Showing papers on "Seedling published in 1996"

Comparative ecology of seed size and dispersal

Abstract: Seed mass is correlated with a number of other plant traits, including dispersal mode, growth form and specific leaf area. Specific leaf area is the main determinant of potential relative growth rate and an indicator of the site quality to which a species is adapted. The relationships with dispersal mode and growth form have consistent form in five datasets from three continents, and each account for about 20-30% of variation in log seed mass. Thus, there is also very substantial variation within growth form and dispersal categories. Much, but not all, of the 20-30% is associated with shifted family composition between growth forms or dispersal modes. Experiments have shown that seedlings of larger-seeded species are better able to survive hazards including deep shade, drought, physical damage and the presence of competing vegetation. If there is a common mechanism under these different hazards, it seemingly must be a `reserve effect', whereby during deployment and early growth larger-seeded species hold a bigger percentage of seed reserves uncommitted to seedling structure and available to support respiration or repair damage. A reserve effect has not yet been demonstrated directly. It remains possible that different mechanisms operate under different hazards. Under a reserve effect, advantages of larger seed size should be temporary, and temporary advantage has indeed been observed with regard to seedling survival under dense shade. Although larger seed mass confers benefits on seedlings, larger seeds must necessarily be produced in smaller numbers per unit of resource allocated. Seed mass is presumed to have evolved as a compromise between these counterposed pressures. Yet there has proved to be surprisingly little difference in average seed mass between very different vegetation regions, at least in temperate climates. Rather, there is startlingly wide variation in seed mass among species growing interspersed with each other. Recent applications of game theory may be capable of accounting for this wide variation between coexisting species, but at present these models are driven by competition among seedling species (as opposed to between seedlings and adults). It remains unclear whether competition among seedlings is a decisive influence on species composition in most of the world's vegetation types.

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.

Seedling growth, allocation and leaf attributes in a wide range of woody plant species and types.

Abstract: 1 Innate mean relative growth rate (mean RGR) of seedlings is a key attribute for the performance of species in their natural habitats. This study aimed firstly at identifying easily measurable correlates of mean RGR of temperate zone woody species. Secondly, it tested the hypothesis that functional groups of woody plants could be characterized by their mean RGR and associated allocation and leaf attributes. 2 In a standardized experiment, 80 woody species from the British Isles and North Spain, ranging widely in leaf habit and life-form, were screened for seed weight and potential seedling mean relative growth rate (RGR), biomass allocation and leaf attributes. 3 Mean RGR, when based on plant weights excluding any attached thick cotyledons, was linearly and closely correlated with leaf area ratio (LAR, total leaf area/plant dry weight) and one of the two components of LAR, specific leaf area (SLA, leaf area/leaf dry weight). The other component, leaf weight fraction (leaf weight/plant dry weight), was only correlated with mean RGR when based upon true leaves, disregarding leafy cotyledons. These relationships were also demonstrated when taxonomic relatedness was accounted for. 4 The data supported the hypothesis that differentiation, as seen among functional groups of species in terms of leaf habit and life-form, corresponded with differentiation in mean RGR and other seedling attributes. For instance, deciduous species grew consistently faster than evergreens. 5 When SLA was split into its two components, specific saturated leaf area (SSLA, total leaf area/total saturated leaf weight) and leaf saturated weight/dry weight ratio (SW/DW), it was found that SSLA was consistently smaller in evergreens than in deciduous species, both for sclerophyllous and succulent leaves. Among species of the same leaf habit, variation in SLA among life-forms could be explained by variation in leaf SW/DW. 6 SSLA and leaf SW/DW, both easy to measure, together could help to categorize growth rate within the evergreen or deciduous species. This may be useful in vegetation monitoring. 7 The data may provide useful predictive tools to infer potential growth rates and nutrient conservation strategies of real vegetation from the functional attributes and composition of its functional species groups.

The effects of fire and cover on seedling establishment in a neotropical savanna

Abstract: 1 It was hypothesized that if facilitation is important for seedling establishment in savanna, then fire should reduce seedling establishment. 2 This was tested in the cerrado savanna of Brazil with a factorial experiment designed to evaluate the effects of cover and prescribed burning on seedling establishment. 3 Seeds of 12 species of trees and shrubs were sown in plots located in sites providing three densities of woody cover and four times since last burning. 4 Seedling establishment generally was greater under the crowns of trees than in open grassland, but individual species responded differently to cover. Eight of the 12 species responded favourably to cover, but a single species showed reduced establishment with increasing cover. 5 Burning had an overall negative effect on seedling establishment in the first year following burning. By the second year following burning, establishment returned to control levels. 6 An irrigation experiment with two species demonstrated that rain-free periods within the wet season had negative effects on seedling establishment. 7 A litter-removal experiment with Miconia albicans revealed that, at least for this species, litter has a negative effect upon seedling establishment in densely wooded sites and positive effects in open grassland.

