Showing papers on "Seedling published in 1984"

Seedling Survival of Tropical Tree Species: Interactions of Dispersal Distance, Light-Gaps, and Pathogens

Abstract: This study tested the hypotheses that seed dispersal enhances seedling survival by (1) escape of distance—dependent or density—dependent mortality or both, and (2) colonization of light—gaps. Spatial patterns of seedling survival for 1 yr in shade and in light—gaps and causes of seedling mortality for the first 2 mo were determined for one tree of each of nine species that use wind dispersal on Barro Colorado Island, Panama. Dispersal was advantageous to all nine trees, but for different reasons. The colonization hypothesis was supported by all nine trees. At some time interval during the 1st yr, the escape hypothesis was supported by eight of the nine trees; dispersal away from the parent to shaded conditions lowered the probability of seedling mortality. However, the trees differed greatly in (1) when the dispersal advantage became apparent, (2) whether the advantages persisted through time, (3) the distance at which survival was most enhanced, and (4) the strength of the advantage. By 1 yr only four trees showed a moderate to strong advantage of dispersal to shaded areas; two trees showed a very weak advantage, and three trees showed no advantage. Pathogens caused the largest proportion of deaths among shaded seedlings in six of the nine trees. For the two trees with the strongest support for the escape hypothesis, pathogens caused distance— and/or density—dependent mortality; those trees had high seedling densities near the parent. Four other trees had moderate to high mortality by pathogens, irrespective of dispersal distance; those trees had low seedling densities at all distances from the parent. For these six trees in which pathogens were responsible for the largest proportion of deaths in the shade, this mortality was significantly reduced in light—gaps, even though seedlings occurred in high densities. The study indicates that the location where seedlings survive is greatly influenced by pathogen activity. Only the four trees with moderate to strong support for the escape hypothesis had large numbers of seedlings survive to 1 yr in the shade. Seedling recruitment is expected to occur away from the parent tree, in shaded conditions for trees with distance— and/or density—dependent mortality, and in light—gaps for all trees studied here.

Seed variation in wild radish: effect of seed size on components of seedling and adult fitness

Abstract: Seed mass in wild radish (Raphanus raphanistrum) varies up to sixfold within single fruits. The impact of such variation on subsequent growth and fecundity was studied. Seeds from single fruits were used to minimize genetic differences among individuals. Weighed seeds were planted close together in a disturbed area typical of Raphanus habitats. Large seeds (> 6 mg) were more likely to emerge as seedlings than were small seeds (<4 mg). Seed size had no effect on emergence time. Seedlings from large seeds grew more rapidly and produced more flowers than did those from related smaller seeds. Results from the field experiment contrasted with those obtained in greenhouse growth studies, where seed size had no significant effect on final plant size. This study points out two factors that have led to inconsistent results in previous studies of seed size and seedling success: (1) differences in the timing of growth measurements, and (2) the presence or absence of competitive inequities among neighbors within the experimental design.

Pathogen mortality of tropical tree seedlings: experimental studies of the effects of dispersal distance, seedling density, and light conditions

Abstract: We present results of two experiments designed to identify the relative importance of dispersal distance, seedling density, and light conditions on pathogen-caused mortality of tropical tree seedlings. The field experiment on Barro Colorado Island, Panama, demonstrated that both an increase in dispersal distance and a decrease in seedling density reduce levels of damping-off disease among seedlings of Platypodium elegans, and that there is an interaction between the two factors. The results indicated significant variation among sites in pathogen activity and suggested that seedlings are more vulnerable to disease when establishing around their parent tree than around other conspecific trees.The second experiment in a screened enclosure used potted seedlings of 18 wind-dispersed tree species exposed to two levels of sunlight and seedling density. The results indicated that environmental conditions similar to those in light-gaps significantly reduce pathogen activity. They also confirmed that high seedling density increases disease levels, especially under shaded conditions.Seedlings of 16 of the 18 species experienced pathogencaused mortality, but in widely varying amounts. Seed weight was not a good predictor of a species' vulnerability to pathogens. Adult wood density, an indicator of growth rate and successional status, was inversely correlated with a species' vulnerability to pathogens. Fast-growing, colonizing species, whose seedlings require light-gaps, lacked strong resistance to seedling pathogens, relative to slow-growing species able to tolerate shade and escape seedling pathogens. We discuss these results in the context of seed dispersal as a means of escaping from seedling pathogens.

Effects of seed size and growth form on seedling establishment of six monocarpic perennial plants

