全文获取类型
收费全文 | 999篇 |
免费 | 30篇 |
专业分类
1029篇 |
出版年
2016年 | 4篇 |
2015年 | 9篇 |
2014年 | 16篇 |
2013年 | 25篇 |
2012年 | 27篇 |
2011年 | 49篇 |
2010年 | 69篇 |
2009年 | 70篇 |
2008年 | 71篇 |
2007年 | 72篇 |
2006年 | 68篇 |
2005年 | 55篇 |
2004年 | 28篇 |
2003年 | 21篇 |
2002年 | 5篇 |
2001年 | 11篇 |
2000年 | 12篇 |
1999年 | 10篇 |
1998年 | 34篇 |
1997年 | 23篇 |
1996年 | 22篇 |
1995年 | 13篇 |
1994年 | 14篇 |
1993年 | 11篇 |
1992年 | 12篇 |
1991年 | 15篇 |
1990年 | 22篇 |
1989年 | 16篇 |
1988年 | 16篇 |
1987年 | 15篇 |
1986年 | 11篇 |
1985年 | 7篇 |
1984年 | 10篇 |
1983年 | 8篇 |
1982年 | 7篇 |
1981年 | 10篇 |
1980年 | 6篇 |
1979年 | 8篇 |
1976年 | 4篇 |
1975年 | 4篇 |
1972年 | 5篇 |
1971年 | 6篇 |
1959年 | 7篇 |
1958年 | 12篇 |
1956年 | 12篇 |
1955年 | 5篇 |
1954年 | 6篇 |
1953年 | 10篇 |
1952年 | 8篇 |
1950年 | 5篇 |
排序方式: 共有1029条查询结果,搜索用时 12 毫秒
141.
142.
143.
Scaling of respiration from the leaf to the canopy level currently depends on identification of physiological parameters that are tightly linked to respiration and that can readily be determined. Several recent studies have helped provide guides to predicting whole canopy respiration on the basis of foliar nitrogen (N). This approach is potentially powerful owing to the well‐described patterns of allocation of N that follow interception of radiation. In the present study, we investigated the sensitivity of the N–respiration correlation to environmental and developmental factors, in order to evaluate its usage for attempts to scale respiration to the organism and ecosystem level. We studied fully expanded, 1 and 2‐year‐old, and current‐year needles from canopies of Pinus radiata that had been treated (unthinned, thinned and thinned+fertilized treatments) in ways likely to induce a wide range of growth and respiratory responses. We examined respiration in detail during the growth period in spring and again at the end of summer, using calorespirometric methods (combined measurements of CO2 and heat rates) to determine the respiration rates , instantaneous enthalpic growth rates (RSGΔHB, a measure of the conservation of electrons in anabolic products) and the enthalpy conversion efficiency (ηH) of needles differing in age. A general linear model revealed that was positively correlated with needle N, but this correlation was strongly dependent on the season and the needle age – indicating an important physiological difference between expanding young needles and fully expanded old needles. Furthermore, the strength of the correlation between needle N and respiration was comparatively weak for the current year, expanding foliage, indicating that factors other than foliage N significantly influenced the respiration of young needles. The analysis of instantaneous growth rates revealed two general processes. Older, nonexpanding foliage showed considerable rates of enthalpic growth (increases in enthalpy) that was mainly caused by the increment of lignin during secondary growth. Secondly, canopy development appeared dynamic and to be optimized according to environmental drivers and constraints – such as light and water availability. In late spring, needle extension slowed in the upper, but not the lower canopy, because the upper canopy appeared to be affected first by the onset of drought stress in late spring. Growth rates were reduced in the upper canopy despite greater rates of respiration, indicating higher demand of ATP for the maintenance of protein and for export of sugars. Consequently, the enthalpy conversion efficiency and enthalpic N productivity (enthalpic growth per unit N) were comparatively poor indicating advanced development of needles in the upper canopy. We suggest that the growth and maintenance paradigm of respiration is, at best, only moderately useful when applied to whole trees, and is not valid at the cellular level or that of the plant organ. A different concept, namely that of respiratory efficiency, seems a more suitable way to represent respiration in carbon (C) balance models and should help provide a better mechanistic understanding of how respiration affects the C conversion efficiency of plants, and ultimately the net primary productivity of ecosystems. 相似文献
144.
