Showing papers on "Germination published in 1996"

An improved method for seed-bank analysis: Seedling emergence after removing the soil by sieving

Abstract: 1. The seedling emergence method for assessing the size of the seed bank is improved by washing soil samples on a fine sieve and spreading the thus concentrated samples in a 3-5 mm thick layer on sterilized potting compost. 2. The method largely increases the number of seedlings that emerge as compared to unconcentrated samples. Hand-sorting afterwards shows that the germination rates vary between 81 and 100% of the viable seeds present. 3. Ninety-five per cent of the seedlings will emerge within 6 weeks using this method. 4. The method greatly reduces the greenhouse space needed and enables examination of large sample volumes.

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

Abstract: 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 envi- ronments 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

Principles of Seed Pathology

Abstract: IntroductionTerminologyHistorical DevelopmentDevelopment of Seed Health TestingSignificance Reduction in Crop Yields Loss in Germination and Vigor Development of Plant Diseases Discoloration and Shrivelling Biochemical Changes in Seeds Alteration in Physical Properties of SeedsSeedborne PathogensFungiBacteriaMycoplasmalike OrganismsFastidious Vascular BacteriaSpiroplasmasVirusesViroidsNematodesLocation of Seedborne InoculumHistopathology of Some Seedborne PathogensSeed Infection Embryo Endosperm and Perisperm Seed Coat and Pericarp Glume InfectionSeed Infestation or ContaminationMechanism of Seed InfectionSeed Infection Systemic Infection through Flower, Fruit, or Seed Stalk Penetration through the Stigma Penetration through the Ovary Wall and Seed Coat Penetration through the Natural Openings and InjuriesSeed Infestation or Contamination Pathogens Adhering to the Seed Surfaces Concomitant Seed ContaminationFactors Affecting Seed InfectionHost GenotypeEnvironment Moisture Temperature Wind Velocity Rainfall IrrigationCrop Management Seed Production Area Tillage Plant Population Fertilizers Growth Regulators Weeds Irrigation Frost Harvest TimeType of InoculumSeed QualityPlant Growth StageMother Plant InfectionInsect InfestationPathogen InteractionLongevity of Seedborne PathogensLongevityFactors Influencing Longevity Host Genotype Inoculum Seed Storage Containers Storage Environment Storage Period Presence of Antagonistic MicrofloraSeed Transmission and InoculationSeed Transmission Systemic Seed Transmission Nonsystemic Seed TransmissionSeed InoculationFactors Affecting Seed TransmissionCrop SpeciesEnvironment Moisture Temperature Wind-Blown Rain LightInoculum Minimum Effective Level of Inoculum for Seed Transmission and Establishment in Seedlings or Plants Inoculum Location Type of InoculumSurvival of InoculumCultural Practices Soil Type Soil Reaction Seeding Rate Depth of Sowing Sowing Time Fertilizers Planting MethodSeed AbnormalitiesSeed Germination Epigeal HypogealSeed LeachatesPresence of Other MicrofloraEpidemiology and Inoculum Thresholds of Seedborne PathogensEpidemiology Classification of Seedborne Diseases Based on EpidemiologyInoculum Thresholds Fungi Bacteria VirusesDevelopment of Inoculum Thresholds Planting Area Role of Seedborne Inoculum in Disease Development Host Susceptibility Economic Loss Due to Seedborne Pathogens Perpetuation of Inoculum by Other Means Cropping Practices Influence of Seed Treatment on Seedborne Infection Certification Program Seed Proce

Phytotoxicity and Distribution of Sorgoleone in Grain Sorghum Germplasm

Abstract: The relative phytotoxicity of sorgoleone as measured by seed germination and seedling growth of selected crop and weed species and inhibition of photosynthetic oxygen evolution in atrazine-resistan...

