全文获取类型
收费全文 | 1665篇 |
免费 | 87篇 |
国内免费 | 71篇 |
出版年
2023年 | 21篇 |
2022年 | 17篇 |
2021年 | 28篇 |
2020年 | 45篇 |
2019年 | 34篇 |
2018年 | 46篇 |
2017年 | 34篇 |
2016年 | 46篇 |
2015年 | 50篇 |
2014年 | 69篇 |
2013年 | 84篇 |
2012年 | 40篇 |
2011年 | 66篇 |
2010年 | 57篇 |
2009年 | 101篇 |
2008年 | 96篇 |
2007年 | 112篇 |
2006年 | 101篇 |
2005年 | 57篇 |
2004年 | 67篇 |
2003年 | 71篇 |
2002年 | 41篇 |
2001年 | 34篇 |
2000年 | 34篇 |
1999年 | 53篇 |
1998年 | 47篇 |
1997年 | 35篇 |
1996年 | 24篇 |
1995年 | 33篇 |
1994年 | 29篇 |
1993年 | 38篇 |
1992年 | 27篇 |
1991年 | 23篇 |
1990年 | 21篇 |
1989年 | 18篇 |
1988年 | 10篇 |
1987年 | 9篇 |
1986年 | 9篇 |
1985年 | 10篇 |
1984年 | 15篇 |
1983年 | 5篇 |
1982年 | 14篇 |
1981年 | 12篇 |
1980年 | 6篇 |
1979年 | 6篇 |
1978年 | 12篇 |
1977年 | 4篇 |
1976年 | 5篇 |
1972年 | 2篇 |
1966年 | 1篇 |
排序方式: 共有1823条查询结果,搜索用时 15 毫秒
231.
Photoinhibition of photosynthesis at low temperatures was investigated in two species of subalpine eucalypt, Eucalypts nitens (Deane and Maiden) Maiden and E. pauciflora Sieb. ex Spreng. Imposition of an artificial cold-hardening treatment increased the frost tolerance of leaf tissue and increased
tolerance to excess light. Cold-hardened seedlings of both species had a higher photosynthetic capacity than non-hardened
seedlings at 6 and 16°C and lower levels of non-photochemical quenching (NPQ) at 20 and 5°C. Furthermore, hardened seedlings
had faster rates of NPQ development at 5 and −3.5°C. An increase in minimal fluorescence, which indicates slowly reversible
photoinhibition, was evident in all seedlings at −1.5 and −3.5°C but was less pronounced in hardened seedlings, with a threefold
faster rate of development of NPQ, at −3.5°C than non-hardened seedlings. Hardened seedlings also recovered faster from photoinhibition
at −3.5°C. Thus cold hardening increased tolerance to high light in these species. Differences between E. nitens and E. pauciflora in their response to excess light were small and significant only at −3.5°C. Faster recovery from photoinhibition of E. pauciflora was consistent with its occurrence in colder habitats than E. nitens.
Received: 27 April 1997 / Accepted: 9 September 1997 相似文献
232.
M. Sandsund M. Sue-Chu J. Helgerud R. E. Reinertsen L. Bjermer 《European journal of applied physiology and occupational physiology》1998,77(4):297-304
The effects of whole-body exposure to ambient temperatures of −15°C and 23°C on selected performance-related physiological
variables were investigated in elite nonasthmatic cross-country skiers. At an ambient temperature of −15°C we also studied
the effects of the selective β2-adrenergic agonist Salbutamol (0.4 mg × 3) which was administered 10 min before the exercise test. Eight male cross-country
skiers with known maximal oxygen uptakes (V˙O2
max
) of more than 70 ml · kg−1 · min−1 participated in the study. Oxygen uptake (V˙O2), heart rate (f
c), blood lactate concentration ([La−]b) and time to exhaustion were measured during controlled submaximal and maximal running on a treadmill in a climatic chamber.
Lung function measured as forced expiratory volume in 1 s (FEV1) was recorded immediately before the warm-up period and at the conclusion of the exercise protocol. Submaximal V˙O2 and [La−]b at the two highest submaximal exercise intensities were significantly higher at −15°C than at 23°C. Time to exhaustion was
significantly shorter in the cold environment. However, no differences in V˙O2
max
or f
c were observed. Our results would suggest that exercise stress is higher at submaximal exercise intensities in a cold environment
and support the contention that aerobic capacity is not altered by cold exposure. Furthermore, we found that after Salbutamol
inhalation FEV1 was significantly higher than after placebo administration. However, the inhaled β2-agonist Salbutamol did not influence submaximal and maximal V˙O2, f
c, [La−]b or time to exhaustion in the elite, nonasthmatic cross-country skiers we studied. Thus, these results did not demonstrate
any ergogenic effect of the β2-agonist used.
Accepted: 18 August 1997 相似文献
233.
Sirkka Rissanen Hannu Rintam?ki 《European journal of applied physiology and occupational physiology》1998,78(6):560-564
The development of thermophysiological responses during four consecutive exercise/rest sessions in the cold was studied in
men wearing chemical protective clothing and a face mask. Six men repeated four exercise/rest sessions during 8 h at –10°C.
