Sorghum bicolor were treated with ring D-modified gibberellins (GAs), and the effects on endogenous GA levels were determined. The growth and timing of floral initiation in 58M plants grown under 18-h days (which significantly delays floral initiation in this short day plant) following treatment with these compounds, relative to GA₃ and GA₅ treatments, were also investigated. Application of the endo-isomer of C16,17-dihydro-GA₅ (endo-DiHGA₅), the exo-isomer of C16,17-dihydro-GA₅ (exo-DiHGA₅), and C16α,17-dichloromethanodihydro-GA₅ (DMDGA₅) altered GA levels in both genotypes. Each ring D-modified GA significantly inhibited shoot growth while significantly decreasing levels of GA₁ and increasing levels of its immediate precursor, GA₂₀. Gibberellin A₈ levels also decreased. Tillering was not affected by any treatment. For the early flowering genotype 58M, grown under noninductive long days, both dihydro-GA₅ isomers promoted floral initiation while shoot growth was strongly inhibited, and floral development was strongly advanced beyond floral stage 4. Gibberellin A₃ and GA₅, applied under the same conditions, promoted shoot growth slightly and gave ``floral-like' apical meristems that did not develop past floral stage 1. These results suggest that the reduced shoot growth of sorghum, which follows application of those ring D-modified GAs, is due to their inhibiting the 3β hydroxylation of GA₂₀ to GA₁, thereby reducing the GA₁ content. That floral initiation was hastened and floral development promoted in genotype 58M by application of both isomers of DiHGA₅ are in contrast to the effects of other GA biosynthesis inhibitors, which act earlier in the GA biosynthesis pathway, but are consistent with results seen for long day grasses. This suggests that endo-DiHGA₅ and exo-DiHGA₅ may be acting directly in promoting floral initiation and subsequent floral apex development of this short day plant under long day conditions. Received October 3, 1996; accepted January 22, 1997     

The Response to Gibberellin in Pisum sativum Grown under Alternating Day and Night Temperature

The application of gibberellins (GA) reduces the difference in stem elongation observed under a low day (DT) and high night temperature (NT) combination (negative DIF) compared with the opposite regime, a high DT/low NT (positive DIF). The aim of this work was to investigate possible thermoperiodic effects on GA metabolism and tissue sensitivity to GA by comparing the response to exogenously applied GA (in particular, GA₁ and GA₃) in pea plants (Pisum sativum cv. Torsdag) grown under contrasting DIF. Control plants not treated with growth inhibitors or additional GA were 38% shorter under negative (DT/NT 13/21°C) than positive DIF (DT/NT 21/13°C) because of shorter internodes. Additional GA₁ or GA₃ decreased the difference between positive and negative DIF. In pea plants dwarfed with paclobutrazol, which inhibits GA biosynthesis at an early step, the response to GA₁ was reduced more strongly by negative compared with positive DIF than the response to GA₃. The induced stem elongation by GA₁₉ and GA₂₀ did not deviate significantly from the response to GA₁. Plants treated with prohexadione-calcium, an inhibitor of both the production and the inactivation of GA₁, grew equally tall under the two temperature regimes in response to both GA₁ and GA₃. We hypothesize that the reduced response to GA₁ compared with GA₃ in paclobutrazol-treated plants grown under negative DIF is caused by a higher rate of 2β-hydroxylation of GA₁ into GA₈ under negative than positive DIF. This contributes to lower levels of GA₁ and consequently shorter stems and internodes in pea plants grown under negative than positive DIF. Differences in tissue sensitivity to GA alone cannot account for this specific thermoperiodic effect on stem elongation. Received May 28, 1998; accepted May 29, 1998     

Gibberellins in Leaves of Alstroemeria hybrida: Identification and Quantification in Relation to Leaf Age

