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.     

How much genetic variation is stored in the seed bank? A study of Atriplex tatarica (Chenopodiaceae)

We investigated to what extent the soil seed bank differed genetically and spatially in comparison to three consecutive life history stages (seedlings, mature plants, and fruiting plants) in a natural population of Atriplex tatarica. Representatives of particular life history stages from twenty subunits within a large population were randomly collected and subjected to allozyme analysis. Comparison of population polymorphism among various life history stages showed significant differences in observed heterozygosity (H(O)) and F statistics (F(IS) and F(ST)), but nonsignificant ones in the cases of number of alleles per polymorphic locus (A) and gene diversity (H(S)). These results indicate an increasing number of heterozygotes, a decreasing level of inbreeding and an increase of the partitioning genetic diversity among populations with increasing population age. Spatial autocorrelation was used to calculate f, the average co-ancestry coefficient between individuals within distance intervals of two meters along a 39 m long transect. Significant positive fine scale genetic structure was detected in mature and fruiting plants but not in soil seeds and seedlings stages. The results of the presented study on A. tatarica indicated that significant differences exist in genetic differentiation, differentiation in allele frequencies and spatial autocorrelation among early (soil seeds and seedlings) and late (mature and fruiting plants) life history stages but not within early and late ones. This pattern suggests that, rather than storing genetic variability in the soil or germination and establishment success, self-thinning might be the major microselective force in populations of A. tatarica.     

Effects of plant density and nutrient levels on fruit polymorphism in Atriplex sagittata

Atriplex sagittata is a heterocarpic species producing three types of fruits which differ in morphology and ecological properties. This study focused on variation in biomass allocation into particular fruit types under different density and fertilization levels. The reduction in total weight under stressful conditions was accompanied by a reduction of reproductive structures in terms of both total fruit production and mean fruit weight. Allocation of biomass to particular fruit types under different density-fertilization treatments showed considerable variation. The non-dormant, bracteolate fruit type (termed C) contributed 80–90% to the total fecundity and its production was not affected by density-fertilization treatments. The production of this fruit type is higher in the upper part of the maternal plant stem. Production of the very dormant ebracteate fruit type (termed A) increased under favourable conditions and was greater on the lower part of the stem, whereas more of the so-called type B fruit (covered by bracteoles and exhibiting dispersal and dormancy characteristics intermediate between types A and C) was produced under suboptimal conditions and was concentrated in the middle part of the plant body. This pattern contradicts to some extent the theory that deeply dormant and less dispersible fruits will be produced under stressful conditions and may be considered a new model for the behaviour of plants with more than two heterocarpic fruits. The type B fruit, intermediate in both morphology and ecological behaviour, provides a continuum between extreme strategies. Received: 8 February 1998 / Accepted: 19 November 1998     

Loss of genetic variation in geographically marginal populations of Atriplex tatarica (Chenopodiaceae)

BACKGROUND AND AIMS: Genetic variability was estimated for Atriplex tatarica from 25 populations in the Czech Republic. Since its north-western range margin is in central Europe, a relationship between marginality and low within-population genetic diversity was tested in accordance with the Central-Marginal Model. METHODS: Population genetic diversity was expressed by assessing patterns of variation at 13 putatively neutral allozyme loci (comprising 30 putative alleles) within and between 25 natural populations of A. tatarica along a north-west-south-east transect in the Czech Republic. KEY RESULTS: Atriplex tatarica is a species of human-made habitats with a mixed mating system and wide geographic distribution. Overall, A. tatarica displayed moderate levels of genetic diversity in comparison with other herbaceous plants. The percentage of loci that were polymorphic was 47.1%, with average values of 1.55, 0.151 and 0.155 for the average number of alleles per polymorphic locus (A), observed heterozygosity (Ho) and expected heterozygosity (He), respectively. There was only weak evidence of inbreeding within populations (FIS=0.031) and significant population differentiation (FST=0.214). Analysis of the data provides no evidence for isolation-by-distance for the whole study area. However, Mantel tests were highly significant for the marginal Bohemian region and non-significant for the central Moravian region. While northern populations of A. tatarica showed significantly lower allelic richness (A=1.462) than populations from the southern part of the study area (A=1.615), they did not differ in observed heterozygosity (Ho), gene diversity (HS), inbreeding within populations (FIS) or population differentiation (FST), despite generally lower values of particular genetic measurements in the marginal region. CONCLUSIONS: Genetic diversity, with the exception of allelic richness, was not significantly lower at the margins of the species' range. This, therefore, provides only weak support for the predictions of the Central-Marginal Model.     