Seed and Seedling Characteristics Contributing to Variation in Early Vigor among Temperate Cereals

Abstract: Seedling characteristics between germination and the two leaf stage are principally responsible for variation in early leaf area development, or vigor, among temperate cereals. There is compelling evidence that this greater vigor confers substantial benefits in crops grown in dryland Mediterranean-type environments. Experiments were conducted to determine the important factors that are likely to be responsible for variation in early vigor among barley (Hordeum vulgare L.), bread wheat (Triticum aestivum L.), durum wheat (T. turgidum var durum), triticale (Triticosecale Wittm.), and oat (Avena saliva L.) cultivars. Variation among and within species in the following factors was examined: (i) time to emergence, (ii) efficiency of utilization of seed reserves, (iii) shoot biomass formed from seed reserves and attributed to a larger plumule relative to the radicle, (iv) embryo size, (v) size of leaf epidermal cells, and (vi) leaf expansion rate during the lower temperatures in winter. Seeds of a similar mass were used in all experiments to eliminate the effects of seed size. The superior vigor of barley and triticale compared with the two wheat species and oat previously found in field experiments were confirmed. The size of the embryo was the single most important factor to account for differences in vigor among the species. It accounted for 90% of the variation in vigor between the species. Earlier emergence in barley accounted for the remaining variation in vigor. The breadth of the first seedling leaf is suggested as an indirect estimate of early vigor that integrates embryo size and specific leaf area and could be used in a breeding program to increase the vigor of cereals.

Aleppo Pine (Pinus Halepensis) Postfire Regeneration: the Role of Canopy and Soil Seed Banks

Abstract: Pinus halepensis (Aleppo pine), is the dominant Introduction tree of a large fraction (26%) of the Greek coniferous forests; this species is an endemic pine of the Mediterra- In Greece, during the last three decades (1964-89), nean Rim and well adapted to fire. Its regeneration is total burned areas amount to an average of 29300 hectares accomplished exclusively through seeds, thus its soil and per year, a large fraction of which (ca. 25%) is Mediter- canopy seed banks are of paramount importance forpostflre ranean pine forest (Efthymiou 1993). Aleppo pine, Pinus resilience. Cone opening and seed dispersal were investi- ha/epensis, one of the major Mediterranean pine species, gated in unburned forests of Attica (Greece) and it was has evolved adaptative mechanisms towards "catastrophic" found that Pinus halepensis trees maintain a significant fire events. Being an obligate reseeder (i.e. being usually percentage of the yearly cone crop (40-80%) closed, thus killed by fire), Aleppo pine depends for its survival upon creating a persistent, canopy seed bank. Full viability of a very powerful regenerative capacity based on both the enclosed seeds was maintained for at least three years in yearly production of prolific seed crops and the safe- canopy storage; moreover, preliminary results concerning guarding of seed banks. In addition, Aleppo pine seed the viability of seeds enclosed within the cones for four to germination and early seedling establishment are both more than 50 years showed a gradual reduction of both final well adapted to exploit the postflre conditions. percentage and rate of germination. Nevertheless, cones of Out of the 95 species in the genus Pinus, 23 produce up to 20 years of age contained a considerable fraction of serotinous cones and only six species are considered germinable seeds. On the other hand, Aleppo pine forms obligately pyriscent (Lamont et al. 1991). Aleppo pine, only a short-lived (transient) soil seed bank; this bank was although not included in the latter group, does produce particularly abundant after a fire, as a result of the flre- serotinous cones, i.e. its seeds are produced, stored and induced cone opening. The germinable seed portion, al- protected in the canopy, within the woody cones. Cone though quite important prior to the start of the rainy season, opening and seed release are controlled mostly by envi- was rapidly depleted, and at the end of the rainy season it ronmental conditions (high temperatures and low air was virtually absent in both burned and unburned forests. humidity). Nevertheless, a significant portion of seeds Itis therefore concluded thatpostfire Aleppopine seedling remains enclosed within the cones (cone serotiny), thus recruitment takes place almost exclusively during the fIrSt forming a so-called canopy seed bank. Despite the gen- year after the fire and depends upon the germination of era! recognition of the occurrence of cone serotiny in seeds in a transient soil bank which is produced by the Aleppo pine (e.g. Moulopoulos 1933, Panetsos 1981, postfIre dispersal of pine seeds stored in the canopy seed Dafis 1987, Klaus 1989, Richardson 1988) it has recently bank. been emphasized that "this phenomenon has not been formally proven" (frabaud 1987).