"Windows of opportunity" for recruitment in long-lived clonal plants: experimental studies of seedling establishment in Vaccinium shrubs

Abstract: Two experiments were performed to analyse seedling recruitment in Vaccinium myrtillus, Vaccinium vitis-idaea, Vaccinium uliginosum, and Vaccinium oxycoccos The purpose was to examine the probability of recruitment at "windows of opportunity," ie, spatially or temporally unpredictable conditions in which seedling recruitment is possible within stands of established conspecific adults Seedling emergence in all four investigated species depended on a combination of seed and microsite (disturbance) availability In V myrtillus, which was studied in more detail than the other species, emergence and survivorship were similar on non wood substrate and on decaying wood, but seedling growth was favored on the latter substrate An experiment along an environmental gradient with all four species showed that recruitment required approximately similar conditions even though the adults of the species exhibited a clear zonation along the gradient For all species except V oxycoccos, recruitment was confined to onl

Solar Ultraviolet-B Radiation Affects Seedling Emergence, DNA Integrity, Plant Morphology, Growth Rate, and Attractiveness to Herbivore Insects in Datura ferox.

Abstract: To study functional relationships between the effects of solar ultraviolet-B radiation (UV-B) on different aspects of the physiology of a wild plant, we carried out exclusion experiments in the field with the summer annual Datura ferox L. Solar UV-B incident over Buenos Aires reduced daytime seedling emergence, inhibited stem elongation and leaf expansion, and tended to reduce biomass accumulation during early growth. However, UV-B had no effect on calculated net assimilation rate. Using a monoclonal antibody specific to the cyclobutane-pyrimidine dimer (CPD), we found that plants receiving full sunlight had more CPDs per unit of DNA than plants shielded from solar UV-B, but the positive correlation between UV-B and CPD burden tended to level off at high (near solar) UV-B levels. At our field site, Datura plants were consumed by leaf beetles (Coleoptera), and the proportion of plants attacked by insects declined with the amount of UV-B received during growth. Field experiments showed that plant exposure to solar UV-B reduced the likelihood of leaf beetle attack by one-half. Our results highlight the complexities associated with scaling plant responses to solar UV-B, because they show: (a) a lack of correspondence between UV-B effects on net assimilation rate and whole-plant growth rate, (b) nonlinear UV-B dose-response curves, and (c) UV-B effects of plant attractiveness to natural herbivores.

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).

The ecology of tropical forest tree seedlings

Abstract: Review of Some Aspects of Tropical Rain Forest Seedling Ecology with Suggestions for Further Enquiry. Photosynthetic Responses to Light in Tropical Rain Forest Tree Seedlings. Functional Morphology of Tropical Tree Seedlings. Demography of Tropical Tree Seedlings. Seedling Ecology and Tropical Forestry. Persistence in a Tropical Understory: Clonal Growth in Psychotria Horizontalis. Seedlings, Saplings, and Tree Temperaments: Potential for Agroforestry in the African Rain Forest. Cotyledon Functional Morphology, Patterns of Seed Reserve Utilization, and Regeneration Niches of Tropical Tree Seedlings. Differential Responses to Nutrients, Shade, and Drought Among Tree Seedlings of Lowland Tropical Forest in Singapore. Seedling Growth of Shorea Section Doona (Dipterocarpaceae) in Soils from Topographically Different Sites of Sinharaja Rain Forest in Sri Lanka. The Significance of Seedling Size and Growth Rate of Tropical Rain Forest Tree Seedlings for Regeneration in Canopy Openings. Seedling Demography in Undisturbed Tropical Wet Forest in Costa Rica. Rates of Mortality and Growth in Three Groups of Dipterocarp Seedlings in Sabah, Malaysia. Index.

