Do seeds sense each other? Testing for density‐dependent germination in desert perennial plants

The timing of seedling emergence may strongly affect fitness in competitive environments. Therefore, selection should favour mechanisms that allow sensing neighbours prior to emergence. We tested whether or not germination is affected by density and identity of neighbouring seeds or seedlings of desert perennial plants. Based on theory, we predicted that germination fractions are independent of neighbouring seeds, that germination is accelerated in dense interspecific neighbourhoods, and neighbour effects are caused by seedlings, not by seeds. We examined germination fraction and timing of four naturally coexisting sandy desert perennial species in low versus high seed densities in both intra- and interspecific neighbourhoods, and with and without removal of newly emerged seedlings. Neighbours accelerated germination independent of density and this pattern was apparently caused by the presence of early emerging seedlings. Germination fractions were lower in high seed densities even when neighbours did not germinate, indicating that seeds were able to sense each other prior to emergence. Early germination may be adaptive because fast emerging seedlings may gain a competitive edge over slow emerging ones. However, since seeds that did not germinate died, delayed germination may only be advantageous for mother plants when sib competition is intense. Another key finding was a competitive hierarchy with late successional species germinating faster and inhibiting germination of pioneer species. This indicates that successional processes may be directed as early as during germination.     

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.     

The fitness benefits of germinating later than neighbors

Premise of the Study

Phenology, the seasonal timing of development, can alter biotic interactions. Emergence from dormant or quiescent stages often occurs earlier when neighbors are present, which may reduce the neighbors' competitive effects. Delayed emergence in response to neighbors also has been observed, but the potential benefits of such delays are unclear. Further, emergence time may respond to neighbors experienced by parents, which may predict future competition in offspring.

Methods

In the annual plant Arabidopsis thaliana (Brassicaceae), we quantified seed germination responses to neighbors in parental and offspring (seed) environments. To examine how observed changes in germination affect interactions with neighbors, we performed an outdoor experiment using neighbors of different sizes to represent different germination times.

Key Results

Seeds were more likely to germinate if their parent had neighbors, but they were less likely to germinate if they themselves experienced a neighbor cue (canopy). As seeds lost dormancy over time, they gained the ability to germinate under a canopy, which suggests that they germinate later in the presence of neighbors. Neighbors of both sizes reduced growth, survival to reproduction, fecundity, and total fitness, but large neighbors increased seedling survival. Smaller neighbors provided no such benefit and had stronger negative effects.

Conclusions

Delayed germination in response to neighbors can reduce negative interactions and promote positive ones if it occurs late enough to expose seedlings to larger neighbors. By altering relative phenologies and, in turn, the outcomes of biotic interactions, phenological responses to environmental change may influence species interactions and community dynamics.     

Effects of germination time on seed morph ratio in a seed-dimorphic species and possible ecological significance

Background and Aims Diaspores of heteromorphic species may germinate at different times due to distinct dormancy-breaking and germination requirements, and this difference can influence life history traits. The primary aim of this study was to determine the effect of germination time of the two seed morphs of Suaeda corniculata subsp. mongolica on life history traits of the offspring.Methods Germinated brown and black seeds were sown on the 20th of each month from April to September in a simulated but near-natural habitat of the species. Phenological and vegetative traits of the maternal plants, and number, size and germination percentage of the offspring were determined.Key Results Germinated seeds sown late in the year produced smaller plants that had a higher proportion of non-dormant brown than dormant black seeds, and these brown seeds were larger than those produced by germinated seeds sown early in the year. The length of the seedling stage for brown seeds was shorter than that for black seeds, and the root/shoot ratio and reproductive allocation of plants from brown seeds were more variable than they were for plants from black seeds. Late-germinating brown seeds produced larger plants than late-germinating black seeds.Conclusions Altering the proportion of the two seed types in response to germination timing can help alleviate the adverse effects of delayed germination. The flexible strategy of a species, such as S. corniculata, that produces different proportions of dimorphic seeds in response to variation in germination timing may favour the maintenance and regeneration of the population in its unpredictable environment.     

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.     

