全文获取类型
收费全文 | 236篇 |
免费 | 18篇 |
国内免费 | 5篇 |
专业分类
259篇 |
出版年
2015年 | 3篇 |
2014年 | 4篇 |
2013年 | 5篇 |
2012年 | 3篇 |
2011年 | 7篇 |
2010年 | 12篇 |
2009年 | 6篇 |
2008年 | 7篇 |
2007年 | 13篇 |
2006年 | 6篇 |
2005年 | 6篇 |
2004年 | 3篇 |
2003年 | 2篇 |
2001年 | 2篇 |
1998年 | 8篇 |
1997年 | 5篇 |
1996年 | 3篇 |
1995年 | 4篇 |
1994年 | 2篇 |
1993年 | 5篇 |
1992年 | 6篇 |
1991年 | 8篇 |
1990年 | 5篇 |
1989年 | 5篇 |
1988年 | 5篇 |
1987年 | 3篇 |
1986年 | 4篇 |
1983年 | 2篇 |
1982年 | 2篇 |
1979年 | 2篇 |
1977年 | 2篇 |
1974年 | 4篇 |
1973年 | 3篇 |
1971年 | 3篇 |
1969年 | 1篇 |
1967年 | 1篇 |
1963年 | 1篇 |
1959年 | 2篇 |
1958年 | 9篇 |
1957年 | 12篇 |
1956年 | 12篇 |
1955年 | 10篇 |
1954年 | 8篇 |
1953年 | 6篇 |
1952年 | 6篇 |
1951年 | 6篇 |
1950年 | 3篇 |
1949年 | 6篇 |
1948年 | 6篇 |
1945年 | 1篇 |
排序方式: 共有259条查询结果,搜索用时 0 毫秒
1.
2.
3.
4.
5.
6.
Salt Stress Causes Acceleration of Purine Catabolism and Inhibition of Pyrimidine Salvage in Zea mays Root Tips 总被引:1,自引:0,他引:1
PETERSON TODD A.; LOVATT CAROL J.; NIEMAN RICHARD H. 《Journal of experimental botany》1988,39(10):1389-1395
Nucleotide metabolism was studied in apical 5.0 mm root tipsof corn plants (Zea mays L., cv. Pioneer 3906) hydroponicallycultured for 7 d and then salinized for 19 d at a rate calculatedto reduce the osmotic potential (o) of the solutions by O.1MPad1 to a final o = -0.4 MPa. Saline treatments withtwo different molar ratios of Ca2+/Na+ were employed, viz.,003 (2.5 mol m3 CaCl2 + 86.5 mol m3 NaCl)for the NaCl treatment and 0.73 (31.5 mol m3 CaCl2 +43.1 mol m3 NaCl) for the NaCl + CaCl2 treatment. Bothsalt treatments reduced root growth by more than 30%. The capacityof roots to provide purine nucleotides either by de novo synthesisor by re-utilization of existing bases, e.g. salvage of hypoxanthineto adenine nucleotides, was not affected by either salt treatment.However, catabolism of hypoxanthine was accelerated more than3.5-fold by both salt treatments, demonstrating an increasedcapacity for purine catabolism which would shift the normal1: 1 ratio of synthesis: degradation of purine nucleotides observedfor the roots of healthy control plants to less than 0.2 duringsalt stress. The ratio of pyrimidine nucleotide synthesis: degradationwas also reduced. In this case, the unfavourable shift towardnucleotide degradation resulted because both salt treatmentsreduced salvage capacity by more than 25%, but had no compensatingeffect on de novo synthesis or catabolism of pyrimidines. Key words: Salinity, osmotic potential, nucleotide metabolism 相似文献
7.
8.
CHRISTOPHER J. PERUMALLA CAROL A. PETERSON DARYL E. ENSTONE 《Botanical journal of the Linnean Society. Linnean Society of London》1990,103(2):93-112
PERUMALLA, C. J., PETERSON, C. A. & ENSTONE, D. E., 1990. A survey of angiosperm species to detect hypodermal Casparian bands. I. Roots with a uniseriate hypodermis and epidermis. Roots of 181 species from 53 families were surveyed to determine the frequency of Casparian bands in hypodermal layers. For six species, inconclusive data were obtained. The roots of the remaining 175 species were divided into three categories on the basis of this survey. In the first, a hypodermis is absent (12 species): no wall modifications were observed in the outer cortex and this region was permeable to the apoplastic dye Cellufluor. In the second, a hypodermis is present, but a hypodermal Casparian band is absent (seven species). In roots of six species, no wall modifications were detected in the hypodermis; the one remaining species had lignified phi thickenings which were permeable to Cellufluor. In the third, both a hypodermis and a hypodermal Casparian band are present (156 species). These Casparian bands consisted of suberin deposits throughout the width of the anticlinal walls of the hypodermis. The tangential walls of the hypodermis were also suberized, indicating that suberin lamellae were probably also present. Hypodermal Casparian bands were found in roots of hydrophytic, mesophytic and xerophytic species and in members of primitive as well as advanced families. The widespread occurrence of these bands (in 89% of the species surveyed) suggests that they were present in the type ancestral to the flowering plants and that this feature has been retained by many species in this group. The epidermal cell walls of the majority of species examined were suberized but were permeable to Cellufluor. 相似文献
9.
10.
Selenium Transport in Root Systems of Tomato 总被引:11,自引:1,他引:10
Selenate and selenite transport through tomato root systemswere followed for periods up to 4 h after removal of the planttops, using 75Se as a tracer. With selenate, 75Se concentrations in the xylem exudate were6 to 13 times higher than in the external solution, and chromatographicanalysis showed that the selenium was transported as inorganicselenate ( ). With selenite, 75Se concentrations in the exudate were alwayslower than in the external solution. Analyses of exudate samplesshowed that negligible amounts of selenium were transportedas inorganic selenite ( except at very high external selenite concentrations (500 ?M), when up to 7 percent was transported as selenite. Most of the selenium transportin selenite-fed plants was as selenate or as an unknown seleniumcompound, the relative proportions of these two forms varyingboth with time and with external selenite concentration. Additionof a 5-fold excess of sulphate over selenite had no detectableeffect on the concentrations of selenate in the exudate, butcaused substantial decreases in the maximum concentrations ofboth total selenium (c. 47 per cent decrease) and the unknownselenium compound (c. 69 per cent decrease). Addition of a 5-foldexcess of sulphite decreased the concentration of the unknown(c. 39 per cent) but caused a large (2.7-fold) increase in themaximum total selenium concentration in the exudate and a 7.9-foldincrease in the maximum concentration of selenate. The resultssuggest metabolic involvement in the uptake and long distancetransport of solenium supplied as selenite, despite lower 75Seconcentrations in the xylem exudate than in the external solution.An attempt is made to incorporate the new and existing informationinto a selenium transport model. 相似文献