In alstroemeria (Alstroemeria hybrida), leaf senescence is retarded effectively by the application of gibberellins (GAs). To study the role of endogenous GAs in leaf senescence, the GA content was analyzed by combined gas chromatography and mass spectrometry. Five 13-hydroxy GAs (GA₁₉, GA₂₀, GA₁, GA₈, and GA₂₉) and three non-13-hydroxy GAs (GA₉ and GA₄) were identified in leaf extracts by comparing Kováts retention indices (KRIs) and full scan mass sprectra with those of reference GAs. In addition, GA₁₅, GA₄₄, GA₂₄, and GA₃₄ were tentatively identified by comparing selected ion monitoring results and KRIs with those of reference GAs. A number of GAs were detected in conjugated form as well. Concentrations of GAs in alstroemeria changed with the development of leaves. The proportion of biologically active GA₁ and GA₄ decreased with progressive senescence and the fraction of conjugated GAs increased. Received May 26, 1997; accepted August 12, 1997     

Effect of Gibberellin on Growth, Protein Secretion, and Starch Accumulation in Maize Endosperm Suspension Cells

The physiologic effect of gibberellins (GA) in seed development is poorly understood. We examined the effect of gibberellic acid (GA₃) on growth, protein secretion, and starch accumulation in cultured maize (Zea mays L.) endosperm suspension cells. GA₃ (5 and 30 μm) increased the fresh weight, dry weight, and protein content of the cultured cells, but the effect of GA₃ at 50 μm was not significantly different. However, the protein content in the culture medium was increased by these three concentrations of GA₃. The effect of GA₃ on the amount of cellular structural polysaccharides was not significant, but GA₃ had a dramatic effect on the starch content. At 5 μm, GA₃ caused an increase in the starch content, but at 50 μm the starch accumulation was reduced. Chlorocholine chloride (CCC), an inhibitor of GA biosynthesis, significantly increased the starch content and decreased the structural polysaccharide content of the cultured cells. The effects of CCC at 500 μm on the starch and polysaccharide content were partially reversed by 5 μm GA₃ applied exogenously. Based on these results we suggest that GA does not favor starch accumulation in the cell cultures and that the addition of lower concentrations of GA₃ in the medium may provide an improved balance among the endogenous GA in the cultured cells. Received October 31, 1995; accepted March 25, 1997     

Effect of Pot Size and Timing of Plant Growth Regulator Treatments on Growth and Tuber Yield in Greenhouse-Grown Norland and Russet Burbank Potatoes

The effects of pot size, timing of the application of paclobutrazol (PTZ) and gibberellic acid (GA₃), and the counteractive effect of these two compounds on growth and tuber yield of greenhouse-grown Norland and Russet Burbank potatoes were investigated. Plants were grown either in 1.5-liter pots (15 cm deep) or 3.0-liter pots (18 cm deep) and received a foliar application of either 1.5 mm PTZ or 9 × 10⁻³ mm GA₃ at early or late stolon initiation. Some plants that had been foliar treated with 1.5 mm PTZ at early stolon initiation were foliar treated with 9 × 10⁻³ mm GA₃ at late stolon initiation. PTZ reduced haulm length in both cultivars significantly, particularly when the treatment was applied at early stolon initiation, but the late treatment reduced haulm length only when growing in 3.0-liter pots. Irrespective of the timing of treatment, GA₃ increased haulm length in Norland growing in both pot sizes, but the treatment increased haulm length in Russet Burbank only when applied at late stolon initiation. GA₃ applied after PTZ did not overcome the growth-inhibiting effect of the PTZ treatment. The PTZ treatment effectively increased usable tuber number/plant (UTN) in Norland, but PTZ had no effect on UTN in Russet Burbank. PTZ reduced usable tuber weight/plant (UTW) only in Norland growing in 1.5-liter pots. By contrast, GA₃ increased UTN only when treated at late stolon initiation of 1.5-liter pot-grown Norland, whereas the same treatment was effective when applied only at early stolon initiation for Russet Burbank. For Norland, the increase in UTN by early applied PTZ was reduced by the subsequent application of GA₃. The use of 3.0-liter pots for minituber production in both Norland and Russet Burbank appears to have no advantage over growing in 1.5-liter pots, particularly when PTZ or GA₃ is used to enhance tuberization. Received May 30, 1997; accepted February 3, 1998     

