Showing papers on "Seedling published in 2002"

Shoot-derived auxin is essential for early lateral root emergence in Arabidopsis seedlings

Abstract: Lateral root formation is profoundly affected by auxins. Here we present data which indicate that light influences the formation of indole-3-acetic acid (IAA) in germinating Arabidopsis seedlings. IAA transported from the developing leaves to the root system is detectable as a short-lived pulse in the roots and is required for the emergence of the lateral root primordia (LRP) during early seedling development. LRP emergence is inhibited by the removal of apical tissues prior to detection of the IAA pulse in the root, but this treatment has minimal effects on LRP initiation. Our results identify the first developing true leaves as the most likely source for the IAA required for the first emergence of the LRP, as removal of cotyledons has only a minor effect on LRP emergence in contrast to removal of the leaves. A basipetal IAA concentration gradient with high levels of IAA in the root tip appears to control LRP initiation, in contrast to their emergence. A significant increase in the ability of the root system to synthesize IAA is observed 10 days after germination, and this in turn is reflected in the reduced dependence of the lateral root emergence on aerial tissue-derived auxin at this stage. We propose a model for lateral root formation during early seedling development that can be divided into two phases: (i) an LRP initiation phase dependent on a root tip-localized IAA source, and (ii) an LRP emergence phase dependent on leaf-derived IAA up to 10 days after germination.

Applications of hydrothermal time to quantifying and modeling seed germination and dormancy

Abstract: Knowledge and prediction of seasonal weed seedling emergence patterns is useful in weed management programs. Seed dormancy is a major factor influencing the timing of seedling emergence, and once dormancy is broken, environmental conditions determine the rate of germination and seedling emergence. Seed dormancy is a population-based phenomenon, because individual seeds are independently sensing their environment and responding physiologically to the signals they perceive. Mathematical models based on characterizing the variation that occurs in germination times among individual seeds in a population can describe and quantify environmental and after-ripening effects on seed dormancy. In particular, the hydrothermal time model can describe and quantify the effects of temperature and water potential on seed germination. This model states that the time to germination of a given seed fraction is inversely proportional to the amount by which a given germination factor (e.g., temperature or water potential) exceeds a threshold level for that factor. The hydrothermal time model provides a robust method for understanding how environmental factors interact to result in the germination phenotype (i.e., germination pattern over time) of a seed population. In addition, other factors that influence seed dormancy and germination act by causing the water potential thresholds of the seed population to shift to higher or lower values. This relatively simple model can describe and quantify the germination behavior of seeds across a wide array of environmental conditions and dormancy states, and can be used as an input to more general models of seed germination and seedling emergence in the field.

Seed size, growth rate and gap microsite conditions as determinants of recruitment success for pioneer species

Abstract: Summary 1 We examined how seed size and seedling growth rate influence recruitment success of neotropical pioneer species. We investigated the soil seed bank, and followed seedlings from emergence and establishment to the sapling stage in artificially created gaps in secondary forest on the Barro Colorado Nature Monument, Panama. 2 To simulate microsite variation within gaps, litter addition/removal and soil disturbance treatments were applied to seedling plots in a factorial design. 3 Seedling emergence was almost three-fold higher in litter-free plots than in control plots. 4 We found a negative relationship between seed mass and seed abundance in the soil, seed mass was positively correlated with seedling emergence success from the seed bank and with seedling survival through to the end of the first dry season. 5 In the 18 months following the first dry season we observed an uncoupling of seed mass from seedling performance. Seedling mortality was now a function of maximum relative growth rate, with higher mortality of fast-growing species attributable to herbivory, notably by shoot-borers. 6 We propose that a seed size-dependent trade-off between dispersal success (selecting for large seed number), and emergence‐establishment success (selecting for large seed size) can explain the maintenance of wide variation in seed size among pioneer species. Secondarily, a trade-off between growth rate and susceptibility to herbivores acting at the post-establishment phase may contribute to observed differences in light requirements among pioneer species.

Arabidopsis Seedling Growth, Storage Lipid Mobilization, and Photosynthetic Gene Expression Are Regulated by Carbon:Nitrogen Availability

Abstract: The objective of the current work was to establish the degree to which the effects of carbon and nitrogen availability on Arabidopsis seedling growth and development are due to these nutrients acting independently or together. Growth of seedlings on low (0.1 mM) nitrogen results in a significant reduction of seedling and cotyledon size, fresh weight, chlorophyll, and anthocyanin content but a slight increase in endogenous sugars. The addition of 100 mM sucrose (Suc) to the nitrogen-depleted growth media results in a further reduction in cotyledon size and chlorophyll content and an overall increase in anthocyanins and endogenous sugars. Storage lipid breakdown is almost completely blocked in seedlings grown on low nitrogen and 100 mM Suc and is significantly inhibited when seedlings are grown on either low nitrogen or high Suc. Carbohydrate repression of photosynthetic gene expression can only be observed under low nitrogen conditions. Low (0.1 mM) nitrogen in the absence of exogenous carbohydrate results in a significant decrease in chlorophyll a/b-binding protein and ribulose bisphosphate carboxylase small subunit gene transcript levels. Thus, carbon to nitrogen ratio rather than carbohydrate status alone appears to play the predominant role in regulating various aspects of seedling growth including storage reserve mobilization and photosynthetic gene expression.

