全文获取类型
收费全文 | 153631篇 |
免费 | 9125篇 |
国内免费 | 812篇 |
专业分类
林业 | 6382篇 |
农学 | 5409篇 |
基础科学 | 1207篇 |
19030篇 | |
综合类 | 27314篇 |
农作物 | 5755篇 |
水产渔业 | 7745篇 |
畜牧兽医 | 79159篇 |
园艺 | 2129篇 |
植物保护 | 9438篇 |
出版年
2020年 | 1379篇 |
2019年 | 1728篇 |
2018年 | 2526篇 |
2017年 | 2909篇 |
2016年 | 2632篇 |
2015年 | 2375篇 |
2014年 | 2778篇 |
2013年 | 5962篇 |
2012年 | 5039篇 |
2011年 | 6165篇 |
2010年 | 4134篇 |
2009年 | 3982篇 |
2008年 | 5726篇 |
2007年 | 5407篇 |
2006年 | 5118篇 |
2005年 | 4610篇 |
2004年 | 4267篇 |
2003年 | 4364篇 |
2002年 | 4024篇 |
2001年 | 5077篇 |
2000年 | 4981篇 |
1999年 | 3886篇 |
1998年 | 1585篇 |
1997年 | 1584篇 |
1996年 | 1379篇 |
1995年 | 1575篇 |
1994年 | 1432篇 |
1993年 | 1411篇 |
1992年 | 2987篇 |
1991年 | 3250篇 |
1990年 | 3082篇 |
1989年 | 3067篇 |
1988年 | 2741篇 |
1987年 | 2795篇 |
1986年 | 2837篇 |
1985年 | 2606篇 |
1984年 | 2205篇 |
1983年 | 1873篇 |
1982年 | 1292篇 |
1979年 | 1923篇 |
1978年 | 1534篇 |
1976年 | 1296篇 |
1975年 | 1474篇 |
1974年 | 1770篇 |
1973年 | 1720篇 |
1972年 | 1626篇 |
1971年 | 1555篇 |
1970年 | 1544篇 |
1969年 | 1478篇 |
1967年 | 1330篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
Downy mildew (Plasmopara viticola) is one of the most important diseases in grape-growing areas worldwide, including Brazil. To examine pathogen population biology and structure, P. viticola was sampled during the 2015/16 growing season from 516 lesions on nine grape cultivars in 11 locations in subtropical areas of São Paulo State, Brazil. For identification of cryptic species, a subsample of 130 isolates was subjected to cleaved amplified polymorphic sequence (CAPS) analysis, and for 91 of these isolates the ITS1 region was sequenced. These analyses suggest that the population of P. viticola in São Paulo State consists of a single cryptic species, P. viticola clade aestivalis. Seven microsatellite markers were used to determine the genetic structure of all 516 P. viticola isolates, identifying 23 alleles and 55 multilocus genotypes (MLGs). Among these MLGs, 34.5% were clonal and represented 93% of the isolates sampled. Four dominant genotypes were present in at least five different locations, corresponding to 65.7% of the isolates sampled. Genotypic diversity (Ĝ = 0.21–0.89) and clonal fraction (0.58–0.96) varied among locations (populations). Most populations showed significant deviation from Hardy–Weinberg expectations; in addition, excess of heterozygosity was verified for many loci. However, principal coordinate analysis revealed no clusters among locations and no significant isolation by distance was found, suggesting high levels of migration. The results indicate that downy mildew epidemics result from multiple clonal infections caused by a few genotypes of P. viticola, and reproduction of P. viticola in São Paulo State is predominantly asexual. 相似文献
992.
Earth tides can trigger shallow thrust fault earthquakes 总被引:6,自引:0,他引:6
We show a correlation between the occurrence of shallow thrust earthquakes and the occurrence of the strongest tides. The rate of earthquakes varies from the background rate by a factor of 3 with the tidal stress. The highest correlation is found when we assume a coefficient of friction of mu = 0.4 for the crust, although we see good correlation for mu between 0.2 and 0.6. Our results quantify the effect of applied stress on earthquake triggering, a key factor in understanding earthquake nucleation and cascades whereby one earthquake triggers others. 相似文献
993.
