Showing papers on "Germination published in 1994"

Seeds: Physiology of Development and Germination

Abstract: Seeds: Germination, Structure, and Composition. Seed Development and Maturation. Development-Regulation and Maturation. Cellular Events during Germination and Seedling Growth. Dormancy and the Control of Germination. Some Ecophysiological Aspects of Germination. Mobilization of Stored Seed Reserves. Control of the Mobilization of Stored Reserves. Seeds and Germination: Some Agricultural and Industrial Aspects. Index.

Recruitment of a Mast‐Fruiting, Bird‐Dispersed Tree: Bridging Frugivore Activity and Seedling Establishment

Abstract: The recruitment of Phillyrea latifolia L. (Oleaceae), a bird-dispersed tree of Mediterranean forest, is described. Fruit removal by birds, seed rain, post-dispersal seed predation, seed germination, and seedling emergence, survival, and establishment were studied. The main objective was testing whether seed dispersal by birds produced a pre- dictable seedling shadow as a result of coupled patterns of seed rain, seedling emergence, and seedling establishment. P. latifolia is a mast-fruiting species and large fruit crops were produced in only 2 (1981 and 1989) out of 15 yr (1978-1992). We report here on the 1989 fruiting event at one scrubland and one forest site. Ripe fruits were available from mid-September to early June. Extensive removal by birds started after fruit crops of other species were depleted. Seed dispersers were more abundant, and fruit predators more scarce, in scrubland than in forest. P. latifolia fruits were a major component in the diet of principal seed dispersers (Sylvia atricapilla and Erithacus rubecula) that depended almost exclusively on them for food late in the season. Fruit removal levels were higher, crops were depleted earlier, and individual plants dis- persed more seeds in scrubland than in forest. Crop size was the best predictor of number of seeds dispersed by individual plants in scrubland, while fruit characteristics were more influential in forest. Seed dispersal was largely a within-population phenomenon, as no seed fall occurred in traps set beyond the distributional limits of P. latifolia in the study region. Frugivores produced a spatially predictable seed rain at the two sites. Seed rain was greatest beneath fleshy fruit-producing species (under female individuals in dioecious species) in scrubland and at forest-gap interfaces in forest. Post-dispersal seed predation was low at the two sites (39 and 54% after 1-yr exposure). In forest, seed survival was lower in gaps than in forest interior or forest edges. In scrubland, seed survival differed widely among microhabitats (defined by overlying plant species), ranging from 19% (open ground) to 61% (beneath Rosmarinus officinalis). In forest, density of emerging seedlings was unrelated to location in the habitat mosaic (gap, forest edge, interior). Seedling density did differ among microhabitats in scrubland, where emergence was greatest under fleshy fruit-producing species. Seedling survival was higher in forest than in scrubland, where seedlings incurred greater mortality due to desiccation. In both sites, seedling survival depended significantly on microhabitat and was depressed under adult conspecifics. The activity of frugivores directly impacted seedling distribution in scrubland, as spatial patterns of seed deposition were not overshadowed by later-acting factors, such as rodent seed predation or variation in germination. In forest, there was spatial discordance between seed rain and seedling distribution, as a consequence of uncoupled seed rain and seedling emergence. Spatial patterns of seed deposition by birds may thus have a lasting impact on the population dynamics of P. latifolia, but this will vary among populations depending on the extent of coupling of the different stages in the recruitment process (dispersal-seed rain-germination and seedling establishment).

Review article no. 92 a survey of antifungal compounds from higher plants, 1982-1993

Abstract: Index-Flowering plants; antifungal agents; constitutive compounds; phytoalexins; second- ary metabolites. Abstract-Recent work on the characterization of antifungal metabolites in higher plants is reviewed. Interesting new structures are discussed and the distribution of those substances in different plant families is outlined. Distinction is made between constitutive antifungal agents and phytoalexins, which are specifically formed in response to fungal inoculation. The literature survey covers the 12 years since 1982. INTRODUCTION A fungal spore landing on the leaf surface of a plant has to combat a complex series of defensive barriers set up by the plant before it can germinate, grow into the plant tissues and survive. The arsenal of weapons against the fungus includes physical barriers (e.g. a thick cuticle) and chemical ones, i.e. the presence or accumulation of anti- fungal metabolites. These can be preformed in the plant, the so called ‘constitutive antifungal substances’, or they are induced after infection involving de

