Physical dormancy (PY), resulting from a water-impermeable seed coat, regulates germination timing in many angiosperms, including Dodonaea viscosa L. (Sapindaceae), a woody shrub widely distributed in tropical to warm temperate regions and coastal and inland habitats. Although PY has been previously documented in D. viscosa, the precise anatomical structure acting as the site for water entry, i.e., water gap, during dormancy release remains unclear. This study investigated the water gap’s morphology and function using microscopy, scanning electron microscopy (SEM) and imbibition assays. It also evaluate the effects of liquid nitrogen (LN₂) freeze–thaw cycles on seed coat integrity, dormancy break, and viability. Seeds possess a distinct hilar slit, which opens in response to hot-water treatment and serves as the exclusive water gap. Imbibition experiments showed that treated seeds (hot water , 10s) increased nearly 100% in mass over seven days, while control seeds absorbed no water. Further, covering the hilar slit with Vaseline restricted water uptake, confirming the absence of water-gap complex. Because the water gap opening was a small circular structure without any lid-like covering, it is classified as Type II. Seeds subjected to one or more liquid nitrogen (LN₂) freeze–thaw cycles experienced extensive seed coat cracking and severe damage to the embryo and cotyledons, leading to reduced viability and little to no germination. These findings demonstrate that while hot-water treatment effectively breaks dormancy, LN₂ exposure causes extensive mechanical injury and is ineffective for dormancy alleviation in D. viscosa. Thus, cryopreservation of some PY should be considered with caution.

The Malvaceae is the 12th largest angiosperm family with ten subfamilies, 243 genera and c. 4000 species of trees, shrubs, herbs and a few climbers. Subfamilies originated in the Upper Cretaceous-Palaeocene, and their divergence times range from 71.6 to 33.0 Ma. Seeds have a folded, investing or spatulate embryo, and they may be nondormant (ND) or have physical (PY) and/or physiological (PD) dormancy. Of the 365 species for which dormancy/germination data were found, 34.0% had ND seeds and 46.6% PY; 1.6%, PD&ND; 13.1%, PY&ND; and 4.7% PY+PD. Seeds with PY have a palisade layer of Malpighian cells (with a light line) in the outer epidermis of the inner integument, and a chalazal plug is the water gap. Seeds of 168 species of wet tropical trees, in all ten subfamilies, were ND (57.2%) or had PY (19.7%), but seed collections of many species were a mixture of ND & PY (20.2%); 2.9% had PD&ND. We found 13 tree species in wet tropics with recalcitrant seeds and 57 species in 28 genera in seven subfamilies in various habitats with persistent soil seed banks. Malvoideae is the most species rich and widely distributed subfamily and is found in tropical and temperate regions but rarely in subalpine/boreal or Arctic/alpine tundra vegetation. Few if any Malvaceae, in particular Malvoideae, grow as herbaceous perennials in tundra vegetation; possible reasons for this are considered.

The classification of acacias has gone through recent upheaval. The latest phylogenies indicate that Acacia sensu stricto is only relatively distantly related to the species with which it was once grouped. Its sister group is the monospecific Paraserianthes. This study concerns P. lophantha subsp. lophantha, a species from SW Western Australia that is widely invasive. Both genera have seeds with physical dormancy (PY) and a lens-type water gap. Seed structure, particularly that of the lens, was assessed in Paraserianthes and compared with Acacia. Seed batch viability was almost 100%, all seeds had PY and average seed mass was 73 mg. The seed coat and the embryo made almost equal contributions to seed mass, indicating a substantial seed coat. Average testa (410 µm) and palisade layer (163 µm) thicknesses were greater than in most investigated Acacia species. Unpopped lenses were small (0.11 mm2, about 0.15% of the seed surface area). With a 1 min boiling water treatment, the lens detached from the seeds. The palisade cells of the lens were about 100% larger in area after detaching, which indicates that they previously were under considerable tension. With other PY-breaking treatments, the lens formed a mound or a slight change in colour occurred. The seeds of Paraserianthes lophantha had the same basic construction as most Acacia seeds, although they were relatively large and heavy, the testa made up a large proportion of the seed and the palisade cells were long. Different lens morphologies, associated with different dormancy-breaking treatments, have rarely been described.

