Changes in abscisic acid levels in embryo axes of Norway maple and sycamore seeds during maturation and dehydration

Changes in the abscisic acid (ABA) levels in embryo axes of seeds, belonging to the orthodox (Norway maple — Acer platanoides L.) and recalcitrant (sycamore — Acer pseudoplatanus L.) categories, were investigated throughout maturation using an ELISA (enzyme-linked immunosorbent assay) test. Concentration of ABA in embryo axes substantially differed depending on species and sampling date. ABA was always higher in Norway maple except at the end of seed maturation when ABA content was similar in both species. During maturation ABA decreased in both species but the decline was more marked in Norway maple than in sycamore (11 vs. 3 fold). These species also differed in the pattern of ABA changes, which in sycamore embryo axes was very regular, while in Norway maple a sharp decrease was recorded after acquisition by the seeds of tolerance to desiccation. Dehydration of embryo axes of Norway maple caused a further significant decrease of ABA level. In contrast, in dehydrated sycamore embryo axes ABA content did not decrease, but slightly increased. The role of ABA in desiccation tolerance and dormancy of Norway maple and sycamore seeds is discussed.     

Control of macaw palm seed germination by the gibberellin/abscisic acid balance

The hormonal mechanisms involved in palm seed germination are not fully understood. To better understand how germination is regulated in Arecaceae, we used macaw palm (Acrocomia aculeata (Jacq.) Lodd. Ex Mart.) seed as a model. Endogenous hormone concentrations, tocopherol and tocotrienol and lipid peroxidation during germination were studied separately in the embryo and endosperm. Evaluations were performed in dry (D), imbibed (I), germinated (G) and non‐germinated (NG) seeds treated (+GA₃) or not treated (control) with gibberellins (GA). With GA₃ treatment, seeds germinated faster and to a higher percentage than control seeds. The +GA₃ treatment increased total bioactive GA in the embryo during germination relative to the control. Abscisic acid (ABA) concentrations decreased gradually from D to G in both tissues. Embryos of G seeds had a lower ABA content than NG seeds in both treatments. The GA/ABA ratio in the embryo was significantly higher in G than NG seeds. The +GA₃ treatment did not significantly affect the GA/ABA ratio in either treatment. Cytokinin content increased from dry to germinated seeds. Jasmonic acid (JA) increased and 1‐aminocyclopropane‐1‐carboylic acid (ACC) decreased after imbibition. In addition, α‐tocopherol and α‐tocotrienol decreased, while lipid peroxidation increased in the embryo during germination. We conclude that germination in macaw palm seed involves reductions in ABA content and, consequently, increased GA/ABA in the embryo. Furthermore, the imbibition process generates oxidative stress (as observed by changes in vitamin E and MDA).     

Quantifying the sensitivity of barley seed germination to oxygen, abscisic acid, and gibberellin using a population-based threshold model

Barley (Hordeum vulgare L.) seeds (grains) exhibit dormancyat maturity that is largely due to the presence of the glumellae(hulls) that reduce the availability of oxygen (O₂) to the embryo.In addition, abscisic acid (ABA) and gibberellins (GAS) interactwith O₂ to regulate barley seed dormancy. A population-basedthreshold model was applied to quantify the sensitivities ofseeds and excised embryos to O₂, ABA, and GA, and to their interactiveeffects. The median O₂ requirement for germination of dormantintact barley seeds was 400-fold greater than for excised embryos,indicating that the tissues enclosing the embryo markedly limitO₂ penetration. However, embryo O₂ thresholds decreased by anotherorder of magnitude following after-ripening. Thus, increasesin both permeability of the hull to O₂ and embryo sensitivityto O₂ contribute to the improvement in germination capacityduring after-ripening. Both ABA and GA had relatively smalleffects on the sensitivity of germination to O₂, but ABA andGA thresholds varied over several orders of magnitude in responseto O₂ availability, with sensitivity to ABA increasing and sensitivityto GA decreasing with hypoxia. Simple additive models of O₂–ABAand O₂–GA interactions required consideration of theseO₂ effects on hormone sensitivity to account for actual germinationpatterns. These quantitative and interactive relationships amongO₂, ABA, and GA sensitivities provide insight into how dormancyand germination are regulated by a combination of physical (O₂diffusion through the hull) and physiological (ABA and GA sensitivities)factors. Key words: Abscisic acid, barley, germination, gibberellin, Hordeum vulgare L., model, oxygen, sensitivity Received 2 August 2007; Revised 14 November 2007 Accepted 19 November 2007     

