Showing papers on "Seedling published in 1992"

Effect of Shrubs on Recruitment of Quercus Douglasii and Quercus Lobata in California

Abstract: Biologically derived safe sites appear to strongly affect spatial and temporal patterns in terrestrial plant communities; however, few experimental field studies have been conducted to investigate the importance of such interactions or the mechanisms by which they operate. Quercus douglasii and Quercus lobata are winter-deciduous oaks en- demic to California. I examined associations between seedlings of these oaks and several species of shrubs and conducted field experiments that tested for facilitative effects of two shrubs, Salvia leucophylla and Artemisia californica, on seedling survival. Naturally oc- curring Q. douglasii seedlings were relatively common (280 seedlings/ha) and were strongly associated with shrub canopies. In two experimental plantings, 30% and 55% of Q. douglasii seedlings that emerged under shrubs survived for > 1 yr, whereas no seedlings in the open survived in either experiment. Survival of Q. douglasii where shrubs were removed was similar to survival in the open grassland, indicating that association between shrubs and oak seedlings was not due to shared microsite requirements. Q. douglasii seedling survival was not different between the two shrub species. Only three Q. lobata seedlings were found in 26 study plots, although reproductive adults were present. The seedlings were under shrub canopies. However, the survival of Q. lobata was not facilitated by shrub cover or shrub simulation and no other evidence was found for nurse-shrub interactions with this oak species. Artificial shade was critical to the survival of Q. douglasii seedlings in simulated shrub environments. When grown in artificial shade, photosynthetic capacities and root elon- gation rates of Q. douglasii seedlings were significantly higher than those of Q. lobata. These differences may partially explain why nurse-plant interactions exist between shrubs and Q. douglasii seedlings but not between the same shrubs and Q. lobata seedlings. Causes of acorn and seedling mortality differed between the shrub and open grassland habitats. Under shrubs, acorn predation was the primary cause of mortality, whereas mortality due to shoot herbivory was much more frequent in the open grassland. Thus, propagules under shrubs that avoided predation until shoot emergence benefitted from canopy shade and protection from herbivores. The shade tolerance and nurse-plant utilization of Q. douglasii seedlings have parallels with shade tolerance and late successional roles of other oak species in temperate deciduous forests, and, as in these forests, may affect the long-term spatial dynamics in California woodlands.

Experimental Studies of Seedling Recruitment from Contrasting Seed Distributions

Abstract: To examine how selection may act on seed dispersal, we experimentally manipulated seed distributions and compared levels and patterns of seedling recruitment in three altered distributions, relative to the nonmanipulated, leptokurtic distribution. We picked up, then redistributed the wind-dispersed seeds of two isolated parents of Tachigalia versicolor, a tropical tree species growing on Barro Colorado Island (BCI), Panama. Three experimental distributions were created in long columns adjacent to or well beyond the undisturbed column of the natural distribution: (1) a nonleptokurtic "even" distribution of uniform density equal to the mean density throughout the natural column, (2) a "mixed" distribution with the same total number and kurtosis as the natural column, but composed of an equal proportion of seeds from the two parent trees, and (3) an "extended-tail" distribution spread in low uniform density up to 1.8 km beyond the natural distribution. Thus, the study addressed the relative importance of density, distance, and genetic makeup on offspring recruitment. The results of the 2 yr observations of seed and seedling survival indicate that, relative to the natural distribution, seedling recruitment at 2 yr was greater in the even distribution, was minimally affected by a mixture of genotypes from overlapping distributions of two parent trees, and was very infrequent from seeds dispersed to a wide variety of distant sites. Although the mixture of genotypes from overlapping distributions did enhance the probability of survival at one site, that effect occurred only during the seedling and not the seed stage. For the extended-tail and mixed distributions, in which offspring of both parents occurred at both sites, the impact of site differences (survival in Site A tail > Site B tail; survival in Site A mixed > Site B mixed) exceeded that of parent differences (Parent A > Parent B). The major differences among distributions arose in the seed and early seedling stages (<2 mo), and were due to heavy mortality primarily from seed predators (postdispersal by terrestrial mammals) and mammalian herbivores eating young seedlings. Mammalian predation and herbivory were particularly heavy in the extended-tail distributions. It ap- peared that either predator satiation and/or incomplete searching occurred in the three, much more dense distributions adjacent to the parent trees. In contrast, density-dependent mortality at the spatial scale of -iM2 experimental quadrats was limited to very high densities and occurred only in early seedling stages (0-2 mo). Distance effects on the probability of survival were inconsistent both between sites and among distributions. However, the absolute number of seedlings surviving at 2 yr declined with distance from the parent in all three distributions, except the even distribution at Tree A. Additionally, seedling recruitment at any distance < 100 m from the parent for all three distributions greatly exceeded recruitment at extended-tail (100-1800 m) sites. In summary, offspring recruitment was enhanced by the high cohort density at a large spatial scale (i.e., within a circle of 100 m radius) because of predator satiation. At the same time, it was enhanced slightly by the evenness of the distribution within this scale because of density-dependent mortality at a smaller spatial scale (i.e., 1 M2). Therefore, we conclude that selection on dispersal by Tachigalia versicolor on BCI should act to increase the uniformity of the seed distribution, but not to increase dispersal distance to the point that the density of the seed shadow is diluted.

