Compensatory reproduction in a biennial herb following insect defloration

Summary The ability of the biennial herb, Pastinaca sativa L. (wild parsnip), to respond to and compensate for destruction of primary umbel seeds by the larvae of Depressaria pastinacella (Lepidoptera: Oecophoridae) was analyzed by comparing umbel and seed production of damaged and undamaged plants collected from five populations. Plants with a basal stem diameter smaller than 8 mm suffer a reduction in seed set of about 50% when the primary umbel is destroyed but larger plants are able to compensate for loss of primary umbel seeds by increased seed set of tertiary umbels. Depending on plant size, this is due to either an increase in the number of tertiary umbels that reach maturity or an increase in the number of seeds per tertiary umbel. Although seeds of tertiary umbels are significantly smaller than those of primary or secondary umbels, their viability is equivalent to that of secondary seeds and may be greater than that of primary seeds. Characteristics of P. sativa's reproduction, such as the long flowering period and the initiation of more umbels than the plant is normally able to bring to maturity, are important to P. sativa's ability to compensate for the effects of herbivore damage.     

DORMANCY AND GERMINATION IN SEEDS OF COMMON RAGWEED WITH REFERENCE TO BEAL'S BURIED SEED EXPERIMENT

Common ragweed (Ambrosia artemisiifolia L.) was one of 19 herbaceous weedy species used by Beal in his buried viable seed experiment started in 1879. No seeds germinated during the first 35 years of the experiment when germination tests were performed in late spring, summer or early autumn. Germination did occur in seeds buried for 40 years when seeds were exhumed and tested for germination in early spring. Data obtained in more recent research provide the probable explanation for these results. Seeds of common ragweed that do not germinate in spring enter secondary dormancy by mid to late spring and will not germinate until dormancy is broken the following late autumn and winter. Thus, during the first 35 years of the experiment seeds were dormant when tested for germination, whereas seeds buried for 40 years were nondormant. Seeds buried 50 years or longer did not germinate when tested in spring, probably because they had lost viability and/or seeds germinated during burial and seedlings died.     

Relationships between position on the parent plant and germination characteristics of seeds of parsley (Petroselinium crispum Nym.)

The percentage germination of seeds of parsley cv. Imperial Curled was higher in the light than in the dark, the high temperature limits for germination being 30 and 28°C for light and dark respectively. At the higher temperatures, the germination rate was slower in the dark. At 30°C, treatment with a gibberellin A_4/7 mixture at 2 × 10^–4 M partially alleviated the inhibiting effect of darkness on the germination percentage. Pre-incubation of parsley seeds at 35°C in the dark for 30 h increased the rate, but decreased the percentage, of germination of seeds incubated at 15°C in the light. Germination and seedling emergence studies were made on seed harvested from four different umbel positions. Although heavier seeds were produced from primary umbels than from other umbel orders, they were less viable as measured by seedling emergence in the glasshouse. The rate of emergence was decreased with increasing umbel order i.e. with later seed development: this was reflected in subsequent seedling weights, with seedlings from quarternary umbel seeds being about half the weight of those from primary umbel seeds. The upper temperature limit for dark germination was only slightly affected by umbel order, with quarternary umbel seeds being the most thermo-inhibited.Abbreviations BA N⁶-benzyladenine - GA_4/7 a mixture of gibberellins A₄ and A₇ - SD8339 6-benzyl-amino-9-(tetrahydropyran-2-yl)-9H-purine     

GERMINATION ECOLOGY OF SEDUM PULCHELLUM MICHX. (CRASSULACEAE)

Factors controlling the timing of seed germination were investigated in the small succulent winter annual Sedum pulchellum Michx. (Crassulaceae) in its natural habitat on unshaded limestone outcrops in northcentral Kentucky. At maturity in early July the dormant seeds are not dispersed but are retained in the fruits on the standing dead plants until September and October. Many, but not all, of the seeds afterripen in the fruits during summer, and at the time of dispersal some of them are dormant and some are nondormant. Germination and annual population establishment occur in September and October from seed reserves that have been in the soil for one or more years and from seeds produced in the current year. Germination of nondormant seeds may be prevented in autumn by lack of the appropriate combination of environmental factors including light, temperature and soil moisture in the seed's microsite. The effect of low winter temperatures on ungerminated seeds in the population is to induce nondormant seeds into secondary dormancy and to prevent afterripening of dormant seeds. Thus, in spring all the seeds in the population's seed reserve are dormant. During spring and summer some of these seeds afterripen, and they germinate in autumn when, and if, germination requirements are fulfilled.     