Breakage of Pseudotsuga menziesii seed dormancy by cold treatment as related to changes in seed ABA sensitivity and ABA levels

The main aims of the present work were to investigate whether a chilling treatment which breaks dormancy of Douglas fir ( Pseudotsuga menziesii (Mirb.) Franco) seeds induces changes in the sensitivity of seeds to exogenous ABA or in ABA levels in the embryo and the megagametophyte, and whether these changes are related to the breaking of dormancy. Dormant seeds germinated very slowly within a narrow range of temperatures (20–30°C), the thermal optimum being approximately 25°C. The seeds were also very sensitive to oxygen deprivation. Treatment of dormant seeds at 5°C improved further germination, and resulted in a widening of the temperature range within which germination occurred and in better germination in low oxygen concentrations. In dry dormant seeds the embryo contained about one-third of the ABA in the megagametophyte. ABA content of both organs increased during the first 4 weeks of chilling. It then decreased sharply in the megagametophyte to the level in the embryo after 7–15 weeks of chilling. At 15°C, a temperature at which dormancy was expressed, the ABA level increased in the embryo and the megagametophyte of dormant unchilled seeds whereas it decreased in the organs of chilled seeds. The longer the chilling treatment, the faster the decrease in ABA after the transfer of seeds from 5°C to higher temperatures, and the decrease was faster at 25 than at 15°C. These results suggest that the breaking of dormancy by cold was associated with a lower capacity of ABA biosynthesis and/or a higher ABA catabolism in the seeds subsequently placed at 15 or 25°C. Moreover, the chilling treatment resulted in a progressive decrease in the sensitivity of seeds to exogenous ABA. However, seeds remained more sensitive to ABA at 15 than at 25°C. The possible involvement of ABA synthesis and of responsiveness of seeds to ABA in the breaking of dormancy by cold treatment is discussed.     

The Role of Maturation Drying in the Transition from Seed Development to Germination: VII. EFFECTS OF PARTIAL AND COMPLETE DESICCATION ON ABSCISIC ACID LEVELS AND SENSITIVITY IN RICINUS COMMUNIS L. SEEDS

During mid-development (25–40 d after pollination: DAP)of the castor bean seed the amount of abscisic acid (ABA) increasesin both the endosperm and the embryo, declining substantiallythereafter until there is little present in the mature dry (60DAP) seed. Premature desiccation of the seed at 35 DAP alsoleads to a major decline in ABA within the embryo and endosperm.Partial water loss from the seed at 35 DAP which, like naturaland premature desiccation, leads to subsequent germination uponreturn of the seed to full hydration, causes a much smallerdecline in ABA levels. In contrast, ABA declines substantiallyin the non-dried (hydrated) control at 35 DAP, but the seedsdo not germinate. Hence, a clear negative correlation betweenABA content and germinability is not observed. Both drying,whether natural or imposed prematurely, and partial drying decreasethe sensitivity of the isolated embryo to exogenous ABA by about10-fold. The protein synthetic response of the castor bean embryo exposedto 0.1 mol m^–3 ABA following premature desiccation exhibitssome similarity to the response of the non-dried developingembryo—in both cases the synthesis of some developmentalproteins is enhanced by ABA, and germination is suppressed.Germination of mature seeds is also suppressed by 0.1 mol m^–3ABA, but the same developmental proteins are not synthesized.In the cotyledons of prematurely-desiccated seed, some proteinsare hydrolysed upon imbibition in 0.1 mol m^–3 ABA, a phenomenonthat occurs also in the cotyledons of similarly treated matureembryos, but not in developing non-dried embryos. Hence theembryo exhibits an ‘intermediate’ response uponrehydration in 0.1 mol m^–3 ABA following premature desiccation;viz. some of the responses are developmental and some germinative.Following natural or imposed drying, the isolated embryo becomesrelatively insensitive to 0.01 mol m^–3 ABA: germinationis elicited and post-germinative reserve breakdown occurs inthe radicle and cotyledons. The reduced sensitivity of the embryoto ABA as a consequence of desiccation may be an important factorin eliciting the switch to germination and growth within thewhole seed. Key words: Abscisic acid, desiccation, astor bean endosperm, seed development, germination, protein synthesis, isolated embryos, hormone sensitivity     

