Gibberellins and the Legume-Rhizobium Symbiosis : III. Quantification of Gibberellins from Stems and Nodules of Lima Bean and Cowpea

Lima bean (Phaseolus lunatus L.) plants inoculated with Bradyrhizobium sp. strain 127E14 displayed a period of marked internode elongation that was not observed in plants inoculated with other compatible bradyrhizobia, including strain 127E15. When strain 127E14 nodulated an alternate host, cowpea (Vigna unguiculata L. Walp), a similar, although less dramatic growth response induced by the bacteria was observed. It has been speculated that the elongative growth promotion brought about by inoculation with strain 127E14 is mediated by gibberellins (GAs). Using deuterated internal standards and gas chromatography-mass spectroscopy analysis, we have quantified the levels of GA₁, GA₂₀, GA₁₉, and GA₄₄ in nodules and stems of two varieties of lima bean (bush and pole) and one variety of cowpea that were inoculated with either strain 127E14 or 127E15. In nodules formed by strain 127E14 on lima bean, endogenous levels of GA₂₀ and GA₁₉ were 10 to 40 times higher (35-88 ng/g dry weight) than amounts found in nodules formed by strain 127E15 (2.2-3.9 ng/g dry weight). Relative amounts of GA₄₄ were also higher (4- to 11-fold) in 127E14 nodules, but this increase was less pronounced. The rhizobial-induced increase of these GAs in the nodule occurred in both pole and bush varieties and seemed to be independent of host morphology. Regardless of rhizobial inoculum, levels of the “bioactive” GA₁ in the nodule (0.3-1.1 ng/g dry weight) were similar. In cowpea nodules, a similar, although smaller, difference in GA content due to rhizobial strain was observed. The concentration of GA₁ in lima bean stems was generally higher than that observed in the nodule, whereas concentrations of the other GAs measured were lower. In contrast with the nodule, GA concentrations in lima bean stems were not greater in plants inoculated with strain 127E14, and in some cases the slower growing plants inoculated with strain 127E15 actually had higher levels of GA₂₀, GA₁₉, and GA₄₄. Thus, there were major differences in concentrations of the precursors to GA₁ in nodules formed by the two bacterial strains, which were positively correlated with the observed elongation growth. These results support the hypothesis that the rhizobial strain modifies the endogenous GA status of the symbiotic system. This alteration in GA balance within the plant, presumably, underlies the observed growth response.     

Increase in Internode Length of Phaseolus lunatus L. Caused by Inoculation with a Nitrate Reductase-deficient Strain of Rhizobium sp

Dramatic differences in the height of lima beans (Phaseolus lunatus L.) treated with two different Rhizobium strains were studied. Lima beans were grown in Perlite in the greenhouse or in a minus-N culture solution in the growth chamber. The plants were inoculated with either Rhizobium sp. (lima bean) strain 127E15, which contains the constitutive nitrate reductase activity, or strain 127E14, which lacks that activity. For up to 3 weeks, no growth differences were observed in the plants inoculated with either strain. Five weeks after inoculation, however, those plants inoculated with strain 127E14 were significantly taller and had a larger number of leaves than those inoculated with strain 127E15. The difference in plant height was the result of increased internode elongation caused by inoculation with Rhizobium sp. 127E14. This response was observed with all lima bean cultivars tested, including Henderson, Fordhook, Allgreen, and Early Thorogreen. The growth difference occurred in plants cultured in the greenhouse or in the growth chamber.     

Gibberellin physiology of safflower: endogenous gibberellins and response to gibberellic acid

Endogenous gibberellins (GAs) were extracted from safflower (Carthamus tinctorius L.) stems and detected by capillary gas chromatography-mass spectrometry from which GA₁, GA₃, GA₁₉,, GA₂₀, GA₂₉, and probably, GA₄₄ were detected. The detection of these GAs suggests that the early 13-OH biosynthetic pathway is prevalent in safflower shoots. Deuterated GAs were used as internal standards and GA concentrations were determined in stems harvested at weekly intervals. GA₁ and GA₁₉ levels per stem increased but concentrations per gram dry weight decreased over time. GA₂₀ was only detected in young stem tissue.Gibberellic acid (GA₃) was also applied in field trials and both GA₃ and the GA biosynthetic inhibitor, paclobutrazol, were applied in growth chamber tests. GA₃ increased epidermal cell size, internode length, and increased internode cell number causing stem elongation. Conversely, paclobutrazol reduced stem height, internode and cell size, cell number and overall shoot weight. In field tests, GA₃ increased total stem weight, but decreased leaf weight, flower bud number and seed yield. Thus, GA₃ promoted vegetative growth at the expense of reproductive commitment. These studies collectively indicate a promotory role of GAs in the control of shoot growth in safflower, and are generally consistent with gibberellin studies of related crop plants. Author for correspondence     

