Showing papers on "Germination published in 1993"

Isolation and characterization of actinomycete antagonists of a fungal root pathogen.

Abstract: By use of selective media, 267 actinomycete strains were isolated from four rhizosphere-associated and four non-rhizosphere-associated British soils. Organic media with low nutrient concentrations were found to be best for isolating diverse actinomycetes while avoiding contamination and overgrowth of isolation media by eubacteria and fungi. While all isolates grew well at pHs 6.5 to 8.0, a few were unable to grow at pH 6.0 and a significant number failed to grow at pH 5.5. Eighty-two selected isolates were screened for in vitro antagonism towards Pythium ultimum by use of a Difco cornmeal agar assay procedure. Five isolates were very strong antagonists of the fungus, four were strong antagonists, and ten others were weakly antagonistic. The remaining isolates showed no antagonism by this assay. Additional studies showed that several of the P. ultimum antagonists also strongly inhibited growth of other root-pathogenic fungi. Twelve isolates showing antifungal activity in the in vitro assay were also tested for their effects on the germination and short-term growth of lettuce plants in glasshouse pot studies in the absence of pathogens. None of the actinomycetes prevented seed germination, although half of the isolates retarded seed germination and outgrowth of the plants by 1 to 3 days. During 18-day growth experiments, biomass yields of some actinomycete-inoculated plants were reduced in comparison with untreated control plants, although all plants appeared healthy and well rooted. None of the actinomycetes significantly enhanced plant growth over these short-term experiments. For some, but not all, actinomycetes, some correlations between delayed seed germination and reduced 18-day plant biomass yields were seen. For others, plant biomass yields were not reduced despite an actinomycete-associated delay in seed germination and plant outgrowth. Preliminary glasshouse experiments indicated that some of the actinomycetes protect germinating lettuce seeds against damping-off caused by P. ultimum.

Patterns of Seed Longevity and Germination in the Tropical Rainforest

Abstract: Tropical rainforest plants produce seeds showing a wide range of sizes, shapes, structures, chemical composition, water content, dormancy mechanisms, and patterns of longevity. Prompt germination seems to be the most common behavior, although there are many cases where germination is delayed by a hard coat or endogenous or enforced dormancy mechanisms. Some gap colonizers show sophisticated light or temperature regulated dormancy. Most rainforest seeds remain alive for a short time in the soil, even those that form part of the soil seed bank.

Seed Germination Ecology in Southwestern Western Australia

Abstract: Germination responses of species from the native plant communities of southwestern Western Australia can be related to syndromes of life history, fire response, and seed storage, and also to factors related to environmental stress. The Mediterranean-type climate of the region with periodic drought and recurrent fires affects the production of viable seeds in plants of limited stature and rooting depth. Fire response ephemerals and species cued to flower by fire tend to produce viable, readily germinable seeds, but there are instances where seed production is aborted in these predominantly herbaceous life forms. Clonal, rhizomatous species often produce mainly inviable seeds. Production of viable seeds in woody species of these highly diverse communities may also be restricted by limitations to cross pollination. Obligate post-fire seeding species tend to produce a greater proportion of viable seeds than species which are capable of resprouting following fire. Serotinous species, whether post-fire re-seeders or post-fire resprouting species, produce mainly viable seeds, which germinate readily once freed from protective fruits. Species of the legume families and a few others of the soil seed bank produce innately dormant seeds which can be germinated following heat shock treatments which simulate the effects of fire. Heat shock in these species appears mainly as a mechanism to crack the hard seed coats, but the effect of heat to denature seed coat inhibitors has not been eliminated. Western Australian species do not seem to break dormancy when exposed to leachates from burned wood as has been observed in comparable habitats in California and South Africa, but further research is advised. Germination in many native southwestern Australian species is cued by temperatures that correspond to the winter rainfall period. There are also indications that an after-ripening period of warm, dry storage increases percentage of germinable seeds. Stimulation of germination by hormones is almost unresearched in Western Australia, but germination percentages have been increased in a small number of species of horticultural potential. Stimulation of germination by soil nutrient concentrations is almost unresearched in Western Australia, except for the inhibitory effect of excess sodium chloride levels inEucalyptus andMelaleuca. These species only germinate when osmotic effects are reduced to lower levels as would occur when winter rains dilute soil salts. Application of research on seed germination has already enhanced the establishment of seedlings in the restoration of mine sites and is becoming important in aspects of the breeding and selection of native plants for the cut flower, bedding plant and essential oil industries.