Seedling growth in relation to seed size among species of arid Australia

Abstract: It has been reported that species from environments where seedlings are prone to drought during establishment tend to have larger seeds. One proposed reason for this is that larger-seeded species might be able to allocate a larger proportion of mass to roots rather than shoots during early growth. Seedlings of 32 species from arid central Australia were grown in coarse sand under standard conditions in a glasshouse, and harvested 10 days after germination. Seedlings from heavier-seeded species did not allocate relatively more resources to roots than lighter-seeded species. Complementary experiments confirmed this result for plants grown on sandy loam, in drying as compared to well-watered soil, and under shaded conditions (...)

Seeds: The Ecology of Regeneration in Plant Communities

Abstract: Reproductive allocation and reproductive effort in plants, F.A. Bazzaz and D.D. Ackerly maternal effects on seeds during development, Y. Gutterman the ecology of seed dispersal, M.F. Wilson animals as seed dispersers, E.W. Stiles fruits and frugivory, P. Jordano seed predators and plant population dynamics, M.J. Crawley longevity, viability and dormancy, A.J. Murdoch and R.H. Ellis the functional ecology of seed banks, K. Thompson seed responses to light, T.L. Pons the role of temperature in germination ecophysiology, R.J. Probert effect of chemical environment on seed germination, C.M. Karssen and H.W.M. Hilhorst the contribution of seedling regeneration to the structure and dynamics of plant communities and larger units of landscape, J.P. Grime and S.H. Hillier.

Regeneration in Phaseolus vulgaris L.: High-frequency induction of direct shoot formation in intact seedlings by N(6)-benzylaminopurine and thidiazuron.

Abstract: Induction of prolific shoot formation in Phaseolus vulgaris L. cv. Kinghorn Wax was achieved by germinating mature seeds and growing seedlings on a medium supplemented with 10 μM thidiazuron (TDZ), a substituted phenylurea, or 80 μM N6-benzylaminopurine (BAP). Culture for 7 d in the presence of 10 μM TDZ was sufficient to induce maximal shoot formation, whereas a continuous presence of BAP was required for the induction and development of shoots. The differentiation of adventitious shoots occurred within four weeks of seed culture, from tissues in the regions of axillary buds on the cotyledonary node and also areas surrounding the shoot apex of the intact seedling. The number of shoots regenerated from intact seedlings was significantly higher than that obtained with expiants. Regenerated shoots developed into flowering plants. Similar results were obtained in six other bean cultivars.

Estimating the composition of a forest seed bank: a comparison of the seed extraction and seedling emergence methods

Abstract: The composition of a forest seed bank was estimated using two methods: (i) seed extraction, i.e., the physical separation of the seeds from the soil via flotation in a salt solution, and (ii) seedl...

