Showing papers on "Seedling published in 2001"

A postgermination developmental arrest checkpoint is mediated by abscisic acid and requires the ABI5 transcription factor in Arabidopsis

Abstract: Seed dormancy is a trait of considerable adaptive significance because it maximizes seedling survival by preventing premature germination under unfavorable conditions. Understanding how seeds break dormancy and initiate growth is also of great agricultural and biotechnological interest. Abscisic acid (ABA) plays primary regulatory roles in the initiation and maintenance of seed dormancy. Here we report that the basic leucine zipper transcription factor ABI5 confers an enhanced response to exogenous ABA during germination, and seedling establishment, as well as subsequent vegetative growth. These responses correlate with total ABI5 levels. We show that ABI5 expression defines a narrow developmental window following germination, during which plants monitor the environmental osmotic status before initiating vegetative growth. ABI5 is necessary to maintain germinated embryos in a quiescent state thereby protecting plants from drought. As expected for a key player in ABA-triggered processes, ABI5 protein accumulation, phosphorylation, stability, and activity are highly regulated by ABA during germination and early seedling growth.

Proteomic Analysis of Arabidopsis Seed Germination and Priming

Abstract: To better understand seed germination, a complex developmental process, we developed a proteome analysis of the model plant Arabidopsis for which complete genome sequence is now available. Among about 1,300 total seed proteins resolved in two-dimensional gels, changes in the abundance (up- and down-regulation) of 74 proteins were observed during germination sensu stricto (i.e. prior to radicle emergence) and the radicle protrusion step. This approach was also used to analyze protein changes occurring during industrial seed pretreatments such as priming that accelerate seed germination and improve seedling uniformity. Several proteins were identified by matrix-assisted laser-desorption ionization time of flight mass spectrometry. Some of them had previously been shown to play a role during germination and/or priming in several plant species, a finding that underlines the usefulness of using Arabidopsis as a model system for molecular analysis of seed quality. Furthermore, the present study, carried out at the protein level, validates previous results obtained at the level of gene expression (e.g. from quantitation of differentially expressed mRNAs or analyses of promoter/reporter constructs). Finally, this approach revealed new proteins associated with the different phases of seed germination and priming. Some of them are involved either in the imbibition process of the seeds (such as an actin isoform or a WD-40 repeat protein) or in the seed dehydration process (e.g. cytosolic glyceraldehyde-3-phosphate dehydrogenase). These facts highlight the power of proteomics to unravel specific features of complex developmental processes such as germination and to detect protein markers that can be used to characterize seed vigor of commercial seed lots and to develop and monitor priming treatments.

Proteomic Analysis of Arabidopsis Seed Germination and Priming 1

Abstract: To better understand seed germination, a complex developmental process, we developed a proteome analysis of the model plant Arabidopsis for which complete genome sequence is now available. Among about 1,300 total seed proteins resolved in two-dimensional gels, changes in the abundance (up- and down-regulation) of 74 proteins were observed during germination sensu stricto (i.e. prior to radicle emergence) and the radicle protrusion step. This approach was also used to analyze protein changes occurring during industrial seed pretreatments such as priming that accelerate seed germination and improve seedling uniformity. Several proteins were identified by matrix-assisted laser-desorption ionization time of flight mass spectrometry. Some of them had previously been shown to play a role during germination and/or priming in several plant species, a finding that underlines the usefulness of using Arabidopsis as a model system for molecular analysis of seed quality. Furthermore, the present study, carried out at the protein level, validates previous results obtained at the level of gene expression (e.g. from quantitation of differentially expressed mRNAs or analyses of promoter/reporter constructs). Finally, this approach revealed new proteins associated with the different phases of seed germination and priming. Some of them are involved either in the imbibition process of the seeds (such as an actin isoform or a WD-40 repeat protein) or in the seed dehydration process (e.g. cytosolic glyceraldehyde-3-phosphate dehydrogenase). These facts highlight the power of proteomics to unravel specific features of complex developmental processes such as germination and to detect protein markers that can be used to characterize seed vigor of commercial seed lots and to develop and monitor priming treatments.