Thermomorphogenic and Photomorphogenic Control of Stem Elongation in Fuchsia Is Not Mediated by Changes in Responsiveness to Gibberellins

Stem elongation in Fuchsia × hybrida was influenced by cultivation at different day and night temperatures or in different light qualities. Internode elongation of plants grown at a day (25°C) to night (15°C) temperature difference (DIF+10) in white light was almost twofold that of plants grown at the opposite temperature regime (DIF−10). Orange light resulted in a threefold stimulation of internode elongation compared with white light DIF−10. Surprisingly, internode elongation in orange light was similar for plants grown at DIF−10 and DIF+10. Flower development was accelerated at DIF−10 compared with DIF+10 in both white and orange light. To examine whether the effects of DIF and light quality on shoot elongation were related to changes in gibberellin metabolism or plant sensitivity to gibberellins (GAs), the stem elongation responses of paclobutrazol-treated plants to applied gibberellins were determined. In the absence of applied gibberellins paclobutrazol (>0.32 μmol plant⁻¹) strongly retarded shoot elongation. This inhibition was nullified by the application of about 10–32 nmol of GA₁, GA₄, GA₉, GA₁₅, GA₁₉, GA₂₀, GA₂₄, or GA₄₄. The results are discussed in relation to possible effects of DIF and light quality on endogenous gibberellin levels and gibberellin sensitivity of fuchsia and their effects on stem elongation. Received October 4, 1997; accepted December 17, 1997     

Thermo- and Photoperiodicity and Involvement of Gibberellins during Day and Night Cycle on Elongation Growth of Begonia x hiemalis Fotsch

The effects of thermo- and photoperiodicity on elongation growth and on endogenous level of gibberellins (GAs) in Begonia x hiemalis during various phases of the day-night cycle have been studied. Plant tissue was harvested during the day and night cycle after temperature and photoperiodic treatments and analyzed for endogenous GAs using combined gas chromatography and mass spectrometry. Elongation growth increased when the difference between day and night temperature (DIF = DT − NT) increased from a negative value (−9.0 and −4.5°C) to zero and with increasing photoperiod from 8 to 16 h. When applied to the youngest apical leaf, gibberellins A₁, A₄, and A₉ increased the elongation of internodes and petioles. GA₄ had a stronger effect on elongation growth than GA₁ and GA₉. In relative values, the effect of these GAs decreased when DIF increased from −9 to 0°C. The time of applying the GAs during a day and night cycle had no effect on the growth responses. In general, endogenous levels of GA₁₉ and GA₂₀ were higher under negative DIF compared with zero DIF. The level of endogenous GA₁ in short day (SD)-grown plants was higher under zero DIF than under negative DIF, but this relationship did not appear in long day (LD)-grown plants. The main effects of photoperiod seem to be a higher level of GA₁₉ and GA₁ at SD compared with LD, whereas GA₂₀ and GA₉ show the opposite response to photoperiod. No significant differences in endogenous level of GA₁, GA₉, GA₁₉, and GA₂₀ were found for various time points during the diurnal day and night cycle. Endogenous GA₂₀ was higher in petiole and leaf compared with stem, whereas there were no differences of GA₁, GA₉, and GA₁₉ between plant parts. No clear relationship was found between elongation of internodes and petioles and levels of endogenous GAs. Received December 26 1996; accepted July 1, 1997     

Growth Responses and Allocation of Assimilates of Rice Seedlings by Paclobutrazol and Gibberellin Treatment