Seed Germination in Desert Plants

Abstract: During germination, the most resistant stage of the life cycle - the seed - changes to the most sensitive stage, namely the seedling. Therefore, in desert plant species, seed dispersal and subsequent germination in the optimum place are particularly critical parameters. Discussed here are the ways and means by which desert plants have adapted through the course of evolution to their extreme environment. Two such strategies which have evolved are outlined: plants with relatively large and protected seeds which germinate when the chance of seedling survival is high and the risk relatively low; and those with an opportunistic strategy - minute seeds which germinate after low rainfall. The reader is provided with an understanding of the complex influences acting upon each seed, traced from initial development to germination stage, regarding germination preparation and the subsequent survival.

Vesicular-arbuscular mycorrhizas and soil salinity

Abstract: This review discusses the growth and activity of vesicular-arbuscular (VA) mycorrhizal fungi in saline conditions. The review includes examination of the effects of high concentrations of salts on the occurrence of VA mycorrhizal fungi in field soils, and on spore germination, growth of hyphae, establishment of the symbiosis and production of spores in controlled conditions. Information on the growth and reproduction of VA mycorrhizal fungi under saline conditions is scarce and is often circumstantial. There is clear evidence that germination of spores and subsequent hyphal growth of some VA mycorrhizal fungi are reduced by increasing concentration of salts. However, in plant growth experiments, experimental designs and methodologies have generally not allowed the direct effects of salinity on fungal growth to be separated from plant-mediated effects. There is a need for controlled studies to investigate the responses of VA mycorrhizal fungi to soil salinity. Research is required which distinguishes between effects on different phases of the fungus lifecycle and which includes in its design the ability to separate direct effects from plant-mediated influences on fungal growth and reproduction.

Promotion of germination of fynbos seeds by plant‐derived smoke

Abstract: SUMMARY Fynbos is the dominant vegetation type in the Cape floristic region. Periodic fires are a natural phenomenon in fynbos and fire-stimulated seed germination has been reported for a number of fynbos species. Amongst the factors proposed as being directly responsible for the effects of fire are: heat fracturing hard seed coats, heat stimulating seed embryos, high-temperature desiccation of seed coats, ethylene and ammonia in smoke stimulating seed germination, and unknown chemical factors in plant-derived smoke and smoke extracts stimulating germination. In this study 28 fynbos species were screened for a seed germination response to smoke and/or smoke extract treatments. Twelve of these species showed a statistically significant enhancement of germination in response to treatment. For the first time smoke per se has been shown to act as a seed germination cue for fynbos species in the Asteraceae, Ericaceae and Restionaceae, and for the first time smoke extracts have been shown to enhance germination in fynbos species of Proteaceae. The most marked response to both smoke and smoke extracts was shown by seed of Syncarpha vestita (syn. Helichrysum vestitum) (Asteraceae).

Bet-hedging germination of desert annuals: beyond the first year.

Abstract: Prolonged seed dormancy in desert annuals is thought to be an adaptation to environmental uncertainty. Germination spread over several years could reduce the year-to-year variation in the fitness of a genotype. Previous work has demonstrated that, for a number of species, not all viable seeds germinate under one set of conditions in the first year, but the subsequent fate of the dormant seeds has not been tested. In this study, germination experiments were performed on seeds of six species of winter annuals from Portal, Arizona. Seeds that do not germinate under good conditions in the first year germinate under the same conditions in subsequent years. However, germination fractions in the first 2 yr are not equal fractions of remaining viable seeds, which is the theoretical optimal behavior, and the germination distributions differ greatly among species. Germination under good conditions following a year of bad conditions for germination also differs among species. Germination behavior is age-dependent, and germination trials conducted on seeds of unknown ages from soil samples may give misleading results. These results indicate that seed dormancy acts as a form of bet hedging but that selection for an optimal distribution of germination across years may be weak.

Isolation of strigol, a germination stimulant for Striga asiatica, from host plants

Abstract: The germination of Striga sciatica, a root parasite of many cereal and leguminous crops, is stimulated by several host and nonhost plant derived stimulants. HPLC revealed the presence of three active composeds in root exudates from Striga host plants, maize and sorghum, and also from proso millet. A fourth active composed was present in sorghum exudates Acetate and heptafluorobutyrate derivatives were prepared and analyzed by HPLC and mass spectrometry. Each step involved in the isolation, chromatographic purification, and derivatization was followed by a sensitive Striga seed germination bioassay. We report the isolation of strigol as the major Striga seed germination stimulant in maize and proso millet root exudates and as a minor component of the total activity in sorghum root exudates

Elevated CO2 and plant nitrogen-use: is reduced tissue nitrogen concentration size-dependent?