Intra-inflorescence variation in floral traits and reproductive success of the hermaphrodite Silene acutifolia

BACKGROUND AND AIMS: Intraspecific variation in floral components and reproductive success is often located at the intra-individual level. The arrangement of flowers within inflorescences may explain a great deal of this variation. The variation in number of ovules, fruit set, number of seeds per fruit, seed set, seed weight and seed germination is investigated at different positions within the inflorescence of Silene acutifolia. METHODS: Data were obtained in natural populations, and germination experiments were conducted in a germination chamber. KEY RESULTS: The number of ovules, fruit set, number of seeds, seed set and seed weight, showed a significant decline from early (primary) position to later (tertiary) position. The patterns of intra-inflorescence variation were consistent in different populations and years of study. Seed germination showed an opposite pattern, seeds from primary position showed the lowest germination percentages and seeds from tertiary position the highest, although the effect of position on germination was only marginally significant. There was significant among-population variation in number of ovules per flower. Fruit set also varied significantly among populations, with lower fruit set in the smaller and more isolated population. No significant among-population differences were detected in number of seeds per fruit and seed set. Seeds from the smallest and more isolated population (Arnado) were the lightest. Seed germination showed strong differences between populations, seeds from Arnado started to germinate later, and showed the lowest final germination percentages. CONCLUSIONS: Architectural effects or resource competition are the most commonly proposed hypothesis to explain these patterns. Data suggest that there is less pollen available to pollinate tertiary flowers, and that there is not enough outcross pollen in Arnado. The germination percentages suggest that there is variation in the source of pollen within inflorescences, with high probability of receiving outcross pollen in flowers from primary position, and higher probability of geitonogamous crosses in tertiary flowers.     

Seed Germination Biology of the Narrowly Endemic Species Lesquerella stonensis (Brassicaceae)

Abstract Lesquerella stonensis (Brassicaceae) is an obligate winter annual endemic to a small portion of Rutherford County in the Central Basin of Tennessee, where it grows in disturbed habitats. This species forms a persistent seed bank, and seeds remain viable in the soil for at least 6 years. Seeds are dormant at maturity in May and are dispersed as soon as they ripen. Some of the seeds produced in the current year, as well as some of those in the persistent seed bank, afterripen during late spring and summer; others do not afterripen and thus remain dormant. Seeds require actual or simulated spring/summer temperatures to come out of dormancy. Germination occurs in September and October. Fully afterripened seeds germinate over a wide range of thermoperiods (15/6–35/20°C) and to a much higher percentage in light (14 h photoperiod) than in darkness. The optimum daily thermoperiod for germination was 30/15°C. Nondormant seeds that do not germinate in autumn are induced back into dormancy (secondary dormancy) by low temperatures (e.g., 5°C) during winter, and those that are dormant do not afterripen; thus seeds cannot germinate in spring. These seed dormancy/ germination characteristics of L. stonensis do not differ from those reported for some geographically widespread, weedy species of winter annuals and thus do not help account for the narrow endemism of this species.     

EFFECT OF SEED DIMORPHISM ON THE DENSITY-DEPENDENT DYNAMICS OF EXPERIMENTAL POPULATIONS OF ATRIPLEX TRIANGULARIS (CHENOPODIACEAE)