High temperature and drought stress effects on survival of Pinus ponderosa seedlings

Abstract: We studied the effects of high temperature and drought on the survival, growth and water relations of seedlings of Pinus ponderosa (Dougl.) Lawson, one of few coniferous tree species that can successfully colonize drought-prone sites with high soil surface temperatures. Temperature profiles were measured with 0.07-mm thermocouples in a sparse ponderosa pine forest in northern Idaho. The soil surface and the adjacent 5 mm of air reached maximum temperatures exceeding 75 degrees C, well above the lethal temperature threshold for most plants. Air temperatures 50 mm above the soil surface (seedling needle height) rarely exceeded 45 degrees C. Pinus ponderosa seedlings that survived maintained basal stem temperatures as much as 15 degrees C lower than the surrounding air. The apparent threshold temperature at the seedling stem surface resulting in death was approximately 63 degrees C for less than 1 min. No correlation between seedling mortality and needle temperature was found, although some needles reached temperatures as high as 60 degrees C for periods of

The influence of vegetation, salinity, and inundation on seed banks of oligohaline coastal marshes

Abstract: Sea level rise may alter salinity and inundation regimes and create patches of open water in oligohaline coastal marshes, potentially affecting the composition and germination of seed bank species. We conducted seedling emergence experiments to: (1) examine the effects of standing vegetation on the seed banks of three oligohaline marsh communities in coastal Louisiana (dominated by Paspalum vaginatum Sw., Sagittaria lancifolia L., or Spartina patens (Ait.) Muhl., respectively); and (2) investigate the effects of salinity and inundation regime on germination of seed bank species. We also studied the effect of a temporary increase in salinity (to simulate a salt water intrusion event) on the viability of buried seeds. We found that the presence or absence of vegetation within a community affected the abundance of some species in the seed bank but had little effect on species composition. Also, the seed banks of the three communities exhibited considerable overlap in species composition and had similar species richness (10-11) and diversity (antilog Shannon-Weaver diversity index = 6.5-7.1), despite differences in vegetation type. Higher salinities and flooding reduced seedling emergence for most species; few species emerged at salinities above four parts per thousand (ppt), and only Sagittaria lancifolia and Eleocharis parvula germinated well under flooded conditions. A temporary increase in salinity did not affect species richness or seedling emergence of most species. Our results suggest that differences in vegetation may have little effect on the composition of seed banks of oligohaline marshes. However, higher salinities and greater depth and duration of inundation (anticipated as global sea level continues to rise) may decrease recruitment of seed bank species, reducing their abundance in oligohaline marsh communities.

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.

Importance of seed size for the establishment of seedlings of five deciduous broad-leaved tree species

Abstract: Effects of seed size and phenology on the establishment of five deciduous broad-leaved tree species were examined in deciduous woodland. Treatments included absence and presence of litter in the forest understory, a small gap, and a large gap. Seedling emergence of large-seeded speciesQuercus mongolica var.grosseserrata andAcer mono was not reduced by accumulation of litter in the forest understory, but was promoted in the large gap where litter was less. Seedling emergence of small-seeded species,Alnus hirsuta, Cercidiphyllum japonicum andBetula platyphylla var.japonica, was reduced by the litter in almost all of the sites. Seedlings of large-seeded species avoid shade stress phenologically by unfolding all of their large leaves in a short period before canopy closure in the forest understory. These species had little mortality after seedling emergence. In contrast, small-seeded species have a longer duration of leaf emergence, shorter leaf longevity, and rapid leaf turnover in all the sites. These seedlings attained similar height to those of the large-seeded species at the end of the second year in the large gap, but survival and height growth rate decreased after canopy closure in the forest understory. We suggest that the importance of seed size in determining seedling establishment largely depends on the relationships between seasonal changes of environmental conditions and phenological traits of seedlings, which are related to seed size.

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.

Changes in growth, leaf abscission, and biomass associated with seasonal tropospheric ozone exposures of Populus tremuloides clones and seedlings

Abstract: The effects of single-season tropospheric ozone (O3) exposures on growth, leaf abscission, and biomass of trembling aspen (Populustremuloides Michx.) rooted cuttings and seedlings were studied. Plants were grown in the Upper Peninsula of Michigan in open-top chambers with O3 exposures that ranged from 7 to 92 ppm-h. Depending on the genotype, total seasonal O3 exposure in the range of 50–92 ppm-h had negative impacts on stem, retained leaf, and root biomass accumulation and on diameter growth. Leaf abscission generally increased with increasing O3 exposure and was the principal cause of the decrease in leaf biomass of the O3-treated plants. Considerable genetic variation in O3 responses occurred, as shown by differences in sensitivities among clones and among seedlings. However, the responses to O3 of rooted cuttings and seedlings were similar when seedling means were compared with clonal means for leaf abscission, diameter growth, retained leaf biomass, and root biomass. Comparison of a single square-wav...

