GERMINATION ECOPHYSIOLOGY OF ARENARIA GLABRA,A WINTER ANNUAL OF SANDSTONE AND GRANITE OUTCROPS OF SOUTHEASTERN UNITED STATES

The germination ecophysiology of Arenaria glabra Michx., a characteristic winter annual plant species of granite and sandstone outcrops of southeastern United States, was investigated. Seeds germinate in early autumn, plants overwinter in the rosette stage and then flower, set seeds, and die in late spring; seeds are dispersed soon after maturity. Eighty-five to 90% of freshly-matured seeds were innately dormant, and the other 10–15% germinated only at temperatures lower than those that occur in the habitat at the time of seed dispersal in June. During the summer after-ripening period, seeds stored dry under ambient laboratory conditions exhibited progressive increases in rates and total percentages of germination, a widening of the temperature range for germination, and a loss of the light requirement. At a 14-hr daily photoperiod, seeds kept on continuously moist soil germinated to 83% at simulated July and August temperatures during July and August, and the remainder germinated at September temperatures in September. On the other hand, seeds subjected to alternate wetting and drying during July and August germinated to only 9% during those 2 months, and the remainder germinated after the soil was kept continuously moist, beginning on 1 September, at simulated habitat temperatures during September and October. Thus, the timing of germination of A. glabra in the field is controlled by an interplay of the seeds' physiological state with the dynamics of temperature and soil moisture conditions.     

Influence of an abscisic acid (ABA) seed coating on seed germination rate and timing of Bluebunch Wheatgrass

Semi‐arid rangeland degradation is a reoccurring issue throughout the world. In the Great Basin of North America, seeds sown in the fall to restore degraded sagebrush (Artemisia spp.) steppe plant communities may experience high mortality in winter due to exposure of seedlings to freezing temperatures and other stressors. Delaying germination until early spring when conditions are more suitable for growth may increase survival. We evaluated the use of BioNik? (Valent BioSciences LLC) abscisic acid (ABA) to delay germination of bluebunch wheatgrass (Pseudoroegneria spicata). Seed was either left untreated or coated at five separate rates of ABA ranging from 0.25 to 6.0 g 100 g^?1 of seed. Seeds were incubated at five separate constant temperatures from 5 to 25°C. From the resultant germination data, we developed quadratic thermal accumulation models for each treatment and applied them to 4 years of historic soil moisture and temperature data across six sagebrush steppe sites to predict germination timing. Total germination percentage remained similar across all temperatures except at 25°C, where high ABA rates had slightly lower values. All ABA doses delayed germination, with the greatest delays at 5–10°C. For example, the time required for 50% of the seeds to germinate at 5°C was increased by 16–46 d, depending on the amount of ABA applied. Seed germination models predicted that the majority of untreated seed would germinate 5–11 weeks after a 15 October simulated planting date. In contrast, seeds treated with ABA were predicted to delay germination to late winter or early spring. These results indicate that ABA coatings may delay germination of fall planted seed until conditions are more suitable for plant survival and growth.     

Seed germination of exotic and native winter annuals differentially responds to temperature and moisture,especially with climate change scenarios

Seeds of winter annuals require a summer after-ripening period for dormancy loss and low autumn temperatures for germination. With current and future changes in moisture and temperature, we tested the effects of warming along a relative humidity (RH) gradient on dormancy loss and effects of decreased diurnal temperature range (DTR) on germination. We further reasoned that the effects of changes in these variables would be disproportionate between the exotic and native winter annuals. Seeds of exotic species (Buglossoides arvensis, Lamium purpureum and Ranunculus parviflorus) and co-occurring native species (Galium aparine, Paysonia stonensis and Plantago virginica) were collected in middle Tennessee. After-ripening occurred over a 15–100% RH gradient at 25 and 30°C and germination was tested at 20/10 and 20/15°C. Niche breadth was calculated using Levins' B. Fresh Ranunculus seeds had high germination and those of other species did not. Germination for these species increased with after-ripening, mostly across the RH gradient irrespective of temperature. A decrease in DTR showed mixed results – the extreme being Ranunculus with no germination at 20/15°C. Most exotic species had wider germination niche breadths than native species. With climate change, we suggest that a decrease in DTR may have a larger effect on germination than increasing moisture or warming on dormancy break. Moreover, there is not a clear-cut winner with climate change when we compare exotic versus native species because the responses of our six species were species specific.     

