Seed Dormancy: Induction, Maintenance and Seed Technology Approaches to Break Dormancy

Abstract

Dormancy is the major cause of erratic germination, patchy emergence and uneven seedling establishment in the field. These traits are exceedingly undesirable in crop production as future phases of growth and development are strongly linked to uniform seedling development at early growth phases. Variations in maturation time, and difficulty in managing abiotic and biotic stresses during pre- and postharvest are common consequences of uneven germination and seedling emergence. Minimizing this negative impact of dormancy in a seed lot is the major concern of all seed production companies. Generally, mature seeds show some considerable dormancy during which embryo growth is halted momentarily because one or more internal and external stimuli for growth resumption is/are absent. If the inhibition of seed germination is solely due to insufficient or complete absence of external signals, then the seed is in a state of quiescence. Otherwise, if linked to internal factors, then the seed is in a state of dormancy. Induction, maintenance, and release of dormancy are therefore related to Seed-dependent factors such as morphology, hormones, state of embryo maturity at seed dispersal and chemical inhibitors. This chapter focuses on species-dependent methods currently used to break dormancy, reduce germination time and improve emergence and seedling establishment.

1. Introduction

Generally, the concept of dormancy is centered on the absence of growth in any plant organ having a meristem like bulbs, corms, axillary buds, seeds and even in other living organisms like fungi spore, spirogyra zygospore etc. In the discipline of seed biology, dormancy could be considered simply as a block to the completion of germination of an intact viable seed under favorable conditions. The process might look simple but several authors have reported that dormancy is one of the least understood phenomena in the field of seed biology and needs further elucidation [1, 2]. The resting condition of many seeds, especially those of different grasses and garden crops, is maintained only as long as the seeds are in dry storage. As soon as a suitable moist medium and a favorable temperature are provided germination proceed almost immediately. Dormancy is a seed characteristic, manifesting as a block or series of blocks that prevent germination under otherwise favorable moisture, temperature and gaseous conditions. Dormancy is thus considered as an adaptive life history trait to seasonally unfavorable environmental conditions [3]. Dormancy is just one among quite a wide range of reasons why a seed may not germinate [4]. The concept of dormancy signifies that the miniature plant has life but require other factors beyond external factors like temperature and water to resume growth. Inhibition of germination is therefore completely or partially linked to a combination of three seed borne mechanisms, notably Chemical inhibitors that prevent growth, physical barriers that prevent the uptake of water, gases or chemicals and incomplete development of embryo prior to seed dispersal. In the later mechanism, the embryo of the seed needs extra time after dispersal to ripen [5, 6], and optimum levels of internal hormones [7]. Seed germination is the most critical part in the life cycle of seed bearing plants and seed dormancy is an excellent capability to increase the chance of survival by optimizing the distribution of germination in time or space [8].

The significance of seed dormancy can be seen from the ecological and agronomic stand points. Ecologically, seed dormancy can be beneficial for propagation and dissemination of plant populations while in agronomic systems, dormancy is a problem for seed evaluation and seedling establishment. For example it has been reported that when seed dormancy level is too low, it can trigger pre harvesting sprouting leading to either yield loses in cereals or germination before the start of a favorable growth season, risking seedling mortality. In contrast, deep seed dormancy levels limiting production in many field crops as it turn to delay germination and reduce the length of the growing season [3, 9].

1.1 Definition

There is yet no consensual definition for dormancy therefore many definitions and classifications exist [1]. It is a situation whereby a viable seed fails to germinate though given presumably favorable conditions [10]. Dormancy in a strict sense will refer to the inability of an intact viable seed to resume growth even when the environmental conditions are most favorable [1, 11]. Dormancy is delay of a viable seed to germinate, under a given set of environmental conditions and will only germinate if restricting factors are eliminated either via natural or artificial means [12]. Based on the later definition, it is evident that the state of dormancy can only be experimentally proven when the seed is placed under suitable external growth conditions and nonetheless germination process is not initiated, even though embryo is a life. The process is thus complex as it is difficult to appreciate the degree of restriction imposed by each seed borne characteristics contributing to delay resumption of growth by the embryo.

