Hypoxia interferes with ABA metabolism and increases ABA sensitivity in embryos of dormant barley grains

Two mechanisms have been suggested as being responsible for dormancy in barley grain: (i) ABA in the embryo, and (ii) limitation of oxygen supply to the embryo by oxygen fixation as a result of the oxidation of phenolic compounds in the glumellae. The aim of the present work was to investigate whether hypoxia imposed by the glumellae interferes with ABA metabolism in the embryo, thus resulting in dormancy. In dormant and non-dormant grains incubated at 20 degrees C and in non-dormant grains incubated at 30 degrees C (i.e. when dormancy is not expressed), ABA content in the embryo decreased dramatically during the first 5 h of incubation before germination was detected. By contrast, germination of dormant grains was less than 2% within 48 h at 30 degrees C and embryo ABA content increased during the first hours of incubation and then remained 2-4 times higher than in embryos from grains in which dormancy was not expressed. Removal of the glumellae allowed germination of dormant grains at 30 degrees C and the embryos did not display the initial increase in ABA content. Incubation of de-hulled grains under 5% oxygen to mimic the effect of glumellae, restored the initial increase ABA in content and completely inhibited germination. Incubation of embryos isolated from dormant grains, in the presence of a wide range of ABA concentrations and under various oxygen tensions, revealed that hypoxia increased embryo sensitivity to ABA by 2-fold. This effect was more pronounced at 30 degrees C than at 20 degrees C. Furthermore, when embryos from dormant grains were incubated at 30 degrees C in the presence of 10 microM ABA, their endogenous ABA content remained constant after 48 h of incubation under air, while it increased dramatically in embryos incubated under hypoxia, indicating that the apparent increase in embryo ABA responsiveness induced by hypoxia was, in part, mediated by an inability of the embryo to inactivate ABA. Taken together these results suggest that hypoxia, either imposed artificially or by the glumellae, increases embryo sensitivity to ABA and interferes with ABA metabolism.     

Possible involvement of volatile compounds in the after-ripening of cocklebur seeds

The mechanism of emergence from primary dormancy, the process of after-ripening, in cocklebur (Xanthium pennsylvanicum) seeds was examined in relation to the involvement of volatile compounds and to the relative humidity (RH) in which the seeds were stored. The after-ripening of these seeds proceeds only at water contents between 7 and 14% which are conditioned under RHs of 33% to 53% and are identified with water-binding region II. After-ripening of cocklebur seeds occurred even in water-binding region I. imposed by 12% RH. when exposed to HCN gas during the storage period. Exposure of dormant seeds to acetaldehyde (ethanal) retarded after-ripening. even in water-binding region II. thus decreasing germinability. This decrease of germinability by ethanal was found also in the after-ripened seeds, suggesting that ethanal accelerates seed deterioration rather than retarding the after-ripening. The contents of ethanal. ethanal and HCN were high only in the dormant seeds held at 12% RH. Regardless of RH. a possible conversion of ethanal to ethanol. perhaps via alcohol dehydrogenase. was far larger in dormant than in non-dormant seeds. In contrast, the reverse conversion of ethanol to ethanal was more profound in non-dormant seeds. Pre-exposure of both types of seeds to HCN reduced the contents of both ethanal and ethanol at 12% RH. The contents of various adenylales including ATP in seed tissues were higher in dormant seeds stored at 12% RH than in non-dormant seeds after-ripened at 44% RH. It is suggested that emergence of cocklebur seeds from primary dormancy by HCN treatment at 12% RH may result from the reduction in the contents of ethanal via an unknown mechanism incurring the consumption of ATP. This implies involvement of volatile compound metabolism at the water-binding region II in the after-ripening process of cocklebur seeds.     

Ethylene as a Component of the Emanations From Germinating Peanut Seeds and Its Effect on Dormant Virginia-type Seeds

The embryonic axes of Spanish-type peanut seeds that do not exhibit dormancy to any extent were found to produce ethylene during germination. Virginia-type peanut seeds of the extremely dormant variety NC-13 produced low levels of ethylene when imbibed but not germinating. Treatments that released dormancy of NC-13 peanut seeds resulted in increased ethylene production by the embryonic axis. The estimated internal concentration of ethylene in Virginia-type peanut seeds was 0.4 ppm at 24 hr of germination. Fumigation with an external concentration of 3.0 to 3.5 ppm for 6 hr was sufficient to break dormancy of Virginia-type peanut seeds. These results suggest that ethylene is associated with the germination processes of non-dormant seeds and participates in the breaking of seed dormancy of dormant peanut varieties.     

