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Fruit wings accelerate germination in Anacyclus clavatus
Abstract
Premise
The lateral membranous expansions of fruits, commonly referred to as wings, have long been theorized to serve only dispersal functions. Alternatively, because winged fruits typically have earlier seed germination than unwinged fruits, we hypothesized that wings could increase the contact surface with water, ultimately triggering earlier germination.
Methods
We investigated this alternative hypothesis by exploring the potential role of fruit wings on germination in the heterocarpic species Anacyclus clavatus (Desf.) Pers. (Asteraceae), which produces both winged and unwinged fruits. First, we measured the speed and degree of water absorption in winged and unwinged fruits. Second, we investigated the effects of wings on germination performance, by either reducing wing size or by preventing water absorption by sealing wings with wax. Next, we tested the influence of water availability on the germination performance of winged and unwinged fruits by reducing the water potential.
Results
Winged fruits absorbed more water at a faster rate than unwinged fruits. The sealing of wings delayed germination, whereas germination time was not significantly altered by wing cutting. The restriction of water availability by decreasing water potential significantly delayed seed germination of unwinged fruits, whereas winged fruits remained unaffected.
Conclusions
Altogether, our results support the effect of wings on germination and cast doubt on the unique role of wings in dispersal. Whether or not wings contribute to dispersal, we propose that they also improve seed germination and seedling establishment by facilitating water absorption after the release from their mother plants.
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The competition–colonisation trade‐off is recognised as a key mechanism for diversity maintenance, whereby weak competitors can avoid competition with stronger competitors due to their greater dispersal ability. To date, most theoretical and empirical work has focused on trade‐offs among different species yet a within‐species perspective is crucial to assess the scope for natural selection on competition–colonisation trade‐offs.
For six wind‐dispersed Pinus species encompassing a wide range of seed masses, we examined whether within and among species there was evidence that heavier seeds typically disperse shorter distances but produce seedlings that are stronger competitors with a higher likelihood of successful establishment, under different treatment conditions.
Within species, we found seedling establishment to be independent of diaspore terminal velocity (a measure of dispersal potential), which would maximise the variation in potential dispersal distances and locations reached by the seeds shed from a parent tree, and therefore the chances that some offspring will reach a favourable recruitment site. However, among Pinus species the likelihood of seedling emergence and survival was higher in species that produce heavier seeds with higher terminal velocity.
We found a marked competition–colonisation trade‐off among all six species consistent with expectations from coexistence theory. In contrast, within species, there was no evidence of competition–colonisation trade‐offs for any species, despite differences in the relationships between diaspore mass and terminal velocity. This is consistent with weak selection within species for a competition–colonisation trade‐off. The results suggest that within‐species trade‐offs do not influence the risk of invasive spread.
A free Plain Language Summary can be found within the Supporting Information of this article.
Brandisia hancei (Paulowniaceae) is a widely distributed shrub in karst regions in southwestern China. Its seeds have a membranous wing, and they mature just before the rainy season begins. To assess the effect of the wing on seed dispersal and germination of B. hancei, we measured the dispersal distance at varying wind speeds and release heights, falling duration from different release heights, floating duration on still water, rates of imbibition of water, and drying and soil adherence to seeds. Germination experiments were conducted on intact and de-winged seeds immediately after harvest. The wing increased the falling duration in still air and the floating ability on water. Dispersal distance of winged and de-winged seeds did not differ at a wind speed of 2.8 m s⁻¹, but at 3.6 and 4.0 m s⁻¹ dispersal distances were greater for de-winged than for winged seeds. Seed wing had little effect of absorption and retention of water, but significantly increased soil adherence to the seeds. Mature seeds were non-dormant and germinated to over 90% with a mean germination time of about 10 days. By combining the environmental conditions in karst habitat with the seed traits of B. hancei, we conclude that dispersal and germination of winged seeds are adapted to the precipitation seasonality in heterogeneous habitats absence of soil.
The production of two or more distinct fruit types by an individual, i.e. heterocarpy, is considered as a mixed dispersal strategy in which a proportion of the offspring is able to colonize new sites, whilst others remain near the maternal location. Here, we aimed to explore the effects of achene morphology (winged vs. unwinged achenes) and achene size -measured here as achene mass-on post-dispersal life-history traits (probability and time of seedling emergence) in three heterocarpic Anacyclus species (Anthemideae, Asteraceae). Morphology, size and germination performance were studied in achenes from six populations of Anacyclus clavatus (Desf.) Pers., A. homogamos (Maire) Humphries, and A. valentinus L. Our results show that achene morphology and size were related to their position within the capitulum, such that outer, winged achenes were significantly heavier than the inner, unwinged ones. Additionally, winged achenes germinated faster than unwinged ones. This pattern may be related to the sequential achene time of release displayed by these species. Finally, our findings cast doubt on the role of wings as structures that favor dispersal by wind in these three species of Anacyclus.
Maximum likelihood or restricted maximum likelihood (REML) estimates of the
parameters in linear mixed-effects models can be determined using the lmer
function in the lme4 package for R. As for most model-fitting functions in R,
the model is described in an lmer call by a formula, in this case including
both fixed- and random-effects terms. The formula and data together determine a
numerical representation of the model from which the profiled deviance or the
profiled REML criterion can be evaluated as a function of some of the model
parameters. The appropriate criterion is optimized, using one of the
constrained optimization functions in R, to provide the parameter estimates. We
describe the structure of the model, the steps in evaluating the profiled
deviance or REML criterion, and the structure of classes or types that
represents such a model. Sufficient detail is included to allow specialization
of these structures by users who wish to write functions to fit specialized
linear mixed models, such as models incorporating pedigrees or smoothing
splines, that are not easily expressible in the formula language used by lmer.
