Showing papers on "Seedling published in 1989"

Effect of nurse plants on the microhabitat and growth of cacti.

Abstract: (1) Seedlings of the cactus Carnegiea gigantea at two sites in the Sonoran Desert were found only in sheltered microhabitats, 89% occurring under the canopy of Ambrosia deltoidea and Cercidium microphyllum. In contrast, 29% of the seedlings of Ferocactus acanthodes, which tolerates higher temperatures than C. gigantea, occurred in unsheltered microhabitats, where maximum soil surface temperatures reached 71?C. Most (70%) of the sheltered seedlings of F. acanthodes occurred under the canopy of the perennial bunchgrass Hilaria rigida. (2) Shading by nurse plants reduced the total daily photosynthetically active radiation (PAR) available for their associated seedlings. Near an equinox, a seedling of C. gigantea located at the centre of an A. deltoidea shrub received 77% less total daily PAR, which reduces its predicted net CO2 uptake by 90% compared with an unshaded seedling. Similarly, a seedling of F. acanthodes located at the centre of an H. rigida plant received 64% less total daily PAR and could fix 65% less CO2 than could an unshaded seedling. (3) H. rigida and F. acanthodes have overlapping shallow root systems (mean root depth of 0 08 m for H. rigida and 0 05 m for seedlings of F. acanthodes), which accentuates competition for water. A water uptake model, which closely predicted the soil water potential in the root zone of H. rigida, indicated that a seedling of F. acanthodes located at the centre of the bunchgrass took up 32% less water than did an exposed seedling. (4) Assuming that the effects of temperatures, PAR, and soil water on net CO2 uptake are multiplicative, the predicted net CO2 uptake of a seedling of F. acanthodes under the canopy of H. rigida was only 36% of that of an exposed seedling. However, areole production by seedlings of F. acanthodes located under the nurse plant was 68% of that measured in exposed areas. This discrepancy probably reflects the 60% higher soil nitrogen levels under H. rigida. (5) Therefore, nurse plants facilitate seedling establishment by reducing high temperatures near the soil surface and provide a microhabitat with a higher soil nitrogen level. However, shading and competition for water with the nurse plants markedly reduce seedling growth. The magnitude of the reduction depends on seedling size and location under the nurse plant.

Tree Seedlings on Logs in Picea-Tsuga Forests of Oregon and Washington

Abstract: Logs are the major seedbed for trees in coastal Picea sitchensis–Tsuga heterophylla forests. Field experiments were conducted at Cascade Head, Oregon, and Hoh River, Washington, to examine pathogens, predation, competition, and standing water as causes for this close seedling–log association. More seedlings survived on log blocks than on soil blocks, regardless of whether the blocks were raised or placed flush with the soil surface. Standing water was therefore an unlikely cause of the seedling–log association. Comparisons of plots protected from and exposed to predation revealed that predation was minor and of equal intensity on soils and logs. Sterilizing soils did not consistently increase seedling survival above controls. Clearing ground–layer vegetation from soil plots significantly increased the survival of conifer seedlings compared with that on uncleared soils. The seed penetration rates through moss mats indicated that <1% of the seedlings germinated within moss mats. Competition with herbs and mosses on the forest floor therefore appears to be responsible for the disproportionate number of tree seedlings found on logs. Recently fallen logs represent sites where competition is low enough for tree seedling recruitment within many Picea–Tsuga forests.

Woody seedling dynamics in an east Texas floodplain forest.

Abstract: We monitored woody plant seed deposition, seedling emergence, and the survival and growth of seedlings (i.e., plants ≤0.5 m tall regardless of age) in an East Texas river floodplain forest from 1979 through 1984. In addition, we estimated the relative importance of flooding, drought, fungal attack, herbivory, proximity to a conspecific adult, and shade in causing seedling mortality. Tree species fell into two major groups on the basis of their demographic characteristics and responses to unfavorable conditions. The first group was composed of heavy—seeded species, of which water oak (Quercus nigra) was the primary example. They produced few seeds, but had high seedling survival. Seedlings of these species emerged late in the summer, thereby avoiding peak periods of flooding and damping—off mortality. Seedling survival was little affected by drought, herbivory, or proximity to a conspecific adult. The second group included most of the common tree species (e.g., ironwood, Carpinus caroliniana; sweetgum, Liq...

