Seed dormancy profiles for forest dynamics plot data: focusing on a tropical seasonal rainforest in Xishuangbanna, southwest China.

Long-term forest dynamics monitoring plots provide information on number of individual species in the plot, allowing us for the first time to construct seed dormancy profiles at the species and individual levels for a specific site. Focusing on the Xishuangbanna tropical season rainforest plot (XTRDP), we used data from nine forest dynamics plots (two for tropical, four for subtropical and three for temperate) and information on kind of seed dormancy to generate seed dormancy profiles for communities across tropical to temperate latitudes at the species and individual levels. Seed dormancy information was collected from previous publications, and some data were provided by two germplasm banks that test seed germination of wild plants in China. In XTRDP, 35% of the species and 58% of individuals have non-dormant seeds; the dominant species have non-dormant seeds. In all plots, the most common kind of dormancy among species and individuals with dormant seeds was physiological dormancy. At the species level, the profile for tropical, subtropical and temperate plots was similar to profiles for each of these vegetation regions. In all plots, except one subtropical plot, the percentage of species versus individuals with dormant seeds differed. All temperate plots had a higher percentage of individuals than species with dormant seeds, but this pattern was not consistent for tropical or subtropical plots. We show that dormancy increases with latitude at both the species and individual levels. Dormancy profiles at the individual tree level provide new insight into seed dormancy relationships within plant communities.

Seed dormancy of Cardiospermum halicacabum (Sapindaceae) from three precipitation zones in Sri Lanka.

This study investigated seed germination of Cardiospermum halicacabum, a medicinally important invasive species. We compared mass, moisture content (MC), dormancy and dormancy-breaking treatments and imbibition and germination of scarified and non-scarified seeds of C. halicacabum from a low-elevation dry zone (DZ), low-elevation wet zone (WZ1) and mid-elevation wet zone (WZ2) in Sri Lanka to test the hypothesis that the percentage of seeds with water-impermeable seed coats (physical dormancy, PY) decreases with increased precipitation. Seed mass was higher in WZ2 than in DZ and WZ1, while seed MC did not vary among the zones. All scarified DZ, WZ1 and WZ2 and non-scarified DZ and WZ1 seeds imbibed water, but only a few non-scarified WZ2 seeds did so. When DZ and WZ1 seeds were desiccated, MC and percentage imbibition decreased, showing that these seeds have the ability to develop PY. GA3 promoted germination of embryos excised from fresh DZ and WZ1 seeds and of scarified WZ2 seeds. At maturity, seeds from DZ and WZ1 had only physiological dormancy (PD), while those from WZ2 had combinational dormancy (PY+PD). Thus, our hypothesis was not supported. Since a high percentage of excised embryos developed into normal seedlings; this is a low-cost method to produce C. halicacabum plants for medicinal and ornamental purposes.

Linking seed germination and plant height: a case study of a wetland community on the eastern Tibet Plateau.

Seed germination is the earliest trait expressed in a plant's life history, and it can directly affect the expression of post-germination traits. Plant height is central to plant ecological strategies, because it is a major determinant of the ability of a species to compete for light. Thus, linking seed germination and plant height at the community level is very important to understanding plant fitness and community structure. Here, we tested storage condition and temperature requirements for germination of 31 species from a wetland plant community on the eastern Tibet Plateau and analysed correlation of germination traits with plant height in relation to storage condition. Germination percentage was positively related to plant height, and this relationship disappeared when seeds were incubated at a low temperature (i.e. 5 °C) or after they were stored under wet-cold conditions. The response of seeds to dry+wet-cold storage was negatively related to plant height. Based on the scores of each species on the first two principal components derived from PCA, species were classified into two categories by hierarchical clustering, and there was a significant difference between germination and plant height of species in these two categories. These results suggest that the requirements for seed germination together with seasonal change in environmental conditions determine the window for germination and, in turn, plant growth season and resource utilisation and ultimately plant height.

Sensitivity cycling in physically dormant seeds of the Neotropical tree Senna multijuga (Fabaceae).

