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21.
Cultivars of common bean (Phaseolus vulgaris L.) capable of yielding well at low levels of native or added phosphorus (P)
are highly desirable in many tropical production systems. The objective of the present study was to identify geographical
regions which might be sources of such genotypes. A total of 364 landraces, cultivars and wild genotypes, drawn from a broad
geographic range, were divided on the basis of growth habit into four field trials, each comprising two levels of P, stressed
and unstressed, on an infertile Andosol in Popayan, Colombia. The regression relationship between grain yield per plant in
the presence and in the absence of stress was determined, and each genotype's deviation from this relationship was used as
a measure of P-efficiency. There was highly significant variation in efficiency among genotypes in all growth habits, and
in climbing beans there were consistent regional differences, superior genotypes being identified with greater frequency among
those from Bolivia, West Mexico and South Mexico-West Guatemala. The latter region was promising for prostrate bush genotypes
also. Wild beans in general performed relatively poorly; it appears that P-efficiency traits in P. vulgaris have been acquired
during or after domestication. These results confirm that genetic differences in P-efficiency exist among common bean genotypes
and suggest that these are related to geographic origin. Furthermore, the use of a representative sample of germplasm can
help to identify segments of the gene bank that are especially promising as sources of desirable traits.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
22.
Bartels RA Paul A Green H Kapteyn HC Murnane MM Backus S Christov IP Liu Y Attwood D Jacobsen C 《Science (New York, N.Y.)》2002,297(5580):376-378
We present spatial coherence measurements of extreme ultraviolet (EUV) light generated through the process of high-harmonic up-conversion of a femtosecond laser. With a phase-matched hollow-fiber geometry, the generated beam was found to exhibit essentially full spatial coherence. The coherence of this laser-like EUV source was shown by recording Gabor holograms of small objects. This work demonstrates the capability to perform EUV holography with a tabletop experimental setup. Such an EUV source, with low divergence and high spatial coherence, can be used for experiments involving high-precision metrology, inspection of optical components for EUV lithography, and microscopy and holography with nanometer resolution. Furthermore, the short time duration of the EUV radiation (a few femtoseconds) will enable EUV microscopy and holography to be performed with ultrahigh time resolution. 相似文献
23.
Min JH Yang H Ivan M Gertler F Kaelin WG Pavletich NP 《Science (New York, N.Y.)》2002,296(5574):1886-1889
The ubiquitination of the hypoxia-inducible factor (HIF) by the von Hippel-Lindau tumor suppressor (pVHL) plays a central role in the cellular response to changes in oxygen availability. pVHL binds to HIF only when a conserved proline in HIF is hydroxylated, a modification that is oxygen-dependent. The 1.85 angstrom structure of a 20-residue HIF-1alpha peptide-pVHL-ElonginB-ElonginC complex shows that HIF-1alpha binds to pVHL in an extended beta strand-like conformation. The hydroxyproline inserts into a gap in the pVHL hydrophobic core, at a site that is a hotspot for tumorigenic mutations, with its 4-hydroxyl group recognized by buried serine and histidine residues. Although the beta sheet-like interactions contribute to the stability of the complex, the hydroxyproline contacts are central to the strict specificity characteristic of signaling. 相似文献
24.
Modern laser technology has revolutionized the sensitivity and precision of spectroscopy by providing coherent light in a spectrum spanning the infrared, visible, and ultraviolet wavelength regimes. However, the generation of shorter-wavelength coherent pulses in the x-ray region has proven much more challenging. The recent emergence of high harmonic generation techniques opens the door to this possibility. Here we review the new science that is enabled by an ability to manipulate and control electrons on attosecond time scales, ranging from new tabletop sources of coherent x-rays to an ability to follow complex electron dynamics in molecules and materials. We also explore the implications of these advances for the future of molecular structural characterization schemes that currently rely so heavily on scattering from incoherent x-ray sources. 相似文献
25.
Sallon S Solowey E Cohen Y Korchinsky R Egli M Woodhatch I Simchoni O Kislev M 《Science (New York, N.Y.)》2008,320(5882):1464
An ancient date seed (Phoenix dactylifera L.) excavated from Masada and radiocarbon-dated to the first century Common Era was germinated. Climatic conditions at the Dead Sea may have contributed to the longevity of this oldest, directly dated, viable seed. Growth and development of the seedling over 26 months was compatible with normal date seedlings propagated from modern seeds. Preliminary molecular characterization demonstrated high levels of genetic variation in comparison to modern, elite date cultivars currently growing in Israel. As a representative of an extinct date palm population, this seedling can provide insights into the historic date culture of the Dead Sea region. It also has importance for seed banking and conservation and may be of relevance to modern date palm cultivation. 相似文献
26.
