Changes In Dormancy Research Articles

Changes in dormancy of Sisymbrium officinaleseeds do not depend on changes in respiratory activity

Effects of dark incubation at different temperatures were studied on dormancy and respiratory activity of seeds of Sisymbrium officinale (L.) Scop. Because germination of this species absolutely depends on the simultaneous action of light and nitrate, changes in dormancy could be studied in darkness without the interference of early germination events. Upon the start of incubation rates of O2 uptake and CO2 release rose. This was followed by a gradual decrease until stable levels of O2 uptake and CO2 release were achieved. Seeds kept for prolonged periods at 24°C, showed neither a change in germination capacity nor in rates of O2 uptake and CO2 release. Respiratory quotients were 0.55–0.7. The initial rise in O2 uptake correlated with the rate of water uptake and with breaking of primary dormancy. However, the subsequent decline in O2 uptake was not generally linked to induction of secondary dormancy. An increased O2 uptake was not required during breaking of secondary dormancy. It is concluded that changes in dormancy are not generally related to changes in respiratory activity. However, germination strongly depends on respiration. The increase in O2 uptake started well before radicle protrusion. A far red irradiation only reversed this increase when it was given before germination escaped from its red light antagonising action. The contribution of different respiratory pathways was followed during prolonged incubation at 24°C in darkness. KCN at 1.5 mM was needed to inhibit the cytochrome pathway (CP) and benzohydroxamic acid (BHAM) at 30 mM to inhibit the alternative pathway (AP). These concentrations did not exert any side effects. Electron flow was predominantly via the CP, maximally 10% was via the AP. Flow through the CP declined during the first 6 days and residual respiration remained constant. Therefore, the contribution of residual respiration became relatively more important with prolonged incubation. KCN at concentrations that almost completely inhibited flow through the CP, did not dramatically reduce germination. BHAM already inhibited germination at concentrations that do not inhibit oxygen uptake.

Seasonal Periodicity in Germination of Seeds of Chenopodium album L.

Seeds of Chenopodium album L. were buried under field and controlled conditions. The germination capacity of these seeds was tested over a range of conditions at regular intervals. Seed buried in the field only showed small seasonal changes in germination capacity when tested at constant temperatures in incubators. However, when germination was tested at field temperatures, seasonal changes in germination were more obvious. Nitrate and light always promoted germination. There was a strong positive interaction between the effects of the two factors. When nitrate and light were combined, exhumed seeds germinated over a much longer period of the year than in water with or without light. Desiccation only stimulated under particular conditions, for example, when germination was tested in nitrate in darkness. A regression model was developed with the data from the germination tests in incubators. The model describes the changes in dormancy and germination and estimates germination at field temperatures accurately throughout the year. Despite the absence of clear seasonal changes in the temperatures suitable for germination (computed with the model), germination in the field showed seasonal periodicity, because the field temperature and the germination-temperature range only overlapped from spring to late summer.

Heat Tolerance, Cold Hardiness, and Bud Dormancy Relationships in Seedlings of Selected Conifers

Greenhouse-cultured, container-grown seedlings of interior Douglas fir [Pseudotsuga menziesii var. glauca (Beissn.) France], Engelmann spruce [Picea engelmannii (Parry) Engelm.], and ponderosa pine (Pinus ponderosa var. scopulorum Engelm.) were acclimated and deacclimated to cold in growth chambers over 19 weeks. Heat tolerance and cold hardiness of needles, and bud dormancy, were measured weekly. Heat tolerance of Douglas fir and Engelmann spruce needles increased with development through the first complete annual cycle: new needles on actively growing plants; mature needles, not cold-hardy, on dormant plants; cold-hardy needles on dormant and quiescent plants; and mature, needles, not cold-hardy, on actively growing plants. Heat tolerance of ponderosa pine needles differed in two respects. New needles had an intermediate tolerance level to heat, and fully cold-hardy needles were the least tolerant. Thus, the physiological changes that conferred cold hardiness were not associated with greater heat tolerance in all the conifers tested. In none of these species did the timing of changes in heat tolerance coincide consistently with changes in cold hardiness or bud dormancy.