Cycling of sensitivity to physical dormancy (PY) break has been documented in herbaceous species. However, it has not been reported in tree seeds, nor has the effect of seed size on sensitivity to PY-breaking been evaluated in any species. Thus, the aims of this study were to investigate how PY is broken in seeds of the tropical legume tree Senna multijuga, if seeds exhibit sensitivity cycling and if seed size affects induction into sensitivity. Dormancy and germination were evaluated in intact and scarified seeds from two collections of S. multijuga. The effects of temperature, moisture and seed size on induction of sensitivity to dormancy-breaking were assessed, and seasonal changes in germination and persistence of buried seeds were determined. Reversal of sensitivity was also investigated. Fresh seeds were insensitive to dormancy break at wet-high temperatures, and an increase in sensitivity occurred in buried seeds after they experienced low temperatures during winter (dry season). Temperatures ≤20 °C increased sensitivity, whereas temperatures ≥30 °C decreased it regardless of moisture conditions. Dormancy was broken in sensitive seeds by incubating them at 35 °C. Sensitivity could be reversed, and large seeds were more sensitive than small seeds to sensitivity induction. Seeds of S. multijuga exhibit sensitivity cycling to PY-breaking. Seeds become sensitive during winter and can germinate with the onset of the spring-summer rainy season in Brazil. Small seeds are slower to become sensitive than large ones, and this may be a mechanism by which germination is spread over time. Sensitive seeds that fail to germinate become insensitive during exposure to drought during summer. This is the first report of sensitivity cycling in a tree species.

Effect of population, collection year, after-ripening and incubation condition on seed germination of Stipa bungeana.

Knowledge of the germination behavior of different populations of a species can be useful in the selection of appropriate seed sources for restoration. The aim of this study was to test the effect of seed population, collection year, after-ripening and incubation conditions on seed dormancy and germination of Stipa bungeana, a perennial grass used for revegetation of degraded grasslands on the Loess Plateau, China. Fresh S. bungeana seeds were collected from eight locally-adapted populations in 2015 and 2016. Dormancy and germination characteristics of fresh and 6-month-old dry-stored seeds were determined by incubating them over a range of alternating temperature regimes in light. Effect of water stress on germination was tested for fresh and 6-month-old dry-stored seeds. Seed dormancy and germination of S. bungeana differed with population and collection year. Six months of dry storage broke seed dormancy, broadened the temperature range for germination and increased among-population differences in germination percentage. The rank order of germination was not consistent in all germination tests, and it varied among populations. Thus, studies on comparing seed dormancy and germination among populations must consider year of collection, seed dormancy states and germination test conditions when selecting seeds for grassland restoration and management.

A graphical method for identifying the six types of non-deep physiological dormancy in seeds.

We present a new seed dormancy classification scheme for the non-deep level of the class physiological dormancy (PD), which contains six types. Non-deep PD is divided into two sublevels: one for seeds that exhibit a dormancy continuum (types 1, 2 and 3) and the other for those that do not exhibit a dormancy continuum (types 4, 5 and 6). Analysis of previous studies showed that different types of non-deep PD also can be identified using a graphical method. Seeds with a dormancy (D) ↔ conditional dormancy (CD) ↔ non-dormancy (ND) cycle have a low germination percentage in the early stages of CD, and during dormancy loss the germination capacity increases. However, seeds with a CD/ND (i.e. D→CD↔ND) cycle germinate to a high percentage at a narrow range of temperatures in the early stages of CD. Cardinal temperatures for seeds with either a D/ND or a CD/ND cycle change during dormancy loss: the ceiling temperature increases in seeds with Type 1, the base temperature decreases in seeds with Type 2 and the base and ceiling temperatures decrease and increase, respectively, in seeds with Type 3. Criteria for distinguishing the six types of non-deep PD and models of the temperature functions of seeds with types 1, 2 and 3 with both types of dormancy cycles are presented. The relevancy of our results to modelling the timing of weed seedling emergence is briefly discussed.

