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Showing papers on "Seedling published in 1979"


Journal ArticleDOI
TL;DR: There was positive correlation between seed size and maximum depth from which seeds of this species and those of five other associated annuals emerge, and there may be strong selection for early germination and emergence in the field and within a patch.
Abstract: Summary Ambrosia trifida L. is a fast growing annual colonizer of disturbed ground. When present it drastically reduces kinds, numbers and growth of other annuals. Its seedlings emerge before any other annual in early spring. We studied seed germination in relation to temperature, moisture, depth of sowing, and seedling emergence, survivorship and performance of mature plants in relation to time and depth of sowing and seed size. Seeds germinated under a wide range of temperature (8 to 41°C) with an optimum between 10 to 24 °C. Further germination occurred after restratification suggesting germination polymorphism. The seeds also germinated under a wide range of soil moisture conditions (17 to 55 % dry wt) but highest germination occurred at 20 to 33 % soil moisture. Optimum depth of sowing was 2 cm; very little germination occurred very close to soil surface, and some occurred at the 16 cm depth. In the field seedlings emerged from shallow depths first and then from progressively deeper parts of the soil bank. There was positive correlation between seed size and maximum depth from which seeds of this species and those of five other associated annuals emerge. Time required for seedlings to emerge after germination was directly related to depth. Seedlings that emerged first had the highest probability of survival and those that emerged late suffered progressively higher mortality, irrespective of whether late emergence was caused by late germination or deep germination. Among the survivors that reached maturity, plant weight, height, and number of seeds per plant declined with delayed emergence. However, experimental manipulation showed that delayed emergence per se was not the cause of the mortality but rather competition with individuals which emerged earlier. Furthermore, delayed emergence of cohorts in separate plots did not significantly reduce plant height, weight and number of seeds per plant. Thus in the field and within a patch there may be strong selection for early germination and emergence. However, this selection may be opposed by the unpredictability of the time of disturbance, the spatial patchiness of disturbance, and the longevity and induced dormancy of Ambrosia seeds. Compared to its associated annuals, A. trifida has the largest seeds and seedlings, the earliest germination and emergence and a very high photosynthetic rates. These properties give A. trifida a decisive advantage over the other species.

179 citations


01 Jan 1979
TL;DR: In this paper, the characteristics of seed germination and of seedlings showed that water stress in the seedling stage may be the most important factor affecting establishment of the common desert succulent, Agave deserti, required unusually wet years and protection afforded by nurse plants or other shelters.
Abstract: Survival of seedlings of the common desert succulent, Agave deserti, required unusually wet years and the protection afforded by nurse plants or other shelters. The characteristics of seed germination and of seedlings showed that water stress in the seedling stage may be the most important factor affecting establishment. Leaves wilted irreversibly at an osmotic potential of -1.6 MPa (-16 bars). Survival of seedlings required the generation of enough biomass during the first wet season to survive drought periods when the water potential of the soil was below -1.6 MPa. Lengths of individual growing seasons and subsequent droughts were obtained from rainfall records for 1961 through 1978 for a site in the western Colorado desert. The length of the first major drought following germination apparently limited establishment of Agave deserti to only one year in the last 17. The favorable year, 1967, was consistent with the estimated year of germination of the six smallest Agave deserti observed in 1,500 m2 at the field site. WATER RELATIONSHIPS play a critical role in the survival of the seedling stage of desert succulents. Their roots are often quite shallow (Steenbergh and Lowe, 1969; Nobel, 1976), and so during an extended drought the seedlings can obtain no water from the soil. To survive such a dry period, the seedlings must develop sufficient water storage tissue during the previous wet season. Desert succulents exhibit Crassulacean acid metabolism (CAM), characterized by nocturnal stomatal opening and CO2 uptake (Nishida, 1963; Nobel, 1976). Although important for water conservation, CAM is associated with slow growth rates, e.g., Carnegiea gigantea grew only 1.3 cm

