全文获取类型
收费全文 | 2557篇 |
免费 | 202篇 |
国内免费 | 108篇 |
专业分类
2867篇 |
出版年
2024年 | 21篇 |
2023年 | 88篇 |
2022年 | 68篇 |
2021年 | 117篇 |
2020年 | 98篇 |
2019年 | 100篇 |
2018年 | 84篇 |
2017年 | 93篇 |
2016年 | 87篇 |
2015年 | 100篇 |
2014年 | 136篇 |
2013年 | 180篇 |
2012年 | 97篇 |
2011年 | 111篇 |
2010年 | 89篇 |
2009年 | 82篇 |
2008年 | 110篇 |
2007年 | 93篇 |
2006年 | 67篇 |
2005年 | 72篇 |
2004年 | 84篇 |
2003年 | 91篇 |
2002年 | 83篇 |
2001年 | 65篇 |
2000年 | 54篇 |
1999年 | 46篇 |
1998年 | 50篇 |
1997年 | 44篇 |
1996年 | 54篇 |
1995年 | 34篇 |
1994年 | 41篇 |
1993年 | 52篇 |
1992年 | 29篇 |
1991年 | 29篇 |
1990年 | 25篇 |
1989年 | 17篇 |
1988年 | 23篇 |
1987年 | 20篇 |
1986年 | 16篇 |
1985年 | 21篇 |
1984年 | 19篇 |
1983年 | 9篇 |
1982年 | 16篇 |
1981年 | 15篇 |
1980年 | 16篇 |
1979年 | 6篇 |
1978年 | 4篇 |
1977年 | 5篇 |
1973年 | 2篇 |
1967年 | 2篇 |
排序方式: 共有2867条查询结果,搜索用时 0 毫秒
1.
2.
3.
4.
Microsclerotium formation by six isolates of Verticillium dahliae was studied at different temperatures both in vitro and in Arabidopsis thaliana . In vitro mycelial growth was optimal at 25°C, but microsclerotium formation was greatest at 20°C (two isolates) or 15–20°C (one isolate). Seedlings of A. thaliana were root-dipped in a conidial suspension, planted, and either placed at 5, 10, 15, or 25°C, or left at 20°C until the onset of senescence, after which some of the plants were placed at 5, 10, 15, or 25°C. The amount of microsclerotia per unit of shoot weight was assessed in relation to isolate and temperature. The optimal temperature for production of microsclerotia was 15–25°C. Two isolates each produced about 10 times more microsclerotia than each of the other four isolates. For these isolates, high R 2 adj. -values of 0.77 and 0.66 were obtained, with temperature and its square as highly significant (P < 0.001) independent variables. R 2 adj. -values for the other isolates varied between 0.28 and 0.39. Moving plants to different temperatures at the onset of senescence led to microsclerotial densities that were intermediate between densities on plants that had grown at constantly 20°C and plants grown at other temperatures. This suggests that vascular colonization rate and rate of microsclerotium formation are similarly affected by temperature. The senescence rate of plants appeared unimportant except for plants grown at 25°C, which showed the highest amounts of microsclerotia per unit of plant weight in the most rapidly senescing plants. 相似文献
5.
Latency and development of Botrytis cinerea were assessed under field conditions and after artificial inoculation of two grape varieties, Gamay (susceptible) and Gamaret (resistant). When the percentage of latent Botrytis was the same for both varieties, severity of visible grey mould remained very low in Gamaret berries, while Gamay clusters were destroyed by the disease to a high percentage. Some biochemical parameters were measured in berries, such as constitutive and induced anti‐fungal compounds, polymeric proanthocyanidins and lipid peroxidation products as markers of senescence. Differences were observed in polymeric proanthocyanidins (PPRA) of Gamaret compared with those of Gamay. Concentration and mean degree of polymerization (mDP) of PPRA were always higher in the berries of the resistant variety. The inhibitory effect of Gamaret PPRA on enzyme activity remained until harvest whereas Gamay PPRA lost their inhibitory activity at the beginning of véraison. Based on these results, resistance to B. cinerea seems to be linked to the maintainance of the fungus in its latent form in berries, mainly due to the ability of Gamaret PPRA to inhibit macerating fungal enzyme activities. 相似文献
6.
Published data on, and hypotheses regarding the effect of NO− 3 on functioning of legume root nodules are reviewed. It is concluded that a short-term reversible effect of NO− 3 may act via an increased resistance to O2 diffusion in nodules; this is coupled to decreased bacteroid respiration. For longer exposures to NO− 3 nodule activity is irreversibly lost, but how this relates to carbohydrate deprivation or NO-2 accumulation is unclear. Complicating factors include denitrification reactions and the interaction of NO− 2 with leghaemoglobin. 相似文献
7.