Abstract: (1) Effects of seed size and seedling morphology on the establishment of six monocarpic perennials were examined in glasshouse experiments. Both withinand between-species comparisons of seed-size effects were made as seed weight varied by more than two orders of magnitude among species and by 3 to 20-fold within a species. Experiments were conducted in four ground-cover types: bare soil, litter cover, vegetated, and vegetated plus litter. (2) In vegetated cover, emergence of the two small-seeded species, Verbascum thapsus and Oenothera biennis, was significantly lower than in litter and bare soil. In contrast, emergence of the medium (Dacus carota and Dipsacus sylvestris) and large-seeded species (Tragopogon dubius and Arctium minus), was not significantly reduced in the presence of vegetation. Although the rate of emergence of all six species was significantly reduced in vegetated cover, there were no between species differences in seedling emergence rates in any cover type. (3) Relative growth rates of all six species were significantly lower in vegetated cover compared with litter and bare soil and the effect was greatest on the small-seeded species. At the end of the experiment, seedling weight in vegetated cover was positively correlated with seed weight. In non-competitive cover types (litter and bare soil), seedling weight was independent of initial seed weight. (4) Relative growth rates of seedlings in non-competitive cover types were inversely related to seed size. In bare soil and litter, the small-seeded species had relative growth rates twice those of the large-seeded species. In vegetated cover this pattern was reversed; the species with large seeds had the highest relative growth rates. (5) The growth form of a seedling did not have any effect on its probability of establishment in any cover type. Species with different growth forms and similar seed sizes had equal emergence, survival, and relative growth rates in all four cover types. (6) Within species differences in seed size had a significant effect on seedling growth in non-competitive cover, but had no effect on seedling growth in competitive cover. Thus, both withinand between-species differences in seed size had similar effects on seedling establishment success in different types of ground cover.

Light requirements of neotropical tree seedlings: a comparative study of growth and survival

Abstract: (1) Eighteen species of wind-dispersed trees on Barro Colorado Island, Panama, are compared with respect to germination, survival, and growth of seedlings during 1 year under sun and shade conditions in a screened enclosure. The species vary in their mean dry weight of seed from 1.9 mg to 686 mg. (2) High and synchronous germination occurs in both sun and shade for sixteen of the eighteen species. Most species have epigeal germination with leafy green cotyledons. (3) Seedlings of fifteen species survive better in sun than shade; none survive better in shade than sun. Survival in shade and in sun are positively correlated. Shade tolerance varies widely and continuously among the species; it correlates with adult wood density, an indicator of growth rate and successional status, but not with the dry weight of seed reserves. The more shade-tolerant species have a lower proportion of seedlings dying from disease in the shade. (4) Seedling height after 1 year is greater in sun than in shade for all species. In both sun and shade, growth rate and height at 1 year are not correlated with adult wood density or shade tolerance. Seed dry weight correlates with seedling height in both sun and shade at 4 weeks, but with height in shade only at 1 year. Seedlings in shade show no or slow growth after the initial growth has used the seed reserves. (5) In the shade, number and length of leaves show little or no change with time; leaf turnover is negligible. In the sun, leaf number increases and successively younger leaves mature at larger sizes. Leaf size is greater in the sun than in the shade Senescence of the leafy cotyledons is more rapid in sun than shade for all species. (6) This experimental study predicts that all species examined benefit from seed dispersal to light-gaps. Some species are restricted as seedlings to gaps, while others persist, but do not grow appreciably, in the shaded understorey. The implications of these differences in light requirements for the size-age structure and for the spatial pattern of the populations are discussed.

The effect of salinity and temperature on the germination of polymorphic seeds and growth of atriplex triangularis willd.

Abstract: Polymorphic seeds of Atriplex triangularis were germinated at various temperatures (5-15 C, 5-25 C, 10-20 C, 20-30 C) and salinity regimes (0 to 1.5% NaCl) in order to determine their germinability and early seedling growth under these conditions. Larger seeds generally had a higher germination percentage in saline medium. The rate and percentage of germination decreased with increased salinity stress. A thermoperiod of 25 C day and 5 C night, 12 hr/12 hr, temperature enhanced germination of seeds. Early seedling growth is promoted in larger seeds at lower salinity, and at high-day and low-night temperatures. Polymorphic seeds have different physiological requirements which provide alternative situations for seed germination in natural habitats. ATRIPLEX TRIANGULARIS WILLD. (Chenopodiaceae) is an annual species of halophyte which is widely distnbuted in coastal and inland salt marshes of North America (Chapman, 1974; Ungar, 1974; Osmond, Bjorkman and Anderson, 1980).

The Effects of Seed Burial and Soil Disturbance on Emergence and Survival of Arable Weeds in Relation to Minimal Cultivation

Abstract: (1) Seeds of sixteen weed species (six grasses and ten herbs) were collected from arable field margins during 1977. Species included were Agrostis gigantea, Alopecurus myosuroides, Arrhenatherum elatius, Avena fatua, Bromus sterilis, Poa trivialis, Capsella bursa-pastoris, Galium aparine, Papaver rhoeas, Plantago major, Polygonum aviculare, Stellaria media, Tripleurospermum inodorum, Veronica arvensis, Veronica persica, and Viola arvensis. Seeds were sown in soil in pots or boxes at various depths, and either disturbed periodically or left undisturbed. Periodicity of emergence was recorded for each species. (2) Four patterns of emergence were recorded: (i) emergence in spring alone; (ii) predominantly in autumn; (iii) in both spring and autumn; (iv) emergence indifferent to season. Most of the grasses emerged predominantly in autumn whereas the main period of emergence for many of the dicotyledonous species was in spring. (3) Burial of small-seeded species both delayed and reduced seedling emergence as compared to surface-sown seed. In contrast germination and emergence of the larger-seeded species was increased by burial. Soil disturbance increased the total number of seedlings which emerged, but in general it did not alter the relative periodicity of germination, nor was emergence promoted by cultivation at times outside the period of natural emergence. (4) The number of viable but ungerminated seeds which remained after 2 years varied between species. It was least for surface sown seed and greatest for buried undisturbed seed. Most frequent were A. myosuroides, Viola arvensis, P. rhoeas and P. aviculare. No seeds of A. elatius, B. sterilis or G. aparine remained after 12 months. (5) Relatively few species emerged from below 50 mm with the exception of several large-seeded species. These species generally failed to establish from seed on the soil surface. (6) The results are discussed in relation to the use of minimal cultivations for cereal production. It is concluded that the majority of annual grass-weeds, despite having little innate dormancy, will continue to be encouraged by current agronomic practices. In contrast, many annual dicotyledonous species will decline in importance, partly as a result of their susceptibility to herbicides and partly because their periodicity of germination does not coincide with the establishment phase of autumn-sown cereals.