In vitro fertilization and cortical granule distribution of bovine oocytes having heterogeneous ooplasm with dark clusters. 总被引:1,自引:0,他引:1
M Nagano Y Takahashi S Katagiri 《The Journal of veterinary medical science / the Japanese Society of Veterinary Science》1999,61(5):531-535
In vitro maturation, fertilization and subsequent development of oocytes with homogeneous (category 1), or heterogeneous ooplasm (category 2) were investigated. No significant differences were observed in the nuclear maturation and total fertilization rates between the two categories. However, category 2 oocytes showed a higher normal fertilization rate due to their lower incidence of polyspermy as compared to category 1 oocytes. Electron microscopic study revealed that all category 2 oocytes had cortical granules lined up next to the plasma membrane, and that some category 1 oocytes still had small clusters of cortical granules after maturation. Although the proportion of cleaved zygotes was higher in category 2, the percentages of cleaved zygotes that developed to the blastocyst stage did not differ between the two categories. These results demonstrate that oocytes with heterogeneous ooplasm have a higher capacity for normal fertilization due to the reduction in polyspermy. This can be attributed to the normal distribution of cortical granules in category 2 oocytes after maturation. 相似文献
994.
The effect of the seed cut of the uniform shelterwood silvicultural system on white pine seed production, seed characteristics, and seed viability during 2 mast seeding events was examined in operationally harvested second growth, white pine-dominated forest stands in central Ontario. Seed traps placed along transects in unmanaged and shelterwood stands in each of 3 blocks were used to monitor seed production in 2000 and 2006 (4 and 10 years after harvesting). During these 2 mast seed years between 386,000 and 2,730,600 seed ha?1 were produced among study stands. Total seed production expressed on a per hectare and unit pine basal area basis did not differ by harvest treatment or among blocks in either year. Variability in seed production among stands was primarily due to differences in stand structure, with seed production positively related to white pine basal area. Seed characteristics were largely similar between harvested and unmanaged stands and between seed years. Seed viability was relatively high in both years, with seed from shelterwoods germinating slightly slower than those from unmanaged stands. Seed quality, as estimated by laboratory germination performance, was higher in 2006 than 2000, likely due to improved seed development and maturation in the warmer, wetter growing season of 2006. Our results suggest that the seed cut of the uniform shelterwood system applied to second growth white pine stands is unlikely to adversely affect white pine seed production, seed quality, or potential for natural regeneration during mast seeding events. 相似文献
995.
The impacts of tillage and cropping sequences on soil organic matter and nutrients have been frequently reported to affect the uppermost soil layers, but there is little published information concerning effects at greater depth. This article reports results on the distribution of soil organic carbon (SOC), active carbon (AC), N, Olsen‐P and extractable K within 100 cm in short (4 yr) and long (16 yr) term experiments under different tillage systems. Short (TT4) and long (TT16) traditional tillage are compared with conservation tillage, reduced (RT16) and non‐tillage (NT4). The results show more accumulation of SOC in the near‐surface under RT16 and NT4 in both experiments compared with traditional tillage. Moreover, greater C content occurs to 40 cm depth in the long‐term experiment. The results demonstrate the importance of time on C accumulation, not only in near‐surface layers but also at greater depths. Active C is an indicator of the increase in soil quality in the long‐term experiment. This trend is only apparent for the first 10 cm in the short‐term experiment. Patterns in N, Olsen‐P and extractable K are similar to that of SOC. However, only extractable K is significantly greater in soil under conservation tillage (RT16 and NT4) after short and long periods. Potassium availability is a good indicator of the changes caused by tillage. Our results indicate that studies of soils at depth could be very useful in long‐term experiments to demonstrate the effect of conservation tillage on C and nutrient distribution. 相似文献
996.
