全文获取类型
收费全文 | 1890篇 |
免费 | 128篇 |
国内免费 | 169篇 |
专业分类
2187篇 |
出版年
2024年 | 13篇 |
2023年 | 15篇 |
2022年 | 21篇 |
2021年 | 25篇 |
2020年 | 19篇 |
2019年 | 26篇 |
2018年 | 33篇 |
2017年 | 40篇 |
2016年 | 26篇 |
2015年 | 41篇 |
2014年 | 54篇 |
2013年 | 48篇 |
2012年 | 43篇 |
2011年 | 48篇 |
2010年 | 39篇 |
2009年 | 117篇 |
2008年 | 107篇 |
2007年 | 112篇 |
2006年 | 113篇 |
2005年 | 103篇 |
2004年 | 73篇 |
2003年 | 57篇 |
2002年 | 49篇 |
2001年 | 39篇 |
2000年 | 59篇 |
1999年 | 67篇 |
1998年 | 64篇 |
1997年 | 60篇 |
1996年 | 47篇 |
1995年 | 43篇 |
1994年 | 54篇 |
1993年 | 52篇 |
1992年 | 49篇 |
1991年 | 53篇 |
1990年 | 48篇 |
1989年 | 37篇 |
1988年 | 43篇 |
1987年 | 37篇 |
1986年 | 29篇 |
1985年 | 33篇 |
1984年 | 33篇 |
1983年 | 10篇 |
1982年 | 23篇 |
1981年 | 29篇 |
1980年 | 20篇 |
1979年 | 15篇 |
1978年 | 7篇 |
1977年 | 8篇 |
1974年 | 2篇 |
1973年 | 2篇 |
排序方式: 共有2187条查询结果,搜索用时 0 毫秒
241.
Summary Two alternative estimators of individual tree leaf area (A1) area are used to derive estimates of leaf-area index (L) for 40 plots in Pinus contorta Dougl. stands. One estimator of A1 is based on the common assumption of a constant ratio between A1 and sapwood cross-sectional area at breast height (As). The second estimator of A1 accounts for tree-to-tree variation in the relation between A1 and As. The apparent relationship between stand growth and leaf-area index is strongly dependent on the way leaf area is estimated. When L is derived from a constant A1As ratio, stand growth appears to be strongly correlated with L. However, when L is based on estmates of A1 that account for tree-to-tree variation in the A1 — As relation, stand growth is seen to be only weakly related to L. Stand structure, quantified as percent live-crown, accounts for a great deal of the observed variation in leaf-area efficiency. These contrasting relationships illustrate the importance of unbiased estimates of L in interpreting the link between stand-level processes and leaf area.Utah Aggriculural Experiment Station Journal Paper No. 3333 相似文献
242.
The red light-stimulated component of unrolling in sections from 7-d-old dark-grown barley (Hordeum vulgare L.) leaves is inhibited by ethyleneglycol-bis-(-aminoethyl ether)-N,N,N,N-tetracetic acid (EGTA). A free-Ca2+ activity of less than 40 M restores the ability to respond to red light, but only if supplied within 1 h of red light. Magnesium ions are an ineffective substitute. At least two processes in unrolling appear to be Ca2+-sensitive.Fluence-response measurements indicate that the levels of the far-red-absorbing from of phytochrome (Pfr) still present 4 h after red-light treatment should be above saturation for the unrolling response; consequently, loss of Pfr does not explain the loss in effectiveness of Ca2+ during prolonged EGTA treatment. However, if a further red-light treatment is given simultaneously with Ca2+ addition 4 h after the initial light stimulus, then full unrolling occurs in EGTA-treated sections. These data indicate that, under normal circumstances, a functional change in the properties of Pfr must occur, uncoupling it from the transduction chain.Abbreviations EGTA
ethyleneglycol-bis-(-aminoethylether)-N,N,N,N,-tetracetic acid
- FR
far-red light
- Mes
2-(N-morpholino)ethanesulphonic, acid
- Pfr
far-red absorbing form of phytochrome
- Pr
red-absorbing form of phytochrome
- R
red light 相似文献
243.
We examined whether the effects of elevated CO2 on growth of 1-year old Populus deltoides saplings was a function of the assimilation responses of particular leaf developmental stages. Saplings were grown for 100 days
at ambient (approximately 350 ppm) and elevated (ambient + 200 ppm) CO2 in forced-air greenhouses. Biomass, biomass distribution, growth rates, and leaf initiation and expansion rates were unaffected
by elevated CO2. Leaf nitrogen (N), the leaf C:N ratio, and leaf lignin concentrations were also unaffected. Carbon gain was significantly
greater in expanding leaves of saplings grown at elevated compared to ambient CO2. The Rubisco content in expanding leaves was not affected by CO2 concentration. Carbon gain and Rubisco content were significantly lower in fully expanded leaves of saplings grown at elevated
compared to ambient CO2, indicating CO2-induced down-regulation in fully expanded leaves. Elevated CO2 likely had no overall effect on biomass accumulation due to the more rapid decline in carbon gain as leaves matured in saplings
grown at elevated compared to ambient CO2. This decline in carbon gain has been documented in other species and shown to be related to a balance between sink/source
balance and acclimation. Our data suggest that variation in growth responses to elevated CO2 can result from differences in leaf assimilation responses in expanding versus expanded leaves as they develop under elevated
CO2.