Chilling tolerance of maize, cucumber and rice seedling leaves and roots are differentially affected by salicylic acid

Abstract: Salicylic acid (SA) is one component of a complex signalling pathway that is induced by a number of biotic and abiotic stresses. Exposing seedling radicles to aqueous solutions of 0.5 mM salicylic acid for 24 h before chilling at 2.5ae Cf or 1–4 days reduced the chilling-induced increase in electrolyte leakage from maize and rice leaves, and cucumber hypocotyls, but not from their radicles. The SA treatments that induced chilling tolerance in the aerial portion of the seedlings did not

Role of dispersal in the recruitment limitation of neotropical pioneer species

Abstract: Summary 1 We examined the importance of seed dispersal in predicting the pioneer seedling composition in recent gaps in a forest plot on Barro Colorado Island (BCI), Panama. We characterize seed dispersal for 13 taxa from seed rain data collected over 13 years in 200 mesh traps, and for an additional species from germination from soil samples collected in one year. We describe seedling distribution patterns from a complete census of all seedlings of these 14 taxa present in 36 treefall gaps. 2 A maximum likelihood model, incorporating both distance to adult trees and tree size, fitted observed seed rain to traps quite well for all taxa. 3 The ability to predict seedling recruit number per gap varied greatly among taxa. For 8 of 14 taxa, regression models incorporating predicted seed rain were significantly better predictors of seedling recruitment than models in which recruitment probability was assumed constant in all sites. 4 To see if variation in local dispersal patterns determined the community composition of gaps, we examined the relative abundances of these 14 pioneer taxa in the 36 gaps. We found that taxon abundances were significantly positively correlated with abundances predicted from seed dispersal models and seed–seedling regressions for 27 out of 36 gaps. 5 Overall, we find evidence that limited seed dispersal is an important factor contributing, together with factors affecting post-dispersal recruitment success, to seedling distribution patterns in gaps.

Proteomics of Arabidopsis seed germination. A comparative study of wild-type and gibberellin-deficient seeds

Abstract: We examined the role of gibberellins (GAs) in germination of Arabidopsis seeds by a proteomic approach. For that purpose, we used two systems. The first system consisted of seeds of the GA-deficient ga1 mutant, and the second corresponded to wild-type seeds incubated in paclobutrazol, a specific GA biosynthesis inhibitor. With both systems, radicle protrusion was strictly dependent on exogenous GAs. The proteomic analysis indicated that GAs do not participate in many processes involved in germination sensu stricto (prior to radicle protrusion), as, for example, the initial mobilization of seed protein and lipid reserves. Out of 46 protein changes detected during germination sensu stricto (1 d of incubation on water), only one, corresponding to the cytoskeleton component α-2,4 tubulin, appeared to depend on the action of GAs. An increase in this protein spot was noted for the wild-type seeds but not for the ga1 seeds incubated for 1 d on water. In contrast, GAs appeared to be involved, directly or indirectly, in controlling the abundance of several proteins associated with radicle protrusion. This is the case for two isoforms of S -adenosyl-methionine (Ado-Met) synthetase, which catalyzes the formation of Ado-Met from Met and ATP. Owing to the housekeeping functions of Ado-Met, this event is presumably required for germination and seedling establishment, and might represent a major metabolic control of seedling establishment. GAs can also play a role in controlling the abundance of a β-glucosidase, which might be involved in the embryo cell wall loosening needed for cell elongation and radicle extension.

Comparative Effects of NaCl and Polyethylene Glycol on Germination, Emergence and Seedling Growth of Cowpea

Abstract: Seeds of Paceno and Cuarenteno cultivars of cowpea (Vigna unguiculata L. Walp.) were tested for salt and drought tolerance at germination, seedling emergence and early seedling growth in NaCl and PEG-8000 solutions of different osmotic potentials (0, −0.2, −0.4, −0.6, and −0.8 MPa). Daily and final germination and emergence percentage, as well as germination and seedling emergence rate, and seedling growth were recorded under controlled conditions. Results showed that germination and emergence rate were delayed by both solutions in both cultivars, with differences between cultivars among growth stages, given that cultivar Cuarenteno, showed a higher germination rate than Paceno in NaCl, but Paceno was less affected by NaCl and PEG solutions at the emergence stage. Sodium chloride had a lesser effect on both cultivars in terms of germination rate, emergence rate and the final germination and emergence percentage than did PEG-8000. This conclusively proves that the adverse effect of PEG-8000 on germination, emergence and early seedling growth was due to the osmotic effect rather than the specific ion. Seedling growth was reduced by both stresses, but NaCl usually caused less damage than PEG to cowpea seedlings, suggesting that NaCl and PEG acted through different mechanisms.

Relative toxicity of arsenite and arsenate on germination and early seedling growth of rice (oryza sativa l.)