The effect of fruit age on seed germinability of a heterocarpic species, Atriplex sagittata

Atriplex sagittata is an annual heterocarpic plant that produces three different fruit types (termed A, B, and C). To examine the consequence of heterocarpy on germinability patterns over long time periods, we compared seed germinability of different fruit types that had been stored for up to 8 years. While germinability of non-dormant type C fruits in distilled water was high (up to 100 %) in the first 2 years, it rapidly decreased over time. Dormant fruit types A and B showed increased germinability up to 7 years, though loss of germinability was lower for type B than for type A fruits. Eight-year-old fruits of all types had significantly lower germinability than younger fruits, probably due to loss of viability. Heterocarpy, therefore, ensures that emergence rates for seedlings of A. sagittata will be maintained over relatively long periods, even in years of strong disturbance when all adult plants may be destroyed. The experiment further showed that germinability of all fruit types in high concentrations of salt, as compared with water treatment, changed over the course of 8 years. Whilst dormant types (A and B) of A. sagittata show increased germinability with age of the seed in water treatment, they significantly lose germinability over time with salinity treatment. Type C fruit was not influenced by salt in the first year, but germinability rapidly decreased with time. It follows that the species is able to germinate under high salt concentration in the first year, but this advantage gradually disappears.     

Drought tolerance and germination response to light and temperature for seeds of saline and non-saline habitats of the habitat-indifferent desert halophyte <Emphasis Type="Italic">Suaeda vermiculata</Emphasis>

Few plants are habitat-indifferent halophytes (i.e., grow well in both saline and non-saline soils). These plants offer a good opportunity to study drought and salinity tolerances during germination for seeds developed and matured in soils differ in salinity. Here, we assessed drought tolerance during germination, as simulated with PEG, and response of germination to light and temperature for Suaeda vermiculata, a habitat-indifferent shrub. Seeds matured in saline and non-saline soils were germinated in six PEG concentrations (0 to ? 1.0 MPa) and put in three incubators adjusted at different temperatures in both light and dark regimes. Drought tolerance was greater for seeds of the non-saline than those of saline soils, especially at higher temperatures. Seeds of the saline soils germinated in the lowest osmotic potentials (? 0.8 and ??1.0 MPa) only at lower temperatures, but seeds of the non-saline soils germinated to higher levels at all temperatures. Tolerances to drought and high temperatures were greater in light for seeds of saline soils, but in darkness for seeds of non-saline soils. Germination rate index did not differ significantly between seeds of the two soil types in higher osmotic potentials, but was significantly greater in seeds of non-saline at lower osmotic potentials. Most seeds that failed to germinate in the PEG concentrations recovered their germination when transferred to distilled water. Germination recovery levels and speeds increased with the decrease in osmotic potentials. Seeds of the saline soil postpone their germination until arrival of suitable temperatures and effective rainfalls that ensure seedling survival in salty habitats of the arid unpredictable deserts.     

紫翅猪毛菜的种子多型性及其结实格局

紫翅猪毛菜(Salsola affinis)主要分布于新疆准噶尔荒漠, 具有很强的抗干旱和耐盐碱能力, 其种子具有多型现象。我们对紫翅猪毛菜的种子形态、萌发特性及结实格局进行了观测研究。结果表明: (1) 紫翅猪毛菜3种类型的种子在形状、大小、颜色、着生方式及包被其花被片背部是否具翅方面均有明显不同; (2) 3种类型的种子具有不同的散布特性和萌发行为: A型种子容易从母体脱落, 发达的果翅能够借助风进行远距离传播; B、C型种子成熟时紧连母体, 不易脱落。在变温下A型种子和B型种子能够快速萌发, 且B型种子比A型种子的萌发速度快; 而C型种子萌发缓慢, 最终萌发率小于10%, 表明其处于休眠状态; (3)随植株个体的增大, A型种子所占比例逐渐增多, 由0.43%增加到51.07%; B型种子所占比例逐渐减少, 由65.80%减少到18.06%; C型种子所占比例没有显著变化, 约为30–35%。紫翅猪毛菜的种子多型性以及种子输出的灵活性有利于其在荒漠异质环境中成功定居。对种子多型性及其结实格局的研究将为探讨其生态适应机制及生活史进化研究提供科学依据。     