Received: 28 September 1998 / Accepted: 23 June 1999 相似文献
244.
Hydrogen isotopic compositions of n-alkanes from terrestrial plants correlate with their ecological life forms 总被引:1,自引:0,他引:1
Stable hydrogen isotopic compositions (δD) of compound-specific biomarkers, such as n-alkanes from plant leaf waxes, can be used as a proxy for paleoclimatic change. However, the relationship between hydrogen isotopes of plant leaf wax and plant ecological life forms is not well understood. Here, we report the δD of n-alkanes from 34 modern terrestrial plants, including twenty-one C3 plants and thirteen C4 plants from northwestern China, determined using gas chromatography/thermal conversion/isotope ratio mass spectrometry. Our data show that the stable hydrogen isotopes are poorly correlated with the plant photosynthetic pathway (C3 vs. C4) and that they do not give clear regional precipitation signals. Together with a comparative analysis of published δD values from plant leaf waxes in other regions, we believe that the stable hydrogen isotope of plant leaf waxes is more closely related to ecological life forms of these terrestrial plants (i.e. tree, shrub, and grass). In general, the grasses have more negative δD values than the co-occurring trees and shrubs. Our findings suggest that the δD values of sedimentary leaf waxes from higher plants may record changes of a plant ecosystem under the influence of environmental alteration and imply that reconstruction of the paleoclimate using δD values from plant n-alkanes should be based upon specific plant taxa, and comparison should be made among plants with similar ecological life forms. 相似文献
245.
Adams WW Watson AM Mueh KE Amiard V Turgeon R Ebbert V Logan BA Combs AF Demmig-Adams B 《Photosynthesis research》2007,94(2-3):455-466
The potential role of foliar carbon export features in the acclimation of photosynthetic capacity to differences and changes
in light environment was evaluated. These features included apoplastic vs. symplastic phloem loading, density of loading veins,
plasmodesmatal frequency in intermediary cells, and the ratio of loading cells to sieve elements. In initial studies, three
apoplastic loaders (spinach, pea, Arabidopsis thaliana) exhibited a completely flexible photosynthetic response to changing light conditions, while two symplastic loaders (pumpkin,
Verbascum phoeniceum), although able to adjust to different long-term growth conditions, were more limited in their response when transferred
from low (LL) to high (HL) light. This suggested that constraints imposed by the completely physical pathway of sugar export
might act as a bottleneck in the export of carbon from LL-acclimated leaves of symplastic loaders. While both symplastic loaders
exhibited variable loading vein densities (low in LL and high in HL), none of the three apoplastic loaders initially characterized
exhibited such differences. However, an additional apoplastic species (tomato) exhibited similar differences in vein density
during continuous growth in different light environments. Furthermore, in contrast to the other apoplastic loaders, photosynthetic
acclimation in tomato was not complete following a transfer from LL to HL. This suggests that loading vein density and loading
cells per sieve element, and thus apparent loading surface capacity, play a major role in the potential for photosynthetic
acclimation to changes in light environment. Photosynthetic acclimation and vein density acclimation were also characterized
in the slow-growing, sclerophytic evergreen Monstera deliciosa. This evergreen possessed a lower vein density during growth in LL compared to HL and exhibited a more severely limited potential
for photosynthetic acclimation to increases in light environment than the rapidly-growing, mesophytic annuals. 相似文献
246.
Kerstin Huss-Danell 《Plant and Soil》1986,91(1):43-49
Summary A pot experiment withAlnus incana (L.) Moench growing in sand was set up to compare the amounts of nitrogen released from plants shoot litter with that released below ground as root litter and/or root exudation. No nitrogen fixation by free-living microorganisms was found in the sand and the increased nitrogen content of the plant + soil system was therefore due to nitrogen fixation byFrankia in the alder root-nodules. Most of the nitrogen released from the plants was in the nitrogen-rich leaf and other shoot litter. Only small amounts of nitrogen were found in the drainage water from the pots and were recorded as increased nitrogen content of the sand. 相似文献
247.
