Germination Fraction Research Articles

Evolutionarily stable germination strategies with time-correlated yield

We investigate the effect of auto-correlated yield on the evolutionarily stable germination fraction of dormant seeds. By using both analytics and numerics, we first show that in a regime of small fluctuations a positive correlation reduces dormancy and a negative correlation enhances dormancy. By extending the numerical analysis we also show that in the regime of large fluctuations a more complex picture emerges where also negative correlations can reduce dormancy.

Hydrothermal time model of germination: Parameters for 36 Mediterranean annual species based on a simplified approach

Summary Germination rates and germination fractions of seeds can be predicted well by the hydrothermal time (HTT) model. Its four parameters HTT, minimum soil temperature, minimum soil moisture, and variation of minimum soil moisture, however, must be determined by lengthy germination experiments at combinations of several levels of soil temperature and moisture. For some applications of the HTT model it is more important to have approximate estimates for many species rather than exact values for only a few species. We suggest that minimum temperature and variation of minimum moisture can be estimated from literature data and expert knowledge. This allows to derive HTT and minimum moisture from existing data from germination experiments with one level of temperature and moisture. We applied our approach to a germination experiment comparing germination fractions of wild annual species along an aridity gradient in Israel. Using this simplified approach we estimated HTT and minimum moisture of 36 species. Comparison with exact data for three species shows that our method is a simple but effective method for obtaining parameters for the HTT model. HTT and minimum moisture supposedly indicate climate related germination strategies. We tested whether these two parameters varied with the climate at the site where the seeds had been collected. We found no consistent variation with climate across species, suggesting that variation is more strongly controlled by site-specific factors.

ENVIRONMENTAL AND GENETIC SOURCES OF DIVERSIFICATION IN THE TIMING OF SEED GERMINATION: IMPLICATIONS FOR THE EVOLUTION OF BET HEDGING

Environmental variation that is not predictably related to cues is expected to drive the evolution of bet-hedging strategies. The high variance observed in the timing of seed germination has led to it being the most cited diversification strategy in the theoretical bet-hedging literature. Despite this theoretical focus, virtually nothing is known about the mechanisms responsible for the generation of individual-level diversification. Here we report analyses of sources of variation in timing of germination within seasons, germination fraction over two generations and three sequential seasons, and the genetic correlation structure of these traits using almost 10,000 seeds from more than 100 genotypes of the monocarpic perennial Lobelia inflata. Microenvironmental analysis of time to germination suggests that extreme sensitivity to environmental gradients, or microplasticity, even within a homogeneous growth chamber, may act as an effective individual-level diversification mechanism and explains more than 30% of variance in time to germination. The heritability of within-season timing of germination was low (h(2) = 0.07) but significant under homogeneous conditions. Consistent with individual-level diversification, this low h(2) was attributable not to low additive genetic variance, but to an unusually high coefficient of residual variation in time to germination. Despite high power to detect additive genetic variance in within-season diversification, it was low and indistinguishable from zero. Restricted maximum likelihood detected significant genetic variation for germination fraction (h(2) = 0.18) under homogeneous conditions. Unexpectedly, this heritability was positive when measured within a generation by sibling analysis and negative when measured across generations by offspring-on-parent regression. The consistency of dormancy fraction over multiple delays, a major premise of Cohen's classic model, was supported by a strong genetic correlation (r = 0.468) observed for a cohort's germination fraction over two seasons. We discuss implications of the results for the evolution of bet hedging and highlight the need for further empirical study of the causal components of diversification.

Comparison of Seed Bank Size and Composition in Fringing, Restored, and Impounded Marsh in Southwest Louisiana

