全文获取类型
收费全文 | 460篇 |
免费 | 21篇 |
国内免费 | 30篇 |
专业分类
511篇 |
出版年
2024年 | 1篇 |
2022年 | 2篇 |
2021年 | 4篇 |
2019年 | 1篇 |
2018年 | 6篇 |
2017年 | 4篇 |
2016年 | 6篇 |
2015年 | 11篇 |
2014年 | 4篇 |
2013年 | 13篇 |
2012年 | 7篇 |
2011年 | 13篇 |
2010年 | 5篇 |
2009年 | 17篇 |
2008年 | 16篇 |
2007年 | 20篇 |
2006年 | 36篇 |
2005年 | 28篇 |
2004年 | 18篇 |
2003年 | 16篇 |
2002年 | 22篇 |
2001年 | 25篇 |
2000年 | 19篇 |
1999年 | 21篇 |
1998年 | 10篇 |
1997年 | 31篇 |
1996年 | 12篇 |
1995年 | 3篇 |
1994年 | 11篇 |
1993年 | 15篇 |
1992年 | 13篇 |
1991年 | 11篇 |
1990年 | 16篇 |
1989年 | 11篇 |
1988年 | 10篇 |
1987年 | 13篇 |
1986年 | 12篇 |
1985年 | 6篇 |
1984年 | 5篇 |
1982年 | 5篇 |
1981年 | 8篇 |
1980年 | 3篇 |
1972年 | 1篇 |
排序方式: 共有511条查询结果,搜索用时 15 毫秒
1.
2.
This paper presents information about the release of nitrogen and phosphorus from dying grass roots and the capture of phosphorus by other, living plants. We have paid particular attention to the part played by mycorrhizas in this phosphorus capture, and the possible importance of mycorrhizal links between dying and living roots.WhenLolium perenne plants were grown with ample nutrients and their roots then detached and buried in soil, about half the nitrogen and two-thirds of the phosphorus was lost in three weeks, but only one-fifth of the dry weight. The C:N and C:P ratios suggest that microbial growth in the roots would at first be C-limited but would become N- and P-limited within three weeks.Rapid transfer of32P can occur from dying roots to those of a living plant if the two root systems are intermingled. The amount transferred was substantially increased in two species-combinations that are known to form mycorrhizal links between their root systems. In contrast, in a species-combination where only the living (receiver) plant could become mycorrhizal no significant increase of32P transfer occurred. This evidence, although far from conclusive, suggests that mycorrhizal links between dying and living roots can contribute to nutrient cycling. This research indicates a major difference in nutrient cycling processes between perennial and annual crops. 相似文献
3.
Fructan biosynthesis in excised leaves of Lolium temulentum L. 总被引:10,自引:10,他引:0
4.
5.
O. I. Joarder N. H. Joarder P. J. Dale 《TAG. Theoretical and applied genetics. Theoretische und angewandte Genetik》1986,73(2):286-291
Summary Immature gramineous leaves provide a convenient system for comparing the response of cells in culture with their state of differentiation. Callusing frequency is compared with leaf segment position, leaf age and in vivo mitotic activity in Lolium multiflorum. (1) In a succession of one millimeter sections from the immature leaf base, callus was formed from the first and second sections but not the third or subsequent sections. The frequency of those explants callusing decreased with distance from the base of the leaf and with leaf age (or leaf extension growth). (2) In vivo, the proportion of cells in mitosis declined from around 10–14% at the base of young leaves to zero at 5 mm from the base and beyond. Mitotic activity also declined in leaves as they aged, and dividing cells were not observed in leaves 30 days from initiation or older. (3) A high frequency of callus formation was associated with a high mitotic index in the explant. But for corresponding mitotic indices, cells further away from the leaf base were less responsive in culture. (4) It is proposed that cells are becoming differentiated even in highly meristematically active regions of the leaf and concomitantly losing their ability to respond in culture. 相似文献
6.
Z. Rengel 《Plant and Soil》1990,128(2):185-189
Ammonium acetate and BaCl2-triethanolamine were used to desorb Mg2+ from the root Donnan free space (DFS) of 23-d-old ryegrass (Lolium multiflorum Lam. cvs. Gulf and Wilo). Amounts of desorbed Mg2+ increased with the increase in Mg2+ activity of the nutrient solution. Slightly less Mg2+ was desorbed by Ba2+ than by NH4
+. Previously published data on short-term net Mg2+ uptake by intact 23-d-old ryegrass plants of the two cultivars were linearly related to the amount of exchangeable Mg+ desorbed from the root DFS (r2=0.90 and 0.81 for the desorption by NH4
+ and Ba2+, respectively). A sward of Mg2+ ions attracted to the negative charges of the cell surface is suggested to represent a part of a pool of Mg2+ available for active transport through the plasmalemma. 相似文献
7.