Characterization of Root Exudates at Different Growth Stages of Ten Rice (Oryza sativa L.) Cultivars

Abstract: Plant root exudates play important roles in the rhizosphere. We tested three media (nutrient solution, deionized water and CaSO4 solution) for three periods of time (2, 4 and 6 h) for collecting root exudates of soil-grown rice plants. Nutrient culture solution created complications in the analyses of exudates for total organic C (TOC) by the wet digestion method and of organic acids by HPLC due to the interference by its components. Deionized water excluded such interference in analytical analyses but affected the turgor of root cells; roots of four widely different rice cultivars excreted 20 to 60 % more TOC in deionized water than in 0.01 M CaSO4. Furthermore, the proportion of carbohydrates in TOC was also enhanced. Calcium sulfate solution maintained the osmotic environment for root cells and did not interfere in analytical procedures. Collection for 2 h avoided under-estimation of TOC and its components exuded by rice roots, which occurred during prolonged exposure. By placing plants in 0.01 M CaSO4 for 2 h, root exudates of soil-grown traditional, tall rice cultivars (Dular, B40 and Intan), high-yielding dwarf cultivars (IR72, IR52, IR64 and PSBRc 20), new plant type cultivars (IR65598 and IR65600) and a hybrid (Magat) were collected at seedling, panicle initiation, flowering and maturity and characterized for TOC and organic acids. The exudation rates were, in general, lowest at seedling stage, increased until flowering but decreased at maturity. Among organic acids, malic acid showed the highest concentration followed by tartaric, succinic, citric and lactic acids. With advancing plant growth, exudation of organic acids substituted exudation of sugars. Root and shoot biomass were positively correlated with carbon exudation suggesting that it is driven by plant biomass. As root exudates provide substrates for methanogenesis in rice fields, large variations in root exudation by cultivars and at different growth stages could greatly influence CH4 emissions. Therefore, the use of high-yielding cultivars with lowest root excretions, for example IR65598 and IR65600, would mediate low exudate-induced CH4 production. The screening of exciting rice cultivars and breeding of new cultivars with low exudation rates could offer an important option for mitigation of CH4 emission from rice agriculture to the atmosphere.

Ecology of seed and seedling growth for conservation and restoration of tropical dry forest : a review

Abstract: Dry forests are among the most threatened ecosystems and have been extensively converted into grasslands, secondary forest, savanna or agricultural land. Knowledge of seed germination and seedling establishment is required for the success of efforts on restoration of these forests. This review focuses on the ecological requirements at seed and seedling stages, and collates the current knowledge of seed viability, dormancy, germination pattern and seedling behaviour of dry tropical tree species. The spatio-temporal variations within the tropical dry forest biome in soil moisture, light, temperature, nutrients and intensity of predation, significantly affect the seed and seedling traits of component species. The majority of dry tropical species possess orthodox seeds which are characterized by dormancy, while a few have recalcitrant seeds which possess little or no dormancy. Seed coat dormancy, which can be overcome by mechanical or acid scarification or sometimes by transit through animal guts, is most prevalent in the dry tropical forest species. Persistent species dominating the undisturbed portions of the forest have bigger seeds compared to those that mostly occur in disturbed regions and require shade for the survival of their seedlings. Shade demand is associated with drought endurance, and may be absolute in species such as Guettarda parviflora and Coccoloba microstachya, or facultative as in Plumeria alba and Bursera simaruba. The fluctuation in temperature significantly affects seed germination in several species of dry Afromontane forest trees of Ethiopia. Seedling mortality is primarily a function of moisture stress during the dry period. Adaptive responses of seedlings to drought stress include increased chlorophyll content, for example in Acacia catechu, and root biomass, as in several dry forest species (for example Drypetes parvifolia, Teclia verdoornia) of Ghana. Mulching, application of fertilizers, interplanting of leguminous species and mycorrhizal inoculation are useful tools for promoting seedling establishment in nutrient-poor dry tropical soils. Periodic forest fires, and predation affect recruitment and seedling development according to their intensity. Many species experiencing frequent fires have evolved thick seed coats, produce fire-hardy seedlings, or escape the effect by temporal separation of seed dispersal and fire events. Predation may result in abortion of fruits or may enhance germination and recruitment by scarification and dispersal, as in most species of the Guanacaste dry forest. Exposure to elevated CO2 has increased relative growth rate, total leaf area and water use efficiency in most of the dry tropical seedlings tested, but the magnitude of the effect has varied markedly among species. Due to the availability of a large source of energy, large seeds show higher germination percentage, greater seedling survival and increased growth. Seeds originating from different provenances exhibit differences in germination and seedling growth (for example Prosopis cineraria, Albizia lebbeck, Eucalyptus camaldulensis and Acacia mangium), efficiency of nodulation (for example Acacia nilotica, A. auriculiformis), and stress resistance (for example Populus deltoides, Dalbergia sissoo). The review points out the need for coordinated, long-term, field-based studies for identification of multiple cues and niches for germination, on seed and seedling dynamics in response to fire, and on within-species genetic variability for selection of suitable provenances. Field-based studies at species and community levels are also needed to permit manipulations of biotic components to augment the recruitment of desired species and to suppress that of undesirable species.