Control of Seed Germination by Abscisic Acid: I. Time Course of Action in Sinapis alba L

The germination process of mustard seeds (Sinapis alba L.) has been characterized by the time courses of water uptake, rupturing of the seed coat (12 hours after sowing), onset of axis growth (18 hours after sowing), and the point of no return, where the seeds lose the ability to survive redesiccation (12 to 24 hours after sowing, depending on embryo part). Abscisic acid (ABA) reversibly arrests embryo development at the brink of radicle growth initiation, inhibiting the water uptake which accompanies embryo growth. Seeds which have been kept dormant by ABA for several days will, after removal of the hormone, rapidly take up water and continue the germination process. Seeds which have been preincubated in water lose the sensitivity to be arrested by ABA after about 12 hours after sowing. This escape from ABA-mediated dormancy is not due to an inactivation of the hormone but to a loss of competence to respond to ABA during the course of germination. The sensitivity to ABA can be restored in these seeds by redrying. It is concluded that a primary action of ABA in inhibiting seed germination is the control of water uptake of the embryo tissues rather than the control of DNA, RNA, or protein syntheses.     

Abscisic acid relationships in the chill-related dormancy mechanism in apple seeds

Abscisic acid (ABA) levels in seeds from three cultivars of apple (Malus domestica Borkh.) which have substantially different chilling requirements were investigated by gas chromatography mass-spectrometry selected ion monitoring (GCMS-SIM) during stratification. The ABA content of dormant unchilled seeds was similar in the three cultivars, suggesting no relationship between the chilling requirement of those seeds and their ABA status. That chilling is not related to ABA changes during stratification was confirmed by warm (20°C) and cold (5°C) stratification experiments. ABA content dropped rapidly and nearly identically under both temperature regimes, but only cold stratification promoted germination. The decline in ABA during stratification was due in large part to leaching from the seed coat and nucellar membrane; the ABA content of the embryo remained nearly constant. The radicle in intact seeds stratified at 5°C began growing 20–30 days after the ABA in the seed coat and nucellar membrane had nearly disappeared. Radicle growth did not occur in unchilled seeds, even though ABA had leached from them as well. It is possible that the leaching of ABA from the seed allows certain promotive forces to develop, but if so, these can develop only at chilling temperatures. Studies were also conducted on 2-trans ABA relationships to apple seed dormancy, but no association was evident.Report No. 12, Department of Fruit and Vegetable Science, Cornell University.     

Indirect effect of abscisic acid on the induction of secondary dormancy in lettuce seeds

During temporary incubation at 25°C in buffered solutions (pH 4.0) of abscisic acid (ABA) seeds of lettuce ( Lactuca sativa L. cv. Olof) lost the red-light initiated ability to germinate in buffer. The development of secondary dormancy required an inhibitory ABA content in the seeds during a number of days. A temporary incubation in ABA during 24 h met these requirements only if the solution was about 100-fold more concentrated than during continuous incubation. Studies with 2-¹⁴C-ABA showed that the amount of ABA which had penetrated in 24 h was reduced by a factor 100 within 3 to 4 days during subsequent incubation in buffer. Both leaching and metabolic changes were involved in the reduction process. The nature of the metabolic products remained obscure. A shift to 2°C after incubation in ABA prevented the induction of secondary dormancy, but inhibited ABA metabolism. ABA did not interfere with the induction rate of secondary dormancy, and it was not required to maintain the state of dormancy. The sole function of ABA was the non-specific inhibition of germination, which indirectly facilitated the development of an ABA independent secondary dormancy. – The level of endogenous ABA was compared to the amount of ABA found in the embryo during and after incubation in ABA solutions marked with 2-¹⁴C-ABA. The level of endogenous ABA in air-dry seeds (0.11 ng/mg dry weight) corresponded to the minimal level at which penetrated ABA inhibited germination. This level had to be present at least during 4 to 5 days to inhibit the effect of red light. Since endogenous ABA was quickly reduced upon imbibition, a regulatory function of endogenous ABA in the inhibition of red light induced germination can be ruled out. A function in the temporary inhibition of dark germination and, consequently, in the development of secondary light irresponsiveness cannot be excluded, however.     