Differences in Endogenous Levels of Gibberellin-Like Substances in Nodules of Phaseolus lunatus L. Plants Inoculated with Two Rhizobium Strains

Lima bean plants (Phaseolus lunatus L.) inoculated with Rhizobium sp. strain 127E14, which lacks constitutive nitrate reductase activity, were significantly taller after 4 weeks of age than plants inoculated with strain 127E15, which contains constitutive nitrate reductase activity. Plants inoculated with either strain responded to application of 5 micrograms gibberellic acid per plant with rapid internode elongation; plants inoculated with strain 127E15 became less responsive to gibberellic acid from 3 to 5 weeks of age, while plants inoculated with strain 127E14 did not. The height of plants inoculated with strain 127E14 was reduced by 20% with application of gibberellin biosynthesis inhibitors to the roots, while height of plants inoculated with strain 127E15 was unaffected.     

Recognition of Leguminous Hosts by a Promiscuous Rhizobium Strain

The lima bean (Phaseolus lunatus L.) and the pole bean (Phaseolus vulgaris L.) are nodulated by rhizobia of two different cross-inoculation groups. Rhizobium sp. 127E15, a cowpea-type Rhizobium, can induce effective nodules on the lima bean and partially effective nodules on the pole bean. Rhizobium phaseoli 127K14 can induce effective nodules on the pole bean but does not reciprocally nodulate the lima bean. Root hairs of the lima bean when inoculated with Rhizobium sp. 127E15 showed tip curling and swelling and infection thread formation as observed by light microscopy and scanning electron microscopy. When lima bean root hairs were inoculated with R. phaseoli 127K14, no host-specific responses were observed. Pole bean root hairs that had been inoculated with R. phaseoli 127K14 or Rhizobium sp. 127E15 also showed tip curling and swelling and infection thread formation. Colonization of lima bean root hairs by Rhizobium sp. 127E15 and pole bean root hairs by R. phaseoli 127K14 or Rhizobium sp. 127E15 appeared to involve the elaboration of microfibrils. This study showed that when Rhizobium sp. 127E15 nodulates a host of a different cross-inoculation group, it elicits the same specific host responses as it does from a host of the same cross-inoculation group.     

Gibberellins and the Legume-Rhizobium Symbiosis : I. Endogenous Gibberellins of Lima Bean (Phaseolus lunatus L.) Stems and Nodules

The content of gibberellin-like substances in nodules formed by Bradyrhizobium species strain 127E14 on roots of lima bean (Phaseolus lunatus L.) has been previously found to be relatively high. The objectives of the present study were to purify and identify the endogenous gibberellins from the stems and nodules of lima bean. By sequential silica gel partition column chromatography, C₁₈ reverse-phase high performance liquid chromatography, and combined gas chromatography-mass spectrometry, the gibberellins A₁, A₃, A₁₉, A₂₀, A₂₉, and A₄₄ were identified from root nodules. Gibberellins A₁, A₃, A₁₉, A₂₀, and A₄₄ were also identified from lima bean stem tissue. These data provide the first mass spectral-based evidence that gibberellins are present in leguminous root nodules. The presence of the gibberellins identified indicates that the early 13-hydroxylation gibberellin biosynthetic pathway predominates in stem and nodule tissue. However, it is not known if the gibberellins within the nodules are produced in situ, or if they are imported from some remote host plant tissue.     

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     

Gibberellins synthesized by the entomopathogenic bacterium,Photorhabdus temperata M1021 as one of the factors of rice plant growth promotion

In the present study, different types of gibberellins (GAs) in the culture filtrate (CF) of Photorhabdus temperata M1021 were quantified. The analysis of CF helped in profiling various bioactive GAs: GA₁, GA₃, GA₄, and GA₇. Several physiologically inactive GAs: GA₉, GA₁₂, and GA₂₀ were detected as well. Siderophore production was also investigated by growing P. temperata M1021 on chrome azurol-S blue agar plates. Furthermore, the strain was inoculated into ‘Waito-C’ (Oryza sativa L.) rice plants, which significantly (P < 0.05) increased plant growth attributes such as plant length, chlorophyll content, and fresh and dry biomass compared with those in controls. In a separate experiment, canola (Brassica napus L.) seeds treated with CF of M1021 were significantly (P < 0.05) accelerated germination rate as well as biomass production. Findings of the present study suggest that the strain M1021 contributes an important role in the plant growth by synthesizing a wide array of bioactive metabolites.     