Delayed dehiscence of the pericarp: role in germination and retention of viability of seeds of two cold desert annual Brassicaceae species.

Considerable variation occurs in post-maturity timing of dehiscence in fruits of Brassicaceae species, and several studies have shown that the pericarp plays an important role in seed germination and retention of viability in species with indehiscent fruits. However, little is known about the significance to seed biology of delay in pericarp dehiscence for <1 year in the field. Thus, we determined the role of the pericarps of Leptaleum filifolium and Neotorularia korolkovii, which open in <1 year after fruit maturity and dispersal, in seed germination and retention of seed viability. We compared dormancy-break via after-ripening in the laboratory and germination phenology and retention of seed viability in intact siliques and isolated seeds buried in an experimental garden. Seeds of both species have Type 6 non-deep physiological dormancy, which is enhanced by the pericarp. Seeds of both species after-ripened during summer 2013, and some of them germinated in autumn and some in the following spring in watered and non-watered soil. Germination percentages of seeds in siliques increased in soil in spring 2014, after the pericarps had opened. Most isolated seeds of L. filifolium and N. korolkovii had germinated or were dead by spring 2014 and summer 2015, respectively, whereas 60% of the seeds of both species in the (opened) pericarps were viable after 24 months. Thus, although the pericarp opened 9-10 months after burial, its presence had a significant effect on seed dormancy, germination phenology and retention of viability of seeds of L. filifolium and N. korolkovii.

Identification and characterization of ten new water gaps in seeds and fruits with physical dormancy and classification of water-gap complexes.

BACKGROUND AND AIMS: Physical dormancy (PY) occurs in seeds or fruits of 18 angiosperm families and is caused by a water-impermeable palisade cell layer(s) in seed or fruit coats. Prior to germination, the seed or fruit coat of species with PY must become permeable in order to imbibe water. Breaking of PY involves formation of a small opening(s) (water gap) in a morpho-anatomically specialized area in seeds or fruits known as the water-gap complex. Twelve different water-gap regions in seven families have previously been characterized. However, the water-gap regions had not been characterized in Cucurbitaceae; clade Cladrastis of Fabaceae; subfamilies Bombacoideae, Brownlowioideae and Bythnerioideae of Malvaceae; Nelumbonaceae; subfamily Sapindoideae of Sapindaceae; Rhamnaceae; or Surianaceae. The primary aims of this study were to identify and describe the water gaps of these taxa and to classify all the known water-gap regions based on their morpho-anatomical features. METHODS: Physical dormancy in 15 species was broken by exposing seeds or fruits to wet or dry heat under laboratory conditions. Water-gap regions of fruits and seeds were identified and characterized by use of microtome sectioning, light microscopy, scanning electron microscopy, dye tracking and blocking experiments. KEY RESULTS: Ten new water-gap regions were identified in seven different families, and two previously hypothesized regions were confirmed. Water-gap complexes consist of (1) an opening that forms after PY is broken; (2) a specialized structure that occludes the gap; and (3) associated specialized tissues. In some species, more than one opening is involved in the initial imbibition of water. CONCLUSIONS: Based on morpho-anatomical features, three basic water-gap complexes (Types-I, -II and -III) were identified in species with PY in 16 families. Depending on the number of openings involved in initial imbibition, the water-gap complexes were sub-divided into simple and compound. The proposed classification system enables understanding of the relationships between the water-gap complexes of taxonomically unrelated species with PY.

Quantitative analysis of the thermal requirements for stepwise physical dormancy-break in seeds of the winter annual Geranium carolinianum (Geraniaceae).