Adaptations enhancing survival and establishment of seedlings on coastal dune systems

Abstract: Studies on the survival and establishment of seedlings in coastal and lacustrine sand dune systems suggest that nutrient deficiency, lack of moisture, sand accretion, salt spray and predation are probably the most important limiting factors. Seedlings employ both avoidance and tolerance strategies to with-stand the different stresses. For example, seedling recruitment of dune species coincided with periods of high moisture availability and occurred in years with high well distributed rainfall. The seedlings exhibited rapid vertical elongation of roots, avoided moisture stress by leaf rolling, dense hair on leaves, fleshy cotyledonary leaves, and by growing in the shade of nurse plants. A certain proportion of seedlings of all species survived partial burial and showed stimulation of leaf growth, total leaf area, number of tillers and total dry matter. Seedlings of most species could not survive complete burial, however, some species did grow through the sand deposit by an increase in the number of nodes and length of internodes. Plants responded to salt spray by inhibition of seed germination, developing resistance, increase in leaf thickness and reduced uptake of Na and Cl ions. Several characters such as higher vigour of seedlings, larger seed size and seed polymorphism were also of adaptive significance to the species.

The Induction of Seed Germination in Arabidopsis thaliana Is Regulated Principally by Phytochrome B and Secondarily by Phytochrome A.

Abstract: We examined whether spectrally active phytochrome A (PhyA) and phytochrome B (PhyB) play specific roles in the induction of seed germination in Arabidopsis thaliana (L.) Heynh., using PhyA- and PhyB-null mutants, fre1-1 (A. Nagatani, J.W. Reed, J. Chory [1993] Plant Physiol 102: 269-277) and hy3-Bo64 (J. Reed, P.Nagpal, D.S. Poole, M. Furuya, J. Chory [1993] Plant Cell 5: 147-157). When dormant seeds of each genotype imbibed in the dark on aqueous agar plates, the hy3 (phyB) mutant did not germinate, whereas the fre1 (phyA) mutant germinated at a rate of 50 to 60%, and the wild type (WT) germinated at a rate of 60 to 70%. By contrast, seeds of all genotypes germinated to nearly 100% when plated in continuous irradiation with white or red light. When plated in continuous far-red light, however, frequencies of seed germination of the WT and the fre1 and hy3 mutants averaged 14, nearly 0, and 47%, respectively, suggesting that PhyB in the red-absorbing form prevents PhyA-dependent germination under continuous far-red light. When irradiated briefly with red or far-red light after imbibition for 1 h, a typical photoreversible effect on seed germination was observed in the fre1 mutant and the WT but not in the hy3 mutant. In contrast, when allowed to imbibe in the dark for 24 to 48 h and exposed to red light, the seed germination frequencies of the hy3 mutant were more than 40%. Immunoblot analyses of the mutant seeds showed that PhyB apoprotein accumulated in dormant seeds of the WT and the fre1 mutant as much as in the seeds that had imbibed. In contrast, PhyA apoprotein, although detected in etiolated seedlings grown in the dark for 5 d, was not detectable in the dormant seeds of the WT and the hy3 mutant. The above physiological and immunochemical evidence indicates that PhyB in the far-red-absorbing form was stored in the Arabidopsis seeds and resulted in germination in the dark. Hence, PhyA does not play any role in dark germination but induces germination under continuous irradiation with far-red light. Finally, we examined seeds from a signal transduction mutant, det1, and a det1/hy3 double mutant. The det1 seeds exhibited photoreversible responses of germination on aqueous agar plates, and the det1/hy3 double mutant seeds did not. Hence, DET1 is likely to act in a distinct pathway from PhyB in the photoregulation of seed germination.