Growth-active gibberellins overcome the very slow shoot growth of Hancornia speciosa, an important fruit tree from the Brazilian “Cerrado”

Shoot elongation of Hancornia speciosa, an endangered tree from the Brazilian savannah “Cerrado”, is very slow, thus limiting nursery production of plants. Gibberellins (GAs) A₁, A₃, and A₅, and two inhibitors of GA biosynthesis, trinexapac-ethyl and ancymidol were applied to shoots of Hancornia seedlings. GA₁ and GA₃ significantly stimulated shoot elongation, while GA₅ had no significant effect. Trinexapac-ethyl and ancymidol, both at 100 μg per seedling, inhibited shoot elongation up to 45 days after treatment, though the effect was statistically significant only for ancymidol. Somewhat surprisingly, exogenous GA₃ more effectively stimulated shoot elongation in SD-grown plants, than in LD-grown plants. The results from exogenous application of GAs and inhibitors of GA biosynthesis imply that Hancornia shoot growth is controlled by GAs, and that level of endogenous growth-active GAs is likely to be the limiting factor for shoot elongation in Hancornia. Application of GAs thus offer a practical method for nursery production of Hancornia seedlings for outplanting into the field.     

Exophiala sp. LHL08 reprograms Cucumis sativus to higher growth under abiotic stresses

Endophytic fungi are potential sources of secondary metabolites; however, they are little known for phytohormones secretion and amelioration of plant growth under abiotic stresses. We isolated a novel endophyte from the roots of Cucumis sativus and identified it as a strain of Exophiala sp. by sequencing internal transcribed spacer/large subunit rDNA and phylogenetic analysis. Prior to identification, culture filtrate (CF) of Exophiala sp. has shown significant growth promotion of Waito‐C [a gibberellins (GAs)‐deficient mutant cultivar] and Dongjin‐byeo (normal GAs biosynthesis cultivar) rice seedlings. CF analysis of Exophiala sp. showed the presence of physiologically active GAs (GA₁, GA₃, GA₄ and GA₇) and inactive GAs (GA₅, GA₈, GA₉, GA₁₂ and GA₂₀). Exophiala sp. had higher GAs in its CF than wild‐type strain of Gibberella fujikuroi except GA₃. Influence of Exophiala sp. was assessed on cucumber plant's growth and endogenous abscisic acid (ABA), salicylic acid (SA) and bioactive GAs under salinity and drought stresses. Exophiala sp.‐treated plants have shown significantly higher growth and rescued the host plants from stress promulgated water deficit, osmotic and cellular damage. The altered levels of stress‐responsive ABA showed low level of stress confined to endophyte‐applied plants than control. Elevated levels of SA and bioactive GAs (GA₃ and GA₄) in endophyte‐associated plants suggest stress‐modulating response toward salinity and drought. In conclusion, symbiotic relations between Exophiala and cucumber have reprogrammed the host plant growth under abiotic stresses, thus indicating a possible threshold role of endophytic fungi in stress alleviation. This study could be extended for improving agricultural productivity under extreme environmental conditions.     

Conversion of [14C]gibberellin A12-aldehyde to C19- and C20-gibberellins in a cell-free system from immature seed of Phaseolus coccineus L.

A cell-free system prepared from developing seed of runner bean (Phaseolus coccineus L.) converted [¹⁴C]gibberellin A₁₂-aldehyde to several products. Thirteen of these were identified by capillary gas chromatography-mass spectrometry as gibberellin A₁ (GA₁), GA₄, GA₅, GA₆, GA₁₅, GA₁₇, GA₁₉, GA₂₀, GA₂₄, GA₃₇, GA₃₈, GA₄₄ and GA₅₃-aldehyde, all giving mass spectra with ¹⁴C-isotope peaks. GA₈ and GA₂₈ were also identified but contained no ¹⁴C. All the [¹⁴C]GA₁₂-aldehyde metabolites, except GA₁₅, GA₂₄ and GA₅₃-aldehyde, are known endogenous GAs of P. coccineus.Abbreviations GA_n gibberellin A_n - GC-MS combined gas chromatography-mass spectrometry - HPLC highperformance liquid chromatography - MVA mevalonic acid - S-2 2000-g supernatant     

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     

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.     