Offspring Performance in Oxalis acetosella, a Cleistogamous Perennial Herb

Abstract: Seed weight, seed germination, seedling survival, and juvenile/adult fitness in chasmogamously (CH) and cleistogamously (CL) derived offspring of Oxalis acetosella were compared during three growing seasons, to test hypotheses of fitness differences between the offspring types accounting for the maintenance of cleistogamy. In plots at three field sites, CH and CL seeds originating from all sites were sown to compare the performance of offspring growing in their habitat of origin and offspring growing in new habitats. Seeds were also sown in pots in a common garden, to test for effects of sibling competition. CL seeds had significantly lower germination than CH seeds in the field, possibly because of lower mean seed weight due to later flowering. Since the outcrossing rate in the CH flowers of O. acetosella is not known, it is uncertain whether the lower CL germination is a consequence of inbreeding depression. CH seeds had higher germination if sown at their home sites than at new sites, while for CL seeds this made no difference; this contradicts the local adaptation hypothesis for cleistogamy. No other fitness differences were found between the offspring types, and the findings did not support the sibling competition or local adaptation hypotheses. We suggest that the maintenance of the dimorphic reproductive system in O. acetosella is not explained by offspring characteristics, but rather by the two flowering phases complementing each other in maximizing annual seed production in the face of environmental variability. It is, therefore, important to include temporal and spatial variation in studies of reproductive strategies.     

Effect of seed position in spikelet on life history of Eremopyrum distans (Poaceae) from the cold desert of north-west China

Background and Aims

Most studies on seed position-dependent effects have focused on germination characteristics. Our aim was to determine the effects of seed position in the spikelet on differences in timing of germination and on the ecological life history of the grass Eremopyrum distans in its cold desert habitat.

Methods

For seeds in three spikelet positions, morphology, mass and dormancy/germination characteristics were determined in the laboratory, and seeds planted in field plots with and without watering were followed to reproduction to investigate seedling emergence and survival, plant size and seed production.

Key Results

After maturation, of the seeds within the spikelet, basal ones (group 1) are the largest and have the highest proportion with physiological dormancy, while distal ones (group 3) are the smallest and have the highest proportion of non-dormant seeds. A higher percentage of seeds after-ripened in groups 2 and 3 than in group 1. Seeds sown in the field in early summer and watered at short, regular intervals germinated primarily in autumn, while those under natural soil moisture conditions germinated only in spring. Both cohorts completed their life cycle in early summer. Seeds in group 1 had lower percentages of seedling emergence and higher percentages of seedling survival than those in groups 2 and 3. Also, plants from group 1 seeds were larger and produced more seeds per plant than those from groups 2 and 3.

Conclusions

Seed position-dependent mass was associated with quantitative differences in several life history traits of E. distans. The environmentally enforced (low soil moisture) delay of germination from autumn to spring results in a reduction in fitness via reduction in number of seeds produced per plant.     

The effect of bat (Rousettus aegyptiacus) dispersal on seed germination in eastern Mediterranean habitats

The fruit-bat Rousettus aegyptiacus (Pteropodidae) in Israel consumes a variety of cultivated and wild fruits. The aim of this study was to explore some of its qualities as a dispersal agent for six fruit-bearing plant species. The feeding roosts of the fruit-bat are located an average of 30 m from its feeding trees and thus the bats disperse the seeds away from the shade of the parent canopy. The bat spits out large seeds but may pass some (2%) of the small seeds (<4 mg) through its digestive tract. However, neither the deposited seeds nor the ejected seeds (except in one case) had a significantly higher percentage germinating than intact seeds. Although the fruit-bat did not increase the percentage germinating, seeds of three plant species subject to different feeding behaviors (deposited in feces or spat out as ejecta) had a different temporal pattern of germination from the intact seeds. The combined seed germination distribution generated by these different treatments is more even over time than for each treatment alone. It is sugested that this increases asynchronous germination and therefore enhances plant fitness by spreading the risks encountered during germination, especially in eastern Mediterranean habitats where the pattern of rainfall is unpredictable.     

