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61.
Genetic insertion and exposure of a reporter epitope in the ferrichrome-iron receptor of Escherichia coli K-12. 总被引:9,自引:7,他引:2
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G S Moeck B S Bazzaz M F Gras T S Ravi M J Ratcliffe J W Coulton 《Journal of bacteriology》1994,176(14):4250-4259
The ferrichrome-iron receptor of Escherichia coli K-12 is FhuA (M(r), 78,992), the first component of an energy-dependent, high-affinity iron uptake pathway. FhuA is also the cognate receptor for bacteriophages T5, T1, phi 80, and UC-1, for colicin M and microcin 25, and for albomycin. To probe the topological organization of FhuA which enables recognition of these different ligands, we generated a library of 16 insertion mutations within the fhuA gene. Each insertion spliced a 13-amino-acid antigenic determinant (the C3 epitope of poliovirus) at a different position within FhuA. Immunoblotting of outer membranes with anti-FhuA and anti-C3 antibodies indicated that 15 of 16 FhuA.C3 proteins were present in the outer membrane in amounts similar to that observed for plasmid-encoded wild-type FhuA. One chimeric protein with the C3 epitope inserted after amino acid 440 of FhuA was present in the outer membrane in greatly reduced amounts. Strains overexpressing FhuA.C3 proteins were subjected to flow cytometric analysis using anti-FhuA monoclonal antibodies. Such analysis showed that (i) the chimeric proteins were properly localized and (ii) the wild-type FhuA protein structure had not been grossly altered by insertion of the C3 epitope. Twelve of sixteen strains expressing FhuA.C3 proteins were proficient in ferrichrome transport and remained sensitive to FhuA-specific phages. Three FhuA.C3 proteins, with insertions after amino acid 321, 405, or 417 of FhuA, were detected at the cell surface by flow cytometry using anti-C3 antibodies. These three chimeric proteins were all biologically active. We conclude that amino acids 321, 405, and 417 are surface accessible in wild-type FhuA. 相似文献
62.
Summary To determine the effects of elevated CO2 and soil moisture status on growth and niche characteristics of birch and maple seedlings, gray birch (Betula populifolia) and red maple (Acer rubrum) were experimentally raised along a soil moisture gradient ranging from extreme drought to flooded conditions at both ambient and elevated atmospheric CO2 levels. The magnitude of growth enhancement due to CO2 was largely contingent on soil moisture conditions, but differently so for maple than for birch seedlings. Red maple showed greatest CO2 enhancements under moderately moist soil conditions, whereas gray birch showed greatest enhancements under moderately dry soil conditions. Additionally, CO2 had a relatively greater ameliorating effect in flooded conditions for red maple than for gray birch, whereas the reverse pattern was true for these species under extreme drought conditions. For both species, elevated CO2 resulted in a reduction in niche breadths on the moisture gradient; 5% for gray birch and 23% for red maple. Species niche overlap (proportional overall) was also lower at elevated CO2 (0.98 to: 0.88: 11%). This study highlights the utility of of experiments crossing CO2 levels with gradients of other resources as effective tools for elucidating the potential consequences of elevated CO2 on species distributions and potential interactions in natural communities. 相似文献
63.
Cost of reproduction as reduced growth in genotypes of two congeneric species with contrasting life histories 总被引:1,自引:0,他引:1
Summary We examined the effect of reproduction on growth in 33 genotypes of Plantago major and 14 genotypes of P. rugelii. These two herbaceous perennials have contrasting life histories; P. major reproduces at a smaller size, and allocates a larger proportion of its biomass to reproduction, than P. rugelii. The effect of reproduction on frowth was determined experimentally using photoperiod manipulations to control level of reproduction. The difference in growth between reproductive treatments was divided by the difference in capsule weight to produce a measure of reproductive cost per g of capsule for genotypes of the two species. In both species there was substantial variation among genotypes in the effect of reproduction on growth. Much of this variation could be correlated with differences among genotypes in the extent of reproductive investment and plant size. Cost in terms of reduction in growth per g of capsule increased with reproductive investment in P. rugelii, and with plant size in P. major. We suggest the differences between species in timing and extent of reproduction are related to the differences between species in effect of reproduction on growth. Plantago rugelii may reproduce to a lesser extent than P. major because cost per g of capsule in terms of reduced vegetative biomass, increases with reproductive output in the former species, but not in the latter. Similarly, P. major may reproduce earlier than P. rugelii because cost per g of capsule increases with plant size in P. major, but not in P. rugelii. 相似文献
64.