Stored proteinases and the initiation of storage protein mobilization in seeds during germination and seedling growth

Abstract: Though endopeptidases and carboxypeptidases are present in protein bodies of dry quiescent seeds the function of these proteases during germination is still a matter of debate. In some plants it was demonstrated that endopeptidases of dry protein bodies degrade storage proteins of these organelles. Other studies describe cases where this did not happen. The role that stored proteinases play in the initiation of storage protein breakdown in germinating seeds thus remains unclear. Numerous reviews state that the initiation of reserve protein mobilization is attributed to de novo formed endopeptidases which together with stored carboxypeptidases degrade the bulk of proteins in storage organs and tissues after seeds have germinated. The evidence that the small amounts of endopeptidases in protein bodies of embryonic axes and cotyledons of dry seeds from dicotyledonous plants play an important role in the initiation of storage protein mobilization during early germination is summarized here.

Gaps in a gappy forest: plant resources, longleaf pine regeneration, and understory response to tree removal in longleaf pine savannas

Abstract: Resource availability and planted longleaf pine (Pinus palustris Mill.) seedling and understory vegetation re- sponse within and among three sizes of experimentally created canopy gaps (0.11, 0.41, 1.63 ha) in a mature longleaf pine savanna were investigated for 2 years. Longleaf pine seedlings and understory vegetation showed increased growth in gaps created by tree removal. Longleaf pine seedling growth within gaps was maximized approximately 18 m from the uncut savanna. Increased longleaf pine seedling survival under the uncut savanna canopy observed after the first year suggests that the overstory may facilitate establishment of longleaf pine seedlings rather than reduce survival through competition. Despite the relative openness of the uncut longleaf pine forest, light quantity was increased by tree removal. Light was also the resource most strongly correlated with seedling and understory vegetation growth. Al- though net N mineralization was correlated to seedling response, the amount of variation explained was low relative to light. Belowground (root) gaps were not strong, in part because of non-pine understory roots increasing in biomass fol - lowing tree removal. These results suggest that regeneration of longleaf pine may be maximized within gap sizes as small as approximately 0.10 ha, due largely to increases in light availability.

Effect of brassinosteroids on salinity stress induced inhibition of seed germination and seedling growth of rice (Oryza sativa L.)

Abstract: The effect of 24-epibrassinolide and 28-homobrassinolide on the inhibitionof germination and seedling growth of rice (Oryza sativa) induced bysalinity stress was studied. Brassinosteroids were found to reverse theinhibitory effect on germination and seedling growth. The activation ofseedling growth by brassinosteroids under salinity stress was associatedwith enhanced levels of nucleic acids and soluble proteins.

The Effect of Distance from Forest Edge on Seed Rain and Soil Seed Bank in a Tropical Pasture1

Abstract: Although lack of seeds can limit forest recovery in abandoned agricultural lands, few studies have documented the patterns of seed rain and soil seed bank in active pastures. To determine if lack of seeds is a limiting factor, we studied the woody species composition of the seed rain and soil seed bank in an active pasture in Barranquitas, Puerto Rico. In addition, fruit production of 43 common shrub and tree species was monitored in the surrounding secondary forest. Seed rain was monitored for one year at −2, 0, 2, 4, 8, 16, and 32 m from the forest edge along six transects. The soil seed bank was sampled in January/February 1995 and October/November 1995 at the same distances from the forest edge along ten transects. Of the 35 species that produced fruits in the forest, 14 species were detected in the seed rain study and only 0.3 percent of the seeds and 3 species dispersed to more than 4 m from the forest edge. Two of the three species were dispersed by wind. The seed bank study showed a similar pattern with a dramatic decrease in seedling density and species richness with distance from the forest edge. More than 50 percent of the seedlings in the seed bank study were Trema lamarckianum. This study demonstrated that few seeds disperse into the pastures and even when a rare dispersal event occurs, species do not accumulate because of short-term seed viability and, possibly, high seed predation. Studies of early secondary forest regeneration have shown that in some cases, forest can recover rapidly in abandoned agricultural lands; but our results suggest that only a small subset of the forest species will contribute to the initial recovery process.

