全文获取类型
收费全文 | 5193篇 |
免费 | 332篇 |
国内免费 | 473篇 |
学科分类
生物科学 | 5998篇 |
出版年
2024年 | 8篇 |
2023年 | 50篇 |
2022年 | 50篇 |
2021年 | 79篇 |
2020年 | 142篇 |
2019年 | 125篇 |
2018年 | 101篇 |
2017年 | 123篇 |
2016年 | 127篇 |
2015年 | 152篇 |
2014年 | 163篇 |
2013年 | 202篇 |
2012年 | 138篇 |
2011年 | 160篇 |
2010年 | 122篇 |
2009年 | 183篇 |
2008年 | 222篇 |
2007年 | 218篇 |
2006年 | 178篇 |
2005年 | 190篇 |
2004年 | 197篇 |
2003年 | 223篇 |
2002年 | 220篇 |
2001年 | 183篇 |
2000年 | 165篇 |
1999年 | 151篇 |
1998年 | 171篇 |
1997年 | 159篇 |
1996年 | 175篇 |
1995年 | 142篇 |
1994年 | 142篇 |
1993年 | 132篇 |
1992年 | 140篇 |
1991年 | 111篇 |
1990年 | 133篇 |
1989年 | 108篇 |
1988年 | 91篇 |
1987年 | 97篇 |
1986年 | 81篇 |
1985年 | 83篇 |
1984年 | 70篇 |
1983年 | 63篇 |
1982年 | 69篇 |
1981年 | 51篇 |
1980年 | 39篇 |
1979年 | 26篇 |
1978年 | 14篇 |
1977年 | 9篇 |
1976年 | 9篇 |
1975年 | 8篇 |
排序方式: 共有5998条查询结果,搜索用时 174 毫秒
991.
FOTEINI HASSIOTOU JOHN R. EVANS MARTHA LUDWIG & ERIK J. VENEKLAAS 《Plant, cell & environment》2009,32(11):1596-1611
In some plants, stomata are exclusively located in epidermal depressions called crypts. It has been argued that crypts function to reduce transpiration; however, the occurrence of crypts in species from both arid and wet environments suggests that crypts may play another role. The genus Banksia was chosen to examine quantitative relationships between crypt morphology and leaf structural and physiological traits to gain insight into the functional significance of crypts. Crypt resistance to water vapour and CO2 diffusion was calculated by treating crypts as an additional boundary layer partially covering one leaf surface. Gas exchange measurements of polypropylene meshes confirmed the validity of this approach. Stomatal resistance was calculated as leaf resistance minus calculated crypt resistance. Stomata contributed significantly more than crypts to leaf resistance. Crypt depth increased and accounted for an increasing proportion of leaf resistance in species with greater leaf thickness and leaf dry mass per area. All Banksia species examined with leaves thicker than 0.6 mm had their stomata in deep crypts. We propose that crypts function to facilitate CO2 diffusion from the abaxial surface to adaxial palisade cells in thick leaves. This and other possible functions of stomatal crypts, including a role in water use, are discussed. 相似文献
992.
ANTHONY R. AMBROSE STEPHEN C. SILLETT & TODD E. DAWSON 《Plant, cell & environment》2009,32(7):743-757
We examined changes in branch hydraulic, leaf structure and gas exchange properties in coast redwood ( Sequoia sempervirens ) and giant sequoia ( Sequoiadendron giganteum ) trees of different sizes. Leaf-specific hydraulic conductivity ( k L ) increased with height in S. sempervirens but not in S. giganteum , while xylem cavitation resistance increased with height in both species. Despite hydraulic adjustments, leaf mass per unit area (LMA) and leaf carbon isotope ratios ( δ 13 C) increased, and maximum mass-based stomatal conductance ( g mass ) and photosynthesis ( A mass ) decreased with height in both species. As a result, both A mass and g mass were negatively correlated with branch hydraulic properties in S. sempervirens and uncorrelated in S. giganteum . In addition, A mass and g mass were negatively correlated with LMA in both species, which we attributed to the effects of decreasing leaf internal CO2 conductance ( g i ). Species-level differences in wood density, LMA and area-based gas exchange capacity constrained other structural and physiological properties, with S. sempervirens exhibiting increased branch water transport efficiency and S. giganteum exhibiting increased leaf-level water-use efficiency with increasing height. Our results reveal different adaptive strategies for the two redwoods that help them compensate for constraints associated with growing taller, and reflect contrasting environmental conditions each species faces in its native habitat. 相似文献
993.
