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     

Effects of temperature and gibberellic acid on phospholipid composition of Avena sativa stem segments

Stem segments taken from Avena sativa plants grown at 10°, 20° or 30° varied in their phospholipid composition depending on the growth temperature; as temperature was lowered, there was a shift towards a greater proportion of unsaturated fatty acids. A significant increase was observed in the concentration of linolenic acid (18:3) as growth temperature was lowered. Although prolonged treatment of oat plants with GA₃ produced marked changes in phospholipid composition of stem segments, these changes did not always accompany the GA₃-induced growth response of segments. Treatment of stem segments with GA₃ for only 20 hr produced a significant growth response with little or no effect on phospholipid composition over this time. The data support the hypothesis that GA₃-induced growth in Avena stem segments can occur without a concomitant change in phospholipid composition.     

Environmental control of growth and development of Scrophularia marilandica

Summary Seedlings of Scrophularia marilandica were grown at different combinations of day/night temperature and photoperiod under controlled conditions. The species flowered in long days. The stems of plants grown at low temperature and short photoperiod failed to elongate. Treatment with gibberellic acid (GA₃) simulated the effect of increasing temperature and photoperiod and caused stem elongation in plants which would otherwise not have elongated. Application of GA₃ to plants grown at high temperature and long photoperiod resulted in increased stem elongation and flowering. The growth retardant (2-chloroethyl)trimethylammonium chloride (CCC) had little effect on rosette plants grown at low temperature and short photoperiod. Application of CCC to +GA₃ plants grown at a higher temperature and long photoperiod gave a significant increase in stem height. The interaction between temperature and applied GA is described in an experiment using plants grown at high and low temperatures for varying periods of time.This work was supported by National Science Foundation Grant GB 17483.     

Effect of temperature regimes on gibberellin levels in thermosensitive dwarf apple trees

Orchard-grown dwarf apple (Malus domestica Borkh.) trees selected from a hybrid population were propagated by tissue culture but had a growth pattern similar to standard cv. Golden Delicious plants when grown at constant 27°C instead of the expected dwarf pattern of growth. Shoot elongation was markedly reduced, with or without gibberellin A₁ (GA₁) or GA₄ treatment, when trees were grown in an environment where day temperature was maintained at 35°C for 2 h in a ramped regime (night 20°C day ramped to 35°C, held for 2 h and ramped down to 20°C night over a 14-h photoperiod). Application of GA₁ or GA₄ partially overcame growth retardation resulting from prior paclobutrazol treatment of both standard and dwarf trees grown at constant 27°C and of standard trees grown in the ramped environment. However, these GAs had no effect on paclobutrazol-treated or untreated dwarfs grown in the ramped regime. Gas chromatography-mass spectrometry with labelled internal standards was used to quantify GA₁, GA₃, GA₈, GA₁₉, GA₂₀ and GA₂₉ in extracts from standard and dwarf plants grown either at a constant 27°C or in a 20-30-20°C ramped temperature regime. Standard plants, which elongate quite rapidly in either environment, had similar levels of these GAs in both temperature regimes. The slowly growing dwarfs in the ramped temperature environment contained three times more GA₁₉ than the rapidly elongating dwarfs grown at 27°C. The concentrations of the other GAs were reduced to ca 40% or less in plants grown in the ramped temperature regime compared with those grown at 27°C. These data suggest that shoot elongation of dwarf plants is sensitive to elevated temperatures both as a result of reduced responsiveness to GAs and because of a reduction in the concentration of GA₁, apparently as a result of a lower rate of conversion of GA₁₉ to GA₂₀. It is possible that the altered GA metabolism may be a consequence of the change in GA sensitivity.     