GerN, an antiporter homologue important in germination of Bacillus cereus endospores

A homologue of the grmA spore germination gene of Bacillus megaterium and of a NaH-antiporter gene (napA) of Enterococcus hirae has been identified in Bacillus cereus 569 (ATCC 10876). The putative protein product has 58 and 43% amino acid identity with GrmA and NapA, respectively. Insertional inactivation of this B. cereus gene, named gerN, did not affect vegetative growth or sporulation. The null mutant spores were 30-fold slower to germinate in inosine (5 mM) but germinated almost normally in response to L-alanine (10 mM). The null mutant spores germinated after several hours with inosine as the sole germinant, but germination was asynchronous and the normal order of germination events was perturbed. At a suboptimal germinant concentration (50 microM), inosine germination was completely blocked in the mutant, while the rate of germination in 50 microM L-alanine was reduced to one-third of that of the wild type. The requirement for GerN function in the response to a particular germinant suggests that a germination receptor may have a specifically associated antiporter, which is required at the initiation of germination and which, in the case of the inosine receptor, is GerN. Since germination in suboptimal concentrations of L-alanine shows a delay, additional germination transporters may be required for optimal response at low germinant concentrations.     

Effect of Salinity on Seed Germination,Growth, and Ion Accumulation of Atriplex patula (Chenopodiaceae)

Seeds and seedlings of the halophyte Atriplex patula were exposed to 0–2% NaCl to determine the effect of salt stress on germination and growth. Seeds germinated and plants survived and grew in solutions of up to 2.0% NaCl. Both seed germination and dry mass production were negatively affected by increased salinity. Dry mass production declined to 1% of controls and seed germination to 17% of controls in the 2% NaCl treatments, indicating that seeds were less inhibited than growing plants. Also, recovery treatments indicated that high salinity did not permanently injure seeds. Percent ash, and Na⁺ and Cl⁻ ions increased in shoots with each salt increment, while the K⁺ ion content decreased sharply. Atriplex patula is a facultative halophyte, and is limited to low and moderately saline sites because both seed germination and growth are severely reduced at salinities > 1% NaCl.     

The Germination of Vavraia culicis Spores1

Buffered solutions of KCl and NaCl were tested for their stimulatory effect on the germination of variously-aged spores of Vavraia culicis. Germination was optimal in 0.2 M KCl, pH 6.5 for one isolate, and, for another isolate, peaks of germination occurred at pH 7.0 and 9.5. Spores incubated for several hours in suboptimal solutions became unable to germinate under optimal conditions. After being returned to water, they regained their ability to germinate. Calcium chloride, magnesium chloride, and ammonium chloride inhibited germination. After ingestion by mosquito larvae, spores germinated near the posterior end of the midgut.     

Ethylene as a possible cue for seed germination of Schoenoplectus hallii (Cyperaceae), a rare summer annual of occasionally flooded sites

The purpose of our research was to determine why seeds of Schoenoplectus hallii germinate only in some wet years. Seeds mature in autumn, at which time they are dormant. Seeds come out of dormancy during winter, if buried in nonflooded, moist soil, but they remain dormant if buried in flooded soil. Nondormant seeds require flooding, light, and exposure to ethylene to germinate. One piece of apple in water (1/12 of an apple in 125 mL of water in a glass jar for a depth of 5 cm) or a 1-μmol/L solution of ethephon elicited very similar (high) germination percentages and vigor of seedlings. Apple, which was shown to produce ethylene in the air space of the jar, was used in a series of experiments to better understand germination. Seeds germinated to 72% if apple was removed from the water after 1 d of incubation, and they germinated to 97% if seeds were washed and placed in fresh water after 3 d of exposure to apple. No seeds germinated in control with no apple. Seeds incubated in apple leachate for 5 d and then transferred to filter paper moistened with distilled water germinated to 90%. Minimum depth of flooding in apple leachate (no soil in jars) for optimum germination was ≥3 cm. Buried seeds of S. hallii exhibited an annual conditional dormancy/nondormancy cycle. Regardless of the month in which seeds were exhumed, they germinated to 59-100% in light in water with apple at daily alternating temperature regimes of 25°/15°, 30°/15°, and 35°/20°C, but germination at 20°/10°C (and to some extent at 15°/6°C) tended to peak in autumn to spring. Thus, seeds can germinate throughout the summer if flooded (ethylene production) and exposed to light. An ethylene cue for germination serves as a "flood-detecting" mechanism and may serve as an indirect signal that water is available for completion of the life cycle and competing species are absent.     