FEDERICO BRILLI PAOLO CICCIOLI MASSIMILIANO FRATTONI MARCO PRESTININZI ANTONIO FRANCO SPANEDDA & FRANCESCO LORETO 《Plant, cell & environment》2009,32(5):542-552
Chrysomela populi beetles feed on poplar leaves and extensively damage plantations. We investigated whether olfactory cues orientate landing and feeding. Young, unexpanded leaves of hybrid poplar emit constitutively a blend of monoterpenes, primarily ( E )- β -ocimene and linalool. This blend attracts inexperienced adults of C. populi that were not previously fed with poplar leaves. In mature leaves constitutively emitting isoprene, insect attack induces biosynthesis and emission of the same blend of monoterpenes, but in larger amount than in young leaves. The olfactometric test indicates that inexperienced beetles are more attracted by adult than by young attacked leaves, suggesting that attraction by induced monoterpenes is dose dependent. The blend does not attract adults that previously fed on poplar leaves. Insect-induced emission of monoterpenes peaks 4 d after the attack, and is also detected in non-attacked leaves. Induced monoterpene emission is associated in mature leaves with a larger decrease of isoprene emission. The reduction of isoprene emission is faster than photosynthesis reduction in attacked leaves, and also occurs in non-attacked leaves. Insect-induced monoterpenes are quickly and completely labelled by 13 C. It is speculated that photosynthetic carbon preferentially allocated to constitutive isoprene in healthy leaves is in part diverted to induced monoterpenes after the insect attack. 相似文献
994.
Lars Olof Björn George C. Papageorgiou Robert E. Blankenship Govindjee 《Photosynthesis research》2009,99(2):85-98
Chlorophyll a (Chl a) serves a dual role in oxygenic photosynthesis: in light harvesting as well as in converting energy of absorbed photons to
chemical energy. No other Chl is as omnipresent in oxygenic photosynthesis as is Chl a, and this is particularly true if we include Chl a
2, (=[8-vinyl]-Chl a), which occurs in Prochlorococcus, as a type of Chl a. One exception to this near universal pattern is Chl d, which is found in some cyanobacteria that live in filtered light that is enriched in wavelengths >700 nm. They trap the
long wavelength electronic excitation, and convert it into chemical energy. In this Viewpoint, we have traced the possible reasons for the near ubiquity of Chl a for its use in the primary photochemistry of Photosystem II (PS II) that leads to water oxidation and of Photosystem I (PS
I) that leads to ferredoxin reduction. Chl a appears to be unique and irreplaceable, particularly if global scale oxygenic photosynthesis is considered. Its uniqueness
is determined by its physicochemical properties, but there is more. Other contributing factors include specially tailored
protein environments, and functional compatibility with neighboring electron transporting cofactors. Thus, the same molecule, Chl a in vivo, is capable of generating a radical cation at +1 V or higher (in PS II), a radical anion at −1 V or lower (in PS
I), or of being completely redox silent (in antenna holochromes).
相似文献
Govindjee (Corresponding author)Email: |
995.
996.
We investigated seasonal changes in dry mass and CO2 exchange rate in fruit and leaves of the evergreen tree Cinnamomum camphora with the aim of quantitatively determining the translocation balance between the two organs. The fruit dry mass growth peaked
in both August and October: the first increase was due to fruit pulp development and the second to seed development. Fruit
respiration also increased with the rapid increase in fruit dry mass. Therefore, the carbohydrates required for fruit development
showed two peaks during the reproductive period. Fruit photosynthesis was relatively high in early August, when fruit potentially
re-fixed 75% of respired CO2, indicating that fruit photosynthesis contributed 15–35% of the carbon requirement for fruit respiration. Current-year leaves
completed their growth in June when fruit growth began. Current-year leaves translocated carbohydrates at a rate of approximately
10–25 mg dry weight (dw) leaf−1 day−1 into other organs throughout the entire fruit growth period. This rate of translocation from current-year leaves was much
higher than the amount of carbohydrate required for reproduction (ca. 3 mg dw fruit−1 day−1). Given the carbon balance between fruit and current-year leaves, carbohydrates for reproduction were produced within the
current-year fruit-bearing shoots. C. camphora would be adaptive for steadily supplying enough amount of carbohydrate to the fruits, as there was little competition for
carbohydrates between the two organs. As assimilates by leaves are used for processes such as reproduction and the formation
of new shoots, photosynthesis by reproductive organs is considered to be important to compensate for reproductive cost. 相似文献
997.