The importance of seed size and density in determining individual plant performance and plant population dynamics in experimental populations of the halophyte Atriplex triangularis was studied. Two distinct seed morphs—large, light seeds and small, dark seeds—are produced by individual A. triangularis plants. Experimental populations consisting of seed size monocultures (large or small seeds) and seed size mixtures were established at three different densities, and the time of germination, plant size, plant survivorship, and plant fecundity were monitored. Marked variation in time of germination was observed among treatments and between seed sizes, but germination within any given treatment occurred over a five- to ten-day period. Large seeds produced larger plants than small seeds did, and this dichotomy was maintained over the course of the entire experiment. Germination date and seed size interacted such that larger plants grew from seeds which germinated earlier than those which germinated later, regardless of seed size. Germination date had a more pronounced effect than seed size did on plant mortality in high density populations. At high density, large seed monocultures experienced greater mortality than small seed monocultures did, but in seed size mixtures, the mortality was evenly distributed between plants from the two seed sizes. Regardless of density conditions and parentage, large and small seeds were produced in equal proportion by the plants. Total seed production, however, was dramatically affected by plant density, and to a lesser degree by germination date. Although seed size effects alone did not appear to affect directly final plant biomass and fecundity, effects of seed size early in ontogeny may have contributed to differences in fecundity.     

The origin of variation in seed density in celery (Apium graveolens L.)

Seeds were harvested separately from the sequentially formed umbels to obtain information about the origin of variation in seed density in celery. The study involved seeds from the lowest order (primary) to the highest order (quarternary) umbels from the cvs Mars, Monarch, Selfira and Tall Utah. With increasing umbel order the percentage germination decreased and the mean germination time increased. Seed density increased with increasing umbel order, but was more highly correlated with the length of the seed ripening period which differed for each umbel order due to simultaneous harvesting. The decrease in seed density during seed development was correlated with an increase in air volume in the seeds. It is concluded that the variation in seed density in celery seed lots originated from differences in seed maturity.     

The effects of ripening and drying of carrot seed (Daucus carota) crops on germination

Carrot seeds taken from the parent plant were capable of germinating before the stage when maximum seed dry weight was reached but even after this stage, when the seed moisture content had fallen below 20%, improvement in seed germination characteristics continued. In the latter stages of seed growth losses by shedding were 12–20 kg/ha/day. For low density crops (10 plants/m²) the yield of viable seed was at a maximum in crops whose seed was harvested with a moisture content of between 20 and 40% but no consistent relationship could be established for high density crops (80 plants/m²). There were no effects of umbel order or plant density on mean germination time or spread of germination. At any early harvest, percentage germination was highest for primary-umbel-seeds and seeds from low density crops but the differences between the seed origins diminished with later harvests. Drying the seeds on the umbels improved the percentage germination, reduced the mean germination time and the spread of germination particularly at the early harvests compared with seeds removed from the umbels and germinated immediately without drying.     

Diaspore ecology of Mertensia maritima: effects of physical treatments and their relative timing on dispersal and germination

This paper reports an experimental study of water dispersal potential and germination of the shingle beach plant Mertensia maritima in which we consider the effects of physical factors (cold treatment, mechanical wear of the pericarp and salt-water exposure) on the diaspores. Our approach also includes testing effects of different orders of the treatments, in contrast to most earlier studies of diaspore ecology. A cold period was necessary to break seed dormancy, and prolonged cold treatment (stratification at 2°C) enhanced germination. Mechanical wear of the pericarp before cold treatment did not affect germination, whereas mechanical wear after cold treatment increased germination significantly. Seeds exposed to 6 weeks of cold treatment before floating in salt water for 6 weeks did not germinate. In contrast, for seeds given the same cold treatment after floating, the germination was more than 50%. Most undamaged and slightly damaged nutlets stayed afloat throughout the dispersal experiment (9 weeks) in 3% salt water, whereas seeds that fell out of damaged nutlets sunk immediately. Thus, the results suggest that the potential for long-distance dispersal is high unless the diaspores (nutlets) are severely damaged, but the order of cold treatment and water dispersal seems to be of great importance for germination: seeds dispersed in autumn (before cold periods) have a much higher probability of germinating than seeds dispersed in winter or early spring (after a cold period). Similar effects of the relative timing of physical processes have hitherto only been reported for two other water-dispersed beach plants. Future studies in diaspore ecology should consider such timing effects as they may be important determinants of the distribution and abundance of plants.     