The assessment of seed quality and physiological potential is essential in seed production and crop breeding. In the process of rapid detection of seed viability using tetrazolium (TZ) staining, it is necessary to spend a lot of labour and material resources to explore the pretreatment and staining methods of hard and solid seeds with physical barriers. This study explores the TZ staining methods of six hard seeds (Tilia miqueliana, Tilia henryana, Sassafras tzumu, Prunus subhirtella, Prunus sibirica, and Juglans mandshurica) and summarizes the TZ staining conditions required for hard seeds by combining the difference in fat content between seeds and the kinship between species, thus providing a rapid viability test method for the protection of germplasm resources of endangered plants and the optimization of seed bank construction. The TZ staining of six species of hard seeds requires a staining temperature above 35 °C and a TZ solution concentration higher than 1%. Endospermic seeds require shorter staining times than exalbuminous seeds. The higher the fat content of the seeds, the lower the required incubation temperature and TZ concentration for staining, and the longer the staining time. And the closer the relationship between the two species, the more similar their staining conditions become. The TZ staining method of similar species can be predicted according to the genetic distance between the phylogenetic trees, and the viability of new species can be detected quickly.

The plant species Sesbania tomentosa (‘ōhai; Fabaceae) is endemic to the Hawaiian Islands, federally listed as endangered in the USA and has been proposed for categorization as Vulnerable on the IUCN Red List. In 2021, c. 12,000 seeds from 12 seed lots collected during 1990–1992 from across the Hawaiian Islands were discovered in ambient herbarium conditions (55% relative humidity (RH) at 20 °C). International gene bank standards suggest drying seeds in equilibrium with 15% RH and stored at −18 °C. To investigate seed viability, we mechanically scarified then sowed 15 seeds from each accession at daily alternating regimes of 12 h light and 12 h dark at temperatures of 25/15 °C, respectively. Germination was observed after 7 days and ended after 34 days. Mean final germination was 88.9 ± SD 0.1% (range 73–100%). Each seed lot was accessioned into the National Tropical Botanical Garden's Seed Bank and Laboratory. In seeds with a water-impermeable seed coat (i.e. physical dormancy), such as S. tomentosa, seeds can desorb but not absorb water. Therefore, if the seeds were initially dried, although exposed to high RH for up to 32 years, seed equilibrium RH may have remained low, which may in part explain the observed high germinability. This study holds significance for managers who are working to conserve this endangered Hawaiian species and suggests that even suboptimal conditions may still yield highly viable seeds several decades into the future.

This review provides a revised and expanded word-formula system of whole-seed primary dormancy classification that integrates the scheme of Nikolaeva with that of Baskin and Baskin. Notable changes include the following. (1) The number of named tiers (layers) in the classification hierarchy is increased from three to seven. (2) Formulae are provided for the known kinds of dormancy. (3) Seven subclasses of class morphological dormancy are designated: ‘dust seeds’ of mycoheterotrophs, holoparasites and autotrophs; diaspores of palms; and seeds with cryptogeal germination are new to the system. (4) Level non-deep physiological dormancy (PD) has been divided into two sublevels, each containing three types, and Type 6 is new to the system. (5) Subclass epicotyl PD with two levels, each with three types, has been added to class PD. (6) Level deep (regular) PD is divided into two types. (7) The simple and complex levels of class morphophysiological dormancy (MPD) have been expanded to 12 subclasses, 24 levels and 16 types. (8) Level non-deep simple epicotyl MPD with four types is added to the system. (9) Level deep simple regular epicotyl MPD is divided into four types. (10) Level deep simple double MPD is divided into two types. (11) Seeds with a water-impermeable seed coat in which the embryo-haustorium grows after germination (Canna) has been added to the class combinational dormancy. The hierarchical division of primary seed dormancy into many distinct categories highlights its great diversity and complexity at the whole-seed level, which can be expressed most accurately by dormancy formulae.