BACKGROUND AND AIMS: Physical dormancy (PY)-break in some annual plant species is a two-step process controlled by two different temperature and/or moisture regimes. The thermal time model has been used to quantify PY-break in several species of Fabaceae, but not to describe stepwise PY-break. The primary aims of this study were to quantify the thermal requirement for sensitivity induction by developing a thermal time model and to propose a mechanism for stepwise PY-breaking in the winter annual Geranium carolinianum. METHODS: Seeds of G. carolinianum were stored under dry conditions at different constant and alternating temperatures to induce sensitivity (step I). Sensitivity induction was analysed based on the thermal time approach using the Gompertz function. The effect of temperature on step II was studied by incubating sensitive seeds at low temperatures. Scanning electron microscopy, penetrometer techniques, and different humidity levels and temperatures were used to explain the mechanism of stepwise PY-break. KEY RESULTS: The base temperature (Tb) for sensitivity induction was 17·2 °C and constant for all seed fractions of the population. Thermal time for sensitivity induction during step I in the PY-breaking process agreed with the three-parameter Gompertz model. Step II (PY-break) did not agree with the thermal time concept. Q10 values for the rate of sensitivity induction and PY-break were between 2·0 and 3·5 and between 0·02 and 0·1, respectively. The force required to separate the water gap palisade layer from the sub-palisade layer was significantly reduced after sensitivity induction. CONCLUSIONS: Step I and step II in PY-breaking of G. carolinianum are controlled by chemical and physical processes, respectively. This study indicates the feasibility of applying the developed thermal time model to predict or manipulate sensitivity induction in seeds with two-step PY-breaking processes. The model is the first and most detailed one yet developed for sensitivity induction in PY-break.

The autumn effect: timing of physical dormancy break in seeds of two winter annual species of Geraniaceae by a stepwise process.

BACKGROUND AND AIMS: The involvement of two steps in the physical dormancy (PY)-breaking process previously has been demonstrated in seeds of Fabaceae and Convolvulaceae. Even though there is a claim for a moisture-controlled stepwise PY-breaking in some species of Geraniaceae, no study has evaluated the role of temperature in the PY-breaking process in this family. The aim of this study was to determine whether a temperature-controlled stepwise PY-breaking process occurs in seeds of the winter annuals Geranium carolinianum and G. dissectum. METHODS: Seeds of G. carolinianum and G. dissectum were stored under different temperature regimes to test the effect of storage temperature on PY-break. The role of temperature and moisture regimes in regulating PY-break was investigated by treatments simulating natural conditions. Greenhouse (non-heated) experiments on seed germination and burial experiments (outdoors) were carried out to determine the PY-breaking behaviour in the natural habitat. KEY RESULTS: Irrespective of moisture conditions, sensitivity to the PY-breaking step in seeds of G. carolinianum was induced at temperatures ≥20 °C, and exposure to temperatures ≤20 °C made the sensitive seeds permeable. Sensitivity of seeds increased with time. In G. dissectum, PY-break occurred at temperatures ≥20 °C in a single step under constant wet or dry conditions and in two steps under alternate wet-dry conditions if seeds were initially kept wet. CONCLUSIONS: Timing of seed germination with the onset of autumn can be explained by PY-breaking processes involving (a) two temperature-dependent steps in G. carolinianum and (b) one or two moisture-dependent step(s) along with the inability to germinate under high temperatures in G. dissectum. Geraniaceae is the third of 18 families with PY in which a two-step PY-breaking process has been demonstrated.

Pollen and seed desiccation tolerance in relation to degree of developmental arrest, dispersal, and survival.

In most species, arrest of growth and a decrease in water content occur in seeds and pollen before they are dispersed. However, in a few cases, pollen and seeds may continue to develop (germinate). Examples are cleistogamy and vivipary. In all other cases, seeds and pollen are dispersed with a variable water content (2-70%), and consequently they respond differently to environmental relative humidity that affects dispersal and maintenance of viability in time. Seeds with low moisture content shed by the parent plant after maturation drying can generally desiccate further to moisture contents in the range of 1-5% without damage and have been termed 'orthodox'. Pollen that can withstand dehydration also was recently termed orthodox. Seeds and pollen that do not undergo maturation drying and are shed at relatively high moisture contents (30-70%) are termed 'recalcitrant'. Since recalcitrant seeds and pollen are highly susceptible to desiccation damage, they cannot be stored under conditions suitable for orthodox seeds and pollen. Hence, there are four types of plants with regard to tolerance of pollen and seeds to desiccation. Orthodoxy allows for dispersal over greater distances, longer survival, and greater resistance to low relative humidity. The advantage of recalcitrance is fast germination. Orthodoxy and recalcitrance are often related to environment rather than to systematics. It has been postulated that certain types of genes are involved during presentation and dispersal of pollen and seeds, since molecules (sucrose, polyalcohols, late embryogenic abundant proteins, antioxidants, etc.) that protect different cell compartments during biologically programmed drying have been detected in both.