In coastal Louisiana many restoration projects are approved based on assumed regeneration of submerged aquatic species (SAV) in shallow marsh interior ponds. In this study, we estimated seed bank size and composition of shallow water areas in oligohaline fringing and restored (terraced) marsh, and a freshwater managed (impounded) marsh, located in Sabine NWR, LA, using the sieving method. For the same marshes, we also provided an estimate of the readily germinable fraction of the seed bank using the germination method. Sieving results indicated that restored marsh edges had very low seed densities (5034 seeds/m2) compared to fringing marsh (331,185 seeds/m2), although species composition was similar. Managed freshwater marsh ponds had more diverse seed banks and mid-range seed densities (80,500 seeds/m2). Viability estimates of dominant species in the seed bank reduced seed density estimates at all sites by as much as 10 fold (fringing marsh: 36,185 seeds/m2; restored marsh: 859 seeds/m2; managed marsh 44,388 seeds/m2) suggesting that a correction factor should be applied to future seed density estimates in this region. Seedling emergence was significantly higher in the managed marsh under drawdown conditions (> 2500 seedlings/m2) as compared to flooded conditions (< 500 seedlings/m2; ANOVA, p = 0.0001). Seedling emergence in oligohaline marsh was significantly affected by salinity and management (fringing, restored) (ANOVA, p = 0.0186). Fringing marsh at 0 g/L had the highest seedling emergence (> 500 seedlings/m2). At higher salinities, fringing and restored marsh had similar emergence (< 150 seedlings/m2). Results indicate that recruitment is likely to be more successful under drawdown conditions, and in low salinity conditions. However, both a lack of SAV emergence in the germination experiment and a lack of SAV seeds in the seed banks using the sieving method suggest that reliance on seed banks for the restoration of shallow water areas in southwest Louisiana may prove unsuccessful.

Algebraic determination of the evolutionary stable germination fraction

In this article, we consider a bet-hedging model with density dependence for germination behavior of dormant seeds. We show how to compute accurate values of the evolutionary stable germination fraction using a method based on expansion around the average seed bank size. The expansion is made up to the fourth order and is expressed in terms of the first four moments of the distribution of the year-to-year variation in the average yield per adult plant. This method leads to a simple algebraic equation, which can be solved without recurring to simulations of the seed bank dynamics. We first show the analytical derivation under general assumptions about the effect of density dependence. We then test our results with a numerical analysis where an explicit form of the density dependence has been taken.

DORMANCY AND GERMINATION IN A GUILD OF SONORAN DESERT ANNUALS

To investigate bet hedging and species coexistence in a guild of Sonoran Desert winter annuals, we subjected seeds of eight species to factorial combinations of summer treatments (varying temperature and precipitation), germination conditions (representative of early, middle, and late germination season temperatures and day lengths), and experiment trial dates (spanning the germination season). In keeping with bet hedging theory, we found that many viable seeds would not germinate in response to any combination of treatments (germination usually <50%). In keeping with the storage effect model, we found that these coexisting species differed in their germination response to our experimental manipulations and also differed in how the experimental variables interact. Field germination data from a long-term project on population and community dynamics of this guild show that germination fractions are similar between field and growth chamber and that species that tend to germinate under early season conditions in the growth chamber also tend to do so under unmanipulated field conditions. Some species are nondormant during the summer and only acquire dormancy at the onset of the autumn germination season, while others appear to have either innate or conditional dormancy until the onset of the germination season.

Soil seed banks, habitat heterogeneity, and regeneration strategies in a Mediterranean coastal sand dune

This study investigated the effects of microhabitat characteristics on soil seed bank structure, regeneration strategies, and plant composition in a Mediterranean coastal sand dune in Israel. Three different microhabitats were selected: (a) open patches between shrubs, (b) shrub understorey, and (c) open, disturbed patches in pedestrian trails. In each microhabitat two types of sampling plots were established: (a) seedling removal and (b) control (seedlings not removed). Emerging seedlings in each microhabitat were identified, counted, and removed every two weeks. In the controls, the vegetation was sampled only once at the peak of the herbaceous growing season. At the end of the growing season in plots where seedlings were removed, the upper soil was collected and sieved before seeds set. Viable non-germinated seeds were identified and counted. An estimation of persistent seed banks was obtained for each plot. Species composition, species richness, species diversity, and aboveground biomass by species were estimated. Germination fractions among species and plant functional groups were measured. Microhabitats differed in soil seed bank density, germination characteristics, above-ground biomass production, and species composition. The open patches between shrubs generally had the highest densities of seeds, seedlings, and mature individual herbaceous plants. Important density differences between total seedlings removed and mature vegetation may indicate strong plant mortality in all microhabitats. Annual and perennial grasses, composites, annual crucifers, and annual forbs produced transient seed banks, while legumes, umbelifers, and perennial forbs had persistent seed banks. Disturbance by public trampling in trails led to low seed and plant densities in this microhabitat. No effects of facilitation of shrubs on the annual vegetation were noted in shub understorey. Habitat heterogeneity had a strong impact on vegetation structure and regeneration strategies.