144 citations


Journal ArticleDOI
TL;DR: Comparison of depletion of the seed-bank in the soil and increase of plant density showed that seeds germinating in the first week of the growing season produced fewer established seedlings than seeds germination in the second or third weeks.
Abstract: (1) Data for plant density, germinable seed in the soil, and seed production in annual grassland were obtained at Hopland Field Station, California, U.S.A., in 1973 and 1974. The study combined indirect estimates of numbers of seed in the soil, germination in soil samples containing natural seed, and estimation of plant density. (2) Autumn patterns of establishment differed significantly between the two study years. Plant density increased through the autumn, reaching peaks of 261 8 and 345*3 plants per dM2 in the seventh week after germination began, in 1973 and 1974 respectively. (3) The numbers of germinable seed in the top 6.4 cm of the soil prior to the start of the growing season were 6705 per dM2 in 1973 and 6102 per dM2 in 1974, and thus showed little difference between years. (4) Comparison of depletion of the seed-bank in the soil and increase of plant density showed that seeds germinating in the first week of the growing season produced fewer established seedlings than seeds germinating in the second or third weeks. The few seeds remaining in the fifth and sixth weeks had a high probability for successful establishment. (5) Six species-groups exhibiting contrasting strategies for germination and establishment are discussed in detail.

143 citations


Journal ArticleDOI
TL;DR: Test the hypothesis that interference with chlorophyll metabolism may be one mechanism of inhibition of plant growth in allelopathic interactions by reducing dry weights of soybean seedlings by both 10−3 M and 5 × 10−4 M treatments of ferulic,p-coumaric and vanillic acids.
Abstract: Experiments were designed to test the hypothesis that interference with chlorophyll metabolism may be one mechanism of inhibition of plant growth in allelopathic interactions. Effects of ferulic,p-coumaric, and vanillic acids on soybean and grain sorghum growth and chlorophyll content were quantified and compared after seedlings were treated with these compounds in a nutrient culture. Following a 6-day treatment cycle, dry weights of soybean seedlings were reduced by both 10−3 M and 5 × 10−4 M treatments of ferulic,p-coumaric and vanillic acids. Soybean weight reductions in each case were paralleled by a significant reduction in the concentration (μg Chl/mg dry wt) of chlorophylls a and b and total chlorophyll in the unifoliate leaves. Sorghum seedling growth was also reduced by each of the compounds at the 5 × 10−4 M level, but leaf chlorophyll concentration was not below that of control plants.

113 citations


Journal ArticleDOI
TL;DR: It is suggested that plants must reach a critical size before they can respond to the environmental stimuli of vernalization and photoperiod.
Abstract: (1) Wild parsnip (Pastinaca sativa L.) is a monocarpic perennial herb which grows as a weed in North America in grassy habitats that are usually mown in summer. Flowering occurs in May and June, and seeds are mature by early July. Seeds remain attached to the dead erect shoots until autumn, unless the plants are disturbed, as by mowing. (2) At maturity small numbers of seeds can germinate at summer temperatures. As the seeds after-ripen during summer their ability to germinate at high temperatures increases, but there is little increase in ability to germinate at autumn temperatures. Stratification (moist chilling) over winter lowers the temperature requirement for germination, and therefore the majority of seeds in the field germinate in early spring. (3) Seedling mortality is high; of 3980 seedlings marked in a natural population in Kentucky, only thirty-three (0.83%4) flowered before dying. Cohorts of seedlings marked in August, November, December, January and April had concave survivorship curves; cohorts marked in February and March initially had concave survivorship curves, but the curves subsequently became linear. (4) Plants produce rosettes and grow until the second, third, fourth or fifth growing season before producing flowers. Vernalization is required for flowering, and plants reach the flowering stage quicker under long than under short days. (5) An old-field population showed a wide range of individual plant sizes in late May: flowering plants had root-crown diameters (RCDs) of 5-39 mm, and non-flowering plants had RCDs of < 1 (first-year seedlings) to 10 mm. It is suggested that plants must reach a critical size before they can respond to the environmental stimuli of vernalization and photoperiod.