Fitness in Naturally Occurring and Restored Populations of a Grassland Plant Lychnis flos‐cuculi in a Swiss Agricultural Landscape

Wildflower seed mixtures are widely used for restoration of grasslands. However, the genetic and fitness consequences of using seed mixes have not been fully evaluated. Here, we studied the role of genetic diversity, origin (commercial regional seed mixtures, natural populations), and environmental conditions for the fitness of a grassland species Lychnis flos‐cuculi. First, we examined the relationship between genetic diversity, environmental parameters, and fitness in sown and natural populations of this species in a Swiss agricultural landscape. Second, we established an experiment in the study area and in an experimental garden to study the implications of local adaptation for plant fitness. Third, to examine the response of plants to different soil properties, we conducted an experiment in climate chambers, where we grew plants from sown and natural populations of L. flos‐cuculi as well as from seed suppliers on soils with different nutrient and moisture content. We detected no significant effect of genetic diversity on the fitness of sown and natural populations. There was no clear indication that plants from natural populations were better adapted to local environment than plants from sown populations or seed suppliers. However, plants of natural origin invested more into generative reproduction than plants from sown populations or seed suppliers. Furthermore, in the climate chamber, plants originating from natural populations tended to flower earlier. Our results indicate that using nonlocal seeds for habitat recreation may influence restoration success even if the seeds originate from the same seed zone as the restored site.     

The germination and emergence of seeds of winter oilseed rape stored and sown in admixture with pelleted methiocarb

The possibility that pellets of the molluscicide Draza (a.i. methiocarb) might reduce field emergence of seeds of winter oilseed rape (Brassica napus, cv. Jet Neuf) when sown admixed, laboratory germination when stored admixed, or field emergence when both stored and sown admixed was the subject of three separate investigations. Admixture at sowing did not influence the emergence of five seed lots (P > 0·05). Neither did admixture influence loss in germination or change in moisture content during 2 months' storage of one lot in four different regimes (P > 0·05). Field emergence following 2 months' storage in paper bags was not influenced by admixture (P > 0·05), whereas a 12% reduction in field emergence resulting from storage in admixture within polyethylene bags was detected (P < 0·05). Thus short-term storage within paper sacks of winter oilseed rape seeds in admixture with slug pellets containing methiocarb did not damage the seeds. The emergence of five seed lots differed considerably within and between field sowings. Tentative advice on the calculation of sowing rates for different seed lots of winter oilseed rape is provided to account for this variation.     

Seed Dormancy and Germination in Cimicifuga nanchuanensis

Seed anatomy, dormancy breakage, the temperature effect to seed germination and seed life-span of Cimicifuga nanchuanensis Hsiao were studied and the endangerment of this plant in association to these biological characteristics was explored. The embryos were at the globular stage at the time of seed shedding in late November. Low temperature and humid conditions or treatment with exogenous GA3 stimulated the development of embryos and sped up the process of seed germination. The optimum temperature of germination was 20 ℃, but the seeds almost lost their viability after 9 months of storage. Nevertheless, in its natural habitation, the seeds could not acquire enough environmental humidity to accomplish their after-ripening during the dry and cold winter from late November to the following March; after then the temperature in the spring (averaged 10.1 ℃ in April and May) was much lower than 20 ℃ or so which is favorable for seed germination. Moreover, the testa could not provide adequate protection for the embryos and the short life-span of the seeds prevents their survival until the next germination. Therefore it seems reasonable to infer that the unfavorable environmental condition during the process of after-ripening until seed maturation is involved in the cause of endangerment of this plant species.     