Passage through bird guts causes interspecific differences in seed germination characteristics

Abstract: Summary 1 Seed germination characteristics are often modified after seeds are ingested by frugivores. Factors that are intrinsic either to the plant or to the frugivore’s digestive tract are responsible for the great variation observed in germination response. 2 Our objectives were to determine whether and how the seed germination patterns of five common western Mediterranean plant species are affected by seed passage through the guts of their major dispersers, and to elucidate the mechanism by which such patterns are changed. 3 We used captive birds (Turdus merula and Sylvia melanocephala) to obtain ingested seeds and compared their germination rate (speed) and germinability (final percent germination) with those of controls (uningested, pulp-removed seeds), controlling for seed age, size and source. Germination was monitored for 2 years in an experimental garden. We evaluated the possible changes in seed traits after ingestion by measuring weight and coat thickness, and by observing seed coat sculpture. 4 Rate of seed germination, but not germinability, changed in all species after gut treatment. The greatest effect was in Osyris, in which germination was much enhanced. A great acceleration of germination, which is likely to translate into a seedling size advantage, was also found in Asparagus. In the other three species tested, germination was slower for ingested than for control seeds. 5 For Rubus and Rubia seeds, we found a different germination response depending upon the frugivore species tested. A different degree of seed coat scarification caused by differences in gut retention time, chemical and/or mechanical abrasion probably account for such responses. 6 In three of the species (Osyris, Rubia and Phillyrea), seed weight decreased after gut treatment. Such weight loss was not caused by any change in coat thickness, but may have been because of the scarification and consequent alteration of the seed coat structure. 7 The five Mediterranean species studied germinate when rains are most likely to fall (mostly autumn and spring). The different speed of germination promoted by gut treatment within frugivores may increase the probability that seeds can recruit successfully at a given time and in a given place. 8 This study suggests that frugivores contribute to the heterogeneity in germination characteristics not only within plant populations but also within plant communities, each frugivore species having a particular effect on the seeds of each plant consumed.

Effect of salinity on seed germination and early seedling growth of sugar beet (Beta vulgaris L.)

Abstract: Five varieties of sugar beet (Beta vulgaris L.) were studied in order to assess the effect of NaCl on seed germination and growth of young seedlings. Five salt treatments, 0, 50, 100, 150, and 200 mM NaCl were applied and germination was carried out at 21°C for 14 days. The results showed that germination percentage, rate of germination, and the relative germination percentage, were all inhibited by NaCl treatments. The strongest inhibition of germination occurred at the higher salt concentrations. Growth of young seedlings was also reduced, especially at the higher salt concentrations. The use of isotonic mannitol solutions indicated that the inhibitory influence of NaCl on sugar beet seed germination was principally a specific ionic effect and only a small portion of the inhibition could be attributed to an osmotic effect.

Polycomb repression of flowering during early plant development.

Abstract: All plants flower late in their life cycle. For example, in Arabidopsis, the shoot undergoes a transition and produces reproductive flowers after the adult phase of vegetative growth. Much is known about genetic and environmental processes that control flowering time in mature plants. However, little is understood about the mechanisms that prevent plants from flowering much earlier during embryo and seedling development. Arabidopsis embryonic flower (emf1 and emf2) mutants flower soon after germination, suggesting that a floral repression mechanism is established in wild-type plants that prevents flowering until maturity. Here, we show that polycomb group proteins play a central role in repressing flowering early in the plant life cycle. We found that mutations in the Fertilization Independent Endosperm (FIE) polycomb gene caused the seedling shoot to produce flower-like structures and organs. Flower-like structures were also generated from the hypocotyl and root, organs not associated with reproduction. Expression of floral induction and homeotic genes was derepressed in mutant embryos and seedlings. These results suggest that FIE-mediated polycomb complexes are an essential component of a floral repression mechanism established early during plant development.

Light, temperature and burial depth effects on Rumex obtusifolius seed germination and emergence

Abstract: Trials were carried out to investigate the effects of light and temperature on germination of Rumex obtusifolius L. After several months of storage, seeds gradually lost dormancy and became photosensitive. Thermal optima for germination were between 20 °C and 25 °C in light or in darkness. At lower temperatures there was a greater demand for light, so that the greatest differences in germination percentage (between low and high temperatures) were found within the 10-15 °C temperature range. The calculated thermal minimal (x-intercept method) in light and darkness were 8.3 °C and 6.1 °C respectively. Daily temperature fluctuation increased germination even after seed irradiation with far-red light, suggesting a lower demand for the far-red-absorbing form of phytochrome. Seed burial inhibited germination in proportion to depth; however, germination inhibition was independent of seed phytochrome photo-equilibrium, which had been diversified by seed pretreatment with light Seedlings did not emerge when seeds were buried > 8 cm deep. Recovery of ungerminated seeds showed that excessive burial did not impede seedling emergence but rather prevented seed germination. However, this induction of dormancy was lost once germination processes were activated (24-48 h at 20 °C) that made germination irreversible. Temperature was also involved in inhibition, and low temperature ( < 15 °C) induced the least inhibition. This is discussed in terms of processes of respiration and fermentation in buried seeds.