Seedling establishment in an epiphytic orchid: an experimental study of seed limitation.

Abstract: Will increased fruit and seed production in a severely pollination-limited orchid stimulate population growth? We tested whether safe sites for germination and seedling establishment are limiting for the twig epiphyte, Tolumnia variegata, by manipulating fruit set and monitoring subsequent seedling establishment for two seasons (1991-1992, 1992-1993). In the Cambalache Forest Reserve of Puerto Rico, we established 36 plots along a transect. Each plot consisted of nine trees. A center tree was designated as the site for attaching Tolumnia and manipulating fruit set. The other eight potential host trees were 1-3 and 3-5 m from the center tree in each of the cardinal directions. A 1-m length of stem 1 m from the ground was monitored for recruits on each of the nine trees of 24 fruit-enhanced plots and 12 controls (23 and 13, respectively for the 1992-1993 season). Fruit enhancement plots were divided among two treatments: one-fruit and five-fruit additions for the 1st year and one to five and more than five fruits for the 2nd year. Availability of suitable host species was not limiting. T. variegata showed little specificity for host tree species, good host trees and shrubs were common, and there was no evidence that the orchid had a preference for small branches, despite possessing the entire suite of characteristics thought to respresent "obligate" twig epiphytes. Fruit enhancement increased seed rain and seedling establishment consistently in only the high-fruit treatment plots. Most recruitment occurred near fruiting plants. Over the 2-year period, mortality was 18% for adults and 85.5% for the 1991-1992 cohort of recruits. Net recruitment was positive for both the treatment (average = 1.74) and control plots (average = 0.67). Seedling establishment at our study site was not microsite-limited. If selection for increased pollinator attraction occurs, then an increase in seed output should result in population growth.

Arabidopsis mutants with a reduced seed dormancy

Abstract: The development of seed dormancy is an aspect of seed maturation, the last stage of seed development. To isolate mutants of Arabidopsis thaliana that are affected in this process, we selected directly for the absence of dormancy among freshly harvested M2 seeds. The screen yielded two mutants exhibiting a reduced dormancy, rdo1 and rdo2, that are specifically affected in dormancy determined by the embryo. The rdo1 and rdo2 mutants show normal levels of abscisic acid and the same sensitivity to abscisic acid, ethylene, auxin, and cytokinin as the wild type. The rdo2 mutant but not the rdo1 mutant has a reduced sensitivity to the gibberellin biosynthesis inhibitor tetcyclacis. Double-mutant analysis suggested that the RDO1 and RDO2 genes are involved in separate pathways leading to the development of dormancy. We assume that the RDO2 gene controls a step in the induction of dormancy that is most likely induced by abscisic acid and is expressed as an increase of the gibberellin requirement for germination.

Nucleation processes in a mediterranean bird-dispersed plant

Abstract: 1. The spatial distribution of Pistacia lentiscus, a Mediterranean bird-dispersed plant, in abandoned orchards was found to be strongly linked to the presence of trees or shrubs that act as perches. 2. These perches not only attract seed-disperser birds but also produce favourable microenvironmental conditions for seed germination and seedling establishment. 3. Soil moisture content after a rainfall was always greater beneath perches than not beneath perches. Favourable water potentials for seed germination were maintained for a longer time beneath a perch than elsewhere. 4. After a rainfall, soil was compacted faster where not beneath perches. Seedling radicle penetration into soil was strongly associated with soil compactation.