Comportamento Alla Germinazione Delle Cariossidi Delle Linee Primaverile e Invernale del Triticum Esaploide «Denti de Cani»

Abstract

The germination of spring and winter wheat lines of exaploid Triticum « Denti de Cani ». — The dormancy in the seeds of two lines of Triticum « Denti de Cani » (which is spontaneous in Sardinia), one with solid stem (CP line), a spring line, the other with hollow stem (CV line), an winter line, has been studied. Germination was carried out in the dark, in Petri dishes at the constant temperatures of 5°, 10°, 20°, 23°, 26°, 30° and 35°C, using full ripe seeds, and seeds in different stages of after-ripening up to one year of age. The increase in % germination, for increasing temperatures above 5°C, is clearly conditioned by the progress of after-ripening in the seeds. In fact it was seen that, in general for the two lines, percentages over 50% of seeds germinated at 3 days were reached: at 10° and 20° after 15 days from the full ripening; at 23°C after 30 days; at 26°C after 50 days; at 30°C after about 100 days and at 35°C only after about 4–5 months from the harvest. During the experiment at 5°C it was observed that, during the first year of life of seeds and especially in the CP line, this temperature produces a clear slowing down in germinations after first year from the ripening, only the CV seeds — not the CP which remain very much inhibited — reach germination values over 50% at 3 days. It has also been demonstrated that the CV are more sensitive than the CP, in the first initial period of after-ripening (15 and 30 days), to the non-inhibiting activity of low temperatures (5° and 10°C) and that, between these, the 10°C temperature promotes the germination more clearly than the 5°C temperature. The results obtained have shown that the dormancy wears off in the spring CP-line much more slowly than in the winter CV-line. The CP-seeds remain in a relative dormancy condition for a long time, which causes a significative delay in germination, up to 100 days from the full ripening stage.     

Germination ecology of the weed Parthenium hysterophorus in eastern Ethiopia

Germination ecology of Parthenium hysterophorus, recently introduced to Ethiopia, was studied in a series of experiments. Viability of the seeds was greater than 50% after 26 months of burial in the soil indicating the potential build‐up of a substantial persistent soil seed bank. A short period of dry storage was sufficient to overcome a light requirement for germination in a minor fraction of the seeds. Likewise, seeds exhumed from burial showed an increase in germination ability in darkness over time, with a weak tendency for seasonal cyclicity in dormancy level at one of two sites. Germination occurred at the mean minimum (10°C) and maximum (25°C) temperatures of the collection sites, as well as over a wide range of fluctuating (12/2°C‐35/25°C) temperatures in light. No germination of P. hysterophorus seeds occurred at osmotic potentials < ‐0.52 MPa (at 27°C), the species being less tolerant to moisture stress than sorghum grains. Most seedlings emerged from shallowly buried (< 0.5 cm) seeds and none from more than 5 cm depth. Naturally dispersed seeds required about 60 days, at a hot lowland site, to start emergence despite the presence of adequate rainfall, and higher number of seedlings emerged in undisturbed plots than in hand hoed plots. These experiments and field observations suggest that there are no obvious climatic conditions that may limit the germination of Parthenium hysterophorus in Ethiopia, but a high moisture requirement of the seeds for germination could be the major factor limiting germination during the dry season.     

