In Situ Abscisic Acid Synthesis : A Requirement for Induction of Embryo Dormancy in Helianthus annuus

When applied to young nondormant embryos of sunflower (Hellanthus annus) (7-10 day[s] after pollination [DAP]), abscisic acid (ABA) inhibited germination as long as it was present. However, whatever the dose used and the duration of its application, ABA was unable to induce dormancy because after transfer of treated embryos to control (without ABA) medium, germination occurred. Thereafter, exogenous ABA became effective and allowed the dormancy to develop in 13 and 17 DAP embryos, i.e. in embryos which after isolation were still able to germinate in high percentage. After embryo dormancy was well established (21 DAP), application of fluridone allowed the germination to occur very quickly on control medium. Isolated dormant axes were also induced to germinate by an application of fluridone. Radioimmunological analysis showed that 24 hours after these treatments, endogenous ABA levels were drastically reduced in the axes. When these fluridone-treated embryos were cultured on ABA medium, germination was again inhibited as long as exogenous ABA was present but germination occurred as soon as embryos were transferred to control medium. Such behavior suggested that in situ ABA synthesis is necessary to impose and maintain the embryo dormancy.     

Continuous biosynthesis of abscisic acid (ABA) may be required for maintaining dormancy of isolated embryos and intact seeds of Euonymus alatus

Euonymus alatus (Thunb.) Sieb. is a popular landscape plant in the United States due to its brilliant red fall foliage. It is also an important ornamental plant in many other areas of the world such as China, Japan and Europe. However, E. alatus is considered as a highly invasive plant species in the US. Mutation breeding can be used to create sterile, non-invasive cultivars. Seeds are the most commonly used explants for mutagen treatments, but E. alatus mature seeds possess prolonged dormancy and only a low percentage of them germinate even after 18?months of cold stratification. Here we report an immature embryo culture method for E. alatus ??Compactus?? to circumvent the seed dormancy problem. Also, we have found that activated charcoal, gibberellic acid (GA₃) and 6-benzyladenine (BA) can reduce the dormancy of isolated embryos, which suggests that abscisic acid (ABA) might play a role in controlling seed dormancy. We have further demonstrated that exogenous ABA enhances dormancy of isolated E. alatus embryos while fluridone, an inhibitor for ABA biosynthesis, can effectively break their dormancy. These results, particularly the effect of fluridone, suggest that continuous ABA biosynthesis plays an important role in controlling the dormancy of E. alatus seeds.     

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.     

Involvement of ABA in induction of secondary dormancy in barley (Hordeum vulgare L.) seeds

At harvest, barley seeds are dormant because their germination is difficult above 20 degrees C. Incubation of primary dormant seeds at 30 degrees C, a temperature at which they do not germinate, results in a loss of their ability to germinate at 20 degrees C. This phenomenon which corresponds to an induction of a secondary dormancy is already observed after a pre-treatment at 30 degrees C as short as 4-6 h, and is optimal after 24-48 h. It is associated with maintenance of a high level of embryo ABA content during seed incubation at 30 degrees C, and after seed transfer at 20 degrees C, while ABA content decreases rapidly in embryos of primary dormant seeds placed directly at 20 degrees C. Induction of secondary dormancy also results in an increase in embryo responsiveness to ABA at 20 degrees C. Application of ABA during seed treatment at 30 degrees C has no significant additive effect on the further germination at 20 degrees C. In contrast, incubation of primary dormant seeds at 20 degrees C for 48 and 72 h in the presence of ABA inhibits further germination on water similarly to 24-48 h incubation at 30 degrees C. However fluridone, an inhibitor of ABA synthesis, applied during incubation of the grains at 30 degrees C has only a slight effect on ABA content and secondary dormancy. Expression of genes involved in ABA metabolism (HvABA8'OH-1, HvNCED1 and HvNCED2) was studied in relation to the expression of primary and secondary dormancies. The results presented suggest a specific role for HvNCED1 and HvNCED2 in regulation of ABA synthesis in secondary seed dormancy.     