A Comparison of Methods to Predict Weed Seedling Populations from the Soil Seedbank

Abstract: Accurate prediction of potential weed seedling density would allow growers to implement control measures more effectively and could help avoid inappropriate and over application of preemergence herbicides. We compared three methods for handling soil samples to predict potential weed seedling emergence in plow-disk and no-tillage corn: seedling emergence from greenhouse trays, emergence from intact cores, and seed extraction following sieving. Seedbank numbers were highest for common lambsquarters followed by annual grasses and redroot pigweed, and seed numbers were higher in no-tillage than plow-disk plots. Coefficients of variation typically exceeded 60%. Density of seedling emergence from cores was more similar to field density than was emergence from trays. The percent of seeds in the seedbank that emerged was commonly more than 90% for annual grasses and usually less than 20% for common lambsquarters. All methods gave equivalent and relatively poor predictions of field population density. Spearman rank correlation between predicted and actual populations ranged from low negative values for common lambsquarters in no-tillage to 0.60 for annual grass emergence from trays in 1991. No method consistently gave highest correlations in both years and both tillage systems. Seedling emergence from intact cores, evaluated for 4 yr in plow-disk and no-tillage soybean fields, was significantly correlated (rs = 0.15 to 0.68) with emergence in the field. Pooling data from three to five neighboring sample sites increased the correlation between core and field emergence densities.

Frugivory and the fate of dispersed and non-dispersed seeds of six African tree species

Abstract: The relationship between seedling recruitment (under and away from parent trees) and the behaviour of seed dispersers and predators, was explored in a three-year study in Kibale National Park, Uganda. On the basis of 1261 hours of observation, the foraging activity of the diurnal frugivores which fed on fruit from six tree species was quantified. The fate of dispersed and non-dispersed seeds and seedlings was examined experimentally. The findings suggest that a trade-off exists between factors that promote seedling growth in areas with high seedling density and factors that promote dispersal by frugivores. For example, dispersal of Mimusops bagshawei increases both seed and seedling survival; seeds placed away from adult conspecifics had a 8% lower probability of disappearing than seeds placed under adults and seedlings away from adults had a 30% greater probability of surviving than seedlings grown under adults. In contrast, for Uvariopsis congensis, dispersed seeds had a 56% greater probability of disappearing than seeds directly under a parent tree, while the survival of dispersed and non-dispersed seedlings was similar. Non-dispersed seed and seedling disappearance were correlated with the percentage of the fruit crop removed from focal trees, suggesting that the ability to survive under an adult maybe related to other aspects of the tree's life history.

Sabal palmetto seed size: causes of variation, choices of predators, and consequences for seedlings.

Abstract: High variation in seed size, as is common among angiosperms, may be maintained in a plant species when several factors select for seed size. Variation may also result from differences among adult plants, such as nutrient and water availability or the amount of photosynthetic tissue. In a study of Sabal palmetto seed ecology I found high seed size variation both within- and among-palms, and investigated possible factors maintaining this variation. Seed size was positively correlated with the number of leaves on parent palms. Larger seeds produced more vigorous seedlings that had greater leaf length, area, and mass, and greater root mass. Caryobruchus gleditsiae (Bruchidae: Coleoptera), whose larvae develop within palm seeds, preferentially oviposited on larger seeds, which in turn produced larger beetle offspring. By choosing the largest seeds available, ovipositing beetles thus affect both the quantity and the quality of seeds available for recruitment. I conclude that because beetle predation selects against large seeds, while larger seeds promote seedling vigor, the maintenance of seed size variation may be an adaptation of S. palmetto promoting both seed escape from predators and seedling vigor.

Early post-fire regeneration of a Pinus halepensis forest on Mount Párnis, Greece

Abstract: . The post-fire regeneration of a 45-yr-old Pinus halepensis (Aleppo pine) forest, burned in July 1989, has been studied on Mount Parnis, Attiki, Greece. Four experimental plots at various slopes and exposures were established at altitudes of 400 - 450 m, and monitored for 3 yr at 3-month intervals. Early regeneration took place abundantly, through both resprouting and seed germination of mostly hard-seeded herbs and shrubs; the floristic richness was high with 80 taxa. Pine seedling emergence took place during the winter of the first post-fire year. The mean pine seedling density by the end of the recruitment period (March 1990) was 5–6 seedlings/m2. This density decreased slightly during late spring and considerably during summer. During the second post-fire year only a relatively slight decline was observed; thereafter the density was stabilized to 1 - 2 seedlings/m2. Mortality follows a negative exponential curve that levels off at ca. 20 %. Height distributions throughout the three post-fire years were all positively skewed as a result of the presence of few very tall saplings. A considerable fraction (20 %) of very short (5–15 cm) saplings were still alive 39 months after the fire; these may constitute the sapling bank. Based on the analysis of height distribution curves, it is concluded that the taller seedlings survived significantly better than the shorter ones.