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 and the photoperiodic control of leaf growth in Poa pratensis

The role of gibberellins (GAs) in photoperiodic control of leaf elongation in Poa pratensis was studied by both application of exogenous GAs and analysis of endogenous GAs. Leaf elongation was strongly increased under long day (LD, 24 h) conditions at both 9 and 21°C, leaf length at 9°C LD being similar to that in plants grown in short days (SD, 8 h) at 21°C. However, even at 21°C leaf elongation was enhanced by LD. Exogenous GA₁ could completely compensate for LD at both 9 and 21°C. Gibberellins A₂₀, A₁₉ and A₄₄ could also partly replace LD, but they were significantly less active than GA₁, GA₅₃ was inactive when applied to plants grown at 9°C in SD and exhibited only marginal activity at 9°C LD and 21°C SD. The total level of GAs of the early 13-hydroxylation pathway (A₅₃, A₄₄, A₁₉, A₂₀ and A₁) increased rapidly when plants were transferred from SD to LD at 9°C. After transfer from 9 to 21°C, there was an increase in GA levels at both LD and SD, followed by a decrease under LD conditions. In all cases, GA₁₉ was the predominant GA, accounting for 60 to 80% of the analysed GAs. Levels of the bioactive GA₁ were low and increased transiently by LD four days after transfer from SD to LD. At both temperatures, the ratio GA₁₉ to GA₂₀ and GA₂₀ to GA₁ at 9°C was enhanced by LD compared with SD. Taken together, these results support the hypothesis that photoperiodic regulation of leaf elongation in Poa pratensis is GA-mediated, and they indicate a photoperiodic control of oxidation of GA₅₃ to GA₄₄ and GA₁₉ to GA₂₀, and perhaps also of 3β-hydroxylation of GA₂₀ to GA₁.     

Role of Gibberellins in the Environmental Control of Stem Growth in Thlaspi arvense L

Field pennycress (Thlaspi arvense L.) is a winter annual that requires a cold treatment for the induction of stem elongation. An inbred line was selected in which no stem elongation was observed in plants grown for 6 months at 21°C regardless of the prevailing photoperiod. Increased exposure time of plants grown initially at 21°C to cold (2°C) induced a greater rate of stem elongation when the plants were returned to 21°C. Moreover, longer cold treatments resulted in a greater maximum stem height and reduced the lag period for the onset of measurable internode elongation. The optimal temperature range for thermoinduced stem growth was broad: rates of stem growth in plants maintained for 4 weeks at either 2° or 10°C were virtually identical. However, a 4-week thermoinductive treatment at 15°C resulted in a greater lag period for the initiation of stem elongation and a decreased growth rate. The rate of cold-induced stem elongation was greater in plants subjected to long days than for plants subjected to short days following the cold treatment. Thus, photoperiod does not control the induction of stem elongation, but does regulate stem elongation in progress. Exogenous gibberellin A₃ (GA₃) was able to substitute for the cold requirement, but elicited a greater response in plants maintained under long days than short days. This indicates that photoperiod influences the plant's sensitivity to GAs. The GA biosynthesis inhibitor, 2-chloroethyltrimethyl ammonium chloride, inhibited low temperature-induced stem elongation, and this inhibition was completely reversed by exogenous GA₃. These results suggest that cold-induced stem elongation in field pennycress is mediated by some change in the endogenous GA status.     

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     

Etude de l’influence de l’acide gibbérellique (GA3) et de l’acide N- diméthylamino- succinamique (B9) sur la croissance et la floraison de laScrophularia vernalis L.

In this report, B₉ treatment had no effect on the growing of rosette biennialScrophularia vernalis L.; it inhibited or slowed stem elongation. Applications of GA₃ to B₉ treated plants produced a significant increase of stem elongation, in relation to GA₃ treated plants. Plants treated with only GA3 failed to flower; otherwise, the flowering of vernalized plants was not altered by GA₃. Thus, B₉ effect on flowering was tested by using GA₃. B₉ by itself induced flowering, it increased inflorescence formation in vernalized plants without altering stem growth pattern in the most of cases. The induction or the stimulation of flowering brought about by B₉ was not reversed by GA₃; we may thus hypothesize that flowering by B₉ oannot be traced back to gibberellin biosynthesis.     