Regulation of Embryo Dormancy by Manipulation of Abscisic Acid in Kernels and Associated Cob Tissue of Zea mays L. Cultured in Vitro

Sectors of Zea mays cobs, with and without kernels were cultured in vitro in the presence and absence of fluridone. Cultured kernels, cob tissue, and embryos developed similarly to those grown in the field. Abscisic acid (ABA) levels in the embryos were evaluated by enzyme-linked immunosorbant assay. ABA levels in intact embryos cultured in the presence of fluridone were extremely low and indicate an inhibition of ABA synthesis. ABA levels in isolated cob tissue indicate that ABA can be produced by cob tissue. Sections containing kernels cultured in the presence of fluridone were transferred to medium containing fluridone and ABA. Dormancy was induced in more than 50% of the kernels transferred from 13 to 15 days after pollination, but all of the kernels transferred at 16 days after pollination or later were viviparous. ABA recovered from kernels that were placed in medium containing fluridone and ABA suggest that ABA can be transported through the cob tissue into developing embryos and that ABA is required for induction of dormancy in intact embryos.     

Changes in endogenous abscisic acid levels during dormancy release and maintenance of mature seeds: studies with the Cape Verde Islands ecotype,the dormant model of<Emphasis Type="Italic"> Arabidopsis thaliana</Emphasis>

Mature seeds of the Cape Verde Islands (Cvi) ecotype of Arabidopsis thaliana (L.) Heynh. show a very marked dormancy. Dormant (D) seeds completely fail to germinate in conditions that are favourable for germination whereas non-dormant (ND) seeds germinate easily. Cvi seed dormancy is alleviated by after-ripening, stratification, and also by nitrate or fluridone treatment. Addition of gibberellins to D seeds does not suppress dormancy efficiently, suggesting that gibberellins are not directly involved in the breaking of dormancy. Dormancy expression of Cvi seeds is strongly dependent on temperature: D seeds do not germinate at warm temperatures (20–27°C) but do so easily at a low temperature (13°C) or when a fluridone treatment is given to D seeds sown at high temperature. To investigate the role of abscisic acid (ABA) in dormancy release and maintenance, we measured the ABA content in both ND and D seeds imbibed using various dormancy-breaking conditions. It was found that dry D seeds contained higher amounts of ABA than dry ND after-ripened seeds. During early imbibition in standard conditions, there was a decrease in ABA content in both seeds, the rate of which was slower in D seeds. Three days after sowing, the ABA content in D seeds increased specifically and then remained at a high level. When imbibed with fluridone, nitrate or stratified, the ABA content of D seeds decreased and reached a level very near to that of ND seeds. In contrast, gibberellic acid (GA₃) treatment caused a transient increase in ABA content. When D seeds were sown at low optimal temperature their ABA content also decreased to the level observed in ND seeds. The present study indicates that Cvi D and ND seeds can be easily distinguished by their ability to synthesize ABA following imbibition. Treatments used here to break dormancy reduced the ABA level in imbibed D seeds to the level observed in ND seeds, with the exception of GA₃ treatment, which was active in promoting germination only when ABA synthesis was inhibited.Abbreviations ABA Abscisic acid - Cvi Cape Verde Islands - D Dormant - GA Gibberellin - GA₃ Gibberellic acid - ND Non dormant     

An Analysis of Seed Development in Pisum sativum: VIII. DOES ABSCISIC ACID PREVENT PRECOCIOUS GERMINATION AND CONTROL STORAGE PROTEIN SYNTHESIS?

The influence of abscisic acid (ABA) on the precocious germinationand storage protein production of pea seeds has been examinedusing embryo and pod culture. The precocious germination ofembryos in culture could not be inhibited fully by ABA on apermissive medium (2% sucrose) even at 0.1 mol m^–3. However,increasing the sucrose concentration to 5% caused near completeinhibition when ABA was added to the medium. Embryos of differentweights cultured on a high osmoticum (mannitol-containing medium),equivalent to 10% sucrose, did not show any consistent differencein ABA content. When fluridone was added to a non-permissiveculture medium, no decrease in ABA content of the embryos couldbe observed and no precocious germination was induced. In contrast,fluridone was able to prevent the accumulation of ABA in seedspresent in pods cultured in its presence from an early stageof development. These seeds, however, grew normally and reachedmaturity, did not germinate precociously in vivo, were desiccationtolerant and still produced storage protein message whetheror not ABA was included in the culture medium. It does not appear,therefore, that ABA regulates normal development or storageprotein synthesis in pea embryos. Key words: Abscisic acid, peas, Pisum sativum, seed development     

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.     

