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     

Selection for low or high primary dormancy in Lolium rigidum Gaud seeds results in constitutive differences in stress protein expression and peroxidase activity

Seed dormancy in wild Lolium rigidum Gaud (annual ryegrass) populations is highly variable and not well characterized at the biochemical level. To identify some of the determinants of dormancy level in these seeds, the proteomes of subpopulations selected for low and high levels of primary dormancy were compared by two-dimensional polyacrylamide gel electrophoresis of extracts from mature, dry seeds. High-dormancy seeds showed higher expression of small heat shock proteins, enolase, and glyoxalase I than the low-dormancy seeds. The functional relevance of these differences in protein expression was confirmed by the fact that high-dormancy seeds were more tolerant to high temperatures imposed at imbibition and had consistently higher glyoxalase I activity over 0-42 d dark stratification. Higher expression of a putative glutathione peroxidase in low-dormancy seeds was not accompanied by higher activity, but these seeds had a slightly more oxidized glutathione pool and higher total peroxidase activity. Overall, these biochemical and physiological differences suggest that L. rigidum seeds selected for low dormancy are more prepared for rapid germination via peroxidase-mediated cell wall weakening, whilst seeds selected for high dormancy are constitutively prepared to survive environmental stresses, even in the absence of stress during seed development.     

Tolerance of kikuyu grass to long term salt stress is associated with induction of antioxidant defences

Primary dormancy in A. retroflexus seeds wascompletely broken by dry storage or ethylene treatment and partially removedwith GA₃. Norbornadiene counteracted the dormancy breaking action ofethylene and GA₃. The GA₃ effect was lowered bycobaltous ions. ABA increased the ethylene requirement in primary dormant seeds.Dormant seeds had a similar or different ability to produce ethylene and ACCoxidase in vivo activity than did non-dormant seeds,depending on the period of incubation. Dormant seeds contained less endogenousACC than non-dormant seeds. Thus, ethylene seems to play an essential role inthe release of primary dormancy in A. retroflexus seeds.Ethylene also participates in the release of dormancy achieved by GA₃treatment. The results indicate that both ethylene biosynthesis and action isinvolved in the control of primary dormancy in Amaranthusretroflexus seeds.     

Dormancy release during hydrated storage in Lolium rigidum seeds is dependent on temperature, light quality, and hydration status

The influence of temperature, light environment, and seed hydration on the rate of dormancy release in Lolium rigidum (annual ryegrass) seeds during hydrated storage (stratification) was investigated. In a series of experiments, seeds were subjected to a range of temperatures (nine between 5 degrees C and 37 degrees C), light (white, red, far-red, and dark), and hydration (4-70 g H(2)O 100 g(-1) FW) during stratification for up to 80 d. Samples were germinated periodically at 25/15 degrees C or constant 15, 20, or 25 degrees C with a 12 h photoperiod to determine dormancy status. Dark-stratification was an alternative, but not equivalent dormancy release mechanism to dry after-ripening in annual ryegrass seeds. Dormancy release during dark-stratification caused a gradual increase in sensitivity to light, but germination in darkness remained negligible. Germination, but not dormancy release, was greater under fluctuating diurnal temperatures than the respective mean temperatures delivered constantly. Dormancy release rate was a positive linear function of dark-stratification temperature above a base temperature for dormancy release of 6.9 degrees C. Dormancy release at temperatures up to 30 degrees C could be described in terms of thermal dark-stratification time, but the rate of dormancy release was slower at < or =15 degrees C (244 degrees Cd/probit increase in germination) than > or =20 degrees C (208 degrees Cd/probit). Stratification in red or white, but not far-red light, inhibited dormancy release, as did insufficient (<40 g H(2)O 100 g(-1) FW) seed hydration. The influence of dark-stratification on dormancy status in annual ryegrass seeds is discussed in terms of a hypothetical increase in available membrane-bound phytochrome receptors.     