BRENDAN C. EBNER JASON D. THIEM DEAN M. GILLIGAN MARK LINTERMANS IAN J. WOODEN SIMON LINKE 《Austral ecology》2008,33(7):891-901
Abstract Biodiversity estimates are typically a function of sampling effort and in this regard it is important to develop an understanding of taxon‐specific sampling requirements. Northern hemisphere studies have shown that estimates of riverine fish diversity are related to sampling effort, but such studies are lacking in the southern hemisphere. We used a dataset obtained from boat electro‐fishing the fish community along an essentially continuous 13‐km reach of the Murrumbidgee River, Australia, to investigate sampling effort effects on fish diversity estimates. This represents the first attempt to investigate relationships between sampling effort and the detection of fish species in a large lowland river in Australia. Seven species were recorded. Species‐specific patterns in catch per unit effort were evident and are discussed in terms of solitary and gregarious species, recreational fishing and the monitoring of rare and threatened species. There was a requirement to sample substantial lengths of river to describe total species richness of the fish community in this river reach. To this end, randomly allocated sampling effort and use of species richness estimators produced accurate estimates of species richness without the requirement for excessive levels of effort. Twenty operations were required to estimate species richness at this site, highlighting the need for comparable studies of river fish communities in lowland rivers elsewhere in Australia and the southern hemisphere. 相似文献
145.
MARK BOLTON ANDREA L. SMITH ELENA GÓMEZ‐DÍAZ VICKI L. FRIESEN RENATA MEDEIROS JOËL BRIED JOSE L. ROSCALES ROBERT W. FURNESS 《Ibis》2008,150(4):717-727
The existence of two seasonally distinct breeding populations of Oceanodroma storm‐petrels in the Azores islands was first documented in 1996. The discovery of morphological differences between the populations led to the suggestion that they may represent cryptic sibling species. Recent mtDNA and microsatellite analysis from storm‐petrel populations has considerably advanced our understanding of their taxonomic relationships. Here we present new information on the timing of breeding and moult of the two Azores populations, the extent of exchange of individuals between seasons, and diet from feather isotopes. We conclude that the hot‐season Azores population should be considered a new species for which we propose the name Oceanodroma monteiroi, Monteiro's Storm‐petrel. The species is both genetically distinct and genetically isolated from the sympatric cool‐season population of Madeiran Storm‐petrel Oceanodroma castro, and from all other populations of Oceanodroma castro in the Atlantic and Pacific Oceans examined to date. Differences in the vocalizations permit species recognition, and the extent of primary feather wear and stage of moult aids separation of the two species in the Azores, which is especially valuable during August when both attend the breeding colonies in large numbers. Feather carbon and nitrogen isotopes reveal that the diet of Monteiro's Storm‐petrel differs from that of the sympatric Madeiran Storm‐petrel during both breeding and non‐breeding seasons, and unlike the Madeiran Storm‐petrel, Monteiro's Storm‐petrel appears to maintain the same foraging environment during the summer and winter months, though it shows a dietary shift to higher trophic levels during the non‐breeding season. Monteiro's Storm‐petrel is thought to be confined to the Azores archipelago, where it is currently known to nest on just two small neighbouring islets. The total population size was estimated at 250–300 pairs in 1999. 相似文献
146.
147.
DAVID T. TINGEY ROBERT B. MCKANE DAVID M. OLSZYK MARK G. JOHNSON PAUL T. RYGIEWICZ E. HENRY LEE 《Global Change Biology》2003,9(7):1038-1050
The effects of elevated CO2 and temperature on principal carbon constituents (PCC) and C and N allocation between needle, woody (stem and branches) and root tissue of Pseudotsuga menziesii Mirb. Franco seedlings were determined. The seedlings were grown in sun‐lit controlled‐environment chambers that contained a native soil. Chambers were controlled to reproduce ambient or ambient +180 ppm CO2 and either ambient temperature or ambient +3.5 °C for 4 years. There were no significant CO2 × temperature interactions; consequently the data are presented for the CO2 and temperature effects. At the final harvest, elevated CO2 decreased the nonpolar fraction of the PCC and increased the polar fraction and amount of sugars in the needles. In contrast, elevated temperature increased the nonpolar fraction of the PCC and decreased sugars in needles. There were no CO2 or temperature effects on the PCC fractions in the woody tissue or root tissue. Elevated CO2 and temperature had no significant effects on the C content of any of the plant tissues or fractions. In contrast, the foliar N content declined under elevated CO2 and increased under elevated temperature; there were no significant effects in other tissues. The changes in the foliar N concentrations were in the cellulose and lignin fractions, the fractions, which contain protein, and are the consequences of changes in N allocation under the treatments. These results indicate reallocation of N among plant organs to optimize C assimilation, which is mediated via changes in the selectivity of Rubisco and carbohydrate modulation of gene expression. 相似文献
148.