Weed Seedbanks of the U.S. Corn Belt: Magnitude, Variation, Emergence, and Application

Abstract: Seedbanks and seedling emergence of annual weeds were examined in arable fields at eight locations in the Corn Belt. Seed densities were estimated by direct seed extraction from each of several soil cores in each sampled plot. Average total seedbank densities ranged from 600 to 162 000 viable seed m-2 among locations. Coefficients of variation (CV) typically exceeded 50%. CV for seed densities of individual species usually exceeded 100%, indicating strongly aggregated distributions. CV were lower for species with dense seed populations than those with sparse seed populations. Variance of total seedbank densities was unstable when < 10 cores were examined per plot, but stabilized at all locations when ≥ 15 cores were analyzed, despite a 12-fold difference in plot size and 270-fold difference in seed density among locations. Percentage viable seed that emerged as seedlings in field plots ranged from < 1% for yellow rocket to 30% for giant foxtail. Redroot pigweed and common lambsquarters were the most frequently encountered species. Emergence percentages of these species were related inversely to rainfall or air temperatures in April or May, presumably because anoxia and/or high temperatures induced secondary dormancy in nondormant seed. From 50 to 90% of total seed in the seedbank were dead. This information can be employed by bioeconomic weed management models, which currently use coarse estimates of emergence percentages to customize recommendations for weed control.

Salt stress limitation of seedling recruitment in a salt marsh plant community.

Abstract: Seedling recruitment in salt marsh plant communities is generally precluded in dense vegetation by competition from adults, but is also relatively rare in disturbance-generated bare space. We examined the constraints on seedling recruitment in New England salt marsh bare patches. Under typical bare patch conditions seed germination is severely limited by high substrate salinities. We examined the germination requirements of common high marsh plants and found that except for one notably patch-dependent fugitive species, the germination of high marsh plants is strongly inhibited by the high soil salinities routinely encountered in natural bare patches. Watering high marsh soil in the greenhouse to alleviate salt stress resulted in the emergence of up to 600 seedlings/225 cm2. The vast majority of this seed bank consisted of Juncus gerardi, the only common high marsh plant with high seed set. We tested the hypothesis that salt stress limits seedling contributions to marsh patch secondary succession in the field. Watering bare patches with fresh water partially alleviated patch soil salinities and dramatically increased both the emergence and survival of seedlings. Our results show that seedling recruitment by high marsh perennial turfs is limited by high soil salinities and that consequently their population dynamics are determined primarily by clonal growth processes. In contrast, populations of patch-dependent fugitive marsh plants which cannot colonize vegetatively are likely governed by spatially and temporally unpredictable windows of low salinities in bare patches.

Relationship between endophyte infestation level of tall fescue seed lots and Rhizoctonia zeae seedling disease

Abstract: The objective of this research was to define the relationship between endophyte (Acremonium coenophialum) infestation level of tall fescue (Festuca arundinacea) seed lots and a seedling disease caused by Rhizoctonia zeae. Seeds were planted in soilless medium or medium amended with R. zeae cultures, and number of seedlings per three 5.0-cm cores was determined. In a preliminary experiment, percentage of loss (61%) was greater for Forager (a low-endophyte cultivar) than for a Kentucky 31 seed lot with an 85% endophyte infestation level (32%) (.)

Evolution of seed dispersal and recruitment in clonal plants

Abstract: Previous studies have shown that seedling recruitment occurs sporadically in the population dynamics of many clonal plant species, and that a low rate of seed recruitment is associated with high seed dispersal ability. The aim of the present study was to examine these features of clonal plant life histories from an evolutionary point of view. No significant relationships were detected between recruitment pattern, seed size, seed dispersal, and possession of seed bank. However, a tendency was found that species characterized by a persistent seed bank, lack of seed dispersal traits, or repeated seedling recruitment, possess smaller seeds than do other clonal species. Distributions of fruit and life form characters in 35 genera of clonal plants indicated that traits affecting seed dispersal generally are evolutionarily ancestral in relation to the clonal growth form possessed by extant species. This sequence was found in 32 genera whereas the opposite sequence was found in 3 genera. Thus, seed dispersal features have not evolved as a result of recruitment difficulties in the vicinity of adult plants possessing the extant clonal life form. It is suggested that clonal propagation may have been augmented in lineages where long distance seed dispersal predominates, in order to make genet fitness less dependent on local dispersal by seed. A low rate of seedling recruitment within patches of adult plants is the expected outcome of this scenario. Finally, some aspects of how long term environmental changes may affect seedling recruitment in clonal plants are briefly discussed.

Seed and seedling vigour in relation to crop growth and yield

Abstract: In theory, seed germination, vigour and size (three aspects of seed quality) may influence crop yield through both indirect and direct effects. The indirect effects include those on percentage emergence and time from sowing to emergence. These influence yield by altering plant population density, spatial arrangement, and crop duration. Direct effects on subsequent plant performance are more difficult to discern. A number of different approaches to testing the hypothesis that seed quality affects subsequent plant performance (implicit in some definitions of vigour) are illustrated. The results show that it is possible to detect such effects in some circumstances.