Seedling longevity under deep shade in relation to seed size

Abstract: 1 Seedlings were grown in dense shade, below the compensation point. At cotyledon stage (i.e. when placed in dense shade from germination onwards), species with larger seed mass tended to survive longer. This was true within 11 phylogenetically independent contrast (PIC) pairs of plant species, as well as across all 22 species. 2 RGR of early seedlings in full light (to day 10 after germination), and cotyledon dark respiration, were measured as potential indicators of the speed with which metabolic resources in the seed were used during early seedling development. These indicators proved not to be such good predictors of longevity under dense shade as was seed mass itself. 3 Seedlings were also grown in the light until five days after true leaves appeared, before being transferred to dense shade. The different behaviour of these true-leaf seedlings was consistent with the idea that extra resources in seeds are the direct cause of longer dense-shade survival. Longevity in dense shade at leaf stage was often shorter than for cotyledon-stage seedlings, especially for larger-seeded species. Longevity in dense shade at leaf stage was best correlated with a slow potential RGR in full light measured over days 10-30 of growth. Correlation with seed mass was marginal and probably an indirect correlation via RGR, and there was no correlation with leaf dark respiration.

Soil temperatures after the passage of a fire : Do they influence the germination of buried seeds ?

Abstract: Soil temperatures down to a depth of 5 cm were measured in the days following one fire in summer, one fire in winter and in unburnt vegetation during summer. Soil temperatures did not rise above 40°C after the winter fire or in unburnt vegetation during summer. Consequently, no impact on seed dormancy in the soil seedbank was expected. After a summer fire, soil temperatures above 40°C were found up to 4.5 cm in depth, while temperatures above 60°C were found only in the top 0.5 cm of soil. These temperatures are sufficient to break seed dormancy in some legume species in the seedbank. Hence, the season of burn may influence the number of seeds in the soil that have their dormancy broken and subsequent germination levels.

The effects of manure, genotype, seed priming, depth and date of sowing on the emergence and early growth of Sorghum bicolor (L.) Moench in semi-arid Botswana

Abstract: Surveys of research reports and farmers' opinions in semi-arid Botswana suggested that poor stand establishment of cereals was common and a major cause of low yields. Of 146 researcher-managed trials conducted since 1979, 40% failed to germinate, emerge or establish properly. Analysis of a subset of 84 trials for which more detailed data were available showed that the most common reason for failure was ‘soil too dry’ at, or just after, sowing. Therefore we tried to improve post-seeding conditions by agronomic means. Sorghum seeds were sown on nine planting opportunities during the 1990–1991 wet season, all after substantial rainstorms. Adding manure did not affect seedling emergence but enhanced growth over the first 25 days after sowing (DAS) when the soil did not dry out rapidly. Two varieties (Segaolane and 65D) differed significantly and consistently throughout the experiment, with Segaolane emerging more quickly and growing more vigorously. The effect of depth of sowing was inconsistent between sowings. Establishment was more successful if rain fell or if the soil dried out slowly after sowing. Consequently, both stand count and dry matter (plot −1 and plant −1 ) at 25 DAS were directly proportional to the rate at which seedlings emerged. Individual plant vigour in relation to rates of emergence was explained in terms of the interaction between sorghum morphology and rapid soil-drying. Successful establishment was achieved when the soil dried slowly or when seeds germinated and emerged quickly. Soaking seed in water immediately prior to sowing (seed-priming), as a way of speeding up germination, was explored in detail in controlled environments. The time taken for seeds to germinate at 30°C decreased as the soaking time increased from 0 to 10–12 h, a treatment in which a 50% saving in time could be achieved. Germination of seeds soaked for 16 h or more was found to continue even after soaking ceased, suggesting that they would be susceptible to damage in the event of any delays in sowing. Emergence from soil at 30°C was significantly hastened by 23% when seeds were pre-soaked for 6 h or longer. The rate of imbibition during soaking was found to be proportional to temperature. The practicality of seed-priming for improving establishment in the field is discussed.