Strategies of seed dispersal and germination in plants inhabiting deserts

Abstract: Massive seed consumption is typical in many deserts. The “escape” or “protection” strategies of seed dispersal are important, as they prevent massive seed consumption. The more extreme the desert, the more unpredictable the low amounts and distribution of the rains as well as the beginning and length of the season or seasons with rains. Seeds, which have the highest resistance to extreme environmental conditions, develop during germination into seedlings, which are the most sensitive. Therefore, germination of parts of the seed population at their respective proper times spreads the risk over time and is thus very important for plant survival, especially in those plants inhabiting the more extreme deserts. Each of the plant species studied was found to have its own survival strategies of seed dispersal and germination. At least two extreme seed-dispersal and germination strategies have been observed: 1) the “escape” seed dispersal and “opportunistic” portioned seed-germination strategies, such as inSchismus arabicas andSpergularia diandra, and 2) the “cautious” portioned dispersal by rain of the protected seeds, such as inAsteriscus hierochunticus and portioned dispersal and rapid germination strategies such as inBlepharis spp. The fate of future generations, as far as the germinability of seeds of some species is concerned, depends on the influences of maternal and environmental factors when the seeds are still on the mother plant, mostly during the final stage of seed maturation, as inTrigonella arabica. It may even depend on the position of the caryopsis from which the mother plant originated, as inAegilops geniculata. The dry post-maturation conditions and the environmental factors during seed wetting and germination may also affect the percentage of seed germination, as inSchismus arabicus.

Delayed Germination of Seeds: A Look at the Effects of Adult Longevity, the Timing of Reproduction, and Population Age/Stage Structure

Abstract: The effects of adult longevity, the timing of reproduction, and population age/stage structure on the evolution of seed dormancy are explored in both constant and variable environment models. In the constant environment models complete germination is the evolutionarily stable strategy (ESS) regardless of adult longevity. Incorporating a cost of reproduction on subsequent survival does not alter this result. In contrast, in a variable environment changes in adult longevity can exert a strong selection pressure against seed dormancy. Incorporating a cost of reproduction for iteroparous species reduces adult longevity, which selects for more seed dormancy. The magnitude of the change in ESS germination probability depends on several factors, including which life-history stage is variable (e.g., fecundity, seedling survival), whether seeds can detect favorable sites for establishment, and the age/stage structure of the population. In general, increases in adult longevity select against seed dormancy, but exce...

Effect of salinity, temperature, and growth regulators on the germination and early seedling growth of Atriplex griffithii var. stocksii

Abstract: Seed germination and early seedling growth of Atriplex griffithii var. stocksii from seeds collected from a saline desert habitat in Karachi, Pakistan, were studied. The seeds of A. griffithii did ...

Cytokinins and plant development : an overview

Abstract: This chapter presents a summary of the involvement of cytokinins in seed germination, de novo bud formation, release of buds from apical dominance, leaf expansion, reproductive development, and senescence. The effects of cytokinins on seed germination are not as pronounced; however, numerous studies have shown that cytokinins can influence germination, particularly when conditions are suboptimal for germination. Endogenous cytokinins have been measured in germinating seeds of various species. The level of cytokinin is generally low in dry seeds and initially decreases even further, but then increases during germination. The classic studies by F. Skoog and C. O. Miller demonstrated the dramatic effects of cytokinins on shoot differentiation in tobacco callus. The endogenous cytokinins as well as the metabolism of exogenous cytokinins were determined in relation to adventitious bud regeneration. Cytokinins can inhibit oxidation and prevent the sharp rise in respiration usually occurring during senescence.

Bird-dispersed seed rain and seedling establishment in patchy Mediterranean vegetation

Abstract: The composition and spatial patterns of the seed rain produced by bird dispersers and of the seedlings of fleshy-fruited plants have been studied in patchy Mediterranean vegetation in southern France. The seeds of fleshy-fruited plants were collected in seed traps during a 17-month period (1981-1983) and the fecal samples of bird dispersers mist-netted on the same 2.5-ha site over a 25-month period (1981-1984) were analysed. The locations of the 5 sets of seed traps and the 6 mist-nets were chosen in order to take into account the various plant communities and the successional gradient from old fields to Quercus ilex coppice. Seedling establishment of fleshy-fruited plants was censused in 1983 adjacent to the seed traps. In 1992 seedlings were again censused in the open coppice and in the old fields (...)

Up in smoke: I. Smoke-derived germination cues for postfire annual,Nicotiana attenuata torr. Ex. Watson.