110 citations


Journal ArticleDOI
01 Apr 1979-Ecology
TL;DR: Investigating the nature of the distribution of plant species along a soil salinity gradient at Rittman, Ohio, indicates that there is a definite relationship between soilSalinity concentrations and the position of plantspecies along this environmental gradient.
Abstract: The nature of the distribution of plant species along a soil salinity gradient at Rittman, Ohio, indicates that there is a definite relationship between soil salinity concentrations and the position of plant species along this environmental gradient Only a single species of flowering plant, Salicornia europaea, was found growing at the most extreme portion of the salinity gradient Soil samples collected from sites dominated by S europaea had salinity levels which yielded in situ electrical conductivities ranging from 189 to 1428 mmhos/cm, Atriplex triangularis grew at sites with inter- mediate levels of soil salinity, 39 to 400 mmhos/cm, and Hordeum jubatum dominated the least saline sites, 11 to 140 mmhos/cm, which were directly adjacent to a nonsaline meadow Salicornia europaea had high levels of mortality during the spring and early summer months of the growing season, with the mean number of seedlings ranging from 58 plants per 100-cm2 quadrat in May to a mean value of 18 mature plants per quadrat at the end of the growing season in October At the most highly saline sites containing S europaea, seedling numbers ranged from a mean of 5 plants per 100- cm2 quadrat in May to 1 mature individual per quadrat by October Transplants of S europaea had the highest standing crop biomass levels in the Atriplex and Hordeum zones, with dry weight production decreasing in zones of both higher and lower soil salinity Laboratory investigations indicated that S europaea reached its optimal growth in nutrient solution treatments containing 170 meq/l and 340 meq/l NaCl Germination of seeds of S europaea under field conditions occurred over an extended period of time, from February through June, when soil salinity levels were lowest Seed germination was not stimulated by treatments with salt and maximum germination percentages occurred in distilled water controls

90 citations


Journal ArticleDOI
TL;DR: Seeds of eleven woody species were exposed to 2.3 cm/wk of simulated sulfuric acid rain at pH values of 3.0, 3.5, or 4.0 as mentioned in this paper.
Abstract: Seeds of eleven woody species were exposed to 2.3 cm/wk of simulated sulfuric acid rain at pH values of 3.0, 3.5, or 4.0, or to a simulated control rain at approximately pH 5.6. All treatments also contained a neutral mixture of cations and anions based on concentrations reported for Hubbard Brook, N.H. Seeds or seedlings were subject to ambient conditions, except deposition. Seeds were planted in winter, 1977; seedlings were harvested the following summer. The dry weights of the top and roots of each seedling were recorded. Although eight species were affected by simulated acid rain, the direction and magnitude of effects varied with species and with treatment. Seedling emergence was stimulated by at least one acid treatment for four species, while root growth was inhibited for one species. Except for one species, whose emergence rate and top growth were both affected, effects were confined to one measured parameter.

84 citations


Journal ArticleDOI
TL;DR: Fungi mainly affected seed germination and the development of young seedlings; at later stages, herbivory became more important in plant survival.
Abstract: SUMMARY (1) An abundant mycoflora was found on the seed of Abutilon theophrasti, Datura stramonium, Ipomoea hederacea, and Polygonum pensylvanicum, which are colonizing annuals of early successional habitats. The fungal species differed among the plant species and most were specialists with respect to their hosts. (2) A more extensive mycoflora occurred within seed of Polygonum pensylvanicum and D. stramonium than within seed of I. hederacea and A. theophrasti. Ipomoea hederacea and A. theophrasti had greater resistance to fungal infection than P. pensylvanicum and D. stramonium. Fungi differed in their ability to penetrate seeds. (3) Seed germination and seedling development were affected, both quantitatively and qualitatively, by the fungal isolates. Polygonum pensylvanicum seedlings were more susceptible to fungal invasion than those of A. theophrasti or I. hederacea. (4) Extracts from all tissues of I. hederacea and from the roots of A. theophrasti had antifungal activity. Little activity was detected in extracts from P. pensylvanicum. (5) Plant densities of I. hederacea, D. stramonium and P. pensylvanicum remained fairly constant in the field, after seedling emergence, irrespective of density or species diversity. (6) Fungi mainly affected seed germination and the development of young seedlings; at later stages, herbivory became more important in plant survival.