Premise of the study: In heterocarpy, fruits with different morphologies have been associated with alternative strategies of dispersal, germination, dormancy, and seedling competitive ability. In heterocarpic species, it is common to find fruits with competitive or dispersal syndromes. The competitive advantage of nondispersing fruits has been frequently attributed to their larger size, but recent studies have suggested that this could also be mediated by germination time. The main objective of our study was to investigate which factor, fruit type or germination time, most affects plant performance and, consequently, competitive ability, using the heterocarpic species Anacyclus clavatus.
• Methods: To explore the effects of achene type and germination time on plant performance, we followed an innovative experimental approach including two experiments: one allowing for differences in germination time, and the other evaluating the effect of achene type alone by synchronizing germination time.
• Key results: A significant effect of germination time on several postdispersal life-history traits was observed: Achenes that germinated earlier produced plants with higher biomass and reproductive effort. When germination time was controlled, no significant differences were observed in any of the traits.
• Conclusions: The competitive advantage of achenes with different morphologies was mainly mediated by germination time and not by differences in size or other intrinsic traits. The consequences of these results are discussed in light of the dispersal–competition trade-off. Our experimental approach (i.e., the synchronization of germination time) revealed the importance of manipulative experiments for testing the effects of germination time on plant survival and performance.
The present study of the winter annual Crepis tectorum examines the relationship between seed (achene) size and the extent to which seeds resist germination during the year of their production. I carried out two seed burial experiments, one at an outcrop site occupied by a small-seeded population, and another in an experimental garden with soil from the same field site, with seeds representing the local population and a segregating generation of a cross between two other populations. Using logistic regression with data corrected for seed viability, I found an association between small seed size and failure to germinate in the first autumn. The small seed size characterizing many outcrop populations may have evolved as a response to selection for delaying germination in a habitat subject to unpredictable droughts during the growth season. Keywords: Crepis tectorum, germination, seed bank, seed size.
Background and AimsMost of the diversity in the pseudanthia of Asteraceae is based on the differential symmetry and sexuality of its flowers. In Anacyclus, where there are (1) homogamous capitula, with bisexual, mainly actinomorphic and pentamerous flowers; and (2) heterogamous capitula, with peripheral zygomorphic, trimerous and long-/short-rayed female flowers, the floral ontogeny was investigated to infer their origin.Methods
Floral morphology and ontogeny were studied using scanning electron microscope and light microscope techniquesKey ResultsDisc flowers, subtended by paleae, initiate acropetally. Perianth and androecium initiation is unidirectional/simultaneous. Late zygomorphy occurs by enlargement of the adaxial perianth lobes. In contrast, ray flowers, subtended by involucral bracts, initiate after the proximal disc buds, breaking the inflorescence acropetal pattern. Early zygomorphy is manifested through the fusion of the lateral and abaxial perianth lobes and the arrest of the adaxials. We report atypical phenotypes with peripheral 'trumpet' flowers from natural populations. The peripheral 'trumpet' buds initiate after disc flowers, but maintain an actinomorphic perianth. All phenotypes are compared and interpreted in the context of alternative scenarios for the origin of the capitulum and the perianth identity.Conclusions
Homogamous inflorescences display a uniform floral morphology and development, whereas the peripheral buds in heterogamous capitula display remarkable plasticity. Disc and ray flowers follow different floral developmental pathways. Peripheral zygomorphic flowers initiate after the proximal actinomorphic disc flowers, behaving as lateral independent units of the pseudanthial disc from inception. The perianth and the androecium are the most variable whorls across the different types of flowers, but their changes are not correlated. Lack of homology between hypanthial appendages and a calyx, and the perianth double-sided structure are discussed for Anacyclus together with potential causes of its ray flower plasticity.
Background and aims:
The competition-colonization trade-off theory postulates that the competitive and colonizing abilities of organisms are negatively related; this trade-off has been proposed as a major force in the maintenance of diversity. In plants, the competition-colonization trade-off is often considered to result from variation in resource partitioning, thus generating heavy competitive (non-dispersing) seeds and light (dispersing) non-competitive seeds. Here, the possibility is explored that early germination provides a competitive advantage, thus mediating competitive interactions.
Methods:
Using eight populations of the heterocarpic species Crepis sancta (Asteraceae), the possibility was tested that dispersing and non-dispersing achenes differ in germination timing, and the impact of early germination on individual fitness components was analysed in the context of intraspecific competition. To evaluate whether seed reserve varies among achene types, endosperm size was also measured by analysing photographs of cross-sections taken under a binocular microscope.
Key results and conclusions:
The results show that non-dispersing achenes germinated 4 d earlier (on average) than dispersing achenes. It is also shown that early germination provides a positive advantage for the survival and final biomass of individuals, a pattern that was consistent over the eight populations and independent of achene type. Dispersing and non-dispersing achenes did not differ in terms of seed reserve (endosperm size). It is proposed that germination phenology may mediate the competition-colonization trade-off in Crepis sancta and the evolutionary significance of this phenomenon is discussed.
For the past 25 years NIH Image and ImageJ software have been pioneers as open tools for the
analysis of scientific images. We discuss the origins, challenges and solutions of these two programs,
and how their history can serve to advise and inform other software projects.
Intrageneric relationships in the genusBrachycome were investigated by the comparison of nucleotide sequences of the chloroplast genematK. Evolutionary trends of some morphological and cytological characters were estimated based on thematK tree. The results indicate thatBrachycome is divided into four major clades, and circumscription of superspecies based primarily on fruit morphology is not fully supported. Fruit morphology has evolved in parallel from simple to complex structures in several lineages. The ancestral basic chromosome number isx = 9, and lower chromosome numbers are the products of several dysploid reductions fromn = 9 in two of four major clades ofBrachycome.
