全文获取类型
收费全文 | 1776篇 |
免费 | 116篇 |
国内免费 | 188篇 |
出版年
2023年 | 26篇 |
2022年 | 34篇 |
2021年 | 41篇 |
2020年 | 57篇 |
2019年 | 56篇 |
2018年 | 56篇 |
2017年 | 43篇 |
2016年 | 52篇 |
2015年 | 58篇 |
2014年 | 128篇 |
2013年 | 121篇 |
2012年 | 79篇 |
2011年 | 85篇 |
2010年 | 71篇 |
2009年 | 73篇 |
2008年 | 73篇 |
2007年 | 104篇 |
2006年 | 64篇 |
2005年 | 56篇 |
2004年 | 66篇 |
2003年 | 61篇 |
2002年 | 59篇 |
2001年 | 55篇 |
2000年 | 38篇 |
1999年 | 47篇 |
1998年 | 41篇 |
1997年 | 38篇 |
1996年 | 24篇 |
1995年 | 23篇 |
1994年 | 23篇 |
1993年 | 22篇 |
1992年 | 23篇 |
1991年 | 23篇 |
1990年 | 15篇 |
1989年 | 24篇 |
1988年 | 21篇 |
1987年 | 12篇 |
1986年 | 9篇 |
1985年 | 18篇 |
1984年 | 21篇 |
1983年 | 15篇 |
1982年 | 17篇 |
1981年 | 26篇 |
1980年 | 16篇 |
1979年 | 8篇 |
1978年 | 9篇 |
1976年 | 8篇 |
1975年 | 8篇 |
1974年 | 12篇 |
1973年 | 11篇 |
排序方式: 共有2080条查询结果,搜索用时 906 毫秒
21.
22.
23.
Elly P. H. Best 《Physiologia plantarum》1986,68(3):502-510
The photosynthetic and growth characteristics of Ceratophyllum demersum L. were investigated under laboratory conditions which simulated those encountered in the plants' normal environment. The carbon fixation rate of C. demersum measured with 14 C at light and carbon saturation at pH 8.0 was 4.48 mg C (g ash-free dry weight)−1 h−1 . It was lower at pH 6.5 than at pH 8.0. The light use efficiencies in quiescent plants and actively growing plants were 6.3 and 8.7 × 10−9 kg CO2 J−1 , respectively, with corresponding maximum photosynthetic rates of 2.67 and 4.36 mg C (g ash-free dry weight)−1 h−1 . Photorespiration in actively growing plants consumed 24% of the carbon fixed. Incubation with DCMU demonstrated that about one-third was refixed. The optimum temperature for carbon fixation was 25°C. The C3 -photosynthetic pathway was the main operational route as indicated by the early photosynthetic products (largely C3 -acids) and the absence of Krantz anatomy and the chlorophyll a:b ratio (2.7). The maximum relative growth rates ranged from 0.025 to 0.041 g ash-free dry weight (g ash-free dry weight)−1 day−1 in the field (Lake Vechten, 1 to 3 m depth classes). 相似文献
24.
25.
Effects of irradiation level on leaf growth of sunflower 总被引:1,自引:0,他引:1
George A. Pieters 《Physiologia plantarum》1985,65(3):263-268
Sunflower, Helianthus annuus L. cv. INRA 6501, plants were grown in a gravel culture subirrigated with Hoagland nutrient solution, at photosynthetically active radiation levels of 15, 30 and 60 W m-2 at a daylength of 16 h, a temperature of 20°C and a relative humidity of 60% throughout. Development of the plant and growth of the leaves were measured. High irradiance accelerated development proportionally in all phases from germination, through leaf initiation, primordial flower formation and the maturation of all plant organs until anthesis. High irradiance levels stimulated the expansion of the growing shoot, which produced more and larger primordia. Under constant conditions the ratio between leaf initiation rate and mature length of a leaf remained constant, although the growth patterns [relationship between relative growth rate (RGR) and organ age] of successive leaves were not similar. Consequently, it may be assumed that, as in poplar, the increasing size of the growing shoot reflects the increase of the vascular system of sunflower. The growth patterns of the leaves depend on the developmental stage of the plant and, in the young primordial stage, also on irradiance level. In the linear phase of growth the growth pattern is independent of irradiance level. 相似文献
26.