Water stress and Sphaeropsissapinea as a latent pathogen of red pine seedlings.

Abstract: Summary • The role of water stress in the initiation of collar rot by Sphaeropsis sapinea in asymptomatically colonized Pinus resinosa seedlings is reported. • Mortality and frequency of identification of the pathogen was quantified for seedlings subjected to different water regimes or watering regime–fungicide (benomyl) combinations in glasshouse experiments. • In experiment 1, seedling mortality ranged from 8% of repeatedly watered seedlings to 50% of those in the driest regime; data analysis indicated a high probability that mortality was not independent of watering regime. Seedlings developed symptoms resembling those of Sphaeropsis collar rot, and S. sapinea was identified from living (42%) and dead (92%) seedlings. In experiment 2, mortality of repeatedly watered seedlings was low, irrespective of fungicide application. For nonwatered seedlings, however, mortality was greater among seedlings not treated with fungicide (61%) than among benomyl-treated seedlings (37%); data analysis indicated a high probability that mortality was not independent of fungicide treatment. • Sphaeropsis sapinea can act as a latent pathogen; physiological alteration, through water stress, can effect release from the quiescent condition to result in rapid disease development.

Pinus monophylla establishment in an expanding Pinus-Juniperus woodland: Environmental conditions, facilitation and interacting factors

Abstract: . The tree species comprising Pinus-Juniperus woodlands are rapidly expanding into shrub-grasslands throughout their range. Observational studies indicate that establishment is facilitated by nurse plants, but little information exists on the mechanisms involved. I examined both abiotic and biotic factors influencing Pinus monophylla establishment in Artemisia tridentata steppe with expanding populations of P. monophylla and Juniperus osteosperma. I determined soil water contents, temperatures, and nutrient characteristics for the primary establishment microhabitats, i.e. under Pinus, under Juniperus, under Artemisia, tree interspace and sage interspace, and evaluated the emergence and survival response of two seedling cohorts over a 3-yr period for the different microhabitats. I also examined the effects of seed burial and predation on seedling establishment. Microhabitats under trees and shrubs had higher extractable P and K, higher organic matter, total nitrogen and cation exchange capacity than interspace microhabitats. Soil water contents (0–15 cm) were lower in interspaces than under shrubs or trees due to dry surface (0–5 cm) soils. Soil temperatures (at 1 and 15 cm) were lowest under trees, intermediate under shrubs, and highest in interspaces. Timing and rate of seedling emergence were temperature dependent with the order of emergence paralleling mean growing season temperatures: tree interspace = shrub interspace > under shrub > under Juniperus under Pinus. Seed burial was required for rooting and the highest emergence occurred from depths of 1 and 3 cm indicating that caching by birds and rodents is essential and that animals bury seeds at adequate if not optimal depths for emergence. Seedlings required microenvironmental modification for survival; all seedlings, including those that emerged from seeds and transplants, died within the first year in interspace microhabitats. Survival in under-tree or under-shrub microhabitats depended on soil water availability and corresponded closely to soil water contents over the 3-yr study. Under-shrub microhabitats had more favourable soil and micro-environmental characteristics than under-tree microhabitats and had the highest seedling life spans for the first-year seedling cohort. Predation of Pinus seedlings by rodents was a significant cause of mortality with caged transplants exhibiting life spans that were 74% longer overall than uncaged transplants. Emergence and survival of P. monophylla within the expanding woodland were dependent upon a complex set of interacting factors including growing season conditions, microhabitat characteristics, and animal species.

Effects of seedling size, El Niño drought, seedling density, and distance to nearest conspecific adult on 6-year survival of Ocotea whitei seedlings in Panamá

Abstract: We present an analysis of the long-term survival of two cohorts of seedlings of the tropical canopy tree Ocotea whitei (Lauraceae) on a 1-ha plot of mature, lowland moist forest on Barro Colorado Island, Panama. In 1980, we counted an even-aged cohort of seedlings that germinated in 1979, then measured and tagged survivors in 1981. We also measured and tagged a second, smaller cohort of seedlings that germinated in 1981. We followed the subsequent survival of all seedlings through 1985. Seedling mortality was phenotypically, temporally, and spatially non-random. Important correlates of non-random mortality included: (1) seedling size and age, (2) an El Nino drought, and (3) biotic neighborhood. Larger and older seedlings survived better than smaller and younger seedlings, respectively, and the El Nino-related drought of 1982–1983 was associated with elevated mortality rates. Seedling density, which was strongly correlated with the proximity to the nearest conspecific adult, increased mortality. The observed mortality patterns suggest that processes consistent with the Janzen-Connell hypothesis operate during the recruitment phase of O. whitei population dynamics. However, the processes causing the observed density- and distance-dependent mortality may vary with factors such as total seed number, seedling size, and climatic variation, making it difficult to determine whether time-integrated seedling-to-adult spacing mechanisms other than self-thinning operate on a given plant population. After 6 years in the hectare studied, survivors remained densest and most numerous underneath the adult trees. We conclude that only long-term demographic data, collected at a variety of scales on a variety of species, will ultimately answer the question: do Janzen-Connell effects contribute substantially to structuring tropical forests?