Variation in demography of juvenile tsuga heterophylla across the substratum mosaic

Abstract: SUMMARY (1) Recruitment and survival of juvenile Tsuga heterophylla were followed on different substrata on the west slope of the Cascade Range, central Oregon, U.S.A. (2) Whilst seeds fell mainly from October to March, some were shed in all but six of thirty-two consecutive months. (3) Almost all juveniles occurred on decaying logs even though fallen timber may cover only 10-30% of the forest floor. (4) Recruitment on all substrata varied widely from year to year, as expected in a conifer with mast years of seed production. (5) Cohorts emergent in different months during one calendar year often showed greatly different survivorship curves even on the same rooting substratum. (6) Mortality was much higher for juveniles during the first 2 yr after emergence than in subsequent years. Microtine rodents probably account for most seed and seedling deaths. (7) The extent of decay of woody rooting-substrata does not influence percentage emergence, although survival of juveniles was most prolonged on Pseudotsuga menziesii logs with rotten heartwood. (8) The age-class structures of juvenile populations were functions of the fraction of the forest floor covered by fallen wood in each decay class.

Variation in seedling recruitment of Cape Proteaceae after fire

Abstract: (1) Seedling recruitment of closed cone (serotinous) Proteaceae was surveyed in thirty-one burnt areas in mature mountain fynbos of the Southern Cape, South Africa. (2) The number of seedlings per pre-burn individual (parent) varied greatly from fire to fire. (3) This variability could be partially explained by pre-burn densities since these were frequently negatively correlated with number of seedlings per parent. (4) Seedling recruitment differed significantly with season of fire. The most successful seedling establishment followed autumn burns. Summer burns were often less favourable and had a greater variability from fire to fire. Winter and spring burns lead to very poor seedling establishment, mostly well below replacement levels, so that successive fires in these seasons would rapidly lead to local extinction. (5) Density-dependent regulation of seedling populations is apparently due to the reduction of the reproductive effort, and thus seed reserves, per individual with increased pre-burn crowding. (6) The causes of seasonal variation, which appear to be density independent, have not been satisfactorily explained though they are important for the prediction of fire succession in all Proteaceae which store seed in the canopy.

Predator Satiation and Site Alteration Following Fire: Mass Reproduction of Alpine Ash (Eucalyptus Delegatensis) in Southeastern Australia

Abstract: This study examines seedfall, seed predation, and seedling recruitment in a relatively fire—sensitive tree, the alpine ash (Eucalyptus delegatensis), in the Brindabella Range, Australian Capital Territory (A.C.T.), Australia. An experimental high—intensity fire was used at Bushrangers Creek (BC) to determine whether massive, synchronized seedfall and subsequent seedling recruitment are linked to (1) satiation of postdispersal seed predators and/or (2) reduction of the physical and biotic resistance to seedling growth and survivorship. Results were contrasted with findings at Moonlight Hollow (MH), a site which served as a natural, unburned control. Overall results wee consistent with the predator—satiation hypothesis. High—intensity fire killed 93% of the alpine ash trees at BC and caused a massive, synchronized release of seeds, averaging 405 viable seeds/m2 (4021 total ovular structures/m2) in the 3 wk after fire. Fire precipitated an immediate drop in weekly removal of seeds from trays, from an average of 65.3% prefire to 13.6% postfire. Postfire seed removal at BC dropped to significantly lower levels than corresponding measures at unburned MH. Caging experiments demonstrated that ants were the only important removal agents of alpine ash seed at BC and MH. Although abundance and species richness of ants trapped on the seedbed increased following fire, the number of ants trapped per available seed significantly declined due to the massive input of seed. Fire and subsequent seedfall resulted not only in a greater absolute number of emergent seedlings (66.1 seedlings/m2 at BC; 0.8 seedlings/m2 at MH) but also in a significantly greater proportion of emergence from seed input from the canopy (14.7% at BC; 2.7% at MH). Seedling mortality was great, and after 72 wk mean density at BC was 1.9 seedlings/m2. Successful recruitment did not occur at unburned MH. A phytometer experiment demonstrated that fire neither enhanced the physical environment for early growth and survivorship of seedlings nor reduced the effect of natural enemies on E. Delegatensis seedlings. Greater frequency of grazing on seedlings growing in burned soil and a more rigorous physical environment at BC appeared to account for lack of enhanced growth and survivorship at postfire BC. Massive, synchronized release of stored seed from shrub and tree species following fire is a common feature in many fire—prone environments. Parallels in seed storage, postdispersal seed predation, and regeneration following fire in Eucalyptus and Pinus suggest that postdispersal seed predators are important selective agents in the evolution of seed storage and induction of mass seed release by fire.