Thermoperiodic control of stem elongation and endogenous gibberellins in Campanula isophylla

The effect of day/night temperature regimes on stem elongation and on the content of endogenous gibberellins (GAs) in vegetatively propagated plants of Campanula isophylla cv. Hvit have been studied. Compared with a constant temperature regime at 18°C (18/18°C), stem and internode elongation was enhanced significantly by a combination of high day/low night temperature (21/15°C) and inhibited by an opposite regime (15/21°C). Gibberellins A₁, A₁₉, A₄₄, A₅₃, and A₉₇ were identified as endogenous components in Campanula. (GA₉₇ was earlier referred to as 2-OH-GA₅₃.) Quantitative analysis of the endogenous GAs indicates that temperature regimes that stimulate elongation growth are accompanied by an increase in the level of GA₁, GA₁₉, and GA₄₄. On the other hand, in plants grown under conditions that reduced stem elongation growth, there was an increased level of GA₉₇.Abbreviations DIF difference between day temperature and night temperature - GA gibberellin - HPLC high performance liquid chromatography - GC-MS gas chromatography-mass chromatography - SPE solid phase extraction - TMS trimethylsilyl - MSTFA N-methyl-N-TMS-trifluoroacetamide - KRI Kovats retention index - SIM selected ion monitoring - D2 deuterated     

Interactions between abscisic acid, ethylene and gibberellin in internodal elongation in floating rice: the promotive effect of abscisic acid at low humidity

The enhancement of internodal elongation in floating or deepwater rice (Oryza sativa L. cv. Habiganj Aman II) by treatment with ethylene or gibberellic acid (GA₃) at high relative humidity (RH) is inhibited by abscisic acid (ABA). Here, we examined the interactive effects of ethylene, gibberellin (GA) and ABA at low RH on internodal elongation of deepwater rice stem segments. Although ethylene alone hardly promoted internodal elongation of stem sections at 30% RH, it enhanced the internodal elongation induced by GA₃. Application of ABA alone to stem segments had no effect on internodal elongation. However, in the presence of ethylene and GA₃ at 30% RH, ABA further promoted internodal elongation. This promotive effect of ABA was not found in the internodes of stem segments treated either with ethylene or with GA₃ at 30% RH or in the internodes of stem segments treated with ethylene and/or GA₃ at 100% RH.     

Participation of GA3, ethylene,NO and HCN in germination of Amaranthus retroflexus L. seeds with various dormancy levels

Amaranthus retroflexus seeds were dormant at 25 °C in the darkness and in the light, and also at 35 °C in the darkness. GA₃ and ethylene partially removed dormancy at 35 °C in the darkness and at 25 °C in the light. Dormancy was removed by 1–5 days of treatment with nitric oxide or cyanide. The effect of NO and HCN was inhibited by cPTIO, thus the effect of HCN was NO dependent. Dry storage for 16 weeks could partially release dormancy only at 35 °C, but not at 25 °C. Dry storage increased the response to light, GA₃ and ethylene. The response to GA₃ and ethylene at 25 °C was enhanced with increasing storage temperature. GA₃, ethylene and nitric oxide could substitute dry storage and stratification in partially dormant seeds.     

The effect of low temperature and GA3 treatments on dormancy breaking and activity of antioxidant enzymes in Fritillaria meleagris bulblets cultured in vitro