Abstract: Plants often respond to elevated atmospheric CO2 levels with reduced tissue nitrogen concentrations relative to ambient CO2-grown plants when comparisons are made at a common time. Another common response to enriched CO2 atmospheres is an acceleration in plant growth rates. Because plant nitrogen concentrations are often highest in seedlings and subsequently decrease during growth, comparisons between ambient and elevated CO2-grown plants made at a common time may not demonstrate CO2-induced reductions in plant nitrogen concentration per se. Rather, this comparison may be highlighting differences in nitrogen concentration between bigger, more developed plants and smaller, less developed plants. In this study, we directly examined whether elevated CO2 environments reduce plant nitrogen concentrations independent of changes in plant growth rates. We grew two annual plant species. Abutilon theophrasti (C3 photosynthetic pathway) and Amaranthus retroflexus (C4 photosynthetic pathway), from seed in glass-sided growth chambers with atmospheric CO2 levels of 350 μmol·mol−1 or 700 μmol·mol−1 and with high or low fertilizer applications. Individual plants were harvested every 2 days starting 3 days after germination to determine plant biomass and nitrogen concentration. We found: 1. High CO2-grown plants had reduced nitrogen concentrations and increased biomass relative to ambient CO2-grown plants when compared at a common time; 2. Tissue nitrogen concentrations did not vary as a function of CO2 level when plants were compared at a common size; and 3. The rate of biomass accumulation per rate of increase in plant nitrogen was unaffected by CO2 availability, but was altered by nutrient availability. These results indicate that a CO2-induced reduction in plant nitrogen concentration may not be due to physiological changes in plant nitrogen use efficiency, but is probably a size-dependent phenomenon resulting from accelerated plant growth.

Cd-tolerant arbuscular mycorrhizal (AM) fungi from heavy-metal polluted soils

Abstract: Spores of arbuscular mycorrhizal (AM) fungi were isolated from two heavy-metal polluted soils in France via trap culture with leek (Allium porrum L.). Preliminary identification showed that the predominant spore type of both cultures (P2 and Cd40) belongs to the Glomus mosseae group. Their sensitivity to cadmium was compared to a laboratory reference strain (G. mosseae) by in vitro germination tests with cadmium nitrate solutions at a range of concentrations (0 to 100 mg L−1) as well as extracts from a metal-polluted and unpolluted soils. Both cultures of AM fungi from heavy-metal polluted soils were more tolerant to cadmium than the G. mosseae reference strain. The graphically estimated EC50 was 0.8 mg L−1 Cd (concentration added to the test device) for G. mosseae and 7 mg L−1 for P2 culture, corresponding to effective Cd concentrations of approximately 50–70 μg L−1 and 200–500 μg L−1, respectively. The extract of the metal-polluted soil P2 decreased germination of spores from the reference G. mosseae but not from P2 culture. However, the extracts of two unpolluted soils with different physico-chemical characteristics did not affect G. mosseae, whereas germination of P2 spores was markedly decreased in the presence of one of the extracts. These results indicate a potential adaptation of AM fungi to elevated metal concentrations in soil. The tested spores may be considered as metal-tolerant ecotypes. Spore germination results in presence of soil extracts show the difficulty of assessing the ecotoxic effect of metals on AM fungi without considering other soil factors that may interfere in spore germination and hyphal extension.

Management of Weed Seed Banks with Microorganisms

Abstract: Successful weed management in agroecosystems centers on manipulating the weed seed bank in soil, the source of annual weed infestations. Despite advances in aboveground weed control and decreases in the production of new seed, weed infestations continue to be generated from a small portion of the seed bank that persists as a result of dormancy and resistance to decay. Depletion of the persistent seeds using soil-applied chemicals to stimulate germination has received much attention while the search for microorganisms selective for seed decay has been largely overlooked. This paper provides an overview of the effects of microorganisms on weed seed viability relative to seed bank depletion, and how this information can be applied to weed management. Limited studies indicate that microorganisms associated with weed seeds can contribute to seed bank depletion through attraction to seeds by chemotaxis, rapid colonization of the spermosphere and production of enzymes and/or phytotoxins to kill seeds prior to germination. It is recognized, however, that the best opportunity for success will be through integration of selected microorganisms or microbial products with other approaches including germination stimulation, application of low rates of herbicides, manipulation of the soil environment (e.g., solarization), and biological control agents for effectively eliminating dormant, persistent seeds from soil. To achieve success, more in-depth research on microbial factors affecting weed seed banks is required.

Ultra‐violet radiation damage to Metarhizium flavoviride conidia and the protection given by vegetable and mineral oils and chemical sunscreens

Abstract: Summary Metarhizium flavouiride conidia formulated in oil or water were exposed to simulated solar radiation. Radiation below 320 nm killed conidia and caused delays in the germination of survivors; germination was greater after 48 h of incubation than after 24 h. UV exposure of conidia formulated in oil for 2 h reduced germination from 99% to 37.5% after incubation for 48 h. Exposure of conidia in water to UV for 1 h resulted in 4.7% germination after 24 h incubation compared with 36.5% for conidia formulated in oil. The addition of 1% oxybenzone resulted in 81.9% conidial germination after 3 h exposure and 48 h incubation compared with 28.1% in oil without the sunscreen.