77 citations



Journal ArticleDOI
TL;DR: The main conclusion is that fire is not a succession-initiating disturbance in this ecosystem because the long association with fire has allowed the evolution of specific survival and recruitment strategies.
Abstract: This study investigates the impact of insect grazing on the establishment of plant populations from seed after bushfires in coastal plain woodland near Perth, Western Australia. Seedling germination was measured and individual seedlings were observed frequently to estimate survivorship and causes of mortality. Invasion rates of acridid grasshoppers into both small and large burnt areas were measured and palatabilities of seedlings to the most abundant grasshopper species were estimated. Escape of seedlings from insect grazing was influenced by both seedling palatability and area of burning. Grasshoppers were absent from large burnt areas for one to two years. In small burnt patches of vegetation, the patterns of grazing on seedlings were related to seedling palatabilities. These results are discussed in the light of current models of plant succession and the main conclusion is that fire is not a succession-initiating disturbance in this ecosystem because the long association with fire has allowed the evolution of specific survival and recruitment strategies.

67 citations


Journal ArticleDOI
TL;DR: Water loss was studied in regenerated plantlets of Brassica oleracea var.

Journal ArticleDOI
TL;DR: Results from field and experimental sowing of seeds both indicate that this perennial species has minimal defense against germination when conditions are suboptimal for seedling survival.
Abstract: Seeds of the Panamanian shrub, Hybanthus prunifolius (Schult.) Schulze (Violaceae) are dispersed at different times in different years ((March to June) and are exposed to the irregular rainfall of the dry season in some years. Fluctuations in soil moisture in the dry season represent suboptimal conditions for germination and seedling survival. There are no mechanisms to prevent germination prior to the arrival of consistent rains in the wet season. Among three natural cohorts of seeds followed in two years, the cohort experiencing the longest time from sowing to consistent rains had the highest germination, but it also had the longest time lag from sowing to, beginning of germination, longest germination period, and lowest survival of seedlings 3 months after sowing. Seeds were also induced experimentally to germinate under 14 different moisture patterns. The patterns encompassed 1) varying lengths of moisture before a dry period, 2) inconsistent moisture, and 3) varying lengths of dryness prior to any moisture. Mortality of seeds by fungal infection occurred if the wet period was delayed. But germination was less affected by fluctuations than was seedling survival. Length of the first wet period and frequency of occurrence of the wet period both affected germination levels. Survival and development of seedlings was influenced by the number of days exposed to dry conditions and by the stage of development at the beginning of the dry period. Young seedlings suffered attrition due to drought stress, and older seedlings died from fungal attack. Results from field and experimental sowing of seeds both indicate that this perennial species has minimal defense against germination when conditions are suboptimal for seedling survival. Undoubtedly there is more recruitment in some years than in others.