Seeds of Abrus precatorius L. (Fabaceae) were used as weight measure by Indigenous people. Where, the seeds were referred as Ratti; a traditional Indian unit of mass measurement. Seed weight fluctuates depending upon age, moisture, storage-period/conditions. Therefore, use of seeds as a weighing unit become dubious and need to be validated. For this purpose, seeds of A. precatorious were subjected to different moisture conditions and periodically monitored. Surprisingly, there was no change in seed weight was observed, indicating the impermeability of seed coat. The later was confirmed by scarification of seed coat which resulted in 53% increase in seed weight against 0% in control. Further, presence of a potent toxin (abrin) in the seed coat protects it from pests and microbes, and contributes to the maintenance of impermeability for longer period of time. The data validates the use of A. precatorious seeds as a weighing unit (ratti) by the indigenous people and discussed.

Astragalus is the largest genus of seed plants with 3000 or more species that occurs naturally on several continents. The genus has some use as a forage and medicine and in industry, and many of the species are rare endemics threatened with extinction. The seeds are reported to be dormant at maturity, and various treatments have been used in an attempt to germinate them. Our primary aim was to determine via a meta-analysis the most effective way(s) to break dormancy in seeds of this species-rich genus. Mechanical and chemical (conc. sulphuric acid) scarification were by far the best of 12 treatments for breaking seed dormancy of the 40 species included in our meta-analysis, whereas prechilling, gibberellin and smoke were ineffective. These results along with those of imbibition tests confirm that seeds of the examined Astragalus species have physical dormancy (PY). Further, PY in these 40 species and (its documented occurrence) in 118 species that could not be included in our meta-analysis transcends climatic and geographic boundaries, edaphic conditions, life cycle/life form types and infrageneric phylogeny. Thus, it seems likely that most species of Astragalus have PY. However, in addition to PY, physiological dormancy (PD), that is, combinational dormancy (PY + PD), has been reported in a few species of Astragalus. This study should be useful to both basic and applied scientists who want to germinate seeds of Astragalus.

The effects of cold atmospheric plasma (CAP) of dielectric barrier discharges on the wettability, imbibition and germination of Leucaena leucocephala were investigated. It was established that CAP treatment markedly hydrophilized the seed coat, especially at longer treatment times. From the profile of the imbibition curve and visual observation, it was possible to verify that there are two resistance barriers to water penetration: integument surface and region of the macrosclereid cell wall (light line). Although the plasma interacts only in the integument, increasing the density of hydrophilic sites increases the capacity of water absorption, producing enough driving force to overcome the second resistance barrier. The existence of these two barriers changes the three-phase pattern generally observed during seed germination. Despite an increase in imbibition, the plasma treatment conditions used in this work, were not enough to overcome completely the dormancy barrier.

The seeds of most Australian acacias have pronounced physical dormancy (PY). While fire and hot water (HW) treatments cause the lens to ‘pop’ almost instantaneously, for many Acacia species the increase in germination percentage can be gradual. If PY is broken instantly by HW treatment, why is germination often an extended process? Control and HW treatments were performed on seeds of 48 species of Acacia. Seeds were placed on a moist substrate and imbibition was assessed by frequently weighing individual seeds. In the two soft-seeded species all control seeds were fully imbibed within 6–24 h, while in hard-seeded species very few control seeds imbibed over several weeks. In 10 species over 50% of the HW-treated seeds imbibed within 30 h, but mostly the percentage of imbibed seeds gradually increased over several weeks. Some seeds in a replicate would imbibe early, while others would remain unimbibed for many days or weeks then, remarkably, become fully imbibed in less than 24 h. While HW treatment broke PY almost instantaneously, it appeared that in many Acacia species some other part of the testa slowed water from reaching the embryo. This process of having staggered imbibition may be a way of ensuring not all seeds in a population germinate after small rain events. Thus it appears the lens acts as a ‘fire gauge’ while some other part of the seed coat acts as a ‘rain gauge’.