A new role for phytochromes in temperature-dependent germination

Germination timing is a fundamental life-history trait, as seedling establishment predicates realized fitness in the wild. Light and temperature are two important cues by which seeds sense the proper season of germination. Using Arabidopsis thaliana, we provide evidence that phytochrome-mediated germination pathways simultaneously respond to light and temperature cues in ways that affect germination. Phytochrome mutant seeds were sown on agar plates and allowed to germinate in lit, growth chambers across a range of temperatures (7 degrees C to 28 degrees C). phyA had an important role in promoting germination at warmer temperatures, phyE was important to germination at colder temperatures and phyB was important to germination across a range of temperatures. Different phytochromes were required for germination at different temperatures, indicating a restriction or even a potential specialization of individual phytochrome activity as a function of temperature. This temperature-dependent activity of particular phytochromes reveals a potentially novel role for phytochrome pathways in regulating the seasonal timing of germination.     

VARIATION IN SEED WEIGHT AND ITS EFFECTS ON GERMINATION IN PASTINACA SATIVA L. (UMBELLIFERAE)

Pastinaca sativa (wild parsnip) produces seeds on the primary, secondary, and tertiary umbels of the flowering stalk. Within plants, variation in seed weight is about twofold. Secondary and tertiary seed weight is 73% and 50% of primary seed weight, respectively. Maximum variation in seed weight between plants is sixfold when tertiary seeds from a small plant are compared to primary seeds from a large plant. Within an umbel order, variation in seed weight between plants is correlated with plant size. Under autumn germinating conditions in the laboratory, final germination of seeds from different umbel orders does not differ but smaller seeds germinate more rapidly than larger seeds. Under spring germination conditions in the laboratory, significantly more primary and secondary seeds germinate than tertiary seeds and the rate of germination is independent of seed weight. Field germination of seeds from different umbel orders produces similar results except that in the spring both secondary and tertiary seed germination is lower than that of primary seeds. These results suggest that with respect to seed germination characteristics small seeds may have a competitive advantage over large seeds in the autumn because they germinate more quickly, but in the spring small seeds are at a disadvantage because they have lower overall germination. Because most germination in the field occurs in the spring, population recruitment from small seeds is likely to be substanially less than that from large seeds.     

Germination biology of selected central Australian plants

Abstract Risk spreading of germination may be particularly common in environments with unpredictable climates. Germinability, propensity to germinate at different temperatures and germination speed were classified for seeds of 105 species from the central Australian arid zone, and related to plant growth form, perenniality, seed size and seed dispersal mode. Almost all species had at least some seeds which were dormant, consistent with the idea that risk spreading is important in arid zones. Dispersal mode and plant perenniality were not found to be associated with germinability. Seeds of most species germinated rapidly relative to what has been recorded from higher-rainfall environments, as might be expected in an environment where wet soils are usually temporary. Faster germination tended to be associated with low germinability, suggesting a spectrum of strategies from species that risk a small number of their seeds in many rainfall events, to those that germinate only in large rainfall events but then risk large numbers of seeds.     

Soil water availability effects on seed germination account for species segregation in semiarid roadslopes

Previous studies report that the low colonisation success on eroded roadslopes of semiarid environments is controlled by microsite limitations. We predicted that soil water availability, through its effect on seed germination, is a determinant factor in the colonisation process of roadslopes in semiarid environments. Moreover, we predicted that the success of species establishment on the harshest roadslope conditions (i.e., south-facing roadcuts) is either due to the ability of seeds to germinate fast at low water potentials (colonising species) or to the ability of plants to sprout (resistant species). Specifically we present evidence for: (1) soil drying occurs faster on roadcuts than on roadfills after a rainfall event; (2) germination is a filtering process that influences the success of species establishment on roadslopes; (3) species able to colonise successfully south-facing roadcuts have higher germination rates and a shorter time to germination under water-stress conditions than species able to colonise successfully but exclusively the most favourable roadslopes (i.e., roadfills); (4) species that live on south-facing roadcuts and have the ability to sprout do not necessarily germinate with germinating rates and speeds as high as species that colonise successfully these slopes but are unable to sprout. To test these hypotheses we compared water dynamics in the soil among roadslope types and aspects as well as the seed ability to germinate at low water potentials among species showing different regeneration strategies and establishment success on roadslopes. Soil water availability after rainfalls occurring during the germination period played a major role in the germination of seeds. The patterns of seed germination under water-stress conditions were consistent with the success of colonising species on roadslopes and with the distribution of adult plants in the roadslopes 8 years after these latter were built. We discuss the usefulness of these results for the improvement of revegetation projects in semiarid areas by means of an appropriate selection of species adapted to the local environmental conditions. We suggest that the ability of species to germinate under water stress could be an indication of a species’ potential for success under semiarid conditions. Responsible Editor: John McPherson Cheeseman.     