Seed banks, fire season, safe sites and seedling recruitment in five co-occurring Banksia species

Abstract: Quantity of seed stored in the canopy of 5 co-occurring Banksia species varied by nearly 2 orders of magnitude. The 3 species which resprout vegetatively after fire, produced less seeds and retained smaller seed banks than 2 non-sprouting species. Contiguous patches of scrub-heath were burned in spring and autumn. The spring fire was cooler, and both sets of seed released subsequently did not germinate until the following (common) winter. Rate of seed release was higher after the autumn fire. The non-sprouting species released more seeds after each fire and yielded more seedlings per parent than the resprouting species. Percentage (field) germination of the non-sprouters was not consistently different from that of the resprouters. Seeds exposed on the soil surface during summer soon lost viability compared with buried seeds. The large number of seedlings established up to 8 months after the spring fire resulted from many seeds escaping exposure by dispersal into litter-covered "safe' sites. Although litter microsites covered only 30% of the ground surface after the spring burn, they accounted for 80% of seedlings both before and after the summer drought. Litter microsites covered only 14% of the autumn-burned site but accounted for 60% of seedlings before summer and 40% after summer, suggesting density-dependent thinning of seedlings. The autumn-burned site supported more than twice the density of seedlings than the spring-burned site by the end of the 1st winter. Summer mortality of seedlings was 32% in the spring-burned site and 65% in the autumn-burned site, equalizing the seedling:parent ratio at both sites. Net recruitment of 1-yr-old seedlings varied from 0 for 2 resprouters to >100 per parent for a non-sprouter.

Woody plant invasion of grasslands: establishment of honey mesquite (Prosopis glandulosa var.glandulosa) on sites differing in herbaceous biomass and grazing history

Abstract: Emergence and survival of honey mesquite (Prosopis glandulosa var.glandulosa Torr.) seedlings was quantified on sites with contrasting grazing histories: long-term continuous grazing (LTG) and long-term protection (LTP) from grazing by cattle. On each site, different levels of heroaceous defoliation were imposed at monthly intervals (no defoliation=ND, moderate=MD and heavy=HD). The two weeks following seed dissemination appeared to be the most critical toProsopis establishment on LTP-ND plots. Openings in the herbaceous layer created by moderate defoliation of grasses on the LTP site increased germination and/or survival 7-to 8-fold during this period. However, increasing the degree of defoliation from moderate to heavy did not stimulate additional emergence on either the LTP or LTG site. Emergence from scarified seed placed in cattle dung (17 to 30%) was lower than that of bare seed placements in various microhabitats (43–60%). However, deposition of scarifiedProsopis seed in dung in conjunction with graminoid defoliation may be the most likely combination of events when livestock are present. Emergence from seeds transported into grasslands by other fauna likely would be low, unless seeds were deposited in areas where grasses had been defoliated.Prosopis survival was comparably high in dung and bare seed placements after one growing season. survival of seedlings present two weeks after seed dissemination ranged from 74 to 97% at the end of the second growing season. Seedling survival and shoot development (biomass, leaf area and height) were similar on LTP and LTG sites, regardless of the level of herbaceous defoliation or seed placement. In addition, the magnitude and patterns of net photosynthesis, stomatal conductance and xylem water potential were comparable among one-year-old seedtings on ND, MD and HD plots, even though differences in herbaceous species composition and above- and below-ground biomass between these treatments were substantial. Such data suggest competition for soil resources between grasses andProsopis may be minimal early in the life cycle ofProsopis. High rates ofProsopis emergence and establishment on LTP-MD plots are counter to the widespread assumption that long-term and/or heavy grazing is requisite forProsopis encroachment into grasslands. Results are discussed with regard to factors contributing to the recent, widespread invasion of this woody legume into grasslands of southwestern North America.