Germination and Dormancy of Abscisic Acid- and Gibberellin-Deficient Mutant Tomato (Lycopersicon esculentum) Seeds (Sensitivity of Germination to Abscisic Acid, Gibberellin, and Water Potential)

Abstract: Germination responses of wild-type (MM), abscisic acid (ABA)-deficient (sitw), and gibberellin (GA)-deficient (gib-1) mutant tomato (Lycopersicon esculentum Mill. cv Moneymaker) seeds to ABA, GA4+7, reduced water potential ([psi]), and their combinations were analyzed using a population-based threshold model (B.R. Ni and K.J. Bradford [1992] Plant Physiol 98: 1057–)1068). Among the three genotypes, sitw seeds germinated rapidly and completely in water, MM seeds germinated more slowly and were partially dormant, and gib-1 seeds did not germinate without exogenous GA4+7. Times to germination were inversely proportional to the differences between the external osmoticum, ABA, or GA4+7 concentrations and the corresponding threshold levels that would either prevent ([psi]b, log[ABAb]) or promote (log[GAb]) germination. The sensitivity of germination to ABA, GA4+7, and [psi] varied widely among individual seeds in the population, resulting in a distribution of germination times. The rapid germination rate of sitw seeds was attributable to their low mean [psi]b (-1.17 MPa). Postharvest dormancy in MM seeds was due to a high mean [psi]b (-0.35 MPa) and a distribution of [psi]b among seeds such that some seeds were unable to germinate even on water. GA4+7 (100 [mu]M) stimulated germination of MM and gib-1 seeds by lowering the mean [psi]b to -0.75 MPa, whereas ABA inhibited germination of MM and sitw seeds by increasing the mean [psi]b. The changes in [psi]b were not due to changes in embryo osmotic potential. Rather, hormonal effects on endosperm weakening opposite the radicle tip apparently determine the threshold [psi] for germination. The analysis demonstrates that ABA- and GA-dependent changes in seed dormancy and germination rates, whether due to endogenous or exogenous growth regulators, are based primarily upon corresponding shifts in the [psi] thresholds for radicle emergence. The [psi] thresholds, in turn, determine both the rate and final extent of germination within the seed population.

Environmental and genetic maternal effects on seed characters in nemophila menziesii.

Abstract: Nuclear genetic, maternal genetic and maternal environmental effects on seed characters were estimated in the California native annual plant Nemophila menziesii using two greenhouse crosses. In one cross, according to a nested mating design, the narrow sense heritability of seed weight was small (3.9%). A subset of full-sib progenies produced in this cross was grown singly and in competition with the introduced grass Bromus diandrus. In a second cross, these plants were used as mothers (dams) and were each mated to the same three sires. Seeds produced by mothers competing with B. diandrus showed a significant reduction in weight, increase in time to germination, and increase in the incidence of dormancy, when compared to seeds from mothers grown singly. Significant sire components were found for time to germination and incidence of dormancy. Maternal genetic variation for seed weight was largely expressed as maternal genotype by maternal environment interaction, and showed no significant maternal genetic main effect. Time to germination and dormant fraction showed a relatively large maternal genetic effect. Evolution of seed characters in N. menziesii is more likely to occur via indirect response to selection among maternal plants than among the seeds themselves. Maternal genotype by maternal environment interaction could potentially contribute to the maintenance of maternal genetic variation in seed weight, but this does not appear likely for dormancy.