Seedling recruitment patterns in a tropical dry forest in Ghana

Abstract: . Studies of seedling demography and dynamics were carried out in a dry forest at Pinkwae, Ghana (rainfall 1100 mm). Seedlings of tree and liana species in permanent transects were tagged, identified, measured and recensused at bimonthly intervals. A total of 1931 seedlings in 53 species were followed over 14 censuses during a 2-yr period. Germination and mortality were seasonal in their distribution; mortality was highest in dry periods. Seedling density varied seasonally, with the highest densities in comparatively wet periods. Diversity of seedlings was low, corresponding to that of the adult assemblage. Survivorship of tagged seedlings was recorded during the study period. The cohort tagged at the first census, comprising 1033 individuals of mixed ages and species, lost 61.9% during a 24-month period. Many seedlings were found to be rather large at the initial post-germination stage, with strong root development. Vegetative reproduction by root suckers or epicormic shoots was quite common at the site, although the majority of regeneration was by seeds. The current seedling assemblage appears to be representative, in terms of composition and diversity, of the adult species assemblage at Pinkwae.

Seed removal and seed fate in Gustavia superba (Lecythidaceae).

Abstract: The fate of the large (mean 12.6 g) seeds of Gustavia superba, an understory tree, was analyzed on Barro Colorado Island (BCI), Panama. Eight hundred thread-marked seeds were placed on the ground at two contrasting 1-ha forest sites (Gustavia-rich vs Gustavia-poor) during two periods (June and July) of the fruiting season of G. superba. These months correspond to maximum food availability on BCI. On average, 85.5 percent of the seeds were removed within 28 days, 47.5 percent and 3.8 percent of them being found scatterhoarded (buried) by agouti (Dasyprocta punctata) and gnawed by rodents, respectively, within 10 m of their origins. The effect of site and the interaction of site with month significantly affected seed removal rates, but not scatterhoarding rates. Proportions of seeds removed were greater where G. superba trees were rare. Because freshly fallen seeds were not infested by bruchid weevils, unburied and germinating seeds provide an abundant short-term food supply for terrestrial mammals. Burying seeds allowed agoutis to later consume cotyledons of germinating seeds from mid-August throughout October, when food is scarce on BCI. Overall seed dispersal effectiveness (% seed dispersal multiplied by % seedling survival) contrasts dramatically between forest sites, being 10.1 percent and 0.75 percent at Gustavia-rich and Gustavia-poor areas, respectively. This result suggests that predator-disperser satiation occurred, maybe due to greater G. superba seed availability and other alternative food supply, allowing greater G. superba seedling survival at the Gustavia-rich area.

Seed bank dynamics in a Mediterranean grassland

Abstract: 1. Changes in the seed bank, seedling emergence and plant density of the most common grass species, and all legume species, were monitored in a grassland in north Syria from October 1986 to May 1989. 2. The seed bank was greatest in May-June, following pasture maturity, then declined to its minimum in March-April. Losses of seeds during summer were mainly due to sheep grazing, while losses in winter and spring were mainly due to germination, although some seeds were eaten by soil fauna and some seeds died. The seed bank, particularly of the small-seeded species (Trifolium tomentosum and T. campestre) was greater under the high stocking rate than under the low stocking rate

Changes in seed quality during seed development and maturation in tomato

Abstract: Changes in tomato (Lycopersicon esculentum Mill.) seed quality were monitored during seed development and maturation in glasshouse experiments in 2 years. The end of the seedfilling period (mass maturity) occurred 35–41 d after anthesis (differing among trusses) in 1989 and 42 d after anthesis in 1990. Seed moisture contents at this developmental stage were 53–72% (wet basis), while the onset of ability to germinate (during 21-d tests at 20°/30°C) and the onset of tolerance to rapid enforced desiccation occurred just before (1990) or just after (1989) mass maturity. In 1989, seed quality was assessed primarily by seedling size in a glasshouse experiment; maximum mean seedling dry weight 25 d after sowing was not achieved until 24–40 d after mass maturity. In 1990, seed quality was assessed primarily by germination following storage; maximum normal germination after 35 d in storage at 40 °C with 14 ± 0.5% moisture content was attained 23 d after mass maturity, but with little difference among seed lots harvested 10 d earlier or up to 30 d later. The results contradict the hypothesis that maximum seed quality is attained at the end of the seed-filling period and that seed viability and vigour begin to decline immediately thereafter.