South American Polylepis mountain forests are recognised as being one of the most endangered forest ecosystems in the world. Reforestation measures
have been strongly recommended but may be hampered due to the very low seed germination reported for several Polylepis species. In order to facilitate reforestation we analysed the influence of seed mass on germination probability for Polylepis australis seeds in the Córdoba mountains (central Argentina). We collected seeds from 43 trees distributed throughout five woodland
fragments located within two regions differing in size, topographical position, and altitude (1,900m a.s.l. and 2,200m a.s.l.).
Seeds of Polylepis australis exhibited a great variation in terms of mass and percent seed germination among individual trees and among geographical regions.
The results of logistic regression showed that germination probability was highly correlated with seed mass. However, the
explained deviance significantly increased by including the region, the woodland fragment and especially the individual tree
in addition to seed mass in the regression models. We conclude that selecting seeds on the basis of mass is an appropriate
way to enhance germination prospects for reforestation projects. However, no absolute mass values are applicable in this context
as the highest germination probabilities were reached at varying seed mass values depending on geographical region, woodland
fragment or individual tree. We suggest collecting the relatively heaviest available seeds, even though the absolute seed
mass may be low.
Fin-winged fruits have two or more wings aligned with the longitudinal axis like the feathers of an arrow, as exemplified by Combretum, Halesia, and Ptelea. Such fruits vary in dispersal mode from those in which the fruit itself is the ultimate disseminule, to schizocarps dispersing two or more mericarps, to capsules releasing multiple seeds. At least 45 families and more than 140 genera are known to possess fin-winged fruits. We present an inventory of these taxa and describe their morphological characters as an aid for the identification and phylogenetic assessment of fossil and extant genera. Such fruits are most prevalent among Eudicots, but occur occasionally in Magnoliids (Hernandiaceae: Illigera) and Monocots (Burmannia, Dioscorea, Herreria). Although convergent in general form, fin-winged fruits of different genera can be distinguished by details of the wing number, texture, shape and venation, along with characters of persistent floral parts and dehiscence mode. Families having genera with fin-winged fruits and epigynous perianth include Aizoaceae, Apiaceae, Araliaceae, Asteraceae, Begoniaceae, Burmanniaceae, Combretaceae, Cucurbitaceae, Dioscoreaceae, Haloragaceae, Lecythidiaceae, Lophopyxidaceae, Loranthaceae, and Styracaceae. Families with genera having fin-winged fruits and hypogynous perianth include Achariaceae, Brassicaceae, Burseraceae, Celastraceae, Cunoniaceae, Cyrillaceae, Fabaceae, Malvaceae, Melianthaceae, Nyctaginaceae, Pedaliaceae, Polygalaceae, Phyllanthaceae, Polygonaceae, Rhamnaceae, Salicaceae sl, Sapindaceae, Simaroubaceae, Trigoniaceae, and Zygophyllaceae. This survey has facilitated the identification of fossil winged fruits such as Combretaceae and Araliaceae in the late Cretaceous of western North America and provides additional evidence toward the identification of various Cenozoic fossils including Brassicaceae, Fabaceae, Polygonaceae, Rutaceae, and Sapindaceae.
Seed dormancy is an innate seed property that defines the environmental conditions in which the seed is able to germinate. It is determined by genetics with a substantial environmental influence which is mediated, at least in part, by the plant hormones abscisic acid and gibberellins. Not only is the dormancy status influenced by the seed maturation environment, it is also continuously changing with time following shedding in a manner determined by the ambient environment. As dormancy is present throughout the higher plants in all major climatic regions, adaptation has resulted in divergent responses to the environment. Through this adaptation, germination is timed to avoid unfavourable weather for subsequent plant establishment and reproductive growth. In this review, we present an integrated view of the evolution, molecular genetics, physiology, biochemistry, ecology and modelling of seed dormancy mechanisms and their control of germination. We argue that adaptation has taken place on a theme rather than via fundamentally different paths and identify similarities underlying the extensive diversity in the dormancy response to the environment that controls germination.
Ecological research and the way that ecologists use statistics continues to change rapidly. This second edition of the best-selling Design and Analysis of Ecological Experiments leads these trends with an update of this now-standard reference book, with a discussion of the latest developments in experimental ecology and statistical practice. The goal of this volume is to encourage the correct use of some of the more well known statistical techniques and to make some of the less well known but potentially very useful techniques available. Chapters from the first edition have been substantially revised and new chapters have been added. Readers are introduced to statistical techniques that may be unfamiliar to many ecologists, including power analysis, logistic regression, randomization tests and empirical Bayesian analysis. In addition, a strong foundation is laid in more established statistical techniques in ecology including exploratory data analysis, spatial statistics, path analysis and meta-analysis. Each technique is presented in the context of resolving an ecological issue. Anyone from graduate students to established research ecologists will find a great deal of new practical and useful information in this current edition.
Background: Pallenis spinosa (Asteraceae) produces both winged and wingless achenes. Both achene morphs are non-dormant and show a similar embryo size, rendering dispersal ability as their only apparent functional difference.
Aims: We studied morph-specific release and spatial dispersal patterns to ascertain whether the common view of seed dimorphism as a mixed strategy, that is functionally fully differentiated morphs, is appropriate for this system.