Paclobutrazol [(2RS,3RS)-1-(4-chlorophenyl)methyl-4,4-dimethyl-2-(1h-1,2,4-trizol-1-yl)penten-3-ol] effectively decreased vegetative growth of rice (Oryza sativa L.) seedlings and increased the chlorophyll content. The number of veins in a leaf, the calculated number of stomata per leaf, and the length of guard cells were not altered by the paclobutrazol treatment, suggesting an effect on cell elongation. The allocation pattern of carbohydrates was changed by either gibberellin (GA) or paclobutrazol treatment. GA₃ induced more shoot growth and less accumulation of starch than the control and paclobutrazol-treated seedlings. Photosynthetic ability was not affected by either paclobutrazol or GA₃ treatment. Paclobutrazol-treated plants allocated a smaller amount of photosynthates for vegetative shoot growth and stored more as starch in the crowns than the control and GA₃-treated plants. The same starch degrading activity in the crown tissue of paclobutrazol-treated seedlings as in control plants suggests that the accumulated starch is utilized in a normal activity for growth including leaf emergence, tiller formation, and root production, resulting in improved seedling quality. Received May 30, 1996; accepted December 10, 1996     

Possible Involvement of Altered Gibberellin Metabolism in the Induction of Tomato Irregular Ripening in Dwarf Cherry Tomato by Silverleaf Whitefly

External and internal tomato irregular ripening (TIR) symptoms were associated with the feeding of silverleaf whitefly (SLW), Bemisia argentifolii Bellows and Perring. Four experiments consisting of various soil drench applications of GA₃ (100 ppm) and cycocel (CCC, an inhibitor of GA biosynthesis; 1,000 and 2,000 ppm) were applied to dwarf cherry tomato cv. Florida Petite in the presence and absence of SLW in an attempt to mimic the disorders induced by the SLW. The application of GA₃ induced external and internal TIR symptoms similar to the SLW-induced disorder. Minimal TIR-like symptoms also occurred in the control and CCC treatments. Internal TIR symptoms in GA₃, GA₃ plus SLW, and GA₃ plus CCC treatments ranged from 66% to 97% throughout the experiments. The incidence of external TIR symptoms was highest in the GA₃ plus SLW treatment compared with the other treatments. CCC reduced the incidence of external TIR symptoms induced by GA₃ or GA₃ plus SLW treatments. However, CCC-treated plants also attracted more oviposition and higher populations of SLW and consequently induced a greater incidence of TIR symptoms than SLW treatment alone. Furthermore, although low SLW populations may be associated with low external TIR symptoms, internal TIR symptoms almost always remained high in infested plants. The results suggest that the TIR disorder in dwarf cherry tomato which is induced by the SLW may be a gibberellin-regulated disorder. Received May 27, 1997; accepted September 26, 1997     

The Effect of Gibberellins on Flowering in Roses

The gibberellins A₁, A₃, A₅, A₈, A₁₉, A₂₀, and A₂₉ were identified in vegetative shoot tips of Rosa canina by comparing their mass spectra and Kovats retention indices with those of standards. Most wild roses have a short flowering season of 2–4 weeks in spring, whereas most modern cultivars flower recurrently. `Félicité et Perpétue' is a short-season hybrid from a cross between a wild rose and a recurrent-flowering rose, whereas its sport, `Little White Pet,' flowers recurrently. The concentrations of gibberellins (GAs) were measured in shoot apices of both cultivars. In March (before floral initiation in spring) the concentrations of GA₁ and GA₃ were respectively threefold and twofold higher in `Félicité et Perpétue' than in `Little White Pet.' In April (after floral initiation) the concentrations of both gibberellins were substantially greater than in March, and concentrations of GA₁ and GA₃ were, respectively, 17-fold and 12-fold greater in `Félicité et Perpétue' than in `Little White Pet.' It is postulated that, in `Félicité et Perpétue,' floral initiation occurs when concentrations of GAs are low and is inhibited when concentrations of GAs are high, whereas in `Little White Pet' concentrations of GAs remain at permissive levels throughout the growing season. Applications of GA₁ and GA₃ to axillary shoots in March inhibited floral development in `Félicité et Perpétue' but not in `Little White Pet.' This suggests that the combined concentration of exogenous and endogenous gibberellins might have been raised to inhibitory levels in the former but not in the latter cultivar. Received January 10, 1999; accepted June 16, 1999     

Gibberellic Acid Treatment Reduces the Tolerance of Field-Grown Common Bean to Leaf Removal