Journal ArticleDOI
TL;DR: It is suggested that tolerance of R. mangle seedlings to high concentrations of Cd, Pb, and Hg may be due to formation of non-toxic sulfides in the root or on its surface, detoxification in tissues, an ion-exclusion mechanism in the roots, or a combination of these factors.
Abstract: Seedlings of red mangrove (Rhizophora mangle) were treated twice with 25, 250, or 500 Ag Cd/g soil; 62.5, 125, or 250 jtg Pb/g soil; or 10, 100, or 500 Ag Hg/g soil. Survival of seedlings was affected only by 500 ,ug Hg/g soil. There was no effect of any metal on final weight and size of hypocotyls, stems, roots, or leaves; the time at which the stem emerged from the plumule; or the time at which the first pair of leaves unfurled from the stem. Lead was not translocated by the seedlings but Cd and Hg were. It is suggested that tolerance of R. mangle seedlings to high concentrations of Cd, Pb, and Hg may be due to formation of non-toxic sulfides in the root or on its surface, detoxification in tissues, an ion-exclusion mechanism in the roots, or a combination of these factors. HIGHER PLANT SPECIES VARY WIDELY in resistance to high concentrations of heavy metals in soils, and resistance may vary at the subspecies level. Gregory and Bradshaw (1965) described metal-tolerant ecotypes of Agrostis tenuis that grew on toxic mining soil, and Wu and Antonovics (1976) showed that an ecotype of Plantago lanceolata that grew near a roadside contaminated by lead was more resistant to lead than were unexposed ecotypes. The mechanism of tolerance to heavy metals by higher plants is not known, but Antonovics et al. (1971) suggested that they detoxify heavy metals by chelation at the cell wall. In mangrove soil, where there is an abundance of sulfides, heavy metals may be precipitated in sulfide complexes and thus be made unavailable to the plants. Another mechanism for resistance could be exclusion of ions by roots. Scholander et al. (1962) showed that roots of the mangrove genus Rhizophora contain an osmoregulatory mechanism for exclusion of ions. We exposed seedlings of Rhizophora mangle L. to very high concentrations of lead, cadmium, and mercury with subsequent measurements of residues in plant parts and of increases in biomass. We hypothesized that lack of translocation would indicate metal exclusion and that high tissue residues without effect would indicate detoxification at the tissue level. MATERIALS AND METHODS Five hundred ripe R. mangle seedlings were picked from trees near Naples, Florida, and shipped to, the Gulf Breeze laboratory. There, they were separated into 10 groups of 40 seedlings, each of similar average hypocotyl length and weight. Each seedling was planted in a drainless clay not. 90 mm tall and 63 mm in diameter. Each pot contained between 269 and 281 g (average 272 g) of muddy sand from the head of Escambia Bay, Florida. The sediment was passed through a 1 cm2-mesh screen and was, kept under natural estuarine water of 28.5 ppt (parts per thousand) salinity. Soil pH and Eh were determined on a Corning model 7 pH meter. The sediment was analyzed for chlorinated insecticides and polychlorinated biphenyls (PCBs) by extraction with 10 percent acetone in petroleum ether, cleaned-up in a Florisil column, and analyzed by electron capture gas chromatography (Duke et al. 1970). The organic content of four substratum aliquots was determined by drying for 16.5 hrs at 106?C and ashing for 6 hrs at 525?C. The loss of weight upon ashing was considered to be equal to the weight of organic matter. The results are given as the average of the four samples. Lead and cadmium in soil and plant parts were analyzed by flame and flameless atomic absorption spectrophotometry (Segar, in press). Samples were wet ashed with HN03-H2S04 ( 1:1 ) in a microwave oven (Abu-Samra et al. 1975). Mercury was determined after wet ashing by the method of Hatch and Ott (1968), and total sulfur by the method of Blanchar et al. (1965). Before planting, weight and hypocotyl length were recorded for each seedling. During the tests, total weight and weight of roots, hypocotyl, stem, and leaves were determined. Also, length of hypocotyl and stem and length and width of leaves were measured. Times of appearance of the stem and leaves were also noted. Seedlings were grown at 250?+1?0C under 6000 lux from cool white fluorescent tubes with alternating 12-hr periods of light and darkness. Water was 1Publication No. 324 from the Gulf Breeze Laboratory. 22 BIOTROPICA 11(1): 22-27 1979 This content downloaded from 207.46.13.122 on Thu, 19 May 2016 05:05:30 UTC All use subject to http://about.jstor.org/terms added twice daily to each pot. On the first day, the pots were watered with natural estuarine water of 28.5 ppt salinity. Thereafter, deionized water was used. Soil was treated twice with lead, cadmium, or mercury. The first treatment occurred one week after planting; the second, when seedlings had one pair of leaves. Treatment concentrations were: lead, 62.5, 125, and 250 jug/g soil; cadmium, 25, 250, and 500 ,ug/g soil; mercury, 10, 100, and 500 jug/g soil. All were added as the chloride salt dissolved in deionized water. One-half of the seedlings were collected one week after the first pair of leaves unfolded from the stem. The other half was treated a second time and collected two weeks later. They were examined for chlorosis, epinasty, abscission, and other gross changes. Data were analyzed by the t statistic for two means (Brownlee 1965).


Journal ArticleDOI
TL;DR: When cells or the culture filtrate of Azotobacter chroococcum grown on a medium without a source of fixed nitrogen were added to barley seeds, they sometimes stimulated the extension of seedling roots and inhibition of germination appears to be due to competition between viable bacteria and seed for available oxygen.
Abstract: When cells or the culture filtrate of Azotobacter chroococcumgrown on a medium without a source of fixed nitrogen were added to barley seeds, they sometimes stimulated the extension of seedling roots. The stimulation appeared to be due to a bacterial metabolite as the cells used were non-viable (due to a decrease in pH of the growth medium). There was no evidence that 3-indoleacetic acid or gibberellic acid, which were produced by the bacterium, were involved in the stimulation. When the bacterium was grown on a medium containing nitrate, the cells, but not the culture filtrate, always inhibited germination and the extension of seedling roots. This inhibition of germination appears to be due to competition between viable bacteria and seed for available oxygen.