Methods: For three years, at the onset of achene release, in early autumn, we placed achene traps at different distances from source plants, censusing achene arrival at 3–4 day intervals. We constructed morph-specific dispersal kernels and related release intensity to prevailing meteorological conditions in census intervals. Selected kernel models were used to describe dispersal effects of observed changes in the proportion of winged achenes (pw) in successive released fractions.
Results: Achene release extended up to early-mid winter, peaking in rainy, windy intervals. Throughout the season, pw decreased progressively. Unexpectedly, the wingless morph produced the longest dispersal tails and it only showed ability for fat-tailed dispersal. Consequently, maximum dispersal distances steadily increased throughout the season.
Conclusions: Achene dimorphism in P. spinosa appears to allow a within-season continuous reshaping of the seed-dispersal kernel instead of representing a mixed strategy.
Strobe photographs were taken of over 200 spinning samaras from seven species of trees. These were used to measure the rate of descent, angular velocity, orientation, and other parameters of the samaras as they fell. These data were then used to compare the aerodynamic behavior of samaras, helicopters, and theoretical ideal rotors. Plotting morphological data for each samara against its rate of descent showed that this rate was highly correlated with the square root of the samara's wing loading (samara weight divided by wing-surface area). This plot demonstrated the existence of two distinct groups of samaras, distinguished by their morphology, spinning motion, and rate of descent. These results are of greatest use in characterizing local dispersal patterns.
The differential germination responses of ray and disc achenes of Hemizonia increscens (Asteraceae) were compared in field and laboratory investigations in order to gain insight into the ecological and evolutionary significance of heterocarpy.
In the field, 200 ray and 200 disc achenes were placed in native, sterilized soil in a series of cleared, randomized, replicated plots. In a nearby plot a similar number of achenes were placed in plastic petri dishes in which high moisture conditions were maintained. Disc achene germination occurred under relatively minimal moisture conditions (<1 cm rainfall for 19 days) and relatively mild temperature regimes (21–7 C). Disc achene germination began three days after planting in the field plot and four days after they were put into the field petri dishes. In contrast, the onset of ray achene germination occurred 21 days after planting in the field plot and 19 days after planting in the field petri dishes. Averages of 2.05 and 2.71 disc achenes/day germinated in the field plot and field petri dishes, respectively. These contrasted with averages of 0.57 and 0.50 ray achenes/day germinated in the field plot and in the field petri dishes, respectively. A total of 67.5% and 69% disc achenes germinated in the field plot and the field petri dishes, but only 18% and 16.5% ray achenes germinated in the field plot and field petri dishes, respectively.
Three separate treatments, using 100 ray and 100 disc achenes in each, were performed in laboratory growth chambers: 1) nicking the fruit coat, 2) excising the embryo, and 3) leaving the fruit coat intact. Onset of germination for all disc achene treatments occurred after three days. No significant differences were found among the three disc achene treatments in timing, rate, or germination percentage. All three disc treatments in the laboratory closely paralleled those for disc achenes in the field plots in time and germination percentage, but rates of germination were not as high. Germination of the nicked and excised ray achenes treatments began after four days, while germination of the untreated ray achenes began after 27 days. Untreated ray achenes in the laboratory paralleled the ray achenes in both field experiments in rate and germination percentage, but were delayed in time of germination. The nicked and excised ray achene treatments, however, were similar to the disc achene treatments in time of germination, and were not significantly different from disc achenes in rate of germination. These data suggest 1) that ray and disc achenes are markedly different in germination under identical conditions in field and laboratory experiments, and 2) this difference in germination response may be due to the thicker pericarp of the ray achenes.
Tall plant species disperse further distances than do short species, within and across dispersal syndromes, yet the driver underpinning this relationship is unclear. The ability of taller plants to invest more in dispersal structures may explain the positive relationship between plant height and dispersal distance. Here, we quantify the cross‐species relationships between presence of dispersal structures, dispersal investment plant height and dispersal distance.
Plant height, dispersal syndrome and dispersal investment data were collated for 1613 species from the literature, with dispersal distance data collated for 114 species.
We find that species with high dispersal investment disperse further than do species with low dispersal investment. Tall species have a greater probability of having dispersal structures on their seeds compared with short species. For species with dispersal structures on their seeds, plant height is very weakly related to dispersal investment.
Our results provide the first global confirmation of the dispersal investment–distance hypothesis, and show dispersal investment can be used for predicting species dispersal distances. However, our results and those of previous studies indicate plant height is still the best proxy for estimating species dispersal distances due to it being such a readily available plant trait.
The seed shadow created by wind dispersal around parent trees may be affected by functional traits, as well as wind conditions and surrounding vegetation.
This study of one mature tree each of 12 Neotropical species determined the extent to which species variation in diaspore traits vs. tree height and crop size explains (i) rate of diaspore descent in still air, (ii) distributions of diaspores dispersed from a 40‐m tower in the forest, and (iii) natural seed shadows around the parent tree.
A model of diaspore wing‐loading to a fitted power explained 66·9% of the variation among species in the geometric mean rate of descent in still air. For a subset of four species, expected dispersal distance was not correlated with actual dispersal distance from the forest tower. For a subset of seven species, variation in wing‐loading 0·5 of individual diaspores explained ≤4·3% of the dispersal distance from the parent tree.
Measured seed shadows, particularly their distribution edges and area, differed significantly among the 12 species (range in maximum tree height 19–42 m) and were best fit by 12 separate anisotropic dispersal kernels and surveyed fecundities. Measured mean distance was highly correlated with simulated distances from the kernels.
The best models, explaining 57·6% of the variation among species in shadow area, and 59·6, 61·6 and 61·7% of variation in mean, median and maximum distances, included maximum tree height, either alone or in combination with crop size, and not diaspore rate of descent.