Different properties of the inward rectifying potassium conductance of aleurone protoplasts from dormant and non-dormant barley grains

Isogenic dormant and non-dormant barley grains provide a useful system to study the molecular mechanisms of grain dormancy and the role of plant hormones in this process. As ion fluxes are associated with dormancy-related plant hormone responses, we compared the properties of the inward rectifying potassium conductance in aleurone protoplasts isolated from dormant and non-dormant Triumph grains and in germinating Himalaya grains. Maximal conductance, voltage dependency of steady-state activation, activation and deactivation kinetics were studied in the whole-cell patch-clamp configuration. Activation and deactivation time courses were single exponential. No differences in the above described properties were found between the protoplasts isolated from non-dormant Triumph and Himalaya grains. However, the maximal conductance (corrected for cell size) in protoplasts from dormant Triumph grains was much smaller (65%), and activation time constants were much larger as compared to protoplasts from non-dormant grains. No differences were found in the deactivation kinetics in the three different types of protoplasts. The half-maximal activation potential was slightly more negative in protoplasts from dormant grains than from non-dormant grains.     

Abscisic Acid-Like Inhibitors and Cytokinins in the Developing Seeds of Dormant and Non-Dormant Varieties of Peanut (Arachis hypogea L.)

Abscisic acid-like inhibitors and cytokinin activities werestudied in the dormant and non-dormant peanut seeds during theirdevelopment from 20 to 50 d after anthesis. ABA-like inhibitorsincreased continuously from 20 to 50 d of seed development inthe dormant variety whereas in non-dormant seeds they increasedup to 40 d and then declined. Cytokinin-like compounds of boththe varieties increased from 20 to 30 d and then decreased upto maturity. Larger quantities of cytokinins were found at 30d of seed development in both the varieties. At the time ofmaturity dormant seeds contained greater amounts of ABA-likeinhibitors and less cytokinins than the non-dormant variety.     

Clonal dynamics and evolution of dormancy in the leafy hepatic Lophozia silvicola

Dead shoots of colonies of a leafy hepatic species, Lophozia silvicola , are replaced by shoots developing from asexual propagules, the gemmae. Observations of two populations of L. silvicola showed a strong decreasing seasonal trend in germinability of the gemmae. We suggest that the non-germinating gemmae enter dormancy, and that the proportion of gemmae entering dormancy is season-specific. We assume that there are two types of gemmae, dormant and non-dormant and that only the dormant gemmae can survive during winter. Using a stochastic individual-based cellular automaton model, we investigated whether selection on season-specific dormancy fraction would lead to a decreasing proportion of germinating gemmae. Thus the germination schedule is the evolving trait in the model. Parameter estimates for the model were based on data collected from a population of L. silvicola in southern Finland over a three-year study period. In the simulations, the germination schedule shaped by evolutionary change was similar to the observed pattern. Thus the modelling results give support to the dormancy hypothesis. The qualitative pattern of decreasing germinability towards the end of the growing season is robust. Quantitative predictions are influenced by changes in parameters; for example, if winter mortality of shoots increases relative to mortality during the growing season, production of an increased fraction of dormant gemmae is favoured, especially at the end of the season.     

Studies on the Germination of Cereals: I. The Germination of Wheat Grains in the Ear during Development, Ripening, and After-ripening

The ability of individual grains to germinate in the ears ofa red and a white wheat variety has been determined at differentperiods after anthesis, and at different moisture contents,before the stage of full maturity. No grains germinated while active growth was taking place, butafter desiccation during ripening, 88·5 per cent. ofthe white grains and 7 per cent. of the red grains were ableto germinate in the ear; the percentage germination of the redgrains increased to 83 per cent., when further desiccation occurredduring the first 5 weeks of after-ripening, but some grainsin the basal spikelets of the ears of both varieties failedto germinate until they had been subjected to the same desiccationfrom 13 to 23 weeks after anthesis. The ability of the grains to germinate has been correlated withtheir desiccation at different stages during maturation, andthe effect of certain factors, which inhibit the germinationof immature grains, are discussed in relation to varietal differencesin the colour of the grains and their position in the ear.     