Herbicides used in combination can reduce the probability of herbicide resistance in finite weed populations

SummaryA simulation study was conducted to examine the effect of pattern of herbicide use on development of resistance to two herbicides with different modes of action in finite weed populations. The effects of the size of the treatment area (analogous to initial weed population), germination fraction and degree of self‐pollination in the weed were investigated. The results indicate that the probability of developing resistance to one or both herbicides decreases as the size of the area/initial population decreases. For treatment areas of 100 ha or less with an initial weed seedbank of 100 seeds m−2 and initial frequencies of the resistance genes of 10−6, development of resistance to both herbicides (double‐resistance) is uncommon within 50 years for all types of weeds if both herbicides are used in all years (used in combination). If herbicides are used in alternate years (rotated) double‐resistance almost always occurs in 100 ha areas but is uncommon in areas of 1 ha or less. The results suggest that adoption of practices that limit movement of weeds in conjunction with using herbicides in combination rather than in rotation can substantially delay development of herbicide resistance.

TEMPERATURE - GERMINATION RESPONSES OF SUNFLOWER (Helianthus annuus L.) GENOTYPES / GERMINACION COMO REACCION A LA TEMPERATURA EN LOS GENOTIPOS DEL GIRASOL (Helianthus annuus L.) / GERMINATION - RÉACTION À LA TEMPÉRATURE DANS LES GÉNOTYPES DE TOURNESOL (Helianthus annuus L.)

SUMMARY Seed germination of six sunflower (Helianthus annuus L.) hybrids was investigated across a range of eleven constant temperatures between 5°C and 45°C. Large temperature differences in germination rate 1/t (d-1), cardinal temperature (°C) and thermal time θ (°cd) were observed among hybrids. Base temperatures (Tb) varied between 3.3°C and 6.7°C whereas maximum germination temperatures (Tm) varied between 41.7°C and 48.9°C. Final germination fraction was attained at 15°C - 25°C whereas the maximum rate of germination was attained at 30.4°C - 35.6°C. The maximum germination rate of hybrid USDA 894, the cultivar with the slowest germination rate, was only 50% of that of hybrid EX 47. The low Tb and high Tm of sunflower appear to be one of the factors which explain the successful adaptation of sunflower to a wide range of temperature. These findings are discussed in relation to the origin of the crop and its wide adaptations in diverse habitats and climatic zones.

Seed Germination in Desert Annuals: An Empirical Test of Adaptive Bet Hedging

Temporal variability in survivorship and reproduction is predicted to affect the evolution of life-history characters. Desert annual plants experience temporal variation in reproductive success that is largely caused by precipitation variability. We studied several populations of the desert annual Plantago insularis along a precipitation gradient. Whereas models of bet hedging in unpredictable environments generally predict one optimal germination fraction for a population, empirical studies have shown that environmental conditions during germination can cause a range of germination fractions to be expressed. In a 4-yr field study, we found that populations in historically more xeric environments had lower mean germination fractions, as is predicted by bet-hedging models. However, populations exhibited significant variation in germination among years. Two experimental studies measuring germination under several environment conditions were conducted to elucidate the source of this in situ variation. Germination fractions exhibited phenotypic plasticity in response to water availability and date within the season. Populations differed in their norms of reaction such that seeds from more xeric populations germinated under less restrictive conditions. A pattern of delayed germination consistent with among-year bet-hedging predictions arose in the field through the interaction of seed germinability and the distribution of environmental conditions during germination.

Effect of Temperature on Germination Behavior of Horsenettle (Solanum carolinense L.) Seeds.