Among 10 species, seed shadow area was not related to rank of seedling shade tolerance. In their highly skewed distributions, most seeds were much closer than the distance of greatest seedling recruitment and in very high density, thus enhancing later density‐ and/or distance‐related seedling mortality.
Tree functional traits, rather than the historically emphasized diaspore traits, explain distance distributions of these wind‐dispersed species. Additional exploration of diaspore abscission in relation to wind and the influence of wind patterns after release are needed.
A lay summary is available for this article.
Ecologists, both practical and theoretical, now appreciate that to understand biological systems they must consider the spatial dimension. Consequently, dispersal has become one of the hottest topics in plant ecology. However, in the midst of so much research output on dispersal, there is a need for a stock-take to determine the needs of future research: what has been achieved to date, where do current studies fit in, and what still needs to be determined? What are the implications of dispersal for those engaged in managing plant populations and communities? This is the first book for many years to present a synthesis of research on dispersal and its implications for plant population dynamics. The book consists of three sections: Section A reviews information on the biological and environmental processes that determine the path of an individual dispersing propagule, usually a seed, and the theory that has been developed to predict these trajectories; Section B discusses the distributions of seeds resulting from dispersal from an entire plant, theoretical research predicting the shapes of these distributions and design issues for future dispersal studies; Section C explores the implications of dispersal for expansion of populations, structure within existing populations and communities, and the evolution of dispersal traits.
1. An understanding of seed persistence in the soil is important to vegetation management and weed control, but experimental collection of seed bank data is tedious and expensive. We report a rapid, simple method for predicting seed persistence in the soil. The method is tested on a range of British, mostly herbaceous, species. 2. Diaspore (seed or fruit) weight is plotted against variance of the three linear dimensions of the diaspore. All diaspores within an area of the graph defined by a maximum weight and variance are persistent in the soil. The critical weight is the same for fruits and seeds, but the critical variance of diaspore dimensions appears slightly higher for fruits. The great majority of diaspores outside this region are short lived, and the relatively few ambiguous cases can be resolved by reference to habitat. 3. The generality of the suspected underlying mechanism suggests that the method can also be applied to floras outside north-west Europe.
(1) Freshly matured seeds of the native North American winter annual Chaerophyllum tainturieri have morphophysiological dormancy (MPD), which is a combination of morphological (underdeveloped embryos) and physiological dormancy of the non-deep type. (2) Seeds after-ripen at high diurnally fluctuating temperatures (25/15⚬C, 30/15⚬ C and 35/20⚬C) but not at low-constant or fluctuating temperatures (5⚬ C, 15/6⚬C and 20/10⚬C); thus, the breaking of physiological dormancy occurs in the field during summer. Gibberellic acid promotes after-ripening. (3) Light is required for embryo growth, but it is ineffective while seeds are physiologically dormant. Thus, embryo growth and germination occur in autumn, after seeds are no longer physiologically dormant. However, if seeds are exposed to light for two or more days in summer, after the breakage of dormancy has started, they will germinate in darkness at normal autumn temperatures. (4) October habitat temperatures (c. 20/10⚬C) induce dormancy, and consequently no germination occurs in spring. Thus, the species behaves as a strict winter annual, germinating only in autumn. (5) Buried seeds exhibit an annual dormancy-non-dormancy cycle. Seeds become non-dormant during summer and re-enter dormancy in autumn. This is the first species in which it clearly has been demonstrated that seeds with MPD can go into and out of dormancy annually. (6) Embryos did not grow while seeds were buried for thirty-two months. A light requirement for embryo growth ensures that buried seeds do not germinate in autumn while they are non-dormant. This is one reason why seeds can form persistent seed banks. (7) Seeds have a type of MPD that has not been described before, and we suggest that it be called `non-deep simple MPD.' This is the eighth type of MPD so far described for seeds.
The relationship between the energy expended per offspring, fitness of offspring, and parental fitness is presented in a two-dimensional graphical model. The validity of the model in determining an optimal parental strategy is demonstrated analytically. The model applies under various conditions of parental care and sibling care for the offspring but is most useful for species that produce numerous small offspring which are given no parental care.
Species of the South American genus Soliva are troublesome weeds because they invade turf areas and have spinous achenes. Soliva is now widely naturalised in New Zealand and the plants are known here as Onehunga weed. The four species usually accepted for subgenus Soliva, three of which are recorded for New Zealand, are distinguished by characters of the achenes, but are identical in vegetative and floral characters. Examination of herbarium and field material of naturalised New Zealand plants has shown that these achene characters are continuously variable. Many forms previously referred to separate species, as well as intermediates between them, co-occur within populations. Individuals are self-pollinating and produce seedlings with the same achene form. Experimental crosses between plants with the most distinctive achene forms produced fully fertile progeny with achenes intermediate between those of the pollen and seed parents. It is concluded that subgenus Soliva consists of a series of inbreeding forms best regarded as a single species, S. sessilis, which is very variable in achene morphology. It is also suggested that there may be fewer good species in subgenus Gymnostyles than are usually accepted, and that the elevation of subgenus Gymnostyles to generic rank, as proposed by some authors, may not be warranted.
The interrelationships among achene weight, allocation to embryo and pericarp, and germination time were determined for 500 stratified achenes of tetraploid Aster pilosus Willd. var. pilosus. Only 52.6% of the achenes germinated. Germinated achenes were significantly heavier than ungerminated achenes. Germination time was independent of achene weight and embryo weight, but varied inversely with pericarp weight. Variable achene weight is evolutionarily advantageous. Heavy achenes are at an advantage in that their proportionately larger embryos and thinner pericarps facilitate germination, promoting competitive establishment of seedlings. Lighter achenes are also at an advantage through increased dispersibility, and their relatively thick pericarp provides a persistent seed bank. Evolutionary pressures presumably maintain the variability in achene weight of var, pilosus. These results are discussed in the context of the early midsuccessional ecology of var. pilosus.