Abstract: Elevated soil arsenic levels resulting from long-term use of arsenic contaminated ground for irrigation in Bangladesh may inhibit seed germination and seedling establishment of rice, the country's main food crop A germination study on rice seeds and a short-term toxicity experiment with different concentrations of arsenite and arsenate on rice seedlings were conducted Percent germination over control decreased significantly with increasing concentrations of arsenite and arsenate Arsenite was found to be more toxic than arsenate for rice seed germination There were varietal differences among the test varieties in response to arsenite and arsenate exposure The performance of the dry season variety Purbachi was the best among the varieties Germination of Purbachi was not inhibited at all up to 4 mg l−1 arsenite and 8 mg l−1 arsenate treatment Root tolerance index (RTI) and relative shoot height (RSH) for rice seedlings decreased with increasing concentrations of arsenite and arsenate Reduction of RTI caused by arsenate was higher than that of arsenite In general, dry season varieties have more tolerance to arsenite or arsenate than the wet season varieties

Salinity Tolerance of Phaseolus Species during Germination and Early Seedling Growth

Abstract: Salinity tolerance during germination and early seedling growth was evaluated for 24 accessions representing four wild Phaseolus species (P. angustissimus A. Gray, P. filiformis Bentham, P. leptostachyus Bentham, and P. microcarpus Mart.) and four accessions of cultivated common bean (P. vulgaris L.) at 0, 60, 120, and 180 mM NaCl. Salinity stress delayed germination in all accessions to varying degrees. Eight accessions of P. filiformis germinated fastest under high salinity (120 mM NaCl). Additional wild accessions exhibiting rapid germination at 120 mM NaCl were P. angustissimus, PI535272, P. leptostachyus, PI535336, and P. microcarpus, PI430196. Among accessions, median germination time (days to 50% germination, T50) at 120 mM NaCl was correlated positively (r2 = 0.55, P less than or equal to 0.01) with germination in the control treatments. Seeds that germinated rapidly at 60 mM NaCl also germinated rapidly at 120 mM NaCl. At 180 mM NaCl, several accessions reached 50% germination by 6 d, demonstrating high genetic potential within Phaseolus for salinity tolerance during germination. The biomass of radicles plus hypocotyls decreased with increasing salinity. Cluster analysis separated the accessions into three groups. Group I included salt sensitive accessions with late germination, high sensitivity index (ratio of median germination time at 120 mM NaCl versus control), and reduced seedling growth. Group II included salt tolerant accessions with rapid germination, high sensitivity index, and enhanced seedling growth. Group III included cultivated accessions corresponding to the Mesoamerican and Andean gene pool with rapid germination, low sensitivity index, and intermediate seedling growth. The results confirm that wild Phaseolus species, and in particular P. filiformis, represent a genetic resource for improvement of salinity tolerance in common bean.

Effect of shrubs on tree seedling establishment in an abandoned tropical pasture

Abstract: Summary 1 The objective of this study was to evaluate whether early successional shrubs facilitate, tolerate or inhibit different stages of tree establishment in abandoned tropical pasture. 2 Seed rain, seed predation, seed germination and seedling survival of tropical forest trees in pasture grass, below small ( 40 m 2 ) shrub patches, were compared in one abandoned pasture in Costa Rica over 2 years. 3 Seed rain of animal-dispersed trees was higher below both large and small shrub patches than below grass. Seed rain of wind-dispersed trees did not differ in the three patch types. 4 Predation of all animal-dispersed seeds combined and of three individual species was significantly higher below large and small shrub patches compared with below grass; predation of five species did not differ significantly in the three patch types. 5 Germination did not differ significantly in the three patch types for any of the species. 6 Seedling survival was highest below large shrub patches for three of four species. 7 Computer simulations of probabilities of seeds arriving in the pasture and surviving to the seedling stage suggest that early successional shrubs have a net facilitative effect on the early stages of forest tree seedling establishment compared with areas without shrubs in the pasture studied, although variance was high. Shrubs may facilitate, inhibit and tolerate different stages of tree seedling establishment.

The combined impacts of deep shade and drought on the growth and biomass allocation of shade-tolerant woody seedlings.

Abstract: To test whether the impact of drought on the growth and biomass allocation of first-season shade-tolerant woody seedlings in low irradiance differs from that in high irradiance, seedlings of Viburnum lantana, V. opulus, V. tinus and Hedera helix were grown in pots at two watering frequencies × three irradiances. Hypotheses in the recent literature variously predict that drought will have a stronger, weaker or equal impact on seedling relative growth rate (RGR) in deep shade relative to that in moderate shade. Experimental irradiance levels were selected in the typical range for temperate deciduous forest seedlings in either understorey or clearings: 3–4% daylight (low red: far-red shade), 3–4% daylight (neutral shade), and 30–40% daylight (neutral shade). Watering was 'frequent' (every 3–4 days) or 'infrequent' (five times during the 8-week experiment), producing soil matric potentials as low as –0.03 MPa, and –2 MPa. To prevent the interaction of irradiance and watering treatments, each seedling was grown in a 'shade tower' that was surrounded by an uncovered sward of grass (Festuca rubra), which depleted pot water at the same rate regardless of the species of seedling, or its irradiance treatment. Shading affected all species: seedlings in 3.5% daylight grew at 56–73% of their dry-mass RGR in 35% daylight. Low red: far-red shade reduced the RGR of Hedera to 68% of its value in neutral shade. Infrequent watering significantly reduced the RGR of only V. lantana and V. opulus, by approximately the same proportion across irradiance treatments. Infrequent watering did not significantly alter any species' biomass allocation across irradiance treatments. Such orthogonal impacts of deep shade and drought on seedling growth and biomass allocation indicate a large potential for niche differentiation at combinations of irradiance and water supply for species of forest seedlings, and suggest a multiplicative-effects approach for modelling seedling performance in microsites with different combinations of irradiance and water supply.