We investigated the effect of low temperature and gibberellic acid (GA₃) treatment on dormancy in Fritillaria meleagris L. bulbs. Also, we studied the effect of dormancy breaking on the antioxidant enzymes activity. To overcome dormancy, bulbs require a period (4–8 weeks) of exposure to low temperature. Bulbs regenerated in vitro were grown in the dark on medium without growth regulators at the standard (24 °C) or at low temperatures (4 and 15 °C) for 4, 6, 8 and 10 weeks. Bulbs were collected after 3, 4 and 5 weeks of cooling at 4 °C. To investigate the influence of GA₃ on dormancy, bulbs were treated for 24 h with GA₃ solutions with 1, 2 and 3 mg l^?1 concentrations. During the period of growth of bulbs at 4 °C, regeneration of bulbs was very weak, while at 15 °C the number of regenerated bulbs increased significantly. Improved bulb sprouting was achieved by a short treatment with gibberellin. Low temperature also represents a kind of oxidative stress for the plant. The activity of superoxide dismutase, catalase (CAT) and peroxidase (POX) in bulbs of F. meleagris L. grown in vitro and ex vitro increased with decreasing temperature in contrast to glutathione reductase. POX showed generally lower activity than CAT which indicates that major role in the breaking dormancy and preparing bulbs for sprouting have catalases.     

Auxin-Gibberellin Interactions in Pea: Integrating the Old with the New

Recent findings on auxin-gibberellin interactions in pea are reviewed, and related to those from studies conducted in the 1950s and 1960s. It is now clear that in elongating internodes, auxin maintains the level of the bioactive gibberellin, GA₁, by promoting GA₁ biosynthesis and by inhibiting GA₁ deactivation. These effects are mediated by changes in expression of key GA biosynthesis and deactivation genes. In particular, auxin promotes the step GA₂₀ to GA₁, catalyzed by a GA 3-oxidase encoded by Mendel’s LE gene. We have used the traditional system of excised stem segments, in which auxin strongly promotes elongation, to investigate the importance for growth of auxin-induced GA₁. After excision, the level of GA₁ in wild-type (LE) stem segments rapidly drops, but the auxin indole-3-acetic acid (IAA) prevents this decrease. The growth response to IAA was greater in internode segments from LE plants than in segments from the le-1 mutant, in which the step GA₂₀ to GA₁ is impaired. These results indicate that, at least in excised segments, auxin partly promotes elongation by increasing the content of GA₁. We also confirm that excised (light-grown) segments require exogenous auxin in order to respond to GA. On the other hand, decapitated internodes typically respond strongly to GA₁ application, despite being auxin-deficient. Finally, unlike the maintenance of GA₁ content by auxin, other known relationships among the growth-promoting hormones auxin, brassinosteroids, and GA do not appear to involve large changes in hormone level.     

CONTROL OF FLOWER FORMATION BY GROWTH RETARDANTS AND GIBBERELLIN IN SAMOLUS PARVIFLORUS,A LONG-DAY PLANT

The growth retardants AMO–1618 and CCC inhibited flower formation and stem elongation in Samolus parviflorus, a long-day rosette plant, under inductive conditions. The vegetative growth of the plants, as measured by leaf formation, was affected only slightly, or not affected at all. Application of gibberellic acid (GA₃) reversed completely the inhibition both of flower formation and of stem elongation caused by AMO, but relatively larger amounts of GA were required to reverse the CCC inhibition of stem elongation than that of flower formation. When applied under short-day conditions, AMO had no effect on the level of applied GA required for flower induction. When applied following long-day treatment the retardant caused some reduction of flower formation after marginal numbers of long days, but had no effect when enough long days to cause 100% flower formation were given. Other evidence indicates that the growth retardants act by inhibiting the synthesis of endogenous gibberellin. In LD plants, at least part of the action of inductive environmental conditions consists in causing an increase of gibberellin synthesis, supporting the hypothesis that relatively high GA levels are necessary for the production of the floral stimulus in this group of plants, as in long-short-day plants. The experiments with CCC indicate that stem elongation and flower formation in Samolus can be separated, and that the effect of GA on flower formation is not necessarily dependent on its effect on stem elongation.     