HENDRIK TSCHOEP YVES GIBON PETRONIA CARILLO PATRICK ARMENGAUD MAREK SZECOWKA ADRIANO NUNES-NESI ALISDAIR R. FERNIE KARIN KOEHL & MARK STITT 《Plant, cell & environment》2009,32(3):300-318
We have established a simple soil-based experimental system that allows a small and sustained restriction of growth of Arabidopsis by low nitrogen (N). Plants were grown in a large volume of a peat–vermiculite mix that contained very low levels of inorganic N. As a control, inorganic N was added in solid form to the peat–vermiculite mix, or plants were grown in conventional nutrient-rich solids. The low N growth regime led to a sustained 20% decrease of the relative growth rate over a period of 2 weeks, resulting in a two- to threefold decrease in biomass in 35- to 40-day-old plants. Plants in the low N regime contained lower levels of nitrate, lower nitrate reductase activity, lower levels of malate, fumarate and other organic acids and slightly higher levels of starch, as expected from published studies of N-limited plants. However, their rosette protein content was unaltered, and total and many individual amino acid levels increased compared with N-replete plants. This metabolic phenotype reveals that Arabidopsis responds adaptively to low N by decreasing the rate of growth, while maintaining the overall protein content, and maintaining or even increasing the levels of many amino acids. 相似文献
149.
YVES GIBON EVA-THERESA PYL RONAN SULPICE JOHN E. LUNN MELANIE HÖHNE MANUELA GÜNTHER & MARK STITT 《Plant, cell & environment》2009,32(7):859-874
Arabidopsis was grown in a 12, 8, 4 or 3 h photoperiod to investigate how metabolism and growth adjust to a decreased carbon supply. There was a progressive increase in the rate of starch synthesis, decrease in the rate of starch degradation, decrease of malate and fumarate, decrease of the protein content and decrease of the relative growth rate. Carbohydrate and amino acids levels at the end of the night did not change. Activities of enzymes involved in photosynthesis, starch and sucrose synthesis and inorganic nitrogen assimilation remained high, whereas five of eight enzymes from glycolysis and organic acid metabolism showed a significant decrease of activity on a protein basis. Glutamate dehydrogenase activity increased. In a 2 h photoperiod, the total protein content and most enzyme activities decreased strongly, starch synthesis was inhibited, and sugars and amino acids levels rose at the end of the night and growth was completely inhibited. The rate of starch degradation correlated with the protein content and the relative growth rate across all the photoperiod treatments. It is discussed how a close coordination of starch turnover, the protein content and growth allows Arabidopsis to avoid carbon starvation, even in very short photoperiods. 相似文献
150.
R. DOUGLAS HOLT L. WES BURGER JR. STEPHEN J. DINSMORE MARK D. SMITH SCOTT J. SZUKAITIS K. DAVID GODWIN 《The Journal of wildlife management》2009,73(6):989-995
ABSTRACT Radiotelemetry is used extensively in zoographic studies of wildlife species, including northern bobwhite (Colinus virginianus). These studies assume that radiomarking does not affect survival of marked individuals. However, most researchers implicitly acknowledge that capture, handling, and radiomarking may have short-term deleterious effects on individuals and, therefore, include in analyses only animals that survive an adjustment period of arbitrary length (often 7 days) following capture and marking. Length of adjustment period is rarely empirically based and may potentially bias survival estimates. We outline an analytical approach to determine an appropriate adjustment period and illustrate this approach by examining effects of time-since-marking on survival of 410 northern bobwhite captured during winter from 1997 to 2001, in Mississippi, USA. We modeled daily survival rates using time-since-marking as a covariate in the nest-survival model of Program MARK. Although survival varied among and within years, we found no evidence to suggest that standard adjustment periods of 7–14 days were appropriate for our sample. If adjustment periods are used in radiotelemetry studies, those that are empirically based may be more appropriate than arbitrarily set periods. 相似文献