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11.
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The growth of garden orache, A triplex hortensis was studiedunder conditions of mild NaCl or Na2SO4 salinity. Growth, drymatter production and leaf size were substantially stimulatedat 10 mM and 50 mM Na+ salts. Increased growth, however, appearedto be due to a K+-sparing effect of Na+ rather than to salinityper se. The distribution of K+ and Na+ in the plant revealeda remarkable preference for K+ in the roots and the hypocotyl.In the shoot the K/Na ratio decreased strongly with leaf age.However, the inverse changes in K+ and Na+ content with leafage were dependent on the presence of bladder hairs, which removedalmost all of the Na+ from the young leaf lamina. Measurementsof net fluxes of K+ and Na+ into roots and shoots of growingAtriplex plants showed a higher K/Na selectivity of the netion flux to the root compared to the shoot. With increasingsalinity the selectivity ratio SK, Na* of net ion fluxes tothe roots and to the shoots was increased. The data suggestthat recirculation of K+ from leaves to roots is an importantlink in establishing the K/Na selectivity in A. hortensis plants.The importance of K+ recirculation and phloem transport forsalt tolerance is discussed. Key words: Atriplex hortensis, Salinity, Potassium, Sodium, K+ retranslocation, Bladder hairs, Growth stimulation  相似文献   
13.
Nutritional ecology forms the interface between environmental variability and large herbivore behaviour, life history characteristics, and population dynamics. Forage conditions in arid and semi‐arid regions are driven by unpredictable spatial and temporal patterns in rainfall. Diet selection by herbivores should be directed towards overcoming the most pressing nutritional limitation (i.e. energy, protein [nitrogen, N], moisture) within the constraints imposed by temporal and spatial variability in forage conditions. We investigated the influence of precipitation‐induced shifts in forage nutritional quality and subsequent large herbivore responses across widely varying precipitation conditions in an arid environment. Specifically, we assessed seasonal changes in diet breadth and forage selection of adult female desert bighorn sheep Ovis canadensis mexicana in relation to potential nutritional limitations in forage N, moisture and energy content (as proxied by dry matter digestibility, DMD). Succulents were consistently high in moisture but low in N and grasses were low in N and moisture until the wet period. Nitrogen and moisture content of shrubs and forbs varied among seasons and climatic periods, whereas trees had consistently high N and moderate moisture levels. Shrubs, trees and succulents composed most of the seasonal sheep diets but had little variation in DMD. Across all seasons during drought and during summer with average precipitation, forages selected by sheep were higher in N and moisture than that of available forage. Differences in DMD between sheep diets and available forage were minor. Diet breadth was lowest during drought and increased with precipitation, reflecting a reliance on few key forage species during drought. Overall, forage selection was more strongly associated with N and moisture content than energy content. Our study demonstrates that unlike north‐temperate ungulates which are generally reported to be energy‐limited, N and moisture may be more nutritionally limiting for desert ungulates than digestible energy.  相似文献   
14.
Endemic species and ecosystem sensitivity to climate change in Namibia   总被引:1,自引:0,他引:1  
We present a first assessment of the potential impacts of anthropogenic climate change on the endemic flora of Namibia, and on its vegetation structure and function, for a projected climate in ~2050 and ~2080. We used both niche‐based models (NBM) to evaluate the sensitivity of 159 endemic species to climate change (of an original 1020 plant species modeled) and a dynamic global vegetation model (DGVM) to assess the impacts of climate change on vegetation structure and ecosystem functioning. Endemic species modeled by NBM are moderately sensitive to projected climate change. Fewer than 5% are predicted to experience complete range loss by 2080, although more than 47% of the species are expected to be vulnerable (range reduction >30%) by 2080 if they are assumed unable to migrate. Disaggregation of results by life‐form showed distinct patterns. Endemic species of perennial herb, geophyte and tree life‐formsare predicted to be negatively impacted in Namibia, whereas annual herb and succulent endemic species remain relatively stable by 2050 and 2080. Endemic annual herb species are even predicted to extend their range north‐eastward into the tree and shrub savanna with migration, and tolerance of novel substrates. The current protected area network is predicted to meet its mandate by protecting most of the current endemicity in Namibia into the future. Vegetation simulated by DGVM is projected to experience a reduction in cover, net primary productivity and leaf area index throughout much of the country by 2050, with important implications for the faunal component of Namibia's ecosystems, and the agricultural sector. The plant functional type (PFT) composition of the major biomes may be substantially affected by climate change and rising atmospheric CO2– currently widespread deciduous broad leaved trees and C4 PFTs decline, with the C4 PFT particularly negatively affected by rising atmospheric CO2 impacts by ~2080 and deciduous broad leaved trees more likely directly impacted by drying and warming. The C3 PFT may increase in prominence in the northwestern quadrant of the country by ~2080 as CO2 concentrations increase. These results suggest that substantial changes in species diversity, vegetation structure and ecosystem functioning can be expected in Namibia with anticipated climate change, although endemic plant richness may persist in the topographically diverse central escarpment region.  相似文献   
15.
Abstract

We have developed stereocontrolled transesterification for the synthesis of chiral phosphite mester backbone under basic conditions. Substrate phosphite triesters with phenoxy derivative substituents as leaving groups were separated into individual stereoisomers and reacted with hydroxyl-containing compounds to form desired backbone.