Shrub clumps of the Chilean matorral vegetation: structure and possible maintenance mechanisms

Abstract: Previous studies have claimed that the Chilean matorral is more open than the Califonia chaparral, and have attributed this dissimilarity largely to the role of man in Chile. In this paper we show that in general the Chilean matorral has a structure better described as shrub clumps that merge to form a continuous vegetation matrix only in very mesic habitats, where it is comparable to the Califonia chaparral. We also present evidence that these clumps have been present for at least the last 26 years and that even without human disturbance they are likely to maintain themselves. Evidence for the latter pertains to seed dispersal, seed germination and establishment, seedling survival, and the diameter size structure of shrub clumps. Finally, we propose that differences between the California chaparral and Chilean matorral are more profound than previously thought and are due not only to different degrees of human disturbance, but also to the presence of periodical natural fires in California and not in Chile, and to different shrub recruitment patterns and mammalian herbivore activity in the two areas.

Fire survival of Cape Proteaceae - influence of fire season and seed predators

Abstract: Many Cape Proteaceae store seed reserves in closed cones on the plant and rely entirely on these reserves for episodic recruitment after fires. Population size is sensitive to intervals between fires but also to fire season. Populations can be nearly eliminated by successive winter or spring fires. Three hypotheses explaining seasonal variation in recruitment were tested: (a) seeds germinate immediately after fire but seedlings die from summer drought; (b) seeds remain dormant over summer but the longer the delay between seed release after fire and germination 1) the greater the competition between seedlings and resprouts, or 2) the greater the seed losses to predators and/or decay before germination. Drought-avoiding dormancy occurred in 9 of 11 Cape Proteaceae studied, all of which delayed germination to autumn or winter. Seedling emergence and survival was not significantly increased after removal of competitors by methyl bromide poisoning. Seed predation, measured by exclosures, however, significantly reduced seed reserves before germination and also number of seedlings emerging. Post emergence seedling predation was negligible in the burn in contrast to adjacent mature vegetation where seedling predation was very heavy. The role of germination cues and rodent behaviour in controlling population recruitment is discussed and it is concluded that a knowledge of both is essential for predicting vegetation dynamics in this system.

The effect of seed size and maternal source on individual size in a population of ludwigia leptocarpa (onagraceae)

Abstract: Seed size is normally distributed for many annual species, while mature plant size is frequently positively skewed. A study was conducted to determine the influence of seed size and the role of genetic differences in determining relative seedling size for Ludwigia leptocarpa. Seed size had a significant effect on percentage germination and time of seed germination but no effiect on dry weight or leaf area of seedlings. Seed size and spacing had a significant effiect on seedling dry weight for plants grown under competition, while relative day of emergence had no effect. Familial (genetic) diffierences were found in average seed weight between maternal plants, but not in average number of days to germination, average weight of seeds which germinated, or shoot dry weight. It is concluded that neither seed size alone nor genetic differences between plants are directly responsible for the development of size hierarchies in L>dwigia leptocarpa populations. Large seed size does convey an advantage in growth when plants from seeds of differing initial size interact.

Developmental and genetic sources of seed weight variation in raphanus raphanistrum l. (brassicaceae)

