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In this review, we explore the origin of the rudimentary embryo, its relationship to other kinds of plant embryos and its role in the diversification of angiosperms. Rudimentary embryos have a length:width ratio of ≤2.0, and they have organs, including cotyledon(s) and a primary root. A literature survey failed to reveal rudimentary embryos in the pre-angiosperms, suggesting that this kind of embryo is an angiosperm invention. Although proembryos of some gymnosperms and angiosperms have a length:width ratio of ≤2.0, they have not formed meristems or organs. Thus, rudimentary embryos are not proembryos. During the development of rudimentary embryos in monocots and dicots (all non-monocots), the growth pattern of the epicotyledonary cells differs, resulting in differences in the placement of the shoot meristem and in one versus two cotyledons, respectively, but the embryo size is similar. Rudimentary embryos grow inside the seed prior to germination, which is true for linear-underdeveloped embryos, including those in some gymnosperms. Rudimentary embryos served as the starting point for the great diversification of embryos, and ultimately of seeds, in angiosperms, and they are still present in many families of extant angiosperms. The rudimentary embryo is part of the syndrome of changes, including increased speed of pollen germination and pollen tube growth, simplification of the female gametophyte, development of endosperm and elimination of multiple embryo production from each zygote, that distinguish angiosperm seed production from that of gymnosperms. We conclude that the rudimentary embryo was one of many new developments of angiosperms that contributed to their great success on earth.
The Asteraceae with up to 30,000 species occurs on all continents except Antarctica and in all major vegetation zones on earth. Our primary aim was to consider cypselae dormancy-break and germination of Asteraceae in relation to ecology, vegetation zones and evolution. Cypselae are desiccation-tolerant and in various tribes, genera, species and life forms of Asteraceae are either non-dormant (ND) or have non-deep physiological dormancy (PD) at maturity. All six types of non-deep PD are found among the Asteraceae, and dormancy is broken by cold or warm stratification or by afterripening. Soil cypselae banks may be formed but mostly are short-lived. Much within-species variation in dormancy-break and germination has been found. Using data compiled for 1192 species in 373 genera and 35 tribes of Asteraceae, we considered ND and PD in relation to life form, vegetation zone and tribe. Senecioneae and Astereae had the best representation across the vegetation zones on earth. In evergreen and semi-evergreen rainforests, more species have ND than PD, but in all other vegetation zones, except alpine/high-latitude tundra (where ND and PD are equal), more species have PD than ND. Tribes in the basal and central grades and those in the Heliantheae Alliance have both ND and PD. The high diversity and lability of non-deep PD may have enhanced the rate of species diversification by promoting the survival of new species and/or species in new habitats that became available following globally disruptive events since the origin of the Asteraceae in the Late Cretaceous.
Information in the literature and unpublished results of the authors on Dobinea were used to determine the kind [class(es)] of seed (true seed + endocarp) dormancy and of non-dormancy of genera in all five tribes of Anacardiaceae, and the results were examined in relation to the taxonomic position and endocarp anatomy within the family. Reports of both seed germination and endocarp anatomy were found for 15 genera in tribe Spondiadeae, 6 in tribe Anacardieae, 30 in tribe Rhoeae, 3 in tribe Semecarpeae and 1 in tribe Dobineeae. In Spondiadeae (Spondias-type endocarp), Anacardieae, Semecarpeae and Dobineeae (Anacardium-type endocarp), seeds are either non-dormant (ND) or have physiological dormancy (PD). In Rhoeae (Anacardium-type Rhoeae Groups A, B, C and D endocarps), on the other hand, seeds are ND or have physical dormancy (PY), PD or PY + PD. PY/PY + PD in this tribe seems to be restricted (or nearly so) to Rhus s.s. and closely related genera (e.g. Cotinus, Malosma and Toxicodendron) with an Anacardium-type Rhoeae Group A endocarp. However, seeds of other genera (e.g. Astronium and Schinus) with this type of endocarp and those with Rhoeae Group B (e.g. Pistacia), Group C (e.g. Pentaspadon) and Group D (e.g. Heeria) endocarps are either ND or have PD. The fossil fruit record strongly suggests that present-day relationships between diaspore dormancy (or non-dormancy), endocarp structure and taxonomic position within Anacardiaceae extend back to at least the Palaeogene.