Dormancy and Germination of Abscisic Acid-Deficient Tomato Seeds : Studies with the sitiens Mutant

The role of abscisic acid (ABA) in the dormancy induction of tomato (Lycopersicon esculentum) seeds was studied by comparison of the germination behavior of the ABA-deficient sitiens mutant with that of the isogenic wild-type genotype. Freshly harvested mutant seeds, in contrast to wild-type seeds, always readily germinate and even exhibit viviparous germination in overripe fruits. Crosses between mutant and wild-type and self-pollination of heterozygous plants show that in particular the ABA fraction of embryo and endosperm is decisive for the induction of dormancy. After-ripened wild-type seeds fully germinate in water but are more sensitive toward osmotic inhibition than mutant seeds. Germination of both wild-type and mutant seeds is equally sensitive toward inhibition by exogenous ABA. ABA content of mature wild-type seeds is about 10-fold the level found in mutant seeds. Nevertheless, it is argued that the differences in dormancy between the seeds of both genotypes are not a result of actual ABA levels in the mature seeds or fruits but a result of differences in ABA levels during seed development. It is hypothesized that the high levels of ABA that occur during seed development in wild-type seeds induce an inhibition of cell elongation of the radicle that can still be observed after long periods of dry storage.     

Endogenous abscisic acid during the germination of chick-pea seeds

Over the past twenty years many studies have been undertaken to elucidate the regulation of seed germination. Abscisic acid (ABA) and the gibberellins (GAs) are the hormones proposed to control this process, the first by inhibiting and the second by inducing germination. It has been proposed that a high water potential increases the growth potential of the embryo, presumably permitting the production or activation by GA of the cell wall hydrolases and thus decreasing the yield threshold of the endosperm close to the radicle tip. A low water potential, e.g., imbibition in an osmoticum. imposes a stress on cell metabolism, by reducing the turgor of the radicle cells, and there is a decrease in growth potential. Exogenous ABA also causes a decline in growth potential of the radicle: however, the actions of low water potential in preventing germination are not mediated through an increase in ABA in the seeds. In the present paper an attempt is made to asses the role of ABA and polyethylene glycol (PEG) in the germination of chick-pea (Cicer arietinum L.) seeds. The endogenous ABA of chick-pea seeds was purified by reversed-phase HPLC and quantified by GC-ECD. The variations in the ABA levels in the embryonic axes and the cotyledons were studied during 120 h. of imbibition. The highest ABA level in the embryome axes was found at 18 h. coinciding with an increase in fresh weight and a high germination percentage. ABA was not detected in the cotyledons during incubation which probably indicates that the hormone is more involved in the active growth of the embryonic axes itself than in the mobilization process of the reserves. When seeds were treated with different PEG-cycles. PEG delayed germination, reduced the fresh weight of embryonic axes, and retarded the onset of ABA synthesis. It is concluded that endogenous ABA is related to the onset of germination and the growth of the embryonic axis. In addition, there is no correlation among the different PEG-cycles and the level of ABA and germination. Germination was related more to the water conditions inside the embryo's cells than to ABA levels.     

Abscisic acid and osmoticum prevent germination of developing alfalfa embryos,but only osmoticum maintains the synthesis of developmental proteins