Competition for soil water between annual plants and blue oak (Quercus douglasii) seedlings.

Abstract: We examined the competitive effects of two annual species on soil water potential and blue oak (Quercus douglasii Hook & Arn.) seedling growth and water relations. Two densities of the annual grass Bromus diandrus (Roth.) (100/dm2, 3.6/dm2) and one density of the annual forb Erodium botrys (Cav.) (3.6/dm2) comprised plant neighborhoods around the oak seedlings grown in 1 m deep boxes. Rates of soil water depletion differed among neighborhoods. Soil in the Erodium neighborhoods dried significantly more slowly than did soil in the Bromus neighborhoods at either density. Differences in the rates of soil water depletion were correlated both with the 30% lower root biomass developed by Erodium, and the lower water extraction rates of Erodium relative to Bromus roots at constant root biomass. These results suggest that the annual species are not equivalent competitors for water: fibrous grass roots had greater competitive effect than did forb tap-roots. In a control container without an annual neighborhood, soil water potentials remained high for the duration of the experiment. Oak seedling emergence and growth responses were significantly affected by annual plant density. High density of annual plants suppressed oak root growth and shoot emergence. Only 20% of the acorns planted in high density Bromus neighborhoods showed aboveground shoot growth; 56% of those planted in low density Bromus or Erodium emerged. Ninety percent emerged in the control box. Relative growth rates of oak seedling roots and shoots were directly dependent on soil water potentials. Soil water was also closely correlated with oak seedling predawn water potentials and gas conductance measurements. Higher soil water potentials greater dry weights, and longer growing seasons were found for oak seedlings in the Erodium neighborhood and the container with no annuals than in Bromus neighborhoods of either density. These results suggest that competition for soil water with introduced annual species contributes to the increased rate of blue oak seedling mortality currently observed in California woodland systems.

Interactions of Factors Affecting Seedling Recruitment of Blue Oak (Quercus Douglasii) in California

Abstract: Acorn germination and seedling recruitment of blue oak (Quercus douglasii) were studied in relation to postdispersal predators, planting depth, oak canopy cover, slope angle and aspect, and herb layer. Removal of acorns and seedling recruitment from surface—sown and buried acorns were measured for 2 yr at two sites, a north—slope forest and a ridgetop savanna. Nested exclosures were used to measure acorn predation by birds, mice, gophers, deer, and cattle. The rate of seedling recruitment (Ps) was related to treatment variables using cluster analysis combined with stepwise logistic regression. Average acorn fall ranged from 3.5 to 58.7 acorns/m2, with germination rates varying from 28 to 85% in different years. 8172 sown acorns yielded 2922 (35.76%) seedlings, but Ps varied from 0.09 to 0.71 among the different treatments. Year, site, and rodents interacted strongly to affect seedling recruitment. Ps averaged 0.48 during a cool wet year compared to 0.23 in drier years. Low correlations between seedling recruitment rates for the same plot in different years were due to annual changes in the distribution of favorable microsites and patchy, unpredictable acorn predation. Recruitment rates for buried acorns were twice those of surface—sown acorns due to improved germination and reduced predation. Pocket gophers (Thomomys bottae) were the major predator of buried acorns. Surface acorns suffered high mortality from dying and overheating, as well as from predation by mice, gophers, and cattle. Seedling recruitment was positively associated with increasing canopy cover and more mesic microsites at the low elevation site, but was negatively associated with these factors at the cooler, high elevation site. Hierarchichal classification combined with stratum—specific logistic regression models was important in revealing strong interactions among seedling recruitment factors.