Leo S. Luckinbill Michael J. Clare Walter L. Krell William C. Cirocco Paula A. Richards 《Evolutionary ecology》1987,1(1):37-46
Summary Although many different physiological and biochemical changes characterize the process of senescence, little is understood of the genetic elements that determine its age of onset. We provide here the first estimates of the number of genetic factors that extend longevity inDrosophila melanogaster. Life span was measured in F1, F2 and backcrosses of true-breeding long and short-lived stocks ofD. melanogaster, established by selection. Estimates of the number of effective factors delaying senescence range from about 0.3 to 1.5, indicating control by a single factor. The distribution of longevity shows this to arise as selection acts on the short-lived parental stock. Life span is extended at the cost of early fecundity. 相似文献
8.
Changes in gene expression during foliar senescence and fruit ripening in tomato (Lycopersicon esculentum Mill.) were examined using in-vitro translation of isolated RNA and hybridization against cDNA clones.During the period of chlorophyll loss in leaves, changes occurred in mRNA in-vitro translation products, with some being reduced in prevalence, whilst others increased. Some of the translation products which changed in abundance had similar molecular weights to those known to increase during tomato fruit ripening. By testing RNA from senescing leaves against a tomato fruit ripening-related cDNA library, seven cDNA clones were identified for mRNAs whose prevalence increased during both ripening and leaf senescence. Using dot hybridization, the pattern of expression of the mRNAs corresponding to the seven clones was examined. Maximal expression of the majority of the mRNAs coincided with the time of greatest ethylene production, in both leaves and fruit. Treatment of mature green leaves or unripe fruit with the ethylene antagonist silver thiosulphate prevented the onset of senescence or ripening, and the expression of five of the seven ripening- and senescence-related genes.The results indicate that senescence and ripening in tomato involve the expression of related genes, and that ethylene may be an important factor in controlling their expression.Abbreviations cDNA
copy-DNA
- MW
molecular weight
- PAGE
polyacrylamide gel electrophoresis
- SDS
sodium dodecyl sulphate 相似文献
9.
Leaf senescence and ribulose-1,5-bisphosphate carboxylase/oxygenase (RuBP carboxylase, EC 4.1.1.39) degradation in orange [ Citrus sinensis (L.) Osbeck cv. Washington Navel] explants have been investigated. Explants consisted of a segment of stem (ca 15 cm) and 5 mature leaves. In vitro RuBP carboxylase degradation was determined by culturing the explants in water for different periods of time (3 days usually) and quantifying the two RuBP carboxylase subunits in the extracts following sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE). In vitro RuBP carboxylase degradation was estimated by autodigestion of leaf extracts and SDS-PAGE. The extent of in vivo RuBP carboxylase degradation in explants cultured under 16 h light/8 h dark photoperiod varied throughout the year and showed a cyclic behaviour correlated with the growth cycle of Citrus. The highest proteolytic activity both in vivo and in vitro was found in explants made from April to August coinciding with the maximum vegetative growth period of the tree.
Leaf senescence and abscission could be retarded significantly at any time of the year by maintaining the explants continuously in the dark. Treatment of the explants in the dark with a continuous flow of ethylene enhanced both leaf abscission and rate of RuBP carboxylase degradation, proportionally to ethylene concentration (0.1-0.6 ppm). Ethylene-induced senescence of Citrus leaf explants in the dark appears to be a convenient model system to study the regulation of the proteolytic degradation of RuBP carboxylase. 相似文献
Leaf senescence and abscission could be retarded significantly at any time of the year by maintaining the explants continuously in the dark. Treatment of the explants in the dark with a continuous flow of ethylene enhanced both leaf abscission and rate of RuBP carboxylase degradation, proportionally to ethylene concentration (0.1-0.6 ppm). Ethylene-induced senescence of Citrus leaf explants in the dark appears to be a convenient model system to study the regulation of the proteolytic degradation of RuBP carboxylase. 相似文献
10.
It has been known for several decades that cultured murine cells undergo a defined series of changes, i.e., anin vitro evolution, which includes crisis, spontaneous transformation (immortalization), aneuploidy, and spontaneous neoplastic transformation. These changes have been shown to be caused by thein vitro environment rather than an inherent instability of the murine phenotype or genotype. Serum amine oxidases were recently identified as a predominant cause of crisis. These enzymes generate hydrogen peroxide from polyamine substrates that enter the extracellular milieu. This finding implicates free-radical toxicity as the underlying cause ofin vitro evolution. We propose an oxyradical hypothesis to explain each of the stages ofin vitro evolution and discuss its significance for cytotechnology and long-term cultivation of mammalian cell types.ORR, CDER, FDA Mod-1, Room 2023, 8301 Muirkirk Road, Laurel MD 20708, USA 相似文献