The role of the seed bank in gap regeneration in a calcareous grassland community

Abstract: We conducted a factorial field experiment in order to investigate the role of the soil seed bank in gap recolonization in a calcareous grassland, using 240 experimental gaps (10 X 10 cm). Experimental manipulations included the elimination of the seed bank (by sterilizing the soil), the elimination of short-distance seed rain by removing all flow- erheads in 2 X 2 m plots, and the prevention of lateral clonal spread by surrounding gap soil with 41-pum nylon mesh. The influence of competition on regeneration was also studied, by planting an adult individual of Filipendula vulgaris or Festuca rubra in the center of some gaps. There were 130 species of vascular plants in the established vegetation. Twenty-five species also occurred as seedlings; 51 species were recorded in the seed bank, of which only two were not present in the growing vegetation. There were on average 2362 seeds/ m2 in a 10 cm deep soil layer. Species relative abundances in the established vegetation and in the seed bank were not significantly correlated. Eighty-four vascular plant species emerged from seeds in the experimental gaps. The seed bank contributed on average 5.4 seedlings per gap, short-distance seed rain ( 0.5 m) seed rain 2.7 seedlings. Thirty-one vascular plant species arrived in experimental gaps by lateral clonal spread. Of these, only one was not recorded as a seedling in the experimental gaps. There were on average 2.7 clonally spread shoots per gap. Species with smaller seeds were present in significantly greater numbers in the seed bank than in the vegetation and were also more common colonizers of experimental gaps. There was no evidence of competition or facilitation between seed- lings, or between seedlings and adults. It was concluded that the soil seed bank has an important functional role in a perennial grassland community as a means for population maintenance for many species. Thirty-six percent of the regeneration in small gaps was due to the seed bank, and one may expect that this proportion will increase in larger gaps.

The influence of priming on germination of rice (Oryza sativa L.) seeds and seedling emergence and performance in flooded soil

Abstract: The effects of priming treatments (CaCl 2 , CaCl 2 + + NaCl (1:1.4 mol/mol), CaCl 2 + NaCl (1:1.4 mol/mol)+10, 100, or 1000 ppm GA 3 , at -1.25 MPa for 2 d and polyethylene glycol (PEG 8000) at -1.25 MPa for 3 d at 20°C) were investigated on rice seeds (Oryza sativa L. cv 'Zhenongda 454') in germination tests in the laboratory. Priming did not improve rice seed germination, but significantly increased germination energy and germination index and slightly reduced mean germination time (MGT). Seeds primed with salt solutions had a significantly higher germination index and lower MGT than those primed with PEG8000. There were no significant differences in energy of germination and germination percentage among seeds primed with salt solutions and those primed with PEG8000. The addition of GA 3 to CaCl 2 + NaCl solution did not significantly improve the rate of germination or germination percentage as compared with CaCl 2 + NaCl priming solution. In a greenhouse trial, priming improved the seedling vigour index and seedling and stand establishment in flooded soil. The addition of GA 3 to CaCl 2 + NaCl did not significantly increase either the speed of emergence or stand establishment as compared with CaCl 2 + NaCl priming solution. It is suggested that priming of rice seeds may contribute to improved seedling establishment in anaerobic soil.

Influences of established trees on mycorrhizas, nutrition, and growth of quercus rubra seedlings

Abstract: Established plants may facilitate the regeneration of closely related seedlings if they increase populations of mutualistic symbionts that would otherwise be limiting. In this study we examined the influence of ectomycorrhizal and vesicular-arbuscular mycor- rhizal (VAM) trees on Quercus rubra seedlings to determine how trees influence mycorrhizal infection, nutrient uptake, and growth of seedlings. In two related experiments, we planted Q. rubra acorns adjacent to stump sprouts of Q. montana (=Near-Quercus) and Acer rubrum (=Near-Acer), and, in the second experiment, near Quercus spp. stumps that had not re- sprouted (=Near-Dead-Quercus). Congeneric Quercus were used to prevent root grafting; using stump sprouts minimized aboveground differences between treatments by limiting canopy size. In both experiments, Near-Quercus seedlings were infected by ectomycorrhizal fungi to a significantly greater extent than Near-Acer or Near-Dead-Quercus seedlings. Near-Quercus seedlings were also infected by a different and more diverse community of ectomycorrhizal fungi. Cenococcum geophilum, the only ectomycorrhizal fungus more abundant on Near-Acer seedlings than on Near-Quercus seedlings, appears to be relatively ineffective at increasing seedling nutrient uptake. Near-Quercus seedlings had greater con- centrations and contents of N and P than other seedlings in both experiments. In the first experiment, Near-Quercus seedlings had greater growth than Near-Acer seedlings, although it was not clear if this represented beneficial influences of Q. montana, or an undetermined negative influence of A. rubrum. No significant growth responses were found in the second experiment; severe drought may have prevented the expression of growth potential. The results demonstrate indirect facilitation of seedlings by established congeneric trees through increased seedling ectomycorrhizal infection.