Takeshi?IzutaEmail author Taeko?Yamaoka Tatsuro?Nakaji Tetsushi?Yonekura Masaaki?Yokoyama Ryo?Funada Takayoshi?Koike Tsumugu?Totsuka 《Trees - Structure and Function》2004,18(6):677-685
To obtain basic information for evaluating critical loads of acid deposition for protecting Japanese beech forests, growth, net photosynthesis and leaf nutrient status of Fagus crenata seedlings grown for two growing seasons in brown forest soil acidified with H2SO4 or HNO3 solution were investigated. The whole-plant dry mass of the seedlings grown in the soil acidified by the addition of H2SO4 or HNO3 solution was significantly less than that of the seedlings grown in the control soil not supplemented with H+ as H2SO4 or HNO3 solution. However, the degrees of reduction in the whole-plant dry mass and net photosynthetic rate of the seedlings grown in the soil acidified by the addition of H+ as H2SO4 solution at 100 mg l–1 on the basis of air-dried soil volume (S-100 treatment) were greater than those of the seedlings grown in the soil acidified by the addition of H+ as HNO3 solution at 100 mg l–1 (N-100 treatment). The concentrations of Al and Mn in the leaves of the seedlings grown in the S-100 treatment were significantly higher than those in the N-100 treatment. A positive correlation was obtained between the molar ratio of (Ca+Mg+K)/(Al+Mn) in the soil solution and the relative whole-plant dry mass of the seedlings grown in the acidified soils to that of the seedlings grown in the control soil. Based on the results, we concluded that the negative effects of soil acidification due to sulfate deposition are greater than those of soil acidification due to nitrate deposition on growth, net photosynthesis and leaf nutrient status of F. crenata, and that the molar ratio of (Ca+Mg+K)/(Al+Mn) in soil solution is a suitable soil parameter for evaluating critical loads of acid deposition in efforts to protect F. crenata forests in Japan. 相似文献
248.
为阐释竹子开花过程中的生理功能变化,探寻竹子花后复壮更新的途径,本研究选取红哺鸡竹(Phyllostachys iridescens)竹林换叶后出现开花征兆的竹株异形叶和外观正常叶,并以未开花竹株正常叶为对照,进行了叶片结构观察和光响应测定。结果发现:红哺鸡竹从营养生长向生殖生长转变的过程中,叶片的发育有变小变薄的特征,相邻维管束距离增大,维管束横切面积和气孔密度有显著变小的趋势(p0.05)。开花竹异形叶的叶片厚度、上表皮厚度、维管束横切面积和气孔密度是未开花竹正常叶的71.59%、87.40%、77.79%和73.56%,相邻维管束距离增加了19.34%,这种结构的变化相应地导致了开花竹叶片光合和蒸腾作用的显著降低(p0.05)。开花竹异形叶的光响应参数,如光饱和点、表观量子效率和最大光合速率是未开花竹正常叶的67%、40.5%和8.27%,分别为900、0.015、1.22μmol·m-2·s-1;光补偿点比未开花竹正常叶高出208.5%,达85.11μmol·m-2·s-1,而开花竹外观正常叶的光响应参数介于两者之间。开花竹株的叶片结构弱化和随之产生的生理功能异化可能是多年生竹子无法像树木那样连年开花、持续生长的重要原因之一。 相似文献
249.
To understand the functions of antioxidant enzymes during leaf development in sweetpotato, we investigated the activities
of several antioxidant enzymes such as superoxide dismutase (SOD), peroxidase (POX), ascorbate peroxidase (APX) and catalase
(CAT). Significant increases were observed in the activities of SOD, POX and APX during the late stage of leaf development,
whereas CAT activity increased during the early developmental stage. By RT-PCR analysis, various POX and APX genes showed
differential expression patterns during leaf development. Four POX genes swpa3, swpa4, swpa6, swpb4 and one APX gene swAPX1 exhibited high levels of gene expression during the senescence stage of leaf development, but two POX genes, swpa1 and swpa7 were preferentially expressed at both the mature green and the late senescence stages of leaf development. These results
indicate that hydrogen peroxide (H2O2)-related antioxidant enzymes are differentially regulated in the process of leaf development of sweetpotato. 相似文献
250.
Shubhi Avasthi Ajay Kumar Gautam Rekha Bhadauria 《Archives Of Phytopathology And Plant Protection》2017,50(1-2):62-69
A leaf spot disease caused by Phoma eupyrena Sacc. on Aloe vera was reported from Madhya Pradesh, India. Disease symptoms were observed as irregular to elongated, sunken lesion on both leaf surfaces which later turns creamish brown with maroon margin. The fungal colonies were whitish or light pale to dark grey colour. The pycnidia were glabrous and solitary, papillate, and indistinctly uniostiolate averages 100–260 μm in diameter. Conidia were ellipsoidal, 4.5–7.5 × 2.5–3.75 μm in size, with two large guttules. To the best of our knowledge, this is the first report of P. eupyrena causing leaf spot disease on A. vera in India. 相似文献