Journal Article
TL;DR: The influences of a vesicular-arbuscular mycorrhiza (Glomus etunicatus) and burrowing nematode (Radophohts similis) alone and in combination on the growth of rough lemon seedlings were studied in the greenhouse.
Abstract: The influences of a vesicular-arbuscular mycorrhiza (Glomus etunicatus) and burrowing nematode (Radophohts similis), alone and in combination, on the growth of rough lemon (Citrus limon) seedlings were studied in the greenhouse. Growth of mycorrhizal seedlings was significantly greater than that of nonmycorrhizal seedlings or seedlings inoculated with R. sindlis. Mycorrhizal stimulation of seedling growth was inhibited by nematode infection. When seedlings were inoculated with G. etunicatus arid R. similis, suppression of seedling growth by R. similis was less on VAM seedlings than on nonmycorrhizal seedlings, Nonmycorrhizal seedlings infected with R. similis were significantly smaller than nonmycorrhizal seedlings free of R. similis. Vesicle formation and mycelia growth were less in nematode-infected roots.

Journal ArticleDOI
TL;DR: Immunohistochemical determination of the pattern of enzyme movement during germination and early seedling growth revealed that at the commencement of germination the α-amylase protein moiety moves away from the entire face of the scutellum and intermediate crushed cell layer.
Abstract: A procedure for the isolation of a germination-specific α-amylase isoenzyme from Hordeum vulgare (cv. Nordal) is described. The pure α-amylase isoenzyme was used to obtain a monospecific anti-α-amylase antibody. The antibody was subsequently used to localise α-amylase in sections of germinating barley seeds (cv. Nordal). Immunohistochemical determination of the pattern of enzyme movement during germination and early seedling growth revealed that at the commencement of germination the α-amylase protein moiety moves away from the entire face of the scutellum and intermediate crushed cell layer. As early seedling growth continues, the protein begins to be synthesised in the aleurone layer and transported away from this layer into the endosperm.




Journal ArticleDOI
01 Mar 1979-Nature
TL;DR: It is reported here that, after germination, the amount of linamarin in the total seedling remains essentially constant during a 26-day experimental period and moreover corresponds to the amounts present in the seed before germination.
Abstract: WHEN a seed germinates, we expect the defensive secondary compounds it contains to be transferred intact to the growing seedling, or variously decomposed to produce resources for the growing seedling. The seeds of wild indigenous Costa Rican lima beans (Phaseolus lunatus L.) contain about 3.45% fresh weight of linamarin1, a cyanogenic glucoside that can enzymatically decompose to produce 0.37% fresh weight hydrocyanic acid (HCN). We report here that, after germination, the amount of linamarin in the total seedling (roots, cotyledons, and shoots) remains essentially constant during a 26-day experimental period and moreover corresponds to the amount of linamarin present in the seed before germination (Fig. 1).

Journal ArticleDOI
15 May 1979-Botany
TL;DR: It appears that Kochia phytotoxins do not inhibit germination but retard growth later, which also supports the high density but drastically reduced growth of Kochia in its 2nd year of revegetation in the field.
Abstract: The autotoxic properties of Kochia scoparia phytotoxins on its germination and on radicle and seedling growth were studied. Inhibition of seed germination, a commonly known allelopathic phenomenon, did not occur and seeds reached close to 100% germination within 24 h when tested against different phytotoxins isolated from Kochia leaves regardless of the concentrations applied. Similarly, the radicle length of Kochia seeds measured was not significantly affected. However, when radicles were allowed to grow for an extended period of time in various concentrations of isolated phytotoxins, their length was significantly reduced in almost all cases. Such autotoxic growth retardation patterns were more pronounced in 10-day-old seedlings. It appears that Kochia phytotoxins do not inhibit germination but retard growth later, which also supports the high density but drastically reduced growth of Kochia in its 2nd year of revegetation in the field. The autotoxins isolated and identified from Kochia leaves were phen...