Seed and seedling ecology of neotropical melastomataceae

Abstract: Models of gap-phase regeneration in tropical wet forests are based primarily on studies of canopy trees. These paradigms are derived from studies of a small number of species whose life history traits place them easily into "pioneer" or "climax" species. Woody plants of the forest understory, subcanopy trees and shrubs, are known to affect significantly regeneration dynamics in treefall gaps, yet we lack fundamental information on life history characteristics of understory plants that could permit their incorporation into gap regeneration models. We investigated intra- and interspecific variability in early life history characteristics of understory Melastomataceae species of different growth forms and adult distributions from the lowland tropical rain forest at La Selva Biological Station, Costa Rica. We examined germination patterns in vitro (22 species), effects on germination of passage through birds (5 species), seedling establishment in the field (3 species), and growth in different light environments (8 species). Established seedlings were significantly more abundant in gaps than in nearby under- story sites. This pattern was generally applicable to all melastomes studied, regardless of the habitat where adults were normally most abundant. Seedlings were more common in the root pits and on the root mounds of uprooted trees, even though there was less light available to the seedlings in these microsites than elsewhere in the gaps. In a series of germination trials in vitro, most seeds germinated readily within 1-2 wk. Comparisons of germination rates between seeds collected from bird droppings and seeds extracted from fruits by hand gave varied results. With one exception, seeds of shade-tolerant species germinated more slowly following passage through birds, while seeds from high-light- demanding species germinated more rapidly after passing through birds. Seeds of three common species were sown directly into different microsites within gaps and understory control sites. In contrast to the germination trials, emergence percentages in the field ranged from 4% in understory sites to 21 % in gap centers. Survival after 1 yr was low (0.1%- /0.4%). After 1 yr, these seedlings were small (<2 cm tall) and produced few leaves. In similar light environments in the shade-house, melastome seedling survival was between 80% and 100%, and growth increased with increasing light levels from 2% (corresponding to understory sites) to 20% (small gap centers) of full sun. With the exception of two species common to abandoned pastures and river banks, the species used in the shade-house growth experiments did not show significant increases in growth with increases in light from 20% to 40%. Suites of early life history traits described for tropical tree species are not consistent with the variation in pattern seen within this one family of understory species.

Seed banks as a neglected area of biogeographic research: a review of literature and sampling techniques

Abstract: The article highlights a comparatively neglected area of biogeographical research - seed banks and the distribution of seeds in the soil. The article reviews some of the relevant literature on seed banks and the methods for their study. Attention is focused on aspects of seed banks of particular relevance to biogeographers, with detailed examples drawn from seed bank studies in both temperate and tropical environments. In the review of the seed bank literature, the topics covered include the seed banks of successional communities and the size of seed banks in different vegetation types. The species composition of seed banks in different plant communities is discussed, particularly the degree of correlation between the species composition of seed banks and associated ground flora. The relationships between seed persistence, depth of burial in the soil and soil properties, such as moisture and pH, are explored. Seed bank heterogeneity is examined and a number of studies which have attempted to describe and measure the spatial variability of seed banks are summarized. Ways of classifying seed banks in terms of seed bank strategies are explained. The role of seed banks in conservation is discussed, for example in restoration projects, where preferred species have been lost from the vegetation but survive in the seed bank. The relevance of seed banks for the conservation of rare species and in landscape management is considered. Lastly, the contribution of seed banks to the recovery of vegetation following disturbance in various plant communities is discussed. In the review of seed bank sampling techniques, the subjects considered include methods of sample collection, the sampling intensity required for reliable estimates of seed density, a consideration of the relative merits of random and systematic sample distribution, as well as the importance of the timing of sampling. Various methods for the estimation of seed numbers in samples are appraised; these either involve extraction of seeds from the soil, followed by seed identification or enumeration by germination and seedling identification. Problems of analysing seed bank data are considered and several useful techniques for data analysis are suggested. Finally, the article draws attention to areas of future seed bank research for biogeographers and plant ecologists.

Effects of salicylic acid on plant-water relationships.

Abstract: Soybean seedlings [Glycine max (L.) Merr.] were used as the test species to study the allelopathic influence of salicylic acid (SA) on short- and long-term plant water status. Plants were grown in greenhouse conditions in nutrient culture medium amended with SA. Treatments were initiated 10 days after germination and continued for either 14 or 28 days. The threshold for inhibition of seedling growth over a 28-day treatment was 0.15 mM SA. Seedlings grown with 0.3 mM SA consistently had higher leaf diffusive resistance and lower transpiration and water potentials than control plants. The stable carbon isotope ratio (13C:12C) in tissue from both the 0.15 and 0.30 mM SA-treated plants was significantly higher than control seedlings, indicating SA caused a chronic water stress during the 28-day treatment. These data show that an interference with plant-water relationships is one mechanism whereby this allelochemical inhibits plant growth.

Bet-hedging germination of desert annuals: variation among populations and maternal effects in

Abstract: Prolonged seed dormancy in desert annuals has been modeled as a bet-hedging adaptation to environmental uncertainty. Several aspects of that theory were tested in the preceding article; further results using Lepidium lasiocarpum are reported here. Seeds from different populations of Lepidium differed in their germination behavior. These differences were weakly correlated with differences in the mean winter rainfall among the collection sites: as predicted, sites with greater mean precipitation had less dormancy. Precipitation in the year of seed production was a strong predictor of germination behavior: sites with greater precipitation produced seeds with more dormancy. In a second set of experiments, maternal plant size had a strong effect on germination: plants that produced many seeds had a smaller proportion germinate in the first year. While these results support the general theories of bet-hedging germination in desert annuals, they indicate that selection due to year-to-year variation in popu- lation size and sibling competition may be more important in the evolution of seed dormancy than previously thought.