Autogamous seed set in a critically endangered orchid in Japan: pollination studies for the conservation of Nervilia nipponica

The pollination biology of the critically endangered Nervilia nipponica was investigated at two sites on Shikoku Island, Japan, to shed light on the species' ecological strategy and to help prioritise management decisions for its conservation. Though the solitary flowers failed to attract pollinators, high rates of fruit set were observed in control plants at both sites. Comparable rates were obtained in bagged plants and following induced autogamous and xenogamous pollinations, indicating that the species is capable of outbreeding, but self-compatible and not pollinator-limited for fruit set under natural conditions; rates were significantly lower following emasculation. The number of seeds set per fruit and their fertility did not differ significantly between open-, self- and cross-pollinated treatments. Epifluorescence microscopy revealed that, in open-pollinated plants, pollen grains germinate within the anther on the first day of anthesis, and that fertilisation occurs within the following 96 hours. The position of the stigma, which overlaps the clinandrium, friable pollinia, and the absence of a rostellum ensure efficient, non-mechanical autopollination. Though this confers reproductive assurance, inbreeding depression may be limiting fitness at subsequent stages of the recruitment cycle and low genetic diversity is inferred.     

CHANGE IN DORMANCY STATUS OF FRASERA CAROLINIENSIS SEEDS DURING OVERWINTERING ON PARENT PLANT

Seeds of the monocarpic perennial Frasera caroliniensis ripen in late summer, and most of them are dispersed in late autumn and winter. However, some viable seeds may remain undispersed for more than a year. Embryos are underdeveloped (ca. 1.1–1.3 mm long) at seed maturity and do not grow while seeds remain on plants in the field. Dormancy in freshlymatured seeds was broken by 12 to 14 weeks of cold stratification at 5 C, during which the embryos elongated. On the other hand, seeds collected in January and March required a period of warm stratification followed by a period of cold stratification to germinate. Seeds collected in September and sown in a nonheated greenhouse germinated to 83% the first spring after maturation, whereas those collected and sown in January and March did not germinate until the second spring. Thus, seeds that remained on plants in the field until winter entered a deepened state of dormancy, and a warm (summer) followed by a cold (winter) stratification period was required to overcome it.     

Germination and Flowering in Arabidopsis thaliana in Sterile Culture

The optimal conditions for the germination, growth, and flowering of an Indian strain of Arabidopsis thaliana were investigated in sterile culture. Seeds require a cold treatment to germinate, and the most effective temperature is 8?C for 48 hours. Germination after vernalization is promoted by red light and inhibited by far-red. Unvernalized seeds germinated after 31 days and flower buds appeared in 61 days. On verbalization and subsequent transfer to a temperature of 25?C and a light intensity of 4300 lux of fluorescent light, plants flowered in 25 days. Under 7000 lux of light rich in both blue and red region of the spectum, plants flowered in only 12 days. A minimum of five long-day photocyeles appeared to be necessary for flowering. Kinetin (10^?7M) and gibberellic acid (10^?7M, 10^?6M) accelerated flower formation. Kinetin and 2,4-D also catised specific types of callussing from different regions of the plant.     

Consistent variation in seed germination across an environmental gradient in a Neotropical savanna