Wheat seed proteins regulated by imbibition independent of dormancy status

Seed dormancy is an important trait in wheat (Trticum aestivum L.) and it can be released by germination-stimulating treatments such as after-ripening. Previously, we identified proteins specifically associated with after-ripening mediated developmental switches of wheat seeds from the state of dormancy to germination. Here, we report seed proteins that exhibited imbibition induced co-regulation in both dormant and after-ripened seeds of wheat, suggesting that the expression of these specific proteins/protein isoforms is not associated with the maintenance or release of seed dormancy in wheat.     

Annual dormancy cycles in buried seeds of shrub species: germination ecology of Sideritis serrata (Labiatae)

The germination ecology of Sideritis serrata was investigated in order to improve ex‐situ propagation techniques and management of their habitat. Specifically, we analysed: (i) influence of temperature, light conditions and seed age on germination patterns; (ii) phenology of germination; (iii) germinative response of buried seeds to seasonal temperature changes; (iv) temperature requirements for induction and breaking of secondary dormancy; (v) ability to form persistent soil seed banks; and (vi) seed bank dynamics. Freshly matured seeds showed conditional physiological dormancy, germinating at low and cool temperatures but not at high ones (28/14 and 32/18 °C). Germination ability increased with time of dry storage, suggesting the existence of non‐deep physiological dormancy. Under unheated shade‐house conditions, germination was concentrated in the first autumn. S. serrata seeds buried and exposed to natural seasonal temperature variations in the shade‐house, exhibited an annual conditional dormancy/non‐dormancy cycle, coming out of conditional dormancy in summer and re‐entering it in winter. Non‐dormant seeds were clearly induced into dormancy when stratified at 5 or 15/4 °C for 8 weeks. Dormant seeds, stratified at 28/14 or 32/18 °C for 16 weeks, became non‐dormant if they were subsequently incubated over a temperature range from 15/4 to 32/18 °C. S. serrata is able to form small persistent soil seed banks. The maximum seed life span in the soil was 4 years, decreasing with burial depth. This is the second report of an annual conditional dormancy/non‐dormancy cycle in seeds of shrub species.     

The viability and germination characteristics of exhumed Solanum mauritianum seeds buried for different periods of time

The site, depth and duration of burial significantly influenced the viability and state of dormancy of Solanum mauritianum seeds. Burial at a depth of 15 cm was most effective in reducing the level of conditional dormancy. Secondary dormancy was not induced at any of the environmental (burial) sites when seeds were maintained at 15 cm, where light and temperature fluctuations were minimal. When buried at 4 cm or maintained on the soil surface secondary dormancy was induced, particularly at the inland sites where environmental conditions such as temperature and moisture were more extreme. Conditional dormancy could generally be overcome by incubating seeds at 15/30 °C in the light, even after prolonged burial at unfavourable germination conditions. Gibberellic acid (500 mg l^–1) was very effective in breaking secondary dormancy of seeds induced by storage under unfavourable conditions after burial. These results have important implications for the control of this week in commercial forests.     

Shade avoidance and the regulation of leaf inclination in Arabidopsis

As a rosette plant, Arabidopsis thaliana forms leaves near to the ground, which causes the plant to be vulnerable to shading by neighbours. One mechanism to avoid such shading is the regulation of leaf inclination, such that leaves can be raised to more vertical orientations to prevent neighbouring leaves from overtopping them. Throughout Arabidopsis rosette development, rosette leaves move to more vertical orientations when shaded by neighbouring leaves, exposed to low light levels or placed in the dark. After dark-induced reorientation of leaves, returning them to white light causes the leaves to reorient to more horizontal inclinations. These light-dependent leaf movements are more robust than, and distinct from, the diurnal movements of rosette leaves. However, the movements are gated by the circadian clock. The light-dependent leaf orientation response is mediated primarily through phytochromes A, B and E, with the orientation varying with the ratio of red light to far-red light, consistent with other shade-avoidance responses. However, even plants lacking these phytochromes were able to alter leaf inclination in response to white light, suggesting a role for other photoreceptors. In particular, we found significant changes in leaf inclination for plants exposed to green light. This green light response may be caused, in part, by light-dependent regulation of abscisic acid (ABA) biosynthesis.     