We measured the effect of elevated CO2 on populations of the western flower thrips, Frankliniella occidentalis and on the amount of leaf damage inflicted by the thrips to one of its host plants, the common milkweed, Asclepias syriaca. Plants grown at elevated CO2 had significantly greater aboveground biomass and C:N ratios, and significantly reduced percentage nitrogen. The number of
thrips per plant was not affected by CO2 treatment, but the density of thrips (numbers per gram aboveground biomass), was significantly reduced at high CO2. Consumption by thrips, expressed as the amount of damaged leaf area per capita, was significantly greater at high CO2, and the amount of leaf area damaged by thrips was increased by 33%. However overall leaf area at elevated CO2 increased by 62%, more than compensating for the increase in thrips consumption. The net outcome was that plants at elevated
CO2 had 3.6 times more undamaged leaf area available for photosynthesis than plants at ambient CO2, even though they had only 1.6 times the overall amount of leaf area. This study highlights the need for measuring the effects
of herbivory at the whole-plant level and also the importance of taking herbivory into account when predicting plant responses
to elevated CO2.
Received: 9 January 1996 /Accepted: 30 July 1996 相似文献
65.
F. A. Bazzaz 《Oecologia》1973,13(1):73-80
Summary
Prosopis farcta (Banks et Sol.) Eig., is a widespread perennial shrub in a variety of habitats over a large geographic area in the Middle East. In iraq it occurs in non-saline deserts of the west and saline agricultural lands of the Mesopotamian Plain. The species is economically important as fuel for a sizable segment of the population in rural areas. Seeds were collected in three environmentally distinct habitats, with regard to annual precipitation and soil salt content. The seeds were germinated in various concentrations of sodium chloride and their germination, seedling radicle growth, and dark respiration studied. The results indicate that the three populations differend in their response to NaCl with the northern population being generally more sensitive than the other two populations. The three populations represent salt ecotypes of this very widely distributed shrub. The salt ecotypes seem to be of somewhat recent origin and apparently have been further spread by improper agricultural land-use which created vast saline areas. 相似文献
66.
Summary In order to more fully understand carbon dioxide dynamics in a soil-plant-atmosphere system, an in situ sampling technique has been developed to measure carbon dioxide concentration within the soil profile as well as in the atmosphere. Gas samples are automatically pumped in sequence from six porous collectors within the soil profile and five aboveground inlets through an infrared gas analyzer. Field measurements in a first year field, indicated that carbon dioxide concentrations reached a maximum value (1800 ppm) in the deepest soil sampling site (-180 cm). Temporal and spatial variations of carbon dioxide concentration were related to the development of root and vegetation structure as well as the position of the groundwater table.Supported, in part, by a grant from the Graduate Research Board. We acknowledge, with thanks, the competent assistance of Frank W. Schwartz. 相似文献
67.
Feedbacks between canopy composition and seedling regeneration in mixed conifer broad‐leaved forests
To address the role of canopy‐seedling feedbacks in the structure and dynamics of mixed conifer broad‐leaved forests in the eastern US, we monitored seedling regeneration patterns and environmental conditions in the understorey of stands dominated by either hemlock (Tsuga canadensis) or red oak (Quercus rubra) for three years. Hemlock seedlings were favoured over other species’ seedlings in hemlock stands (a true positive feedback), due to a combination of high seed inputs, high seedling emergence and relatively high seedling survival during the growing season, which allowed hemlock to remain dominant under its own canopy. Red oak stands favoured a suite of mid‐successional broad‐leaved species over hemlock. A more even age structure of broad‐leaved species in red oak stands revealed that high seedling survival in such stands were driving this feedback. Canopy‐mediated variations in both understorey light availability (1.5% for hemlock vs 3.5% for red oak) and soil pH (3.9 for hemlock vs 4.4 for red oak) were found to be the primary correlates of stand‐level differences in seedling regeneration dynamics. In mixed temperate forests in the eastern US, canopy‐seedling feedbacks could act to slow successional trajectories and contribute to the maintenance of a stable landscape structure over many generations. 相似文献
68.
J. Cavender-Bares S. Apostol I. Moya J.-M. Briantais F.A. Bazzaz 《Photosynthetica》1999,36(4):587-596
We compared the sensitivity to cold stress, in terms of photosynthetic capacity and changes in chlorophyll fluorescence of
photosystem 2 (PS2), of an evergreen and a deciduous oak species, which co-occur in the southeastern United States. We predicted
that the evergreen species, Quercus virginiana, which must endure winter, is likely to have an inherently greater capacity for energy dissipation and to be less susceptible
to chilling stress than the deciduous species, Quercus michauxii. Short-term cold stress in both species lead to greater than 50 % reduction in maximum photosynthetic rates, 60-70 % reduction
in electron transport, and irreversible quenching of PS2 fluorescence. The kinetics of recovery in the dark after exposure
to 1 h high irradiance (1000 μmol m-2 s-1) and chilling (5 °C) showed that the evergreen Q. virginiana exhibited more protective qE and less irreversible quenching (qI) than the deciduous Q. michauxii. The large qE which we observed in Q. virginiana suggests that the capacity for photoprotection at low temperatures is not induced by a long-term acclimation to cold but
preexists in evergreen leaves. This capacity may contribute to the ability of this species to maintain leaves during the winter.