1.2 Significance of dormancy

Whether a seed should remain dormant or proceed to germination under certain circumstances is important in two aspects; first with respect to its survival as a species under specific ecological conditions and secondly relating to its economic and agronomic importance. From an ecological viewpoint, dormancy is an important survival mechanism that favors propagation and dissemination of seeds to establish plant populations. Because specific conditions are required to break dormancy, it may favor germination and seedling emergence under more favorable conditions [2]. Heterogeneous and asynchronous seed germination in the soil over years due to dormancy is an extra advantage to the survival of some species. This is because individual seeds in a seed population usually have different levels of dormancy, spreading their germination over time. This delay avoids unfavorable environmental events such as a drought that would eliminate the population if all the seeds were to germinate at same time on one hand and intra-specific competition for the available resources within the ecosystem on the other [13]. This explains why weeds are difficult to eliminate in a field because the seed banks provide a vast array of seeds with differing levels of dormancy. Although deep seed dormancy is considered problematic in agricultural species, some level of dormancy is desirable to prevent vivipary, a phenomenon rampant in cereals in which pre-harvest sprouting occurs. Vivipary causes losses in seed quality and quantity in agricultural plants. Regardless of all the advantages of dormancy for natural plant populations, it is an undesirable trait for most economic crops because it makes rapid and uniform germination during crop establishment difficult.

2. Key factors in the induction/maintenance of dormancy and initiation of germination

2.1 Environmental factors

The stimuli needed for seed germination include chemical or hormonal signals or environmental signals such as temperature, soil nitrate levels, or light [1]. It is important to keep in mind when dealing with dormancy and germination that a seed is never just under the control of one factor in nature, but many factors concurrently [5].

2.1.1 Temperature

Seed germination is a complex process involving many individual reactions and phases, each of which is affected by temperature. Because temperature influences both the percentage and rate of germination of seeds, it is one of the most critical factors affecting seed germination. Although seeds of each species have optimal temperatures for attaining maximum germination, between 30 and 40°C for most palms [14]. Alternating temperatures are preferred to constant temperatures because they can overcome shallow seed dormancy and enhance uniform germination. In the wild, most tropical tree seeds generally require higher temperatures to germinate; for example Soil temperatures above 38°C, but below 42°C can reduce the time required for germination of seeds of E. guineensis from years to weeks [15, 16].

2.1.2 Water

Water is a basic requirement for germination and its role as a medium for biochemical processes leading to germination, such as weakening the seed coat, activating enzymes, and breaking down food reserves, scarcely requires emphasis. Physical dormancy has been attributed to a hard or water-impermeable seed cover [17], such as a fibrous mesocarp and/or a stony endocarp, which are very common among palms [18]. It is generally recognized that seed germination is more sensitive to moisture stress than its subsequent seedling development [19]. Though moisture favors germination, excess of it may also become an obstacle to germination due to improper ventilation for the physiological process of germination [18]. While inadequate moisture results in delayed and poor germination, excessive moisture will hinder germination due to decreased aeration. Metabolic activation, preparation for cell elongation, radicle emergence, and seedling growth are subsequent events of seed germination and require different levels of hydration [20].

2.1.3 Oxygen

Seeds of many species will not germinate well at an oxygen level considerably lower than that normally present in the atmosphere [19]. From the physiological point of view studies are still to prove that inadequate oxygen supply induces dormancy [21]. Nevertheless, it has been acknowledged that oxygen is required to break chemical dormancy caused by germination restriction substances located in the endocarp [22]. However, insufficient oxygen supply under excessive moisture in a growth medium delays initiation of germination [23, 24]. In the tropics oxygen availability is usually not a limiting factor since germination usually takes place at room temperatures [25]. It has been reported that germination of oil-palm seed is dependent upon a minimal threshold concentration of oxygen in the embryo [22].

2.1.4 Light

The promotional effect of light in seed germination is through a single photoreaction controlled by the blue pigment phytochrome [26]. Seeds of many temperate tree species are known to be light sensitive, and their germination is promoted by red light and inhibited by far red light [27]. It is generally held that both light intensity (lux) and light quality ‘colour and wavelength’ influence seed germination. Germination is triggered by increases in light as well as by the ratio of red to far-red light and temperature [28]. However, while light has been reported to be a germination inhibitor i.e., show negative photoblastism in some Palmae as it’s the case of Sabal palmetto [29] others are indifferent to light for their germination. Germination of seeds covered by soil suggests that seed of most species are indifferent to light [30].