Dual effects of ethylene on potato dormancy and sprout growth

Dormant potato tubers (Solanum tuberosum L.) of two cultivars were treated with various concentrations of ethylene gas for various exposure periods. As has been shown by others, ethylene caused a rapid but transient increase in respiration rate, which appeared to be independent of any effects on dormancy. All concentrations tested caused accelerated sprouting, 2 microliters per liter being the most effective. Ethylene exerts a dual effect on potato tubers: it markedly shortens the duration of rest, but it inhibits elongation of the sprouts during extended treatment. Comparing these results with published work on seeds, bulbs, and corms suggests that ethylene must have a significant but as yet unexplained role in rest and dormancy. However, since the most effective ethylene treatment did not equal the response elicited by treatment with ethylene chlorhydrin, other factors must also contribute to termination of rest.     

ENVIRONMENTAL CONTROL OF SEED GERMINATION IN THLASPI ARVENSE (CRUCIFERAE)

The effect of environmental conditions during storage and imbibition on germination was investigated in field pennycress (Thlaspi arvense L.), a weed species that can behave as a winter or a summer annual. Freshly harvested seeds of an inbred line with a cold requirement for flowering exhibited primary dormancy that was rapidly lost following 1 month of afterripening in a dry state. Nondormant seeds were positively photoblastic. The light effect was mediated through phytochrome since germination was promoted by red light and inhibited by far red light. Seedling emergence was also inhibited by light filtered through a canopy of wheat leaves. Germination of field pennycress seeds was considerably more sensitive to moisture stress than two sympatric species, wild oat (Avena fatua L.) and wheat (Triticum aestivum L., cv. ERA). Seeds of the latter two species were chosen in order to compare the effect of water potential on germination in field pennycress with that in sympatric species. It was concluded that the major environmental factor limiting nondormant field pennycress seeds on the soil surface was water availability. Imbibition of fully afterripened seeds at low temperatures (6 C) induced a deep secondary dormancy. In contrast to primary dormancy, cold-induced dormancy was not alleviated by red light, alternating temperatures (21/5 C), or 2 months of dry storage at 6, 15, or 35 C. However, exogenous gibberellin A₃ or 24 weeks of dry storage resulted in germination in cold-induced dormant seeds. Secondary dormancy was not observed in fully afterripened seeds that were preincubated at 21 C for 1 or 2 days prior to the cold treatment. These results may explain the failure in field experiments to observe the cold-induced secondary dormancy that limits spring emergence in other winter annuals (J. Baskin, C. Baskin, Weed Res. 1979 19: 285–292).     

Hormonal and environmental regulation of seed germination in flixweed (<Emphasis Type="Italic">Descurainia sophia</Emphasis>)

Flixweed is one of the most abundant weeds in North America and China, and causes a reduction in crop yields. Dormancy of flixweed seeds is deep at maturity and is maintained in soil for several months. To identify regulators of seed dormancy and germination of flixweed, the effect of environmental and hormonal signals were examined using dormant and non-dormant seeds. The level of dormancy was decreased during after-ripening and stratification, but long imbibition (over 5 days) at 4 °C in the dark resulted in the introduction of secondary dormancy. The strict requirement of duration of cold treatment for the break of dormancy may play a role in the seasonal regulation of germination. The germination of non-dormant flixweed seeds was critically regulated by red (R) and far-red (FR) light in a photoreversible manner. Sodium nitroprusside, a donor of nitric oxide (NO), promoted germination of half-dormant seeds, suggesting that NO reduced the level of seed dormancy. As has been shown in other related species, light elevated sensitivity to GA₄ in dark-imbibied flixweed seeds, but cold treatment did not affect GA₄-sensitivity unlike in Arabidopsis. Taken together, our results indicate that seed germination in flixweed and its close relative Arabidopsis is controlled by similar as well as distinct mechanisms in response to various endogenous and environmental signals.     

