Showing papers on "Seedling published in 1997"

Effects of rodents on survival of tree seeds and seedlings invading old fields

Abstract: Tree invasion of old fields is an important process affecting landscape het- erogeneity and species diversity, as well as the quality of wildlife habitat, in eastern North America. We examined the effects of tree seed and seedling predation by small mammals on the rate, spatial pattern, and species composition of tree invasion in old fields. We performed field experiments inside U-shaped enclosures built at forest-field edges, within which we manipulated the density of meadow voles (Microtus pennsylvanicus), which are major seedling predators. Although we did not directly manipulate density of white-footed mice (Peromyscus leucopus), which are major seed predators, mouse density was high wherever vole density was low. We introduced cohorts of seeds and seedlings of red maple (Acer rubrum) or sugar maple (Acer saccharum) (as specified in the Methods for each trial), white ash (Fraxinus americana), white pine (Pinus strobus), and tree-of-heaven (Ailanthus altissima), all common early invaders of old fields in our study area. Rates of seedling predation were higher in enclosures with higher density of voles, and voles showed distinct preferences for red maple, white ash, and Ailanthus over white pine. Predation rates on small seedlings were higher than on larger seedlings. Seedlings near the forest edge were killed at a lower rate than those >5 m into the field. Predation rates on seeds were higher in low-vole-density treatments, where mice were more abundant. Mice showed distinct preferences for white pine, avoided Ailanthus, and consumed sugar maple and white ash at intermediate rates. Predation rates on seeds were higher in shrub-dominated than in herbaceous old fields. Overall density of mice increased dramatically between 1994 and 1995, and seed predation rates increased concomitantly. In 1994, seed predation was most intense 5 m and 10 m into the field, and less intense at the forest-field edge and 20 m into the field; although a similar pattern was observed in 1995, the variation was not statistically significant. We conclude that both of these common small mammal species can have pro- found effects on survival of tree propagules near forest-field edges, but that their effects are strongly dependent on density. Voles had a strong, negative direct effect on seedling survival, but a positive indirect effect on seed survival by inhibiting seed predation by mice. Voles and mice: (1) showed distinctly different food preferences; (2) had their strong- est effects at different positions with respect to the forest-field edge; and (3) had their strongest effects in different old-field types (herbaceous fields for voles, shrub-dominated fields for mice). These two species are not functionally redundant in their effects on old- field succession.

Does mammal community composition control recruitment in neotropical forests? evidence from panama

Abstract: Patterns of seed predation, germination, and seedling herbivory were investigated in Panamanian forests. We hypothesized that seed and seedling survival would vary with differences in mammal community composition. We tested this hypothesis at five sites in mainland forests adjacent to Gatun Lake, full terrestrial mammalian granivore/herbivore communities with top predators; at five sites on Barro Colorado Island (BCI), also a full mammalian granivore/herbivore community but without the two largest cats; at one site each on five medium-sized islands, with rats, agouti, rabbit, and paca present; and on five small islands that support rats only. Experiments were replicated for Dipteryx panamensis, Gustavia superba, and Virola nobilis, all of which have large seeds. To assess seed removal, seeds were placed in wire exclosure cages and nearby outside the cages. There was no difference in removal rates between forest types, with almost all unprotected seeds removed at all sites. To assess post-removal seed fate, seeds of Gustavia and Virola were attached to threads and placed on the forest floor. All threaded seeds were victims of predation on small islands, whereas 34, 43, and 77% of threaded seeds were dispersed and buried on BCI, medium islands, and the mainland, respectively. To assess seedling herbivory, half of the wire exclosure cages were removed after germination, and seedling survival was assessed after 13–14 mo. Protection from mammals increased seedling survivorship by more than sixfold on the smallest islands, by threefold on the medium islands, by twofold on the mainland, and by less than twofold on BCI. The absence of the two largest cats and the exclusion of poachers from BCI was associated with lower seedling herbivory and higher seed predation than observed on the mainland. In contrast, extreme mammal defaunation on the small and medium islands had large and consistent effects on seedling recruitment, including increased seed predation and increased seedling herbivory relative to sites with more intact mammal communities.

Smoke as a germination cue: a review

Abstract: Slow combustion or burning of dry or green plant material from many sources produces volatile compounds that are water soluble and that stimulate the germination of many seeds with different dormancy strategies. The active principals are apparently produced around 160ndash;200 deg;C and are volatilized at higher temperatures. Once dissolved in water the active compounds also stimulate rooting, seedling growth and flowering. The positive effects of smoke have resulted in it being used as a seed pre-treatment for enhancing conservation of threatened or rare species, the horticultural exploitation of desirable plants and in the reclamation of mine spoils and disturbed land. Presently the identity of the active molecules is unknown but their remarkable effect on seed germination is, already widely utilized.

