排序方式: 共有40条查询结果,搜索用时 15 毫秒
1.
2.
3.
LEO W. D. VAN RAAMSDONK WILLEM A. WIETSMA JAAP N. DE VRIES 《Botanical journal of the Linnean Society. Linnean Society of London》1992,109(2):293-303
VAN RAAMSDONK, L. W. D., WIETSMA, W. A. & DE VRIES, J. N., 1992. Crossing experiments in Allium L. section Cepa . A full diallel was carried out with six diploid species of Allium section Cepa and A. roylei of section Rhizirideum , High isolation barriers were found between the related species A. cepa and A. oschaninii , between A. oschaninii and A. vavilovii , and between A. galanthum and A. pskemense . On the contrary, the species A. cepa and A. roylei , belonging to different sections, show only slight isolation barriers. The Wallace effect, which is the development of internal isolation barriers as such, is likely to have taken place in the evolution of A. oschaninii and A. vavilovii , and possibly also between A. galanthum and A. pskemense . 相似文献
4.
SANDER C. DE VRIES GERRIE W. J. VAN DE VEN MARTIN K. VAN ITTERSUM KEN E. GILLER 《Global Change Biology Bioenergy》2012,4(1):20-35
We present an approach for providing quantitative insight into the production‐ecological sustainability of biofuel feedstock production systems. The approach is based on a simple crop‐soil model and was used for assessing feedstock from current and improved production systems of cassava for bioethanol. Assessments were performed for a study area in Mozambique, a country considered promising for biomass production. Our focus is on the potential role of smallholders in the production of feedstock for biofuels. We take cassava as the crop for this purpose and compare it with feedstock production on plantations using sugarcane, sweet sorghum and cassava as benchmarks. Production‐ecological sustainability was defined by seven indicators related to resource‐use efficiency, soil quality, net energy production and greenhouse gas (GHG) emissions. Results indicate that of the assessed systems, sugarcane performed better than cassava, although it requires substantial water for irrigation. Targeted use of nutrient inputs improved sustainability of smallholder cassava. Cassava production systems on more fertile soils were more sustainable than those on less fertile soils; the latter required more external inputs for achieving the same output, affecting most indicators negatively and reducing the feasibility for smallholders. Cassava and sweet sorghum performed similarly. Cassava production requires much more labour per hectare than production of sugarcane or sweet sorghum. Production of bioethanol feedstock on cultivated lands was more sustainable and had potential for carbon sequestration, avoiding GHG emissions from clearing natural vegetation if new land is opened. 相似文献
5.
A new species of marine planarian, Obrimoposthia aparala sp. nov. , is described from the Crozet Islands and two other species are newly recorded for these islands, viz. 0. wandeli and Synsiphonium ernesti. The only known freshwater triclad from the Crozets, Dugesia seclusa , is redescribed from the type material. The morphology, ecology, and distribution of these four species is discussed. 相似文献
6.
Flow Cytometric Determination of Nuclear Replication Stages in Tomato Seeds during Priming and Germination 总被引:14,自引:0,他引:14
Flow cytometric determination of DNA levels in embryos of fullymatured dry tomato (Lycopersicon esculenium) seeds revealedlarge amounts of 2C DNA signals, indicating that most cellshad arrested in the cell cycle at the presynthetic G1 phaseof nuclear division. After imbibition in water, an augmentationof the 4C signal in the embryonic root tip region was found.This increase could be ascribed to cells entering the syntheticphase of nuclear division leading towards the doubling of chromosomalmaterial. In the root tip cells, 4C:2C ratios increased I dafter imbibition in water though radicle emergence started 2d later. Apparently, DNA synthesis preceded germination. Onlya small increase in the number of cells with 4C DNA levels wasfound in the rest of the embryonic tissues. In whole dry seeds,DNA histograms revealed both a 2C signal and a considerable6C peak, the latter originating from the endoreduplicated endosperm. A priming period of 14 d in PEG-6000 considerably enhanced therate and uniformity of germination. In the ungerminated seeds,the 4C DNA signal of root tip cells started to increase after3 d incubation in PEG. The ratio of 4C:2C steadily increasedduring the 14 d priming period, though did not reach the levelobtained after hydration in water. Upon priming, the 4C:2C ratiowas constant after redrying the seeds towards the original moisturecontent, indicating that the chromosomal material in the rootcells had stably ceased cell cycle activity at the G2 phase.The present results indicate that the beneficial effects ofpriming on seedling performance are associated with the actionof replicative DNA synthetic processes prior to germination. Lycopersicon esculeniumMill, tomato, DNA content, flow cytometry, priming, seed, nuclear replication stage, C levels 相似文献
7.