Hydrolysis products of glucosinolates in Brassica napus tissues as inhibitors of seed germination

Abstract: Enzymatic hydrolysis of glucosinolates, a class of compounds found in Brassica species, results in a number of products with potential to inhibit seed germination. To investigate the impact of both volatile and water-soluble allelochemicals, germination bioassays using Lactuca sativa seeds were conducted with root and combined leaf and stem tissues of Brassica napus. Tissues in which glucosinolates were hydrolyzed to remove volatile glucosinolate degradation products were compared with intact tissues and water controls. Only tissues containing glucosinolates produced volatiles that inhibited germination. Volatiles were trapped and identified using GC-MS. Volatiles produced in greater quanitity from intact tissues than from tissues without glucosinolates were almost exclusively glucosinolate hydrolysis products. Water-soluble components also inhibited germination. Chemical analysis of extracts confirmed the presence of glucosinolate hydrolysis products, but indicated the involvement of additional allelochemicals, especially in leaf and stem tissues. Results support the proposal that glucosinolate-containing plant tissues may contribute to reductions in synthetic pesticide use if weed seeds are targeted.

Photophysiology of turion formation and germination inSpirodela polyrhiza

Abstract: Standardized laboratory techniques for the vegetative growth of the duckweedSpirodela polyrhiza (Lemnaceae), and for formation as well as germination of their turions were described. Increasing photon fluence rates of blue or red light increased the yield of turions. A specific stimulating effect of blue light was demonstrated under autotrophic but not under mixotrophic conditions. Therefore the spectral composition of light is not important in mixotrophic formation of turions whereas in autotrophic formation light sources with a higher portion of blue light are recommended. Dark-grown (etiolated) turions showed accelerated germination and higher germination percentage in comparison with light-grown turions after induction by a single red light pulse. This difference was overcome in continuous red light by speeding up the germination response of light-grown turions. Use of Petri dishes (8 cm3 nutrient solution) instead of Erlenmeyer flasks (50 cm3 nutrient solution) retarded germination response. Especially for long term experiments the use of Erlenmeyer flasks is recommended. Storage of turions for 72 h at 25 ‡C following at 5 ‡C in darkness after-ripening resulted in a decreased lag phase of the light-induced germination both after induction by a single light pulse and in continuous light.

Water and temperature relations and microconidial germination of Fusarium moniliforme and Fusarium proliferatum from maize

Abstract: The effects of water activity (aw, 0.994–0.85 = 0.4–21.0 (−)MPa water potential), temperature (5–42 °C), and their interactions on microconidial germination of three isolates each of Fusarium moniliforme and Fusarium proliferatum were determined in vitro on a maize meal extract medium. Temporal germination rates of microconidia of isolates of both species were significantly influenced by both aw and temperature. Germination was very rapid at >0.94 aw with an almost linear increase with time. Germination rates of microconidia of F. moniliforme were slower than those of F. proliferatum isolates at marginal aw levels and 5–25 °C, while at higher temperature (30–37 °C), the former germinated more rapidly than the latter. The aw minima for germination of isolates of both species was 0.88, with none occurring at 0.85 aw over a 40-day incubation period. At 37 °C, isolates of F. moniliforme had slightly lower aw minima than those of F. proliferatum. The narrowest range of aw for germination was at 5 °C, and none ...