Environmental conditions experienced by mother plants determine germination patterns. Here, we investigated the germination ecology of Miconia albicans (Melastomataceae), a widespread apomictic shrub, along a soil fertility gradient in the Brazilian Cerrado (a Neotropical savanna). The mosaic of vegetations in the Cerrado ranges from grasslands to woodlands, which present different conditions for seedling establishment. Cerrado grasslands are a more unpredictable environment because they are more prone to disturbances such as fires and prolonged droughts than closed woodlands. We expect lower reproductive investment but increasing germinability, germination speed and synchrony in areas with more dense vegetation, due to prolonged droughts in grasslands and higher soil properties in woodlands. Seeds of M. albicans were collected in grasslands, shrublands and woodlands for three years in a Cerrado area in southeastern Brazil. Seeds collected in all vegetations were set to germinate under identical experimental conditions. According to our prediction, we found a consistent pattern of increasing germinability, mean germination time and synchrony in areas with more closed vegetation. Both slower and asynchronous germination in seeds from grassland plants are likely to increase seedling survival under a more unpredictable environment. On the other hand, seeds from plants in the woodlands germinated at higher rates and germination was more synchronous, thus increases the chances of survival under a more competitive light-limited environment. This pattern was found in all three years of study. Our study suggests that stressful environmental conditions experienced by mother plants in the Cerrado may act as cues signalizing unfavourable conditions for seed germination and help explain functional divergence in germination traits within species. In conclusion, we showed for the first time a temporally consistent pattern of intraspecific variation in the germination traits of M. albicans, with different germination strategies at the extremes of the ecological gradient.     

Germination ecology of the polycarpic grassland perennials Primula veris and Trollius europaeus

The seed germination behaviour of Primula veris and Trollius europaeus , both perennial, polycarpic grassland plants was compared The species have similar-sized seeds that are dormant at dispersal Seeds buried in soil and exhumed at regular intervals showed that for both species, primary seed dormancy was overcome by cold-stratification Hence, their germination in the field should occur in spring, following dispersal, or later Seeds of P veris became dormant again in the late spring/early summer, and dormancy was broken again in the second winter Seeds of T europaeus did not exhibit such changes in dormancy
Seeds of P veris did not germinate in darkness This suggests that P veris can accumulate a persistent seed bank because buried seeds are prevented from germinating Trollius europaeus , on the other hand, germinated equally well in darkness and in light which suggests that seeds might germinate even when they are too deep in the soil for seedlings to emerge Two lines of evidence confirm this difference in seed bank behaviour (1) Primula veris was detected in the persistent seed bank of a grassland site, whereas T europaeus was not (n) After 16 months burial, 85% of the P veris seeds but only 8% of the T europaeus seeds remained viable     

Parental overwintering history affects the responses of Thlaspi arvense to warming winters in the North

The overwintering conditions of northern plants are expected to change substantially due to global warming. For perennial plants, winter warming may increase the risk of frost damage if the plants start dehardening prematurely. On the other hand, evergreen plants may remain photosynthetically active and thereby benefit from milder winters. The positive and negative effects of mild winters on annual plants remain, however, largely unknown. We postulated that summer annuals may be susceptible to frost damage if the seeds germinate during a mild spell in winter. Winter annuals may utilize a warm period for photosynthesis. These questions were addressed in two consecutive experiments in which pot-grown individuals of Thlaspi arvense that overwintered in the field were exposed to an elevated temperature for 8 days in growth chambers in mid-winter. No premature germination was observed in summer annuals. However, in accordance with our hypothesis, winter annuals started photosynthesising very rapidly upon exposure to elevated temperature. The winter warming treatment affected neither the total number of seeds produced nor the mean seed weight. These seeds, possessing divergent parental overwintering histories, were used as starting material for the second experiment. Seeds originating from both summer and winter annual plants germinated both in the autumn and in the following spring. We observed a major parental effect associated with the winter warming treatment. The warm spell experienced by the mother plant (either as a winter annual rosette or as a summer annual seed) reduced the proportion of autumn germination in the next generation. Only 43% of the seeds of summer annuals possessing a parental warming history germinated before the winter, whereas the germination percentage of seeds with no previous winter warming history was 71%. In the case of seeds collected from winter annual plants, 4% of the seeds germinated in autumn if the mother plants experienced the warming treatment during the previous winter, whereas the corresponding value was 37% if the mother plants did not experience warming. Our results show that summer and winter annual individuals show diverse responses to warm spells in winter. Since the responses are not limited only to the generation that actually experiences the warm spell, but also appear in their offspring, long-term studies consisting of several generations are called for.     