Abstract: Some postfire annuals with dormant seeds use heat or chemical cues from charred wood to synchronize their germination with the postfire environment. We report that wood smoke and polar extracts of wood smoke, but not the ash of burned wood, contain potent cue(s) that stimulate germination in the postfire annual plant,Nicotiana attenuata. We examined the responses of seeds from six populations of plants from southwest Utah to extracts of smoke and found the proportion of viable seeds that germinated in the presence of smoke cues to vary between populations but to be consistent between generations. With the most dormant genotypes, we examine three mechanisms by which smoke-derived chemical cues may stimulate germination (chemical scarification of the seed coat and nutritive- and signal-mediated stimulation of germination) and report that the response is consistent with the signal-mediated mechanism. The germination cue(s) found in smoke are produced by the burning of hay, hardwood branches, leaves, and, to a lesser degree, cellulose. Moreover, the cues are found in the common food condiment, “liquid smoke,” and we find no significant differences between brands. With a bioassay-driven fractionation of liquid smoke, we identified 71 compounds in active fractions by GC-MS and AA spectrometry. However, when these compounds were tested in pure form or in combinations that mimicked the composition of active fractions over a range of concentrations, they failed to stimulate germination to the same degree that smoke fractions did. Moreover, enzymatic oxidation of some of these compounds also failed to stimulate germination. In addition, we tested 43 additional compounds also reported from smoke, 85 compounds that were structurally similar to those reported from smoke and 34 compounds reported to influence germination in other species. Of the 233 compounds tested, 16 proved to inhibit germination at the concentrations tested, and none reproduced the activity of wood smoke. By thermally desorbing smoke produced by cellulose combustions that was trapped on Chromosorb 101, we demonstrate that the cue is desorbed between 125 and 150°C. We estimate that the germination cues are active at concentrations of less than 1 pg/seed and, due to their chromatographic behavior, infer that a number of different chemical structures are active. In separate experiments, we demonstrate that cues remain active for at least 53 days in soil under greenhouse conditions and that the application of aqucous extracts of smoke to soil containing seeds results in dramatic increases in germination of artificial seed banks. Hence, although the chemical nature of the germination cue remains elusive, the stability of the germination cues, their water-solubility, and their activity in low concentrations suggest that these cues could serve as powerful tools for the examination of dormant seed banks and the selective factors thought to be important in the evolution of postfire plant communities.

Seed dispersal by forest chimpanzees in Uganda

Abstract: The role of chimpanzees (Pan troglodytes) as seed dispersers in the Kibale Forest Reserve of western Uganda was assessed by analysing 1849 dung samples from two chimpanzee communities and by conducting germination trials on dispersed and non-dispersed seeds. Of the chimpanzee dung samples, 98.5% contained seeds, and fig seeds were the most common. The number of large seeds (>2 mm) per dung sample was often high, even for species with a relatively large seed. For example, Mimusops bagshawei has a 15-mm diameter seed and had an average of 26.5 seeds per sample. In addition, many seed species were found in the dung repeatedly over several months. Germination trials demonstrated that the species of seed commonly collected from dung were viable. Comparisons of both the rate and success of germination of chimpanzee-dispersed seeds with seeds collected directly from the parent trees, showed that gut passage increased the speed and probability of germination. The number of large seeds dispersed by the chimpanzee population was estimated at 369 large seeds km-' day-'. We suggest that in Kibale, chimpanzees may play a more significant role in primary seed dispersal than implied by their low numbers and biomass.

The role of seed coats in seed viability

Abstract: The seed coat is the seed’s primary defense against adverse environmental conditions. A hard seed coat protects the seed not only from mechanical stress but also from microorganism invasion and from temperature and humidity fluctuations during storage. Phenolic compounds in the seed coat contribute to seed hardness and inhibition of microorganism growth. During germination, the seed coat protects the seed from hydration stress and electrolyte leakage.

In situ and in vitro specificity between Rhizoctonia spp. and Spiranthes sinensis (Persoon) Ames. var. amoena (M. Bieberstein) Hara (Orchidaceae)

Abstract: SUMMARY The relationships between the orchid Spiranthes sinensis (Persoon) Ames. var. amoena (M. Bieberstein) Hara and Rhizoctonia spp. were investigated in situ at germination and in adult plants, Seeds of the orchid placed in cotton gauze were buried at 210 sampling points in turf grassland, the orchid habitat (in situ germination). Eight weeks later, protocorm development of the orchid was confirmed at 67 of the 210 sampling points. Isolation of fungi from protocorms showed that in situ germination was induced mainly by Rhizoctonia rapens Bernard. Similarly, R. repens was the dominant mycorrhizal fungus isolated from roots of adult plants. The number of adult plants within a radius of either 30 or 50 cm of burial points did not influence seed germination. The distribution of Rhizoctonia spp. other than R. repens in the sample site was examined with a baiting method using buckwheat stems. Thirty-two isolates consisting of binucleate Rhizoctonia anastomosis group (AG)-A, AG-B, AG-G, and AG-1, R. solani Kuhn AG-4, Waitea circinata Warcup & Talbot, which anastomozed with WAG-O and WAG-Z, and a multinucteate Rhizoctonia sp. were isolated. Three AG-G isolates were obtained from the points at which protocorm development was induced by R. repens. Seeds of S. sinensis var. amoena were inoculated in vitro with these isolates to test for symbiotic germination. Most Rhizoctonia spp, not associated with the germination in situ induced seed germination in vitro. Seedlings which developed with these isolates in vitro were transferred to ex vitro conditions. New leaves developed and elongated as seedlings continued to grow for 3 months, The seed burial method enabled the clarification of the differences in orchid-fungal specificity in situ and in vitro. We concluded that the specificity between S. sinensis var. amoena and fungi in situ conditions was different to that in vitro.