Endogenous gibberellin A1 levels control thermoperiodic stem elongation in Pisum sativum

Gibberellin (GA) is believed to be involved in thermoperiodic stem elongation. With this in mind, we studied the correlation between gibberellin A₁ (GA₁) levels and stem elongation affected by alternating day (DT) and night temperature (NT) in 5 genotypes of Pisum sativum differing in their degree of dwarfism. The endogenous GA content in the tissue of two of the genotypes was determined by combined gas chromatography and mass spectrometry. The wild genotype developed 40 to 50% shorter stems and internodes under a low DT and high NT combination (negative difference [DIF] between DT and NT, DT/NT 15.5/21.5 or 14/24°C) than under the opposite regime of high DT and low NT (positive DIF, DT/NT 22.5/16.5 or 24/14°C). The GA biosynthetic mutants ls and le, and the auxin and brassinosteroid mutant lkb responded in a similar way, but not as strongly as the wild type. The stem length of the GA-insensitive slender mutant (la cry^s) was reduced by only 8% under negative compared to positive DIF. In the wild type endogenous GA levels decreased by 60% from positive to negative DIF in the upper part of the stem. Further, there was a corresponding decrease in the levels of precursors to GA₁, i.e. GA₅₃, GA₄₄, GA₁₉ and GA₂₀, while 2β-hydroxylated GA₂₀ and GA₁, GA₂₉ and GA₈, respectively, were unaffected by DIF. A similar increase in the ratios of GA₂₉ to GA₂₀ and GA₈ to GA₁ from positive to negative DIF was seen in the stem tissue of the le mutant as in the wild type. The temperature regimes affected the levels of GA₁ and its precursors in combined leaf and petiole samples and in the shoot tip in a similar manner as in the stem tissue. However, the different temperature regimes did not affect the ratio of GA₈/GA₁ in the shoot tip. The results indicate that altered stem elongation of the pea plants in response to diurnal temperature alternations may be mediated by changes in endogenous levels of GA₁. The GA₁ levels may be controlled by an effect of DIF on both biosynthetic and inactivation steps.     

The effects of ABA and GA3 treatments on resistance to frost and high-light treatment in oilseed rape leaf discs

The experiments were performed to check the effects of exogenous ABA and gibberellin on photosynthetic apparatus and leaf resistance to freezing. In the experiment, two cultivars (winter and spring) of oilseed rape were used in the experiment. Discs, cut out from leaves of cold acclimated plants grown at 12 and 20 °C at similar PPFD levels, were immersed for 72 hours in growth regulator solutions. Some of discs were additionally subjected to high radiation. Independently on cultivar studied, the effects of growth regulator treatments were significant only in leaves developed at 20 °C. ABA treatment increased frost resistance, promoted photosynthetic activity measured in cold and inhibited expansion of leaf-disc area, whereas GA₃ evoked opposite effects. The treatment with growth regulators particularly affected the resistance of photosynthetic apparatus to high light. In this case ABA treatment decreased, whereas GA₃ increased photoinhibition of PSII. The outcomes may suggest that in the ABA-treated plants PSII is better protected against photoinduced inactivation both by the increase in effectiveness of photosynthetic dark reactions at high light/low temperature conditions, increased energy dissipation in xantophyll cycle and enhanced accumulation of anthocyanins. GA₃ treatment may affect the resistance to photoinhibition directly via decrease in anthocyanins contents and indirectly through increase of elongation growth rate in the tissue.     

Response of Amaranthus retroflexus L. seeds to gibberellic acid, ethylene and abscisic acid depending on duration of stratification and burial