2.1.5 Soil factor

Edaphic factors like soil structure, texture, humidity, pH and temperature among others influence seed germination and emergence.

2.2 Internal factors

2.2.1 Seed maturity

Seed maturation is the second phase of seed development after embryogenesis during which food reserves accumulate, while dormancy and desiccation tolerance develops [31]. In angiosperms, there is increasing evidence that maternal environmental effects affect both germination percentage and germination phenology [32]. Environmental factors that affect the mother plant, such as photoperiod and temperature, during seed maturation have also been found to influence seed germination of many species [33]. These factors may also influence the size of the seeds, which in turn may influence germination timing and success [34], and within species, seed mass is often associated with probability or time of germination [35]. Identifying the environmental mechanisms involved in the transmission of maternal environmental effects through the seeds is important to understand their ecological function and adaptive value [32].

2.2.2 Plant growth regulator (PGR)

ABA, GA, ethylene, brassinosteroids, auxins, and cytokinins have a tremendous effect on plant development, even at low concentrations among all PGR [36]. Even though PGR like cytokinin, auxins ethylene and brassinosteroids can either promote germination alone or in association with ABA, most literature on hormonal regulation of seed germination suggests that induction and release of dormancy is controlled by two main plant growth regulators notably ABA and GA. However, the influence of these two hormones is mainly at the variation in their ratio than their absolute content. Thus, dormancy induction would depend on ABA and GA metabolism during the process. Any metabolic processes that can change the concentration of the active forms of IBA and GA within the seed like synthesis, degradation, activation or deactivation will either induce or break dormancy. Thus any metabolic process that synthesizes or activates IBA induces dormancy because such processes turn to degrade or deactivate GA, given that these two hormones are antagonistic. Increase of seed to ABA sensitivity, and decrease of GA sensitivity induce dormancy and the opposite events will lead to dormancy alleviation and germination [37].

2.2.2.1 Abscissic acid

ABA is a sesquiterpene hormone that plays an integral part in mediating the adaptation of the plant to biotic and abiotic environmental stresses, regulation of seed development and germination. During seed development, ABA concentration increases intensely during embryogenesis to promote accumulation of reserves, prevent precocious germination, promote embryo tolerance during seed desiccation phase, and induction of dormancy at dispersal [38]. ABA is a main plant growth regulator of dormancy induction and maintenance [1]. Insensitivity to ABA during seed development results to precocious seed germination. This has been observed in maize, tomato, and Arabidopsis mutants deficient in ABA content [39, 40].

In spite of the fact that the relationship between ABA content and its physiological function in seed dormancy and germination is still to be made clear, more light has been shed on mechanisms surrounding ABA-mediated seed germination inhibition. It is the case in Brassica napus, where it was suggested that one of the probable contribution of ABA to inhibiting germination is to prevent loosening of the embryo cell wall which impedes water uptake [41]. In addition, ABA has also been found to specifically inhibit endosperm rupture instead of testa rupture. The effect of ABA on seed dormancy can be efficiently alleviated by chilling ‘stratification treatment’ so that endogenous ABA content drops precipitously with a concurrent increase in germination rate [42]. This inhibitory effect can be partially counteracted by the antagonistic action of GA [43].

2.2.2.2 Gibberellic acid (GA₃)

Gibberellins are a group of tetracyclic diterpenoids endogenous regulators that are well-known for their capability to promote plant growth and development [44]. In the process of seed development, GA levels are usually high during the embryo morphogenesis phase and decreased during the embryo maturation phase [45]. Active GAs may help promote the growth of embryo and later they are deactivated to avoid vivipary [46]. There are reports on the application of GA₃ in alleviating innate and environment-induced dormancy [7, 47] or synchronizing GA with different scarification pre-treatments like stratification, heat and removal of seed coat [5, 48]. In fact, GA is likely to antagonize the ABA effect on seed development, particularly dormancy induction [49].