Effects of sediment load on seedling emergence from wetland seed banks

Abstract: We examined the effects of sediment depth on emergence of seedlings from wetland seed banks, with the goal of understanding potential effects on wetlands of sediment runoff from agricultural fields. Seed germination was studied in the greenhouse using seed bank samples taken from natural wetlands in central Iowa, U.S.A. Sediment loads as low as 0.25 cm significantly reduced the number of species and total number of individuals recruited from seed bank samples. Addition of sediment decreased the number of individuals appearing for most, but not all, species. The change in number of seedlings that occurred in treatments with 1 cm of sediment cover was related to seed mass, with larger-seeded species showing the least effect of burial by sediment.

Assessing toxicity of spent pig litter using a seed germination technique

Abstract: The phytotoxicity of spent litter collected from pig pens employing the ‘[pig-on-litter’ system at various times was evaluated using seed germination and root elongation techniques. The percentage seed germination of four plant species (lettuce, Chinese cabbage, tomato and green beans) was not affected by the water extracts of spent litter samples collected in the first 30 weeks of production. Seed germination was significantly retarded by litter extracts from 34 weeks onwards. The percentages of seed germination at the end of the 45 weeks study were 1% for lettuce, 16% for cabbage, 21% for tomato, and 44% for green beans. Compared with seed germination, root elongation was more sensitive to the toxicity of the spent litter. The root lengths of all seedlings except green beans were less than 50% of the control (deionized water) throughout the experiment. The inhibitory effects of spent litter on root elongation increased with the age of the litter. The final root lengths of lettuce, Chinese cabbage and tomato seedlings were 14%, 24% and 28% of the control, respectively. Green beans behaved very differently from the other species; spent litter extracts stimulated root growth throughout the study, except the last week. The elevated concentrations of heavy metals (in particular extractable Cu) and nutrients (especially NH 4 + N) present in spent litter were the main factors responsible for the phytotocicty of the spent litter. The aged spent litter had accumulated more salts, nutrients and heavy metals and imposed more toxic effects on seed germination than did the young spent litter.

Seeds of Kosteletzkya virginica (Malvaceae): their structure, germination, and salt tolerance. II. Germination and salt tolerance

Abstract: Kosteletzkya virginica (L.) Presl. (Malvaceae) is a perennial that grows in saline or brackish water, and is salt-tolerant in its mature state, but less tolerant during germination. The seeds show a very low permeability to water that increases during storage. The permeability to water differs in seeds harvested in different years. Optimal temperature for germination is 2830 C. The effect of salinity on imbibition is largely osmotic, but germination is inhibited, apparently, by the combined osmotic and "ionic" effects, especially at high NaCl concentrations. Inhibition of germination by high NaCl concentrations is relatively more severe in scarified than in intact seeds, indicating that the seed coat acts as a partial barrier to Na+ influx. External application of proline or betaine did not improve germination under saline conditions. Dry seeds contain a significant amount of betaine and 1ow levels of proline, but during germination and in the presence of NaCl the betaine content decreased while the proline content increased. Thus, the likely compatible solute in the germinating seed seems to be proline. Kosteletzkya virginia (L.) Presl. (Malvaceae) is a perennial that grows in salt or brackish marshes along the warmer part of the Atlantic and Gulf shores of the United States where it flowers during the months of July to October (Radford, Ahles, and Bell, 1968). It is a C3 plant inhabiting ecologically stable habitats such as the high marsh or areas flooded with brackish water (Rumer, 1982). Somers (1982) has reported that K. virginica is more

Accumulation of reactive oxygen species and oxidation of cytokinin in germinating soybean seeds.