Abscisic acid-regulated responses of dormant and non-dormant embryos of Helianthus annuus: Role of ABA-inducible proteins

Embryos of Helianthus annuus L. became dormant 3 weeks after anthesis and their dormancy was lifted during storage in dry conditions. The objectives of this study were to investigate changes in the pattern of soluble proteins associated with the release of embryo dormancy. Sunflower dehydrins and group 3 late embryogenesis-abundant (LEA) proteins were studied in developing embryos. Three dehydrins (17, 21 and 26 kDa) and two group 3 LEA polypeptides (17 and 23 kDa) appeared during dormancy induction. Their levels remained steady until maturity. After imbibition, these polypeptides disappeared within 24 h except for the 23-kDa protein whose levels remained stable for a further 4 d, whatever the culture condition. Analysis of radiolabelled proteins by two-dimensional gel electrophoresis revealed that among dormancy-associated proteins other than dehydrin and group 3 LEA, several low molecular mass (18, 19, 20 and 21 kDa) proteins were expressed in dormant embryos but not detected in non-dormant embryos. After a treatment with fluridone, which inhibits ABA synthesis, or with GA₃, which allows germination to occur, the 19-kDa protein could not be detected. In contrast, application of ABA to non-dormant embryos arrested germination and enhanced the synthesis of the 18- and 21-kDa proteins, but not that of the 19- and 20-kDa polypeptides. These results demonstrate that steady-state levels of specific proteins change during early imbibition of dormant and non-dormant sunflower embryos and indicate that these changes may be associated with differential gene expression responsible for the maintenance of dormancy.     

On the role of abscisic acid in seed dormancy of red rice

Abscisic acid (ABA) is commonly assumed to be the primary effector of seed dormancy, but conclusive evidence for this role is lacking. This paper reports on the relationships occurring in red rice between ABA and seed dormancy. Content of free ABA in dry and imbibed caryopses, both dormant and after-ripened, the effects of inhibitors, and the ability of applied ABA to revert dormancy breakage were considered. The results indicate: (i) no direct correlation of ABA content with the dormancy status of the seed, either dry or imbibed; (ii) different sensitivity to ABA of non-dormant seed and seed that was forced to germinate by fluridone; and (iii) an inability of exogenous ABA to reinstate dormancy in fluridone-treated seed, even though applied at a pH which favoured high ABA accumulation. These considerations suggest that ABA is involved in regulating the first steps of germination, but unidentified developmental effectors that are specific to dormancy appear to stimulate ABA synthesis and to enforce the responsiveness to this phytohormone. These primary effectors appear physiologically to modulate dormancy and via ABA they effect the growth of the embryo. Therefore, it is suggested that ABA plays a key role in integrating the dormancy-specific developmental signals with the control of growth.     

Abscisic Acid and the maturation of cacao embryos in vitro

Abscisic acid (ABA) was tested for its ability to affect development of immature zygotic embryos of cacao (Theobroma cacao) in vitro, by adding exogenous ABA, fluridone, or mefluidide to cultured embryos. Endogenous ABA levels, measured by enzyme-linked immunosorbent assay, were increased by exogenous ABA or by culture on sucrose increasing to 21%, and were decreased by fluridone and, to a lesser extent, by mefluidide. The effects of these on maturation were measured as effects on anthocyanins, lipids, and fatty acid saturation, all of which increase with maturation of the cacao embryo. Maturation was stimulated by increasing sucrose and, to a lesser degree, the addition of ABA, but decreasing endogenous ABA by treating with fluridone significantly inhibited all maturation parameters. Although desiccation tolerance does not develop in cacao embryos, these results suggest that ABA and sucrose are both needed for the initiation of events associated with maturation in vitro.     