The temperature range of germination, thermal time and the effects of temperature fluctuation on horsenettle (Solanum carolinense L.) seeds were investigated. Base temperature (Tb) and thermal time (θ) to germination for seeds treated with gibberellin were calculated by Garcia-Huidobro's method. Tb was estimated to be between 14-16°C. Maximum temperature was surmised to be between 45-50°C from a graph. Gompertz function adequately described the relationship between θ and germination fraction (%), but a modification of the equation was needed in some cases. The germination percentage of seeds exposed to diurnal temperature fluctuation with an upper limit of 15-30°C and amplitudes of 5-15°C for 10 or 20 cycles was significantly higher than that of seeds incubated at a constant 30°C. Seeds incubated at lower temperatures needed more cycles to increase the germination percentage, but the maximum percentage obtained with each temperature regime was similar. Horsenettle seeds appeared to respond even to temperature fluctuation below Tb because exposure to 15/10°C for 20 cycles increased the germination percentage.It was suggested that horsenettle seeds whose dormancy can be broken by temperature fluctuation may be released from dormancy by temperature fluctuation below Tb, and then germination may begin with the temperature above 14-16°C under field conditions.

Cardinal temperatures and thermal requirements for germination of cineraria seed

SummaryEffects of temperature on the rate of germination, measured as the reciprocal of the time taken for half the population to germinate, were studied in cineraria cvs Cindy Blue and Cindy Dark Red. The base, optimum and maximum temperatures for germination were derived respectively as 1.6, 24.8 and 37.8°C in cv. Cindy Blue, close to Tb = 1.9, To = 24.5 and Tm = 38.4°C in cv. Cindy Dark Red. All seeds of each cultivar had a common base temperature but needed different thermal times to germinate. Seed of cv. Cindy Dark Red began to germinate after ca. 30°Cd compared with 35°Cd required by cv. Cindy Blue. A thermal time of ca. 50°Cd was required for half the population to germinate in each cultivar and the thermal requirement for germination fraction >0.5 was similar in both cultivars. The base and maximum temperatures for germination were similar to those determined previously for leaf initiation and flower development.

Effect of pasture renovation on the size and composition of the germinable fraction of the seed pool beneath bent grass (Agrostis castellana) pastures in southern Victoria

This paper investigated the potential for propagation of grass weeds from seed by measuring changes in the size and composition of the germinable fraction of the seed pool beneath 4 bent grass (Agrostis castellana) pastures in southern Victoria during renovation. Soil samples, for the estimation of the seed pool by seedling emergence in the glasshouse, were taken from a control and a renovated pasture on 3 occasions during renovation at each of the 4 sites. The depth distribution of seeds in the top 0–100 mm was assessed by dividing collected soil samples into depths of 0–30 mm and 30–100 mm. The total germinable seed pool (0–30 mm) ranged from 17 620 to 83 540 seedlings/m2, as affected by the time of sampling. There were seedlings from 14 grass genera (12% contribution to the total seed pool), Juncus (60%), 4 leguminous genera (17%), and 22 other dicotyledonous genera (12%). The percentage change in the number of germinated seeds of grasses and dicotyledons between the first and second sampling was negative ( –7·3% to –31·4%) beneath renovated pasture, as opposed to a positive change (29·0–174·4%) beneath undisturbed pasture; this was attributed to the prevention of seeding due to the cultivation of the pasture associated with renovation. The percentage change in the number of germinated seeds of grasses and dicotyledons did not differ significantly between renovated and undisturbed pasture between the second and third sampling, which suggested that no more seed germinated in response to renovation. Re-distribution of germinable seed to lower depths (30–100 mm) with renovation was apparent for grasses and other dicotyledons, and would depress successful emergence of these seedlings. Renovation prevented re-seeding of weeds and buried surface seed, but did not significantly stimulate additional seed to germinate the following autumn after renovation. Weed seeds were still present in the soil seed pool after renovation, which indicated that these species could potentially germinate and establish in new pastures.

Is Decreased Germination Fraction Associated with Risk of Sibling Competition?