Abstract: Seed germination is an important developmental switch when quiescent seed cells initiate oxidative phosphorylation for further development and differentiation. During early imbibition of soybean seeds (Glycine max L. cv. Weber), a superoxide dismutase (SOD) activity peak was observed, in embryonic axes, after 6 h imbibition. Peroxidase activities, including catalase, were significantly increased after 12 h inhibition and during germination phase III. Catalase was the most efficient enzyme in catabolizing H2O2 in embryonic axes. When stored at 42 degrees C and 100% relative humidity, seeds were stressed and lost their viability in a time-dependent manner. A significant increase in the Cu, Zn-superoxide-dismutase activity, and to a lesser extent, Mn superoxide dismutase activity was observed during germination in low-viability (stressed) seeds as compared to high-viability (unstressed) seeds. Northern blot analysis confirmed that superoxide dismutase induction resulted from an accumulation of its transcripts in response to the production of O2-. The induction of catalase did not occur in low-viability seeds, resulting in dramatic accumulation of H2O2. Using capillary electrophoresis, HPLC and NMR we found that the endogenous cytokinin, zeatin riboside, was present in large quantities in the high-viability seeds, but it was oxidized into adenine in the low-viability seeds. In vitro superoxide anion could also oxidize the cytokinin. Zeatin riboside, but not adenine, was found to act as a scavenger of superoxide anions and may help to maintain seed viability by detoxifying reactive oxygen species. Germination of stressed seeds was partially restored by the addition of exogenous cytokinin (zeatin riboside). Protection against oxidative stress by cytokinin seemed to be a general phenomenon, as Escherichia coli cells were also protected against superoxide stress in the presence of cytokinin.

Hydrothermal time analysis of tomato seed germination at suboptimal temperature and reduced water potential

Abstract: Both temperature (T) and water potential (ψ) have consistent and quantifiable effects on the rate and extent of seed germination (radicle emergence). Germination at suboptimal T can be characterized on the basis of thermal time, or the T in excess of a base (Tb) multiplied by the time to a given percentage germination (tg). Similarly, germination at reduced ψ can be characterized on a hydrotime basis, or the ψ in excess of a base (ψb) multiplied by tg. Within a seed population, the variation in thermal times to germination for a specific percentage (g) is based upon the normal distribution of ψb values among seeds (ψb(g)). Germination responses across a range of suboptimal T and ψ might be accounted for by a general hydrothermal time model incorporating both T and ψ components. We tested this hypothesis for tomato (Lycopersicon esculentum Mill.) seeds of two genotypes differing in germination rates and tolerance of suboptimal T and ψ. For combinations of T (10−25°C) and ψ (0 to −0.9 MPa), a general hydrothermal time model accounted for approximately 75% of the variation in times to germination within the seed populations of both genotypes, and over 96% of the variation in median germination rates. However, ψb(g) distributions were sensitive to both the T and ψ of imbibition, resulting in a poor fit of the model to specific time course data. Analysis of germination timing separately for low and high ψ ranges within a given T resulted in specific models accounting for 88−99% of the variation in individual germination times and >99% of the variation in madian germination rates. Thus, for a given T and ψ range, the hydrotime model closely matched tomato seed germination time courses. Accumulated hydrothermal time accounted well for germination rates at ψ> −0.5 MPa across suboptimal T if ψb(g) was allowed to vary with T. Germination did not show a consistent response to T at ψ < −0.5 MPa, and estimated Tb values varied over different T ranges. Generalization of the hydrothermal time model across the entire range of suboptimal T and ψ was limited by physiological adjustments of the seeds to their current environment. The hydrothermal time model detected and quantified these adjustment processes that would otherwise not be evident from inspection of germination time courses. Temperature and water potential influence the time to germination via physiological mechanisms that reciprocally interact.

Germination strategies during grassland succession.