2.2.2.3 Mechanism of GA3 in promoting germination

The role of GA3 in breaking of dormancy has been documented by different workers [50, 51]. The hormone signal (GA₃) activates DNA in the aleurone cells. This is followed by transcription and translation of a gene coding for α-amylase and its eventual production inside the aleurone cells. In the model for gibberellin-induced production of α-amylase, it is demonstrated that GAs produced in the scutellum diffuse to the aleurone cells, where they stimulate the secretion of α-amylase [52].

2.2.2.4 Regulation of dormancy/germination by GA and ABA

The two main plant growth regulators that regulate dormancy-germination cycling are ABA and GA₃. A model for the regulation of dormancy by ABA and GA in response to the environment has been proposed [37]. This model suggests that, the sensitivity of embryo to ABA and GA ratios within the seed is influenced by environmental factors like temperature. If the ABA synthesis and signaling (GA catabolism) dominates then dormant state is induced and maintained while the opposite event promotes transition to germination. Variations in the level of dormancy alter the requirements for germination when these overlap with varying environmental factors. Throughout the process of seed development, dormancy can be induced and maintained depending on GA/ABA balance. During imbibition, GA levels increases while ABA levels decline, suggesting that GA and ABA have antagonistic roles in seed dormancy and germination processes [53]. GA counteracts the effect of ABA by promoting the embryo growth potential and the weakening of tissues covering the embryo [54].

2.3 Multiple control points in dormancy induction and breaking

A block to germination may occur at more than one point in the sequence of steps leading to visible growth [55]. Various dormancy mechanisms occur at different levels in the course of seed development and principally in germination, indicating the concurrent nature of dormancy and germination. For example a dormant seed without a hard seed coat will imbibe water but remain blocked at some stage of metabolic activation or growth until other blocks are removed [56]. For example an impermeable seed coat inhibits water absorption, blocking imbibition, and, in many cases, the seed coat may reduce oxygen availability to the embryo [57]. If a seed successfully completes imbibition, it will normally be metabolically active [1]. However, germination-related activities may further be blocked during activation phase in dormant seeds. Even if this stage is completed successfully, a mechanical restriction of root growth may still impose dormancy on the seed, at the last step in germination. This shows that dormancy induction can occur in more than one control point depending on the plant species. The complex interactions between stimulating, inhibiting and limiting factors demonstrate that a series of requisites must be met for the seed to germinate. A strong line of evidence in support of multiple control points in dormancy induction and breaking is provided by [58] who characterized germination of Sisymbrium and Arabidopsis seeds under different wave lengths of the visible spectrum and growth stimulants.

Secondary dormancy may be induced by metabolic variations, like ABA synthesis due to drought or cold ambient conditions or when the level of primary dormancy decrease, during after-ripening, because of decreases in ABA level or increases in GA or cytokinin levels [59]. At the molecular level, a well-known genetic barrier to germination is DELAY OF GERMINATION1 (DOG1) [60]. Though the binding of (DOG1) to Protein Phosphatase 2C ABSCISIC ACID (PP2C ABA), Hypersensitive Germination (AHG1) and heme are independent processes, they are however both essential for the in vivo functioning of DOG1’s. AHG1 and DOG1 are both involved in the regulatory system for dormancy induction, maintenance and germination. DOG1 has a significant influence on the level of expression of ABA INSENSITIVE5 (ABI5) [61, 62].

3. Classification of dormancy

Generally, different classes of dormancy require different dormancy breaking methods [4]. By misinterpreting the class of seed dormancy, researchers may fail to apply the correct methods to overcome dormancy in a given species. For this reason, method of overcoming dormancy must be directed towards the specific kind of dormancy [63]. Owing to the fact that the notion of dormancy still need to be elucidated, several classifications exist, but the most common are those proposed by Nicolaeva [11], Hilhorst [2] and that of Baskin and Baskin [64].

3.1 Classification based on barrier factors

One of the earliest classifications of dormancy is that proposed by [11] who distinguished three main types of dormancy based on barrier factors within the seed.

3.1.1 Exogenous dormancy

Dormancy is due to some features of the seed located outside the embryo. Exogenous dormancy could be due to the following; impermeability of seed coat to water owing to seed coat structure, which is hard enough to restrict the entry of moisture into the seeds. Secondly seed coat may be impermeability to gases, hence insufficient intake of oxygen necessary for catabolism of reserves. Thirdly, extremely hard seed/fruit structure such as seed coat, endosperm as in Acacia species can impose mechanical resistances on further development of the embryo. Finally, the testa may produce some biochemical substances that serve as inhibitors, blocking germination of embryo.