Dormancy of Paddy Seeds of a 'Non-dormant' Variety

Seeds of a non-dormant rice variety, Satika, were studied underdifferent natural and controlled humid conditions of storageat room temperature. Just after harvest the germination andpercentages of grain moisture were 9.2 and 30.0 respectively.Under humid conditions of storage (94 per cent) there was onlypartial break of dormancy after 42 d (32.8 per cent ger mination)with high grain moisture (23.8 per cent); in all other treatmentswhere the grain moisture varied from 4.4 to 16.4 per cent thegermination percentages were over 96 per cent after 42 d. This delaying effect of high moisture contents on the breakingof dormancy became very pronounced at about 26 per cent. Itis suggested that high moisture content may inhibit dormancy-breakingreactions possibly by interrupting air-diffusion inside theseeds.     

Variation between Emex australis populations in seed dormancy/non-dormancy cycles

Abstract Seeds of four Western Australian accessions of Emex australis were stored outside and their germinability tested at 4–6 week intervals for 22 months. Accessions showed cyclical behaviour in germinability, with peaks in autumn/early winter and troughs in spring/summer. There was considerable variation between accessions in dormancy/non-dormancy cycles, but when plants from accessions with contrasting cycles were grown in a common environment, the cycles shown by their seeds were virtually identical. Thus, the parameters of seed dormancy/non-dormancy cycles in E. australis appear to be under environmental control. Only part of an E. australis seed population demonstrated cyclical changes in dormancy status, in contrast to many other annual species whose entire seed populations undergo such changes. Populations also contained individuals which were either continuously dormant or continuously non-dormant following a period of after-ripening. The latter seeds give E. australis the flexibility to recruit opportunistically after summer rainfall events in mediterranean-climate environments.     

Growth,sugar accumulation,and dark respiration of suspension cell cultures derived from contrasting alfalfa cultivars

Fall dormancy results in decumbent, slow shoot growth of alfalfa (Medicago sativa L.) in autumn and reduced shoot regrowth rates after herbage removal in summer. Although fall dormancy is used to predict alfalfa adaptation, we possess a poor understanding of the biological mechanisms underlying fall dormancy. Our objective was to examine growth and carbohydrate metabolism of suspension cell cultures derived from contrasting alfalfa cultivars that genetically differed in fall dormancy. Suspension cells were grown in B5h media containing 2% sucrose. Cells derived from fall non-dormant plants accumulated sugars more rapidly after transfer to fresh media and to higher concentrations than did cells derived from fall dormant alfalfa cultivars. Dark respiration rates of cells derived from non-dormant plants were similar to those derived from fall dormant plants when growth was limited at low cell sugar concentrations. However, both cell growth and dark respiration rates increased in cells derived from non-dormant cultivars in response to greater cell sugar concentrations. High growth rates of cells derived from rapid growing, fall non-dormant alfalfa cultivars were associated with rapid sugar uptake and higher cell respiration rates when compared to cells derived from dormant alfalfa cultivars.     

Grain dormancy,peroxidase activity and oxygen uptake in Oryza sativa

In developing grains of rice (Oryza sativa L.) of the dormant variety H4, peroxidase activity decreased sharply about a week before grain maturity without any change in grain dormancy and oxygen uptake of intact grain. During storage or after-ripening of mature dormant intact grains of four varieties (H4, H6, Mayang Ebos and Seraup 27) at 25–30°, the critical range in peroxidase activity was 1·0–1·4 μmol purpurogallin/hr/grain above which rice grains were almost completely dormant and below which the grains were almost completely nondormant. The oxygen uptake of intact H4 grain tended to decrease during the loss of dormancy. The decrease in both the peroxidase activity and oxygen uptake could be attributed mainly to the lower activities of the hull. Dehulling of developing and mature H4 grains reduced dormancy and increased the oxygen uptake of the grain. Thus, reduction by the hull of the level of oxygen available to the dehulled grain (embryo) was mainly responsible for grain dormancy in rice.     

Role of reactive oxygen species in the regulation of Arabidopsis seed dormancy

Freshly harvested seeds of Arabidopsis thaliana, Columbia (Col) accession were dormant when imbibed at 25°C in the dark. Their dormancy was alleviated by continuous light during imbibition or by 5 weeks of storage at 20°C (after-ripening). We investigated the possible role of reactive oxygen species (ROS) in the regulation of Col seed dormancy. After 24 h of imbibition at 25°C, non-dormant seeds produced more ROS than dormant seeds, and their catalase activity was lower. In situ ROS localization revealed that germination was associated with an accumulation of superoxide and hydrogen peroxide in the radicle. ROS production was temporally and spatially regulated: ROS were first localized within the cytoplasm upon imbibition of non-dormant seeds, then in the nucleus and finally in the cell wall, which suggests that ROS play different roles during germination. Imbibition of dormant and non-dormant seeds in the presence of ROS scavengers or donors, which inhibited or stimulated germination, respectively, confirmed the role of ROS in germination. Freshly harvested seeds of the mutants defective in catalase (cat2-1) and vitamin E (vte1-1) did not display dormancy; however, seeds of the NADPH oxidase mutants (rbohD) were deeply dormant. Expression of a set of genes related to dormancy upon imbibition in the cat2-1 and vet1-1 seeds revealed that their non-dormant phenotype was probably not related to ABA or gibberellin metabolism, but suggested that ROS could trigger germination through gibberellin signaling activation.     