Abstract: 1. We analysed the germination of 91 herbaceous species in response to a temperature gradient, and to alternating temperatures and stratification (chilling). A principal-components analysis revealed that the species were distributed along two statistically independent axes, the first of which primary represented the optimal temperature for germination and the second the rate of germination. 2 These results were further related to data on species replacement during 25 years of succession in grassland after the cessation of fertilizer application but with continued hay making. Separate successional sequences were distinguished for the dry parts and the wet parts of the fields. Changes in canopy structure were studied by counting gaps and mapping light at the soil surface. 3 Changes in species composition of species present at the different stages were related to the changes in the germination attributes during the succession towards nutrient-poor grassland. The productive grassland was characterized by rapidly germinating species and in the wetter parts, by those germinating at low temperatures. This might enable these species to escape from light competition by germinating in the autumn or winter, i.e. soon after seed set. The species from the less productive, more open stages germinated more slowly, and responded more clearly to stratification and alternating temperatures. It is therefore likely that germination of most seeds of these species is delayed until the following summer. 4 The importance of the observed interspecific differences in germination characteristics in relation to the observed changes in vegetation structure as an explanation for species replacement during succession is discussed.

Potential of Trichoderma spp. as consistent plant growth stimulators

Abstract: In a series of repeated trials, six Trichoderma spp strains, applied as a dried powder from a liquid fermentation in molasses/yeast medium, proved to be consistent at promoting the growth of lettuce (Latuca sativa L) seedlings grown in a peat-sand potting compost in the glasshouse Strains WT, 92, 20, and 75 at 075% or 1% w:w concentrations increased shoot dry weight by up to 26%, although WT did inhibit germination For example, after 4 days only 13% of seeds sown in WT 1% w:w treated compost had germinated, whereas in other treatments germination was consistently greater than 32% WT increased shoot fresh and dry weights by 143 g and 06 g per pot, respectively, without affecting the root dry weights, to give concomitant increases in shoot: root ratios of fresh and dry weight The potential use of these Trichoderma spp strains for plant growth promotion is discussed

Photostimulation of seed germination during soil tillage

Abstract: SUMMARY We conducted intensively replicated field experiments in the Willamette Valley. Oregon, USA, in order to study the role of light in triggering seed germination during soil tillage. We found that the normal practice of cultivating agricultural land during daytime can increase germination of buried seed populations between 70 and 400 above the levels recorded following nighttime cultivations. Experimental reduction of the irradiance under the tillage implements during daytime cultivation decreases the number of dicotyledonous seedlings emerged. while strong artificial illumination (>300μmol m−2 s−1; 400–800nm) of the soil surface under the implements during nighttime tillage significantly increases seedling densities. These results suggest that the enhancement of seed germination caused by daytime tillage, compared with nighttime tillage, is due to light that penetrates into the soil during the actual disturbance. The detection by the seeds of this very short exposure to sunlight requires a high photosensitivity. which provides an adaptive‘purpose’for the evolution of the very-low-fluence response mechanism in phytochrome-controlled seed germination. Seedling emergence induced by nighttime control tillage was considerable in some experiments. suggesting that light perceived by seeds after cultivation or other microenvironmental factors affected by tillage may be important in triggering germination in disturbed soil.

Seeds in soil and worm casts from a neutral grassland

Abstract: Density of germinable seeds was determined in each of 128 blocks of surface soil, each 7×7 cm, together forming a 112x56 cm rectangle, taken from a lowland, species-poor sown grassland in Devon. The turf contained only six species, but seeds of 27 species germinated from the soil. The commonest species in the turf, Festuca rubra, was absent from the seed bank. The second most frequent species in the seed bank, Coronopus didymus, was absent from the turf. Trifolium repens was common in both. The seeds of several weed species had survived in the soil for up to 10 years. Buried seeds of all the common species were significantly clumped. For species common to both, there was no evidence of significant correlation between small-scale abundance in the turf and the seed bank (...)

Differential tolerance to Cd and Zn of arbuscular mycorrhizal (AM) fungal spores isolated from heavy metal-polluted and unpolluted soils

Abstract: Spores of arbuscular mycorrhizal fungi were isolated from two soils of field trials at INRA-Bordeaux (France) polluted by long-term application of a zinc-polluted sewage sludge (S2 soil) or treated with cadmium nitrate (Cd40 soil) and from corresponding unpolluted soils (F and Cd0 soils). These AM fungi were tested for their tolerance to Cd and Zn added as salt solutions with increasing concentrations (0 to 10 mg L−1) in a simple spore germination device. According to preliminary identification the predominant species in S2 and F cultures was Glomus mosseae, whereas Cd40 and Cd0 cultures contained a mixture of at least G. mosseae and G. etunicatum. Germination of Cd40 spores was more tolerant to Cd and Zn than for Cd0 spores, with EC50 values of 73 and 158 μmol L−1 added Cd and Zn corresponding to approximately 10 and 13 μmol L−1 remaining in solution in the device. The S2 spores from the sludge contaminated soil were more tolerant to Zn (EC50=87 μmol L−1), but not to Cd (EC50=7.5 μmol L−1), than the spores from the farmyard manure-treated F soil (EC50=38 and 8.8 μmol L−1, respectively). Thus, S2 culture exhibited a specific tolerance to Zn, which was lower than the unspecific tolerance of Cd40 culture to both Cd and Zn, despite the much higher Zn availability in S2 soil. These results indicate that AM fungi from different soils may differ in their metal susceptibility and that both metal specific and unspecific tolerance mechanisms may be selected in metal polluted soils.