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.

Dispersal of Acacia seeds by ungulates and ostriches in an African savanna

Abstract: The dispersal of African Acacia seeds in the presence and absence of large mammalian herbivores and ostriches was assessed in a savanna ecosystem in South Africa. In the absence of large herbivores, A. tortilis and A. nilotica pods were mainly dispersed in the shade, directly beneath the tree crown and seeds remained in pods for over 18 months. In the presence of large herbivores, A. tortilis, A. nilotica and A. karroo seeds were freed from pods and were dispersed into open, non-shaded habitats. Impala dispersed most A. tortilis seeds (18,900 ha−1), giraffe most A. nilotica seeds (1060 ha−1) and giraffe and kudu most A. karroo seeds (452 and 448 ha−1, respectively). Seedling survival in dung in open environments may exceed that of seedlings in soil shaded beneath the tree crown. It appears that seed dispersal by large herbivores may be advantageous to future seedling recruitment.

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.

The Dynamics of Seedling and Cotyledon Cell Development in Arabidopsis thaliana During Reserve Mobilization

Abstract: We have analyzed seedling and cotyledon cell development in Arabidopsis thaliana during reserve mobilization qualitatively and quantitatively and related development to the time scale "hours after imbibition" (HAI). Cell division and elongation occur simultaneously and are observable 18 HAI just behind the radicle apex where a zone of cell elongation is formed prior to radicle emergence. Structurally and biochemically, the transition from a storage to a photosynthetic tissue occurs 48-60 HAI, evidenced by major changes in organelle sizes and numbers; symplast and apoplast volume increases throughout germination, and plastid number and size increase substantially, with a simultaneous decrease in lipid. Vacuome organization changes drastically as it is converted from a protein store to a central, sap-filled vacuolar system; protein bodies initially swell and then fuse during hydrolysis forming a single large structure. The constant cell number in the developing cotyledons demonstrates the relative independe...

Genetic Variation for Seedling Vigor Traits in Rice

Abstract: Seedling vigor is important for stand establishment in direct-seeded rice (Oryza sativa L.) culture. Genetic variation for, heritability of, and relationships among seedling vigor traits in 27 diverse rice cultivars and two segregating populations from crosses between low- and high-vigor cultivars were studied. Growth chamber slantboard tests were conducted at 18° and 25°C. In a greenhouse test, entries were seeded directly in standing water. Field tests, conducted during 2 yr, used drill-seeding. Genotypic differences were highly significant for all growth chamber, greenhouse, and field seedling vigor traits. Group-wise, temperate japonicas and indicas were more vigorous than tropical japonica types. Italica Livorno and Black Gora were the most vigorous cultivars across traits. For shoot length in the slantboard test, temperate japonica cultivars showed lower response to higher temperature than tropical japonicas and indicas. Response of root length was pronounced in tropical japonicas. Based on correlation analyses, shoot weight, shoot length, and coleoptile length in slantboard tests best predicted greenhouse and field seedling vigor. Heritabilities of slantboard traits were estimated in two segregating populations. Heritability for shoot length, the most important trait, was high (0.80) in the cross Labelle/Black Gora and low (0.26) in the cross Labelle/Italica Livorno. The slantboard test was a reliable screening procedure for seedling vigor, and promising donors were identified for incorporation into breeding programs.

Seedling growth strategies and seed size effects in fourteen oak species native to different soil moisture habitats

Abstract: Seedling growth and morphology are thought to reflect evolutionary responses to habitat or influences of seed size. To test these hypotheses, we selected fourteen species of North American oaks differing in soil moisture habitat preference and seed size. Seedlings were grown for 1 – 2 years with abundant soil water and moderate soil nutrition in pots placed outdoors and in a common garden. Oak species native to xeric environments produced the smallest seedlings. Oaks from hydric soils had more shoot weight per unit of root weight and more height per unit of total plant weight than did mesic or xeric oaks. Essentially no differences in leaf area per unit of total plant weight were detected. Species with thinner and larger individual leaves tended to produce larger seedlings. Within species, seed size was generally unrelated to seedling growth, although results may have been complicated by uncontrolled genotypic variability. However, when species were compared, those with larger mean seed size produced larger seedlings. Root/shoot allometry, height growth and leaf thickness in the tested species may reflect evolutionary responses to soil moisture and flooding. Although seed size influenced seedling growth, no clear relationship between seed size and soil moisture habitat was found.

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.