Seed Ecophysiology of Temperate and Boreal Zone Forest Trees

Abstract: Chapter 1: Introduction Chapter 2: Components of Seed Production Chapter 3: Dispersal Chapter 4: How Seed Germinate Chapter 5: How to Evaluate Germination Chapter 6: Seed Dormancy Chapter 7: Seed Banking Trees Chapter 8: Germination Genetics Chapter 9: Germination and Establishment: Temperature, Water and Light Chapter 10: Germination and Establishment: Chemical Ecology Chapter 11: Germination and Establishment: Animals, Pathogens and Aging Chapter 12: Germination and Establishment: North American Microsites Index

Effect of oxygen availability and salinity on EARLY LIFE HISTORY STAGES OF SALT MARSH PLANTS. I. Different germination strategies of Spartina alterniflora and Phragmites australis (Poaceae)

Abstract: Gradients in oxygen availability and salinity are among the most important environmental parameters influencing zonation in salt marsh communities. The combined effects of oxygen and salinity on the germination of two salt marsh grasses, Spartina alternifora and Phragmites australis, were studied in growth chamber experiments. Germination of both species was initiated by emergence of the shoot and completed by root emergence. Percentage S. alternifora germination was reduced at high salinity (40 g NaCl/L) and in decreased oxygen (5 and 2.5%). In 0% oxygen shoots emerged, but roots did not. P. australis germination was reduced at a lower salinity (25 g NaCl/L) than S. alterniflora, and inhibited at 40 g NaCl/L and in anoxia. However, a combination of hypoxia (10 and 5% 02) and moderate salinity (5 and 10 g NaCl/L) increased P. australis germination. When bare areas in the salt marsh are colonized, the different germination responses of these two species to combinations of oxygen and salt concentrations are important in establishing their initial zonation. In high salinity wetlands S. alterniflora populates the lower marsh and P. australis occupies the high marsh at the upland boundary.

Soybean seed quality reductions by high day and night temperature

Abstract: Soybean Seed Quality Reductions by High Day and Night Temperature Environmental stress during reproductive growth of soybean [Glycine max (L.) Merr.] may reduce seed germination and vigor. Most studies of environmental influence on soybean seed germination and vigor have concentrated on the effects of drought, maximum day temperature, or average daily temperature during seed fill. This study was conducted to determine the effects of different combinations of day temperatures and night temperatures during reproductive growth on soybean seed quality. Day/night temperatures of 30/20, 30/30, 35/ 20, and 35/30°C were imposed at flowering and pod set (R1-R5), seed fill and maturation (R5-R8), and during the entire reproductive growth period (R1-R8). High day and night temperature reduced soybean seed germination and seedling vigor. The extent of reductions in germination and vigor by high temperature was influenced by duration of temperature exposure and phase of reproductive growth. Temperatures during flowering and pod set exerted little influence on soybean seed quality. A combination of 35°C day temperature and 30°C night temperature during seed fill and maturation or 35°C day temperature from flowering to maturity decreased germination. Seedling vigor was reduced by 35/20, 30/30, and 35/30°C during seed fill and maturation and the entire reproductive period. Increase in night temperature from 20 to 30°C decreased seedling vigor at both 30 and 35°C day temperatures. These results suggest that locational and seasonal differences in soybean seed quality may be induced by high temperatures during reproductive growth and indicate that night temperatures along with day temperatures are important in determining soybean seed quality.

Effects of light, moisture, temperature, and litter on the regeneration of five tree species in the tropical montane wet forest of Puerto Rico

Abstract: Field experiments were conducted to investigate the effect of light, moisture, temperature, and litter on the regeneration ot two early-, one mid-, and two late-successional tropical tree species. High light and litter seem to be universally good cues for regeneration, increasing seed/seedling survival for all species except for Cecropia (an early-successional species) whose small seeds may not be able to penetrate the litter layer. In addition, the high temperature environment in both artificially shaded and nonshaded area of a natural gap exhibits less seed loss, an increase in the percent and rate of germination, and an increase in seedling survival for Darcryodes (a late-successional species), than the lower temperature environment under an intact canopy. Low soil water is also a good cue for Dacryodes germination as it is for Prestoea and Cecropia. Finally, the lower temperature environment found under the forest canopy (compared to the natural gap) leads to less seed loss and more germination for Guarea (a mid-successional species). Our results suggest that a good patch for regeneration of many species in this forest, early- as well as late-successional species, would have high light and a litter layer that moderates extremes.