Two Tomato Expansin Genes Show Divergent Expression and Localization in Embryos during Seed Development and Germination

Abstract: Expansins are plant proteins that can induce extension of isolated cell walls and are proposed to mediate cell expansion. Three expansin genes were expressed in germinating tomato (Lycopersicon esculentum Mill.) seeds, one of which (LeEXP4) was expressed specifically in the endosperm cap tissue enclosing the radicle tip. The other two genes (LeEXP8 and LeEXP10) were expressed in the embryo and are further characterized here. LeEXP8 mRNA was not detected in developing or mature seeds but accumulated specifically in the radicle cortex during and after germination. In contrast, LeEXP10 mRNA was abundant at an early stage of seed development corresponding to the period of rapid embryo expansion; it then decreased during seed maturation and increased again during germination. When gibberellin-deficient (gib-1) mutant seeds were imbibed in water, LeEXP8 mRNA was not detected, but a low level of LeEXP10 mRNA was present. Expression of both genes increased when gib-1 seeds were imbibed in gibberellin. Abscisic acid did not prevent the initial expression of LeEXP8 and LeEXP10, but mRNA abundance of both genes subsequently decreased during extended incubation. The initial increase in LeEXP8, but not LeEXP10, mRNA accumulation was blocked by low water potential, but LeEXP10 mRNA amounts fell after longer incubation. When seeds were transferred from abscisic acid or low water potential solutions to water, abundance of both LeEXP8 and LeEXP10 mRNAs increased in association with germination. The tissue localization and expression patterns of both LeEXP8 and LeEXP10 suggest developmentally specific roles during embryo and seedling growth.

The effect of salinity on different developmental stages of an endemic annual plant, Aster laurentianus (Asteraceae)

Abstract: Salinity reduces substrate water potential, thereby restricting water and nutrient uptake by plants; salinity may also cause ionic imbalance and toxicity. Because substrate salinity fluctuates through the growing season, a plant may be exposed to different salinity levels, at various stages of development, with potentially significant consequences on population dynamics. Here, we present the results of a study of the effect of substrate salinity on seed germination, seedling emergence, and growth of Aster laurentianus, an annual marsh plant, endemic to the Gulf of St. Lawrence and potentially threatened. Seed germination was reduced in low salt concentration (10 g sea salt/L) and completely inhibited by salinity levels >/=20 g sea salt/L. However, this inhibiting effect was reversible: seeds from the salt treatments germinated readily after being washed in distilled water. Though seedling emergence was diminished at low salinity levels, postemergence survival was little affected. Plant growth was reduced, but net carbon assimilation rate was not affected by high salinity levels. Increased root respiration and respiratory costs associated with salt tolerance might have contributed to lower C accumulation at higher salinity levels. All developmental processes considered are thus negatively affected by substrate salinity, with potentially significant consequences on population abundance and distribution in salt marshes. Yet, the tolerance of this species to high salinity levels after seedling emergence is remarkable. Seed germination represents a major bottleneck in the species life cycle, potentially controlling local distribution and abundance in the natural habitat.

Seedling functional types in a lowland rain forest in Mexico.