Seed size, dormancy, and dispersal share 3 population-dynamic functions in temporally and spatially varying environments: risk reduction of bet hedging, escape from crowding, and escape from sib competition. A model was developed to explore ways they may interact to reduce risk. The risk-reducing properties of these seed traits evolve only in response to global temporal variance. Thus, to understand how selection impinges on the seed traits, creating fitness interactions, one must understand the factors contributing to global temporal variance and how they are mitigated by the various seed traits. Since the traits interact to reduce variance, arbitarily fixing any 1 trait at different values alters the fitness-maximizing values of the others, resulting in trade-offs among traits. The authors explore how changes in the number of independent environmental patches, probability of favorable conditions, radius of dispersal, and spatial and temporal autocorrelation of environmental conditions alter selection on the interacting syndrome of seed traits. -from Authors
Question: We explored the functional significance of seasonal aerial seed banks in two coexisting, heterocarpic annual Asteraceae with dormant (Chrysanthemum coronarium) and non-dormant (Anacyclus radiatus) achenes. We hypothesised that the plant achene pool is a significant component of total seed reserves, and that within-season seedling emergence timing is shaped by achene release patterns.
Location: SW Spain.
Methods: In an observational study, we established temporal achene release patterns. We also quantified the aerial and soil achene pools throughout the release season, and assessed seedling emergence timing. Sowing experiments were used to explore the influence of achene release dynamics on emergence timing, and to establish achene morph-specific patterns of between-year distribution of germination.
Results: Achene release extended from late spring to late autumn (Chrysanthemum), or from early autumn to early winter (Anacyclus). Within species, achene morphs differed in release timing. Only in Chrysanthemum, a small achene fraction seemed to persist in the soil, and between-year germination distribution differed among morphs. In coexisting populations, the Anacyclus plant achene pool was an order of magnitude higher than the soil pool throughout the release season, whereas in Chrysanthemum both pools were of the same magnitude during autumn. Within-year seedling emergence was significantly staggered beneath parent plants compared with the pattern resulting solely from the germination response in soil, with the exception of Chrysanthemum in one of the two study years.
Conclusions: Results suggest that seasonal aerial seed banks are effective within-season, risk-reducing traits in ruderal Mediterranean habitats characteristic of the study species.
The biogeography of Cineraria (Asteraceae, Senecioneae) is assessed using a chorological approach in terms of its distribution, centres of diversity and endemism. Rare species are identified and categorised according to Rabinowitz's criteria and causes for rarity in the genus are investigated. The conservation status of the species is assessed according to IUCN criteria for Red List categories and compared to levels of rarity. The main phytogeographic affinity of Cineraria is Afromontane in association with seven recognised centres of endemism in South Africa, four in tropical Africa, in Ethiopia and in Madagascar. Fifteen species are endemic and six are near-endemic to a specific centre of endemism or mountain range. Seventy four percent of Cineraria spp. are endemic to southern Africa with the centre of diversity in the KwaZulu-Natal Midlands, South Africa. The rarest species number 11; of these eight are endangered or vulnerable according to IUCN Red Data Criteria and three are data deficient. Causes of rarity in Cineraria are related to narrow habitat specificity, notably soil or rock type and/or altitudinal range. Paired comparisons of the 11 rarest and commonest species reveal no convincing causal links to morphological, reproductive or life history strategy attributes in Cineraria. © 2009 University of the Witwatersrand, Botanical Journal of the Linnean Society, 2009, 160, 130–148.
1. It is often assumed that there is a trade-off between maternal provisioning and dispersal capacity, leading small-seeded species to disperse further than large-seeded species. However, this relationship between dispersal distance and seed mass has only been quantified for species from particular sites or with particular dispersal syndromes.
2. We provided the first large-scale, cross-species quantification of the correlations between dispersal distance and both seed mass and plant height. Seed mass was positively related to mean dispersal distance, with a 100-fold increase in seed mass being associated with a 4.5-fold increase in mean dispersal distance (R2 = 0.16; n = 210 species; P < 0.001). However, plant height had substantially stronger explanatory power than did seed mass, and we found a 5-fold increase in height was associated with a 4.6-fold increase in mean dispersal distance (R2 = 0.54; n = 211 species; P < 0.001).
3. Once plant height was accounted for, we found that small-seeded species dispersed further than did large-seeded species (R2 = 0.54; n = 181 species; slope = −0.130; P < 0.001); however, seed mass only added 2% to the R2 of the model. Within dispersal syndromes, tall species dispersed further than did short species, while seed mass had little influence on dispersal distance.
4. Synthesis. These findings enhance our understanding of plant life-history strategies and improve our ability to predict which species are best at colonizing new environments.