Relationship between Position on the Parent Plant and Dormancy Characteristics of Seeds of Three Cultivars of Celery (Apium graveolens)

Germination and seedling emergence studies were made on seeds harvested from four different umbel positions of three cultivars of celery (Apium graveolens L.). Although heavier seeds were produced from primary umbels than from other umbels, these were less viable as measured by the germination percentage at I8°C in the light. However, germination of viable seeds from quaternary umbels was lower than that of seeds from primary umbels at 18°C in the dark when incubated with GA₄, (2 × 10 ^?4M) and seed from secondary and tertiary umbels tended to be intermediate in response. All viable seeds germinated when N⁶-benzyladenine (10^?2M) was used in combination with GA₄. Seeds from quaternary umbels of two of the cultivars had a lower high-temperature limit for germination in the dark than did seeds from other umbels. In glasshouse experiments the emergence of viable ‘quaternary’ seeds of these cultivars was higher than that of ‘primary’ seeds. Under these conditions the time to 50% of the final emergence as determined after 42 days was similar for seeds from all umbel positions within each cultivar. In two varieties seedling weights were greater from seeds of primary as compared to quaternary umbels, and in general, the largest seedlings arose from the heaviest seeds and the smallest from the lightest seeds in all three cultivars.     

Seeds photoblastism and its relationship with some plant traits in 136 cacti taxa

Seed germination triggered by light exposure (positive photoblastism) has been determined in quantitative studies for numerous plant families and species. For Cactaceae, positive photoblastism is thought to be associated with life form and seed mass, but this association has never been evaluated. To explore hypotheses on associations between seed mass, seed dispersal, seed dormancy, life form, taxa and plant height with Relative Light Germination (RLG) in Cactaceae, we evaluated the effect of light on seed germination of 136 taxa. The taxa studied are native to several countries: México, Chile, Argentina, Brazil, Perú, USA, and Venezuela. Seed traits contrasted with RLG were life form, seed mass, seed dispersal, seed dormancy, adult plant height and taxon. We found some differences between RLG among taxa; Cacteae, Pachycereeae and Trichocereeae had higher RLG than Notocacteae. RLG was lower for seeds from taller than for shorter taxa, and lower for taxa with heavier seeds than for taxa with lighter seeds. Dispersal syndrome groups varied with RLG. RLG did not differ between cylindrical and globose taxa. Trends found here were in agreement with expectations for small-seeded species to have a light requirement to germinate more often than large-seeded species. This is the first time that cactus height is related to photoblastism. It is possible that seeds from tall plants are larger and thus have the capacity to produce taller seedlings than those from small plants, and that seedlings from large seeds with more resources have the ability to emerge from greater soil depths than those from small seeds.     

ENVIRONMENTAL AND GENETIC MATERNAL EFFECTS ON SEED CHARACTERS IN NEMOPHILA MENZIESII

Nuclear genetic, maternal genetic and maternal environmental effects on seed characters were estimated in the California native annual plant Nemophila menziesii using two greenhouse crosses. In one cross, according to a nested mating design, the narrow sense heritability of seed weight was small (3.9%). A subset of full-sib progenies produced in this cross was grown singly and in competition with the introduced grass Bromus diandrus. In a second cross, these plants were used as mothers (dams) and were each mated to the same three sires. Seeds produced by mothers competing with B. diandrus showed a significant reduction in weight, increase in time to germination, and increase in the incidence of dormancy, when compared to seeds from mothers grown singly. Significant sire components were found for time to germination and incidence of dormancy. Maternal genetic variation for seed weight was largely expressed as maternal genotype by maternal environment interaction, and showed no significant maternal genetic main effect. Time to germination and dormant fraction showed a relatively large maternal genetic effect. Evolution of seed characters in N. menziesii is more likely to occur via indirect response to selection among maternal plants than among the seeds themselves. Maternal genotype by maternal environment interaction could potentially contribute to the maintenance of maternal genetic variation in seed weight, but this does not appear likely for dormancy.     

Variation of seed heteromorphism in Chenopodium album and the effect of salinity stress on the descendants

Background and Aims

Chenopodium album is well-known as a serious weed and is a salt-tolerant species inhabiting semi-arid and light-saline environments in Xinjiang, China. It produces large amounts of heteromorphic (black and brown) seeds. The primary aims of the present study were to compare the germination characteristics of heteromorphic seeds, the diversity of plant growth and seed proliferation pattern of the resulting plants, and the correlation between NaCl stress and variation of seed heteromorphism.