Free Air Temperature Increase (FATI): a new tool to study global warming effects on plants in the field 总被引:4,自引:1,他引:3
I. NIJS F. KOCKELBERGH H. TEUGHELS H. BLUM G. HENDREY I. IMPENS 《Plant, cell & environment》1996,19(4):495-502
A new technique, called Free Air Temperature Increase (FATI), was developed to artificially induce increased canopy temperature in field conditions without the use of enclosures. This acronym was chosen in analogy with FACE (Free Air CO2 Enrichment), a technique which produces elevated CO2 concentrations [CO2] in open field conditions. The FATI system simulates global warming in small ecosystems of limited height, using infrared heaters from which all radiation below 800 nm is removed by selective cut-off filters to avoid undesirable photomorpho-genetic effects. An electronic control circuit tracks the ambient canopy temperature in an unheated reference plot with thermocouples, and modulates the radiant energy from the lamps to produce a 2.5°C increment in the canopy temperature of an associated heated plot (continuously day and night). This pre-set target differential is relatively-constant over time due to the fast response of the lamps and the use of a proportional action controller (the standard deviation of this increment was <1°C in a 3 week field study with 1007 measurements). Furthermore, the increase in leaf temperature does not depend on the vertical position within the canopy or on the height of the stand. Possible applications and alternative designs are discussed. 相似文献
8.
Plant regeneration from ryegrass ovules cultivated on endosperm-derived feeder cells 总被引:1,自引:0,他引:1
An efficient method for the regeneration of zygote-derived plants via ovule culture is desirable for overcoming postzygotic cross incompatibility as well as for the development of certain methods for genetic manipulation. High-frequency plantlet regeneration from ovules of Italian ryegrass (Lolium multiflorum Lam.) and a hybrid Italian/perennial ryegrass excised 1 to 4 days post pollination was obtained by culture on endosperm-derived feeder cells. Ovules excised 3 or 4 days after anthesis and grown on feeder cells generally regenerated about twice as frequently as ovules grown directly on nutrient medium. In one of the genotypes tested, ovules excised 1, 2 and 3 days post pollination developed into plantlets at percentages of 38.1, 52.0 and 52.8, respectively, using the feeder-cell system.Abbreviations EM
endosperm multiplications
- OC
ovule culture
- R
regeneration
- 2,4-d
2,4-dichlorophenoxyacetic acid 相似文献
9.
The development of herbicide multiple-resistance in weed species represents a major threat to current agricultural practices. The mechanistic basis for herbicide multiple-resistance has been investigated in a population of the annual grass weed Lolium rigidum Gaud. (annual ryegrass) resistant to herbicides affecting 6 target sites. A subset of the resistant population (R2 subset) has been isolated by germination on a medium containing the acetyl-CoA carboxylase (ACCase, EC 6.4.1.2) inhibiting herbicide, sethoxydim ((2-[1-(ethoxyimino)butyl]-5-[2-(ethylthio)propyl]-3-hydroxy-2-cyclohexen-1-one)). This 12% R2 subset of the population is 600 times more resistant to sethoxydim and between 30 to 200 times more resistant to other ACCase inhibitors than the bulk of the R population. The subset has a form of ACCase which is 6 to 55 times less sensitive to inhibition by these herbicides than the enzyme present in the bulk of the resistant or in the susceptible population. There was no difference in the uptake and metabolic degradation of [4-14 C]sethoxydim between the R2 subset and the unselected R population. These results show the accumulation of different resistance mechanisms in that single population. Furthermore we propose that this accumulation of multiple resistance mechanisms is the basis for herbicide multiple-resistance in this biotype. 相似文献
10.
Hydraulic properties of sphagnum peat moss and tuff (scoria) and their potential effects on water availability 总被引:2,自引:0,他引:2
The importance of macrostructure to root growth of ryegrass (L. perenne) seedlings sown on the soil surface was studied in two soils in which the macrostructure had resulted mainly from root growth
and macro-faunal activity. Sets of paired soil cores were used, one of each pair undisturbed and the other ground and repacked
to the field bulk density.
Undisturbed and repacked soils were first compared at equal water potentials in the range −1.9 to −300 kPa. At equal water
potential, the undisturbed soil always had the greater strength (penetration resistance), and root growth was always greater
in the repacked soil with no macrostructure than it was in the soil with macrostructure intact. At equal high strength (low
water potentials) it appeared that root growth was better when soils were structured. When strength was low (high water potentials),
root growth was better in the unstructured soil.
Soils were then compared during drying cycles over 21 days. The average rate at which roots grew to a depth of 60 mm, and
also the final percentage of plants with a root reaching 60 mm depth, was greatest in repacked soils without macrostructure.
The species of vegetation growing in the soil before the experiment affected root growth in undisturbed soil; growth was slower
where annual grasses and white clover had grown compared with soil which had supported a perennial grass.
It appears that relatively few roots locate and grow in the macrostructure. Other roots grow in the matrix, if it is soft
enough to be deformed by roots. Roots in the matrix of a structured soil grow more slowly than roots in structureless soil
of equal bulk density and water potential. The development of macrostructure in an otherwise structureless soil, of the type
studied, is of no advantage to most roots. However, once a macrostructure has developed, the few roots locating suitable macropores
are able to grow at low water potential when soil strength is high. The importance of macrostructure to establishing seedlings
in the field lies in rapid penetration of at least a few roots to a depth that escapes surface drying during seasonal drought.
ei]{gnB E}{fnClothier} 相似文献