Seed germination ecology in southwestern Western Australia

Germination responses of species from the native plant communities of southwestern Western Australia can be related to syndromes of life history, fire response, and seed storage, and also to factors related to environmental stress. The Mediterranean-type climate of the region with periodic drought and recurrent fires affects the production of viable seeds in plants of limited stature and rooting depth. Fire response ephemerals and species cued to flower by fire tend to produce viable, readily germinable seeds, but there are instances where seed production is aborted in these predominantly herbaceous life forms. Clonal, rhizomatous species often produce mainly inviable seeds. Production of viable seeds in woody species of these highly diverse communities may also be restricted by limitations to cross pollination. Obligate post-fire seeding species tend to produce a greater proportion of viable seeds than species which are capable of resprouting following fire. Serotinous species, whether post-fire re-seeders or post-fire resprouting species, produce mainly viable seeds, which germinate readily once freed from protective fruits. Species of the legume families and a few others of the soil seed bank produce innately dormant seeds which can be germinated following heat shock treatments which simulate the effects of fire. Heat shock in these species appears mainly as a mechanism to crack the hard seed coats, but the effect of heat to denature seed coat inhibitors has not been eliminated. Western Australian species do not seem to break dormancy when exposed to leachates from burned wood as has been observed in comparable habitats in California and South Africa, but further research is advised. Germination in many native southwestern Australian species is cued by temperatures that correspond to the winter rainfall period. There are also indications that an after-ripening period of warm, dry storage increases percentage of germinable seeds. Stimulation of germination by hormones is almost unresearched in Western Australia, but germination percentages have been increased in a small number of species of horticultural potential. Stimulation of germination by soil nutrient concentrations is almost unresearched in Western Australia, except for the inhibitory effect of excess sodium chloride levels inEucalyptus andMelaleuca. These species only germinate when osmotic effects are reduced to lower levels as would occur when winter rains dilute soil salts. Application of research on seed germination has already enhanced the establishment of seedlings in the restoration of mine sites and is becoming important in aspects of the breeding and selection of native plants for the cut flower, bedding plant and essential oil industries.     

Effects of fern thickets on woodland development on landslides in Puerto Rico

Abstract. Thicket-forming ferns are common colonizers of disturbed habitats in the tropics, but little is known about their ecology. The effects of thickets formed by the fern Dicranopteris pectinata on tree seedlings on five landslides in the Luquillo Experimental Forest in northeastern Puerto Rico were both positive and negative. Soil moisture and total soil N were higher under fern thickets than in adjacent open areas and soil bulk density and soil surface temperatures were lower. Germination of seeds of the tree Cecropia schreberiana was higher for seeds sown under fern thickets than for those sown into adjacent open areas. Tree seedlings of Tabebuia hetero-phylla exhibited a threefold reduction in photosynthesis under ferns, probably resulting from a twelvefold reduction of photosynthetic photon flux density. Growth of Tabebuia seedlings was reduced under ferns but the distribution of seedlings of naturally occurring woody plants was not strongly correlated with the presence of fern thickets. Although fern thickets on low-nutrient landslide soils appear to facilitate germination, they inhibit growth of tree seedlings and may, therefore, delay forest development on landslides in Puerto Rico.     

Low allelopathic potential of an invasive forage grass on native grassland plants: a cause for encouragement?

Tall fescue (Festuca arundinacea Schreb.), a highly competitive European grass that invades US grasslands, is reportedly allelopathic to many agronomic plants, but its ability to inhibit the germination or growth of native grassland plants is unknown. In three factorial glasshouse experiments, we tested the potential allelopathic effects of endophyte-infected (E+) and uninfected (E−) tall fescue on native grasses and forbs from Midwestern tallgrass prairies. Relative to a water control, at least one extract made from ground seed, or ground whole plant tissue of E+ or E− tall fescue reduced the germination of 10 of 11 species in petri dishes. In addition, the emergence of two native grasses in potting soil was lower when sown with E+ and E− tall fescue seedlings than when sown with seeds of conspecifics or tall fescue. However, when seeds of 13 prairie species were sown in sterilized, field-collected soil and given water or one of the four tall fescue extracts daily, seedling emergence was lower in one extract relative to water for only one species, and subsequent height growth did not differ among treatments for any species. We conclude that if tall fescue is allelopathic, its inhibitory effects on the germination and seedling growth of native prairie plants are limited, irrespective of endophyte infection. On the other hand, the apparent inability of these plants to detect tall fescue in field soil could hinder prairie restoration efforts if germination near this strong competitor confers fitness consequences. We propose that lack of chemical recognition may be common among resident and recently introduced non-indigenous plants because of temporally limited ecological interactions, and offer a view that challenges the existing allelopathy paradigm. Lastly, we suggest that tall fescue removal will have immediate benefits to the establishment of native grassland plants.     