Plants in Agriculture

Abstract: Preface 1. Plants as the basis of agriculture 2. Plants and water 3. Plants and minerals 4. Plant strength and integrity 5. Plant growth and development: seed and seedling 6. Plant growth and development: the vegetative plant 7. Plant growth and development: flower, fruit and seed 8. Plant growth and development: environmental effects and internal control 9. Plant variation, inheritance and breeding 10. Vegetative propagation 11. Crop production and yield 12. Plants as food Index.

Germination and emergence of hairy beggarticks (Bidens pilosa).

Abstract: Effects of environmental factors on germination and emergence of hairy beggarticks were examined in laboratory and greenhouse studies. Optimum temperature range for germination of hairy beggarticks was 25/20 to 35/30 C (day/night, 12/12 h). Germination decreased above or below this range. Temperatures below 15/10 C and above 45/40 C were unfavorable for germination. Hairy beggarticks seed can germinate under both a 12-h photoperiod and a 24-h dark regime. Seed germinated 78 to 90% in buffer solutions of pH 4 to 9. Radicle growth was more sensitive to extreme pH than germination. Osmotic stress up to −0.1 MPa had little effect on germination, but less than 3% of the seed germinated at an osmotic stress of −0.75 MPa. Hairy beggarticks seed (13%) germinated at NaCl concentration of 100 mM but failed to germinate at 200 mM NaCl. Maximum emergence occurred when seed were planted less than 1 cm deep. No seedlings emerged when planted 10 cm deep. Flooding even for a day following planting decreased emergence to 25% compared to no flooding (56%). Seedling emergence decreased sharply with a further increase in duration of flooding, and no seedlings emerged when flooding was maintained up to 28 d after planting.

Spatial pattern of seedlings 1 year after fire in a Mediterranean pine forest.

Abstract: The spatial distribution of seedlings of the dominant perennial plant species (Pinus halepensis, Cistus salviifolius, Rhus coriaria) and may annual species was studied after a wild fire in an eastern Mediterranean pine forest. The spatial distribution of all seedlings is affected by the location of the old burned pine trees. Seedling density of Pinus and Cistus is higher at a distance from the burned pine canopy and lower near the burned pine trunk. It is also higher beneath small burned pine trees than under big ones. Rhus seedling density is higher under big burned pine trees and also near the burned trunks. Seedlings of Pinus, Cistus and Rhus growing under the burned canopy of big pine trees tend to be taller than seedlings under small ones or outside the burned canopy. Most annual species germinate and establish themselves outside the burned canopies, and only a few annual species are found beneath them. It is suggested that variation in the heat of the fire, in the amount of ash between burned pine trees of different sizes, and in the distance from the burned canopy are responsible for the observed pattern of seedling distribution. The possible ecological significance of the spatial pattern of seedlings distribution and their differential growth rate are discussed.

Spatial relationship between seed and seedling abundance and mortality in a deciduous forest of north-eastern North America

Abstract: The relationship between seed and seedling spatial patterns of abundance was examined for five tree species in a deciduous forest of north-eastern North America. In this low-diversity forest, seed shadows (from different individuals of a given tree species) may overlap to a significant extent. The selective advantages of greater seed dispersal ability in order to avoid high predation and pathogen attack and intraspecific competition at proximity to the parent tree in unclear in this context. On the study site, there were no strong and consistent relationships between seed and seedling spatial patterns of abundance for any of the tree species studied (...)