Post-dispersal seed predation and the establishment of vertebrate dispersed plants in Mediterranean scrublands.

Abstract: The post-dispersal fate of seeds and fruit (diaspores) of three vertebrate-dispersed trees, Crataegus monogyna, Prunus mahaleb and Taxus baccata, was studied in the Andalusian highlands, south-eastern Spain. Exclosures were used to quantify separately the impact of vertebrates and invertebrates on seed removal in relation to diaspore density and microhabitat. The three plant species showed marked differences in the percentage of diaspores removed, ranging from only 5% for C. monogyna to 87% for T. baccata. Although chaffinches (Fringilla coelebs) fed on diaspores, rodents (Apodemus sylvaticus) were the main vertebrate removers of seed and fruit. Two species of ant (Cataglyphis velox and Aphaenogaster iberica) were the only invertebrates observed to remove diaspores. However, the impact of ants was strongly seasonal and they only removed P. mahaleb fruit to any significant extent. While removal of seed by rodents was equivalent to predation, ants were responsible for secondary dispersal. However, their role was limited to infrequent, small-scale redistribution of fruit in the vicinity of parent trees. Rodents and ants differed in their use of different microhabitats. Rodents foraged mostly beneath trees and low shrubs and avoided open areas while the reverse was true of ants. Thus, patterns of post-dispersal seed removal will be contigent on the relative abundance and distribution of ants and rodents. Studies which neglect to quantify separately the impacts of these two guilds of seed removers may fail to elucidate the mechanisms underlying patterns of post-dispersal seed removal. The coincidence of both increased seed deposition by the main avian dispersers (Turdus spp.) and increased seed predation with increasing vegetation height suggested that selection pressures other than post-dispersal seed predation shape the spatial pattern of seed dispersal. Rather than providing a means of escaping post-dispersal seed predators, dispersal appears to direct seeds to microhabitats most suitable for seedling survival. Nevertheless, the reliance of most vertebrate-dispersed trees on regeneration by seed and the absence of persistent soil seed banks imply that post-dispersal seed predators may exert a strong influence on the demography of the plants whose seeds they consume. Even where microsites are limited, the coincidence of the most suitable microhabitats for seedling establishment with those where seed predation is highest provide a means by which selective seed predators can influence community composition.

Seedling recruitment in semi-natural pastures: the effects of disturbance, seed size, phenology and seed bank

Abstract: We examined recruitment patterns in semi-natural pastures and their relationships to disturbance and species composition at different spatial scales (0.01 m2 up to 4 m2) of both the vegetation and the seed bank. Possible associations between seedling recruitment and phenology were also studied. The study was performed in four pastures with different management history. Seedling recruitment was generally enhanced by disturbance, with the greatest effect for small-seeded species that germinate in the autumn. The local species-pool contributed to a large extent to seedling recruitment; 83 % of the recruited species were found in the vegetation, whereas 44 % of the recruited species were found in the seed bank. There were a total of 88 species found in the vegetation at different spatial scales; 61 % were recorded at the smallest spatial scale (0.01 m2). The area sampled at this scale comprised 1.25% of the area examined, indicating a small scale structuring of diversity in the pastures. The species number in the vegetation varied between 7 and 14 for the smallest scale (0.01 m2), and between 23 and 42 for the largest scale (4 m2), in the four pastures. The species number in the seed bank was less variable. Like most studies of perennial grasslands, we found no close correspondence between species distribution in the vegetation and in the seed bank. Two of the pastures had a joint management history and exhibited a close similarity of species in the seed bank, despite that one of the pastures was abandoned already in the beginning of this century.

Pea mutants with reduced sensitivity to far-red light define an important role for phytochrome A in day-length detection

Abstract: In garden pea (Pisum sativum L.), a long-day plant, long photoperiods promote flowering by reducing the synthesis or transport of a graft-transmissible inhibitor of flowering. Previous physiological studies have indicated that this promotive effect is predominantly achieved through a response that requires long exposures to light and for which far-red (FR) light is the most effective. These characteristics implicate the action of phytochrome A (phyA). To investigate this matter further, we screened ethylmethane sulfonate-mutagenized pea seedlings for FR-unresponsive, potentially phyA-deficient mutants. Two allelic, recessive mutants were isolated and were designated fun1 for FR unresponsive. The fun1-1 mutant is specifically deficient in the PHYA apoprotein and has a seedling phenotype indistinguishable from wild type when grown under white light. However, fun1-1 plants grown to maturity under long photoperiods show a highly pleiotropic phenotype, with short internodes, thickened stems, delayed flowering and senescence, longer peduncles, and higher seed yield. This phenotype results in large part from an inability of fun1-1 to detect day extensions. These results establish a crucial role for phyA in the control of flowering in pea, and show that phyA mediates responses to both red and FR light. Furthermore, grafting and epistasis studies with fun1 and dne, a mutant deficient in the floral inhibitor, show that the roles of phyA in seedling deetiolation and in day-length detection are genetically separable and that the phyA-mediated promotion of flowering results from a reduction in the synthesis or transport of the floral inhibitor.