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16.
Abstract

Analogs of the C-nucleoside pyrazofurin were prepared in 7–9 steps using a key Pd(0)- catalyzed coupling reaction between protected iodopyrazoles 6a and 6b and glycal 8 to form the glycosyl bond. Conditions for this reaction were improved from those previously described for related reactions in order to maximize product yields and eliminate the need for triphenylarsine.

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17.
The Aquatic Warbler Acrocephalus paludicola is one of the most threatened Western Palearctic passerine species, classified as globally Vulnerable. With its breeding grounds relatively secure, a clear need remains for the monitoring and protection of the migration and wintering grounds of this rare and endangered migrant. Recent research has shown that the Aquatic Warbler migrates through northwest Africa in autumn and spring. The wintering grounds are apparently limited to wetlands of sub-Saharan West Africa, with records from only about 20 localities in Mauritania, Mali, Senegal and Ghana. Given the lack of knowledge of its whereabouts, we decided to use the available data to predict the wintering distribution of the Aquatic Warbler with the help of Geographic Information Systems (GIS). We used a novel approach to model the distribution of rarely recorded species, which is based on a combination of presence-only and presence–absence modelling techniques. Using the program BIOMOD, we thus generated four progressively more conservative predictions of where the Aquatic Warbler overwinters in Africa. Whereas the most permissive model predicts the Aquatic Warbler to be found in a latitudinal band stretching from the Senegal river delta all the way to the Red Sea coast, the most restrictive model suggests a much smaller area concentrated within the regions around the Senegal river delta in northern Senegal and southern Mauritania and around the Niger inundation zone in southern Mali and eastern Burkina Faso. Such model predictions may be useful guidelines to focus further field research on the Aquatic Warbler. Given the excellent model predictions in this study, this novel technique may prove useful to model the distribution of other rare and endangered species, thus providing a means to guide future survey efforts.  相似文献   
18.
We modelled the future distribution in 2050 of 975 endemic plant species in southern Africa distributed among seven life forms, including new methodological insights improving the accuracy and ecological realism of predictions of global changes studies by: (i) using only endemic species as a way to capture the full realized niche of species, (ii) considering the direct impact of human pressure on landscape and biodiversity jointly with climate, and (iii) taking species' migration into account. Our analysis shows important promises for predicting the impacts of climate change in conjunction with land transformation. We have shown that the endemic flora of Southern Africa on average decreases with 41% in species richness among habitats and with 39% on species distribution range for the most optimistic scenario. We also compared the patterns of species' sensitivity with global change across life forms, using ecological and geographic characteristics of species. We demonstrate here that species and life form vulnerability to global changes can be partly explained according to species' (i) geographical distribution along climatic and biogeographic gradients, like climate anomalies, (ii) niche breadth or (iii) proximity to barrier preventing migration. Our results confirm that the sensitivity of a given species to global environmental changes depends upon its geographical distribution and ecological proprieties, and makes it possible to estimate a priori its potential sensitivity to these changes.  相似文献   
19.
20.
Abstract Normanbya normanbyi (W. Hill) L. H. Bailey (Arecaceae) is a monoecious, arborescent palm with a very small distribution area within the Daintree rainforest in north‐eastern Australia. Our 2‐year study was focused on the reproductive phenology at the individual and population level. At the population level flowering peaked in the dry season, whereas fruiting was confined to the wet season. Each palm can bear up to three inflorescences/infructescences at the same time. Flowering of each inflorescence is separated from each other by a couple of weeks. A single inflorescence consists of about 1900 staminate and 800 pistillate flowers. The flowering of N. normanbyi is protandrous with a staminate phase lasting 40 days and a pistillate phase of approximately 2 weeks. Between both phases is a non‐flowering phase of about 9 days. Fruit ripening takes 21 weeks, with an average of about 280 ripe fruit per tree. Comparison of three study plots revealed a moderate synchrony of flowering and fruiting initiation in this species of palm. The male phase of flowering shows a higher degree of synchrony than the female phase at the population level. Seasonal regularity of flowering and fruiting peaks appears to be predictable. The general flowering and fruiting phenology of N. normanbyi follows a subannual pattern with a strong tendency towards a continual pattern.  相似文献   
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