Acquisition of physical dormancy and ontogeny of the micropyle--water-gap complex in developing seeds of Geranium carolinianum (Geraniaceae).

BACKGROUND AND AIMS: The 'hinged valve gap' has been previously identified as the initial site of water entry (i.e. water gap) in physically dormant (PY) seeds of Geranium carolinianum (Geraniaceae). However, neither the ontogeny of the hinged valve gap nor acquisition of PY by seeds of Geraniaceae has been studied previously. The aims of the present study were to investigate the physiological events related to acquisition of PY and the ontogeny of the hinged valve gap and seed coat of G. carolinianum. METHODS: Seeds of G. carolinianum were studied from the ovule stage until dispersal. The developmental stages of acquisition of germinability, physiological maturity and PY were determined by seed measurement, germination and imbibition experiments using intact seeds and isolated embryos of both fresh and slow-dried seeds. Ontogeny of the seed coat and water gap was studied using light microscopy. KEY RESULTS: Developing seeds achieved germinability, physiological maturity and PY on days 9, 14 and 20 after pollination (DAP), respectively. The critical moisture content of seeds on acquisition of PY was 11 %. Slow-drying caused the stage of acquisition of PY to shift from 20 to 13 DAP. Greater extent of cell division and differentiation at the micropyle, water gap and chalaza than at the rest of the seed coat resulted in particular anatomical features. Palisade and subpalisade cells of varying forms developed in these sites. A clear demarcation between the water gap and micropyle is not evident due to their close proximity. CONCLUSIONS: Acquisition of PY in seeds of G. carolinianum occurs after physiological maturity and is triggered by maturation drying. The micropyle and water gap cannot be considered as two separate entities, and thus it is more appropriate to consider them together as a 'micropyle--water-gap complex'.

Identification and characterization of the water gap in physically dormant seeds of Geraniaceae, with special reference to Geranium carolinianum.

BACKGROUND AND AIMS: Physical dormancy in seeds of species of Geraniaceae is caused by a water-impermeable palisade layer in the outer integument of the seed coat and a closed chalaza. The chalazal cleft has been reported to be the water gap (i.e. location of initial water entry) in innately permeable seeds of Geraniaceae. The primary aim of this study was to re-evaluate the location of the water gap and to characterize its morphology and anatomy in physically dormant seeds of Geraniaceae, with particular reference to G. carolinianum. METHODS: Length, width, mass, anatomy and germination of two seed types (light brown and dark brown) of G. carolinianum were compared. Location, anatomy and morphology of the water gap were characterized using free-hand and microtome tissue sectioning, light microscopy, scanning electron microscopy, dye tracking, blocking and seed-burial experiments. KEY RESULTS: Treatment with dry heat caused a colour change in the palisade cells adjacent to the micropyle. When placed in water, the 'hinged valve' (blister) erupted at the site of the colour change, exposing the water gap. The morphology and anatomy in the water-gap region differs from those of the rest of the seed coat. the morphology of the seed coat of the water-gap region is similar in G. carolinianum, G. columbinum, G. molle and G. pusillum and differs from that of the closely related species Erodium cicutarium. CONCLUSIONS: Dislodgment of swollen 'hinged valve' palisade cells adjacent to the micropyle caused the water gap to open in physically dormant seeds of G. carolinianum, and it was clear that initial water uptake takes place through this gap and not via the chalazal opening as previously reported. This water gap ('hinged valve gap') differs from water gaps previously described for other families in morphology, anatomy and location in the seed coat.