Variation in seed production and germinability in common reed (Phragmites australis) in Britain and France with respect to climate

Abstract: summary Northern populations of Phragmites australis (Cav.) Trin. ex Steud. (common reed) often flower so late that the ability to produce viable seeds before winter dieback can be limited. If climate changes as predicted, the reed may become able to set seed further north in its range. This would have implications for the ability of reed to colonize new sites as well as for the distribution of animals which eat reed seeds in winter. A survey of 34 British populations in 1993 and 1994 showed that seed production is highly variable between sites, ranging from 0 to 100%. Southern and western populations tended to show the highest seed sets, although some southern populations were infertile and some northern sites set seed. Average seed weight tended to be higher in sites which showed good seed set. Seeds from sites with high average seed weight tended to germinate more readily. The seed-setting capability of populations was successfully associated with climatic conditions at the time of flowering. However, seed weight was not found to relate to climate, but depended on seed production and overall plant height. Logistic regression modelling indicated that the best seed sets should occur in P. australis when: (i) August rainfall is low; (ii) the combined rainfalls of September and October are high; and (iii) the combined temperatures of these months are high. The model accounts for approximately one third of the variation in observed seed sets between sites, indicating that other, unmeasured, factors might also play a part in determining seed set in P. australis. At three northern populations which were studied in more detail, a number of genotypes coexisted, and legitimate pollen germination and pollen-tube growth was observed in each case. As no seed set occurred in two of these, self-incompatibility cannot explain poor seed setting.

A hydrothermal time model of seed after-ripening in Bromus tectorum L.

Abstract: Bromus tectorum L. is an invasive winter annual grass with seeds that lose dormancy through the process of dry after-ripening. This paper proposes a model for after-ripening of B. tectorum seeds based on the concept of hydrothermal time. Seed germination time course curves are modelled using five parameters: a hydrothermal time constant, the fraction of viable seeds in the population, base temperature, mean base water potential and the standard deviation of base water potentials in the population. It is considered that only mean base water potential varies as a function of storage duration and incubation temperature following after-ripening. All other parameters are held constant throughout after-ripening and at all incubation temperatures. Data for model development are from seed germination studies carried out at four water potentials (0, −0.5, −1.0 and −1.5 MPa) at each of two constant incubation temperatures (15 and 25°C) following different storage intervals including recently harvested, partially after-ripened (stored for 4, 9 or 16 weeks at 20°C) and fully after-ripened (stored for 14 weeks at 40°C). The model was fitted using a repeated probit regression method, and for the two seed populations studied gave R2 values of 0.898 and 0.829. Germination time course curves predicted by the model generally had a good fit when compared with observed curves at the incubation temperature/water potential treatment combinations for different after-ripening intervals. Changes in germination time course curves during after-ripening of B. tectorum can largely be explained by decreases in the mean base water potential. The simplicity and good fit of the model give it considerable potential for extension to simulation of after-ripening under field conditions.

Bromus tectorum seed germination: between-population and between-year variation

Abstract: Cheatgrass (Bromus tectorum L., Poaceae), an introduced winter annual, has invaded a variety of habitats in western North America. This study examines variation in cheatgrass germination response a...

Effect of enhanced uv-b radiation on pollen quantity, quality, and seed yield in brassica rapa (brassicaceae)