Developing seeds of alfalfa (Medicago sativa L.) acquire the ability to germinate during the latter stages of development, the maturation drying phase. Isolated embryos placed on Murashige and Skoog medium germinate well during early and late development, but poorly during mid-development; however, when placed on water they germinate well only during the latter stage of development. Germination of isolated embryos is very slow and poor when they are incubated in the presence of surrounding seed structures (the endosperm or seed coat) taken from the mid-development stages. This inhibitory effect is also achieved by incubating embryos in 10^?5 M abscisic acid (ABA). Endogenous ABA attains a high level during mid-development, especially in the endosperm. Seeds developing in pods treated with fluridone (1-methyl-3-phenyl-5[3-(trifluoromethyl)-phenyl]-4(1H)-pyridinone) contain low levels of ABA during mid-development, and the endosperm and seed coat only weakly inhibit the germination of isolated embryos. However, intact seeds from fluridone-treated pods do not germinate viviparously, which is indicative that ABA alone is not responsible for maintaining seeds in a developing state. Application of osmoticum (e.g. 0.35 M sucrose) to isolated developing embryos prevents their germination. Also, in the developing seed in situ the osmotic potential is high. Thus internal levels of osmoticum may play a role in preventing germination of the embryo and maintaining development. Abscisic acid and osmoticum impart distinctly different metabolic responses on developing embryos, as demonstrated by their protein-synthetic capacity. Only in the presence of osmoticum do embryos synthesize proteins which are distinctly recognizable as those synthesized by developing embryos in situ, i.e. when inside the pod. Abscisic acid induces the synthesis of a few unique proteins, but these arise even in mature embryos treated with ABA. Thus while both osmoticum and ABA prevent precocious germination, their effects on the synthetic capacity of the developing embryo are quite distinct. Since seeds with low endogenous ABA do not germinate, osmotic regulation may be the more important of these two factors in controlling seed development.     

Induction of dormancy during seed development by endogenous abscisic acid: studies on abscisic acid deficient genotypes of Arabidopsis thaliana (L.) Heynh.

Mutant lines of Arabidopsis thaliana (L.) Heynh., which are characterized by symptoms of withering and the absence of seed dormancy, showed much lower levels of endogenous abscisic acid (ABA) in developing seeds and fruits (siliquae) than the wild type. Reciprocal crosses of wild type and ABA-deficient mutants showed a dual origin of ABA in developing seeds. The genotype of the mother plant regulated a sharp rise in ABA content half-way seed development (maternal ABA). The genotype of the embryo and endosperm was responsible for a second ABA fraction (embryonic ABA), which reached much lower levels, but persisted for some time after the maximum in maternal ABA. The onset of dormancy correlated well with the presence of the embryonic ABA fraction and not with the maternal ABA. Dormancy developed in both the absence and presence of maternal ABA in the seeds. In this respect maternal ABA resembled exogenously applied ABA which did not induce dormancy in ABA-deficient seeds. However, both maternal and applied ABA stimulated the formation of a mucilage layer around the testa, which could be observed during imbibition of the mature seeds. In the mature state, ABA-deficient seeds germinated in the siliquae on the plant, but only when the atmosphere surrounding the plant was kept at high relative humidity. In younger stages germination in siliquae occurred after isolation from the plants and incubation on wet filter paper. Therefore, it seems that limited access to water is the primary trigger for the developmental arrest in these seeds.     

Ascorbic acid and reactive oxygen species are involved in the inhibition of seed germination by abscisic acid in rice seeds

The antagonism between abscisic acid (ABA) and gibberellin (GA) plays a key role in controlling seed germination, but the mechanism of antagonism during this process is not known. The possible links among ABA, reactive oxygen species (ROS), ascorbic acid (ASC), and GA during rice seed germination were investigated. Unlike in non-seed tissues where ROS production is increased by ABA, ABA reduced ROS production in imbibed rice seeds, especially in the embryo region. Such reduced ROS also led to an inhibition of ASC production. GA accumulation was also suppressed by a reduced ROS and ASC level, which was indicated by the inhibited expression of GA biosynthesis genes, amylase genes, and enzyme activity. Application of exogenous ASC can partially rescue seed germination from ABA treatment. Production of ASC, which acts as a substrate in GA biosynthesis, was significantly inhibited by lycorine which thus suppressed the accumulation of GA. Consequently, expression of GA biosynthesis genes was suppressed by the low levels of ROS and ASC in ABA-treated seeds. It can be concluded that ABA regulates seed germination in multiple dimensions. ROS and ASC are involved in its inhibition of GA biosynthesis.     