Identification and characterization of the water gap in the physically dormant seeds of Dodonaea petiolaris: a first report for Sapindaceae.

BACKGROUND AND AIMS: The Sapindaceae is one of 17 plant families in which seed dormancy is caused by a water-impermeable seed or fruit coat (physical dormancy, PY). However, until now the water gap in Sapindaceae had not been identified. The primary aim of this study was to identify the water gap in Dodonaea petiolaris (Sapindaceae) seeds and to describe its basic morphology and anatomy. METHODS: Seed fill, viability, water-uptake (imbibition) and other characteristics were assessed for D. petiolaris seeds. The location and structure of the water gap were investigated using a blocking experiment, time series photography, scanning electron microscopy and light microscopy. Dodonaea petiolaris seeds with PY also were assessed for loss of PY at four ecologically significant temperatures under moist and dry conditions. Seeds of three other species of Sapindaceae were examined for presence of a water gap. KEY RESULTS: The water gap in D. petiolaris seeds was identified as a small plug in the seed coat adjacent to the hilum and opposite the area where the radicle emerges. The plug was dislodged (i.e. water gap opened = dormancy break) by dipping seeds in boiling water for 2.5 min or by incubating seeds on a moist substrate at 20/35 degrees C for 24 weeks. Layers of cells in the plug, including palisade and subpalisade, are similar to those in the rest of the seed coat. The same kind of water gap was found in three other species of Sapindaceae, Diplopeltis huegelii, Distichostemon hispidulus and Dodonaea aptera. CONCLUSIONS: Following dormancy break (opening of water gap), initial uptake of water by the seed occurs only through the water gap. Thus, the plug must be dislodged before the otherwise intact seed can germinate. The anatomy of the plug is similar to water gaps in some of the other plant families with PY.

Occurrence of physical dormancy in seeds of Australian Sapindaceae: a survey of 14 species in nine genera.

BACKGROUND AND AIMS: Sapindaceae is one of 16 angiosperm families whose seeds have physical dormancy (PY). However, the extent and nature of PY within this family is poorly known. The primary aims of this study were: (1) to evaluate seed characteristics and determine presence (or not) of PY within nine genera of Australian Sapindaceae; and (2) to compare the frequency of PY across the phylogenetic tree within Australian Sapindaceae. METHODS: Viability, imbibition and seed characteristics were assessed for 14 taxa from nine genera of Sapindaceae. For five species of Dodonaea, optimal conditions for germination and dormancy break were evaluated. An in situ burial experiment was performed on D. hackettiana seeds to identify the factor(s) responsible for overcoming PY. Classes of dormancy and of non-dormancy for 26 genera of Sapindaceae were mapped onto a phylogenetic tree for the family. KEY RESULTS: Mean seed viability across all taxa was 69.7 %. Embryos were fully developed and folded (seven genera) or bent (two genera); no endosperm was present. Seeds of all five Dodonaea spp. and of Distichostemon hispidulus had PY. Hot-water treatment released PY in these six species. Optimal germination temperature for seeds of the four Dodonaea spp. that germinated was 15-20 degrees C. Following 5 months burial in soil, 36.4 % of D. hackettiana seeds had lost PY and germinated by the beginning of the winter wet season (May). Laboratory and field data indicate that dormancy was broken by warm, moist temperatures (> or =50 degrees C) during summer. CONCLUSIONS: PY occurs infrequently in genera of Sapindaceae native to Australia. Seeds of Dodonaea and Distichostemon had PY, whereas those of the other seven genera did not. Seeds of these two genera and of Diplopeltis (a previous study) are the only three of the 20 native Australian genera of Sapindaceae for which germination has been studied that have PY; all three belong to subfamily Dodonaeoideae.

Cycling of sensitivity to physical dormancy-break in seeds of Ipomoea lacunosa (Convolvulaceae) and ecological significance.