Journal ArticleDOI
TL;DR: Lack of an effective means of achene dispersal, inability to maintain reserves of achenes in the soil and low competitive ability of rosettes prevent the species from growing in intermittently disturbed habitats occupied by weedy species of monocarpic perennials.
Abstract: Grindelia lanceolata (Compositae) grows in open habitats in shallow, rocky soils developed over limestones. Some freshly matured achenes from both disc and ray flowers can germinate in light, but disc achenes germinate over a wider range of temperatures than ray achenes. Stratification lowers the temperature requirement for germination and partially substitutes for the light requirement in both types of achenes. When stratified and incubated in darkness, disc achenes germinated better than ray achenes at all temperatures. In the field some germination occurs in au- tumn, but most of it occurs in early spring. High seedling mortality occurs during the first growing season, and less than 1% of the plants that germinate flower and pro- duce seeds. After germination, a rosette of leaves is produced, and the plants grow from 2-4 years before flowering. There are many more nonflowering than flowering plants in the populations, and both groups of plants are composed of individuals of various ages. Vernalization is an absolute requirement for flowering, and vernalized plants flower under both long and short days. GA3 did not substitute for the chilling require- ment for flowering. The reserve stage in the life cycle is the rosette stage, and rosettes grow slowly, taking advantage of favorable periods for plant growth until they reach the proper size to be vernalized. Lack of an effective means of achene dispersal, inability to maintain reserves of achenes in the soil and low competitive ability of rosettes prevent the species from growing in intermittently disturbed habitats occupied by weedy species of monocarpic perennials.

Journal ArticleDOI
TL;DR: The results from these experiments support the idea that the differential establishment of E. ovata and C. littoralis at Ocean Grove is partly due to differences in their shade tolerance, as sun-grown seedlings of both species had very similar light response curves.
Abstract: The invasion of unburnt Eucalyptus woodland by Casuarina scrub has greatly reduced light penetration, hence seedlings encounter intense shading beneath the scrub and within the grass sward. Artificial shading experiments showed that Acacia pycnantha seedlings were the most shade-tolerant, followed by C. littoralis and C. stricta, while Eucalyptus ovata seedlings all died under prolonged intense shading, apparently owing to Botrytis cinerea. Shading (30% of full daylight) for 9 months (autumn-spring) increased the yield of A. pycnantha, C. littoralis and C. stricta seedlings compared with non-shaded seedlings. Intense shading (less than 8% of daylight) caused etiolation and decreased yields in all species. Chlorophyll a/b ratios of E. ovata and C. littoralis were decreased by shading. The total chlorophyll content of E. ovata plants was increased by shading, but that of C. littoralis was not affected. Shade tolerance of A. pycnantha, C. littoralis and C. stricta was partly morphogenetic in origin (increased leaf area ratios with shading) and partly due to resistance to fungal attack. All species had low compensation points. Shaded A. pycnantha seedlings retained pinnae for much longer than non-shaded seedlings. The differential survival of shaded E. ovata and C. littoralis seedlings is apparently not the result of differences in their photosynthetic efficiency at low light intensities, as sun-grown seedlings of both species had very similar light response curves. Prolonged shading significantly decreased the root/shoot ratios of all species except C. littoralis; hence shading will reduce the drought resistance of seedlings. The results from these experiments support the idea that the differential establishment of E. ovata and C. littoralis at Ocean Grove is partly due to differences in their shade tolerance.