Abscisic acid levels in seeds of the gibberellin-deficient mutant lh-2 of pea (Pisum sativum)

The lh-2 mutation in garden pea ( Pisum sativum L.) blocks an early step in the gibberellin (GA) biosynthesis pathway, the three-step oxidation of ent -kaurene to ent -kaurenoic acid. As a result, only low levels of GAs, including the bioactive GA₁, are found in shoots and seeds of lh-2 plants. Mutant plants are dwarf in stature, and show increased seed abortion and decreased seed weight, compared with seeds of the tall wild-type (WT) progenitor (cv. Torsdag). The aberrant seed development of lh-2 plants is associated with reduced levels of GA₁ and GA₃, and with an accumulation of abscisic acid (ABA) in young seeds (pre-contact point). This ABA accumulation is typically 3- to 4-fold, and can be up to 6-fold, compared with control plants. To investigate whether the accumulation of ABA is partly responsible for causing the observed seed abortion in lh-2 plants, we constructed a double mutant between the lh-2 allele and wil . The wil mutation blocks ABA biosynthesis, and reduces ABA levels in young seeds by 10-fold. Introduction of the wil mutation reduces the endogenous ABA levels in young lh-2 seeds, but fails to rescue the seeds from abortion. This indicates that the effects of lh-2 on seed development are not mediated through increased ABA levels, and is consistent with previous evidence that GAs are the controlling factor underlying the lh-2 seed phenotype in pea.     

Dynamic distribution and the role of abscisic acid during seed development of a lady’s slipper orchid,Cypripedium formosanum

Background and Aims Although abscisic acid (ABA) is commonly recognized as a primary cause of seed dormancy, there is a lack of information on the role of ABA during orchid seed development. In order to address this issue, the localization and quantification of ABA were determined in developing seeds of Cypripedium formosanum.Methods The endogenous ABA profile of seeds was measured by enzyme-linked immunosorbent assay (ELISA). Temporal and spatial distributions of ABA in developing seeds were visualized by immunohistochemical staining with monoclonal ABA antibodies. Fluoridone was applied to test the causal relationship between ABA content and seed germinability.Key Results ABA content was low at the proembryo stage, then increased rapidly from 120 to 150 days after pollination (DAP), accompanied by a progressive decrease in water content and seed germination. Immunofluorescence signals indicated an increase in fluorescence over time from the proembryo stage to seed maturation. From immunogold labelling, gold particles could be seen within the cytoplasm of embryo-proper cells during the early stages of seed development. As seeds approached maturity, increased localization of gold particles was observed in the periplasmic space, the plasmalemma between embryo-proper cells, the surface wall of the embryo proper, and the inner walls of inner seed-coat cells. At maturity, gold particles were found mainly in the apoplast, such as the surface wall of the embryo proper, and the shrivelled inner and outer seed coats. Injection of fluoridone into capsules resulted in enhanced germination of mature seeds.Conclusions The results indicate that ABA is the key inhibitor of germination in C. formosanum. The distinct accumulation pattern of ABA suggests that it is synthesized in the cytosol of embryo cells during the early stages of seed development, and then exported to the apoplastic region of the cells for subsequent regulatory processes as seeds approach maturity.     

Effect of Water Stress, Seed Coat Restraint, and Abscisic Acid upon Different Germination Capabilities of Two Tomato Lines at Low Temperature

Two tomato (Lycopersicon esculentum Mill.) lines with greatly different capabilities to germinate at 10°C were compared with respect to sensitivity to experimental treatments which affect the water status of the embryo. Germination rates and final percentages could be drastically changed (at 25°C) by (a) removing the mechanical constraint from the radicle tip, (b) imposing water stress by an osmoticum, (c) stress hardening of the seeds through osmotic pretreatment, and (d) inhibiting embryo expansion by abscisic acid (ABA). All treatments showed a similar difference in germination vigor between the two lines indicating that cold sensitivity is in fact a matter of water relations rather than of phase transitions in membrane lipids. Inhibition of germination by ABA was completely abolished by removing the mechanical constraint from the radicle tip. Osmotic stress of −3 bar which quantitatively replaced this constraint in inhibiting germination also restored the sensitivity to ABA. It is concluded that all these treatments act on the balance between the hydraulic extension force of the embryo radicle and the opposing force of the seed layers covering the radicle tip. The difference in cold sensitivity between the two seed lines resides either in the osmotic potential or the pressure potential of the germinating embryo.     