Weed seedling emergence and seed survival: separating the effects of seed position and soil modification by tillage

Abstract: Summary Tillage causes both vertical redistribution of weed seeds and changes in soil physical properties. These two factors are rarely distinguished in studies of the impact of tillage on seedling emergence or seed survival. In this study, seeds of Chenopodium album L., Amaranthus retroflexus L., and Abutilon theophrasti Medik, were planted at particular depths in pots of undisturbed or stirred soil to separate these effects. Emergence and survival data were analysed by non-linear regression to determine the nature of significant differences between treatments. Emergence increased with slight burial and then decreased exponentially at greater depths. Average emergence over all depths was generally greater in tilled sou than in unfilled soil, particularly for C. album and A. retroflexus. Seed survival approached a maximum with increasing depth. Average survival of seeds that did not produce emerged seedlings was greater in tilled soil than in untilled soil for C. album and A. retroflexus. Thus, tillage affects emergence and seed survival of weeds through changes in soil conditions independently of effects resulting from redistribution of seeds in the soil profile.

Seed Treatment with a Fungal or a Bacterial Antagonist for Reducing Corn Damping-off Caused by Species of Pythium and Fusarium.

Abstract: Bioassays were conducted under greenhouse conditions to test the efficacy of antagonists applied to corn (Zea mays) seed for protection against seed rot and seedling damping-off at 18 and 25°C in a field soil artificially infested with a combination of Pythium ultimum, P. arrhenomanes, and Fusarium graminearum. Biomass of Gliocladium virens isolates Gl-3 or Gl-21, Trichoderma viride isolate Tv-1, or peat-based slurry of Burkholderia cepacia isolates Bc-B, Bc-T, or Bc-1 was coated individually onto corn seeds in one test, and Gl-3 or Bc-B at four inoculum levels was used in another test. Seed treatments with most of the biocontrol agents, as well as with the fungicide captan, significantly (P ≤ 0.05) increased seedling stand, plant height and fresh weight, and decreased root rot severity compared with untreated seeds in pathogen-infested soil. Coating seeds with the biocontrol fungus G. virens isolate Gl-3 was the most effective treatment, resulting in greater (P ≤ 0.05) seedling stand, plant heig...

Effect of seed predation on seed bank size and seedling recruitment of bush lupine (Lupinus arboreus).

Abstract: Whether seed consumers affect plant establishment is an important unresolved question in plant population biology. Seed consumption is ubiquitous; at issue is whether seedling recruitment is limited by safe-sites or seeds. If most seeds inhabit sites unsuitable for germination, post-dispersal seed consumption primarily removes seeds that would otherwise never contribute to the population and granivory has minimal impacts on plant abundance. Alternatively, if most seeds ultimately germinate before they lose viability, there is greater potential for seed consumption to affect plant recruitment. Of the many studies on seed consumption, few ask how seed loss affects seedling recruitment for species with long-lived seed banks. We examined post-dispersal seed predation and seedling emergence in bush lupine (Lupinus arboreus), a woody leguminous shrub of coastal grasslands and dunes in California. We followed the fate of seeds in paired experimental seed plots that were either protected or exposed to rodent granivores in grassland and dune habitats. Significantly more seeds were removed by rodents in dunes than grasslands. In dunes, where rodent granivory was greatest (65% and 86% of seeds removed from plots by rodents in two successive years), there is a sparse seed bank (6.6 seeds m−2), and granivory significantly reduced seedling emergence (in the same two years, 18% and 19.4% fewer seedlings emerged from exposed versus protected plots), suggesting seed rather than safe-site limited seedling recruitment. In contrast, rodents removed an average of 6% and 56% of seeds from grassland plots during the same two years, and the grassland seed bank is 43-fold that of the dunes (288 seeds m−2). Even high seed consumption in the second year of the study only marginally influenced recruitment because seeds that escaped predation remained dormant. Burial of seeds in both habitats significantly reduced the percentage of seeds removed by rodents. Results suggest that granivores exert strong but habitat-dependent effects on lupine seed survival and seedling emergence.