Effects of light and temperature on seed dormancy and gibberellin‐stimulated germination in Arabidopsis thaliana: studies with gibberellin‐deficient and ‐insensitive mutants.

Effects of light and temperature on gibberellin (GA)‐induced seed germination were studied in Arabidopsis thaliana (L.) Heynh. with the use of GA‐deficient (gal) mutants, mutants with a strongly reduced sensitivity to GA (gai) and with the recombinant gai/gal. Seeds of the gal mutant did not germinate in the absence of exogenous GAs, neither in darkness, nor in light, indicating that GAs are absolutely required for germination of this species. Wild‐type and gai seeds did not always require applied GAs in light. The conclusion that light stimulates GA biosynthesis was strengthened by the antagonistic action of tetcyclacis, an inhibitor of GA biosynthesis. In wild‐type, gal and gai/gal seeds light lowered the GA requirement, which can be interpreted as an increase in sensitivity to GAs. In gai and gai/gal seeds light became effective only after dormancy was broken by either a chilling treatment of one week or a dry after‐ripening period at 2°C during some months. The present genetic and physiological evidence strongly suggests that temperature regulates the responsiveness to light in A. thaliana seeds. The responsiveness increases during dormancy breaking, whereas the opposite occurs during induction of dormancy (8 days at 15°C pre‐incubation). Since light stimulates the synthesis of GAs as well as the responsiveness to GAs, temperature‐induced changes in dormancy may indirectly change the capacities to synthesize GAs and to respond to GAs. GA sensitivity is also directly controlled by temperature. It is concluded that both GA biosynthesis and sensitivity to GAs are not the primary controlling factors in dormancy, but are essential for germination.

Changing sensitivity to light and nitrate but not to gibberellins regulates seasonal dormancy patterns in Sisymbrium officinale seeds

ABSTRACTSeeds of Sisymbrium officinale (L.) Scop, that are buried under natural conditions in soil pass annually through a seasonal pattern of changes in dormancy. Dormancy is broken in autumn‐winter and re‐induced in summer. To elucidate dormancy regulation in this species under natural conditions, a detailed analysis of the changes in sensitivity to some relevant germination factors was carried out Germination data fitted as logistic dose response curves showed that sensitivity to light and nitrate, both indispensable stimuli for germination of this species, varied with the seasons. Patterns of shifts in requirement for light and nitrate were remarkably similar. Sensitivity increased when both primary and secondary dormancy were alleviated, and it was reversed during induction of secondary dormancy. During alleviation of primary dormancy in spring 1991, the fluence response curves exhibited a biphasic character with responses occurring both in the very‐low‐fluence‐range and in the low‐fluence‐range. The nitrate dose response data could all be fitted as monophasic curves, although responses might have occurred in two distinct ranges as well. From interpretation of curve parameters, it is postulated that dormancy is regulated by changes in the number of phytochrome and nitrate receptors, in shifts in the binding characteristics of the receptors and/or in shifts in the response chain initiated by the ligand‐receptor interaction. Somewhere in this response chain, biosynthesis of gibberellins (GAs) is stimulated. By use of the GA biosynthesis inhibitor tetcyclasis, it was indirectly proven that the capacity to synthesize GAs indeed varied with the seasons. Sensitivity to GAs gradually increased from burial onwards and was not particularly related to changes in dormancy. Thus, except for the first few months of burial, GA sensitivity may not be regarded as a limiting factor in controlling dormancy in this species.

Annual changes in dormancy and germination in seeds of Sisymbrium officinale (L.) Scop.

SUMMARYThe effect of environmental conditions on changes in dormancy and germination of seeds of Sisymbrium officinale (L.) Scop, was studied. Seeds were buried in soil in the field or in incubators and at regular intervals, germination of exhumed seeds was tested over a range of conditions. Seeds that were buried in the field showed clear seasonal changes in dormancy. Fluctuations in soil moisture and nitrate content were not required for the changes in dormancy. Temperature seemed to be the only factor regulating these changes. Dormancy was relieved in periods of low temperatures and induced in periods of high temperatures. Fresh seeds and seeds buried for only a few months germinated best at elevated temperatures whereas seeds buried for a longer time germinated best at low‐temperatures. Light, nitrate and desiccation stimulated germination of exhumed seeds. As a consequence, these treatments extended germination over a much longer period of the year. Germination of exhumed seeds in Petri dishes at field temperature was reasonably described by a model based on the dual role of temperature, in the regulation of both dormancy and subsequent germination.