Bet-hedging germination of desert annuals: variation among populations and maternal effects in Lepidium lasiocarpum.

Abstract: Prolonged seed dormancy in desert annuals has been modeled as a bet-hedging adaptation to environmental uncertainty Several aspects of that theory were tested in the preceding article; further results using Lepidium lasiocarpum are reported here Seeds from different populations of Lepidium differed in their germination behavior These differences were weakly correlated with differences in the mean winter rainfall among the collection sites: as predicted, sites with greater mean precipitation had less dormancy Precipitation in the year of seed production was a strong predictor of germination behavior: sites with greater precipitation produced seeds with more dormancy In a second set of experiments, maternal plant size had a strong effect on germination: plants that produced many seeds had a smaller proportion germinate in the first year While these results support the general theories of bet-hedging germination in desert annuals, they indicate that selection due to year-to-year variation in population

The Impact of Fungi on Soil Seeds: Implications for Plants and Granivores in a Semiarid Shrub-Steppe

Abstract: Fungi are ubiquitous in soil and may affect seed survivorship directly by decomposition or pathogenesis, or indirectly through the effects of fungi on seed preference of granivores. We studied the role of fungi in the persistence of soil seeds in a shrub-steppe ecosystem using two experimental approaches. In the first, we examined survivorship of 900 seeds of five shrub-steppe species in nylon bags over a 1 0-mo period and determined viable seed losses to germination, decomposition, and pathogens. The proportion of seeds representing a viable seed bank decreased from 84.4% in the May pre-experimental con- ditions to 4.4% in March. Decomposition and attack by fungi were responsible for the greatest decrease from fall to winter (0% in August, 32.0% in November, 35.2% in March) while germination accounted for the greatest decrease from winter to spring (18.5% in August, 21.9% in November, 55.0% in March). Small seeds were more likely to be lost to decomposition and pathogens, whereas larger seeds had greater losses to germination. A subset of 80 experimental seeds was cultured for seed-infecting fungi. We found fungi in virtually all seeds; the most common fungal isolates were cosmopolitan soil fungi, such as species of Penicillium and Mucor. These species are known producers of extracellular enzymes and mycotoxins. Two sterile (nonfruiting) forms of fungi were also frequently isolated, as were two pathogenic species of Fusarium. In a second experiment we placed seeds in fungal cultures obtained from soil seeds and presented moldy seeds along with control seeds to a shrub-steppe granivore, the western harvester ant (Pogonomyrmex oc- cidentalis). Ants harvested control seeds at nearly twice the rate of moldy seeds, suggesting that they avoided seeds infected with fungi. In this way, fungi may indirectly affect seed mortality due to predation by granivores, in addition to the direct effects fungi can have on seed survivorship. We suggest that fungi may play an important role in soil seed dynamics, but one that is poorly understood.

Volatile monoterpenes as potential parent structures for new herbicides

Abstract: Eighteen volatile monoterpenes, which are among the major constituents of essential oils from plants, were screened for phytotoxicity to several crop and weed species. Several monoterpenes containing an oxygen function severely inhibited germination of four different annual weeds while concurrently displaying little effect on soybean germination. Soybeans varied in their sensitivity to various monoterpenes (measured by seedling growth), with a-terpineol and geraniol selectivity inhibiting weed seed germination while having little or no effect on soybeans. All of the monoterpenes tested exhibited relatively high germination and growth inhibition to corn and wheat. Nomenclature: Cinmethylin, exo-l-methyl­ 4-(I-methylethyl)-2-((2-methylphenyl)methoxy).7-oxabicy. clo (2.2.1) heptane; Italian ryegrass, Lolium multiflorum Lam. #3 LOLMU; large crabgrass, Digitaria sanguinalis (L.) Scop. #DIGSA; redroot pigweed, Amaranthus retroflexus L. #AMARE; velvetleaf, Abutilon theophrasti Medik. #ABUTH; alfalfa, Medicago sativa (L.) 'Vernal'; corn, Zea mays (L.) 'Pioneer 3377'; cucumber, Cucumis sativus (L.) 'Edmonson'; soybeans, Glycine rna;\: (L.) Merr. 'Williams 82'; wheat, Triticum aestivum (L.) 'Coker 68-19'.