Allelopathy as a competitive strategy in persistent thickets of Lantana camara L. in three Australian forest communities

Abstract: Field experiments were established to assess possible allelopathic suppression by Lantana camara L. of two indigenous tree species. The design allowed comparison of allelopathic effects with density-dependent resource competition effects. Fire and its role in competitive interactions was included as an experimental treatment. Allelopathic responses were measured in L. camara thickets by germinating and growing Alectryon subcinereus (A. Gray) Radlk. in dry rainforest ecotones (Macleay River) or Cryptocarya rigida (Meissner) in warm temperate rainforest and wet sclerophyll forest (Lake Macquarie) at 10, 20 and 30 seedlings m-2, where L. camara was either physically removed (LR), burnt (LB), or cut and left in place (LT). Germination for both species increased significantly by completely removing L. camara (LR) whereas burning (LB) was significant only for C. rigida. Seedling growth for both species was negatively related to increasing density when all L. camara was removed (LR) but was positively related in the other two treatments (LB and LT). C. rigida seedling biomass increased 47.4% (1.75%2.58 g) and 68.6% (1.98%2.95 g) with increasing seedling density for LT and LB respectively and decreased 23.2% (2.93–2.25 g) for LR. A. subcinereus seedling biomass increased 29.7% (1.95–2.53 g) and 34.7% (2.25–3.03 g) with increasing seedling density for LT and LB respectively and decreased 27.9% (3.30–2.38 g) for LR. Phytotoxin dilution effects were inferred in LT and LB rather than density-dependent intraspecific competition, whereas the reverse was true for LR. Seedling biomass for C. rigida resulting from potential phytotoxin dilution at high seedling density was not significantly different from the response of LR at low seedling density but, for A. subcinereus, the phytotoxin dilution response was significantly less than LR at low seedling density. Moderately intense fire (LB) was not significantly different from the LT treatment at both locations, emphasising that moderate to low intensity fires should not be used to control existing invasions of L. camara. Competitive strategies for invasive populations are identified that may modify succession following disturbance, thereby allowing thicket formation and long-term persistence to affect community dynamics. Such strategies need to be recognised in managing natural communities, particularly for biodiversity conservation. Nomenclature: Harden (1990).

Amino Acid Utilization in Seeds of Loblolly Pine during Germination and Early Seedling Growth (I. Arginine and Arginase Activity)

Abstract: The mobilization and utilization of the major storage proteins in loblolly pine (Pinus taeda L.) seeds following imbibition were investigated. Most of the seed protein reserves were contained within the megagametophyte. Breakdown of these proteins occurred primarily following radicle emergence and correlated with a substantial increase in the free amino acid pool in the seedling; the majority of this increase appeared to be the result of export from the megagametophyte. The megagametophyte was able to break down storage proteins and export free amino acids in the absence of the seedling. Arginine (Arg) was the most abundant amino acid among the principal storage proteins of the megagametophyte and was a major component of the free amino acid pools in both the seedling and the megagametophyte. The increase in free Arg coincided with a marked increase in arginase activity, mainly localized within the cotyledons and epicotyl of the seedling. Arginase activity was negligible in isolated seedlings. Experiments with phenylphosphorodiamidate, a urease inhibitor, supported the hypothesis that arginase participates in Arg metabolism in the seedling. The results of this study indicate that Arg could play an important role in the nutrition of loblolly pine during early seedling growth.

Interference mechanisms in conifer-Ericaceae-feathermoss communities

Abstract: In northern boreal forests ground level species occur which may have the capacity to suppress tree seedling regeneration substantially, particularly in late successional stages. We investigated, through an experiment involving sequential manipulations, the effects of three biotic components operating at the ground layer, i.e. aboveground effects of the ericaceous shrub Empetrum hermaphroditum and of the feathermoss species Pleurozium schreberi and the below-ground effects of extramatrical mycorrhizal hyphae, on the seedling establishment, growth and nutrient acquisition of Pinus sylvestris during two growing seasons. Germination and seedling establishment of P. sylvestris were enhanced by reduction of the above ground components of E. hermaphroditum and P. schreberi but were not influenced by temporary disruption of mycorrhizal hyphae. Seedling growth and nitrogen content were, however, significantly increased when hyphal connections were disrupted and above ground influence of P. schreberi and E. hermaphroditum were reduced. Higher shoot:root ratios were also found in seedlings when hyphal connections were disrupted. Seedlings in treatments where P. schreberi was left intact and above ground interference by E. hermaphroditum was reduced had unusually low shoot:root ratios (1.6) after two growing seasons and significantly lower total amounts of nutrients after one growing season than were initially present in the seeds, indicative of a net nutrient drain during the early establishment phase. The experiment could not explain the mechanisms behind this unexpected nutrient drain. Possible rhizosphere interactions immobilizing nutrients and causing seedling nutrient losses are discussed. From the results of this study and other indirect evidence we hypothesize that the three biotic components we investigated could act together to immobilize nutrients and to inhibit tree seedling regeneration and growth in late successional stages of boreal forests.