Host specificity among five species in the cowpea cross-inoculation group

Summary In a cross-inoculation experiment using crushed nodules from cowpea (Vigna unguiculata), groundnut (Arachis hypogea), bambara groundnut (Voandzeia subterranea), lima bean (Phaseolus lunatus) and soybean (Glycine max.), it was found that soybean did not nodulate with Rhizobia from any of the other species whilst its Rhizobia nodulated with all species. Cowpea and lima bean, on the other hand, nodulated with Rhizobia from all species, but their Rhizobia nodulated only with each other. Groundnut and bambara groundnut nodulated with Rhizobia from all species except cowpea and lima bean, and their Rhizobia also nodulated with all species except soybean.     

Gibberellin-producing Promicromonospora sp. SE188 improves Solanum lycopersicum plant growth and influences endogenous plant hormones

Plant growth-promoting rhizobacteria (PGPR) producing gibberellins (GAs) can be beneficial to plant growth and development. In the present study, we isolated and screened a new strain of Promicromonospora sp., SE188, isolated from soil. Promicromonospora sp. SE188 secreted GAs into its growth medium and exhibited phosphate solubilization potential. The PGPR produced physiologically active (GA₁ and GA₄) and inactive (GA₉, GA₁₂, GA₁₉, GA₂₀, GA₂₄, GA₃₄, and GA₅₃) GAs in various quantities detected by GC/MS-SIM. Solanum lycopersicum (tomato) plants inoculated with Promicromonospora sp. SE188 showed a significantly higher shoot length and biomass as compared to controls where PGPR-free nutrient broth (NB) and distilled water (DW) were applied to plants. The presence of Promicromonospora sp. SE188 significantly up-regulated the non C-13 hydroxylation GA biosynthesis pathway (GA₁₂→GA₂₄→GA₉→GA₄→ GA₃₄) in the tomato plants as compared to the NB and DW control plants. Abscisic acid, a plant stress hormone, was significantly down-regulated in the presence of Promicromonospora sp. SE188. Contrarily, salicylic acid was significantly higher in the tomato plant after Promicromonospora sp. SE188 inoculation as compared to the controls. Promicromonospora sp. SE188 showed promising stimulation of tomato plant growth. From the results it appears that Promicromonospora sp. SE188 has potential as a bio-fertilizer and should be more broadly tested in field trials for higher crop production in eco-friendly farming systems.     

Internode length in pisum. Mutants lk,lka, and lkb do not accumulate gibberellins

The levels of gibberellin A₁ (GA₁), GA₈, GA₁₉, GA₂₀, GA₂₉, and GA₄₄ in the short Pisum sativum L. mutants lk, lka, and lkb, and comparable wild-type plants, were determined by gas chromatography-selected ion monitoring (GC-SIM) using ²H or ¹³C internal standards. The mutants possessed similar GA₁ levels to wild-type plants, consistent with their classification as GA-sensitivity rather than GA-synthesis mutants. However, these mutants differ from certain sensitivity mutants in other species, in which substantial accumulation of GA₁ occurs. The results suggest that if the proposed feedback model for the regulation of GA synthesis occurs in peas it is not the reduced growth per se that is the trigger for elevated levels of C₁₉ GAs. The results are also consistent with the hypothesis that in those GA-sensitivity mutants which do not accumulate C₁₉ GAs, the biochemical lesion may be well down the transduction pathway which leads from GA₁ reception to stem elongation.     