Seed and Seedling Ecology of a Monocarpic Tropical Tree, Tachigalia Versicolor

Abstract: Tachigalia versicolor is a monocarpic canopy tree species on Barro Colorado Island, Panama, This 2—yr demographic and physiological study examined whether seed and seedling survival of this monocarpic species is high compared to nonmonocarpic species previously studied, as life history theory predicts. For the weighted average of two trees, the percentage of offspring estimated to survive from dispersed seed to germination, 1 yr, and 2 yr was 30, 7.2, and 4.3%, respectively. The large seeds (500—600 mg) suffered greater mortality in absolute numbers than did the seedlings. Major mortality agents of seeds were bruchid beetles (Amblycerus tachygaliae) and terrestrial vertebrates, while vertebrate herbivores and damping—off fungi killed seedlings. Seedlings survival and growth were enhanced under canopies of dying T. versicolor adults, relative to under canopies of living conspecific and nonconspecific adults. The majority of 2—yr—old seedlings occurred within 0.40 m from the parent trees, reflecting the original peak of the distribution of wind—dispersed seeds. Few seedlings persisted beyond 40 m. Seedling survival in the first 2 mo was higher in shaded understory than in light—gaps, due to higher vertebrate herbivory in sun than shade. Seedling growth and survival beyond 2 mo was higher in light—gaps than in shaded understory. T. versicolor seedlings exhibited a wide range in maximum net photosynthetic rate when grown in contrasting light conditions, and were able to acclimate to a sudden increase in light level by means of morphological and physiological adjustments of newly developed leaves. These results indicate that T. versicolor can utilize a wide range of light environments in the forest during seedling establishment. Relative to nomonocarpic wind—dispersed tree species previously studied in the same community. T. versicolor ranks near the top in its seed mass, dispersal capability, photosynthetic flexibility to contrasting light conditions, and probability of seedling survival in the shaded understory through 1 yr. These characteristics enable T. versicolor to establish large numbers of seedlings. The mean total number of offspring of two trees estimated to survive through 2 yr was 4395 out of 103 055 dispersed seeds and 30 529 germinated seeds. In comparison to nonmonocarpic species, juvenile survival of T. versicolor is high as expected for this monocarpic tree species.

Recovery patterns of three chaparral shrub species after wildfire

Abstract: In a mature, even aged stand of mixed chaparral, Rhus laurina (facultative resprouter) had consistently higher water potentials and deeper roots than Ceanothus spinosus (facultative resprouter) and Ceanothus megacarpus (obligate seeder). For two years following a wildfire, the same stand of chaparral had resprouts with higher survivorships, predawn water potentials, stomatal conductances, photosynthetic rates and shoot elongation rates than seedlings. Supplemental irrigation of seedlings during summer months removed differences between resprouts and seedlings suggesting that the cause of such differences was limited water availability to the shoot tissues of seedlings. After two years of postfire regrowth, mean seedling survivorship for the obligate seeder (C. megacarpus) was 42%, whereas seedling survivorship for facultative resprouters was only 18% (C. spinosus) and 0.01% (R. laurina). Our results are consistent with the hypothesis that lack of resprouting ability among obligate seeders is offset by an enhanced ability to establish seedlings after wildfire, allowing obligate seeders to maintain themselves in mixed populations through many fire cycles.