Physical dormancy (PY) occurs in at least 18 angiosperm plant families and is caused by water-impermeable palisade cells in seed (or fruit) coats. Breaking of PY involves disruption or dislodgement of water-gap structures causing the seeds/fruits to become water permeable (non-dormant). The water-gap region is a morphologically distinct area of the seed or fruit coat that forms a water-gap complex. The location, anatomy, morphology and origin of water-gaps can differ between and even within families and genera. Water-gap structures sense environmental conditions that allow seeds with PY to become permeable just prior to the commencement of conditions favourable for germination and plant establishment. There are three basic water-gap morpho-anatomies characterized by the way the water-gap opens: Type-I, Type-II and Type-III. In Type-I water-gaps, specific kinds of cells pull apart to form a surface opening, while in Type-II a portion of the surface structure is pulled away from adjacent cells, opening the water-gap. Type-III is the least common type and has a circular, plug-like structure that is dislodged, whereby water entry occurs. In addition, water-gap complexes are either simple or compound, depending on whether only a single primary water-gap structure is involved in dormancy release or an additional secondary water-gap structure opens, permitting water entry.

Differences in fruit morphology among or within species might indicate differences in other regenerative traits, such as seed dormancy and germination. In species with physical dormancy (PY), environmental conditions are suggested to be responsible for dormancy break in field. Seeds of Vachellia caven have PY. This species exhibits two fruit morphs highly represented in Córdoba forests, Argentina: one is dehiscent and the other is indehiscent. In this study we performed a burial experiment with the aim to determine if the differences in V. caven fruit morphology were related to different patterns of PY break of their seeds in field conditions. We related these patterns to (1) environmental conditions that could influence the loss of PY, and (2) histological features of the lens zone. Seeds of both morphs exhibited dormancy break within 14 months of the start of the experiment, but with different patterns. The dehiscent morph showed an abrupt percentage of seeds that broke dormancy 14 months after the beginning of the experiment, probably after undergoing environmental changes similar to those suggested by the two-stage softening model. The indehiscent morph showed a gradual increase in seeds that broke dormancy, not clearly related to any of the environmental variables studied. No differences in seed coat structure of the lens zone were observed between morphs. The existence of both morphs could confer the species with higher possibilities of establishing and coping with environmental heterogeneity. Those characteristics contribute to the understanding of the success of this species in open and disturbed environments.

Primary physical dormancy caused by seed coat impermeability to water is a major reason for the persistence of velvetleaf in soil seedbanks. Understanding temporal trends in seed dormancy status will help predict potential emergence in the spring. Experiments were begun in 1992 and 1993 to determine the effects of velvetleaf seed maturation time, storage environment, and storage duration on changes in seed dormancy and germination over 20 mo. Seeds buried 1 and 10 cm deep exhibited a 30 to 70% decline in physical dormancy from maturity until winter, little change in dormancy from winter through the following summer, and a further decline the next autumn. The loss of physical dormancy was more rapid for early than for late maturing seeds and more rapid in 1992 than in 1993. Physical dormancy of seeds held at 4 C declined steadily, at a rate of approximately 0.8% per day, over the course of the study. Germination of seeds buried 1 cm averaged 23 to 37% in the first spring after harvest, which was equivalent to 68 to 100% of seeds that had lost physical dormancy over autumn and winter. The percentage of seeds with enforced dormancy reflected the loss of physical dormancy during autumn and the loss of seeds to germination during spring and summer. Additional information on how autumn temperature and moisture conditions influence the pattern of dormancy decline could aid in explaining the variation in velvetleaf infestations over time.