Germination of seeds of some summer annuals in Kentucky (eastern USA) in late-winter lead to the hypothesis that under present climate conditions the whole length of the winter cold stratification (CS) period is not required for dormancy-break of seeds of summer annuals with physiological dormancy (PD). We evaluated our data from germination phenology studies of 45 species (69 datasets) and buried-seed studies of 33 species (44 datasets). We determined time and temperature of germination after CS and percentage of the total number of hours of CS during winter (% of winter CS) seeds received prior to start of germination. In the phenology studies, mean temperature during the week of first germination for C3 and C4 species was 11.1 and 14.4°C, respectively, and % of winter CS was 80.8 and 87.4, respectively. In the buried-seed studies, % of CS for C3 and C4 species was 40.8 and 48.1, respectively, when they germinated to 25% at 20/10°C. For 32 of 33 species in the buried-seed studies, the minimum temperature at which seeds germinated decreased with increased CS; thus, seeds had Type 2 non-deep PD. The time of germination is controlled by a number of hours of CS, a decrease in minimum temperature at which seeds can germinate and a temperature increase in early spring. Seeds can germinate at relatively high temperatures as early as December and January, but they continue to be CS until spring. Temperature increases in eastern North America due to global warming are not likely to inhibit the germination of summer annuals with PD in spring.
To persist (without immigration) in habitats with unpredictable environmental conditions, annuals must produce seeds each year or have a seed bank. Thus, we predicted that compared to perennials, annuals have a wider germination temperature range (GTR, the difference in temperature between the week with the highest and the week with the lowest germination during the natural germination season). We determined the GTR via germination phenology data for 350 herbaceous species in 59 families from the eastern USA: summer annuals (SA), 63; winter annuals (WA), 83; monocarpic perennials (MP), 28; and polycarpic perennials (PP), 176. There was no significant phylogenetic signal for the GTR. The width of the GTR during the first spring germination season was 9.6, 8.7 and 8.8°C for MP, PP and SA, respectively, and during the first autumn germination season 12.8, 11.8 and 12.4°C for MP, PP and WA, respectively. Annuals did not have a wider GTR than perennials in either the spring or the autumn germination season. Our data suggest that selection for early germination in either spring or autumn has resulted in only small differences in the GTR. We predict that global warming will have little or no effect on reshaping the germination phenology of herbaceous species of temperate eastern North America.
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.
Martin placed the lateral embryo, which occurs only in grasses, adjacent to the broad embryo at the base of his family tree of seed phylogeny. Since Poales and Poaceae are derived monocots, we questioned the evolutionary relationship between the lateral embryo and other kinds of monocot embryos. Information was compiled on embryo and seed characteristics for the various families of monocots, kind of embryogenesis for families in Poales and germination morphology of families with lateral (only Poaceae) and broad embryos. The kinds of monocot embryos are broad, capitate, lateral, linear fully developed, linear underdeveloped and undifferentiated, but only broad and lateral embryos are restricted to Poales. Asterad embryogenesis occurs in Poaceae with a lateral embryo and in Eriocaulaceae, Rapataceae and Xyridaceae with a broad embryo. In developing grass seeds, the growing scutellum (cotyledon) pushes the coleoptile, mesocotyl and coleorhiza to the side. In the organless broad embryo, the cotyledonary sector is larger than the epicotyledonary sector. During germination of grass seeds, the coleorhiza and then the coleoptile emerge, while in a seed with a broad embryo the elongating cotyledon pushes the epicotyledonary sector outside the seed, after which a root–shoot axis is differentiated at a right angle to the cotyledon inside the seed. Broad and lateral embryos are closely related; however, the lateral embryo is more advanced in seed/embryo traits and germination morphology than the other kinds of monocot embryos, suggesting that its position on the family tree of seed phylogeny should be higher than of the other monocot embryos.