The effect of environmental conditions on the annual dormancy pattern of seeds of Spergula arvensis

The effect of environmental factors on the germination of exhumed seeds of Spergula arvensis L. after variable periods of burial in soil was investigated. Seeds were buried in the field and exhumed at regular intervals after which germination was tested over a range of conditions. These tests showed clear seasonal changes in dormancy during 3 successive years. Dormancy was broken in spring and reinduced in autumn at rising and falling temperatures, respectively. In experiments in incubators, greater loss of dormancy occurred at 10 and 15 °C than at 2 and 6 °C. As in the field, induction of dormancy occurred when the preincubation temperature was lowered. The expression of the dormancy pattern was strongly influenced by the germination tests conditions. At 15 °C, seeds germinated during a longer period of the year than at 2 or 30 °C. Irradiation with red light, addition of nitrate, and desiccation of the seeds prior to the germination test strongly stimulated germination. All three factors enabled germination of exhumed seeds during a longer period of the year. When light, nitrate, and desiccation were combined, exhumed seeds could germinate in all seasons. The seasonal germination pattern was modelled with a descriptive model based on the dual effect of temperature, which regulates dormancy and also affects germination. This model closely simulated germination at field temperatures. Germination of exhumed seeds in the field was restricted to the period of overlap between the germination temperature range computed with the model and field temperature. The features of the model supported the hypothesis that dormancy of S. arvensis is regulated by the actual changes in temperature rather than the combined effects of a cold and heat sum. Key words: Spergula arvensis, dormancy pattern, germination, regression model, weed seeds.

Seed dormancy and germination: the role of abscisic acid and gibberellins and the importance of hormone mutants

Over the past decades many studies have aimed at elucidating the regulation of seed dormancy and germination. Many hypotheses have been proposed and rejected but the regulatory principle behind changes in dormancy and induction of germination is still a ‘black’ box. The majority of proposed mechanisms have a role for certain plant hormones in common. Abscisic acid and the gibberellins are the hormones most frequently suggested to control these processes. The development of hormone-deficient mutants made it possible to provide direct evidence for the involvement of hormones in germination and dormancy related processes. In the present paper an attempt is made to assess the role of abscisic acid and gibberellins in the transitions between dormant and non-dormant states and germination. First a conceptual framework is presented in which the different states of dormancy and germination are defined in order to contribute to a solution of the semantic confusion about these terms that has existed since the beginning of seed physiology. It is concluded that abscisic acid plays a pivotal role during the development of primary dormancy and gibberellins are involved in the induction of germination. Changes in sensitivity to these hormones occur during changes in dormancy. Both synthesis of and responsiveness to the hormones are controlled by natural environmental factors such as light, temperature and nitrate.

The dual role of temperature in the regulation of the seasonal changes in dormancy and germination of seeds of Polygonum persicaria L.

The role of temperature in the regulation of seasonal changes in dormancy and germination was studied in seeds of Polygonum persicaria. Seeds were buried in the field and under controlled conditions. Portions of seeds were exhumed at regular intervals and germination was tested over a range of conditions. Seeds of P. persicaria exhibited a seasonal dormancy pattern that clearly showed the typical features of summer annuals, i.e. dormancy was relieved at low winter temperatures, the germination peak occurred in spring and dormancy was re-induced in summer. The expression of the dormancy pattern was influenced by the temperature at which germination was tested. At 30°C exhumed seeds germinated over a much longer period of the year than at 20° or 10°C. Nitrate added during the germination test occasionally stimulated germination. The seasonal changes in dormancy of buried seeds were regulated by the field temperature. Soil moisture and nitrate content did not influence the changes in dormancy. The fact that, on the one hand, field temperature determined the changes in dormancy and, on the other hand, germination itself was influenced by temperature, was used to describe the seasonal germination pattern of P. persicaria with a model. Germination of exhumed seeds in Petri dishes at field temperature was accurately described with this model. Germination in the field was restricted to the period where the range of temperatures over which germination could proceed (computed with the model) and field temperature overlapped.