Effects of Dispersers, Gaps, and Predators on Dormancy and Seedling Emergence in a Tropical Herb

Abstract: The influence of biotic and abiotic factors on seed fates are exceptionally difficult to analyze in natural populations because it is not usually possible to identify an emerging seedling with its history as a seed. Large seed size and the unusual feature that the seed body remains attached to the new seedling in the ant—dispersed herb Calathea ovandensis provided an opportunity for field experiments that examine the influence of (1) seed dispersers, (2) treefall gaps, and (3) post—dispersal predators on seed survival, seed dormancy, seedling emergence, and seedling distributions. In a seed depot experiment, seed—dispersing ants significantly affected the seedling emergence patterns in three ways. Seeds with rewards for ants: (a) had improved seedling emergence, by 1.6—fold (17.4 vs. 10.6%); (b) gave rise to seedlings with a mean dispersal distance that was threefold farther (34 vs. 114 cm); and (c) generated a non—leptokurtic seedling shadow (kurtosis: —0.60, not significant vs. 10.84, P < 0.001) in contrast to seeds without ant rewards. There was significant spatial variation in the effect of ant rewards on seedling emergence and also temporal variation in seedling emergence, the greatest number of seedlings emerging in an el Nino year. In two germination box experiments: (1) treefall gaps enhanced seedling emergence 180—fold (72.4 vs 0.4%) compared to the deep shade, and ungerminated seeds in the deep shade were mostly dormant, not dead; (2) post—dispersal seed predation was very low (overall mean of 6.2%), with considerable microspatial variability, and pathogens and small invertebrates accounted for only one—fourth as much seed mortality as larger (vertebrate and large insect) post—dispersal predators. Seedling emergence in this ant—dispersed herb with a relatively long—lived seed bank depends upon a complex of factors, suggesting an interaction of the effects of biotic dispersal to local safe microsites with larger scale effects of spatial and temporal variation in the abiotic environment.

Lipid peroxidation and peroxide-scavenging enzymes associated with accelerated aging of peanut seed

Abstract: Accelerated aging is known to reduce seed viability and vigor in many crop species. The phenomenon is due in part to aging-induced lipid peroxidation, which has the potential to damage membranes of the seed tissues. This study was undertaken to evaluate the effect of accelerated aging on germinability and several physiological characteristics related to peroxidation in the seed of two peanut cultivars. Accelerated aging was achieved by incubating seed at 45°C and 79% relative humidity in a closed chamber for 3, 6, or 9 days. The results indicate that accelerated aging inhibited seed germination and seedling growth. Enhanced lipid peroxidation and increased peroxide accumulation were observed in the axis and cotyledons of aged seed. Accelerated aging also inhibited the activity of superoxide dismutase, peroxidase, ascorbate peroxidase, and lipoxygenase. Seed axes appeared to be more susceptible to aging than cotyledons. The changes in germination and physiological activities, expressed as a function of aging duration, were similar in the two cultivars, despite differences in their seed weight.

Implications of calcium nutrition on the response of Phaseolus vulgaris L. to salinity

Abstract: The effect of Ca2+ level in the growth medium on the response of germination and early seedling growth of Phaseolus vulgaris to NaCl salinity was investigated. When NaCl concentration was increased germination and early seedling growth was decreased. The addition of Ca2+ to the media increased both germination percentage and seedling growth. Chloride concentrations were not affected by the level of Ca2+. Potassium and Ca2+ concentrations and transport from roots to shoots were decreased by NaCl, but were restored by increasing Ca2+ in the medium. The opposite was true for Na+. Leakage of NO3- and H2PO4- was increased by salinity and reduced by high Ca2+ in the medium. The results are discussed in terms of the beneficial effects of calcium for plant growth under saline conditions.