Abstract: Seed weight is known to have a marked impact on emergence and post-emergence productivity in wild radish (Raphanus raphanistrum). In this paper, I describe several levels of seed weight variation in plants taken from a natural population in Hamden, Connecticut. Six maternal plants from the 1981 season were analyzed in detail: the weights and positions of all seeds within a fruit were recorded, and some of these seeds were used the following summer for competition studies and progeny analysis. Within a plant, average seed weight decreased as the number of seeds within a fruit increased, suggesting that developing embryos compete for maternal resources. Seed weight also varied significantly among the six maternal plants used in the study. Comparison of the average weights of seeds produced by offspring of those six plants with the average weights of seeds borne by the maternal plant revealed a significant genetic component to seed weight variation. Seed weight varied up to six-fold within single fruits of R. raphanistrum; large seeds tend to occur near the pedicel or in the middle positions. Seed size variation seen within single fruits is of sufficient magnitude to result in differential reproductive output among closely related seeds under competitive field conditions. DEMOGRAPHIC STUDIES of natural plant populations have consistently demonstrated the importance of early seedling size differences in determining ultimate seedling fitness (Harper, 1977; Cook, 1979; Gross, 1981; Cook and Lyons, 1983). Individuals in a cohort can gain an early size advantage in several ways: through more rapid relative growth rate (Rabinowitz, 1979), earlier emergence (Howell, 1981; Cook and Lyons,1983), or greater seed weight (Black, 1958; Howe and Richter,1982; Stanton,1984). Despite the potential importance of seed size . . . . ln competltlve envlronments we know relatively little about environmental and genetic sources of seed weight variation in non-agricultural crops. Ecological research on intraspecific seed size variation has been discouraged by two observations. First, an influential survey of seed ' Received for publication 17 October, 1983; revision accepted June 1984. 2 Current address: Botany Department, University of California, Davis, CA 95616. This study was initiated with support from the Brown Post-doctoral Fellowship in Botany at Yale University. S. N. Handel gave encouragement and guidance throughout. J. Mishkin, D. Levine, and K. McFadden provided essential field assistance. Later seed weighing and analysis was supported at U. C. Davis by NSFGrant DEP-82 14508. C. McGee, J. Damey, and H. Hasbrouck were of great assistance during the latter stages of the project. An earlier draft of this paper was much improved by the careful criticisms of N. Ellstrand, S. Handel, D. Marshall, A. Snow, and two anonymous reviewers. weight variation, largely using data from crop species, found that flower number and stem number are many times more plastic than seed size (Harper, Lowell and Moore, 1970). Second, in species that do exhibit substantial seed weight variation, much ofthat variation occurs within individual maternal plants (Cavers and Harper,1966; Maun and Cavers,1971; Janzen, 1977a,b,1982;Primack,1978;Waller,1982). These patterns suggest that seed weight should show low levels of heritability in natural populations and be relatively resistant to directional change through natural selection. Given this background, it is not surprising that most research into the effects of seed size on seedling fitness has focused on interspecific, rather than intraspecific, comparisons (e.g. Wilbur, 1977; Rabinowitz, 1978). Studies on intraspecific seed weight variation may prove to be of great ecological value, despite the above problems. First, surveys of non-agricultural plants commonly report considerable range in seed size for a species (e.g., Berggren, 1981). Second, intraspecific seed weight variation may provide real insights into life history models dealing with maternal resource allocation. The basic premise of such models is that maternal resources are finite, leading to a tradeoW between the number of offispring produced and investment in each oSspring (Smith and Fretwell, 1974; Wilbur, 1977). Because maternal investment in animal offspring often includes hard-to-measure pa-

Effects of CO2 enrichment and water stress on growth of Liquidambar styraciflua and Pinus taeda seedlings

Abstract: Mathematical growth analysis techniques were used to assess the possible interactive effects of atmospheric carbon dioxide enrichment and water stress on growth and biomass partitioning of Liquidambar styraciflua L. (sweetgum) and Pinus taeda L. (loblolly pine) seedlings. Plants were grown from seed under 1000 μmol∙m−2∙s−1 photosynthetic photon flux density at CO2 concentrations of 350, 675, and 1000 μL∙L−1 for 56 days. At this time, half the seedlings in each CO2 treatment had water withheld until plant water potentials reached about −2.5 MPa in the most stressed plants, while the remaining plants were well watered. At the end of the drying cycle, stressed plants were returned to well-watered conditions for a 14-day recovery period. The greatest effects of water stress on growth were seen following the recovery period and were most severe for sweetgum seedlings grown at the lowest CO2 concentration. For sweetgum seedlings in particular, the reduction of early seedling growth following exposure to a perio...

Seed dynamics in Calluna-Arctostaphylos heath in north-eastern Scotland.

Abstract: (1) Estimates were made of seed production, dispersal and storage in the soil in Calluna-Arctostaphylos heath in N.E. Scotland. Germination and seedling establishment in recently burned areas were also studied. (2) Seed production and dispersal by individual species were found to follow the abundance of the species in each stand, though the performance of some species declined as the Calluna canopy closed. Little seed input occurred from surrounding areas into closed or recently burned stands. (3) The amount of seed stored in the soil varied according to stand age. Seeds of Calluna vulgaris and Carexpilulifera increased in abundance with stand age. Stored seed of most other species decreased in numbers though in some there was a slight increase in the degenerate Calluna vulgaris phase. (4) The number of seedlings establishing after fire was far fewer than would be expected from the soil seed store, and is correlated with the type of substrate left by the fire. (5) Seed budgets are estimated for a 15 year fire rotation for the major species studied.

Tiller dynamics and assimilate partitioning in Lolium perenne with particular reference to flowering

Abstract: SUMMARY The fate of 100 seedling plants of Lolium perenne L. was studied over a period of 2 years in a field plot. The birth and death of tillers and the production of inflorescences was followed, and the components of seed yield were recorded in detail in the first year. The pattern of distribution of 14CO2 assimilated by the main shoot was examined at monthly intervals and during the first flowering season the distribution of 14C-assimilate from individual leaves and from the inflorescence was also studied. The capacity of individual tillers to assimilate 14CO2 prior to flowering and the re-distribution of previously accumulated assimilate during seed growth were also assessed. Plants died at a more or less constant rate with time and only 54 survived to the end of the 2–yr period. First year mortality was associated with severe grazing or cutting but in the second year the death of ungrazed plants was observed. There was great variability in the production of tillers by surviving plants. In both years the number of live tillers per plant increased from July to the end of April with particularly rapid tillering in March and April establishing the maximum value for each year. There was a similar phase of rapid tillering after flowering in July. The number of live tillers per plant declined by 50% during stem elongation and inflorescence emergence and the majority of dead tillers were young secondary (in the first year) and tertiary (in the second year) tillers with a mean age of 40 days. Such tillers had poor assimilatory capacity prior to the onset of death and were not supplied with assimilate from the main shoot. Most of the plants surviving at the end of the experiment flowered in both years and one quarter of the maximum number of live tillers per plant recorded in April of each year produced inflorescences. The earlier a tiller was produced the greater was its chance of flowering and the greater its production of seed. The greater weight of seed produced was associated with the development of more seed-bearing florets per spikelet. There was relatively little export of “C-assimilate from the flowering main shoot, and the lower internodes formed the major sink for post-anthesis assimilate. The growth of seeds appeared to be relatively independent of the leaves for current assimilate. There was some evidence that assimilate accumulated in lower internodes was remobilised and utilised in the growth of seeds and new tillers. Overall, the results confirm the view that the grass plant is a dynamic population of short-lived tillers and indicate that increasing competition for assimilate at flowering exerts a major influence on the production and survival of tillers.