The Effects of Priming and Ageing on Seed Vigour in Tomato

Abstract: A comparison was made of the effects of seed priming or ageing treatments on the performance of tomato (Lycopersiconesculentum Mill. cv. UC204C) seeds according to a number of indices of seed vigour. A single lot of tomato seeds was primed in120 mol rrT 3 K 2 HPO 4 + 150 mol m~ 3 KNO 3 for 5 d at 20 °C, or aged at 13% moisture content (dry weight basis) and 50 °C for6 d. Germination percentage (>98%) was unaffected by priming and reduced to 85% by ageing. X-ray photographs andlongitudinal sections revealed the formation of free space surrounding the embryo in dry primed seeds, which was not evident incontrol or aged seeds. Priming increased the rate of germination at all temperatures above the base temperature (T b ), while ageingdecreased it. T b was unaffected by priming and only slightly increased by ageing. The variation in individual times to germinationwas approximately doubled in both primed and aged seed compared to the control, based upon the slopes of probit germinationpercentage versus log thermal time curves. Root growth after germination tests and seedling growth in both greenhouse and fieldtests were not influenced by either priming or ageing. The conductivity test was found to be unreliable as a vigour test for tomatoseeds. The results identify several indices which can be used to quantify seed vigour in tomato. They also illustrate that seedpriming can enhance seed performance according to some criteria, while having no effect or decreasing quality according to othercriteria. Seed vigour can apparently be separated into various components which can be independently influenced by seedenhancement treatments.Key words: Tomato, seed germination rate, seed priming, seed vigour.

Species interactions in a successional grassland. i. seed rain and seedling recruitment

Abstract: SUMMARY (1) Seed rain and recruitment for the five most abundant species of grasses were quantified in a California coastal grassland undergoing succession after release from sheep grazing. (2) Species distributions were very patchy, and four patch types accounted for about 950/o of total cover. Three patch types ('perennial patches') were dominated, respectively, by one of three species of perennial grasses, Anthoxanthum odoratum, Holcus lanatus or Deschampsia holciformis. The fourth type ('annual patches') was dominated by annual grasses, including the most abundant annual Vulpia bromoides, the perennial grass Rytidosperma pilosum and forbs. (3) Dispersal limited the seed rain to the species dominating the local vegetation. Species not present in patches contributed little to the seed rain and nothing to recruitment in the interiors of those patches. (4) Species differed significantly in the densities of their seed rain in the patches they dominated, ranging from 2300 m-2 for R. pilosum to 82 300 m-2 for H. lanatus. The density of seed rain was patchy on all spatial scales examined (from cm to km), but was not significantly correlated with densities of recruits on any scale for the perennial species. However, V. bromoides recruitment correlated positively with the density of its seed rain on small spatial scales (up to 1 mi2) in annual patches. The poor relationship between seed rain and recruitment for the perennials probably reflects the over-riding importance of adult interference as an influence on seedling establishment in the perennial vegetation. (5) A. odoratum had relatively high recruitment (30 m-2) in the patches it dominated. Other perennials had very low recruitment in their own stands (less than 5 m-2). V. bromoides had the highest recruitment (904 m-2) in annual patches. (6) The seed bank contribution to recruitment was estimated using seed exclosures. Only those species abundant in the seed rain and in the local vegetation recruited inside the seed exclosures, i.e. there was no evidence of a persistent, functional seed bank of other species. The seed bank could account for no more than 30/o of V. bromoides recruitment in annual patches. However, establishment in the seed exclosures overestimated recruitment from the seed bank for A. odoratum and possibly for other perennial species (7) The implications of the results for population dynamics and succession are discussed.

Early post-fire regeneration in Pinus brutia forest ecosystems of Samos island (Greece)

Abstract: The recovery of the burnt pine (Pinus brutia) forests of Samos island was followed during the first three post-fire years. Samos is characterised by a Mediterranean-type climate with a mild, subhumid winter and a long xerothermic period. The natural regeneration of P. brutia is realised exclusively by seeds and is enhanced by a number of adaptations to fire: (i) the cones are serotinous and xerochasic, so most seeds survive inside the cones and are disseminated by the strong winds immediately after fire; (ii) the seeds are non-dormant and can germinate throughout the rainy season, though two frequency peaks were observed (a major one in spring and a minor one in late autumn); (iii) the heliophilous nature of both seed germination and seedling establishment is promoted by the opening of habitats produced by fire; (iv) from the survival curve it is evident that pine seedlings show a considerable drought tolerance and, once settled, their survival chance is quite high. The average density of pine seedlings measured after 15 months was 0.30 seedlings/m2, high enough to result in complete natural reforestation for most of the area burnt. The ecosystem recovery was additionally characterised by a prolific regeneration through both seed germination and resprouting. Among the reseeders, an eminent role was allocated to Cistus spp. and many leguminous plants. Resprouting was also very important and was especially impressive among the evergreen sclerophylls, the geophytes-hemicryptophytes and several phryganic shrubs.