3.1.2 Endogenous dormancy

The cause of this dormancy is present within the embryo and can be as a result of; incomplete embryo development after ripening period, in such cases, germination does not occur until the embryos develop to their normal size. Common in family Palmaceae, Amgnoliaceae; inhibitors are present within the embryo. In such cases germination can commence only when these inhibitors are leached out of the embryo, this is the case of Xanthium, Fraximus.

3.1.3 Combined dormancy

This type is provoked by a combination of two or more exogenous and endogenous factors which act in complementary fashion.

3.2 Classification based on time of induction

This classification suggested by [2] distinguishes only two types: primary and secondary dormancy.

3.2.1 Primary dormancy

Primary dormancy is induced during the seed maturation phase and reaches a high level in freshly harvested seeds, meaning seeds with primary dormancy are dispersed from the mother plant in a dormant state [1]. During subsequent dry storage of seeds (after-ripening), dormancy slowly reduces [65]. Primary dormancy is maintained by the accumulation of abscisic acid (ABA) during seed maturation to prevent viviparity [66] and requires a period of after-ripening before seeds have the capacity to germinate under favorable conditions. The level of primary dormancy in seeds is determined by several factors of genetic and non-genetic origin [67]. All these factors may cause physiological variability which is matched with differences in seed morphology (size, weight, color etc.) or simply heterogeneity in degree of dormancy [5]. Developing seeds rarely germinate, and when precocious germination does occur, it is frequently associated with deficiencies in ABA synthesis or sensitivity [2]. The induction of primary dormancy is linked to the two peaks of ABA accumulation during developmental phases of seeds. As observed in studies carried out on Arabidopsis, the first ABA peak occurs prior to embryo maturation hence it’s described as maternal. The second peak regulated by the genome of the embryo is observed during maturation hence described as embryonic ABA peak. The accumulation of embryonic ABA, but not maternal ABA, is indispensable for the induction of primary dormancy [68].

Primary dormancy has the advantage of guaranteeing complete development of embryo, and prevents precocious germination in species like maize. Once seeds acquire primary dormancy during late maturation its water content decreases drastically. It is the case with some Arabidopsis accession Cvi, in which water content decreased by almost 5-fold was noticed after acquiring primary dormancy to late maturation phase [69]. When the moisture content is further reduced to a certain level by dry storage, the seed loses primary dormancy. This process of breaking primary dormancy includes a decrease in ABA concentration and sensitivity, an increase in GA and light sensitivity, and a widening of temperature range for seed germination [70].

3.2.2 Secondary dormancy

Secondary dormancy is generally induced by unfavorable environmental conditions following shedding of mature seeds from the parent plant which were not dormant at shedding [2]. Secondary dormancy is induced when changing environmental conditions cause undesirable germination conditions, such as unfavorable temperature, extended light or darkness, water stress, or lack of oxygen. This type of dormancy does not only decrease with time, but it can also be re-induced in non-dormant seeds when conditions for germination like light are lacking [26]. When exposed to inappropriate conditions like critical temperature, anoxia, limited light etc. secondary dormancy might occur after imbibed after-ripening seeds have lost primary dormancy. In the soil seed bank, secondary dormancy enables cycling, through which different depths of dormancy are progressively gained or lost, until the environment is favorable for germination, and then seedling establishment [6].

3.3 Classification according to Baskin and Baskin

Baskin and Baskin [64] based on an initial dormancy classification scheme proposed by Nikolaeva [11] to put forward a more comprehensive hierarchical classification system made up of five classes of seed dormancy which are in turn sub divided into levels and types.

3.3.1 Physiological dormancy (PD) or class A

Physiological dormancy is present among species distributed over the entire phylogenetic tree of gymnosperms, basal angiosperms, monocots and eudicots [64, 70]. PD is caused by low growth potential of embryo, which cannot overcome mechanical constraint of seed (or fruit) coat. It is the most abundant form of dormancy common in seeds of angiosperms and all major angiosperm clades. Class A has three levels: deep, intermediate and non-deep.