The effect of seed dimorphism on the germination and survival of salicornia europaea

Abstract: Seed dimorphism influenced the germination behavior of Salicornia europaea L., with small seeds being more dormant and less salt tolerant than large seeds. All of the large less dormant seeds of S. europaea germinated prior to May, and all seedlings produced after this time were from small seeds. A persistent seed bank was maintained by the small dormant seeds. Survivorship was relatively constant during the normal germination season, but increasing salinity stress at any time during the spring reduces the chance of seedling survival. Little germination occurred from July through September because of the high surface soil salinities during this period. SALICORNIA EUROPAEA L. is an annual halophyte in the family Chenopodiaceae (Fernald, 1950). The plant has an erect habit, with an articulated and apparently leafless stem. Branching is decussate and in the mature plant all of the branches terminate in a spike-like inflorescence of 3-flowered cymules. The central flower of a cymule always produces a larger seed than the two lateral flowers (Konig, 1960; Dalby, 1962; Ungar, 1979). Seeds from the lateral flowers are less salt tolerant and more dormant than the larger seeds of the central flowers (Ungar, 1979). Small seeds have a light and a stratification requirement, similar to the condition found in S. patula studied by Grouzis, Berger and Heim (1976). Ungar (1979) concluded from his germination studies that S. europaea exhibited a somatic seed dimorphism. Somatic seed polymorphism is defined as the production of seeds of different morphologies and behavior on different parts of the same plant (Harper, 1977). The phenomenon has been reported in a number of weedy annuals that occupy unpredictable environments, and seems to be the rule in several genera of the family Chenopodiaceae (Harper, Lovell and Moore, 1 9 70). The diffierential germination behavior of different seed morphs has been discussed by several authors (Harper, 1965; Williams and Harper, 1965; Harper et al., 1970; Baskin and Baskin, 1976; Harper, 1977; Flint

Nursery Cultural Practices: Impacts on Seedling Quality

Abstract: Better understanding and implementation of nurserycultural practices to improve seedling quality will enable better matching of seedlings to forest sites, reducing the chance of regeneration delay and improving future growth of forest stands This chapter reviews a number of important cultural practices and the ways in which they affect indicators of seedling quality (morphology and physiology) and, ultimately, field performance (growth and survival) Early spring sowing produces larger seedlings that can complete their growth and be hardened by midsummer Lowering seedbed density results in more seedlings from a given amount of seed and can improve field survival and growth A nursery irrigation schedule that imposes moderate stress on seedlings in midsummer induces earlier budset and seedling dormancy and increases field-survival potential Mopst often, field survival and growth are improved with growing-season fertilization in the nursery; fall fertilization also may incfease field growth of North west species Root Wrenching in dry soil and (or) hot, dry weather without immediate irrigation can greatly stress Douglas-fir seedlings and should be avoided because of increased chance of seedling mortality in nursery beds or reduced growth later in the field Wrenching to mildly stress seedlings can induce budset and hardening and may benefit field growth and survival Top pruning, to control shoot height and achieve crop-size uniformity, should be done during the period of active seedling growth nal buds Transplant seedlings have more fibrous root systems, larger stem diameters, and lower shoot:root ratios than seedlings of comparable age grown at a standard density; seedlings are most commonly transplanted in spring and are outplanted as 1+1s or 2+1s It is important for nursery managers to be aware of interactions among the various nursery practices they employ; if a current practice is altered or discontinued or a new practice added, careful attention should be given to the effect of this change on other cultural practices in the nursery

Cultivation of banana using plantlets from meristem cultures

Abstract: Developpement en culture et production commerciale de plants de bananiers. Application a la prevention de la fusariose

Seed Polymorphism and Germination Responses to Salinity Stress in Atriplex triangularis Willd.

Abstract: Seeds of Atriplex triangularis exhibited a very pronounced morphological and physiological seed polymorphism. Seed size varied from 1.0 to 2.8 mm and predicted the likelihood of successful establishment through its effect on germination and seedling vigor. Large seeds had a mean dry weight of 2.44 ± 0.16 mg and a mean length of 2.45 ± 0.24 mm; medium seeds, mean dry weight of 1.21 ± 0.10 mg and mean length of 1.78 ± 0.19 mm; small seeds, mean dry weight of 0.64 ± 0.04 mg and mean length of 1.27 ± 0.10 mm. The degree of salt tolerance increased progressively with increasing seed size. Seeds from all size classes that were initially treated with 2%-5% NaCl had from 85% to 100% germination after being immersed in distilled water for 6 days, indicating a transitory adverse effect of salt stress on germination. The amount of water absorbed by all seeds is influenced by change in media salinity but not by hormonal treatments. Small seeds contain more Na+ and Cl- than medium and large seeds. Seedling dry weight ...