Variation in germination response to temperature in rubber rabbitbrush (chrysothamnus nauseosus: asteraceae) and its ecological implications

Abstract: Seed collections from 72 rubber rabbitbrush populations occupying a range of habitats in western North America were incubated at 3 C in the laboratory. Collections from warm desert habitats required less than 2 weeks to achieve 90% relative germination under these conditions, while collections from montane habitats showed delayed germination requiring up to 20 weeks. When 13 representative collections were incubated at constant temperatures from 5 to 30 C, all germinated completely at 30 C within 4 weeks. Collections from warm desert habitats germinated rapidly over the whole range of temperatures. Montane collections sometimes exhibited dormancy at intermediate temperatures (15 and 25 C) even though they were ultimately able to germinate at lower temperatures. Results suggest that dormancy is conditional and temperature-dependent in this species. Chilling the seeds extends the temperature range for germination downward to include the chill temperature itself. Germination response to temperature and its variation as a function of habitat are of apparent adaptive significance, serving to time germination so that the probability of seedling survival is maximized in each habitat. Within populations, response to temperature varied as a function of year of harvest and of within-year harvest date, indicating that germination patterns are probably not under rigid genetic control but represent an integration of genetic and environmental factors.

Drought stress effects during seed fill on soybean seed germination and vigor

Abstract: Seed germination and vigor decreased as SDD increased. The effect of drought on germination and vigor was substantially less than that on yield and was inconsistent across years. Drought reduced standard germination percentage and seedling axis dry weight 5 and 12%, respectively and increased single-seed conductivity 19%. Seed quality loss was correlated with reduced single-seed mass. Severe drought that occurred throughout seed fill reduced yield and seed number at a faster rate than seed mass, germination, or vigor, and maintained the development of at least some viable and vigorous seeds

Effects of Temperature and Water Potential on Germination, Radicle Elongation and Emergence of Mungbean

Abstract: Controlled environment experiments were performed to determine the effects of temperature and water potential on germination, radicle elongation and emergence of mungbean (Vigna radiata (L.) Wilczek cv. IPB-M79-17-79). The effects of a range of constant temperatures (15-45 °C) and water potentials (0 to -2-2 MPa) on germination and radicle elongation rates were studied using an osmoticum technique, in which seeds were held against a semi-permeable membrane sac containing a polyethylene glycol solution. Linear relationships were established between median germination time (Gtso) and water potential at different temperatures, and between reciprocal Gtso (germination rate) and temperature at different water potentials. Germination occurred at potentials as low as — 2-2 MPa at favourable temperatures (30-40 °C), but was fastest at 40 °C when water was not limiting, with an estimated base temperature (Tb) of about 10 °C. Subsequent radicle elongation, however, was restricted to a slightly narrower temperature range and was fastest at 35 °C. The concept of thermal time was used to develop an equation to model the combined effects of water potential and temperature on germination. Predictions made using this model were compared with the actual germination obtained in a related series of experiments in columns of soil. Some differences observed suggested the additional importance of the seed/soil/water contact zone in influencing seed germination in soil. Seedling emergence appeared to reflect further the radicle elongation results by occurring within a narrower range of temperatures and water potentials than germination. Emergence had an estimated Tb of 12-6 °C and was fastest at 35 °C. A soil matric potential of not less than about — 0-5 MPa at sowing was required to obtain 50% or more seedling emergence.