Information from a field perspective on temperature thresholds related to physical dormancy (PY) alleviation and seed resistance to high temperatures of fire is crucial to disentangle fire- and non-fire-related germination cues. We investigated seed germination and survival of four leguminous species from a frequently burned open Neotropical savanna in Central Brazil. Three field experiments were conducted according to seed location in/on the soil: (1) fire effects on exposed seeds; (2) fire effects on buried seeds; and (3) effects of temperature fluctuations on exposed seeds in gaps and shaded microsites in vegetation. After field treatments, seeds were tested for germination in the laboratory, together with the control (non-treated seeds). Fire effects on exposed seeds decreased viability in all species. However, germination of buried Mimosa leiocephala seeds was enhanced by fire in an increased fuel load treatment, in which we doubled the amount of above-ground biomass. Germination of two species (M. leiocephala and Harpalyce brasiliana) was enhanced with temperature fluctuation in gaps, but this condition also decreased seed viability. Our main conclusions are: (1) most seeds died when exposed directly to fire; (2) PY could be alleviated during hotter fires when seeds were buried in the soil; and (3) daily temperature fluctuations in gaps also broke PY of seeds on the soil surface, so many seeds could be recruited or die before being incorporated into the soil seed banks. Thus seed dormancy-break and germination of legumes from Cerrado open savannas seem to be driven by both fire and temperature fluctuations.

Dodder seeds are physically dormant because of hard seed coats and do not readily germinate without scarification. Reliable methods of scarification for small lots of dodder seed are needed to facilitate laboratory, greenhouse, and field research projects. Dodder seed was scarified for varying times using a handheld rotary tool at the 10,000 rpm setting with a conical grinding-stone bit attached. Percentage of germination and weight change of seeds were assessed using scarification times between 0 and 4 min at 0.5-min increments. Mean seed weight loss and mean number of germinated seeds increased quadratically as scarification time increased. Scarifying for 2.5 min was judged the shortest time with maximal germination. Another study evaluated the effect of seed number (100 to 400 seeds sample−1) on the efficacy of rotary tool scarification when scarification time was held constant at 2.5 min. Percentage of germination decreased linearly as seed batch size increased. The handheld rotary tool provides consistent and repeatable scarification of dodder seed with germination rates greater than 80%.

An important factor in controlling invasive plant infestations is frequently the acceleration of the deterioration of their persistent seed bank, which is often associated with physical dormancy mechanisms. We hypothesized that breaking dormancy by heat would enhance the vulnerability of the nondormant seeds to hydrothermal stresses. The aim of the present study was to examine the effect of soil solarization treatments (heating the soil by means of polyethylene mulching) on buried Australian Acacia seeds, with emphasis on Acacia saligna L. The results of three field experiments indicate that soil solarization treatments caused an almost complete eradication of buried seeds of Acacia saligna and two other Australian Acacia species, Acacia murrayana and Acacia sclerosperma. The killing mechanism of solarization was further studied in laboratory experiments. We observed two phases of the heat-induced deterioration of seed persistence: breaking the dormancy of the seeds and exposing the “weakened seeds” to lethal temperatures. From an ecological perspective of conservation, the present study shows for the first time the possible utilization of solar energy, by means of soil solarization, for reducing persistent seed banks of invasive woody plants.

This article briefly reviews how long weed seeds can live in the soil and what happens to them during burial. Freshly matured seeds of many weeds are water-permeable, but those of others are water-impermeable. Water-permeable seeds may have morphological, physiological, or morphophysiological dormancy, with physiological dormancy (PD) being the one most commonly found in buried weed seeds. Further, nondeep PD is very common in buried weed seeds, and many of them exhibit annual dormancy cycles in response to seasonal temperature changes. The time of year when seeds are nondormant varies with the species, i.e., autumn, spring, or spring to summer. A light requirement for germination plays an important role in preventing nondormant seeds from germinating in the soil. To germinate, soil disturbance that exposes seeds to light must occur at a time of year when seeds are nondormant. Buried seeds of some species come out of dormancy and remain nondormant regardless of seasonal changes in environmental conditions; however, a light requirement for germination prevents them from germinating in the soil. Water-impermeable seeds have either physical dormancy (PY) or a combination of PY and PD, with PY being the most common, e.g., in members of the Fabaceae and Malvaceae. Seeds with PY have a water gap in the seed coat that opens in response to an environmental signal, thereby allowing water to enter. When disturbance brings seeds to the soil surface, temperatures that are higher than those in the soil can cause the water gap to open. Consequently, the water gap indirectly serves as a depth sensor. A challenge for the future is to use information about buried weed seeds to better manage weeds in crops, and modeling is an important step in that direction.