Endophytic Cephalotheca sulfurea AGH07 reprograms soybean to higher growth

Abstract

Gibberellins (GAs) are well known for plant growth promotion. GAs production by fungi has received little attention, although substantial work has been carried out on other aspects of plant growth-promoting fungi (PGPF). We investigated GAs production and plant growth-promoting capacity of an endophytic fungus isolated from the roots of soil grown soybean plants. The endophytic fungus is reported as GAs producer and as PGPF for the first time in this study. Nine endophytic isolates were collected from the roots of soybean, and culture filtrates (CFs) obtained from their pure cultures were screened on Waito-C, a dwarf rice cultivar, for the presence of GAs. Of these, seven fungal isolates promoted shoot length as compared to control (distilled water), while one inhibited it. Three fungal isolates were selected on the basis of higher shoot elongation as compared to wild type Gibberella fujikuroi, which was used as positive control. The growth-prompting capacity of selected fungal isolates SB5-1, SB3-2, and SB3-3 was bio-assayed on soybean cv. Hwangkeumkong. Fungal isolate SB5-1 provided maximum plant height (31.6 cm), shoot length (21.1 cm), whole plant fresh biomass (2.41 g), shoot fresh biomass (1.99 g), and leaf area (24.37 cm²). The CF of isolate SB5-1 was analyzed for the presence of GAs, and it was found that all physiologically active GAs were present (GA₁, 0.15 ng/ml, GA₃, 1.2 ng/ml, GA₄, 7.37 ng/ml, and GA₇, 3.18 ng/ml) in conjunction with physiologically inactive GA₅, GA₉, GA₁₅, GA₁₉, GA₂₀, and GA₂₄. The fungal isolate SB5-1 was identified as a new strain of Cephalotheca sulfurea through molecular and phylogenetic approaches.     

Gibberellins and bud break,vegetative shoot growth and flowering in Metrosideros collina cv. Tahiti

Applications of the growth promotive gibberellins (GAs) GA₄ and 2,2-dimethyl GA₄, and of C-16,17 endo-dihydro GA₅, which is known to promote flowering while inhibiting stem growth in the long-day grass Lolium temulentum, were made to micropropagated plants of Metrosideros collina cv. Tahiti, a highly ornamental cultivar with an intermittent flowering pattern. Gibberellin A₄ and 2,2-dimethyl GA₄ stimulated vegetative growth both in elongating shoots, and internodes of shoots developing from buds that were quiescent at the time of GA application. Abscission of the apices of expanding shoots, a feature of mature Metrosideros plants, was inhibited by these GAs, the rejuvenation of micropropagated plantlets being enhanced. However, C-16,17 endo-dihydro GA₅ differed from GA₄ and 2,2-dimethyl GA₄ by having no promotive effects on vegetative growth, and no inhibition of apical abscission. Notwithstanding this contrasting effect on vegetative growth, high doses of GA₄ or C-16,17 endo-dihydro GA₅ similarly reduced flowering on shoots to which either GA was applied. Reduced flowering in response to applied GAs is common in many woody angiosperms, and in this instance was probably the combined result of abortion of developing floral structures in quiescent buds, and a preferential inhibition of bud break for floral buds relative to vegetative buds, particularly by GA₄. Finally, both C-16,17 endo-dihydro GA₅ and GA₄ strongly inhibited bud break in this woody angiosperm, although GA₄ could initially stimulate bud break when applied to vegetative buds close to the expansion stage. The above findings, in toto, highlight the sensitivity of Metrosideros to both classes of GA in a variety of growth and development processes.     

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.     

Changes in Endogenous Gibberellins and the Metabolism of [H]GA(4) after Geostimulation in Shoots of the Oat Plant (Avena sativa)

The recovery from “lodging,” or bending over, by shoots of 42-day-old Avena sativa plants is controlled primarily by a negatively geotropic differential growth of the lower halves of the p-1 node-pulvinus and the base of the p-1 internode, relative to the upper halves. Although geostimulation causes a significant reduction in p-1 internode length, dry matter accumulation in the p-1 node-pulvinus is increased, apparently at the expense of the sheath. Recovery to an angle of 30° is associated with changes in endogenous gibberellin-like substances (GAs), and in differential metabolism of applied [³H]GA₄ (1.4 Curie per millimole). Although geostimulation depressed total GAs (relative to upright plant parts) to 0.40 and 0.13 for node-pulvini and sheaths, respectively, it increased them 2-fold for internodes. Within the plant part geostimulation increased GAs (relative to upper halves) 29- and 7-fold in lower halves of node-pulvini and internodes, respectively, but reduced GAs to 0.3 in lower halves of sheaths. At age 42 days a GA_4/7-like (nonpolar) substance predominates, with lesser amounts of a GA₃-like (polar) substance. Native GAs of Avena include GA₃, GA₄, and GA₇. Geostimulation enhanced the ratio of nonpolar to polar GAs for both halves of internodes, but tended to depress it for sheaths and nodepulvini.