Rainfall seasonality predicts the germination behavior of a tropical dry‐forest vine

Seed dormancy is considered to be an adaptive strategy in seasonal and/or unpredictable environments because it prevents germination during climatically favorable periods that are too short for seedling establishment. Tropical dry forests are seasonal environments where seed dormancy may play an important role in plant resilience and resistance to changing precipitation patterns. We studied the germination behavior of seeds from six populations of the Neotropical vine Dalechampia scandens (Euphorbiaceae) originating from environments of contrasting rainfall seasonality. Seeds produced by second greenhouse‐generation plants were measured and exposed to a favorable wet environment at different time intervals after capsule dehiscence and seed dispersal. We recorded the success and the timing of germination. All populations produced at least some dormant seeds, but seeds of populations originating from more seasonal environments required longer periods of after‐ripening before germinating. Within populations, larger seeds tended to require longer after‐ripening periods than did smaller seeds. These results indicate among‐population genetic differences in germination behavior and suggest that these populations are adapted to local environmental conditions. They also suggest that seed size may influence germination timing within populations. Ongoing changes in seasonality patterns in tropical dry forests may impose strong selection on these traits.     

Comparative germination ecology of two altitudinal vicariant Saxifraga species endemic to the north of Spain

Seeds of high‐mountain species are thought to germinate rapidly, synchronously and at high percentages after a cold period, with limited dependence on the external environment; yet, empirical evidence only partially supports this behaviour. We performed a comparative study of the germination response of two closely related taxa along an altitude gradient in northern Spain. Seeds from several maternal families of six populations of Saxifraga trifurcata (lowland species) and S. canaliculata (highland species) were subjected to temperature and stratification treatments. Germination percentages and germination rates were analysed using generalised linear mixed modelling and accelerated failure‐time modelling. We found that germination percentages and germination rates were high and dependent on incubation temperature in both species. Within species, seeds from higher altitudes had higher germination percentages under all conditions. Cold–wet stratification negatively affected germination success, particularly in the lowland species. Overall, the highland species was less responsive to the experimental treatments and showed more synchronous germination patterns. We conclude that seeds from these two Saxifraga species germinate as efficiently as species from other habitats, but have a narrower germination response, probably due to the stronger selective pressures in their harsh environments. Finally, a cold, wet stratification period is not a prerequisite for the germination of high‐mountain S. canaliculata, and its strong negative effect on the germination of its lowland relative S. trifurcata may contribute to the altitudinal segregation of these two species.     

Morphology and intermittent germination characteristics of synaptospermy of Tribulus terrester

 蒺藜(Tribulus terrester)果实由5个集合繁殖体构成, 是一种常见的具集合繁殖体的一年生植物。该文对不同月份成熟的蒺藜果实及果实集合繁殖体的形态、活力、吸水率、萌发率及萌发速率进行了比较观察, 初步探讨其生态学意义。主要结果如下: 1)果实各集合繁殖体间在质量、附属物质量百分比、种子数、活力、吸水率、萌发率及萌发速率方面随其发育顺序依次减小, 存在显著差异; 2)果实平均活力及萌发率随成熟时间依次升高, 而吸水率依次降低, 平均活力及萌发率与果实成熟时间正相关, 而与吸水率负相关, 成熟时间越早, 活力及萌发率越低, 吸水率越高; 3)除9月最晚成熟的集合繁殖体在最适条件下均能萌发外, 其余月份的果实各集合繁殖体只能部分萌发, 且只有位于集合繁殖体长刺端的种子能萌发, 这种在当季只萌发部分种子, 剩下的种子仍然保持在一个扩散单元中并在数周、数月或数年间不规则地间隔萌发属于间歇性萌发。上述结果表明: 不同月份成熟果实及果实不同发育顺序的集合繁殖体在形态、活力及吸水率方面存在梯度变化, 这种变化可能是造成蒺藜间歇性萌发的主要原因, 这种萌发对策可以降低同胞后代间的竞争, 使其对可变环境中的生存风险进行有效的分摊, 为物种的延续提供保障。     

Does seed dormancy benefit the mother plant by reducing sib competition?

Summary Seed dormancy has been considered, almost without exception, as a bet-hedging strategy in a temporally varying environment. However, in this paper we show that seed dormancy can improve the reproductive success of the mother plant when competition between sibling seedlings and adult plants is intense even if the environment is temporally invariable. We allow a cohort of sibling seeds to germinate simultaneously in the same patch and assume a density dependent survival and fecundity of seedlings. In the model, the mother plant is assumed to control the germination behaviour of the seeds, e.g. by enclosing the seeds in coats of different hardiness. When sib competition is intense, a postponed germination of her seeds can increase the reproductive success of the mother plant up to four times, measured in terms of the number of grandchildren. Consequently, our results suggest that postponed germination may function as a mechanism that alters local interactions in viscous plant populations with limited dispersal.     