Abscisic acid and stress treatment are essential for the acquisition of embryogenic competence by carrot somatic cells

Studies of carrot embryogenesis have suggested that abscisic acid (ABA) is involved in somatic embryogenesis. A relationship between endogenous ABA and the induction of somatic embryogenesis was demonstrated using stress-induced system of somatic embryos. The embryonic-specific genes C-ABI3 and embryogenic cell proteins (ECPs) were expressed during stress treatment prior to the formation of somatic embryos. The stress-induction system for embryogenesis was clearly distinguished by two phases: the acquisition of embryogenic competence and the formation of a somatic embryo. Somatic embryo formation was inhibited by the application of fluridone (especially at 10⁻⁴ M), a potent inhibitor of ABA biosynthesis, during stress treatment. The inhibitory effect of fluridone was nullified by the simultaneous application of fluridone and ABA. The level of endogenous ABA increased transiently during stress. However, somatic embryogenesis was not significantly induced by the application of only ABA to the endogenous level, in the absence of stress. These results suggest that the induction of somatic embryogenesis, in particular the acquisition of embryogenic competence, is caused not only by the presence of ABA but also by physiological responses that are directly controlled by stresses.     

Disrupting Abscisic Acid Homeostasis in Western White Pine (<Emphasis Type="Italic">Pinus monticola</Emphasis> Dougl. Ex D. Don) Seeds Induces Dormancy Termination and Changes in Abscisic Acid Catabolites

To investigate the role of abscisic acid (ABA) biosynthesis and catabolism in dormant imbibed seeds of western white pine (Pinus monticola), ABA and selected catabolites were measured during a combined treatment of the ABA biosynthesis inhibitor fluridone, and gibberellic acid (GA). Fluridone in combination with GA effectively disrupted ABA homeostasis and replaced the approximately 90-day moist chilling period normally required to break dormancy in this species. Individually, both fluridone and GA treatments decreased ABA levels in the embryos and megagametophytes of white pine seeds compared to a water control; however, combined fluridone/GA treatment, the only treatment to terminate dormancy effectively, led to the greatest decline in ABA content. Fluridone treatments revealed that a high degree of ABA turnover/transport occurred in western white pine seeds during the initial stages of dormancy maintenance; at this time, ABA levels decreased by approximately two-thirds in both embryo and megagametophyte tissues. Gibberellic acid treatments, both alone and in combination with fluridone, suggested that GA acted transiently to disrupt ABA homeostasis by shifting the ratio between biosynthesis and catabolism to favor ABA catabolism or transport. Increases in phaseic acid (PA) and dihydrophaseic acid (DPA) were observed during fluridone/GA treatments; however, increases in ABA metabolites did not account for the reduction in ABA observed; additional catabolism and/or transport of ABA and selected metabolites in all probability accounts for this discrepancy. Finally, levels of 7′ hydroxy-ABA (7′OH-ABA) were higher in dormant-imbibed seeds, suggesting that metabolism through this pathway is increased in seeds that maintain higher levels of ABA, perhaps as a means to further regulate ABA homeostasis.     

Control of seed dormancy in Nicotiana plumbaginifolia: post-imbibition abscisic acid synthesis imposes dormancy maintenance

The physiological characteristics of seed dormancy in Nicotiana plumbaginifolia Viv. are described. The level of seed dormancy is defined by the delay in seed germination (i.e the time required prior to germination) under favourable environmental conditions. A wild-type line shows a clear primary dormancy, which is suppressed by afterripening, whereas an abscisic acid (ABA)-deficient mutant shows a non-dormant phenotype. We have investigated the role of ABA and gibberellic acid (GA₃) in the control of dormancy maintenance or breakage during imbibition in suitable conditions. It was found that fluridone, a carotenoid biosynthesis inhibitor, is almost as efficient as GA₃ in breaking dormancy. Dry dormant seeds contained more ABA than dry afterripened seeds and, during early imbibition, there was an accumulation of ABA in dormant seeds, but not in afterripened seeds. In addition, fluridone and exogenous GA₃ inhibited the accumulation of ABA in imbibed dormant seeds. This reveals an important role for ABA synthesis in dormancy maintenance in imbibed seeds. Received: 31 December 1998 / Accepted: 9 July 1999     