BACKGROUND AND AIMS: Although a claim has been made that dormancy cycling occurs in seeds of Ipomoea lacunosa (Convolvulaceae) with physical dormancy, this would seem to be impossible since the water gap cannot be closed again after it opens (dormancy break). On the other hand, changes in sensitivity (sensitive <--> non-sensitive) to dormancy-breaking factors have been reported in seeds of Fabaceae with physical dormancy. The primary aim of the present study was to determine if sensitivity cycling also occurs in physically dormant seeds of I. lacunosa. METHODS: Treatments simulating conditions in the natural habitat of I. lacunosa were used to break seed dormancy. Storage of seeds at temperatures simulating those in spring, summer, autumn and winter were tested for their effect on sensitivity change. Seeds made non-dormant were stored dry in different temperature regimes to test for dormancy cycling. In addition, seeds collected on different dates (i.e. matured under different climatic conditions) were used to test for maternal effects on sensitivity to dormancy-breaking factors. KEY RESULTS: Sensitivity was induced by storing seeds under wet conditions and reversed by storing them under dry conditions at low (< or = 5 degrees C) or high (> or = 30 degrees C) temperatures, demonstrating that seeds of I. lacunosa can cycle between sensitive and insensitive states. Sensitive seeds required > or = 2 h at 35 degrees C on moist sand for release of dormancy. However, there is no evidence to support dormancy cycling per se. Conceptual models are proposed for sensitivity cycling and germination phenology of I. lacunosa in the field. CONCLUSIONS: Seasonal germination behaviour of physically dormant I. lacunosa seeds can be explained by sensitivity cycling but not by dormancy cycling per se. Convolvulaceae is only the second of 16 families known to contain species with physical dormancy for which sensitivity cycling has been demonstrated.

Seed anatomy and water uptake in relation to seed dormancy in Opuntia tomentosa (Cactaceae, Opuntioideae).

BACKGROUND AND AIMS: There is considerable confusion in the literature concerning impermeability of seeds with 'hard' seed coats, because the ability to take up (imbibe) water has not been tested in most of them. Seeds of Opuntia tomentosa were reported recently to have a water-impermeable seed coat sensu lato (i.e. physical dormancy), in combination with physiological dormancy. However, physical dormancy is not known to occur in Cactaceae. Therefore, the aim of this study was to determine if seeds of O. tomentosa are water-permeable or water-impermeable, i.e. if they have physical dormancy. METHODS: The micromorphology of the seed coat and associated structures were characterized by SEM and light microscopy. Permeability of the seed-covering layers was assessed by an increase in mass of seeds on a wet substrate and by dye-tracking and uptake of tritiated water by intact versus scarified seeds. KEY RESULTS: A germination valve and a water channel are formed in the hilum-micropyle region during dehydration and ageing in seeds of O. tomentosa. The funicular envelope undoubtedly plays a role in germination of Opuntia seeds via restriction of water uptake and mechanical resistance to expansion of the embryo. However, seeds do not exhibit any of three features characteristic of those with physical dormancy. Thus, they do not have a water-impermeable layer(s) of palisade cells (macrosclereids) or a water gap sensu stricto and they imbibe water without the seed coat being disrupted. CONCLUSIONS: Although dormancy in seeds of this species can be broken by scarification, they have physiological dormancy only. Further, based on information in the literature, it is concluded that it is unlikely that any species of Opuntia has physical dormancy. This is the first integrative study of the anatomy, dynamics of water uptake and dormancy in seeds of Cactaceae subfamily Opuntioideae.

Ecophysiology of seed dormancy in the Australian endemic species Acanthocarpus preissii (Dasypogonaceae).