Seed dormancy,germination and soil seed bank of Lamiophlomis rotata and Marmoritis complanatum (Labiatae), two endemic species from Himalaya–Hengduan Mountains

Seed dormancy and germination characteristics are important factors determining plant reproductive success. In this study, we aimed to explore the characteristics of seed dormancy and germination of two endemic Labiatae species (Lamiophlomis rotata and Marmoritis complanatum) in the Himalaya–Hengduan Mountains. Germination was first tested in the light using freshly matured seeds at 25/15 and 15/5°C, and then again after dry after-ripening. Dried seeds were incubated in the light at a range of constant temperatures (1–35°C). The effects of dark and GA₃ on germination were tested at several different temperatures. Base temperature (T_b) and thermal times for 50% final germination (θ₅₀) were calculated. Seeds were also buried at the collection site to test seed persistence in the soil. Increased final germination after dry after-ripening indicated that the seeds of the two species exhibited non-deep physiological dormancy; however, they exhibited different germination characteristics and soil seed bank types. In L. rotata, GA₃ only promoted germination at 5°C, producing no significant effect at other temperatures. Dark conditions decreased germination significantly at all temperatures. T_b and θ₅₀ values were 0.6 and 82.7°C d. The soil seed bank of this species was classified as persistent. In M. complanatum, GA₃ significantly promoted germination at all temperatures except 15°C. Dark conditions depressed germination significantly at warmer temperatures (20 and 25°C) but had no effect at lower temperatures. T_b and θ₅₀ values were 0.1 and 92.3°C d. The soil seed bank was classified as transient. Our results suggest that the seed dormancy and germination of the two co-existing species share some commonalities but there are also species-specific adaptations to the harsh alpine environment.     

Soil moisture and sex ratio in a plant with nuclear-cytoplasmic sex inheritance

I investigated whether soil moisture affects relative fitness of females and hermaphrodites and sex ratio in a gynodioecious plant with nuclear-cytoplasmic sex inheritance. I contrast these results with those from species with strictly nuclear sex inheritance. I performed a manipulative watering experiment on seed fitness of the two sexes, and field studies measuring seed fitness and sex ratio as a function of soil moisture. In the dry site, watered hermaphrodites produced approximately twice as many seeds as unwatered hermaphrodites, with little treatment effect on female seed production. Over a natural soil moisture gradient, the ratio of female to hermaphrodite seed production was higher in dry than in wet sites. These data show that the seed fitness advantage of females is a function of soil moisture. Despite this, regression of soil moisture on the sex ratio of 23 populations was not significant. These results indicate a sex-dependent effect of soil moisture on resource allocation to seeds that does not translate into a strong effect on sex ratio. This is consistent with theory based on genomic conflict in which sex ratios are predicted to be only partly determined by fitness differences of the sexes.     

Environmental Heterogeneity and Population Differentiation in Plasticity to Drought in <Emphasis Type="Italic">Convolvulus Chilensis</Emphasis> (Convolvulaceae)

Plant populations may show differentiation in phenotypic plasticity, and theory predicts that greater levels of environmental heterogeneity should select for higher magnitudes of phenotypic plasticity. We evaluated phenotypic responses to reduced soil moisture in plants of Convolvulus chilensis grown in a greenhouse from seeds collected in three natural populations that differ in environmental heterogeneity (precipitation regime). Among several morphological and ecophysiological traits evaluated, only four traits showed differentiation among populations in plasticity to soil moisture: leaf area, leaf shape, leaf area ratio (LAR), and foliar trichome density. In all of these traits plasticity to drought was greatest in plants from the population with the highest interannual variation in precipitation. We further tested the adaptive nature of these plastic responses by evaluating the relationship between phenotypic traits and total biomass, as a proxy for plant fitness, in the low water environment. Foliar trichome density appears to be the only trait that shows adaptive patterns of plasticity to drought. Plants from populations showing plasticity had higher trichome density when growing in soils with reduced moisture, and foliar trichome density was positively associated with total biomass. Co-ordinating editor: F. Stuefer     