Dormancy, Germination and Mortality of Seeds in a Chalk-Grassland Flora

SUMMARY (1) Seeds of thirteen herbaceous species from chalk grassland in The Netherlands were buried in their natural habitat. Samples were exhumed at regular intervals over 3 years. Mortality and germination in situ were recorded. The surviving seeds were tested for germination in light and darkness and with different temperature regimes. (2) Seed mortality was low, except in Gentianella germanica and Origanum vulgare. In Carlina vulgaris, Leontodon hispidus and Scabiosa columbaria disappearance of seeds was mainly caused by germination in the soil. (3) Freshly harvested seeds of Arenaria serpyllifolia and Linum catharticum had an absolute light requirement for germination which was retained during burial. Burial induced a light requirement for germination in Origanum vulgare. (4) All species that germinated showed some degree of primary dormancy and seasonal change in secondary dormancy. This was most pronounced in shortlived species. (5) The seasonal change in secondary dormancy of surviving seeds was most evident either from germinability in light (Arenaria, Daucus carota and Linum), in darkness ( Carlina, Leontodon, Plantago lanceolata and Scabiosa), or both (Centaurea scabiosa). The response of the seeds to temperature also varied seasonally. (6) The amplitude of seasonal change in dormancy tended to decrease during the 3-year of burial in some species (Arenaria, Linum and Daucus). (7) Rhinanthus minor and R. alectorolophus seeds only germinated in late winter and were dormant during the remainder of the year.

The Role of Abscisic Acid in Heat Stress-induced Secondary Dormancy in Apple Seeds

Exposure of stratified apple (Malus domestics Borkh. cv. Golden Delicious) seeds to 30C induces secondary dormancy. To determine if an increase in abscisic acid (ABA) content was associated with the loss in germination capacity, stratified seeds (3,- 6, or 9 weeks at 5C) were held at 30C for 0, 3, or 6 days. Stratification at 5C either had no effect or increased ABA content in embryonic axes, cotyledons, and seed coats. Exposure to 30C after stratification either did not affect or decreased ABA content of embryonic axes and seed coats; in contrast, cotyledonary ABA was increased. Seed coats, cotyledons, and embryonic axes stratified for 3, 6, or 9 weeks at 20C contained the same or higher levels of ABA in comparison with nonstratified seeds or seeds stratified at SC. Changes in ABA levels were not consistently correlated with changes in germination capacity during stratification or after exposure to 30C. These data suggest that changes in ABA are not related to changes in dormancy. Chemical names used: abscisic acid (ABA); butylated hydroxy-toluene (BHT); n-(trichloromethyl) thio-4-cyclohexene-1,2-dicarboximide(Captan).

Dormancy patterns in buried seeds of Datura ferox and D. stramonium

Seasonal changes in seed dormancy of Datura ferox L. and D. stramonium L. were studied during 2 years in seeds buried in the field and compared with changes in dormancy of dry-stored seeds. Dormancy levels were evaluated by measuring the germination of exhumed and dry-stored seeds under 32:27 °C (nondormant seeds) or 32:12 °C (relatively dormant seeds). Levels of relative or full dormancy did not change during 2 years of dry storage in either species. Seed burial at 20 cm accelerated after-ripening, and primary dormancy disappeared earlier than at 5 cm. However, burial depths greater than 10 cm prevented germination in situ. This inhibition was not due to smaller daily fluctuations of soil temperature with increasing depth, since seeds buried at 20 cm and kept at 32:12 °C also failed to germinate. Transferring seeds to shallower soil layers allowed their germination. Cyclic seasonal changes in dormancy were found in buried seeds of D. ferox but not in D. stramonium. In D. ferox, dormancy was alleviated during the winter and germinability was maximal in early spring; the increase in dormancy was preceded by high late spring – summer temperatures. In D. stramonium most seeds near the soil surface germinated in the field in a brief flush in early winter, at temperatures under which D. ferox did not germinate. Key words: Datura, germination, dormancy, buried seeds, temperature.