In fire-prone ecosystems, many species regenerate after fire from persistent soil seed banks. Species with physically dormant (PY) seeds have dormancy broken by fire-related heat. The magnitude of post-fire recruitment, to predict response to varying fire severity, is commonly estimated by testing dormancy-breaking temperature thresholds of fresh PY seeds. However, seeds spend years in the soil during the inter-fire period, and determining whether dormancy-breaking thresholds change over time is essential to accurately predict population persistence. Germination of four south-eastern Australian PY species from the Fabaceae family (Acacia linifolia, Aotus ericoides, Bossiaea heterophylla and Viminaria juncea) were studied. Dormancy-breaking temperature thresholds vary inter-specifically and the species represented either high or low dormancy-breaking threshold classes. Freshly collected seeds, and seeds that had been buried in the field or stored in dry laboratory conditions for 6 and 18 months were subjected to a fire-related range of heat treatments (40–100°C). Seed ageing increased germination response to heat treatments, effectively lowering the dormancy-breaking thresholds of three species. The fourth species, A. linifolia, initially had a relatively large non-dormant fraction which was lost as seeds aged, with older seeds then displaying PY broadly similar to the other study species. Patterns of threshold decay were species-specific, with the thresholds and viability of low-threshold species declining more rapidly than high-threshold species. The non-dormant fraction did not increase over time for any of our study species. Instead of increasing their non-dormant fraction, as is common in other vegetation types, these fire-prone PY species displayed a change of dormancy-breaking temperature thresholds. This is an important distinction, as maintaining dormancy during the inter-fire period is essential for population persistence. While changes in sensitivity to dormancy-breaking treatments have previously been reported as seeds age, our study provides the first test of changes to temperature thresholds, which increases the range of germination response from the seed bank under varying fire severity.

Hypseocharis is a genus endemic to the high Andes and sister to all other Geraniaceae genera. Regarding its basal position in Geraniaceae evolution, its germination ecology can provide important insights into the early evolution of physical dormancy. Imbibition tests performed on seeds of two Hypseocharis populations from Bolivia indicate that their seeds indeed have physical dormancy like all other Geraniaceae. These results indicate that physical dormancy in Geraniaceae evolved during the Eocene before the uplift of the Andes mountains and before the events that led to the cross-Atlantic disjunct distribution of Geraniacae.

Physical dormancy (PY) plays a crucial role in the control of the reseeding process of Vicia villosa Roth, a winter annual species cultivated for pasture and hay, naturalized in several semi-arid temperate agroecosystems. As PY is considered a seed trait modulated by natural selection, populations from different origins are expected to show different responses to environmental regulatory factors. The present study aimed to determine the effect of: (1) water availability on PY-break dynamics of a naturalized population from Argentina (ASC) under both laboratory and field conditions; (2) the seed source on initial PY and dormancy release rate (wet storage at 20°C) of ASC compared to 45 other populations of V. villosa, including wild, naturalized, landraces and cultivars. Water availability increased PY loss rate under both storage and field conditions. ASC PY-break dynamics was adequately described by a Gompertz model with a lower thermal-time requirement estimated for dormancy break under fluctuating soil water conditions compared to seeds buried inside impermeable bags. During the field burial experiment, a considerable proportion of seeds (~70%) became water permeable during the summer season after dispersal, and retained low levels of residual PY for soil seed bank replenishment. Improved populations (i.e. breeding cultivars) showed the lowest percentages of initial PY compared to landraces, naturalized and wild populations. Naturalized populations of Argentina showed similar initial PY compared to landraces, although PY release rate was lower in the former and might be attributed to local environmental selection. Wild types showed the lowest PY release rates.