Methods

The phenotypic characters of heteromorphic seeds, e.g. seed morphology, seed mass and total seed protein were determined. The effects of dry storage at room temperature on dormancy behaviour, the germination response of seeds to salinity stress, and the effect of salinity on growth and seed proliferation with plants derived from different seed types were investigated.

Key Results

Black and brown seeds differed in seed morphology, mass, total seed protein, dormancy behaviour and salinity tolerance. Brown seeds were large, non-dormant and more salt tolerant, and could germinate rapidly to a high percentage in a wider range of environments; black seeds were salt-sensitive, and a large proportion of seeds were dormant. These characteristics varied between two populations. There was little difference in growth characteristics and seed output of plants produced from the two seed morphs except when plants were subjected to high salinity stress. Plants that suffered higher salinity stress produced more brown (salt-tolerant) seeds.

Conclusions

The two seed morphs of C. album exhibited distinct diversity in germination characteristics. There was a significant difference in plant development and seed proliferation pattern from the two types of seeds only when the parent plants were treated with high salinity. In addition, seed heteromorphism of C. album varied between the two populations, and such variation may be attributed, at least in part, to the salinity.     

DEVELOPMENTAL AND GENETIC SOURCES OF SEED WEIGHT VARIATION IN RAPHANUS RAPHANISTRUM L. (BRASSICACEAE)

Seed weight is known to have a marked impact on emergence and post-emergence productivity in wild radish (Raphanus raphanistrum). In this paper, I describe several levels of seed weight variation in plants taken from a natural population in Hamden, Connecticut. Six maternal plants from the 1981 season were analyzed in detail: the weights and positions of all seeds within a fruit were recorded, and some of these seeds were used the following summer for competition studies and progeny analysis. Within a plant, average seed weight decreased as the number of seeds within a fruit increased, suggesting that developing embryos compete for maternal resources. Seed weight also varied significantly among the six maternal plants used in the study. Comparison of the average weights of seeds produced by offspring of those six plants with the average weights of seeds borne by the maternal plant revealed a significant genetic component to seed weight variation. Seed weight varied up to six-fold within single fruits of R. raphanistrum; large seeds tend to occur near the pedicel or in the middle positions. Seed size variation seen within single fruits is of sufficient magnitude to result in differential reproductive output among closely related seeds under competitive field conditions.     

Variation within species and inter-species comparison of seed dormancy and germination of four annual Lamium species

In an ecological context, knowledge of intra-species variation in dormancy and germination is necessary both for practical and theoretical reasons. We used four or five seed batches (replicates) of four closely related annuals co-occurring in arable fields in Sweden: Lamium amplexicaule, L. confertum, L. hybridum and L. purpureum. Seeds used for experiments stemmed from plants cultivated on two sites, each site harbouring one population of each species, thereby ensuring similar environmental history of seeds. Seeds were tested for germination when fresh and after three different pre-treatments (cold or warm stratification, or dry storage) for up to 24 weeks. Seeds were also sown outdoors. Despite substantial intra-species variation, there were clear differences between species. The general seed dormancy pattern, i.e. which environmental circumstances that affect dormancy, was similar for all species; dormancy reduction occurred during warm stratification or dry storage. Even though the response to warm stratification indicates a winter annual pattern, successful plants in Sweden were mostly spring emerged. Germination in autumn occurred, but plants survived winters poorly. Consequently, as cold stratification did not reduce dormancy, strong dormancy in combination with dormancy reduction during dry periods might explain spring germination. It is hypothesised that local adaptations occur through changes mainly in dormancy strength, i.e. how much effort is needed to reduce dormancy. Strong dormancy restricts the part of each seed batch that germinate during autumn, and thus reduces the risk of winter mortality, in Sweden.     

四种短命植物若干生物学生态学特性的研究

 卷果涩芥(Malcolmia scorpioides)、四齿芥(Tetracme quadricornis)、长齿四齿芥(T．recurvata)和狭果鹤虱(Lappula semiglabra)为新疆北部常见的四种短命植物。本文从种子生物学、发育节律、株形和植株寿命等方面研究它们与生存环境的协调与适应。     

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.     

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.     

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.