HEAT STRESS TOLERANCE AND SEASONAL DEVELOPMENT IN CONIFER SEEDLINGS

It is not known when changes in primary direct heat stress tolerance of conifer seedlings occur in relation to other seasonally changing physiological parameters. This information should be incorporated into nursery practices and the matching of genotypes to landscape sites. Greenhouse-cultured, container-grown Douglas-fir, Engelmann spruce, and ponderosa pine. were cold acclimated and reacclimated in growth chambers over 19 weeks. Direct heat stress tolerance of needles, cold hardiness, and bud dormancy were measured weekly. Douglas-fir and Engelmann spruce heat stress tolerance increased with the development of new growth through one complete growth cycle, i.e., bud break, maturation, cold hardening, dehardening, and bud break the following growing season. Ponderosa pine differed in that new needles had intermediate tolerance, and fully cold hardy needles were the most intolerant. In none of the species did the timing of changes in heat stress tolerance coincide consistently with changes in cold hardiness or bud dormancy.

The Fate of Yellow Nutsedge (Cyperus Esculentus) Seed and Seedlings in Soil

Changes in dormancy and viability of freshly harvested seed of yellow nutsedge buried in the field, at depths of 5, 25, and 50 mm, were studied over a 2-yr period. The number of dormant, viable seed decreased most rapidly at 5 mm because more seed lost their dormancy and germinated (up to 30%) than at lower depths. Loss of viability through decay of seed appeared similar at all depths. No seedlings emerged from any depth. In a laboratory experiment, conducted in optimal soil moisture conditions, seedlings emerged from all seed that germinated, demonstrating that adequate soil moisture was critical for seedling establishment. This was substantiated in a second field experiment in which irrigation and mulching greatly increased seedling survival. The number of seedlings surviving in irrigated plots was 0.78% of seed sown and 0.03% in the rain-fed plots. The role of seed in the establishment of yellow nutsedge infestations is probably of little importance in dryland cropping areas despite the longevity and viability of the seed.

Dormancy, germination and mortality of seeds in heathland and inland sand dunes

SUMMARY Seeds of seven dominant species from heathland and inland sand dunes were buried in their natural habitat and exhumed twice a year, in March and September, during a 2- or 3-year period. The number of dead, germinated and surviving seeds was recorded and the latter were subjected to a germination test in a fluctuating temperature regime of 12/22°C in light. All the seeds of Deschampsia flexuosa had germinated in situ within 6 months of burial. This was also the case for almost all seeds of Agrostis vinealis and the majority of the seeds of Corynephorus canescens; the remaining seeds survived until the end of the experiment (2 years). About 35% of the seeds of Molinia caerulea died or germinated in situ; the remaining seeds survived until the end of the experiment (3 years) and showed a seasonal pattern of changing dormancy that has been described for many summer annuals. Dormancy in the soil is enforced by a requirement for temperature fluctuation. The winter annual Spergula morisonii showed a dormancy pattern which was typical for that life form, i.e. alleviation of dormancy in summer and induction of dormancy during winter. The two dwarf shrubs Calluna vulgaris and Erica tetralix had an absolute light requirement for germination and no seeds germinated in the soil during the 3 years; the mortality of C. vulgaris was low, whereas 40% of the E. tetralix seeds died. No seasonal change in dormancy was detected in either species.

Study of seasonal variation in dormancy of Spergula arvensis L. seeds in a condensed annual temperature cycle