Freshly harvested, dormant seeds of Amaranthus retroflexus were unable to germinate at 25 and 35 °C. To release their dormancy at the above temperatures, the seeds were stratified at a constant temperature (4 °C) under laboratory conditions or at fluctuating temperatures in soil or by outdoor burial in soil. Fully dormant, or seeds stratified or buried (2006/2007 and 2007/2008) for various periods were treated with exogenous gibberellic acid (GA₃), ethephon and abscisic acid (ABA). Likewise, the effects of these regulators, applied during stratification, on seed germination were determined. The results indicate that A. retroflexus seed dormancy can be released either by stratification or by autumn–winter burial. The effect of GA₃ and ethylene, liberated from ethephon, applied after various periods of stratification or during stratification, depends on dormancy level. GA₃ did not affect or only slightly stimulated the germination of non-stratified, fully dormant seeds at 25 and 35 °C respectively. Ethylene increased germination at both temperatures. Seed response to GA₃ and ethylene at 25 °C was increased when dormancy was partially removed by stratification at constant or fluctuating temperatures or autumn–winter burial. The response to GA₃ and ethylene increased with increasing time of stratification. The presence of GA₃ and ethephon during stratification may stimulate germination at 35 °C. Thus, both GA₃ and ethylene can partially substitute the requirement for stratification or autumn–winter burial. Both hormones may also stimulate germination of secondary dormant seeds, exhumed in September. The response to ABA decreased in parallel with an increasing time of stratification and burial up to May 2007 or March 2008. Endogenous GA_n, ethylene and ABA may be involved in the control of dormancy state and germination of A. retroflexus. It is possible that releasing dormancy by stratification or partial burial is associated with changes in ABA/GA and ethylene balance and/or sensitivity to these hormones.     

The role of gibberellin biosynthesis in the control of growth and flowering in Matthiola incana

Recently, it was found that stem elongation and flowering of stock Matthiola incana (L.) R. Br. are promoted by exogenous gibberellins (GAs), including GA₄, and also by acylcyclohexanedione inhibitors of GA biosynthesis, such as prohexadione‐calcium (PCa) and trinexapac‐ethyl (TNE). Here, because it was unclear how GA biosynthetic inhibitors could promote stem elongation and flowering, their effect on GA biosynthesis has been examined by quantifying endogenous GA levels; also, the sensitivity of stem elongation and flowering to various GAs in combination with the inhibitors was examined. Stem elongation and flowering were most effectively promoted by GA₄ when combined with PCa and, next in order, by 2,2‐dimethyl‐GA₄, PCa, GA₄+TNE, TNE, GA₉+PCa and by GA₄. There was little or no promotion by GA₁, GA₃, GA₉, GA₁₃, GA₂₀ and 3‐epi‐2,2‐dimethyl‐GA₄. Both the promotive effects of the acylcyclohexanediones on stem elongation and flowering, particularly when applied with GA₄, and the fact that TNE caused a build‐up of endogenous GA₄ imply that one effect of TNE at the lower dose involved an inhibition of 2β‐hydroxylation of GA₄ rather than an inhibition of 20‐oxidation and 3β‐hydroxylation of GAs which were precursors of GA₄. Overall, these results indicate that: (1) GAs with 3β‐OH and without 13‐OH groups (e.g. GA₄) are the most important for stem elongation and flowering in M. incana; (2) growth promotion rather than inhibition can result if an acylcyclohexanedione acts predominantly to slow 2β‐hydroxylation and so slows inactivation of active gibbberellins, including GA₄. It follows that a low dose of an acylcyclohexanedione can be a ‘growth enhancer’ for any applied GA that is liable to inactivation by 2β‐hydroxylation.     

The Influence of Gibberellic Acid and Temperature on the Growth Rate of Avena sativa Stem Segments

A micro-growth measuring technique was used to determine the growth response of stem segments of Avena sativa cv. Avon to a variety of gibberellic acid (GA₃) concentrations over a range of incubation temperatures. Growth rate varied with GA₃ concentration, the temperature at which the rate was measured, and the growth temperature of the plants prior to excision of the segments. The curves relating segment extension rates to temperature were affected by GA₃ such that the linear portion of the curve was shifted to higher rates as GA₃ concentration was increased. The results seem to be analogous to the GA₃-induced shifts of thermally induced phase transitions in glucose leakage from liposomes, observed earlier (Wood, Paleg 1974 Aust J Plant Physiol 1: 31-40).