Two species of jute plants Corchorus capsularis L. (cv. JRC 212) and C. olitorius L. (cv. JRO 632) were subjected to water stress for 2 and 4 days by withholding water. The relative water content (RWC) decreased in both plants under water stress but to a greater extent in C. olitorius . The C. olitorius seedlings also showed greater membrane injury than C. capsularis seedlings under water stress as was evident from injury index data. Water stress increased glycolate oxidase (EC 1.1.3.1.) activity more in C. olitorius than in C. capsularis . The activity of superoxide dismutase (SOD, EC 1.15.1.1.) and catalase (EC 1.11.1.6.) decreased under water stress and their decrease was higher in C. olitorius than in C capsularis . The level of hydrogen peroxide and lipid peroxidation also increased in both plants under water stress and the increase was higher in C. olitorius than in C. capsularis seedlings. Under comparable external water stress, C. capsularis seedlings showed lower membrane damage, lower H2 O2 accumulation and lower lipid peroxidation than C. olilorius which may be taken as indicative of higher water stress tolerance capacity of the former. 相似文献
27.
Nutrient requirements and stress response of Populus simonii and Paulownia tomentosa 总被引:8,自引:0,他引:8
The aim of this investigation was to estimate the optimum nutrient requirements and responses to low relative nutrient addition rates of seedlings of two important broadleaf tree species in China, Populus simonii Carr. and Paulownia tomentosa (Thunb.) Steud. In preliminary experiments the optimum nutrient proportions were estimated under high concentration conditions. The nutrients consumed were replaced by means of daily additions determined by pH and conductivity titrations without changing the nutrient solutions. A relatively high K level was needed in relation to nitrogen; higher than in birch or grey alder seedlings. To obtain a high relative growth rate, suitable proportions by weight were 100 N:70 K:14 P:7 Ca:7 Mg for the Populus seedlings and 100 N:75 K:20 P:8 Ca:9 Mg for the Paulownia seedlings.
In studies of nutrient stress responses the relative nutrient addition rate was used as the treatment variable under low conductivity conditions. The responses and relationships were similar to those described for birch, grey alder and Salix . The relative addition rate, and there was also a strong linear regression between relative growth rate and nitrogen status. Relative growth rates were high and the maximum weight increase was about 19% day−1 in Populus and over 25% day−1 in Paulownia . The nitrogen productivity of Paulownia was very high, 0.26 g dry weight (g N)−1 h−1 , and for Populus it was 0.16 g dry weight (g N)−1 h−1 . 相似文献
In studies of nutrient stress responses the relative nutrient addition rate was used as the treatment variable under low conductivity conditions. The responses and relationships were similar to those described for birch, grey alder and Salix . The relative addition rate, and there was also a strong linear regression between relative growth rate and nitrogen status. Relative growth rates were high and the maximum weight increase was about 19% day
28.
The conformation in solution of derivatives of methyl hexopyranosides has been studied by n.m.r. The esters of methyl 2,3-O-isopropylidene-α-D-manno- and -talopyranosides as well as their 4-deoxy-4-C-methyl analog having a manno configuration exist mainly in a flattened (4,0F) chair conformation (4C1). The presence in the talo epimer of the 4-deoxy-4-C-methyl analog of the bulky methyl group on the endo side of the bicyclic system results in a skew form (3S1). The methyl 4-deoxy-2,3-O-isopropylidene-4-C-methylene-α-D-lyxo-hexopyranosides monosubstituted at C-4′ adopt, in solution, a conformation close to 3S1, whichever their configuration (cis or trans) at the double bond, as indicated by their allylic coupling constants. 相似文献
29.
Growth of Pyrola rotundifolia ssp. maritima in relation to shade 总被引:2,自引:2,他引:0
30.
Y. I. Mladenova 《Plant and Soil》1990,123(2):217-222
Short-term studies for comparing some primary metabolic and growth-responses to salt stress in seedlings of two maize genotypes differing in drought resistance were carried out under controlled conditions. Both genotypes revealed high yielding ability in favourable environments. Treatments: Control (Hoagland-Arnon No 1 solution) and salt stress (Hoagland-Arnon solution plus NaCl, s = –0.84MPa). It was found that in both genotypes the activity of the principal metabolic pathway supplying reduced nitrogen (15N) for the synthesis of amino acids and proteins as well as the assimulatory number (14CO2—assimilation relation rate per chlorophyll unit) were decreased under the effect of the stress. These effects were more marked in the resistant genotype. In this genotype the stress induced metabolic activity decline was accompanied by a corresonding reduction of the relative growth rate. Conversely, continuing growth, resulting probably from accumulation of solutes, was observed in the susceptible genotype.On the basis of these and other observations it is assumed that the resistant genotype manifests short-term energy saving stress reactions. 相似文献