Seed development in relation to desiccation tolerance: A comparison between desiccation-sensitive (recalcitrant) seeds of Avicennia marina and desiccation-tolerant types

Abstract: Development of the highly desiccation-sensitive (recalcitrant) seeds of primarily one species, Avicennia marina, is reviewed and compared with the ontogeny of desiccation-tolerant (orthodox) seeds. A. marina seeds undergo no maturation drying and remain metabolically active throughout development, which grades almost imperceptibly into germination. While PGR control of histodifferentiation is essentially similar to that characterizing desiccation-tolerant seeds, the phase of growth and reserve deposition is characterized by exceedingly high cytokinin levels which, it is proposed, promote a sink for assimilate import. While some starch accumulation does occur, the predominant reserves are soluble sugars which are readily available for the immediate onset of seedling establishment upon shedding. ABA levels are negligible in the embryo tissues during seed maturation, but increase in the pericarp, which imposes a constraint upon germination until these outer coverings are sloughed or otherwise removed. The pattern of proteins synthesized remains qualitatively similar throughout seed development in A. marina, and no LEA proteins are produced. This suggests both that seedling establishment is independent of maturation proteins and that the absence of LEAs and desiccation sensitivity might be causally related. The study on A. marina reveals that for this recalcitrant seed-type, germination per se cannot be defined: rather, it is considered as the continuation of development temporarily constrained by the pericarp ABA levels. This leads to a reexamination of the role of rehydration as key event sensu stricto, in the germination processes in desiccation-tolerant (orthodox) seeds.

Interactions oftrans-cinnamic acid, its related phenolic allelochemicals, and abscisic acid in seedling growth and seed germination of lettuce.

Abstract: Phenolic compounds have been identified as the most common allelochemicals produced by higher plants. Inhibitions of cinnamic acid, its related phenolic derivatives, and abscisic acid (ABA) on seedling growth and seed germination of lettuce were studied.trans-Cinnamic acid, ando-,m-, andp-coumaric acids inhibited the growth of etiolated seedlings of lettuce at concentrations higher than 10−4 M and seed germination above 10−3 M. Coumarin inhibited seedling growth and seed germination at 10−5 M or above. Chlorogenic acid inhibited seedling growth above 10−4 M, but did not inhibit seed germination at 10−5−5×10−3 M. Low concentrations (below 10−3 M) of caffeic and ferulic acids promoted the elongation of hypocotyls, but higher concentrations (over 10−3 M) inhibited seedling growth and seed germination. These phenolic compounds and abscisic acid had additive inhibitory effects both on seedling growth and seed germination. The inhibition on lettuce was reversed by caffeic and ferulic acids at concentrations lower than 10−3 M except for the inhibition of germination by coumarin. These results suggest that in naturetrans-cinnamic acid,o-, m-, p-coumaric acids, coumarin, and chlorogenic acid inhibit plant growth regardless of their concentration. However, caffeic and ferulic acids can either promote or inhibit plant growth according to their concentration.

Establishment of Populus deltoides under simulated alluvial groundwater declines

Abstract: -Establishment, growth and survival of seedlings of Populus deltoides subsp. monilifera (plains cottonwood) were examined in an experimental facility simulating five rates of declining alluvial groundwater. The treatments were permanent saturation, drawdown rates of 0.4, 0.7, 2.9 cm/d and immediate drainage. The experiment was conducted outdoors in planters near Fort Collins, Colorado. Seedling survival was highest under the two slowest drawdown rates and declined significantly with faster drawdown rates. The highest growth rate was associated with the drawdown rate of 0.4 cm/d, in which mean shoot height was 2.4 cm and mean root length was 39 cm 98 days after planting. Growth of shoots and roots was reduced both by saturated conditions and by the more rapid drawdown rates of 0.7 and 2.9 cm/d. No establishment was observed in the immediate drawdown treatment. Whereas maximum biomass accumulation is associated with the most gradual drawdown or saturated conditions, seedlings establishing naturally under such conditions are also most likely to be removed by ice or subsequent flooding. Seedlings establishing in higher topographic positions, in contrast, are subject to increased mortality and reduced shoot growth, resulting from reduced soil moisture. Rapid root extension following establishment allows P. deltoides seedlings to grow across a wide range of groundwater drawdown rates, and thus a variety of positions across a gradient of riparian soil moisture. Our results indicate that in coarse alluvial sands of low fertility, 47% of germinating P. deltoides seeds were able to survive in association with a drawdown rate of 2.9 cm/d and a final water table depth of 80 cm.