Abscisic acid in the thermoinhibition of lettuce seed germination and enhancement of its catabolism by gibberellin

Germination of lettuce (Lactuca sativa L. cv. 'Grand Rapids') seeds was inhibited at high temperatures (thermoinhibition). Thermoinhibition at 28 degrees C was prevented by the application of fluridone, an inhibitor of abscisic acid (ABA) biosynthesis. At 33 degrees C, the sensitivity of the seeds to ABA increased, and fluridone on its own was no longer effective. However, a combined application of fluridone and gibberellic acid (GA3) was able to restore the germination. Exogenous GA3 lowered endogenous ABA content in the seeds, enhancing catabolism of ABA and export of the catabolites from the intact seeds. The fluridone application also decreased the ABA content. Consequently, the combined application of fluridone and GA3 decreased the ABA content to a sufficiently low level to allow germination at 33 degrees C. There was no significant temperature-dependent change in endogenous GA1 contents. It is concluded that ABA is an important factor in the regulation of thermoinhibition of lettuce seed germination, and that GA affects the temperature responsiveness of the seeds through ABA metabolism.     

Interaction of jasmonic acid with some plant growth regulators in the control of apple (Malus domestica) embryo germination

Embryos isolated from dormant apple seeds were treated with jasmonic acid (JA), gibberellin A₃ (GA₃), abscisic acid (ABA) and hydrogen cyanide in darkness and in light. The chemicals were present in the culture medium continuously and simultaneously or applied for 2 days and in different sequences. All treatments stimulated embryo germination except ABA, which was strongly inhibitory. Additive effects of JA with light and with GA₃ on embryo germination were observed, whereas ABA interacted synergically with JA, HCN and light. ABA and GA₃ were most effective when applied early during embryo incubation, but the late JA treatment was more stimulatory. It is concluded that JA does not act on the regulatory pathway that is initiated by light and which leads to embryo germination through gibberellin accumulation and alkaline lipase activation. ABA and HCN appear to be involved in the control of this pathway. JA and ABA may be involved in the control of alkaline lipase activity, independently of this regulatory chain.Abbreviations ABA abscisic acid - GA₃ gibberellin A₃ - JA jasmonic acid     

Inhibition of Amaranthus caudatus seed germination by polyethylene glycol-6000 and abscisic acid and its reversal by ethephon or 1-aminocyclopropane-1-carboxylic acid

The effect of polyethylene glycol (PEG-6000), abscisic acid (ABA), 2-chloroethylphosphonic acid (ethephon) and 1-aminocyclopropane-1-carboxylic acid (ACC) on the germination of Amaranthus caudatus L. seeds was examined. Both PEG-6000 and ABA inhibited the rate and percentage of seed germination. ABA potentiated the effect of PEG. Ethephon was highly effective in reversing the inhibitory effect of PEG and ABA or combinations of both. ACC relieved inhibition by ABA and the combined effect of ABA and PEG. Aminoethoxyvinylglycine (AVG) increased the inhibition of seed germination caused by ABA. The inhibition of seed germination by ABA seems to be related more or less to ethylene biosynthesis or is associated with a change of tissue sensitivity to ethylene. The possibility of ethylene control of water uptake by seeds is also considered.     

Changes in indole-3-acetic acid levels during tomato (Lycopersicon esculentum Mill.) seed development

Summary The changes in the level of indole-3-acetic acid (IAA) were investigated in seeds and fruit tissues-placenta and mesocarp-during tomato (Lycopersicon esculentum Mill.) zygotic embryogenesis, which was characterized through eight morphological embryo stages [from globular (stage 1) to mature embryo (stage 8)]. In whole seeds, IAA levels increased mainly at stage 3 (young torpedo) and at stage 5 (late torpedo stage). As the seed matured and dehydrated, IAA levels decreased and showed a new distribution pattern within seed structures, preferentially in endosperm tissue. IAA contents in fruit tissues were lower but followed the same pattern as those of seeds. These data support the hypothesis of IAA biosynthesis in seeds with a transient role of the endosperm at the end of embryo development and suggest a role of IAA in fruit and seed growth. Moreover a comparison of IAA and ABA changes suggests that IAA could be especially necessary for the beginning of embryo growth, whereas ABA could act mainly at the end of the growth phase.Abbreviations ABA abscisic acid - ABTS 2,2-azino-bis (3-ethylbenzthiazoline-6-sulfonic acid) - BHT butylhydroxytoluene - DW dry weight - ELISA enzyme linked immunosorbent assay - HPLC high performance liquid chromatography - IAA indole-3-acetic acid. PGRs: plant growth regulators