Changes in Abscisic Acid Biosynthesis and Catabolism during Dormancy Breaking in Fagus sylvatica Embryo

At harvest, embryos of Fagus sylvatica are dormant. A cold pretreatment without medium at 30% moisture content allowed them to germinate. A comparison of the abscisic acid (ABA) content before and after the pretreatment has no significant relevance since dormancy is expressed during the culture at 23°C. During this culture, both de novo biosynthesis and conjugate hydrolysis contributed to maintain a high level of ABA in the dormant axis. The level of conjugates and the rate of hydrolysis were not modified substantially by the cold pretreatment. In contrast, the dormancy release was associated with a strong decrease in the capacity for ABA synthesis. Moreover, feeding (+)-[³H]ABA to untreated and pretreated embryos proved that the cold treatment also induced a hastening of ABA catabolism. Received August 15, 1996; accepted December 6, 1996     

Physiological basis of pre-harvest sprouting resistance in Sorghum bicolor (L.) Moench. ABA levels and sensitivity in developing embryos of sprouting-resistant and -susceptible varieties

Pre–harvest sprouting is a major problem in Sorghum cropswhich leads to losses in seed viability and produces importantdecreases in grain weight. In this paper we aimed to have aninsight into the physiological basis of pre-harvest sproutingresistance in this crop by assessing germinability, ABA embryoniccontent and embryonic sensitivity to ABA during seed developmentin three varieties presenting contrasting sprouting behaviour:Redland B2 (very susceptible), SC 650 (moderately resistant)and IS 9530 (very resistant). Redland B2 caryopses were ableto germinate with high germination indices from early stagesof development, while caryopses from IS 9530 did not presentgermination indices different from 0 until near physiologicalmaturity. SC 650 (moderately resistant) grains presented anintermediate pattern of behaviour. In all three varieties isolatedembryos were able to germinate with maximum germination indicesfrom as early as 15 d after pollination (DAP). Differences ingrain dormancy level were not paralleled by a consistently differentendogenous ABA content throughout maturation. However, whenABA embryonic content was measured in incubated 35 DAP caryopses,ABA level in B2 embryos after 24 h of incubation was found tobe less than half that observed in IS 9530 embryo after thesame period of incubation. In addition, B2 embryos were foundto be 10–fold less sensitive to the inhibitory effectof ABA than embryos from the other two varieties. These resultsexplain to a considerable extent differences in germinabilitybetween sproutingresistant and –susceptible varietiesand are consistent with differences in sprouting behaviour. Key words: Sorghum bicolor, abscisic acid, pre-harvest sprouting, seed development, germination     

Endogenous ABA content in relation to maturation of somatic embryos in <Emphasis Type="Italic">Tulip</Emphasis>a (L.) ‘Apeldoorn’ cultures

The aim of the present study was to estimate the endogenous abscisic acid (ABA) content in tulip ‘Apeldoorn’ torpedo and mature somatic embryos. Moreover, the effect of exogenous ABA and/or its inhibitor fluridone on somatic embryo maturation and conversion into plantlets was investigated. Torpedo-stage somatic embryos were subcultured on media containing 5 μM of picloram and 1 μM of 6-benzyl-aminopurine (BAP)—control, and combinations of ABA (0 or 10 μM) and/or fluridone (0 or 30 μM) for 1 week. Then, the torpedo embryos were transferred to a maturation medium containing 0.25 μM of α-naphthaleneacetic acid (NAA) and 2.5 μM of BAP, without ABA and fluridone treatment, and cultivated under darkness or light for ten weeks. Endogenous ABA content (first time measured in tulip somatic embryos) was evaluated by ELISA test. The obtained results revealed that the highest level of endogenous ABA, at 17.45 nmol g^?1 dry weight (DW), was recorded in torpedo-stage of tulip embryo development, only after 1 week of ABA treatment, and was nearly 10 times higher in comparison with the control. Simultaneous addition of ABA and fluridone to the medium resulted in the lowering of the ABA concentration to 9.58 nmol g^?1 DW. During ten weeks of maturation of the embryos, the endogenous ABA content in mature tissue of tulip somatic embryo considerably decreased to an amount 0.87–1.33 nmol g^?1 DW (irrespective of ABA and fluridone treatment) and did not differ significantly from control (0.59 nmol g^?1 DW). Exogenous ABA and fluridone significantly decreased the growth value of fresh weight (FW) of the tulip torpedo-shaped and mature embryos under light conditions. Percentage of the DW of the torpedo embryos treated with exogenous ABA was significantly higher (15.43–17.02) in comparison with the control (10.87). Three to three and a half times more malformed mature embryos were noted under light conditions than in darkness, irrespective of ABA and fluridone treatment. The highest percentage of mature embryos forming shoots (conversion) was observed under light conditions in the control and after fluridone treatment (26 and 20%, respectively).     