Interception by plant canopies during wind dispersal can affect the final destination of diaspores. However, how the interaction of wind speed, canopy type and diaspore attributes affects interception of diaspores by the plant canopy has rarely been studied. We investigated canopy interception for 29 species with different diaspore attributes, six canopy types and six wind speeds in controlled experiments in a wind tunnel. Shrub canopy interception of diaspores were controlled by wind speed and diaspore attributes, but the latter had a greater influence on canopy interception than the former. At low wind speed, diaspore wing loading had a large influence on canopy interception, whereas at high wind speed, diaspore projection area had a large influence. The chance of canopy interception at a particular wind speed was additionally affected by the type of canopy. This study increases our knowledge of the dispersal process, corrects the previous understanding of diaspore dispersal potential and improves the theoretical basis for predicting spatial pattern and dynamics of plant populations.
‘Dust seeds’ with an undifferentiated (organless) embryo are known to be produced by mycoheterotrophic species (MH) in nine families of angiosperms. However, aside from the numerous studies on seed germination of orchids, relatively little is known about germination in MH families. In the Ericaceae, some degree of mycoheterotrophy (full, partial or initial) and dust seeds with an undifferentiated embryo occur in all species in the three tribes of Monotropoideae, the only subfamily of Ericaceae with this combination of characters. In most species, the seed is <0.90 mm in the greatest dimension, the endosperm is absent (Pityopus) or consists of few to many (30–40) cells, and the embryo is minute, consisting of as few as two cells in Monotropa. Germination in Monotropoideae is monopolar, with only the radicular pole of the embryo participating in germination. Thus, germination polarity differs from that of the dust seeds of orchids in which only the plumular pole of the embryo (protocorm) participates in germination. The dust seeds in Monotropoideae require the presence of fungi, either direct contact with a fungus or the presence of a diffusible substance therefrom, to germinate (symbiotic germination). Recently, representatives of the four genera of tribe Pyroleae have been successfully germinated asymbiotically in vitro. We present a broad overview of dust-size seeds in angiosperms and conclude that they should be subdivided into at least two major categories.
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.
Only a few studies have been performed on seed germination of perennial ephemeral species native to the cold deserts of central Asia. We hypothesized that seeds of the cold desert perennial ephemeral Leontice incerta exhibit versatility in the timing of germination, that is, having the capacity to germinate at any time in summer, autumn and next spring. At dispersal in late May, only about 30% of the seeds could germinate; thus, a high percentage of the seeds was dormant. Seeds had a fully developed embryo, and dry storage, cold stratification, warm stratification and gibberellin promoted germination; we concluded that they have non-deep physiological dormancy. Seeds buried under natural conditions during summer germinated to 57–86% in autumn (late October) when exhumed and incubated at 5/2–25/15°C. However, seeds were sown in soil exposed to natural temperature and (low) precipitation did not germinate until next spring when the soil was moist. Thus, like various cold desert annuals, seeds of the perennial L. incerta can germinate in summer, autumn and next spring, depending on the availability of soil moisture (rainfall). Rainfall in cold deserts can play an important role in shaping seed germination traits of desert plants.
Hermaphroditic angiosperms, especially outcrossers, generally produce many more flowers and ovules than they can mature into fruits and seeds. One of the several hypotheses to account for the production of ‘excess’ flowers is selective abortion, which has been shown to increase offspring quality in plants. Our primary aim was to review the literature on the effects of selective abortion on seed germination and post-germination offspring vigour. Of 14 case studies (11 species in 10 genera and four families of flowering plants), germination percentage or rate (speed) increased in six and did not increase in eight, whereas post-germination offspring performance increased in 11 and did not increase in three. In six of the eight cases in which germination was not increased, seedling/juvenile vigour was increased. Seed mass was less likely to influence seed germination than seedling/juvenile vigour. Although selective abortion has been shown to increase progeny vigour of the early life history stages of plants, neither its demographic nor evolutionary importance has been demonstrated.