Journal ArticleDOI
TL;DR: The susceptibility of amino acid uptake and polysome profiles to water stress suggests that the inhibition of betacyanin synthesis, a process dependent on protein synthesis, may be due to inhibition either of precursor (tyrosine) uptake or of the synthesis or activity of some enzyme in the pathway.
Abstract: The use of only the upper part of the hypocotyl and the cotyledons in the Amaranthus tricolor bioassay for cytokinins, instead of the whole seedling, was found to reduce the endogenous response and give a higher benzyladenine-dependent response. There is no marked difference in the uptake or metabolism of benzyladenine in whole seedlings compared with that in excised cotyledons. Analysis of variability in the bioassay showed that water availability to the cut seedlings and to whole seedlings is a major factor in the amounts of betacyanin accumulated during the subsequent induction period. The increase in the amount of betacyanin accumulated in response to benzyladenine, following conditions of water stress, is not correlated with differences in benzyladenine uptake. Endogenous production and fusicoccin stimulation is also increased following water loss by cut seedlings. Possible explanations for this stress induction may be found in responses of active transport to changes in turgor pressure. Although pretreatment of the roots of seedlings with mannitol stimulated subsequent induction by excised cotyledons, the presence of mannitol during the induction period inhibited the accumulation of betacyanin. This inhibition is not due to any effect on benzyladenine uptake. The susceptibility of amino acid uptake and polysome profiles to water stress suggests that the inhibition of betacyanin synthesis, a process dependent on protein synthesis, may be due to inhibition either of precursor (tyrosine) uptake or of the synthesis or activity of some enzyme in the pathway.

Journal ArticleDOI
TL;DR: Results from reciprocal transplant studies indicate that there has been no differentiation into growth ecotypes in this species, and it is concluded that P. riparia is a species of wide ecological amplitude that acts as a gap opportunist.
Abstract: The present study is an attempt to understand biology of Palicourea riparia Bentham relative to the process of recovery following disturbance in a tropical rain forest in Puerto Rico In this species seed germination and seedling maintenance are favored by environmental conditions associated with disturbance, yet germination and seedling persistence are significant in undisturbed forest conditions The pattern of seed germination indicates that (1) the seeds possess a dormancy mechanism, (2) dormany is not evenly distributed within a seed population, and (3) some seeds can remain viable if buried in the soil for at least three years Results from reciprocal transplant studies indicate that there has been no differentiation into growth ecotypes in this species Photosynthetic rates are very low under both field and laboratory conditions In the laboratory, photosynthetic rates are consistently enhanced by increases in light intensity It is concluded that P riparia is a species of wide ecological amplitude that acts as a gap opportunist

Journal ArticleDOI
01 Jan 1979-Planta
TL;DR: It is concluded that the mobilization of storage fat is independent of photomorphogenesis and the surplus of carbohydrate produced from fat in the light seems to be converted to starch grains in the plastids, which function as a secondary storage pool in the cotyledons.
Abstract: The degradation of storage fat in the cotyledons of mustard seedlings is unaffected by phytochrome and photosynthesis (irradiation with continuous red or far-red light from sowing of the seeds) although light imposes a strong constraint on the translocation of organic matter from the cotyledons into the seedling axis. Likewise, the development and disappearance of glyoxysomal enzyme activities (isocitrate lyase, malate synthase, citrate synthase) takes place independently of light. It is concluded that the mobilization of storage fat (fat→carbohydrate transformation) is independent of photomorphogenesis. The surplus of carbohydrate produced from fat in the light seems to be converted to starch grains in the plastids, which function as a secondary storage pool in the cotyledons.

Journal ArticleDOI
TL;DR: It is concluded that the development of a coleoptile tiller may prove to be a useful selection criterion in the breeding of this species.
Abstract: Two characters, previously shown to influence the rate of seedling growth in tall fescue, were studied in detail in three distinct populations as part of a programme to improve seedling vigour in the species. The effect of seed weight was found to be confined to the very early stages of growth and was apparent in seedling dry matter yield but not in tiller number. The development of a tiller in the axil of the coleoptile, which varied significantly between populations, had a far greater and more permanent effect both in terms of dry matter yield and tiller number. The increase in tiller number attributable to the coleoptile tiller and its derivatives increased geometrically in step with the total tiller number, so that the intitial difference of 28% in the first harvest remained practically unchanged in the subsequent three harvests. Similarly, the percentage difference in dry matter yield between plants with and without a coleoptile tiller remained more or less constant at about 22% from the second harvest onwards.

Journal ArticleDOI
TL;DR: Genetic studies indicate that, in the St-R701 line which was derived from a spontaneous mutation in diploid callus culture, the streptomycin resistance is cytoplasmically inherited.