Seed Treatment Optimizes Benefits of Seed Bank Storage for Restoration‐Ready Seeds: The Feasibility of Prestorage Dormancy Alleviation for Mine‐Site Revegetation

Dormant seeds of 18 species from 9 families covering a diverse range of seed dormancy syndromes and life histories from the southwest Australian biodiversity hotspot were assessed for germinability following storage at 15–25°C for 36 months. A total of 10 species with physical dormancy (PY) and 8 with either physiological dormancy (PD) or morphophysiological dormancy (MPD) were assessed as part of the study. Prior to storage, germination from dormant seeds was 1–27%, rising to 41–100% following specific dormancy‐breaking treatments. When seed dormancy was removed prior to storage for 36 months seeds from all species were found to maintain a nondormant state and germinate to a similar level to that observed at the beginning of the experiment (44–100%). Likewise, seeds that did not receive a prestorage dormancy‐breaking treatment maintained a dormant state (0–50% germination) and subsequently responded well to a dormancy‐breaking treatment immediately prior to germination assessment (49–99%). There were minimal differences in response to dormancy‐breaking treatments before and after 36 months storage (average 4–6% difference) and in the germination responses observed between both storage environments assessed (15°C/15% eRH or 15–25°C air dried). Based on these findings, storing seeds in a nondormant state does not alter germinability and this approach provides significant benefits to current seed‐based restoration programs through reduction of double handling and improved seed use efficiency.     

Seed after-ripening is a discrete developmental pathway associated with specific gene networks in Arabidopsis

After-ripening (AR) is a time and environment regulated process occurring in the dry seed, which determines the germination potential of seeds. Both metabolism and perception of the phytohormone abscisic acid (ABA) are important in the initiation and maintenance of dormancy. However, molecular mechanisms that regulate the capacity for dormancy or germination through AR are unknown. To understand the relationship between ABA and AR, we analysed genome expression in Arabidopsis thaliana mutants defective in seed ABA synthesis (aba1-1) or perception (abi1-1). Even though imbibed mutant seeds showed no dormancy, they exhibited changes in global gene expression resulting from dry AR that were comparable with changes occurring in wild-type (WT) seeds. Core gene sets were identified that were positively or negatively regulated by dry seed storage. Each set included a gene encoding repression or activation of ABA function (LPP2 and ABA1, respectively), thereby suggesting a mechanism through which dry AR may modulate subsequent germination potential in WT seeds. Application of exogenous ABA to after-ripened WT seeds did not reimpose characteristics of freshly harvested seeds on imbibed seed gene expression patterns. It was shown that secondary dormancy states reinstate AR status-specific gene expression patterns. A model is presented that separates the action of ABA in seed dormancy from AR and dry storage regulated gene expression. These results have major implications for the study of genetic mechanisms altered in seeds as a result of crop domestication into agriculture, and for seed behaviour during dormancy cycling in natural ecosystems.     

Ethylene in seed dormancy and germination

The role of ethylene in the release of primary and secondary dormancy and the germination of non-dormant seeds under normal and stressed conditions is considered. In many species, exogenous ethylene, or ethephon – an ethylene-releasing compound - stimulates seed germination that may be inhibited because of embryo or coat dormancy, adverse environmental conditions or inhibitors (e.g. abscisic acid, jasmonate). Ethylene can either act alone, or synergistically or additively with other factors. The immediate precursor of ethylene biosynthesis, 1-aminocyclopropane-1-carboxylic acid (ACC), may also improve seed germination, but usually less effectively. Dormant or non-dormant inhibited seeds have a lower ethylene production ability, and ACC and ACC oxidase activity than non-dormant, uninhibited seeds. Aminoethoxyvinyl-glycine (AVG) partially or markedly inhibits ethylene biosynthesis in dormant or non-dormant seeds, but does not affect seed germination. Ethylene binding is required in seeds of many species for dormancy release or germination under optimal or adverse conditions. There are examples where induction of seed germination by some stimulators requires ethylene action. However, the mechanism of ethylene action is almost unknown.
The evidence presented here shows that ethylene performs a relatively vital role in dormancy release and seed germination of most plant species studied.