Each session consisted of step exercise (240 W · m−2) for 60 min and rest for another 60 min. Rectal and skin temperatures were measured continuously and thermal sensations were
obtained at 30-min intervals. Entering the cold from a warm environment and the onset of exercise resulted in a decrease in
skin temperatures during the first session and the decrement in the temperatures of the extremities continued for 10–20 min
during the following period of exercise. Torso skin temperature was at its lowest during the first rest period. After the
first session of cold exposure the range and the level of variation in mean body temperature (Tˉ
b) followed a pattern which was repeated until the end of the experiment. However, the torso skin temperatures increased gradually
until the fourth session, while the temperatures of the extremities, in contrast, tended to decrease up to the third session.
In conclusion, the present results indicated that although Tˉ
b, reflecting the whole body heat balance, showed a typical pattern of change after the first session (2 h), the torso area
was warming until the end of the cold exposure while the extremities continued to cool down up to the third session (6 h),
obviously due to a prolonged redistribution of the circulation.
Accepted: 29 May 1998 相似文献
234.
《Cryobiology》2018
We have developed a novel, intraluminal preservation solution that is tailored to the metabolic requirements of the intestine. This organ-specific solution addresses many of the problems associated with low temperature organ storage including energy, oxidative and osmotic stresses. However, conservation of energy levels remains one of the most difficult obstacles to overcome due to the inherent sensitivity of the mucosa to ischemia. Creatine-loading has become a popular and scientifically proven method of augmenting energy reserves in athletes performing anaerobic burst work activities. We hypothesized that if we could develop a method that was able to augment cellular energy levels, the structure and function of the mucosa would be more effectively preserved. The purpose of this study was to determine if creatine-loading is a feasible and effective strategy for preserving the intestine.Our data indicate that creatine loading has significant impact on energy levels during storage with corresponding improvements in mucosal structure and function. Both of our rodent models, a) continuous perfusion for 4 h and b) a single flush with our intraluminal preservation solution supplemented with 50 mM creatine, demonstrated significant improvements in creatine phosphate, ATP, Energy Charge and ATP/AMP following cold storage (P < 0.05). Notably, after 10 h creatine phosphate was 324% greater in Creatine-treated tissues compared to Controls (P < 0.05). Preferential utilization of glutathione in the Creatine group was effective at controlling oxidative injury after 10 h storage (P < 0.05). Improvements in barrier function and electrophysiology with creatine-treatment reflected superior mucosal integrity after 10 h storage; Permeability and Transepithelial resistance measurements remained at fresh tissue values. This was in stark contrast to Control tissues in which permeability rose to >300% of fresh tissue values (P < 0.005) and transepithelial resistance dropped by 95% (P < 0.005). After 10 h storage, Park's grading of histologic injury reflected extensive villus denudation (grade 4) in control tissues compared to healthy tissue (grade 0) in the Creatine group. This study demonstrates that a strategy of creatine supplementation of our intraluminal preservation solution facilitates the preservation of the intestinal mucosa during storage. 相似文献
235.
Shuling He Kentian Zhao Lingfa Ma Jingjun Yang Yuwei Chang Muhammad Aqeel Ashraf 《Saudi Journal of Biological Sciences》2016,23(2):198-204
To discuss the cold resistance performance of different Herba Rhodiolae and successfully transplant Herba Rhodiolae to the Gansu plateau area for nursing, domestication and planting, this paper systematically studies six physiological and biochemical features of Rhodiola kirilowii, Rhodiola algida, Rhodiola crenulata and Herba Rhodiolae that are closely associated with cold resistance features and concludes with the cold resistance capability of Rhodiola kirilowii. In the selected six main indexes of the Herba Rhodiolae, the POD, SOD and CAT activity and MDA and Pro content in the leaf are the main physiological and biochemical indexes to indicate the cold resistance performance of four Herba Rhodiolae seedlings and can be regarded as the preliminary indexes to assess the winter performance of Herba Rhodiolae. The research work will provide the theoretical basis for the wild variants of Herba Rhodiolae and GAPJ base construction. 相似文献
236.
Sandra Goutte Alain Dubois Samuel D. Howard Rafael Marquez Jodi J. L. Rowley J. Maximilian Dehling Philippe Grandcolas Xiong Rongchuan Frédéric Legendre 《Evolution; international journal of organic evolution》2016,70(4):811-826
Although acoustic signals are important for communication in many taxa, signal propagation is affected by environmental properties. Strong environmental constraints should drive call evolution, favoring signals with greater transmission distance and content integrity in a given calling habitat. Yet, few empirical studies have verified this prediction, possibly due to a shortcoming in habitat characterization, which is often too broad. Here we assess the potential impact of environmental constraints on the evolution of advertisement call in four groups of torrent‐dwelling frogs in the family Ranidae. We reconstruct the evolution of calling site preferences, both broadly categorized and at a finer scale, onto a phylogenetic tree for 148 species with five markers (~3600 bp). We test models of evolution for six call traits for 79 species with regard to the reconstructed history of calling site preferences and estimate their ancestral states. We find that in spite of existing morphological constraints, vocalizations of torrent‐dwelling species are most probably constrained by the acoustic specificities of torrent habitats and particularly their high level of ambient noise. We also show that a fine‐scale characterization of calling sites allows a better perception of the impact of environmental constraints on call evolution. 相似文献
237.