Leaf litter effects on seed and seedling predation of the palm Astrocaryum murumuru and the legume tree Dipteryx micrantha in Amazonian forest

Abstract: The amount of leaf litter fall produced by different tree species in tropical forests varies in space and time. Falling litter may cover seeds and thereby enhance their survival by making their detection by seed predators more difficult. Tests were made to determine whether Astrocaryum murumuru and Dipteryx micrantha seeds survive better in microsites covered by leaf litter. Seed numbers and litter cover on the forest floor were experimentally manipulated. How natural variation in leaf litter cover and thickness affects seed and seedling survival of these two plant species was also examined. Seed survivorship was significantly higher for both plant species in microsites with leaf litter than in those with bare soil. Results from an experiment in which the litter was not disturbed showed that Astrocaryum seed survival was positively correlated with litter thickness (defined as the number of overlying dead leaves). Astrocaryum seedling survival was also significantly affected by leaf litter; more seedlings survived in shallow litter. Leaf litter had no effect on Dipteryx seedling survival. The results of the study suggest that early recruitment of both Astrocaryum and Dipteryx is influenced by the spatial distribution and amount of forest leaf litter.

Effects of altitude and seed size on germination and seedling survival of heathland plants in north Spain

Abstract: The effects of altitude and seed size on germination and seedling survival were studied in Calluna vulgaris, Erica cinerea and Erica vagans. Experiments were carried out in the laboratory over a one year period. Seeds collected from heathlands of different altitudes were divided in two size classes. They were sowed on moist filter paper inside Petri dishes which were placed in chambers at 20 °C and a photoperiod of 12 h light/12 h darkeness. The seeds of Calluna vulgaris were the first to begin germination and had the highest rate and percentage of germination. The germination of Erica vagans was moderate, while seeds of Erica cinerea germinated later and the germination was very low. Seeds of Calluna vulgaris and E. cinerea collected at the highest altitudes had the highest germination percentages. Seed size in Calluna vulgaris and E. cinerea did not affect germination. However, large seeds of Erica vagans had higher germination rates and percentages than small seeds. The large seed size of Calluna vulgaris contributes to a better survival and growth of its seedlings.

Semiochemicals mediating spacing behavior of bird cherry-oat aphid, Rhopalosiphum padi feeding on cereals

Abstract: Olfactometry using an apterous individual of Rhopalosiphum padi (L.) showed an arresting effect by volatiles from a wheat seedling and a repellent effect by volatiles from a wheat seedling infested with aphids at a high population density (ca. 9 aphids/cm2). Four compounds, 6-methyl-5-hepten-2-one, (−)- and (+)-6-methyl-5-hepten-2-ol, and 2-tridecanone, were identified by GC-MS in air entrainments from the wheat seedlings with high aphid density but not from the wheat seedlings alone. The mixture of the four compounds in the natural proportion counteracted the attractivity of the volatiles from the intact uninfested wheat seedling. The likely role of these compounds in the spacing behavior of this aphid species, when present in high densities on wheat, is discussed.

Control of seedling recruitment by land crabs in rain forest on a remote oceanic island

Abstract: We tested the hypothesis that an omnivorous land crab determines the dy- namics of seedling recruitment in rain forest on Christmas Island, Indian Ocean. The red crab (Gecarcoidea natalis) occurs at densities of -1 crab/M2 and consumes seeds, seedlings, and leaf litter. A crab-exclusion experiment below closed canopy and in light gaps showed that red crabs markedly reduced seedling recruitment. Overall, seedling emergence was 29-fold and 21-fold greater on the exclusion plots in the understory and in gaps, respectively. In control plots, mean seedling densities never exceeded 15 seedlings/25 m2 in the under- story and 3 seedlings/25 m2 in gaps. Species richness of seedlings was significantly higher in the absence of crabs, averaging >10 and 12 species per plot in the understory and gaps, respectively. On control plots, mean species richness was less than one species per plot. Two-dimensional ordination (using nonmetric multidimensional scaling) showed that red crabs had a large impact on relative species abundances. Analyses of similarity confirmed that significant differences existed between both treatments and habitats. Seed removal and seedling transplant experiments suggested that most rain forest species are rapidly eaten by red crabs. Seedlings of many of these vulnerable species (e.g., Maclura cochinchinensis, Planchonella nitida, Schefflera elliptical were abundant on exclusion plots. However, they were rare on control plots, and none persisted for >2 mo. Persistent seedlings were limited to two species (Inocarpusfagifer and Tristiropsis acutangula), both of which were resistant as seeds and seedlings to crab predation. Although red crabs are the major biotic deter- minants of seedling recruitment, this impact was not reflected in the overhead canopy. Some tree species unable to recruit in the presence of crabs were common as adults. Shifting mosaics in crab densities over space and time may offer the best explanation for recruitment of vulnerable species. Our results show that a dominant consumer has large effects on plant community structure on a remote oceanic island, consistent with the viewpoint that strong control by a single species is more likely in simple communities.