Abstract: Seedling morphology of 210 species (173 trees and 37 lianas) was studied from a community perspective to identify major patterns of seedling functional types in a Mexican rain forest. Five types of seedlings were distinguished: cryptocotylar with reserve storage or absorption cotyledons (epigeal [CER] and hypogeal [CHR]), phanerocotylar epigeal, either with photosynthetic cotyledons (PEF) or with reserve storage or absorption cotyledons (PER), and phanerocotylar hypogeal with reserve cotyledons (PHR). The most common seedling type was PEF (49.5%), followed by CHR (31.4%), PER (9.5%), PHR (7.2%), and CER (2.4%). Excepting the CER type, seedling type frequencies did not differ between trees and lianas. The PEF seedlings had the lightest seeds, whereas CHR seedlings had the heaviest ones. Pioneer trees showed lighter seeds than persistent trees or lianas in species with PEF but not in species with PER. Pioneer trees (38 species) showed three seedling types and the most common was PEF (82%). Persistent trees (135 species) showed the five seedling types but PEF (43%) and CHR (37%) were the most frequent. Seedling type frequencies differed among dispersal syndrome groups. The animal dispersal syndrome was significantly more frequent in species with CHR. Our results show an evolutionary convergence of seedling types at the community level worldwide and the existence of a phylogenetic inertia in the evolution of initial seedling morphology. A comparison among eight tropical communities indicated on average that PEF is the most frequent type and CER the least common, although the relative frequency of each seedling type differs among communities, particularly between Neotropical and Paleotropical sites.

Seedling herbivory by slugs in a willow hybrid system: developmental changes in damage, chemical defense, and plant performance.

Abstract: We evaluated feeding preference and damage by the slug, Arion subfuscus, on seedlings of two willow species, Salix sericea and S. eriocephala, and their F1 interspecific hybrids. Trays of seedlings were placed in the field and excised leaves were presented to slugs in choice tests. Slugs preferred feeding on and caused the most damage to S. eriocephala seedlings. S. sericea seedlings were least preferred and least damaged. F1 hybrid seedlings were intermediate in preference and damage. Slug preference of and damage to these seedlings decreased over time, suggesting developmental changes in resistance. Seedlings were sampled for phenolic glycoside and tannin chemistry weekly to coincide with the field and laboratory experiments. Concentrations of phenolic glycosides and tannins increased linearly with seedling age, coincident with changes in slug preference and damage, indicating a developmental change in defense. Slug deterrence was not detected at low concentrations of salicortin when painted on leaves or discs, but both salicortin and condensed tannins deterred slug feeding at concentrations between 50 and 100 mg/g, levels found in adult willows. Seedling performance was related to damage inflicted by slugs. Due to lower levels of damage when exposed to slugs in the field, S. sericea plants had significantly greater biomass than S. eriocephala plants. Biomass of F1 hybrids was equal to S. sericea when damaged. However, undamaged S. eriocephala and F1 hybrid plants had the greatest biomass. Because F1 hybrid seedlings performed as well as the most fit parent in all cases, slugs could be an important selective factor favoring introgression of defensive traits between these willow species.

Effect of liana cutting on tree regeneration in a liana forest in amazonian bolivia

Abstract: Lianas, woody climbing plants, are a conspicuous component of tropical forest canopies that might affect prevailing conditions in the forest floor and thus impact tree seedling regeneration. The effects of lianas on tree seedling survival, growth, and density were studied in a lowland liana forest in Bolivia. Gravimetric soil water content and canopy openness were measured to evaluate whether these factors changed as a result of liana cutting. I established 24 square plots of 900 m2 each, and after an initial set of measurements, all lianas were cut in half of them, while the others plots were used as controls. Tree seedling growth and survival of two tree species were evaluated: Clarisia ilicifolia and Astronium fraxinifolium. Eighteen months after liana cutting, seedlings in liana-cut plots grew significantly taller and produced more leaves than did seedlings in control plots, but survival was not affected by treatment. Seedling growth following liana cutting was significantly higher in A. fraxinifolium...

Effects of seed dispersal on Juniperus communis recruitment on a Mediterranean mountain

Abstract: . The recruitment of the relict shrub Juniperus communis on a mountain in SE Spain was studied during the period 1994–1998. The main objective was to determine both the quantitative and qualitative effects of bird dispersal on seedling establishment. Seed removal by birds, seed rain, post-dispersal seed predation, germination, and seedling emergence and survival were analysed in different microhabitats. Birds removed 53 - 89% of the seeds produced by plants. Seed rain was spatially irregular as most seeds accumulated near stones used by birds as perches and below mother plants while a few seeds were dropped in wet meadows and open ground areas. Post-dispersal seed predation by rodents affected < 10% of dispersed seeds but varied significantly among microhabitats. Only 3.6 - 5.5% of dispersed seeds appeared viable, as many seeds had aborted or showed wasp damage. Seeds germinated in the second and third springs after sowing, reaching a germination percentage of 36%. Seedling emergence was concentrated in wet meadows. Seedling mortality was high (75–80%), but significantly lower in wet meadows, the only microhabitat where seedlings could escape from summer drought, the main mortality cause. Seed abortion, germination and seedling mortality proved to be the main regeneration constraints of J. communis on Mediterranean mountains. Birds exerted a strong demographic effect, although their qualitative effect was limited by abiotic factors which caused the pattern of seed rain to differ from the final pattern of recruitment between microhabitats.