Osmotic and ionic effects of NaCl on germination, early seedling growth, and ion content of Atriplex halimus (Chenopodiaceae)

Abstract: The effects of salt and osmotic stresses on the germination processes in seeds of the perennial halophyte species Atriplex halimus L. were compared using iso-osmotic concentrations of NaCl and mannitol. The lowest stress intensity delayed germination, while higher doses of NaCl and mannitol reduced final germination percentages. No significant difference occurred between the effects of these solutes on germination percentages or seedling dry weights. At an external osmotic potential of -0.7 MPa, however, the water content of mannitol-treated seedlings was reduced compared to that of seedlings that developed from NaCl-exposed seeds. The K, Mg, and Pi content decreased in seedlings that developed from mannitol-treated seeds while calcium concentration was strongly reduced in those arising from NaCl-treated seeds. Inhibited seeds were able to germinate at levels similar to those of the control after rinsing in deionized water and imbibition in control conditions. Seedlings produced from NaCl pre-treated seeds had a lower Ca and a higher Na content than control seedlings. The effect of salinity on the germination phase of development is mainly due to its osmotic component, and inhibition of germination is reversible. Both salt and osmotic stresses may have an impact on the mobilization of minerals from the seeds to the young seedling, but this effect does not have any consequence on growth processes analysed on a short-term basis.

Allelopathic Effects of Parthenium hysterophorus Extracts on Seed Germination and Seedling Growth of Eragrostis tef

Abstract: The present study was conducted to investigate the allelopathic effects of Parthenium hysterophorus weed on seed germination and seedling growth of tef. Flower, stem, root and leaf aqueous extracts of Parthenium at 0, 1, 5, and 10 % concentrations were applied to determine their effect on tef seed germination and seedling growth under laboratory conditions. Increasing concentrations of aqueous extracts of Parthenium from leaf and flower inhibited seed germination and complete failure of seed germination was recorded when the extract concentration from the leaf part was 10 %. In contrast, aqueous extracts from stem and root had no effect on tef seed germination. Roots appeared more sensitive to allelopathic effect than shoots. Extracts from flower, root and stem had a stimulatory effect on shoot length at all concentration levels, as against an inhibitory effect of leaf extracts. Root extracts at low concentration (1 %) greatly promoted root length but aqueous extracts from leaf and flower inhibited root length.

Importance of methionine biosynthesis for Arabidopsis seed germination and seedling growth.

Abstract: Proteomics of Arabidopsis seeds revealed the differential accumulation during germination of two housekeeping enzymes. The first corresponded to methionine synthase that catalyses the last step in the plant methionine biosynthetic pathway. This protein was present at low level in dry mature seeds, and its level was increased strongly at 1-day imbibition, prior to radicle emergence. Its level was not increased further at 2-day imbibition, coincident with radicle emergence. However, its level in 1-day imbibed seeds strongly decreased upon subsequent drying of the imbibed seeds back to the original water content of the dry mature seeds. The second enzyme corresponded to S-adenosylmethionine synthetase that catalyses the synthesis of S-adenosylmethionine from methionine and ATP. In this case, this enzyme was detected in the form of two isozymes with different pI and Mr. Both proteins were absent in dry mature seeds and in 1-day imbibed seeds, but specifically accumulated at the moment of radicle protrusion. Arabidopsis seed germination was strongly delayed in the presence of dl-propargylglycine, a specific inhibitor of methionine synthesis. Furthermore, this compound totally inhibited seedling growth. These phenotypic effects were largely alleviated upon methionine supplementation in the germination medium. The results indicated that methionine synthase and S-adenosylmethionine synthetase are fundamental components controlling metabolism in the transition from a quiescent to a highly active state during seed germination. Moreover, the observed temporal patterns of accumulation of these proteins are consistent with an essential role of endogenous ethylene in Arabidopsis only after radicle protrusion.

Effects of disturbance on germination and seedling establishment in a coastal prairie grassland: a test of the competitive release hypothesis

Abstract: Summary 1 We evaluated the responses of native grassland sods to a variety of types of disturbance in order to assess hypotheses about the competitive effects of established vegetation on seed germination and seedling establishment. In particular, we consider whether germination is more responsive to the magnitude and duration of vegetation removal (competitive release) or to individual disturbance types (specific effects). 2 Field-collected sods of coastal tallgrass prairie were subjected to no manipulation, cutting with clippings left, cutting with clippings removed (hayed), burning, and complete destruction of established vegetation under greenhouse conditions. The emergence and fate of seedlings, as well as light penetration through the canopy, were followed for a period of 4.5 months. 3 Total seedling emergence increased from cut to control, hayed, burned and plants-removed treatments. Several periods of increased seedling emergence suggested responses to both light penetration and seasonal change. 4 Species richness was lowest in cut sods and highest in sods that had plants removed or were burned. Rarefaction analysis showed that these differences were largely those expected from differences in seedling number, except for the cut treatment, which produced fewer species per seedling than other treatments. 5 Indicator species analysis and ordination methods revealed that seedling community composition overlapped strongly across all treatments, although the area of ordination space did increase with increasing numbers of seedlings. 6 Overall, most of the effects of disturbance could be explained by cumulative light penetration to the soil surface, an indicator of total competitive release, although a few specific effects could be found (particularly for the cutting treatment). Thus, these results generally support the competitive release hypothesis.