Seed dormancy in Carex canescens: regional differences and ecological consequences

Abstract: We compared the germination of seeds of Carex canescens from populations in four regions: northern and southern Sweden, and northern and southern Germany. Germination behaviour was expected to differ between regions owing to adaptations to the local climates. We compared three or four populations from each region in 31 germination experiments (different pre-treatments, different temperature regimes, light vs darkness). In a parallel experiment, we recorded seedling emergence under outdoor conditions from seeds sown in the late summer in northern Germany. Although there were differences in the level of dormancy between the 13 populations and four regions, we were unable to detect geographic patterns which could be explained in terms of differential adaptation to the local climates. Our data did not support a proposed hypothesis that populations experiencing more severe winter conditions would require a longer cold-stratification period. We compared the results from the outdoor experiment with germination data from the laboratory experiments. The latter could not be used to predict differences in the timing of seedling emergence, the time-span during which seedlings emerged, or the number of seedlings emerging. We conclude that the relevance of detected differences in dormancy level among species or populations must be interpreted with great care. First, the detection of geographical patterns requires extensive sampling, second, the ecological consequences of such differences should be established.

Effects of Endophytic Infection by Fusarium moniliforme on Corn Growth and Cellular Morphology.

Abstract: Kernels of corn, Zea mays, were inoculated with Fusarium moniliforme to analyze seedling growth and development during endophytic, symptomless infection. In planta F. moniliforme distribution and seedling growth, expressed as shoot diameter, plant height, leaf length, and dry weight, were examined weekly for 28 days after planting. Even though no visible disease symptoms developed, F. moniliforme was isolated from most segments taken from seedlings grown from inoculated, but not noninoculated, kernels from the earliest to the latest sampling. F. moniliforme did not alter the rate or percentage of kernel germination, but seedlings grown from inoculated kernels had suppressed shoot diameter, plant height, leaf length, and plant weight 7 days after planting. However, seedling growth from inoculated kernels was similar to or greater than that from noninoculated kernels at 28 days. Histological modifications in seedlings grown from inoculated kernels included accelerated lignin deposition in shoots and modified chloroplast orientation in leaves. In summary, gross morphology and histology were altered in corn seedlings during symptomless, endophytic infection by F. moniliforme.

Effects of Waterlogging on Nitrogen Accumulation and Alleviation of Waterlogging Damage by Application of Nitrogen Fertilizer and Mixtalol in Winter Rape (Brassica napus L.)

Abstract: A study on the physiological and yield effects of waterlogging and the alleviation of waterlogging damage by the application of nitrogen fertilizers and mixtalol in winter rape was conducted in experimental tanks especially designed for controlling soil moisture content. The results showed that waterlogging at the seedling and stem elongation stages causes a significant decrease in nitrogen content and rate of nitrogen accumulation. Leaf chlorophyll content, superoxide dismutase and catalase activities, and root oxidizability (capacity for root oxidation) and root exudate were also reduced by waterlogging. The experiments confirmed that the physiological function of rape plants was retarded during the time of waterlogging at the seedling stage, and its adverse effects remained. Plant height, stem width, and the number of primary branches per plant were decreased significantly by waterlogging at the seedling and stem elongation stages. Pods per plant and seeds per pod were also reduced significantly, giving a 21.3 and 12.5% decrease of seed yield from the control for treatments at the seedling and stem elongation stages, respectively. Foliar sprays of nitrogen fertilizers at the seedling stage or mixtalol at the flowering stage alleviated plant damage caused by waterlogging by retarding chlorophyll and nitrogen degradation, increasing superoxide dismutase and catalase activities and root oxidizability, and improving yield components and seed yield of waterlogged plants. Therefore, besides draining off water, alleviation of waterlogging damage may be controlled by applying nitrogen fertilizer and a suitable plant growth regulator at appropriate growth stages.