Nitrogen nutrition of seedling grain legumes: some taxonomic, morphological and physiological constraints

Abstract: The growth of young plants of the epigeal species Phaseolus vulgaris and Glycine max is compared with that of the hypogeal species Pisum sativum and Vicia faba, with particular reference to synchronization between the exhuastion of seed reserves of N and the availability of fixed N. It is argued that the N stress symptoms which occur when these two processes are not synchronized are more common and obvious in Phaseolus or Glycine than in Pisum or Vicia. This is primarily because in these species (a) the first fixed N is used for nodule growth rather than being exported to the shoot system and (b) the first foliage leaves have a much greater area and contain a larger proportion of N reserves from the seed. It is further suggested that Phaseolus and Glycine may show the greater response to nitrogen fertilizer applied at sowing since (a) most of the applied nitrate is passed directly to the shoots (rather than being reduced in the roots as in Pisum or Vicia) and (b) in addition to being used for growth (following reduction), it may also be used prior to reduction as part of the osmotic force driving cell expansion.

Carbohydrate accumulation and partitioning in Pinus banksiana seedlings and seedling cuttings

Abstract: Tests were conducted to identify possible relations between carbohydrates and callusing-rooting of Pinus banksiana Lamb, cuttings. Terminals, upper stems, and basal (1 cm) stems of 90-day-old untreated seedlings and seedling cuttings were analyzed for sucrose, total soluble reducing sugar, starch and total non-structural carbohydrate during propagation. Seedlings were evaluated in order to determine whether data for cuttings alone properly described carbohydrate-callusing-rooting relations under conditions where stock plants and cuttings were propagated in different environments. Results indicated that seedling terminals and upper stems, but not basal stems, accumulated the measured carbohydrates much like cuttings, though to lesser concentrations. Thus, carbohydrate accumulation by cutting terminals and upper stems would have been overestimated, based on cutting data alone. In terms of rooting, results indicated that: 1) Total carbohydrate accumulation in cutting basal stems was related to callusing-rooting, but a cause-effect relation was not established; 2) The positive relation between callusing-rooting and total carbohydrate accumulation was primarily due to accumulation of reducing sugar and starch, with reducing sugar predominant. 3) Reducing sugar/starch concentration ratios were the most sensitive and convenient indicators of specific carbohydrate differences within and between seedlings and cuttings.

Plant Physiology and Nursery Environment: Interactions Affecting Seedling Growth

Abstract: Environmental factors (such as light, moisture, nutrients, density, and temperature) and plant physiological factors (such as carbohydrate reserves, hormone levels, frost hardiness, and dormancy) interact to shape growth and survival of coniferous seedlings in nursery fields and after outplanting. Nursery managers can manipulate moisture, nutrients, and density to achieve desired seedling morphology and vigor. However, the annual growth cycle of perennial plants has evolved in response to environmental pressures. When the environment is modified, as with heavy irrigation in a nursery, to permit growth at a time when natural seedlings are dormant, the ensuing phases of the growth cycle will not be properly synchronized with their environments. Seedlings so cultivated lack vigor after outplanting. Nursery managers should aim at keying their cultivation schedules to both environmental conditions and endogenous seedling physiology toensure production of high-quality seedlings.

Aspects of Floral Biology, Breeding System, and Seed and Seedling Biology in Podostemum ceratophyllum (Podostemaceae)

Abstract: In New England, Podostemum ceratophyllum flowers at all levels in relation to the water's surface (above, at, below). Only one of three populations studied produces seed, the others mature unfilled capsules. In the seed-producing population pre-anthesis cleistogamy characterizes the breeding system. Outcrossing is unlikely. Capsule maturation takes 2 or 3 weeks. Capsules dehisce largely in response to desiccation, and seeds are initially shed close to the capsules. Hy- drochory is a likely agent of further dispersal. Initially, seeds become attached to the substratum by drying of the outer mucilaginous cells of the seed coat. Seed germination occurs only when the seeds are submersed; the emerging radicle is green and presumably photosynthetic. Seedlings are attached via rhizoidal outgrowths of the radicle epidermis. The functional plant originates from a meristem at the base of the hypocotyl and consists of a prostrate axis from which upright flowering shoots are produced. The time from germination to flowering in the young plant is less than 2 months. Podostemum ceratophyllum Michaux is a pe- rennial member of Podostemaceae, a family of herbaceous annual and perennial aquatics that superficially resemble aquatic bryophytes or freshwater algae. Relatively few reach temper- ate regions, the exceptions being P. ceratophyl- lum in North America, and a few species of

Allelopathic Effects of Lantana (Lantana camara) on Milkweedvine (Morrenia odorata)1