盐生植物种子萌发对环境的适应对策

盐生环境是一种严峻的胁迫环境,对植物的生长、发育、繁殖等生活史的各阶段都产生着重要的影响。盐生植物是生长在盐渍土壤上的一类天然植物区系,它们在长期的进化过程中形成了一系列适应盐生生境的特殊生存策略。一般情况下,盐生植物种子对环境的适应能力,是植物对盐生环境适应性的重要体现;而植物发育早期对盐度的适应能力又是决定物种分布和群落组成的关键因素。在对国内外相关文献进行分析归纳的基础上,从盐分对种子萌发的影响机理及植物种子萌发对盐生环境的适应对策两个方面综述了植物种子休眠萌发与盐生环境的关系。     

密度、物种比例对盐沼植物种子萌发阶段种内、种间相互作用的影响

种间相互作用是影响湿地植物群落构建的关键因子,其形式、强度和机制可能随着生长发育的过程而发生改变。种子萌发是植物生命周期的关键环节,一定程度上决定了植物种群分布和群落结构。野外种子萌发过程受到邻近同种或异种种子间相互作用的影响,但对于种子萌发过程中种内、种间相互作用强度及其影响因素的了解仍十分有限。该研究通过培养皿萌发实验探讨了种子密度(每皿80粒、每皿160粒)及比例(单种、3:1混种、2:2混种、1:3混种)对互花米草(Spartina alterniflora)、海三棱藨草(Scirpus×mariqueter)、芦苇(Phragmites australis)种子萌发率、平均萌发时间的影响,并采用相对邻株效应(RNE)评估了这3个物种的种内、种间相互作用强度。结果表明,密度对互花米草、海三棱藨草、芦苇种子的萌发率均有一定的促进作用。与互花米草混种时,芦苇种子在占比最低(25%)处理下的平均萌发时间显著长于单种及其他混种比例处理。3种植物种子的竞争能力没有明确的等级关系,存在非传递性竞争,萌发时海三棱藨草相对互花米草具有一定的竞争优势,互花米草相对芦苇具有一定优势,而芦苇的竞争能...     

Germination responses of the Rubia fruticosa Ait. seed dispersal system in different experimental seasons

In this paper, we present results on germination patterns of the seed dispersal system of an endemic Macaronesian plant (Rubia fruticosa). Seeds from this plant are mainly dispersed by endemic lizards and native warblers; therefore, we included three different treatments: control seeds, seeds extracted from lizards and seeds found in warbler droppings. Seeds from the same pool of every treatment were germinated in two different seasons, one in autumn, coinciding with the arrival of the first rains, and another in spring, coinciding with the arrival of the dry season. A clear differential pattern of germination success was observed between the two seasons. Seeds planted in autumn achieved a higher percentage of germination than those sown in spring in all treatments. The great robustness of these results seems to indicate that germination timing is strongly selected in R. fruticosa and this evolutionary trend probably extends to other vascular plants growing in xeric coastal environments of the Macaronesian islands.     

Seed ecology of dust seeds in situ: a new study technique and its application in terrestrial orchids

A method is described by which seeds of terrestrial orchids are sown and retrieved in the field under almost natural conditions. For the first time it is possible to conduct a quantitative study of orchid germination in situ and observe seasonal growth and mortality of seedlings. The technique has also enabled us to investigate the relation between the site where the seeds are sown, the availability of an appropriate fungus to infect the seeds, and seedling establishment in the soil. Five local species were studied. Corallorhiza odontorhiza, Goodyera pubescens, and Galearis spectabilis all began to germinate in May–June, after 23-30 weeks in the soil. These species differed in their dependency on infection at germination time, but none of the seedlings developed beyond the point of rupturing the testa except when infected. Seeds of Liparis lilifolia and Tipularia discolor did not germinate within the first 12 months of the experiment. The implications and potential uses of this field sowing technique for further studies and for other kinds of minute seeds are discussed.     