Patterns of Seed Size, Germination and Seed Viability of Tropical Tree Species in Southern India1

Abstract: Seed weight, days to germination and seed viability were observed for 99 species growing in the Western Ghats of Karnataka, India. Seed size was strongly correlated with days to germination; smaller seeds germinated faster than larger seeds. Species which flowered during the rainy season had lighter seeds than species which flowered during the dry season. It was also found that seed size and viability of seeds were related to the season of fruiting. Species which fruit during the rainy season had heavier seeds and shorter viability than species which fruit during the dry season. These flowering and fruiting patterns and varying seed sizes are argued to be adaptations to the time of dispersal, time of moisture availability in the habitat and seedling survival.

Effect of sodium chloride on germination and seedling characters of different types of rice (Oryza sativa L.)

Abstract: The rice eultivars grown in the tropies and sub-tropics can be broadly categorized as aromatic small grain, local coarse grain and modern high yielding varieties (HYV). The first two categories are traditional rice, and are generally palatable. The aromatic rice has better market value than HYV rice, yet farmers are unwilling to expand its cultivation because of the low yield potential. One possibility to expand the cultivation of traditional rice is o t find stress tolerant cultivars for growing in marginal land. The salinity tolerance of nine rice cultivars representing three from each type of aromatic small grains, local coarse grains and HYV types, was analysed at germination and early seedling stage. Seeds were placed for germination and the seedlings were allowed to grow for nine days at NaCl concentrations of 0, 50, 100, 150 and 200 mM. NaCl decreased the germination index (GI), speed of germination, seedling height and seedling dry matter weight. Seedling characteristics were decreased more than GI. In general aromatic small grain type showed more salt sensitivity than the other two types. Among the nine cultivars Shakkorkhora (aromatic type) showed the least salt tolerance in relation to germination and seedling characters while the other two cultivars of the same group showed tolerance as high as the tolerant cultivars in other types. Generalization of group salt tolerance of rice, therefore, has little value from the agronomic point of view at least at early growth stage. As the number of cultivars employed in this study was only three from each type it is suggested that more cultivars be considered for the purpose of salt tolerant improvement programme of various types of rice.

Microclimate above grass adversely affects spring growth of seedling snow gum (Eucalyptus pauciflora)

Abstract: Growth of snow gum seedlings (Eucalyptus pauciflora Sieb. ex Spreng.) was studied in response to differences in microclimate caused by differential heat exchange between seedlings, grass and bare, moist soil during winter and spring. Seedlings were planted in a pasture either directly into grassy groundcover or in circular patches of bare soil of 30, 60 or 120 cm in diameter. There were no differences in maximum air temperatures at seedling leaf height between treatments. However, minimum air temperature increased by 2 °C with increase in patch diameter from 0 to 120 cm such that seedlings surrounded by grass experienced lower minimum temperatures with more frequent and more severe frosts than seedlings growing in large patches of bare soil. These small-scale differences in minimum temperature affected both photosynthetic and growth processes. Over winter, seedlings were photoinhibited, with depression in midday Fv/F m linearly related to minimum temperatures. In spring, repeated frosts and lower minimum temperatures led to a delay in the recovery of F v /F m , a delay in bud-break, damage to elongating stems and developing leaves, lower rates of stem elongation, and ultimately a shorter growing season for seedlings in grass compared to those in bare soil patches. Thus, microclimate above grass adversely affects spring growth of juvenile Eucalyptus pauciflora and may account for much of the competitive inhibition of tree seedling growth by grass during spring.

Combined effects of heat shock, smoke and darkness on germination of Epacris stuartii Stapf., an endangered fire-prone Australian shrub

Abstract: Epacris stuartii (Epacridaceae) is an endangered heathland shrub in which seedling recruitment occurs almost exclusively after fire. Seed viability and multiple seed dormancy mechanisms were examined to explore why levels of seedling recruitment were low after some fires, despite high rates of pre-fire seed production. The individual and combined effects of heat shock, smoke derivatives and darkness on germination were tested in the laboratory using an orthogonal logit-linear model, a form of analysis ideally suited to hypotheses concerning multiple germination cues. Seed viability (56%) was found not to be limiting. After 89 days of incubation, germination was significantly enhanced by heat shock, smoke treatment and continuous darkness but there were no significant interactions. These effects were equal in magnitude and additive such that maximum germination (42% of viable seeds) was stimulated when all three treatments were combined, significantly less germination occurred in response to any two treatments combined (22-23%), any single treatment stimulated germination at levels (10-11%) significantly less than two treatments and the lowest levels of germination occurred among untreated seeds (4%). Relative to the untreated control, germination was stimulated by smoke derivatives in high concentrations but not in low concentrations. The effect of darkness diminished with time so that after 270 days of incubation darkness had no significant effect on germination, while heat and smoke still enhanced germination significantly, additively and interchangeably. More prolonged germination of seeds exposed to light on the soil surface than buried seed may spread the risk of desiccation of emerging seedlings over multiple rainfall events. The role of heat and smoke derivates suggests that fire management is a crucial tool for influencing seedling recruitment and hence the survival of E. stuartii at its only known location.