Seed dormancy is hypothesized to be a risk-spreading strategy that maximizes plant fitness. By spreading germination time of offspring among years, plants may not only vary the environment to which their offspring are exposed, but may also reduce potential fitness losses due to sib competition. The sibling competition hypothesis predicts that large families whose offspring are more likely to experience sibling competition should stagger germination over several seasons (i.e. have greater dormancy) to a greater extent than small families. This should be observed as a lower initial probability of germination for seeds from large families than those from small families. We examined the relationship between family size and germination behavior in the desert mustard Lesquerella fendleri. In a single population in central New Mexico, USA, we estimated family size and germination fraction of 189 randomly selected maternal plants. Although a median test was not supportive, the randomization test showed marginal support for the sibling competition hypothesis. An additional study of twelve large and twelve small families in each of three other populations also showed a trend (significant in one population) for larger families to have lower germination fractions than small families. These results suggest a degree of support for the hypothesis, given that so many selective forces interact to generate patterns of seed dormancy. Thus, we have not ruled out the hypothesis that large families reduce the likelihood of sib competition in good years by producing seed crops with initially low germination. Sibling competition as well as environmental heterogeneity may have influenced the evolution of seed dormancy in this system.

Seed Banks in Desert Annuals: Implications for Persistence and Coexistence in Variable Environments

It is widely believed that desert annual plants maintain between—year seed banks, yet few field studies actually have measured the proportion of the viable seed bank that remains dormant through a season. Dormancy and germination fractions were quantified for a guild of winter annuals on a creosote flat in the Sonoran Desert for three years. Predictions from two types of theoretical models applicable to temporally variable environments were examined: (1) the evolution of life history traits promoting persistence in the face of temporal variation and (2) the role of temporal variation in mediating species coexistence. The density of ungerminated seeds was estimated by collecting soil samples after germination, but prior to new seed set. Seedlings were followed in nearby plots to estimate the density of germinated seedlings and their reproductive success. Long—term data collected from permanent plots over a 10—yr period were used to calculate temporal variation in reproductive success for each species. Species with higher temporal variation in reproductive success had lower germination fractions and smaller seeds, consistent with the theory that seed dormancy and large seed size are partially substitutable bet—hedging strategies. The data also suggested that this system possesses traits that are necessary for temporal variation to promote coexistence. First, between—year seed banks, necessary to buffer populations in unfavorable years, were documented for 17 species. Second, there was a strong tendency for year—to—year variation in germination fractions to vary among species. Finally, plants germinated more in years of higher reproductive success. We discuss how a correlation between germination and reproductive success enhances the role of temporal variance in success hierarchies in promoting species coexistence.

Dynamics of buried seed population and seedling cohorts of two dominant weeds in a hill agroecosystem of the humid subtropics of India

AbstractDynamics of the buried seeds and plant population of two dominant weeds, viz.,Emilia sonchifolia (Linn.) DC. and Richardsonia pilosa HBK were studied in the crop fields of Meghalaya, north‐east India during radish and maize cropping and intervening fallow periods. The total buried seed population of R. pilosa was always larger than that of E. sonchifolia, but the germinable fraction was invariably greater in the latter. A major portion (39–41%) of the viable (germinable+dormant) seed population in both weeds was confined to the surface soil layer (0–5 cm). The viable seed population of E. sonchifolia peaked during April, while that of R. pilosa showed two peaks (during August and December). The survival pattern and half‐lives of seedling cohorts showed, some differences in the two weed species, but both being summer annuals, their populations behaved in a similar manner by showing higher seedling recruitment (K) and survivorship (p) rates in the summer crop (maize) than in the winter crop (radish). However, the density of plants that could attain adulthood was significantly higher in E. sonchifolia than R. pilosa which might have resulted in greater seed input of the former to the soil leading to its greater abundance in the crop fields.

