全文获取类型
收费全文 | 581篇 |
免费 | 42篇 |
国内免费 | 86篇 |
专业分类
709篇 |
出版年
2023年 | 6篇 |
2022年 | 12篇 |
2021年 | 12篇 |
2020年 | 13篇 |
2019年 | 11篇 |
2018年 | 13篇 |
2017年 | 15篇 |
2016年 | 4篇 |
2015年 | 18篇 |
2014年 | 14篇 |
2013年 | 34篇 |
2012年 | 15篇 |
2011年 | 12篇 |
2010年 | 27篇 |
2009年 | 31篇 |
2008年 | 25篇 |
2007年 | 40篇 |
2006年 | 27篇 |
2005年 | 26篇 |
2004年 | 48篇 |
2003年 | 26篇 |
2002年 | 26篇 |
2001年 | 24篇 |
2000年 | 12篇 |
1999年 | 16篇 |
1998年 | 27篇 |
1997年 | 20篇 |
1996年 | 20篇 |
1995年 | 15篇 |
1994年 | 18篇 |
1993年 | 23篇 |
1992年 | 8篇 |
1991年 | 9篇 |
1990年 | 6篇 |
1989年 | 8篇 |
1988年 | 8篇 |
1987年 | 3篇 |
1986年 | 8篇 |
1985年 | 7篇 |
1984年 | 3篇 |
1983年 | 5篇 |
1982年 | 4篇 |
1981年 | 3篇 |
1980年 | 4篇 |
1975年 | 2篇 |
1973年 | 1篇 |
排序方式: 共有709条查询结果,搜索用时 0 毫秒
581.
The impact of elevated atmospheric CO2 on belowground plant growth is poorly understood relative to its effects on aboveground growth. We carried out a study of the seasonal dynamics of gross root production and death to determine how elevated CO2 affected the dynamics of net and gross root production through a full growing season. We quantified gross root production and root loss from sequential, in situ images of fine roots of t Betula papyrifera in ambient (375 ppm.) and elevated (700 ppm) CO2 atmospheres from 2 weeks following germination through leaf senescence. We found that elevated CO2 led to increases in the magnitude of cumulative gross production (P) and cumulative gross loss (L) of roots. However, the effect of elevated CO2 on these processes was seasonally dependent. Elevated CO2 led to greater levels of enhancement in P early in the growing season, prior to maximum standing root length (NP). In contrast, elevated CO2 led to greater levels of enhancement in L in the last half of the growing season, after maximum NP had been reached. This difference in the timing of when elevated CO2 affects P and L led to a transitory, early enhancement in NP. By the end of the growing season, there was no significant effect of elevated CO2 on NP, and P was 87% greater than NP for ambient CO2 and 117% greater in elevated CO2. We conclude that static assessments of belowground productivity may greatly underestimate gross fine root productivity and turnover and this bias can be exaggerated with elevated CO2. 相似文献
582.
583.
Rainer Matyssek Madeleine S. Günthardt-Goerg Theodor Keller Christoph Scheidegger 《Trees - Structure and Function》1991,5(1):5-13
Summary Injury caused by low O3 concentrations (0, 0.05, 0.075, 0.1 l 1-1) was analyzed in the epidermis and mesophyll of fully developed birch leaves by gas exchange experiments and low-temperature SEM: (I) after leaf formation in O3-free and ozonated air, and (II) after transferring control plants into ozonated air. In control leaves, autumnal senescence also was studied in O3-free air (III). As O3 concentration increased, leaves of (I) stayed reduced in size, but showed increased specific weight and stomatal density. The declining photosynthetic capacity, quantum yield and carboxylation efficiency lowered the light saturation of CO2 uptake and the water-use efficiency (WUE). Carbon gain was less limited by the reduced stomatal conductance than by the declining ability of CO2 fixation in the mesophyll. The changes in gas exchange were related to the O3 dose and were mediated by narrowed stomatal pores (overriding the increase in stomatal density) and by progressive collapse of mesophyll cells. The air space in the mesophyll increased, preceded by exudate formation on cell walls. Ozonated leaves, which had developed in O3-free air (II), displayed a similar but more rapid decline than the leaves from (I). In senescent leaves (III), CO2 uptake showed a similar decrease as in leaves with O3 injury but no changes in mesophyll structure and WUE. The nitrogen concentration declined only in senescent leaves in parallel with the rate of CO2 uptake. A thorough understanding of O3 injury and natural senescence requires combined structural and functional analyses of leaves. 相似文献
584.