997.
L. Lundin 《Water, air, and soil pollution》1995,85(3):1695-1700
Water flow and transit in the soil are important to water supply and transport of chemical compounds. Flows through forested till soils are by vertical percolation in the upslope areas, groundwater flows along the slope to discharge in downslope sites. The flow is divided into slow and fast parts both in the unsaturated percolation and the saturated groundwater. Soil development influences the flow and in coarse grained and well drained soils groundwater flow is stable but in finer grained physically stratified soils, variations in flow are considerable. Large flows occur in the upper soil layers and small flows in deep layers. Chemical composition of soil water varies in a similar way. In the upper layers, water is more acid and has a lower content of base cations compared to deeper layers. In downslope areas discharging deep groundwater contributes to less acidic conditions also in the upper soil layers. This discharge and a dominating lateral water flow in the upper layers partly protect deep layers from acidification which is emphasised by slow water turnover in these layers. 相似文献
998.
Triticum urartu is a wild diploid wheat identified as donor of the A genome in the durum and bread wheats. This species has its natural distribution
zone in the Fertile Crescent. Nine populations from this zone (five Lebanese and four Turkish) were analysed for the variation
in the loci coding of endosperm storage proteins: glutenins (Glu-A
u
1 and Glu-A
u
3) and gliadins (Gli-A
u
1 and Gli-A
u
2). Up to 10 alleles for the Glu-A
u
1 locus, 16 for the Glu-A
u
3 locus, 15 for the Gli-A
u
1 locus and 18 for the Gli-A
u
2 locus were found. The Turkish populations showed greater diversity than the Lebanese populations, distributing mainly within
populations, whereas the inverse process takes place for the Lebanese populations. The values of the genetic diversity indicated
that, although these materials showed high level of variability, exist a real risk of genetic erosion. 相似文献
999.
Root respiration may account for as much as 60% of total soil respiration. Therefore, factors that regulate the metabolic activity of roots and associated microbes are an important component of terrestrial carbon budgets. Root systems are often sampled by diameter and depth classes to enable researchers to process samples in a systematic and timely fashion. We recently discovered that small, lateral roots at the distal end of the root system have much greater tissue N concentrations than larger roots, and this led to the hypothesis that the smallest roots have significantly higher rates of respiration than larger roots. This study was designed to determine if root respiration is related to root diameter or the location of roots in the soil profile. We examined relationships among root respiration rates and N concentration in four diameter classes from three soil depths in two sugar maple (Acer saccharum Marsh.) forests in Michigan. Root respiration declined as root diameter increased and was lower at deeper soil depths than at the soil surface. Surface roots (0-10 cm depth) respired at rates up to 40% greater than deeper roots, and respiration rates for roots < 0.5 mm in diameter were 2.4 to 3.4 times higher than those for roots in larger diameter classes. Root N concentration explained 70% of the observed variation in respiration across sites and size and depth classes. Differences in respiration among root diameter classes and soil depths appeared to be consistent with hypothesized effects of variation in root function on metabolic activity. Among roots, very fine roots in zones of high nutrient availability had the highest respiration rates. Large roots and roots from depths of low nutrient availability had low respiration rates consistent with structural and transport functions rather than with active nutrient uptake and assimilation. These results suggest that broadly defined root classes, e.g., fine roots are equivalent to all roots < 2.0 mm in diameter, do not accurately reflect the functional categories typically associated with fine roots. Tissue N concentration or N content (mass x concentration N) may be a better indicator of root function than root diameter. 相似文献
1000.
Zaluzanin D, a sesquiterpene lactone has been isolated from the aerial parts of Vernonia arborea as a major constituent (0.04%). Antifungal activity of zaluzanin D has been studied against six plant-pathogenic fungi. 相似文献