Competition mediates costs of jasmonate-induced defences, nitrogen acquisition and transgenerational plasticity in Nicotiana attenuata.

Abstract: Summary 1 Plants adjust their phenotype in response to environmental signals, but little is known about the interaction of plastic responses to simultaneously occurring environmental stresses. 2 To examine the costs of induced resistance on reproductive performance in plants subjected to other important environmental variables, resistance was elicited with a jasmonate treatment (MeJA) to one, both or neither of two Nicotiana attenuata plants growing competitively in either high- or low-N soils. Half the plants were subjected to leaf removal (LR). K15NO3 was used to quantify differences in N acquisition and allocation. Transgenerational effects were measured with seed germination and seedling performance tests. 3 An induced plant competing with an uninduced plant produced significantly fewer seeds, acquired less 15N and allocated less 15N to seed production. Uninduced plants competing with induced plants realized a comparable fitness benefit. 4 The costs of induction were greater under high N. Plants grown under low N minimized costs by allocating significantly more N to seeds. LR decreased seed production independently of any other effect. Low N and LR both reduced germination rates. 5 The effects of MeJA on seed germination depended on competition and N supply. The differences in germination rates resulted in dramatic fitness differences among offspring. 6 N. attenuata plants appear to use N availability and their induced status to alter their current phenotype and their offspring’s phenotype to adjust to environmental changes that occur predictably over time in their natural environment.

Seed aging, delayed germination and reduced competitive ability in Bromus tectorum

Abstract: In annual plants, increased competitive advantage has often been attributed to rapid germination and early establishment. In contrast, many annual species exhibit some degree of delayed germination (i.e., seed dormancy) that results in the formation of age structure within the seed population. Delayed germination can be an effective bet-hedging strategy in variable or unpredictable environments as a seed bank can buffer against years with reproductive failures and reduce the probability of local extinction. However, there has been little consideration of the direct effects of aging within the seed pool although the potential demographic costs of such a strategy (e.g., mortality in the seed bank or delayed reproduction) are well known. We used aged (4 year-old) and freshly produced seed from meadow steppe and sagebrush steppe populations of an annual grass (Bromus tectorum)to investigate the importance of seed age on seedling vigor and competitive ability. Aged seed from the meadow steppe population exhibited delays in germination that reduced plant growth and final biomass when the plants were grown with competition. Aged seed from the sagebrush steppe population did not exhibit delays in germination. By including a treatment that experimentally delayed the germination of freshly produced meadow steppe seed, we also examined the effects of delayed germination alone. A comparison of results from this delay treatment with those from the aged seed treatment suggested that the reduced competitive ability of meadow steppe plants produced from aged seed, although largely a result of the temporal delay in germination, was partly due to reduced seed vigor. Together these results indicate that physiological costs associated with seed age may affect aboveground competitive interactions and, in turn, the relative fitness of older cohorts in the soil seed bank.

Ecology of fruit-colour polymorphism in Myrtus communis and differential effects of birds and mammals on seed germination and seedling growth

Abstract: Summary 1 The fruit-colour polymorphism of Myrtus communis, a common Mediterranean shrub, is examined. We investigate whether frugivores affect the maintenance of the polymorphism, whether morphs differ in germination or seedling growth, and whether passage through frugivores’ gut affects seed behaviour and seedling growth. 2 Blue berries are very similar in morphological traits and nutrient composition to the rarer white morph. Rates of fruit removal by birds, the main dispersers of this species, did not differ between morphs, suggesting that a colour preference is not involved in maintenance of the polymorphism. 3 Seeds of the two morphs showed the same germinability (final percentage germination) as well as similar rates of germination under controlled conditions (in growth chamber and greenhouse). Outdoors, seeds from blue berries tended to germinate slightly faster (which might give them an early advantage) but differences between morphs disappeared after several weeks of growth. 4 The germination of myrtle seeds is differently affected by passage through the digestive tract of birds, which appeared to significantly increase germinability, and of the carnivorous pine marten, Martes martes, which did not. The different seed retention time in the gut or the chemical composition of the food ingested along with the seeds may be responsible for such differences. 5 The detection of a positive effect of bird ingestion in outdoor conditions, but not in the growth chamber or in the greenhouse, demonstrates the importance of performing germination tests in the natural habitat. 6 Seed size, irrespective of morph, affects the response to passage through the bird gut. Large seeds, which are more rapidly ejected, and are thus less abraded, germinated at a similar speed to non-ingested controls. Variation in seed size within a single species should thus be considered in future studies.