Seed dormancy in Acer: The role of abscisic acid in the regulation of seed development in Acer platanoides L.

Germinability of isolated embryos from developing fruits of Acer platanoides was high at the earliest developmental stage assessed (90 dpa), but fell subsequently and at seed maturity was very low. These observations showed an inverse correlation with changes in endogenous free abscisic acid (ABA) levels in the embryo, which were low during early ontogeny, but reached maximum levels late in development (150–160 dpa). These observations suggest the possibility that dormancy may be induced during development as a result of ABA accumulation in the embryo, an argument strengthened by the obvious inhibitory effect of added ABA on the germinability of isolated embryos. The cotyledons appear to exert an inhibitory influence on embryo germinability that may result from their free ABA content although the embryonic axis itself possesses an innate dormancy that may reflect its own free ABA content. The increased germinability of isolated embryos resulting form added kinetin serves only to emphasise the complexity of the system and the dangers of simplistic interpretation.The correlation between germinability and ABA content is not complete, however, since much of the reduction in germinability had occurred before any appreciable increase in free ABA levels in the embryo was observed. Indeed the failure of the intact seed to respond to endogenous changes in embryonal ABA levels suggests that even though free ABA in the embryo may influence embryo germinability, it has little effect in the intact seed, where the presence of an intact testa may be a more important factor.The absence of a desiccation phase in the embryo during the late stages of development suggests that the large increases in endogenous free ABA did not cause dormancy by inhibiting water uptake, nor did they result from water stress in the embryonal tissues.     

Premature Drying, Fluridone-Treatment, and Embryo Isolation During Development of Maize Kernels (Zea mays L.) Induce Germination, but the Protein Synthetic Responses are Different. Potential Regulation of Germination and Protein Synthesis by Abscisic Acid

Freshly harvested, developing kernels of maize (Zea mays L.)do not germinate up to 77 d after pollination, but can be inducedto do so by fluridone, premature desiccation, and isolationof the developing embryo. The pattern of protein synthesis indeveloping maize embryos is distinct from that during germinationand subsequent seedling growth. Premature desiccation at 35DAP elicits a pattern of protein synthesis upon rehydrationwhich is similar to that in germinated embryos from mature drykernels. Fluridone-induced viviparous germination is accompaniedby changes in the synthesis of some proteins to a post-germinativepattern, but some developmental proteins continue to be synthesized.Embryos isolated from developing kernels at 35 DAP germinatewhen incubated on water; they also produce some developmentalproteins during germination. Kernels from developing cobs at35 DAP which are detached from the mother plant and maintainedin an atmosphere of high relative humidity (moist controls)do not germinate, but neither do they continue a clearly definedpattern of either developmental or germinative protein synthesis.Drying is thus critical to effect a clear transition of proteinsynthesis from a developmental to a germinative mode in maizeembryos. Abscisic acid within the developing embryos is reduced by fluridone,but to a lesser extent by premature drying or maturation drying.Changes in sensitivity to abscisic acid by the developing embryomay be as, or more, important in permitting germination, andthe attendant synthesis of proteins, than changes in abscisicacid content. Key words: Maize (Zea mays L.), germination, vivipary, desiccation, abscisic acid