Sitodiplosis mosellana Géhin, one of the most important pests of wheat, undergoes obligatory diapause as a larva to survive unfavorable temperature extremes during hot summers and cold winters. To explore the potential roles of heat shock proteins (hsp) in this process, we cloned full-length cDNAs of hsp70, hsc70 and hsp90 from S. mosellana larvae, and examined their expression in response to diapause and short-term temperature stresses. Three hsps included all signature sequences of corresponding protein family and EEVD motifs. They showed high homology to their counterparts in other species, and the phylogenetic analysis of hsp90 was consistent with the known classification of insects. Expression of hsp70 and hsp90 were highly induced by diapause, particularly pronounced during summer and winter. Interestingly, hsp70 was more strongly expressed in summer than in winter whereas hsp90 displayed the opposite pattern. Abundance of hsc70 mRNA was comparable prior to and during diapauses and was highly up-regulated when insects began to enter the stage of post-diapause quiescence. Heat-stressed over-summering larvae (⩾30 °C) or cold-stressed over-wintering larvae (⩽0 °C) could further elevate expression of these three genes, but temperature extremes i.e. as high as 45 °C or as low as −15 °C failed to trigger such expression patterns. Notably, hsp70 was most sensitive to heat stress and hsp90 was most sensitive to cold stress. These results suggested that hsp70 and hsp90 play key roles in diapause maintenance and thermal stress; the former may be more prominent contributor to heat tolerance and the latter for cold tolerance. In contrast, hsc70 most likely is involved in developmental transition from diapause to post-diapause quiescence, and thus may serve as a molecular marker to predict diapause termination. 相似文献
238.
239.
The addition of 0.5 mM salicylic acid (SA) to the hydroponic growth solution of young maize (Zea mays L.) plants under normal growth conditions provided protection against subsequent low-temperature stress. This observation
was confirmed by chlorophyll fluorescence parameters and electrolyte leakage measurements. In addition, 1 d of 0.5 mM SA pre-treatment
decreased net photosynthesis, stomatal conductivity and transpiration at the growth temperature (22/20 °C). Since there was
only a slight decrease in the ratio of variable to maximal fluorescence (Fv/Fm) the decrease in photosynthetic activity is not due to a depression in photosystem II. The analysis of antioxidant enzymes
showed that whereas SA treatment did not cause any change in ascorbate peroxidase (EC 1.11.1.11) and superoxide dismutase
(EC 1.15.1.1) activities, there was a decrease in catalase (EC 1.11.1.6) activity, and an increase in guaiacol peroxidase
(EC 1.11.1.7) and glutathione reductase (EC 1.6.4.2) activities after the 1-d SA treatment at 22/20 °C. In native polyacrylamide
gels there was, among the peroxidase isoenzymes, a band which could be seen only in SA-treated plants. It is suggested that
the pre-treatment of maize plants with SA at normal growth temperature may induce antioxidant enzymes which lead to increased
chilling tolerance.
Received: 4 June 1998 / Accepted: 23 November 1998 相似文献
240.
A 25-kDa dehydrin associated with genotype- and age-dependent leaf freezing-tolerance in Rhododendron: a genetic marker for cold hardiness? 总被引:3,自引:0,他引:3
C. C. Lim S. L. Krebs R. Arora 《TAG. Theoretical and applied genetics. Theoretische und angewandte Genetik》1999,99(5):912-920
Dehydrins are plant proteins that may play a critical role in stabilizing cell functions during freezing and other dehydrative
stresses. This study examines whether dehydrin expression in leaves is associated with varying levels of freezing-tolerance
among F2 segregants, species, and cultivars of evergreen Rhododendron. Experiments were also conducted to determine whether physiological and chronological aging affects freezing-tolerance and
dehydrin accumulation in Rhododendron leaf tissues. Our results indicate that in cold-acclimated F2 populations, levels of a 25-kDa dehydrin were closely associated with differences in leaf freezing-tolerance (LFT) among
segregants. Studies of wild and cultivated plants indicated that LFT increased with both chronological age and developmental
phase-change (juvenile to mature plants) and that this trend was accompanied by increased accumulation of the 25-kDa dehydrin.
It is suggested that presence or absence of the 25-kDa dehydrin could serve as a genetic marker to distinguish between super
cold-hardy and less cold-hardy rhododendron genotypes. Similarly, the relative level of this protein within a genotype can
serve as a physiological indicator of freezing-tolerance status under a range of phenological (acclimation) or developmental
(age) conditions.
Received: 5 March 1999 / Accepted: 12 May 1999 相似文献