Pollen source affects female reproductive success and early offspring traits in the rare endemic plant Polemonium vanbruntiae (Polemoniaceae)

Understanding the relative magnitudes of inbreeding and outbreeding depression in rare plant populations is increasingly important for effective management strategies. There may be positive and negative effects of crossing individuals in fragmented populations. Conservation strategies may include introducing new genetic material into rare plant populations, which may be beneficial or detrimental based on whether hybrid offspring are of increased or decreased quality. Thus, it is important to determine the effects of pollen source on offspring fitness in rare plants. We established pollen crosses (i.e. geitonogamous‐self, autonomous‐self, intrasite‐outcross, intersite‐outcross and open‐pollinated controls) to determine the effects of pollen source on fitness (seeds/fruit and seed mass) and early offspring traits (probability of germination, number of leaves, leaf area and seedling height) in the rare plant Polemonium vanbruntiae. Open‐pollinated, intrasite‐outcross and geitonogamous‐self treatments did not differ in fitness. However, plants receiving autonomous‐self pollen had the lowest fitness and the lowest probability of seed germination. Intersite‐outcross plants contained fewer seeds/fruit, but seeds germinated at higher frequencies and seedlings were more vigorous. We also detected heterosis at the seed germination stage. These data may imply that natural populations of P. vanbruntiae exhibit low genetic variation and little gene flow. Evidence suggests that deleterious alleles were not responsible for reduced germination; rather environmental factors, dichogamy, herkogamy and/or lack of competition among pollen grains may have caused low germinability in selfed offspring. Although self‐pollination may provide some reproductive assurance in P. vanbruntiae, the result is a reduction in germination and size‐related early traits for selfed offspring.     

The effects of salinity and osmotic stress on barley germination rate: sodium as an osmotic regulator

Background and Aims

Seed germination is negatively affected by salinity, which is thought to be due to both osmotic and ion-toxicity effects. We hypothesize that salt is absorbed by seeds, allowing them to generate additional osmotic potential, and to germinate in conditions under which they would otherwise not be able to germinate.

Methods

Seeds of barley, Hordeum vulgare, were germinated in the presence of either pure water or one of five iso-osmotic solutions of polyethylene-glycol (PEG) or NaCl at 5, 12, 20 or 27 °C. Germination time courses were recorded and germination indices were calculated. Dry mass, water content and sodium concentration of germinating and non-germinating seeds in the NaCl treatments at 12 °C were measured. Fifty supplemental seeds were used to evaluate the changes in seed properties with time.

Key Results

Seeds incubated in saline conditions were able to germinate at lower osmotic potentials than those incubated in iso-osmotic PEG solutions and generally germinated faster. A positive correlation existed between external salinity and seed salt content in the saline-incubated seeds. Water content and sodium concentration increased with time for seeds incubated in NaCl. At higher temperatures, germination percentage and dry mass decreased whereas germination index and sodium concentration increased.

Conclusions

The results suggest that barley seeds can take up sodium, allowing them to generate additional osmotic potential, absorb more water and germinate more rapidly in environments of lower water potential. This may have ecological implications, allowing halophytic species and varieties to out-compete glycophytes in saline soils.     

Breaking Setaria parviflora seed dormancy by nitrates and light is part of a mechanism that detects a drawdown period after flooding

We explored the hypothesis that, in flood-prone habitats, nitrates can signal to seeds that a drawdown period has begun. To investigate this issue, Setaria parviflora (Poir.) Kerguélen seeds were buried in a never-flooded upland and a nearby, flood-prone lowland grassland. Seeds were exhumed during the flooding period. Additionally, grassland mesocosms with buried S. parviflora seeds were flooded during 20 d (controls drained). After both field and mesocosm pretreatments germination was assayed in laboratory at 25 °C in a medium with or without nitrates, under red light pulses or in darkness. Seeds exhumed from the never-flooded upland showed no specific requirements to germinate. In contrast, seeds exhumed from the flooded lowland germinated ca. 65% when nitrates were combined with red light pulses, significantly higher than in the rest of the treatments. Seeds exhumed from drained mesocosms germinated equally in all treatments. However, in the seeds exhumed from the flooded mesocosms, nitrates increased germination by more than 20% compared with seeds imbibed in water. Seeds germinated ca. 85% when nitrates were combined with red light pulses, significantly higher than in the other treatments. We can conclude that after flooding, S. parviflora seeds require nitrate and light to germinate. Therefore, a large fraction of seeds do not germinate unless nitrates are combined with light, indicating a drawdown period after floods and vegetation gaps.