585.
Natural populations of woody perennials on lead-mining sites in the Mechernich area of the Eifel Mountains were investigated with respect to soil factors determining the degree and type of heavy metal tolerance. Salix caprea L. (Goat Willow) grew on soils with up to 17000 mg kg–1 total lead (ca. 4000 mg kg–1 ammonium acetate-exchangeable Pb). Betula pendula Roth (Silver Birch) was found on soils containing as much as 29000 mg kg–1 total lead (7000 mg kg–1 ammonium acetate-exchangeable Pb). Other woody perennials, with the exception of the dwarf shrub Calluna vulgaris, were not found in the contaminated area even though they did occur in the immediate vicinity. The two lead-tolerant tree species did not form mixed populations.Because of a significantly lower Pb/Ca ratio in Salix soils (2.2) compared with Betula soils (7.4), a calcium-dependent mechanism of lead tolerance is suggested for Salix, but not for Betula.The Betula population could be divided into two groups, each showing a highly significant correlation between root-lead content and exchangeable lead amounts in the soil, but with different levels of lead uptake. The only soil factor distinguishing the two groups was found to be the level of soluble phosphate. A distinctly low level of soil phosphate correlated with a high lead concentration in roots of the one group (30000 mg Pb kg–1 DW), whereas high phosphate amounts corresponded with a much lower lead concentration in roots of the other (12000 mg Pb kg–1 DW–1). Since the correlation between lead in the soil and in plants was similar for the two groups, it is concluded that the type of lead tolerance in Betula is determined by the status of plant phosphate nutrition, rather than by simple phosphate precipitation in the soil.A comparison of growth between different populations of Betula seedlings on homogenized soils from the mining area revealed the Mechaernich population to be a distinct ecotype with respect to lead tolerance. The control population obtained from a non-contaminated area exhibited a lower degree of lead tolerance coupled with a two-step strategy of adaptation to lead. 相似文献
586.
Anna Maria Nuutila Ulrika Kurtén Veli Kauppinen 《Plant Cell, Tissue and Organ Culture》1991,24(2):73-77
The effects of different concentrations of sucrose and inorganic nitrogen on somatic embryogenesis of birch (Betula pendula Roth.) callus cultures were studied using Box-Wilson statistical experimental design. The experimental results were processed by regression analysis, and mathermatical models describing the effects of the selected variables were derived. On the basis of the modeling, maximum embryo production can be achieved with 35 mM total inorganic nitrogen and 20.8 g 1-1 sucrose in the medium. 相似文献
587.
Despite predictions that both atmospheric CO2 concentrations and air temperature will rise together, very limited data are currently available to assess the possible interactive
effects of these two global change factors on temperate forest tree species. Using yellow birch (Betula alleghaniensis) as a model species, we studied how elevated CO2 (800 vs. 400 μl l−1) influences seedling growth and physiological responses to a 5°C increase in summer air temperatures (31/26 vs. 26/21°C day/night),
and how both elevated CO2 and air temperature during the growing season influence seedling ability to survive freezing stress during the winter dormant
season. Our results show that while increased temperature decreases seedling growth, temperature-induced growth reductions
are significantly lower at elevated CO2 concentrations (43% vs. 73%). The amelioration of high-temperature stress was related to CO2-induced reductions in both whole-shoot dark respiration and transpiration. Our results also show that increased summer air
temperature, and to a lesser degree CO2 concentration, make dormant winter buds less susceptible to freezing stress. We show the relevance of these results to models
used to predict how climate change will influence future forest species distribution and productivity, without considering
the direct or interactive effects of CO2.