I studied the influence of gibberellic acid (GA₃) treatment in a field population of common bean on plant tolerance to leaf removal. Individual bean seedlings were treated with a foliar application of 10 μM GA₃ on day 7 and day 14 after emergence, which led to a significant increase in height in GA₃-treated plants. Twenty-eight days after emergence, either zero, one, two, or three leaflets from each trifoliate leaf were removed from each of 20 GA₃-treated and 20 control plants. All pods were harvested from each plant after plants became senescent 6 weeks later. Multivariate analyses revealed that leaf removal produced significant reductions in several yield components in both GA₃-treated and control plants, although the effects were not pronounced until at least two leaflets from each trifoliate leaf (67% of the total leaf area) were removed. However, GA₃-treated plants suffered greater reductions in total pod wall mass and total seed number than control plants after 33 and 67% leaf area removal. These results indicate that GA₃ treatment may have altered the assimilatory capacity or resource allocation pattern of treated plants in such a way as to decrease their ability to tolerate leaf removal, a negative consequence of the hormonal alteration of traits important to plant compensation for biotic stressors. Received December 6, 1996; accepted March 5, 1997     

On the Reversible Conjugation of [17-D2]GA20 in Seedlings of Phaseolus coccineus L.

After administering [17-D₂]GA₂₀ to Phaseolus coccineus L. cv. Preisgewinner seedlings, [17-D₂]GA₂₀-O-glucoside was identified by liquid chromatography (LC)/ESI-tandem mass spectrometry (MS). Likewise, by LC/ESI-tandem MS the metabolic formation of [17-D₂]GA₂₀ glucosyl ester was established. The application of both [17-D₂]-labeled GA₂₀ 13-O-glucoside and GA₂₀ glucosyl ester to Phaseolus coccineus L. seedlings resulted in free [17-D₂]GA₂₀ by gas chromatography/MS. The results demonstrate that conjugation of GA₂₀ and the reconversion of the glucosyl conjugates are concomitant processes in plants. Received October 27, 1998; accepted August 12, 1999     

Gibberellic Acid3 Modifies Some Growth and Physiologic Effects of Paclobutrazol (PP333) on Wheat

Gibberellic acid₃ (GA₃) modification of some growth and physiologic effects of paclobutrazol (PP₃₃₃) was studied by applying PP₃₃₃ alone and in combination with GA₃ at the tillering stage of wheat. Results showed that GA₃ weakened the effect of PP₃₃₃ on tillering and shortening the plant, increased the dry matter accumulation of different tillers, improved plant nitrogen metabolism by favoring nitrogen translocation into tillers at late growth stage, and stimulated tiller development. All of these reduced the difference in spike weight among tillers and the main stem. Compared with control, there were more spikes per plant and fewer grain per spike in the PP₃₃₃ treatment, but all yield components developed positively in the treatment consisting of the mixture of GA₃ and PP₃₃₃. Received May 22, 1996; accepted November 14, 1996     

Roles of Soluble Sugars in Protecting Phytochrome- and Gibberellin A3-Mediated Germination Control in Skotodormant Lettuce Seeds

Skotodormant seeds of Lactuca sativa Grand Rapids imbibed in darkness for 10 days (10-day DS) germinated poorly upon terminal treatment with red light (R) or gibberellin A₃ (GA₃). Soluble sugars in the imbibition solutions influenced the depth of skotodormancy. Ten-day DS seeds, imbibed in 50–500 mm sucrose or 100–500 mm glucose and given terminal GA₃ germinated completely and germinated about 80% when imbibed in 100 mm galactose, mannose, lactose, or maltose. In contrast, terminal R applied to 10-day DS seeds caused only 20–50% germination. If given R at day 0 and imbibed for 10 days in darkness in 500 mm sucrose or glucose, seeds washed free of exogenous glucose or sucrose then germinated about 50% in darkness in water. These seeds responded to terminal R or GA₃ with complete germination. When seeds were given FR at day 0, germination responses following terminal R or GA₃ were significantly lower when the duration of DS was increased from 7–10 day DS to 15 days. In 10-day DS seeds given initial FR and imbibed in either solutions of 50 or 100 mm sucrose and KNO₃, either terminal R or GA₃ treatment gave complete or near complete germination. It is concluded that seed exposure to certain soluble sugars and/or nitrate during a 10-day DS protected certain substrates and thereby extended the sensitivity of the seeds to terminal R or GA₃ treatment. The study provides substantial evidence for nonhormonal factors associated with light and GA action in the control of seed skotodormancy. Received October 30, 1996; accepted April 22, 1997     