Fire severity and seedling establishment in Pinus halepensis woodlands, eastern Iberian Peninsula

Abstract: Given the observed heterogeneity in fire severity produced within wildfires, we asked to what extent this heterogeneity might affect post-fire regeneration. For this purpose, we studied the post-fire dynamics of Pinus halepensis (Aleppo pine) in the eastern Iberian Peninsula. Sampling was stratified on the basis of fire severity. We defined three fire severity classes based on the degree of consumption of the pine canopy. The results suggested that there is no clear relationship between seedling density and fire severity; however, mortality was lower and growth (height, shoot biomass and root biomass) was higher in the high severity class. These results can be explained by soil processes: Sites in the high fire severity class may have sustained higher fire intensities, resulting in higher soil organic matter mineralisation and higher ash deposition, and thus in higher post-fire soil fertility. This higher fertility would produce faster growth in pine seedlings. Independent of the severity class, seedling mortality was higher in quadrats (50 × 50 cm) with higher cover of the perennial grass Brachypodium retusum (Poaceae), suggesting a possible competitive effect. For all plots in all 3 severity classes, spatial analysis suggests an aggregate seedlings pattern, but with independence from the position of the adult (source) trees.

Changes in activities of antioxidant enzymes and lipoxygenase during growth of sunflower seedlings from seeds of different vigour

Abstract: The aim of this study was to investigate whether there was a relationship between growth of sunflower seedlings at 15°C in the dark and activities of enzymes involved in scavenging of reactive oxygen species (ROS), especially superoxide dismutase (SOD), catalase (CAT) and glutathione reductase (GR), or in production of free radicals, namely lipoxygenase (LOX). Untreated control seeds were compared with seeds exposed to accelerated ageing (5 d at 45°C and 100% relative humidity), osmopriming (7 d at 15°C with a polyethylene glycol (PEG) solution at –2 MPa) and accelerated ageing followed by priming. Accelerated ageing decreased seed germinability and slowed down hypocotyl growth, whereas priming resulted in an increase in germination rate and enhanced seedling development. Osmopriming of aged seeds almost completely restored the initial rate of germination and seedling growth. The activity of all the enzymes studied increased during seed germination and seedling development, except that of SOD. Seed imbibition or radicle protrusion were related mainly with an increase in CAT activity and, to a lesser extent, in GR activity. Increase of LOX activity was clearly associated with the onset of hypocotyl elongation. However, in all cases, malondialdehyde measurements did not reveal intense lipid peroxidation. Priming induced a marked stimulation of CAT and GR during seed imbibition or very early during seedling development, as compared to the control seedlings and particularly to the seedlings generated by aged seeds. Hydrogen peroxide (H2O2) contents of seeds and seedlings were closely correlated to the activities of CAT and GR and to the kinetics of seedling development. The results obtained establish a clear relationship between sunflower seed vigour and ROS scavenging.

Cotton Seedling Preemergence Damping-Off Incited by Rhizopus oryzae and Pythium spp. and Its Biological Control with Trichoderma spp.

Abstract: Planting the cotton cv. Sure-Grow 747 in cotton seedling disease plots during the 2001 growing season resulted in high levels of preemergence damping-off among the seedlings. Four cotton pathogens, Pythium aphanidermatum, P. ultimum, an unidentified Pythium sp., and Rhizopus oryzae, were isolated from diseased seed embryos and seedlings. Disease incited by the Pythium spp. could be controlled by seed treatment with Metalaxyl, but disease incited by R. oryzae could not. Seed treatment with Metalaxyl in naturally infested field soil was only partially effective; therefore, symptoms in 47% of the diseased seedlings could be attributed to R. oryzae. Susceptibility to disease appeared to be related to release in the spermosphere, by the germinating seeds, of compounds that stimulate pathogen propagule germination, because exudates from seed of the suscept Sure-Grow 747 and extracts from wheat bran induced pathogen germination and growth, whereas exudates from resistant cv. Stoneville 213 did not. However, even Stoneville 213 became susceptible when infested soil was amended with wheat bran. Seed treatment with preparations of Trichoderma virens parent, mutant, and hybrid strains gave effective biological control of preemergence damping-off. Disease control was attributable to metabolism by the biocontrol agent of pathogen germination stimulants released by the seed, because amendment of pathogen-infested soil with the propagule germination stimulants in wheat bran negated the protective effect of the seed treatment.