The relative roles of abiotic and biotic factors in seedling demography of arroyo willow (salix lasiolepis: salicaceae)

Abstract: The abiotic and biotic factors causing mortality of Salix lasiolepis (Salicaceae) seedlings were studied along a seasonally flowing stream in northern Arizona. Mortality of first-year seedlings approached or equaled 100% for the four cohorts observed. Water-addition experiments demonstrated that lack of soil surface moisture was the primary cause of mortality for seedlings through their first 2 years with > 75 % of seedlings dying in the first month of growth in unwatered plots. Biotic factors, including herbivory and interand intraspecific competition were evaluated, but played a minor role in the dynamics of seedling populations during this study. Shade conditions between sites were inferred to influence growth and survival, and experiments verified the effect of shade in decreasing seedling size. Smaller seedlings were shown experimentally to have a greater likelihood of dying during the winter. The probability of survival between years increased with seedling age, and seedlings that survived three growing seasons were likely to join the population of mature, reproductive plants. Studies of seedling survivorship through the establishment phase have seldom reported mortality levels as extreme as those reported here.

Inhibition of Scots pine seedling establishment byEmpetrum hermaphroditum.

Abstract: Poor establishment and reduced seedling growth of Scots pine (Pinus silvestris L.) in northern Sweden is related to an allelopathic inhibition by the dwarf shrubEmpetrum hermaphroditum Hagerup. Indoor bioassays with green and brown leaves ofEmpetrum have strong negative effects on rooting ability, radicle elongation, and growth of Scots pine seedlings. Bioassays with soil samples show that phytotoxic substances leached fromEmpetrum foliage accumulate in the soil. Field experiments reveal that chemical inhibition byEmpetrum, causing high mortality and slow growth of pine seedlings, can be reduced by adding activated carbon to the soil.

Facilitation of tree seedling establishment in a sand dune succession

Abstract: A nucleated pattern of establishment by pine seed- lings of Pinus strobus and P. resinosa around pre-established oak trees of Quercus rubra in a sand dune succession in Ontario, Canada was examined using field observations, seed planting and habitat manipulation. Densities of young pines beneath oak canopies were approximately six times greater than in treeless areas, and densities on the north sides of the trees were significantly greater than on the south sides. How- ever, oaks younger than 35 yr showed no preferential estab- lishment beneath them, while the pine population structures beneath older oaks indicated single periods of successful recruitment. Pine seed planted beneath and beyond oak cano- pies of three sizes germinated primarily beneath the canopies of medium and large-sized oaks, but subsequent survivorship of seedlings over two growing seasons was poor. Several micro-environmental conditions were changed by oak cano- pies, but only shade showed a pattern closely corresponding to that of seedling establishment. A habitat manipulation experi- ment confirmed the primary role of shade in facilitating pine seedling establishment beneath oaks. Failure of pine to con- tinue recruiting successfully beneath facilitating trees is tenta- tively attributed to intraspecific competition among pine indi- viduals.

Genetic and phenotypic relationships in response to NaCl at different developmental stages in alfalfa.

Abstract: The perennial forage alfalfa (Medicago sativa L.) may be affected by salinity at all stages of development. Selection for increased seed germination or seedling growth in saline environments has not resulted in improved forage yield under salt stress. The purpose of this study was to determine genetic and phenotypic relationships between plant performance in the presence of NaCl at three developmental stages in alfalfa. Understanding these relationships may improve the efficiency of breeding programs aimed at increasing crop survival and yields in saline environments. Fourteen half-sib families were randomly chosen from both an experimental alfalfa population produced from two cycles of mass selection for improved forage regrowth yield at 80 mM NaCl (A80), as well as from an unselected control population (AC1). In two separate experiments, individual plant performance was measured in these families at seed germination (radicle length at 7 days), and during seedling growth (forage yield at 40 days post-planting) and post-harvest regrowth (forage yield at 67 and 95 days post-planting) in the presence of 0 or 80 mM NaCl. Genetic, phenotypic, and family rank correlation coefficients, and broad-sense and narrow-sense heritability estimates were calculated within each growth stage, NaCl level, and population. Radicle length was not highly correlated with seedling or regrowth forage yield within a population or across NaCl levels. Phenotypic correlations between seedling and regrowth yields were also low. Heritability estimates were higher at 0 NaCl in AC1 between all growth stages, but were greater in A80 at 80 mM NaCl. Genetic correlations between seedling and regrowth yields were all positive. This suggests that selection for forage yield in saline environments at harvests-1,-2, or -3 should not decrease performance at other stages. Genetic correlations between seedling and regrowth yields were higher in A80 than in AC1 at 80 mM NaCl. The results indicate that selection for increased alfalfa forage yield in saline environments at germination may not be optimum. Family selection at germination or during seedling growth may be more effective than individual plant selection at any growth stage in saline environments. The results suggest than selection methods which include each critical growth stage may be required to develop alfalfa cultivars with increased forage yield in saline environments.