3.3.1.1 PD deep

Embryos excised from seeds with deep PD either do not grow or produce abnormal seedlings. GA treatment does not break the level of PD dormancy. Three to four months of cold (subtype a) or warm (subtype b) stratification are needed before germination can occur. Examples of plant family of sub type are Aceraceae, and b is Ericaceae [71].

3.3.1.2 PD intermediate

Embryos excised from such seeds produce normal seedlings. GA treatment promotes germination in some but not in all species. Seeds require 2–3 months of cold stratification. Dry storage after-ripening can shorten the cold stratification period. Example of a plant with PD intermediate is Acer Pseudoplatanus (Aceraceae) [70].

3.3.1.3 PD non-deep

Embryos excised from such seeds produce normal seedlings. GA treatment can break this dormancy but depending on the dormancy can also be overcome by scarification, after-ripening in dry storage, and cold (0–10°C) or warm (>15°C) stratification. The sensitivity of seeds to light and GA increases as non-deep PD is released. Members of Asteraceae and poaceae have non-deep PD [72].

3.3.2 Morphological dormancy (MD) or class B

Class B dormancy is evident in seeds whose embryos are under developed in terms of size but well differentiated cotyledon and hypocotyl-radical. The embryos are not physiologically dormant but simply needs time to grow before seed germinates, i.e., growth period of embryo = period of dormancy.

3.3.3 Morpho-physiological dormancy (MPD) or class C

It is common in seeds that have underdeveloped embryos ‘in terms of size’, supplemented with physiological component to their dormancy. These seeds therefore need a dormancy-breaking treatment that will take in to account both morphological and physiological constraints. For example a defined combination of warm and/or cold stratification which in some cases can be replaced by GA treatment. In MPD-seed, embryo growth/emergence requires a considerable longer period of time than in MD-seeds.

3.3.4 Physical dormancy (PY) or class D

PY is known to occur only in angiosperms [17]. Seeds with physical dormancy possess water impermeable fruit coats ‘pericarps’ or seed coats ‘testa’. Impermeability might be enhanced by many factors among which are several layers of tightly spaced, thick-walled cells in the pericarp and testa, the presence of waxes, lignins, and pectins, or a combination of these factors, all impeding water uptake during imbibition [17]. As far as these impermeable layers remain intact, dormancy is maintained until when subjected to natural or artificial conditions that will disintegrate the impervious layers. Wide temperature variations, fire, drying, and passage through gut of animals are among natural factors that can disintegrate impervious layers of the pericarp. Mechanical or chemical scarification is common practices used in seed technology to break PY dormancy. In several cases, specialized structures are associated with the control of water-impermeability, for example, in PY-dormant Anacardiaceae species (Rosids), the endocarp consist of three water impermeable palisade layers (macrosclereids, osteosclereid and brachysclereids) and the outer crystalliferous layer which block water entrance to the carpellary micropyle in dormant seeds [73]. Once the impervious layers becomes permeable following natural or artificial scarification methods, the later can no longer return to their initial stage of absolute impermeability. Thus the timing natural release of dormancy is a more significant in the life cycle of plants with PY, than it is in plants with PD [74].

3.3.5 Combinational dormancy (PY + PD) of class E

Class C dormancy is found in seeds with water impermeable seed or fruit coat (PY) synchronized with physiological embryo dormancy (PD none deep). Release from PY and PD of PY + PD-dormant seeds appears to be independent events and the timely order can be species specific [64].

4. Different methods of breaking dormancy in seed technology

4.1 Scarification

Scarification is a technique used to break mechanical dormancy [75] which acts by impeding the perception of germination elicitors like water, light, temperature and oxygen by the embryo. Scarification techniques used commonly include; mechanical, thermal, hot water and chemical scarification. This type of dormancy breaking has been documented in a number of families e.g. Anacardiaceae, Arecaceae, Cornaceae, Elaeagnaceae, Empeteaceae, Juglandaceae, Meliaceae, Nyssaceae, Oleaceae, Rhamnaceae, Rosaceae and Santalaceae [76].

4.1.1 Mechanical scarification

This method can be accomplished, by abrading the surface of the seed until the endosperm becomes visible, or by using a knife to scrap out the hair plug at the micropylar opening. This method gives the possibilities to bring the best of germination capacity of seeds, although its application is difficult for significant quantities of seeds. Mechanical scarification accelerates germination and some chemical treatments significantly increased germination speed of the mechanically scarified seeds [77].