Diversity and Coexistence of Sonoran Desert Winter Annuals

Abstract Annual plants make up ca. 50% of local floras in the Sonoran Desert. As with most plant communities, there is no shortage of potential coexistence generating mechanisms, and several mechanisms are likely contributors to coexistence at different spatial scales in the Sonoran Desert, e.g. spatial heterogeneity and the behaviors of predators and grazers. We explore one mechanism of likely importance for desert annuals: temporal environmental variation. It is widely recognized that coexistence is promoted by temporal variation if species such as desert annuals have “temporal niches” in the sense that each has years in which it out‐performs the others. It is usually suggested that some resistent life‐history stage, such as a seed bank, is also necessary to buffer each species from the negative population dynamic impact of unfavorable years. Using ten years of demographic data, we document the large year‐to‐year variation in population dynamics of desert annuals and show that ten species respond differently to temporal variation. Competition experiments document reversals in competitive superiority. Also, all species have a between‐year seed bank, such that only a proportion of the seed bank germinates in any given year. Thus this system meets our intuitive requirements for variance‐based coexistence. Dynamic models of this system demonstrate that subtle aspects of the species biology determine whether coexistence criteria are actually met. Specifically, variable germination fractions are required and coexistence is most readily favored with “predictive” germination. Germination fractions in this system do vary among years in a species specific fashion. Also, for the three years of available data, germination was predictive, in that each species had greater germination fractions in year of greater demographic success. Thus all of the population dynamic elements necessary for temporal variance mediated coexistence seem to be present in this system.

Bet-Hedging Germination of Desert Annuals: Beyond the First Year

Prolonged seed dormancy in desert annuals is thought to be an adaptation to environmental uncertainty. Germination spread over several years could reduce the year-to-year variation in the fitness of a genotype. Previous work has demonstrated that, for a number of species, not all viable seeds germinate under one set of conditions in the first year, but the subsequent fate of the dormant seeds has not been tested. In this study, germination experiments were performed on seeds of six species of winter annuals from Portal, Arizona. Seeds that do not germinate under good conditions in the first year germinate under the same conditions in subsequent years. However, germination fractions in the first 2 yr are not equal fractions of remaining viable seeds, which is the theoretical optimal behavior, and the germination distributions differ greatly among species. Germination under good conditions following a year of bad conditions for germination also differs among species. Germination behavior is age-dependent, and germination trials conducted on seeds of unknown ages from soil samples may give misleading results. These results indicate that seed dormancy acts as a form of bet hedging but that selection for an optimal distribution of germination across years may be weak.

Influence of Temperature and Moisture on Growth, Spore Production, and Conidial Germination ofMonilinia laxa

The effect of temperature on mycelial growth and spore production of Monilinia laxa, the brown rot fungus of sweet cherry, was studied in vitro as well as the effect of temperature, moisture, and relative humidity on conidial germination. Mycelial growth was observed at 2.5-30 C. A model was fitted to the data, and the optimum was calculated as 24.8 C. A comparison in a second experiment between observed and predicted growth indicated that diurnal cycles of temperature (3/12 or 101 22 C) did not alter the growth rates compared to growth at constant temperatures. M. laxa produced the highest daily number of conidia at 10 C, with 29.4×10 6 conidia per colony within 15 days. The maximum germination fraction of conidia reached 98% when suspended in deionized water and exposed to 15-25 C [...]

Norms of Reaction of Seed Traits to Maternal Environments in Plantago lanceolata

Environmental maternal effects can have important ecological and evolutionary consequences, yet little is known about genetic variation in the plastic response of offspring phenotype to maternal environment in natural plant populations. We therefore performed two experiments to investigate norms of reaction of seed weight, germination fraction, and germination rate to maternal environment in the common weed Plantago lanceolata. In the first experiment, seven wild-collected maternal genotypes from two sites, a mowed lawn and an abandoned hayfield, were clonally replicated and planted in randomized blocks in a reciprocal transplant design in the two natural source populations. In the second experiment, eight maternal genotypes from the same two populations were clonally replicated over four greenhouse environments in a factorial combination of two light and two fertilizer levels. Both experiments revealed a significant impact of parental environment on offspring traits. There were also significant differences among maternal genotypes within and across maternal environments, which suggests that natural selection can discriminate at this level. In addition, for certain traits, norms of reaction differed among maternal genotypes, which suggests that the plastic response of these traits to maternal environment can evolve. Moreover, in the greenhouse experiment the expression of maternal genotype and environmental effects on germination depended on the offspring's light environment, which suggests that the potential for natural selection to discriminate among maternal genotypes may differ among progeny environments.