Effect of osmo- and hydropriming of chickpea seeds on seedling growth and carbohydrate metabolism under water deficit stress

Abstract: Seven-day-old seedlings obtained from seeds primed with mannitol (4%)and water showed three to four fold more growth with respect to root and shootlength in comparison with seedlings obtained from non-primed seeds. Seedlingswere grown under water deficit stress conditions created by 15% polyethyleneglycol (PEG) 6000 in the medium. Priming of chickpea seeds with NaCl and PEGwasnot effective in increasing seedling growth under these water deficit stressconditions. The activities of amylase, invertases (acid and alkaline), sucrosesynthase (SS) and sucrose phosphate synthase (SPS) were higher in shoots ofprimed seedlings. An increase in the activities of SS, and both the acid andalkaline invertases was also observed in roots of primed seedlings. The twofoldincrease in specific activity of sucrose phosphate synthase was observed incotyledons of primed seedlings. The higher amylase activity in shoots of primedseedlings enhanced the rapid hydrolysis of transitory starch of the shootleading to more availability of glucose for shoot growth and this was confirmedby the low level of starch in shoots of primed seedlings.

Differences in seed germination responses may promote coexistence of four sympatric Piper species

Abstract: Summary 1 Species-specific responses to the range of microsites resulting from canopy gap formation may contribute to coexistence in tropical forests. We investigated the effects of four factors affected by canopy gap formation (red : far-red light, soil nitrate concentrations, soil temperature fluctuations and soil water potential) on the germination response of four pioneer Piper species (P. dilatatum, P. hispidum, P. marginatum and P. peltatum) that are typically found in canopy gaps and clearings. 2 All four Piper species required light for maximum germination. However, the ratio of red : far-red light (R : FR) resulting in maximum germination varied between species. Piper peltatum will germinate in simulated understorey light conditions; P. dilatatum and P. hispidum require conditions typically found in small to medium gaps; while P. marginatum requires the conditions appropriate to large gaps. 3 Only P. marginatum was affected by nitrate concentration: elevated concentrations increased the germination rate. This suggests that this species could detect canopy gaps using a combination of high R : FR and elevated soil nitrate concentrations. 4 The germination rate of P. marginatum was least sensitive to low water potentials and high daytime temperatures characteristic of large gaps. Piper peltatum was most sensitive to these treatments, while P. dilatatum and P. hispidum were intermediate in response. 5 A principal components analysis of the ratios of germination in understorey to large gap conditions, for four variables, generated a significant axis that explained 88·5% of the variance in germination response between species. Differential species distribution along this axis, based on species-specific responses, may allow germination to occur in the most suitable microsite for onward growth of the seedling and contribute to species coexistence by reducing interspecific competition.

Rehabilitation of Degraded Areas of Central Amazonia Using Direct Sowing of Forest Tree Seeds

Abstract: Deforestation in the Amazon Basin is still increasing, and the rehabilitation of these lands continues to be a challenge. Autoecological studies of most Amazonian species are rare, and efficient techniques for restoration of forested habitats have yet to be developed. The aim of this study was to test direct sowing as a rehabilitation technique for sites with different degrees of disturbance: bare soil, pasture, and secondary and mature forests in Central Amazonia, Brazil. At each site, we sowed seeds of 11 native tree species. Throughout the following year we evaluated germination and seedling survival. The germination differed according to the study site and species. Seedling survival in degraded sites was higher than in other areas. After 1 year in the bare soil site, 33% of the sown seeds of eight species developed seedlings; in the pasture the establishment was 23%, in secondary forest 15%, and in mature forest 12% of only four species. The only widespread survivor with more than 45% emergence in all perturbed sites was Caryocar villosum. No pioneer seedlings remained after 1 year. There was a positive correlation between seed size and survival. Large-seeded non-pioneer species seem to be more suitable for direct sowing than small-seeded species. We recommend a combination of direct sowing and planting of seedlings as an appropriate means to accelerate the rehabilitation of degraded areas in Central Amazonia.

Life history of Microstegium vimineum (Poaceae), an invasive grass in southern Illinois.

Abstract: limited to one small area at the edge of the core population. At Dixon Springs, the germinable seed bank was estimated from soil samples collected in Fall 1999 before flowering, and in the subsequent spring. Seedlings emerged from soils placed in a greenhouse from samples collected in the spring. No seedlings emerged from samples collected in the fall. Germinable seed had a 10 times higher density in samples collected near the soil surface than from samples collected at 5-10 cm depth. Seed dispersal occurred within the main population only. Despite the lack of seed dispersal in 1999, seedlings emerged from six of seven subpopulations in spring 2000, with the highest densities where plants had flowered in 1999. These data indicate the presence of a persistent soil seed bank for M. vimineum. Even following summer drought and discounting spring seedling emergence, the size of the seed bank increased by -25% in 1999. Reestablishment of the Dixon Springs population following the 1999 drought was vigorous, but the location of the largest, most fecund plants in 2000 was not the same as in 1999. In 2000, end of season survivorship of M. vimineum among all populations was 40-50%, with 90% of surviving individuals flowering. Performance of M. vimineum varied significantly within and among populations and was related to soil texture and chemistry, and overhead canopy cover. The findings of our study reflect the invasive nature of this plant. A sufficiently large seed bank and a highly plastic morphological response to local microhabitat conditions likely ensures its persistence in invaded sites.