Conventional plant dispersal classification systems use simple binary as-signment schemes classifying each species as either being dispersed by means of a certain dispersal vector or not. However, because the dispersal potential ranges continually, this dichotomy appears to be rather artificial, and the existing systems may not be very useful for addressing ecological questions. To quantify gradual differences in the dispersal potential, we developed a system as-sessing wind dispersal potentials. Wind dispersal potential is defined as the proportion of diaspores exceeding a predefined reference distance under certain weather conditions, to acknowledge that wind dispersal potential is scale and context specific. The system is based on an independently validated simulation model of wind dispersal that was used to compute the proportion of diaspores exceeding predefined reference distances. On an ordinal scale, the proposed system allows one to assess the wind dispersal potential of any plant species with known falling velocity and release height of its diaspores without further computing. The system mainly relies on two traits characterizing the plant species (falling velocity and initial release height of the diaspores) and two context-specific parameters (reference distance and weather conditions). We examined how wind dispersal potential is sensitive to these factors and found that it was most sensitive to weather conditions and falling velocity. The species-specific traits interact with reference distance: the greater the reference distance and the lower the release height in relation, the more relevant a low falling velocity becomes for achieving a high wind dispersal potential. We subsequently applied the system to 335 plant species and found a considerable variation in their wind dispersal potentials. Many species commonly assumed to be wind dispersed exhibit only a low wind dispersal potential. Comparing the wind dispersal po-tentials to the morphology of the diaspores also reveals a considerable variation of the wind dispersal potential of species classified as the same morphological type. The results show that the conventional assignment of a plant species to a certain mode of dispersal, which is primarily based on the morphology of its diaspores, will often result in misleading conclusions regarding the dispersal potential of the respective species.
1 A seed-addition experiment using seven co-occurring annual plant species with a range of seed masses was carried out in a limestone grassland in South Wales.
2 If seedlings compete for establishment sites, then large seed size may confer enhanced competitive ability. However, the simple reciprocal relationship found between seed mass and per capita seed output showed that species producing larger seeds suffer reduced fecundity. Seed size may therefore act as a surrogate in a competition/colonization trade-off.
3 Equal numbers of seeds of all species were sown in a mixture over a range of densities. As sowing density increases, all species should reach a higher proportion of the available microsites. If large-seeded species are the best competitors they are expected to win all the sites they reach, and hence to occupy an increasing proportion of sites as sowing density increases.
4 The three species with the largest seeds made up 49% of individuals at low-density sown plots but 83% of individuals in high-density sown plots. In addition, seed mass and plant density were not correlated in unsown plots, but were strongly correlated in high-density sown plots. However, all small-seeded species maintained a presence in sown plots.
5 Although species were sown at random with respect to one another, individuals were up to five times more likely than expected to have a conspecific as a nearest neighbour. This could be caused by interspecific competition and/or by environmental heterogeneity that favours different species in different patches.
6 The results suggest that seedlings do compete for establishment sites and that large-seeded species generally win when in direct competition. In unsown areas small-seeded species win many sites by forfeit (because large-seeded species are strongly recruitment limited) but there may be a restricted subset of potential sites for which they are the best competitors and which they can win outright.
Marginal and central florets of the capitula of tansy ragwort Senecio jacobaea yield different kinds of fruit. The central (“disk”) achenes are lighter (x±SE=199±5μg), more numerous (x±SE=58±0.6 achenes per head), and are equipped with a pappus aiding wind transport and rows of trichomes aiding animal transport. The marginal (“ray”) achenes are heavier (x±SE=286±7μg), less numerous (virtually invariant at 13 achenes per head), and lack dispersal structures. Whereas disk achenes are relased shortly after they mature, ray achenes are retained by the parent for a period of months following maturity.
Germination at constant temperature (20°C) and with alternating light (12 h light: 12 h dark) demonstrated that disk and ray achenes exhibit different germination syndromes. Germination percentage increases linearly with achene fresh weight in both types; for a given weight, disk achenes have a higher germination percentage than ray achenes. Germination time decreases with increasing achene weight in disk achenes, but increases with achene weight in ray achenes.
The germination percentages and germination times for disk and ray achenes diverge progressively with increasing achene weight. The divergence in behavior is a result of diverging patterns of dry matter allocation in the two achene types. Increase in the size of disk achenes favors the embryo fraction, thereby speeding germination while reducing protection. Increase in the size of the ray achenes favors the pericarp fraction, thereby increasing protection while delaying germination.
Reduced germination percentage and germination speed of the ray achenes were shown by experimental manipulation to be caused by physical inhibition by their thicker pericarps.
Dimorphism in ragwort likely speards germination out in space and time, thereby increasing the number of safe sites an individual parent can exploit in disseminating offspring. The syndrome in other heteromorphic composites resembles that of ragwort, generally combining reduced dispersal-delayed germination in the outer achenes and distance dispersal-quick germination in the central achenes. The outer achenes are generally less numerous and larger. Dispersal traits (large numbers, early release and light wieght) are the direct opposite of dormancy traits (small numbers, delayed release and heavier weight). Thus conflicts between the properties determining dormancy and dispersal appear to require separate dormancy and dispersal phenotypes.
Seed heteromorphism represents the production of different kinds of seeds by a single individual. The morphological differentiation affects either the fruit — heterocarpy — or the seed sensu stricto — heterospermy. In this study the phylogenetic distribution of seed heteromorphism among different families and habitats is investigated for 218 plant species based on existing literature. The ecological consequences of seed heteromorphism are explored as well. Seed heteromorphism is most common in the Asteraceae and Chenopodiaceae, suggesting that these families have morphological characteristics favouring the appearance of seed heteromorphism and ecological features that maintain it. Using the distribution of seed heteromorphism within the genus Crepis, the relationship between seed heteromorphism and life cycle and habitats is examined. From this analysis it appears that heterocarpic species are often monocarpic. In contrast, the relationship between heterocarpy and habitats is not obvious. Finally, a synthesis is presented about the ontogeny of heteromorphism and some guidelines are proposed for future research on this topic.