Status of nuclear division in arbuscular mycorrhizal fungi during in vitro development

Abstract: The number of nuclei in spores and along hyphae of an arbuscular mycorrhizal fungiGigaspora margarita was measured in digital images of fluorescence arising from mithramycin stained cultures. Typical dormant spores (250 μm diameter) contained 2000 nuclei. Eight hundred nuclei were mobilized during the first 3 days of germination. The number of nuclei in the spores nearly returned to the initial number after 22 days of hyphal growth. The average relative DNA content in the nuclei of dormant spores and in the nuclei of spores incubated for 22 days was comparable, as judged from fluorescence intensity. Hyphal elongation occurred with 460 nuclei per cm under a special set of in vitro conditions that promote extensive hyphal growth of arbuscular mycorrhizal fungi. We found an average total of 26000 hyphal nuclei per germinating spore after 22 days. The specific DNA polymerase α inhibitor aphidicolin did not inhibit spore germination but it rapidly reduced the rate of hyphal growth and arrested growth after 4 days. No nuclei were produced de novo during this time. These results demonstrate thatG. margarita replicates nuclear DNA and undergoes nuclear division when grown in vitro even in the absence of a plant host.

Onion (Allium cepa L.) Seedling Emergence Patterns can be Explained by the Influence of Soil Temperature and Water Potential on Seed Germination

Abstract: A theory is outlined that seedling emergence patterns in the field are largely generated by the influence of soil water potential on a critical event (i.e. the initiation of radicle growth) that determines the base water potential for germination. Above this base the mean rate and distribution of seedling emergence are largely determined by temperature. This proposal was expressed as a parsimonious stochastic model to test whether patterns of seedling emergence recorded in field experiments could be explained. The model applies current understanding of the influence of water potential and thermal time on germination under controlled conditions to the variable environment of horticultural seed beds. Where possible, parameter values were taken from the literature, others were inferred from laboratory experiments on seed germination and seedling emergence. The model accurately predicts the onset of seedling emergence and the occurrence and timing of subsequent seedling flushes in a wide range of seed bed conditions. The study indicates that a co-ordinated use of existing seed-bed preparation techniques to retain soil moisture for initial seed imbibition and limited irrigation, timed to coincide with the critical period when radicle growth is initiated in the seed population, may provide an approach to improving crop uniformity that is simple and practical.

Heat effects on seeds and rhizomes of a selection of boreal forest plants and potential reaction to fire.

Abstract: To analyse the potential reaction to firegenerated heat pulses, seeds of 12 species of plants and rhizomes of three species were exposed to elevated temperatures for 10 min The tested material split into three groups with respect to heat tolerance: (1) the rhizomes, for which the lethal temperatures were in the range 55–59° C; (2) the seeds of most of the species tested, for which the lethal temperatures were in the range 65–75° C; (3) The seeds of two species of Leguminosae and three species of Geranium for which the lethal temperatures were around 100° C For all three Geranium species and for one of the legume species, Anthyllis vulneraria, exposure temperatures above ca 45° C resulted in dormancy release, and maximum germination occurred above 60–65° C Speed of germination was little affected for most species, except after exposure to nearlethal temperatures, where it slowed down dramatically, although the seedlings emerging were healthy We conclude that due to sharp temperature gradients in the soil during fire, differences in heat tolerance between species in most cases are not large enough to be a decisive factor in their post-fire colonising success There are exceptions: the seeds of certain taxa that are impermeable to water in the dormant state, some of which have heat triggered germination

Effects of light and temperature on seed dormancy and gibberellin-stimulated germination in Arabidopsis thaliana: studies with gibberellin-deficient and -insensitive mutants.

Abstract: Effects of light and temperature on gibberellin (GA)-induced seed germination were studied in Arabidopsis thaliana (L.) Heynh. with the use of GA-deficient (gal) mutants, mutants with a strongly reduced sensitivity to GA (gai) and with the recombinant gai/gal. Seeds of the gal mutant did not germinate in the absence of exogenous GAs, neither in darkness, nor in light, indicating that GAs are absolutely required for germination of this species. Wild-type and gai seeds did not always require applied GAs in light. The conclusion that light stimulates GA biosynthesis was strengthened by the antagonistic action of tetcyclacis, an inhibitor of GA biosynthesis. In wild-type, gal and gai/gal seeds light lowered the GA requirement, which can be interpreted as an increase in sensitivity to GAs. In gai and gai/gal seeds light became effective only after dormancy was broken by either a chilling treatment of one week or a dry after-ripening period at 2°C during some months. The present genetic and physiological evidence strongly suggests that temperature regulates the responsiveness to light in A. thaliana seeds. The responsiveness increases during dormancy breaking, whereas the opposite occurs during induction of dormancy (8 days at 15°C pre-incubation). Since light stimulates the synthesis of GAs as well as the responsiveness to GAs, temperature-induced changes in dormancy may indirectly change the capacities to synthesize GAs and to respond to GAs. GA sensitivity is also directly controlled by temperature. It is concluded that both GA biosynthesis and sensitivity to GAs are not the primary controlling factors in dormancy, but are essential for germination.