Comparison of different invigoration techniques in wheat (Triticum aestivum L.) seeds

Abstract: The experiment was conducted to investigate the effects of pre-sowing treatments on germination and vigour of wheat (Triticum aestivum L.) cv. Auqab-2000 seeds. Seeds were subjected to hydropriming for 2, 12, 24 or 48 h, osmoconditioning with -1.1 MPa polyethylene glycol (PEG-10,000) for 24 or 48 h, salt priming with -1 .1 MPa KNO3 solution for 24 or 48 h and chilling stress at þ 20 degree centigrade for 12 or 24 h. Hydropriming treatment for 48 h showed maximum invigoration than all other treatments including control with increased germination and seedling root shoot weight and lesser mean germination time (MGT) and time to 50% germination (T50). PEG treated seeds completely lost their germination ability: KNO3 treatment decreased the seed vigour by reducing the germination percentage and increasing T50 and MGT.

Synchronized annual seed production by 16 principal tree species in a temperate deciduous forest, japan

Abstract: To investigate synchronized annual fluctuation of seed production and its advantage for regeneration at the community level, for nine years (1987–1995) we monitored the flowering, seed production, and seedling emergence of the 16 principal tree species in a temperate deciduous forest, Ogawa Forest Reserve, in central Japan. We found that the species with higher synchronized flowering within a population had larger fluctuation of annual seed production at the population level. The coefficient of concordance of flowering and the coefficient of variation of annual seed production were continuously distributed among species, making it difficult to distinguish masting from nonmasting species. The annual seed production patterns of the 16 species were classified, by cluster analysis, into groups that synchronize their fluctuation of annual seed production. This analysis showed a highly synchronized annual seed production, not only among congeneric species, but also among species of different families. Although our results have some insufficiency of statistical significance, they did show that predator satiation, both in a population and a guild, effectively operated for many species to enhance seed survival at the pre-dispersal stage. They also showed that pollination efficiency was likely to be operating at the population level for half of the wind-pollinating species. However, generalist predator satiation at the postdispersal seed stage may not operate in a simple, detectable manner in this species-rich forest community. It is highly probable that there are combined effects of several factors: limited weather triggers for flowering, common flowering physiology among taxonomically related species, and the ecological advantages at the population and guild levels, may cause multiple species to have synchronized fluctuation patterns of seed production.

Azospiriflum Inoculation Mitigates Water Stress Effects in Maize Seedlings

Abstract: Water stress in maize (Zea mays, L) after planting slows down germination and initial seedling growth. This study determined that seed inoculation with Azospirillum brasilense BR11005 could mitigate negative water stress effects in seedlings. Pots containing a mixture of sand and vermiculite were divided in three lots and irrigated with a nutrient solution, to 200 g moist kg - 1 bed dry weight (HM), 100 g kg - 1 bed dry weight (MM), or to 50 g kg - 1 bed dry weight (LM), respectively. The first lot corresponded to well irrigated pots and the other two, to 50% and 75% water supply reduction, respectively. Hybrid DK 636 maize seeds were treated during imbibition with either 10 7 live or autoclaved (control) A. brasilense cells seed - 1 , pre-germinated in a humid chamber, and planted in the pots. All seedlings were then grown in a greenhouse at their corresponding water regimes, for up to 15 d after planting. Total aerial biomass (TAB) and foliar area (FA) was determined in shoots. Relative water content (RWC), water content (WC), and water potential (Ψ) were determined in leaves. Free proline concentration was determined in both leaves and roots. In addition, surface and dry weight (DW) were determined in roots. Seed inoculation with Azospirillum improved both leaf RWC and WC, prevented a significant Ψ drop in leaves, increased root growth, TAB and FA, and promoted proline accumulation in both leaves and roots, in seedlings grown for 15 d at 75% water supply reduction.

Seedling root anatomy and morphology: an examination of ecological differentiation with rainfall using phylogenetically independent contrasts.

Abstract: We examined patterns of seedling root architecture, morphology and anatomy in Australian perennial plants chosen as phylogenetically independent contrasts (PICs) for rainfall in the areas they inhabit. Our objective was to assess whether there are consistent evolutionary patterns in structure of seedling root systems in species from different rainfall environments when examined across multiple evolutionary lineages. Seedlings were grown to a standardised developmental stage under controlled conditions. We found that seedling root systems of species restricted to low rainfall environments are characterised by greater proportional allocation to main root axis and have proportionally smaller main root axis diameter and areas of stele and xylem. Species of low rainfall environments also had higher specific root length (SRL) of the main axis, but lower SRL when the entire root system was considered. Seedling root system elongation rates were higher in species of high rainfall relative to those of low rainfall environments, paralleling expected differences in relative growth rate. The higher root system elongation rates in species of high rainfall environments were associated with greater numbers of growing tips in the root system, but not with differences in elongation rates of individual tips, relative to species of low rainfall environments.