The evolutionary ecology of seed germination of Arabidopsis thaliana: variable natural selection on germination timing

Germination timing of Arabidopsis thaliana displays strong plasticity to geographic location and seasonal conditions experienced by seeds. We identified which plastic responses were adaptive using recombinant inbred lines in a field manipulation of geographic location (Kentucky, KY; Rhode Island, RI), maternal photoperiod (14-h and 10-h days), and season of dispersal (June and November). Transgressive segregation created novel genotypes that had either higher fitness or lower fitness in certain environments than either parent. Natural selection on germination timing and its variation explained 72% of the variance in fitness among genotypes in KY, 30% in June-dispersed seeds in RI, but only 4% in November-dispersed seeds in RI. Therefore, natural selection on germination timing is an extremely efficient sieve that can determine which genotypes can persist in some locations, and its efficiency is geographically variable and depends on other aspects of life history. We found no evidence for adaptive responses to maternal photoperiod during seed maturation. We did find adaptive plasticity to season of seed dispersal in RI. Seeds dispersed in June postponed germination, which was adaptive, while seeds dispersed in November accelerated germination, which was also adaptive. We also found maladaptive plasticity to geographic location for seeds dispersed in June, such that seeds dispersed in KY germinated much sooner than the optimum time. Consequently, bet hedging in germination timing was favorable in KY; genotypes with more variation in germination timing had higher fitness because greater variation was associated with postponed germination. Selection on germination timing varied across geographic location, indicating that germination timing can be a critical stage in the establishment of genotypes in new locations. The rate of evolution of germination timing may therefore strongly influence the rate at which species can expand their range.     

Nurse‐plant effects on the seed biology and germination of desert annuals

Nurse‐plants generally have positive effects on understorey species by creating more suitable conditions for stress‐intolerant plants relative to open micro‐habitats. However, long‐term effects of this plant–plant facilitation system have been rarely examined. Seeds of five desert annual species from Atiquipa coastal desert in Southern Peru were used to examine whether different microenvironmental conditions under the nurse‐plants Caesalpinia spinosa Molina (Kuntze) lead to differences in seed biology and germinability of annual plants relative to open, canopy‐free conditions. Seeds collected from plants associated with nurse‐plants were predicted to be (i) larger due to more favourable growing conditions, (ii) more viable and with greater germination rates, (iii) less variable in size and viability due to reduced environmental heterogeneity, and (iv) to germinate faster to avoid apparent competition with other annuals. Seed attribute measurements and germination trials in growth chambers were used to test these predictions. Although the plant abundance of only 2 of 5 species was strongly facilitated by the nurse‐plant, no significant differences were found in seed mass, viability or relative variability between understorey and open micro‐habitats for any of the species. Contrary to our predictions, final seed germination rates of seeds from open micro‐habitats were higher, and the open micro‐habitat treatment was more favourable for germination of seeds from both open and understorey environments. Taken together, these results suggest that plant–plant facilitation does not necessarily affect seed biology traits. Further studies addressing larger distribution ranges and/or density gradients of understorey species will illuminate the potential evolutionary effects of nurse‐plants.     

Seed germination and plant fitness response of a narrowly endemic,rare winter annual to spatial heterogeneity in microenvironment

Environmental heterogeneity affects distributions of plant species, although the effects of fine-scale heterogeneity on plant demographies are not widely studied. Diminutive winter annuals, especially rare taxa, can be sensitive to spatial variation in microenvironment as a consequence of their small stature above and belowground. To address whether spatial environmental heterogeneity affects demography, germination and fitness of Chorizanthe orcuttiana, an endangered winter annual distributed in distinct easterly and westerly microhabitats within an exceedingly narrow niche in California, we performed multiyear observational and empirical studies. We manipulated after-ripening environment, soil moisture and ambient light at both aspects, and profiled microclimate, soil physicochemistry and soil microbiomes at all sites. We show that easterly aspects host larger plants in larger populations, and have lower air temperatures combined with higher soil moisture in comparison to the west-facing sites. Yet, soil physicochemistry and microbiomes were similar across all sites. Manipulations of after-ripening conditions showed that seeds exposed to low humidity (17%) during dormancy and sown at easterly aspects exhibited the highest germination percentages, whereas seeds incubated in situ and subsequently sown at westerly aspects yielded the lowest germination. Simultaneous manipulations of soil moisture and light showed that at both aspects higher moisture combined with shade led to higher germination, whereas ambient soil moisture combined with shade yielded the lowest germination. Altogether, our studies show that the diminutive, rare winter annual C. orcuttiana exhibits higher germination and plant fitness under cooler soil conditions with higher soil moisture while preferring drier environments during after-ripening.