Selectivity rules in organic chemistry have been inferred largely from nonaqueous environments. In contrast, enzymes operate in water, and the chemical effect of the medium change remains only partially understood. Structural characterization of the "ladder" polyether marine natural products raised a puzzle that persisted for 20 years: Although the stereochemistry of adjacent tetrahydropyran (THP) cycles would seem to arise from a biosynthetic cascade of epoxide-opening reactions, experience in organic solvents argued consistently that such a pathway would be kinetically disfavored. We report that neutral water acts as an optimal promoter for the requisite ring-opening selectivity, once a single templating THP is appended to a chain of epoxides. This strategy offers a high-yielding route to the naturally occurring ladder core and highlights the likely importance of aqueous-medium effects in underpinning certain noteworthy enzymatic selectivities. 相似文献
27.
Attention helps us process potentially important objects by selectively increasing the activity of sensory neurons that represent the relevant locations and features of our environment. This selection process requires top-down feedback about what is important in our environment. We investigated how parietal cortical output influences neural activity in early sensory areas. Neural recordings were made simultaneously from the posterior parietal cortex and an earlier area in the visual pathway, the medial temporal area, of macaques performing a visual matching task. When the monkey selectively attended to a location, the timing of activities in the two regions became synchronized, with the parietal cortex leading the medial temporal area. Parietal neurons may thus selectively increase activity in earlier sensory areas to enable focused spatial attention. 相似文献
28.
The Thirteen Towers of Chankillo run north to south along a low ridge within a fourth-century B.C.E. ceremonial complex in north coastal Peru. From evident observing points within the adjacent buildings to the west and east, they formed an artificial toothed horizon that spanned-almost exactly-the annual rising and setting arcs of the Sun. The Chankillo towers thus provide evidence of early solar horizon observations and of the existence of sophisticated Sun cults, preceding the Sun pillars of Incaic Cusco by almost two millennia. 相似文献
29.
A methylaspartate cycle in haloarchaea 总被引:1,自引:0,他引:1
Access to novel ecological niches often requires adaptation of metabolic pathways to cope with new environments. For conversion to cellular building blocks, many substrates enter central carbon metabolism via acetyl-coenzyme A (acetyl-CoA). Until now, only two such pathways have been identified: the glyoxylate cycle and the ethylmalonyl-CoA pathway. Prokaryotes in the haloarchaea use a third pathway by which acetyl-CoA is oxidized to glyoxylate via the key intermediate methylaspartate. Glyoxylate condensation with another acetyl-CoA molecule yields malate, the final assimilation product. This cycle combines reactions that originally belonged to different metabolic processes in different groups of prokaryotes, which suggests lateral gene transfer and evolutionary tinkering of acetate assimilation. Moreover, it requires elevated intracellular glutamate concentrations, as well as coupling carbon assimilation with nitrogen metabolism. 相似文献
30.
Partridge EA Le Roy C Di Guglielmo GM Pawling J Cheung P Granovsky M Nabi IR Wrana JL Dennis JW 《Science (New York, N.Y.)》2004,306(5693):120-124
The Golgi enzyme beta1,6 N-acetylglucosaminyltransferase V (Mgat5) is up-regulated in carcinomas and promotes the substitution of N-glycan with poly N-acetyllactosamine, the preferred ligand for galectin-3 (Gal-3). Here, we report that expression of Mgat5 sensitized mouse cells to multiple cytokines. Gal-3 cross-linked Mgat5-modified N-glycans on epidermal growth factor and transforming growth factor-beta receptors at the cell surface and delayed their removal by constitutive endocytosis. Mgat5 expression in mammary carcinoma was rate limiting for cytokine signaling and consequently for epithelial-mesenchymal transition, cell motility, and tumor metastasis. Mgat5 also promoted cytokine-mediated leukocyte signaling, phagocytosis, and extravasation in vivo. Thus, conditional regulation of N-glycan processing drives synchronous modification of cytokine receptors, which balances their surface retention against loss via endocytosis. 相似文献