As a tribute to A.C. Martin's classic work on embryos in seeds, we have attempted to gain a better understanding of the peripheral embryo, which puzzled Martin. The peripheral embryo is strongly curved and in contact with the inner surface of the seed coat, and Martin placed it at the base of his family tree of seed phylogeny and called it a ‘phylogenetic orphan’. We evaluated ovule/seed development, kind of embryo and occurrence of perisperm in families with and without a peripheral embryo. All families with a peripheral embryo occur in the Caryophyllales. Seeds with a peripheral embryo have a low cotyledon width:radicle width ratio that coincides with Martin's (full-sized) linear embryo. The peripheral embryo develops in campylotropous and/or amphitropous ovules and is pushed to the side of the seed as the perisperm develops. Linear-full embryos and perisperm are widely distributed across extant angiosperms but are rarely found together, except in core Caryophyllales. The non-core Caryophyllales with endosperm and various kinds of embryos, including the linear-full, diverged before the core Caryophyllales. Thus, the ancestral linear-full embryo appears to have been retained when the core lineage developed campylotropous and/or amphitropous ovules and perisperm. Seeds with a peripheral embryo merit a position on Martin's family tree; however, the position should be a side branch (‘orphan’) slightly above (more advanced than) his linear embryo and not at the base. We conclude that Martin had great insight into the relationships between the kinds of embryos and rightly questioned the position of the peripheral embryo.
It is well documented that the mother plant has much more influence than the father on seed dormancy/germination, especially of the F1 offspring, primarily by providing all material (maternally derived tissue) to the diaspore coat(s); by maternal environmental effects and provisioning of nutrient resources, mRNA transcripts, protein, the hormone abscisic acid and nitrate to the seed during its development; and by determining progeny environment via dispersal and phenology. There is some evidence that the paternal influence on seed dormancy/germination of the offspring (seeds) can be mediated through multiple paternity (including mate number and diversity), non-nuclear (cytoplasmic) and nuclear (genotypic) inheritance and paternal environmental effects. Our primary aim was to determine via a literature review the influence (or not) of the paternal parent on seed germination. Altogether, 37 of 59 studies (62.7%) indicated a positive influence of the father on seed germination, although not all of them were statistically significant. In general, however, results of studies reported in the literature do not offer strong support for the paternal parent having a major role in seed germination (or seed size) of his F1 offspring.
In nature, fruit and seed production in many plants have been shown to be pollen limited. Pollen limitation is demonstrated when open-pollinated plants that are hand-supplemented (Ps) with outcross pollen produce more fruits and/or seeds than open-pollinated controls that are not hand-pollinated (Po). There are three categories of results in such studies: Ps > Po, Ps = Po and Ps < Po, in which case pollen limitation indices are positive, zero and negative, respectively. In an index widely used to calculate pollen limitation, 1 – (Po/Ps), the bounds for Ps ≥ Po are 0 to + 1, whereas the bounds for Ps < Po are 0 to –∞. The first aim of this review was to show how the pollen limitation index can be modified so that the bounds of Ps < Po are 0 and –1, whereupon the index gives equal weight to the best performer (Ps or Po) and worst performer (Ps or Po). In addition to seed quantity, pollen supplementation can affect seed quality, including germinability. Thus, our second aim was to summarize the results of studies that have also tested the effect of pollen limitation on seed germination. In short, the 30 case studies in 15 families, 16 genera and 18 species that we identified show that seed germination percentage increased, was not affected or decreased by pollen supplementation in 12, 11 and seven cases, respectively. The effect of pollen limitation on seed germination, which can be quite large, has not been considered in developing population growth models to determine the effect of pollen limitation on λ.
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.