SummaryChanges in dormancy of Spergula arvensis seeds were studied during pre‐incubation at constant temperatures and under a temperature regime that condensed the annual temperature cycle into 73 days. Each day in the regime represented the mean day and night temperatures and day lengths of 5 successive days of an average year in The Netherlands. Incubation occurred in water or loamy sand, in darkness. Germination of the seeds was tested in water or KNO3 over a range of temperature. Seeds were irradiated with saturating doses of red light. In half of the treatments, pre‐incubated seeds were dehydrated at the transfer to the conditions of the germination test. Breaking of dormancy occurred under conditions of 'spring'. It did not depend on exposure to low‘winter’temperatures, but was induced by rising 'spring’temperatures. Seeds developed secondary dormancy in late‘autumn'. The expression of the changes in dormancy that were induced during pre‐incubation depended on the conditions of the germination test. Light, nitrate and dehydration stimulated germination. The experiments predicted that field emergence from nitrate‐poor soils that have not been dehydrated will be restricted to a short period in autumn, whereas disturbance of nitrate‐rich soils followed by a dry spell will stimulate germination of S. arvensis seeds from early spring to late autumn. The data presented good explanations for the cosmopolitan character and the serious weediness of this species. Its classification as a summer or winter annual is discussed.

Changes in Loblolly Pine Seedling Root Growth Potential, Dry Weight, and Dormancy During Cold Storage

Abstract Loblolly pine (Pinus taeda L.) seedlings from a Virginia nursery were lifted once per month between October and February during the 1983 and 1984 lifting seasons and held in cold storage at 2°C for 3, 6, 9, and 12 weeks. Following cold storage, root growth potential (RGP) and root and shoot characteristics were compared to freshly lifted seedlings. New root growth was similar despite significant differences in dry weight and shoot activity between cold-stored and freshly lifted seedlings. Seedlings placed in cold storage prior to the accumulation of 500 chilling hours in the nursery stored poorly. Cold storage success of seedlings in the spring was variable. For. Sci. 34(1):41-54.

EVIDENCE FOR AN ENDOGENOUS CYCLE OF DORMANCY IN DRY STORED SEEDS OF POA TRIVIALIS L.

Seeds of Poa trivialis L. (rough meadow-grass) were collected from two contrasting habitats during the summer of 1982. Collections from an arable population were obtained on three occasions and seeds from a grassland population were collected once. Although both populations responded similarly to various combinations of light, temperature and nitrate, the arable collections were markedly more dormant. Loss of dormancy of the grassland and early arable populations one month after collection was rapid in seeds stored at ambient temperatures and at 4 °C but more delayed in those stored at 23 °C. Despite a gradual loss of dormancy during storage, cyclic changes in dormancy were also evident, especially at 23 °C. The percentage germination of the various collections 7 d after incubation following storage at 23 °C increased over the first three months, but declined following six and 12 months storage and increased again after 15 months storage. At 4 °C the late arable collection lost dormancy more slowly than the other two collections and germination was greatest after six months storage; a reduction in percentage germination occurred after twelve months. Seeds buried at 5 and 15 cm showed cyclic changes of dormancy, being least dormant in autumn and spring, but most dormant in summer, corresponding to the natural periodicity of germination. It is proposed that cyclic changes in dormancy are regulated by endogenous rhythms and seasonal changes of temperature. The ecological implications of the germination behaviour of this species in various habitats are discussed.

Evolutionary Ecology of Seed-Bank Annuals in Temporally Varying Environments

The production of long-lived seeds by annual plants introduces a unique form of age structure. In a temporally varying environment the dormant seed may experience many years with different weather, whereas the germinating individual experiences only the weather conditions of a single growing season. Natural selection operates on both the between-year dormancy and on non-seed-bank traits that affect the degree of specialization to conditions pertaining in different year types. We have used an integrated model that permits these two aspects of the life history to evolve simultaneously. This leads to predictions that are not attainable by considering the evolution of each in isolation. Changes in the survival probability of the between-year seed bank select for reinforcing changes in between-year dormancy and specialization. Changes in the probability of occurrence of different year types select for damping changes in between-year dormancy and specialization. Across a gradient in environmental quality, we predict that most change will occur in between-year dormancy, with little change in specialization. If between-year dormancy is fixed, however, a greater change in specialization should occur. The predictions of the model are discussed in terms of environmental gradients, seed-bank versus non-seed-bank annuals, and a variety of plant traits modeled elsewhere that may be involved in specialization to different types of years.

Regulation of Pregerminative Rates of Respiration in Seeds of Various Weed Species by Potassium Nitrate

Regulation of Pregerminative Rates of Respiration in Seeds of Various Weed Species by Potassium Nitrate