The development of dormancy in bulblets of Lilium speciosum generated in vitro. II. The effect of temperature

Upon harvest, lily ( Lilium speciosum Thunb. cv. Rubrum) bulblets generated in vitro under standard conditions (11 weeks at 20°C) were dormant and needed a cold treatment prior to planting. During culture in vitro at 20°C, the bulblets proceeded through three phases: (1) at first they were non–viable and non-dormant (up to 5 weeks), (2) then viable and non-dormant (5–9 weeks) and (3) finally viable and dormant (from 9 weeks onwards). At 15°C, the bulblets became viable but did not develop dormancy, even after protracted culture. The results suggest that the development of dormancy depends upon an accumulation of'heat units'occurring at temperatures higher than 15°0. At 25°C, the succession of the three phases occurred more rapidly than at 20°C and heat units were accumulated more rapidly. During the third period, the chilling requirement increased showing that heat units continued to be accumulated during this period.
Dormancy connotes an arrest of growth. In lily bulblets, however, the number of scales continued to increase after the induction of dormancy at 20 or 25°C. Many of the scales initiated before the onset of dormancy were formed by swelling of a petiole, whereas, after the onset of dormancy, all scales were formed directly from a primordium. We conclude that the development of dormancy corresponds to a switch in the development of the primordium. Thus, after the induction of dormancy the primordium lost the ability to become a leaf and always developed into a scale.     

Involvement of endogenous gibberellins in potato tuber dormancy and early sprout growth: a critical assessment

The role of endogenous gibberellins (GAs) in the regulation of potato (Solanum tuberosum) tuber dormancy was examined by determining: 1. changes in endogenous GA levels during natural dormancy progression, 2. the effects of GA biosynthesis inhibitors on tuber dormancy duration and 3. the dormancy status and tuber GA levels in a dwarf mutant of potato. The tubers (cv. Russet Burbank) used in these studies were still completely dormant after 98 days of storage. Between 98 and 134 days of storage, dormancy began to end and tubers exhibited limited (< 2 mm) sprout growth. Tuber dormancy weakened with further storage and tubers exhibited greater rates of sprout growth after 187 days of storage. Tubers stored for 212 days or longer were completely non-dormant and exhibited vigorous sprout growth. Immediately after harvest, the endogenous contents of GA19, GA20, and GA1 were relatively high (0.48-0.62 ng g fresh weight(-1)). The content of these GAs declined between 33 and 93 days of storage. Internal levels of GA19, GA20, and GA, rose slightly between 93 and 135 days of storage reaching levels comparable to those found in highly dormant tubers immediately after harvest. Levels of GA19, GA20, and GA1 continued to increase as sprout growth became more vigorous. Neither GA4 nor GA8 was detected in any tuber sample regardless of dormancy status. Dormant tubers exhibited a time-dependent increase in apparent GA sensitivity. Freshly harvested tubers were completely insensitive to exogenous GAs. As postharvest storage continued, exogenous GAs promoted premature dormancy release with GA1 and GA20 eliciting the greatest response. Injection of up to 5 microg tuber(-1) of kaurene, GA12, GA19 or GA8 had no effect on dormancy release. Sprout growth from non-dormant tubers was also promoted by exogenous GA in the following sequence of activity: GA1 = GA20 > GA19. Kaurene, GA12, and GA8 were inactive. Continuous exposure of developing tubers to inhibitors of GA biosynthesis (AMO-1618, ancymidol, or tetcyclasis) did not extend tuber dormancy but rather hastened dormancy release. Comparison of tuber dormancy and GA1 content in tubers of a wild-type and dwarf mutant of S. tuberosum ssp. andigena revealed a near-identical pattern of dormancy progression in spite of the absence of detectable levels of GA1 in tubers of the dwarf sibling at any time during dormancy progression. Collectively, these results do not support a role for endogenous GA in potato tuber dormancy release but are consistent with a role for GAs in the regulation of subsequent sprout growth.     