Abstract: Three experiments examined the influence of ultraviolet-B radiation (UV-B; 280-320 nm) exposure on reproduction in Brassica rapa (Brassicacaeae). Plants were grown in a greenhouse under three biologically effective UV-B levels that stimulated either an ambient stratospheric ozone level (control), 16% ({open_quotes}low enhanced{close_quotes}), or 32% ({open_quotes}high enhanced{close_quotes}) ozone depletion levels at Morgantown, WV, USA in mid-March. In the first experiment,pollen production and viability per flower were reduced by {approx}50% under both enhanced UV-B levels relative to ambient controls. While plants under high-enhanced UV-B produced over 40% more flowers than plants under the two lower UV-B treatments, whole-plant production of viable pollen was reduced under low-enhanced UV-B to 34% of ambient controls. In the second experiment, the influence of source-plant UV-B exposure on in vitro pollen from plants was examined and whether source-plant UV-B exposure influenced in vitro pollen germination and viability. Pollen from plants under both enhanced-UV-B was reduced from 65 to 18%. Viability of the pollen from plants grown under both enhanced UV-B treatments was reduced to a much lesser extent: only from {approx}43 to 22%. Thus, ambient source-plant pollen was more sensitive to enhanced UV-B levels to fertilize plants growing under ambient-UV-B levels, and assessed subsequent seed production andmore » germination. Seed abortion rates were higher in plants pollinated with pollen from the enhanced UV-B treatments, than from ambient UV-B. Despite this, seed yield (number and mass) per plant was similar, regardless of the UV-B exposure of their pollen source. Our findings demonstrate that enhanced UV-B levels associated with springtime ozone depletion events have the capacity to substantially reduce viable pollen production, and could ultimately reduce reproductive success of B. rapa. 37 refs., 4 figs., 2 tabs.« less

Seed banks and seedling establishment on an experimental productivity gradient

Abstract: A major cause of the lower species richness of nitrogen enriched grasslands with high productivity is the lower colonization rate of such areas by new species. To determine what role the soil seed bank might play, we performed greenhouse germination tests of soil sampled in experimental plots that had received various rates of annual nitrogen addition for 11 yr. Species composition of the soil seed bank depended on the rate of N-addition, even though the species richness and the overall density of viable seeds in soil were independent of N-addition. The proportion of the species shared between the seed bank and the above-ground vegetation decreased with N-addition. The density of forb seedlings emerging in field plots was negatively correlated with the nitrogen input, productivity, and litter depth, but the probability of seedling survival from May through August was not different among treatments. Thus, suppression of seed germination, rather than limitation of the availability of seeds in soil or seedling survivorship, was the primary mechanism of lower colonization rates in the high-N plots. Burning in spring decreased litter depth and promoted germination of forbs, especially in no-N control plots. However, germination was still very low in the high-N plots even after fire decreased litter depth to a level similar to that of controls.

Contribution of benthic cysts to the population dynamics of Scrippsiella spp. (Dinophyceae) in Onagawa Bay, northeast Japan

Abstract: In sjtu germination rate (cells m-2 d-') and cyst deposition rate (cysts m-' d-l) were monitored for Scrippsiella spp. dinoflagellates (mostly S. trochoidea) in Onagawa Bay on the northeastern Pacific coast of Japan, using a 'germinating cell trap/sampler' and sedlment traps, respectively. Seasonal relationships of each rate to the abundance of vegetative cells in the water column were investigated. Germination of the cysts on the surface sediment occurred throughout the year, but the germination rate varied seasonally and was strongly correlated with temperatures of the bottom water and the sediment, indicating that temperature is a principal factor controlling germination. Blooms occurred prior to the increase in germination rate in July, ~nd~ca t lng that bloom initiation is not necessarily a direct consequence of mass cyst germination. Seasonal changes in recruitment ratio (ratio of the germination rate to standing crops of the vegetative cell population In the water column) revealed that, compared to summer, d large part of the winter population of vegetative cells was contributed by cyst germination but increased germination during periods of warmer temperatures contributed little to the bloom population-on the contrary, spring and summer populations appeared to be largely derived from vegetative growth. Sexual reproduction and encystment of Scrippsiella spp. in natural populations seemed to be enhanced by serial, short-term depletion of nutrients during summer. Large encystment events appeared to result in bloom termination. These findings elucidate the population dynamics of Scrippsiella spp. in Onagawa Bay. An annual budget of seed population was also calculated.