Water stress and seedling growth of two eucalypt species from contrasting habitats

Abstract: Seedlings of Eucalyptus maculata Hook (mesic environment) and E. brockwayi C.A. Gardn. (arid environment) were supplied 100, 70 or 40% of their water requirements estimated from leaf area and the water used by well-watered seedlings. Restricting water supply caused large differences in growth rates, which were related to large differences in total leaf area. There was a fivefold range of variation in number of leaves per plant, and a reduction of up to 20% in average leaf size as a consequence of restricting water supply. Eucalyptus maculata seedlings produced more dry matter than E. brockwayi seedlings, but net assimilation rate was higher in E. brockwayi seedlings. Transpiration rates were also higher in E. brockwayi than in E. maculata. Leaf expansion was analyzed as a function of water stress integral (S(Psi)), which is the cumulative integral over time of predawn water potential below a datum. The leaf area achieved at any stress level was not uniquely dependent on total S(Psi), there was a secondary effect associated with reduced leaf growth caused by previous stress. At any value of S(Psi), reductions in leaf growth of water-stressed seedlings relative to leaf growth of well-watered control seedlings were greater in E. maculata seedlings than in E. brockwayi. Treatment differences in both species showed that, within the levels of stress applied, a moderate water stress over a long period of time was more detrimental to dry matter production than a severe stress for a short time.

Early establishment and vegetative growth of understory species in the western hemlock – Sitka spruce forests of southeast Alaska

Abstract: To determine how Vaccinium alaskaense, Cornus canadensis, Coptis asplenifolia, Tiarella trifoliata, and Rubus pedatus invade and maintain themselves in conifer forests, we conducted experiments on seed germination and seedling survival and examined clonal development in old-growth, young-growth, and 4- to 6-year-old stands. Seed of all species germinated well in laboratory and field tests (average 23 – 90%), but low germination rates of C. canadensis and T. trifoliata under actual field conditions may retard their invasion of young stands. Survival of all species at 3 and 4 years was lower in the young stand (40 years) than in the old stand (250 + years) and may be related to incident solar radiation. Regressions of the percentage of survival on percentage of transmitted solar radiation were significant (P ≤ 0.05) for all species except C. canadensis. Seed predation and type of seedbed (moss, logs, duff) had little effect on germination and survival rates. Average annual rhizome or stolon growth was slowe...

Effects of herbivory and competition on survival of Carya tomentosa (Juglandaceae) seedlings

Abstract: We tested the effects of predation, above-ground diffuse competition and belowground diffuse competition on the survival and growth of Carya tomentosa (Juglandaceae) seedlings in a five year oldfield bordering a mature forest. We planted eighty seedlings under cages, in pots or with surrounding vegetation tied back (in all combinations) to realize the treatments. We found that seedlings unprotected from herbivores suffered great mortality (i.e., survival < 10%). Seedlings planted away from the forest edge suffered significantly less predation than those closer to the forest. Seedlings often resprouted after browsing. Because only a few seedlings survived outside the cages, we focused on the caged seedlings to examine competition effects on seedling growth and survival. No significant differences were found in leaf, stem, root, and total biomass of surviving seedlings under any of the competition treatments. Insect herbivores affected 15% of the seedlings by removing 10% of their leaf area during the first year. In the second year, 30% had 1-10% damage to leaf tissue. No seedlings died due to invertebrate herbivores. Browsing by mammalian herbivores appears to be the primary factor determining woody seedling establishment patterns in seral oldfields.

On the possible control of ultraviolet-b induced response in growth and photosynthetic activities in higher-plants

Abstract: When Vigna sinensis L.cv. Walp seedlings were grown under control (from four 40 W white fluorescent tubes) and enhanced ultraviolet‐B (UV‐B) radiation (four 40 W white fluorescent tubes plus one Philips 20W/12 sun lamp) a large inhibition in seedling growth, particularly shoot eelongation and leaf expansion, was observed under enhanced UV‐B radiation. The UV‐B radiation also reduced the overall photosynthetic activity as measured by chlorophyll fluorescence induction. In order to check whether UV‐B causes any destruction of auxins, seedlings with either their shoot tip or primary leaves were covered with black paper and kept under both light conditions. Both the fully exposed and shoot tip‐covered seedlings showed a similar negative response on growth characteristics and physiological activities. Leaf‐covered seedlings showed well preserved photosynthetic activity under both light conditions. However, in these seedlings the pigment content decreased more than under other treatment conditions. Our experiments provide evidence for distinguishing between the UV‐B induced responses on growth and physiological activities; while the former may be controlled through auxins, the latter is probably by direct action on the organelles.