Minna Lintala Yagut Allahverdiyeva Saijaliisa Kangasjärvi Nina Lehtimäki Mika Keränen Eevi Rintamäki Eva-Mari Aro Paula Mulo 《The Plant journal : for cell and molecular biology》2009,57(6):1103-1115
Physiological roles of the two distinct chloroplast-targeted ferredoxin-NADP+ oxidoreductase (FNR) isoforms in Arabidopsis thaliana were studied using T-DNA insertion line fnr1 and RNAi line fnr2 . In fnr2 FNR1 was present both as a thylakoid membrane-bound form and as a soluble protein, whereas in fnr1 the FNR2 protein existed solely in soluble form in the stroma. The fnr2 plants resembled fnr1 in having downregulated photosynthetic properties, expressed as low chlorophyll content, low accumulation of photosynthetic thylakoid proteins and reduced carbon fixation rate when compared with wild type (WT). Under standard growth conditions the level of F0 'rise' and the amplitude of the thermoluminescence afterglow (AG) band, shown to correlate with cyclic electron transfer (CET), were reduced in both fnr mutants. In contrast, when plants were grown under low temperatures, both fnr mutants showed an enhanced rate of CET when compared with the WT. These data exclude the possibility that distinct FNR isoforms feed electrons to specific CET pathways. Nevertheless, the fnr2 mutants had a distinct phenotype upon growth at low temperature. The fnr2 plants grown at low temperature were more tolerant against methyl viologen (MV)-induced cell death than fnr1 and WT. The unique tolerance of fnr2 plants grown at low temperature to oxidative stress correlated with an increased level of reduced ascorbate and reactive oxygen species (ROS) scavenging enzymes, as well as with a scarcity in the accumulation of thylakoid membrane protein complexes, as compared with fnr1 and WT. These results emphasize a critical role for FNR2 in the redistribution of electrons to various reducing pathways, upon conditions that modify the photosynthetic capacity of the plant. 相似文献
998.
运用pH-drift的方法研究了在不同碱度条件下中华水韭(Isoetes sinensis)的沉水叶片昼夜CO2吸收的特征。结果表明中华水韭的沉水叶片具有昼夜吸收水中CO2的能力, 而不具备利用水中的HCO 3–的能力, 进一步证明了水生植物中华水韭的光合碳同化途径具有景天酸代谢(CAM)的特征。中华水韭沉水叶片光照条件下对水中CO2的吸收速率在一定的浓度范围内正相关于水中的CO2浓度。光照条件下, 中华水韭的pH-drift实验的pH补偿点分别为(8.1±0.3)和(7.9±0.1) mmol·L–1, 最终[CT]/Alk值为(1.009±0.01)和(1.022±0.004)。碱度对中华水韭夜晚CO2的吸收速率有显著的影响(F = 38.73, p < 0.000 1)。总碱度1.70 mmol·L–1溶液中的中华水韭沉水叶片在相对较低的CO2浓度(0.04±0.001 mmol·L–1)水平下即表现出对CO2的净吸收。调查了野外一处中华水韭沉水种群的生境pH值及CO2浓度的昼夜变化, 发现水体碱度约为1.59 mmol·L–1, 一昼夜的pH值波动不大, 平均为(6.1±0.04), 昼夜CO2浓度存在波动, 午夜水中的CO2浓度是午后的近3倍。 相似文献
999.
为了比较C4荒漠植物猪毛菜(Salsola collina)和木本猪毛菜(S. arbuscula)的抗旱结构和适应环境的光合作用特征, 在二者混生的群落中, 选择代表性植株, 采集叶片进行叶片解剖结构分析, 在自然条件下测定了二者叶片的气体交换参数。研究结果表明:猪毛菜叶片具表皮毛, 具有更发达的薄壁贮水组织;木本猪毛菜叶片具有更厚的角质层, 表皮下有1层下皮细胞, 其栅栏组织细胞较长, 排列更紧密。猪毛菜的净光合速率明显高于木本猪毛菜, 日平均值分别为21.5和15.7 μmol CO2·m–2·s–1。猪毛菜的蒸腾速率也明显高于木本猪毛菜, 日平均值分别为14.9和10.2 mmol·m–2·s–1。猪毛菜和木本猪毛菜的水分利用效率的日平均值分别为1.39和1.53 μmol CO2·mmol–1 H2O, 特别是在14:00时分别为1.61和2.30 μmol CO2·mmol–1 H2O, 木本猪毛菜高出猪毛菜约42%。猪毛菜的光补偿点低于木本猪毛菜, 而光饱和点和光量子效率较高, 具有更低的CO2补偿点。这表明:二者的旱生结构不同, 木本猪毛菜具有更显著的荒漠植物特征;在适于二者混生的环境下, 猪毛菜比木本猪毛菜的光合能力更强, 而木本猪毛菜的水分利用效率更高。 相似文献
1000.