4.1.2 Thermal scarification

Thermal scarification can be by dry heat or hot water on the other hand can be realized by subjecting seeds under a high and steady temperature of a given duration. In the case of oil palm, dormancy is only broken when seeds are exposed to a constant thermal scarification temperature of 40°C for 80 days [78].

4.1.3 Hot water treatment

This treatment involves soaking seeds in water at 40–100°C depending on the species and seed coat thickness, for a specific period of time or until the boiling water cools to room temperature [79]. A brief soak in 80°C water for 10 minute resulted to 91.26% germination of Acacia catechu while soaking in 100°C water for a period of 12 min for Elaeocarpus floribundus gave 84% seed germination success rates [80]. Improvement of germination via soaking in hot water could be associated to weakening of seed coat. Such weakening probably occurs because lignins and pectins present on epidermal layer of the seed coat are dissolved, hence water and oxygen signals are perceived by the embryo [81].

4.1.4 Acid scarification

This method is recommended only for those seeds that are very hard to germinate, as damage to the embryo during the process can be high [82]. The treatment generally requires soaking seeds in 95% pure (1.84 specific gravity) sulfuric acid. The soaking duration is a factor of the degree of thickness of the pericarp of a given species. Once the soaking time elapses, acid is decanted, then seeds washed severally and dried [83]. The timing of this treatment is critical therefore the soaking period and the post soak washing have to be precisely controlled to avoid seed injury. The acid scarification can be applied either at room temperature or in a heated condition by soaking the seed in different concentrations of sulfuric acid for 10–30 min [79].

4.2 Alternating soaking and drying in water

This is the simplest treatment to give the seeds an early start in the germination process. It is also known as invigoration and its effects are not only on the activation of enzymes and mobilization of reserves in the aleorone layer [84], but also on the softening of hard seed coats and leaching out of chemical inhibitors. Aerated, cold-water soaking for 28 days at 11°C was found to be effective in breaking moderate dormancy and enhancing germination of Pinus taeda seeds [85]. Soaking and drying treatments can have varying effects on germination depending on the rate of drying, the species tested, and the duration of the soaking [86].

4.3 Seed stratification

Stratification is used mainly to alleviate dormancy in temperate plant species. The stratification approach depends on the cause of dormancy in a species. For example dormancy due to immature embryos is broken via warm stratification; physiological dormancy is relieved by cold stratification while combined warm and cold stratification is effective for seeds that have both immature embryos and physiological dormancy [76].

4.4 Embryo rescue

Embryo rescue is a dormancy alleviation method in which immature or mature zygotic embryos are isolated and cultured under aseptic conditions on an aseptic nutrient medium [87]. While the culture of immature embryos has as objective to prevent embryo abortion or sudden arrest of growth during ontogeny, one of the reasons the mature embryos are cultured is to eliminate seed germination inhibitors or to shorten the breeding cycle if dormancy is a problem [88]. The factors which cause dormancy are endogenous inhibitors and embryo immaturity. Breeding cycle can be shortened by removing the embryos from the influences of these factors which localized in the seed coat and endosperm, or both. The breeding cycle of papaya could be shortened via embryo culture from 6 to 9 months to approximately 3 months [89].

5. Conclusion

In this chapter, the definition, significance, classification, key control mechanisms and methods of breaking dormancy have been reviewed. Dormancy is an important survival mechanism that favors propagation and dissemination of a species but it has very strong negative consequences in crop production as it imposes heterogeneous and asynchronous seed germination. Many classification keys of dormancy exist, mainly because the process is generally controlled by more than one seed-dependent factor. For this reason induction and release of dormancy, remain a complex process in the life of plants. At the molecular level, induction and release of dormancy are regulated by a multifaceted network of transcriptional, translational, and epigenetic processes under an integrated control of environmental and hormonal signals. Though the process still needs elucidation, identification of the causes of dormancy is species-specific. In perspective, focus should be on diagnosing the dormancy types in each species with the objective to design a species specific protocol to suppress dormancy and improve on rapid and homogenous seed germination.