Species diversity in spatial and temporal dimensions of fruit-feeding butterflies from two Ecuadorian rainforests 总被引:8,自引:0,他引:8
PHILIP J. De VRIES THOMAS R. WALLA HAROLD F. GREENEY 《Biological journal of the Linnean Society. Linnean Society of London》1999,68(3):333-353
To test the hypotheses that butterflies in an intact lowland rainforest are randomly distributed in space and time, a guild of nymphalid butterflies was sampled at monthly intervals for one year by trapping 883 individuals of 91 species in the canopy and understory of four contiguous, intact forest plots and one naturally occurring lake edge. The overall species abundance distribution was well described by a log-normal distribution. Total species diversity (γ-diversity) was partitioned into additive components within and among community subdivisions (α-diversity and β-diversity) in vertical, horizontal and temporal dimensions. Although community subdivisions showed high similarity (l-β-diversity/γ-diversity), significant β-diversity existed in each dimension. Individual abundance and observed species richness were lower in the canopy man in the understory, but rarefaction analysis suggested that the underlying species richness was similar in both canopy and understory. Observed species richness varied among four contiguous forest plots, and was lowest in the lake edge plot. Rarefaction and species accumulation curves showed that one forest plot and the lake edge had significantly lower species richness than other forest plots. Within any given month, only a small fraction of total sample species richness was represented by a single plot and height (canopy or understory). Comparison of this study to a similar one done in disturbed forest showed diat butterfly diversity at a naturally occurring lake edge differed strongly from a pasture-forest edge. Further comparison showed that species abundance distributions from intact and disturbed forest areas had variances that differed significandy, suggesting mat in addition to extrapolation, rarefaction and species accumulation techniques, the shapes of species abundance distributions are fundamental to assessing diversity among sites. This study shows the necessity for long-term sampling of diverse communities in space and time to assess tropical insect diversity among different areas, and the need of such studies is discussed in relation to tropical ecology and quick surveys in conservation biology. 相似文献
8.
Hydration and dehydration kinetics were investigated in two xerophytic lichens with contrasting thallus morphologies, Chondropsis semiviridis (F.Muell. ex Nyl.) Nyl. and Xanthoparmelia convoluta (Krempehl.) Hale. Pulse‐modulated chlorophyll fluorescence was used to measure photosynthetic activity in thalli hydrated with either liquid water or water vapour in the laboratory and in the field. Water content (WC) and photosynthetic activity of thalli in both species increased rapidly on contact with liquid water. When exposed to water vapour, C. semiviridis hydrated more rapidly and achieved higher WC than X. convoluta. Both lichens achieved maximum Fv/Fm at low WC, regardless of hydration source. Rates of water loss were slower, and Fv/Fm remained high for longer, in X. convoluta than in C. semiviridis. Light saturated electron transport rates of both lichens were low compared with a homoiohydric plant from the same environment. Our results suggest that X. convoluta, which has a more complex morphology, retains water and remains photosynthetically active for significantly longer periods than C. semiviridis, providing X. convoluta with a potential advantage in the semi‐arid environment in which both species are found. 相似文献
9.
10.
PIERRE J. R. de VRIES STEPHAN J. M. de SMET JOOP van der HEIDE 《Freshwater Biology》1985,15(1):95-103
SUMMARY.
- 1 The combined effect of nitrate and phosphate concentrations on the yields of five selected strains of Stigeoclonium is demonstrated in a synthetic medium as well as in natural waters.
- 2 Increase of yield after addition of one nutrient depends on the concentration of the other. A graphic growth model based on the experiments in synthetic medium is presented, which allows prediction of yields at different combinations of nitrate and phosphorus concentrations.
- 3 In general, yields in natural waters before and after addition of nitrate and/or phosphate agree with the model. In some cases, other limiting factors appear to be involved.
- 4 The possible effects of nitrogen and phosphorus limitation on the phosphorus and nitrogen metabolism of the alga are discussed.
- 5 Neither an exponential model (Baule-Mitscherlich) nor a hyperbolic model adequately describe the interaction demonstrated between nitrogen and phosphorus limitation.