Interspecific Competition Between Seeds: Relative Planting Date and Density Affect Seedling Emergence

Abstract: The interactions between seeds in the soil are poorly understood. We per- formed greenhouse experiments to investigate the effects of seed density, relative frequency, and relative planting date on the emergence of seedlings in the species Senecio vulgaris, Capsella bursa-pastoris, and Poa annua. We found that for both Poa and Senecio, the probability of emergence significantly decreased with an increase in total seed density. Neither the density of conspecifics nor heterospecifics alone could explain this decline in the probability of emergence. We also found that, for all three species studied, the rate of emergence accelerated in the presence of previously planted seeds. A second experiment indicated that this acceleration involves a response to leachate from previously germinated seeds.

Interaction of soil bacteria, mycorrhizal fungi and orchid seed in relation to germination of Australian orchids

Abstract: SUMMARY Endotrophic bacteria were isolated from the mycorrhizal tissues of 12 out of 13 tested species of Western Australian terrestrial orchids. The bacteria were placed into eight groups based on u.v. light fluorescence, Gram staining and colony characteristics. The most commonly isolated bacteria from 9 out of the 12 orchid species sampled were strains within the Pseudomonas fluorescens-putida group. The abundance of bacteria followed a seasonal pattern that differed between orchid genera especially on the basis of the morphology of fungus infected tissue. There was little evidence of specificity of bacterial groups to orchid taxa or part of the plant infected by the fungus. Symbiotic germination of Pterostylis vittata seed in association with seven bacterial isolates showed a significant promotion of germination and seedling development with three bacterial strains. The influence of a fourth strain was no different to the control while the remaining three strains significantly suppressed seedling development.

Seed Reserves and Seedling Development in Winter Wheat

Abstract: (...) Excision of endosperm delayed the first two phyllochrons, prevented outgrowth of the coloptilar tiller, an sligtly delayed the appearance of the next two tillers. Plant dry weight, leaf numbers and dimensions, number of nodal roots, and total numbers of tillers were all affected by endosperm excision. The combined area of the first two leaves was linearly correlated with aleurone area for all seed treatments (...)

Effects of phenolic acids on seed germination and seedling growth in soil

Abstract: It is commonly assumed that the adverse effect of plant residues on crop yields is largely or partly due to phytotoxic compounds leached from these residues or produced by their decomposition. There has been substantial support for the hypothesis that the phytotoxic compounds responsible for reduced crop yields are phenolic acids such as p-coumaric acid, p-hydroxybenzoic acid, and ferulic acid. To test the validity of this hypothesis, we studied the effects of nine phenolic acids (caffeic acid, chlorogenic acid, p-coumaric acid, ellagic acid, ferulic acid, gallic acid, p-hydroxybenzoic acid, syringic acid, and vanillic acid) on (1) seed germination of corn (Zea mays L.), barley (Hordeum vulgare L.), oats (Avena sativa L.), rye (Secale cereale L.), sorghum [Sorghum bicolor (L.) Moench], wheat (Triticum aestivum L.), and alfalfa (Medicago sativa L.) on germination paper and soil, (2) seedling growth of alfalfa, oats, sorghum, and wheat on germination paper and soil, and (3) early plant growth of corn, barley, oats, rye, sorghum, and wheat in soil. The results showed that although the phenolic acids tested affected germination and seedling growth on germination paper, they had no effect on seed germination, seedling growth, or early plant growth in soil even when the amounts applied were much greater than the amounts detected in soil. We conclude that the adverse effect of plant residues on crop yields is not due to phenolic acids derived from these residues.