The role of fluctuating temperatures in germination and establishment of Sorghum halepense. Regulation of germination at increasing depths

Abstract: Field and laboratory tests explored the response of Sorghum halepense seeds to fluctuations in soil temperature, and whether that response serves as a mechanism for sensing depth and regulating seed germination. Seedling production in the field was severely curtailed when seeds were planted at increasing soil depths. Soil-temperature fluctuations were strongly dependent on depth. When soil columns containing seeds were incubated at alternating temperatures of 20 and 30°C, however, seed germination did not diminish with increasing depth, and an evaluation of how soil-surface shading affects the perception of depth by the seeds indicated that soil cover reduced germination of only those seeds in the upper strata of the profile (...)

Effect of Elevated CO2 on the Photosynthesis, Respiration and Growth of Perennial Ryegrass

Abstract: Single, seed-grown plants of ryegrass (Lolium perenne L. cv. Melle) were grown for 49 d from the early seedling stage in growth cabinets at a day/night temperature of 20/15 °C, with a 12 h photoperiod, and a C02 concentration of either 340 or 680 (A T1 C02. Following complete acclimation to the environmental regimes, leaf and whole plant C02 effluxes and influxes were measured using infra-red gas analysis techniques. Elevated C02 increased rates of photosynthesis of young, fully expanded leaves by 35-46% and of whole plants by more than 50%. For both leaves and whole plants acclimation to 680 iA 1_1 CO2 reduced rates of photosynthesis in both C02 regimes, compared with plants acclimated to 340 /d 1 "1. There was no significant effect of C02 regime on respiration rates of either leaves or whole plants, although leaves developed in elevated C02 exhibited generally lower rates than those developed in 340iA l"1 C02. Initially the seedling plants in elevated C02 grew faster than their counterparts in 340 iil 1 ~1 C02, but this effect quickly petered out and final plant weights differed by only c. 10%. Since the total area of expanded and unexpanded laminae was unaffected by C02 regime, specific leaf area was persistently 13-40% lower in elevated C02 while, similarly, root/shoot ratio was also reduced throughout the experiment. Elevated C02 reduced tissue nitrogen contents of expanded leaves, but had no effect on the nitrogen contents of unexpanded leaves, sheaths or roots. The lack of a pronounced effect of elevated C02 on plant growth was primarily due to the fact that C02 concentration did not influence tiller (branch) numbers. In the absence of an effect on tiller numbers, any possible weight increment was restricted to the c. 2-5 leaves of each tiller. The reason for the lack of an effect on tillering is not known.

Bacterial fruit blotch of watermelon: association of the pathogen with seed

Abstract: Bacterial fruit blotch, a recently described disease of watermelon, resulted in severe losses to Indiana watermelon growers in 1989. Strains of the pathogen recovered from symptomatic fruit and seedlings exhibited similar phenotypic characteristics. Seedling symptoms (water-soaked lesions becoming necrotic on cotyledons and true leaves) and fruit symptoms (large, watersoaked blotches on the exposed rind surface) were induced by strains from both sources. Seed transmission was demonstrated in both seed from symptomatic fruit and seed soaked in a suspension of the bacterium(.)

Rhizosphere colonization and seedling growth promotion of lodgepole pine by Bacillus polymyxa

Abstract: Lodgepole pine seed was inoculated with Bacillus polymyxa isolate L6-16R, a strain marked with antibiotic resistance to 100 mg rifamycin per litre. Inoculation resulted in statistically significant seedling biomass increases after 8 weeks of pine growth. The L6-16R rhizosphere population declined by an order of magnitude per month, from ca. 106 cfu/g dry weight of pine root tissue 4 weeks past inoculation to ca. 104 cfu/g of root tissue 12 weeks past inoculation. Seed-lings from three of four pine provenances had mean shoot and root dry weight increases of up to 35% due to seed inoculation, but shoot growth of the fourth provenance was inhibited. The size of the L6-16R rhizosphere population was not correlated to the magnitude of the shoot growth response 4 weeks after inoculation, but it showed a strong correlation with shoot growth 8 weeks after inoculation. These results indicate that the promotion of pine seedling growth may be a function of the size of the bacterial rhizosphere population that develo...