Gibberellins and Subapical Cell Divisions in Relation to Bud Set and Bud Break in Salix pentandra

In young plants of Salix pentandra, a temperate zone deciduous woody species, elongation growth ceases and a terminal bud is formed at day lengths shorter than a critical length. This is the first step in dormancy development, making survival under harsh winter conditions possible. Early studies strongly indicate that gibberellin is involved in the photoperiodic control of bud set and bud break. GA₁ action was studied by application under short days to plants where cessation of shoot elongation had occurred, followed by subsequent anatomic investigations of shoot tips. Under short days the frequency of cell division decreased rapidly along with the earlier observed decrease in GA₁ levels. Application of GA₁ to short-day–induced terminal buds rapidly stimulated cell division in apices several days before visible shoot elongation in response to this treatment was observed. One day after GA₁ application a fourfold increase in cell division frequency in apices was observed, increasing to a maximum of sevenfold 2 days after application. Long-day treatment leading to induction of bud break after about 4–6 days was followed by slowly increasing frequency of cell divisions. In earlier studies of this species, short days and gibberellins had no effect on cell elongation. These data show that increased GA₁ content, by application or long-day treatment, results in increased frequency of mitosis. This strongly indicates that GA₁ affects stem elongation in connection with bud set and bud break primarily by affecting cell divisions in subapical tissues. Received February 26, 1999; accepted October 8, 1999     

Role of other plant organs in gibberellic acid-induced delay of leaf senescence in alstroemeria cut flowers

Chlorophyll loss in leaves of cut flowers of alstroemeria (Alstroemeria pelegrina L. cv. Westland) was rapid in darkness and counteracted by irradiation and treatment of the flowers with gibberellic acid (GA₃). The mechanism of the effect of GA₃ under dark conditions was investigated. The content of various carbohydrates in the leaves under dark conditions rapidly decreased; this was not influenced by treatment with GA_3. indicating that the loss of carbohydrates in the leaves did not induce the loss of chlorophyll. Placing the cut flowers in various solutions of organic and inorganic nutrients exhibited no significant effect on the retention of chlorophyll in leaves of dark-senescing flowers. The total nitrogen content in leaves of dark-senescing cut flowers decreased with time. Leaves of GA₃-treated flowers retained more nitrogen. In contrast, the buds of GA₃-treated flowers retained less nitrogen during senescence in the dark than control buds. To investigate whether GA₃ affects export of assimilates from the leaf to various parts of control and GA₃-treated flowers, we labelled one leaf with radioactive carbon dioxide. ¹⁴C-assimilates accumulated preferentially in the flowers, in which the relative specific activity of the youngest floral buds was highest. No significant differences were observed in the distribution of ¹⁴C-labelled compounds between the buds of control and GA₃-treated flowers. To establish the importance of source-sink relations for the loss of leaf chlorophyll we removed the flower buds (i. e. the strongest sink) from the cut flowers. This removal only slightly delayed chlorophyll loss as compared to the large delay caused by GA₃-treatment. In addition, detached leaf tips exhibited chlorophyll loss in the dark, which was delayed by GA₃-treatment in a fashion comparable with that in flowers. Together these data demonstrate that interactions of the leaves with other plant organs are not essential for chlorophyll loss during senescence in the dark. Additionally, we have found no evidence that GA₃ delays the loss of chlorophyll by affecting the transport of nutrients within the cut flowers.