After 120 yr of burial in moist, well-aerated sand, 23 seeds of Verbascum blattaria and two seeds of a Verbascum sp. germinated and produced normal plants (50% germination for Verbascum). After a 6-wk cold treatment, a single seed of Malva rotundifolia germinated also, producing a normal plant (2% germination). Plants were grown to maturity in a greenhouse, and flowering was induced by exposure to a 6-wk cold treatment. Flowers were artificially pollinated to produce seed of both Verbascum blattaria and Malva rotundifolia. The Verbascum sp. failed to set seed. Collected seeds were subsequently germinated, producing normal plants. F(1) seeds of V. blattaria had a germination of 64%. Seeds (6%) of M. rotundifolia germinated after a cold treatment.
Most theoretical treatments of the evolutionary ecology of offspring size assume a simple and direct effect of investment per offspring on offspring fitness. In this paper I experimentally determine the relationship between seed mass and several main fitness components of the oak Quercus ilex, to estimate phenotypic selection acting on seed mass during the early life cycle and to discover any potential selective conflicts occurring between different stages from dispersal to establishment. I found a positive effect of acorn size on most fitness components related to seedling establishment. Large size increased germination rate and seedling survival, accelerated germination timing, and enhanced seedling growth. Nevertheless, there was also a direct negative effect of acorn size on survival to predation, because large acorns were highly preferred by the main postdispersal seed predators at the study site, wild boars and wood mice. Because of the low probability of escape from predation, the fitness of large acorns estimated on this component was significantly lower than the fitness of smaller acorns. Therefore, seed size affected fitness in two different ways, yielding opposing and conflicting selective forces. These findings suggest that the general assumption that offspring fitness is a fixed positive function of seed size needs to be reconsidered for some systems. The existence of conflicting selection might explain the occurrence of an optimal seed size in some plant species without invoking a seed number-size trade-off.
In SW Spain the winter annuals Anacyclus radiatus and Chrysanthemum coronarium (Asteraceae) are found as weeds in diverse crops. Both plant species are heterocarpic, i.e. the peripheral and central achenes of the capitulum are morphologically distinctive. In heterocarpic and heterospermic species the different fruit or seed morphs usually have differential ecological behaviour. In this work we have studied the morphometry, germination and dispersal timing of t he different achene morphs in A. radiatus and C. coronarium. Laboratory germination tests were carried out to evaluate the influence of incubation temperature (light/dark, 27/27, 24/18, 20/10, 15/5, 10/4 degrees C), chilling period (0, 1, 7, 21 and 60 days at 2 degrees C), GA3, nitrate and the dark on the germination percentage and rate (t50). The peripheral achenes of A. radiatus have a significantly broader wing than the central achenes. In this species, germination was inhibited in the dark but viable achenes of both types germinated completely under light exposure irrespective of test conditions. Nevertheless, the peripheral achenes germinated significantly faster compared to the central achenes (t5o 1.04 vs. 1.55 days at 24/18 degrees C). In C. coronarium peripheral achenes have three-winged ribs and are significantly longer, wider and thicker than the central achenes, which have only one- or two-winged ribs. In this species the peripheral achenes showed a much lower germination compared to the central achenes under all conditions tested (0.3-3.5% vs. 41.5-58.0%). Embryos isolated from the pericarp of peripheral achenes germinated quickly in a great extent, and when placed in close contact with a pericarp fragment they did not show any inhibition of germination, suggesting physical dormancy. To determine differences in dispersal timing between achene morphs, we monitored dispersal in a stand of both plant species. In A. radiatus the peripheral achenes were shed first, at the onset of the rainy season in late summer, whereas the central achenes dispersed from late summer to early winter. In contrast, in C. coronarium the central achenes were shed first, from early summer to early autumn, whereas the peripheral achenes dispersed mainly in early autumn. Thus, in both weed species the more dispersible and readier-to-germinate achene morph is dispersed first. The differences in the germination and dispersal behaviour between achene morphs represent an opportunistic, mixed strategy which is favourable under environmental uncertainty, and should be accounted for in order to understand the population dynamics of both weed species.
Published and additional data for polyethylene glycol 8000 (PEG), formerly PEG 6000, solution water potentials (Psi) are compared. Actual bars Psi over the concentration range of 0 to 0.8 gram PEG per gram H(2)O and temperature (T) range of 5 to 40 degrees C are best predicted (probably within +/- 5%) by this equation: Psi = 1.29[PEG](2)T - 140[PEG](2) - 4.0[PEG]. Although transformable through division by [PEG] to virial equation form, results indicate that the coefficients are not virial. Mannitol (MAN) interacts with PEG to produce Psi significantly lower than additive. Vapor pressure osmometer (VPO) data for MAN-PEG synergism compared favorably with those from thermocouple hygrometry; and VPO data showing the interactions between PEG and four salts are presented. The synergism of MAN-PEG and of NaCl-PEG are related linearly to the concentration of solute added with PEG.
- I Álvarez
Álvarez, I. 2019. Anacyclus. L. In C. Benedi, A. Buira, E. Rico, M. B. Crespo, A.
Quintanar, and C. Aedo [eds.], Flora ibérica, vol. XVI, (III). Compositae
(partim), 1777-1788, Editorial CSIC, Madrid, Spain.
emmeans: Estimated marginal means aka least-squares means
- R V Lenth
Lenth, R. V. 2020. emmeans: Estimated marginal means, aka least-squares
means. R package version 1.5.3. Website: https://CRAN.R-project.
org/package=emmeans
Anacyclus clavatus seed germination data [data set
- R Torices
- L Desoto
- J Cerca
- L Mota
- A Afonso
- C Poyatos
Torices, R., L. DeSoto, J. Cerca, L. Mota, A. Afonso, and C. Poyatos. 2022.
Anacyclus clavatus seed germination data [data set]. Zenodo https://
doi.org/10.5281/zenodo.8163459