BACKGROUND AND AIMS: Seedlings of Acanthocarpus preissii are needed for coastal sand dune restoration in Western Australia. However, seeds of this Western Australian endemic have proven to be very difficult to germinate. The aims of this study were to define a dormancy-breaking protocol, identify time of suitable conditions for dormancy-break in the field and classify the type of seed dormancy in this species. METHODS: Viability, water-uptake (imbibition) and seed and embryo characteristics were assessed for seeds collected in 2003 and in 2004 from two locations. The effects of GA(3), smoke-water, GA(3) + smoke-water and warm stratification were tested on seed dormancy-break. In a field study, soil temperature and the moisture content of soil and buried seeds were monitored for 1 year. KEY RESULTS: Viability of fresh seeds was >90 %, and they had a fully developed, curved-linear embryo. Fresh seeds imbibed water readily, with mass increasing approx. 52 % in 4 d. Non-treated fresh seeds and those exposed to 1000 ppm GA(3), 1 : 10 (v/v) smoke-water/water or 1000 ppm GA(3) + 1 : 10 (v/v) smoke-water/water germinated <8 %. Fresh seeds germinated to >80 % when warm-stratified for at least 7 weeks at 18/33 degrees C and then moved to 7/18 degrees C, whereas seeds incubated continuously at 7/18 degrees C germinated to <20 %. CONCLUSIONS: Seeds of A. preisii have non-deep physiological dormancy that is released by a period of warm stratification. Autumn (March/April) is the most likely time for warm stratification of seeds of this species in the field. This is the first report of the requirement for warm stratification for dormancy release in seeds of an Australian species.

Dormancy-breaking and germination requirements for seeds of Symphoricarpos orbiculatus (Caprifoliaceae).

Fruits (drupes) of Symphoricarpos orbiculatus ripen in autumn and are dispersed from autumn to spring. Seeds (true seed plus fibrous endocarp) are dormant at maturity, and they have a small, linear embryo that is underdeveloped. In contrast to previous reports, the endocarp and seed coat of S. orbiculatus are permeable to water; thus, seeds do not have physical dormancy. No fresh seeds germinated during 2 wk of incubation over a 15°/6°-35°/20°C range of thermoperiods in light (14-h photoperiod); gibberellic acid and warm or cold stratification alone did not overcome dormancy. One hundred percent of the seeds incubated in a simulated summer → autumn → winter → spring sequence of temperature regimes germinated, whereas none of those subjected to a winter → spring sequence did so. That is, cold stratification is effective in breaking dormancy only after seeds first are exposed to a period of warm temperatures. Likewise, embryos grew at cold temperatures only after seeds were exposed to warm temperatures. Thus, the seeds of S. orbiculatus have nondeep complex morphophysiological dormancy. As a result of dispersal phenology and dormancy-breaking requirements, in nature most seeds that germinate do so the second spring following maturity; a low to moderate percentage of the seeds may germinate the third spring. Seeds can germinate to high percentages under Quercus leaf litter and while buried in soil; they have little or no potential to form a long-lived soil seed bank.

Morphophysiological dormancy in seeds of two North American and one Eurasian species of Sambucus (Caprifoliaceae) with underdeveloped spatulate embryos.

In contrast to previous reports, the endocarps ("seed coats") of Sambucus species are not impermeable to water; thus, the seeds do not have physical dormancy. Seeds of the North American species Sambucus canadensis and S. pubens and of the European species S. racemosa have spatulate shaped embryos that are ∼60% fully developed (elongated) at seed maturity. The embryo has to extend to the full length of the seed to germinate. Embryos in freshly matured seeds of S. canadensis and in those of S. pubens grew better at 25°/15°C than at 5°C, whereas the rate of embryo growth in S. racemosa was higher at 5°C than at 25°/15°C. Seeds of all three species germinated to significantly higher percentages in light (14-h photoperiod) than in darkness. Fresh seeds of neither species germinated during 2 wk of incubation over a range of thermoperiods. Warm followed by cold stratification broke dormancy in seeds of S. canadensis and in those of S. pubens. Thus, seeds of these two North American species have deep simple morphophysiological dormancy (MPD). In comparison, seeds of the European species S. racemosa required a cold stratification period only for dormancy break, and thus they have intermediate complex MPD. GA(3) was much more effective in breaking dormancy in seeds of S. racemosa than it was in those of S. canadensis or S. pubens.