This revised version was published online in June 2006 with corrections to the Cover Date. 相似文献
69.
W. A. Hoffmann F. A. Bazzaz N. J. Chatterton P. A. Harrison R. B. Jackson 《Oecologia》2000,123(3):312-317
The savannas (cerrado) of south-central Brazil are currently subjected to frequent anthropogenic burning, causing widespread
reduction in tree density. Increasing concentrations of atmospheric CO2 could reduce the impact of such frequent burning by increasing the availability of nonstructural carbohydrate, which is necessary
for resprouting. We tested the hypotheses that elevated CO2 stimulates resprouting and accelerates replenishment of carbohydrate reserves. Using a factorial experiment, seedlings of
a common Brazilian savanna tree, Keilmeyera coriacea, were grown at 350 ppm and 700 ppm CO2 and at two nutrient levels. To simulate burning, the plants were either clipped at 15 weeks or were left unclipped. Among
unclipped plants, CO2 and nutrients both stimulated growth, with no significant interaction between nutrient and CO2 effects. Among clipped plants, both CO2 and nutrients stimulated resprouting. However, there was a strong interaction between CO2 and nutrient effects, with CO2 having a significant effect only in the presence of high nutrient availability. Under elevated CO2, carbohydrate reserves remained at higher levels following clipping. Root total nonstructural carbohydrate remained above
36% in all treatments, so carbohydrate reserves did not limit regrowth. These results indicate that under elevated CO2 this species may be better able to endure the high frequency of anthropogenic burning in the Brazilian savannas.
Received: 19 May 1999 / Accepted: 8 November 1999 相似文献
70.
Changes in drought response strategies with ontogeny in Quercus rubra: implications for scaling from seedlings to mature trees 总被引:1,自引:0,他引:1
We investigated scaling of physiological parameters between age classes of Quercus rubra by combining in situ field measurements with an experimental approach. In the in situ field study, we investigated changes
in drought response with age in seedlings, juveniles, and mature trees of Q. rubra. Throughout the particularly dry summer of 1995 and the unusually wet summer of 1996 in New England, we measured water potential
of leaves (ΨLeaf) and gas exchange of plants at three sites at the Harvard Forest in Petersham, Massachusetts. In order to determine what
fraction of the measured differences in gas exchange between seedlings and mature trees was due to environment versus ontogeny,
an experiment was conducted in which seedlings were grown under light and soil moisture regimes simulating the environment
of mature trees. The photosynthetic capacity of mature trees was three-fold greater than that of seedlings during the wet
year, and six-fold greater during the drought year. The seedling experiment demonstrated that the difference in photosynthetic
capacity between seedlings and mature trees is comprised equally of an environmental component (50%) and an ontogenetic component
(50%) in the absence of water limitation. Photosynthesis was depressed more severely in seedlings than in mature trees in
the drought year relative to the wet year, while juveniles showed an intermediate response. Throughout the drought, the predawn
leaf water potential (ΨPD) of seedlings became increasingly negative (–0.4 to –1.6 MPa), while that of mature trees became only slightly more negative
(–0.2 to –0.5 MPa). Again, juveniles showed an intermediate response (–0.25 to –0.8 MPa). During the wet summer of 1996, however,
there was no difference in ΨPD between seedlings, juveniles and mature trees. During the dry summer of 1995, seedlings were more responsive to a major rain
event than mature trees in terms of ΨLeaf , suggesting that the two age classes depend on different water sources. In all age classes, instantaneous measurements of
intrinsic water use efficiency (WUEi), defined as C assimilation rate divided by stomatal conductance, increased as the drought progressed, and all age classes
had higher WUEi during the drought year than in the wet year. Mature trees, however, showed a greater ability to increase their WUEi in response to drought. Integrated measurements of WUE from C isotope discrimination (Δ) of leaves indicated higher WUE in
mature trees than juveniles and seedlings. Differences between years, however, could not be distinguished, probably due to
the strong bias in C isotope fractionation at the time of leaf production, which occurred prior to the onset of drought conditions
in 1995. From this study, we arrive at two main conclusions:
Received: 14 July 1999 / Accepted: 10 January 2000 相似文献