Effects of primary seed dormancy on lifetime fitness of Arabidopsis thaliana in the field

Background and AimsSeed dormancy determines the environmental niche of plants in seasonal environments, and has consequences for plant performance that potentially go far beyond the seed and seedling stages. In this study, we examined the cascading effects of seed dormancy on the expression of subsequent life-history traits and fitness in the annual herb Arabidopsis thaliana.MethodsWe planted seeds of >200 recombinant inbred lines (RILs) derived from a cross between two locally adapted populations (Italy and Sweden), and both parental genotypes at the native site of the Swedish population in three consecutive years. We quantified the relationship between primary seed dormancy and the expression of subsequent life-history traits and fitness in the RIL population with path analysis. To examine the effects of differences in dormancy on the relative fitness of the two parental genotypes, we planted dormant seeds during the seed dispersal period and non-dormant seeds during the germination period of the local population.Key ResultsIn the RIL population, strong primary dormancy was associated with high seedling survival, but with low adult survival and fecundity, and path analysis indicated that this could be explained by effects on germination timing, rosette size and flowering start. The relationship between primary seed dormancy and germination proportion varied among years, and this was associated with differences in seasonal changes in soil moisture. The planting of dormant and non-dormant seeds indicated that the lower primary dormancy of the local Swedish genotype contributed to its higher germination proportion in two years and to its higher fecundity in one year.ConclusionsOur results show that seed dormancy affects trait expression and fitness components across the life cycle, and suggest that among-year variation in the incidence of drought during the germination period should be considered when predicting the consequences of climatic change for population growth and evolution.     

Studies on the Germination of Cereals: 3. The Effect of the Covering Layers on the Uptake of Water by the Embryo of the Wheat Grain

The uptake of water by imbibition, vacuolation, and growth ofthe embryo, has been determined when wheat grains were placedunder germination conditions before maturation was completed;and the effect of the covering layers has been investigatedin a red and a white variety at three stages during ripening. The water lost during ripening was replace by imbibition withoutcausing the embryo to expand sufficiently to rupture the coveringlayers. The cells of the coleorhiza and epiblast absorbed waterby vacuolation as soon as they were imbibed; but afurther periodelapsed before the scutellum became imbibed and water uptakewas associated with an increase in embryo dry weight. In both varieties, uptake opf water by the imbibed embryo wasprevented by the inner layer of the pericarp and the tesa inunripe grains, and by the epidermal layer of the pericarp duringthe early staes of ripening. But after the final stage of rapiddesiccation the covering layers of the white grains were rupturedwhen the embryo absorbed water by vacuolation; whereas the coveringlayers of the red grains remained intact and the uptake of waterwas delayed until a sufficient period had elapsed for nutrientsfrom the endosperm to be available for growth ofr the embryo. The varietal difference in germinationj is therefore attributedto the greater strength of the covering layers of the red grains,which are not disrupted during ripening, and prevent expansionof the embryo until the water absorbing capacity has been increasedby the transfer of reserves from the endosperm.     

Dormancy and enzyme levels in seeds of wild oats

Nine enzymes were compared in dry and steeped mature dormant and non-dormant seeds of wild oats. In dry seeds only glutamate-pyruvate transaminase and phosphoglycerate kinase were greater in non-dormant seeds. In steeped non-dormant seeds glucose-6-phosphate dehydrogenase activity doubled while the enzyme declined sharply in dormant seeds. Increases in isocitrate dehydrogenase, glutamate-oxaloacetate transaminase and acid phosphatase in non-dormant seeds, during steeping, are consistent with the hypothesis that the pentose phosphate and glycolysis-tricarboxylic acid pathways are involved in the control of dormancy of wild oat seed.     

Secondary dormancy in Avena fatua: Induction and characteristics in genetically pure dormant lines

The induction of secondary dormancy in caryopses of genetically pure dormant lines of Avena fatua L. is described. Seeds harvested from mature plants were after-ripened under controlled conditions (26°C, 25% relative humidity) until fully non-dormant. Secondary dormancy was then induced into these caryopses by incubation on moist filter papers in an aspirated nitrogen atmosphere at 20°C over periods from 3 h to 14 days. These caryopses failed to germinate when returned to an aerobic environment. The dose-response curves for gibberellic acid, sodium azide, sodium nitrite, sodium nitrate and ethanol show that all of these treatments can overcome the induced secondary dormancy. Drying increased the sensitivity of secondary dormant caryopses to these treatments. These treatments overcame secondary dormancy at all times, indicating the presence of only one of the two known blocks to germination that exist during primary dormancy. Similarities between primary and secondary dormancy in A. fatua are discussed.