Population biology of Senecio integrifolius (Compositae), a rare plant in Sweden

In a study of natural populations of Senecio integrifolius in southern Sweden (1979–1983) seeds were found to disperse from mid-June to late July and most of them germinated in autumn. No soil seed bank was observed. On a heavily grazed site few seeds were produced but the percentage of estimated germination was high (c. 75%). In two moderately grazed habitats 8–10% of the estimated number of seeds produced in permanent plots germinated. In a lightly grazed habitat many seeds were produced but only a small percentage germinated (1.4%). In field experiments the average germination was 50–53% when newly harvested seeds were sown where the vegetation had been removed, and 20–33% when sown in undisturbed vegetation at a moderately grazed site. Germination was much lower in a lightly grazed habitat (3–12%). Survival of seedlings was much higher in heavily and moderately grazed habitats than in lightly grazed habitats. Mortality tended to be higher during the growing season (mid-April to early November) than during the winter, and increased markedly during a drought period in the summer of 1982. The half-life of plants established in 1980 varied from 39.3 years at the most intensively grazed site to 7.2 at the lightly grazed site. The number of flowering stems varied between years mainly according to weather. Few plants in the permanent plots flowered every year, the flowering being most frequent at heavily grazed sites. It is concluded that heavy grazing by cattle after seed dispersal is the appropriate management for maintaining S. integrifolius in Sweden.     

GENETIC EFFECTS OF GERMINATION TIMING AND ENVIRONMENT: AN EXPERIMENTAL INVESTIGATION

Seeds of many species do not germinate immediately after dispersal, but instead may remain indefinitely in a dormant but viable state. Although it is well established that seeds often exhibit diversified patterns of dormancy and germination, the causes and consequences of this variation remain poorly understood. In this study, we investigate the extent to which seed genotypes of the desert mustard Lesquerella fendleri differentially germinate and establish under experimental conditions in a greenhouse. We used a two-way factorial design to compare genotypes of Lesquerella plants derived from seeds that germinated and established at different times and under different soil water regimes. Overall allozyme allele frequencies of Lesquerella plants varied significantly with both germination time and initial soil water availability. Single-locus heterozygosity analyses revealed that seeds sown into initially low water conditions produced plants that were significantly more heterozygous than plants derived from seeds experiencing constantly high water conditions, but heterozygosity did not differ significantly among plants originating from early- and late-germinating seeds. This is the first study to experimentally demonstrate that germination timing and environment can significantly affect the genetic structure of emerging plant populations. The study suggests that germination and survival behavior may (1) play an important role in generating and maintaining the genetic structure of natural plant populations and (2) set the stage for subsequent evolution.     

Allelopathic effects of three plant invaders on germination of native species: a field study

The ability of some invasive plant species to produce biochemical compounds toxic to native species, called allelopathy, is thought to be one of the reasons for their success when introduced to a novel range, an idea known as the Novel Weapons Hypothesis. However, support for this hypothesis mainly comes from bioassays and experiments conducted under controlled environments, whereas field evidence is rare. In a field experiment, we investigated whether three plant species invasive in Europe, Solidago gigantea, Impatiens glandulifera and Erigeron annuus, inhibit the germination of native species through allelopathy more than an adjacent native plant community. At three sites for each invasive species, we compared the germination of native species that were sown on invaded and non-invaded plots. Half of these plots were amended with activated carbon to reduce the influence of potential allelopathic compounds. The germination of sown seeds and of seeds from the seedbank was monitored over a period of 9 weeks. Activated carbon generally enhanced seed germination. This effect was equally pronounced in invaded and adjacent non-invaded plots, indicating that invasive species do not suppress germination more than a native plant community. In addition, more seeds germinated from the seedbank on invaded than on non-invaded soil, probably due to previous suppression of germination by the invasive species. Our field study does not provide evidence for the Novel Weapons Hypothesis with respect to the germination success of natives. Instead, our results suggest that if invasive species release allelopathic compounds that suppress germination, they do so to a similar degree as the native plant community.