Received: 5 June 1997 / Accepted: 16 December 1997 相似文献
588.
In many woody plants a short photoperiod triggers the onset of cold acclimation, but the nature of this process has remained obscure. We aimed to establish which physiological and genetic factors have a role in short-day-induced acclimation by comparing two types of birch, Betula pubescens Ehrh. and B. pubescens f. hibernifolia Ulv., the latter being unable to increase its abscisic acid (ABA) levels. In the wild type, short-day or natural autumn conditions in the field appeared to elevate the ABA levels before acclimation, which was accompanied by tissue desiccation, osmotic adjustments and accumulation of Group 2 LEA proteins [responsive to ABA (RAB) 16-like; 24, 30 and 33 kDa] and Group 4 LEA proteins [late embryogenesis abundant (LEA) 14-like; 19 kDa]. Under similar conditions the ABA-deficient birch showed reduced water loss, defective osmoregulation, absence of inducible Group 2 LEA proteins, and delayed or reduced tolerance to freezing. In contrast, both birch genotypes showed similar seasonal production patterns of Group 4 LEA proteins. Our results demonstrate that onset of cold acclimation in birch is based on multiple mechanisms, including molecular pathways that are typical of stress responses. ABA may be important for the accurate timing of cold acclimation in trees that are sensitive to photoperiod. 相似文献
589.
Physiological, stomatal and ultrastructural ozone responses in birch (Betula pendula Roth.) are modified by water stress 总被引:7,自引:3,他引:4
E. Pääkkönen J. Vahala M. Pohjola T. Holopainen & L. Kärenlampi 《Plant, cell & environment》1998,21(7):671-684
The physiological, stomatal and ultrastructural responses to ozone and drought of ozone-sensitive and more ozone-tolerant birch ( Betula pendula Roth.) clones were studied singly and in combination, in a high-stress chamber experiment and in a low-stress open-field experiment. In the chamber experiment, well watered (WW), moderately watered (MW) or drought-stressed (DS) saplings were exposed for 36 d to 0 or 130 nmol mol∠1 ozone. In the open-field experiment, well watered or drought-stressed saplings were grown for one growing season in ambient air or exposed to 1·8 × ambient ozone. Drought stress reduced growth rate, stomatal conductance, stomatal density and the proportion of starch and thylakoids in chloroplasts, but stimulated net photosynthesis, Rubisco and chlorophyll quantity at the end of the growing season, and increased the size and density of plastoglobuli. Ozone fumigations caused more variable, clone- and exposure-dependent responses in growth, decreased stomatal conductance and net photosynthesis, an increased number of stomata, visible and ultrastructural chloroplast injuries, and enhanced autumn yellowing of the leaves. Ozone-induced changes in plastoglobuli, starch and thylakoids resembled drought responses. The two experiments revealed that, depending on the experimental conditions and the variable, the response to drought and ozone stress can be independent, additive or interactive. Drought protected the plants from ozone injuries under high-stress conditions in the chamber experiment. In the low-stress, open-field experiment, however, enhanced ozone damage was observed in birch saplings grown under restricted water supply. 相似文献
590.
The content of natural (S, N, P, Ca, Mg, K, Mn, Zn, Cu) and extraneous elements (Pb, Cd, Al) in leaves of birch growing in
the Krusne hory Mts. (Czech Republic), a territory affected by air pollution (SO2), was evaluated with regard to the altitude and age of birch stands, and in relation to stand nutrition. The contents of
S, Ca, Mg, Zn and Pb culminated at the highest points of the altitudinal transect (elevation 900–1000 m). In contrast, at
high altitudes the content of Cd, Al and Mn decreased. The content of elements in leaves was mostly not dependent on tree
age. In terms of nutrition and growth of birch, the content of Cu was insufficient, the content of Ca, Mg, N, P and K was
optimum, and the content of S, Zn and primarily Mn was higher. The content of Pb and Al was at natural background levels,
that of Cd higher.
Received: 3 July 1997 / Accepted: 19 May 1998 相似文献