Abstract: Allelopathic effects of lantana (Lantana camara L. #3 LANCA) residues (root, shoot), foliar leachates, and the soil (where lantana was grown) on milkweedvine (Mor- renia odorata Lindl. #3 MONOD) seed germination and growth over a 30-day period were examined. Foliar leachates or the soil collected from the field where lantana had been growing had no effect on the final germination percentage or the seedling growth of milkweedvine. Incorporation of dried lantana shoot or root material into soil had no ef- fect on the final percentage germination but caused signifi- cant reductions in milkweedvine growth over a 30-day test period. Roots were more inhibitory than shoots. Fifty percent of milkweedvine seedlings died within 15 days after germination at 1% (w/w) dried lantana root incorpora- tion into the soil, and higher concentrations increased seedling death. Lantana roots incorporated into the soil produced foliar symptoms such as wilting and desiccation, whereas lantana shoots incorporated into the soil produced yellowing of the foliage of milkweedvine. Allelopathic ac- tivity of lantana residues was still strong even after decomposi- tion of lantana residues for 4 weeks prior to the planting of milkweedvine seeds. Additional index words. Allelopathy, germination, LANCA,

A Mathematical Model to Utilize the Logistic Function in Germination and Seedling Growth

Abstract: Several models have been proposed to describe germination rates, but most are limited in statistical analysis and biological meaning of indices. Therefore, a mathematical model is proposed to utilize the logistic function. The function was defined as an overall response including time, temperature, and the interaction between time and temperature. Cumulative germination percentages over time were used to develop the model. Germination tests were conducted on indiangrass (Sorghastrum nutans (L.) Nash) strain TG-2CF,' at constant temperatures of 9,12,15, 20,25, and 30 °C. The function fitted the observed data over six temperatures at r2 = 0-99. Time to reach 10% of final germination (Gim) increased from 2-5 d at 30 °C to 44-0 d at 9 °C, and Gt50 (time to reach 50% of final germination) increased from 3-6 d at 30 °C to 53-8 d at 9°C. True germination rate (% d~ ') for each temperature was maximum at Gt50. A linear model of 1/Gt50 versus temperature was used to estimate the base temperature of 8-3 °C for germination. An Arrhenius plot indicated a change occurred between 20 °C and 25 °C for temperature response of germination. Published data on hypocotyl growth of Cucumis melo L. were recalculated using the model. Absolute growth rates showed a temperature response similar to the published weighted-mean elongation rates. Base temperature for hypocotyl growth of C. melo was estimated as 8-8 °C. The proposed model proved to be useful in calculating and interpreting germination and growth kinetics.

A comparison of different chemicals for osmotic treatment of vegetable seed

Abstract: SUMMARY Samples of carrot, celery, leek and onion seed were treated before sowing by imbibition in osmotic solutions of polyethylene glycol 6000 (PEG), glycerol and potassium dihydrogen orthophosphate (KH2PO4). The solutions were sufficiently concentrated to prevent germination during treatment, and within each species, the amount of water taken up by the seeds during treatment did not vary greatly between solutions. All treatments increased the rates of seed germination and seedling emergence compared to untreated controls, but glycerol-treated seeds germinated and emerged significantly more slowly than did PEG- and KH2PO4-treated seeds. The effects of PEG and glycerol treatments on percentage germination and seedling emergence were small, but KH2PO4 treatment tended to reduce percentage germination and emergence, most markedly in leek and one cultivar of celery. It is concluded that PEG treatment gives the most consistently beneficial effects for the range of species tested.

Quantitative and qualitative comparison of root exudates of mycorrhizal and nonmycorrhizal plant species

Abstract: At 6 weeks after seed germination, the rate of leakage of carbohydrates and amino acids from roots was significantly higher in three plant species (Sorghum vulgare Pers., Lycopersicon esculentum Mill., and Citrus aurantium L.) that normally form vesicular–arbuscular mycorrhizae (VAM) than in three species (Chenopodium quinona Willd., Phytolacca americana L., and Saponaria vaccaria L.) that do not form VAM. There were no sugars, amino acids, or carboxylic acids that were consistently detected in root exudates of VAM-forming species that were also consistently absent from the exudates of non-VAM species. Although rates of root exudation over long time periods were lower in non-VAM species, at early stages of seedling establishment, the rate of exudation was greater in roots of Chenopodium quinona than in roots of Citrus aurantium. These results suggest that the overall rate of exudation may be an important factor in maintaining VAM in most plant species, but no specific compound could be identified as promo...

Detection and Location of Seed‐Borne Inoculum of Didymella bryoniae and its Transmission in Seedlings of Cucumber and Pumpkin

Abstract: Of the 91 tested cucurbit seed samples from thirteen countries nine from four countries were found to be infected with Didymella bryoniae. The pathogen is located on and in the seed coat including the perisperm and in the tissue of the cotyledons. Primary seedling infection occurred on the radicle, hypoctyl and cotyledons. Infection of the radicle generally caused pre-emergence tot while infection on the hypocotyl and cotyledons developed furthere inoculum for infection of the first true leaves and the stem. Experimentally, all the isolates of D. bryoniae could infect cucumber (Cucumis sativus), oriental melon (Cucumis melo var. makuwa), pumpkin (Cucurbita pepo) and watermelon (Citrullus vulgaris) at different growth stages; the susceptibility of cucumber and pumpkin was markedly influenced by previailing humid conditions.The blotter method was found more suitable for detection of seed-borne infection than the agar plate method.