Timing of seedling emergence is a critical aspect of a plant's life cycle, and it may influence the expression of other plant life history traits. However, most studies have been conducted at the population level, and thus little is known about timing of seedling emergence at the community level. In the field, we determined the peak emergence season for seedlings of 144 species collected from a subalpine meadow on the eastern Tibet Plateau in China. The proportion of species with seedlings emerging in autumn, spring and summer, seedling field emergence percentage (FE) and mean emergence time (MET) were analysed in relation to seed mass, life cycle type (annual/biennial and perennial) and phylogeny. The results showed that (1) the proportion of species with seedlings emerging in autumn (33%), spring (44%) and summer (23%) differed significantly; (2) overall, species with seedlings emerging in autumn had higher FE than those emerging during spring/summer; (3) there was a positive relationship between log-seed mass and log-MET, but log-seed mass had no significant effect on log-FE; (4) life cycle type did not affect seedling emergence; and (5) phylogeny significantly explained peak emergence season. These results suggest that seed mass and phylogenetic position are the main determinants of seedling emergence season. However, seedling peak emergence season affected FE, growing season length and resource utilization, and thus may be related to the importance of a species in the community.
The pattern of seed dispersal in time and space can affect plant fitness and the soil seed bank, and thus information is needed on this aspect of the seed biology of a species before it is selected for use in habitat restoration projects. Zygophyllum xanthoxylon is a super-xerophilous shrub that is a potential pioneer species for use in revegetating highly disturbed areas of the cold deserts of northwest China. We studied fruit release and soil seed banks of Z. xanthoxylon for 3 years in two cold desert habitats characterized by different degrees of drought and wind velocity. In our study, fruit (a three-winged capsule) release began in summer (June 2010, August 2011, July 2012) and extended for 9–10 months, but plants can be found in the population with previous- and current-year fruits attached to them. More than 50% of the fruits were released in the first 3–4 months after maturity, while the others were released gradually over a 7–8 month period. The temporal pattern of fruit dispersal varied with habitat but not with amount of precipitation during summer. The pattern of fruit deposition on the soil surface was affected by neighbouring plants, wind velocity, wind direction and topography. In both habitats, >90% of the fruits were deposited beside large and small clusters of plants, mainly Ephedra przewalskii. To facilitate plant community development, we suggest that E. przewalskii should be planted (as a wind break) together with Z. xanthoxylon when native pioneer species are used for restoration of cold desert shrublands.
In central Asia, Agriophyllum squarrosum is the first species to become established during natural succession on sand dunes. However, low germination percentages and thus poor stand establishment greatly inhibit the use of this key species in the stabilization of dunes. The aim of this review is to critically analyse published information on the seed biology of A. squarrosum with particular reference to identifying the factors limiting germination of seeds sown in the field. A conceptual model is used to illustrate the complexities of factors as well as the unknowns we found about the seed/seedling stage of the life cycle of this sand dune annual. A major result of this review is that we now know that high germination percentages can be obtained by storing freshly collected seeds dry at room temperatures for 2 to 3 months to allow dormancy break to occur via afterripening, and then storing them dry at low (e.g. 4–5°C) temperature to prevent them from entering secondary dormancy. Non-dormant seeds should be sown in the field in late spring when wind-blown sand will cover them, thus ensuring that they are in darkness, which promotes germination, at the time summer rains occur.
Various environmental factors were tested under laboratory conditions to determine their effects on germination of seeds of prickly sida (Sida spinosa L. ♯3 SIDSP). Neither freezing and thawing nor moist chilling at 5 C promoted seed germination. However, increasing the incubation temperature and subjecting seeds to wet-dry cycles enhanced germination; high temperatures were more effective than alternate wetting and drying. Shifting seeds from a lower to a higher temperature regime increased germination. Seeds shifted from 15/6, 20/10, 25/15, or 30/15 C to higher regimes of 20/10, 25/15, 30/15, 35/20, or 40/25 C germinated to greater percentages than did seeds kept continuously at the lower thermoperiods. With an increase in length of time seeds were at a lower temperature, there was an increase in the percentage that germinated after they were moved to a higher regime.