Mechanisms of expansion for an introduced species of cordgrass, Spartina densiflora, in Humboldt Bay, California

Abstract: The dominant plant in Humboldt Bay salt marshes in Spartina densiflora, a species of cordgrass apparently introduced from South America. At several salt marshes and restoration sites around Humboldt Bay, distribution of this plant has increased significantly. We investigated the relative contributions of vegetative tiller production and seed germination to the establishment and expansion of S. densiflora. Lateral spread of plants surrounded by competitors were compared to areas without competing plant species. Plants growing in areas without competitors had significantly higher rates of vegetative expansion (p<0.0001). Viable seed production, germination rates, seedling survivorship, and growth of adult plants were measured in six salinity treatments. Approximately 1,977±80 viable seeds are produced per plant (0.25–0.5 m2). The number of germinating seeds was inversely related to increases in salinity. Salinity treatments between 19‰ and 35‰ produced significantly lower germination rates than salinities of 0–18‰ (p<0.0001). Seedling survivorship was 50% at ≤4‰ and 8–14% at ≥11‰. Lateral expansion of adult, greenhouse-grown plants occurred in all salinity treatments, with modest decreases in the highest salinity treatments (p<0.05). Our findings indicate that S. densiflora expands primarily by vegetative expansion, and lateral tillers are produced by throughout the year. Spartina densiflora produces prolific amounts of seed, but recruitment in mature salt marshes may be limited by competitors and higher salinities. At restoration sites, planting of native species such as Salicornia virginica, Distichlis spicata, or Jaumea carnosa may prevent monospecific stands of S. densiflora from developing.

Leaf-cutting ants and early forest regeneration in central amazonia : effects of herbivory on tree seedling establishment

Abstract: The effects of herbivory by the leaf-cutting ant Atta laevigata Fr. Smith on the re-establishment of forest trees in an abandoned farm near Manaus, central Amazonia, were investigated. Experimental seedling transplants and observations on seedlings which emerged naturally in the study area showed that damage by leaf-cutting ants negatively affected tree seedling survival and growth. However, excluding leaf-cutting ants from experimental plots for 20 mo did not significantly increase tree seedling densities. The number of seedlings emerging varied considerably between plots and this obscured any effect the ants may have had on seedling survivorship. Taller seedlings, and seedlings attacked only once, suffered less mortality than smaller seedlings and seedlings attacked twice or more. In general, mortality was greater for those species preferred by the ants, indicating that selective herbivory by leaf-cutting ants affects tree species composition. The number of seedlings attacked by A. laevigata remained approximately constant throughout the period of this study in spite of the fact that the number available for attack increased. Thus, the chance of any individual seedling being attacked declined with time, suggesting that the effect of Atta herbivory on tree establishment is stronger during the first few years of forest regeneration.

Effects of atmospheric CO2 on longleaf pine: productivity and allocation as influenced by nitrogen and water

Abstract: Longleaf pine (Pinus palustris Mill.) seedlings were exposed to two concentrations of atmospheric CO(2) (365 or 720 micro mol mol(-1)) in combination with two N treatments (40 or 400 kg N ha(-1) year(-1)) and two irrigation treatments (target values of -0.5 or -1.5 MPa xylem pressure potential) in open-top chambers from March 1993 through November 1994. Irrigation treatments were imposed after seedling establishment (i.e., 19 weeks after planting). Seedlings were harvested at 4, 8, 12, and 20 months. Elevated CO(2) increased biomass production only in the high-N treatment, and the relative growth enhancement was greater for the root system than for the shoot system. In water-stressed trees, elevated CO(2) increased root biomass only at the final harvest. Root:shoot ratios were usually increased by both the elevated CO(2) and low-N treatments. In the elevated CO(2) treatment, water-stressed trees had a higher root:shoot ratio than well-watered trees as a result of a drought-induced increase in the proportion of plant biomass in roots. Well-watered seedlings consistently grew larger than water-stressed seedlings only in the high-N treatment. We conclude that available soil N was the controlling resource for the growth response to elevated CO(2) in this study. Although some growth enhancement was observed in water-stressed trees in the elevated CO(2) treatment, this response was contingent on available soil N.