收费全文 | 110739篇 |
免费 | 8526篇 |
国内免费 | 6923篇 |
2024年 | 140篇 |
2023年 | 1283篇 |
2022年 | 3226篇 |
2021年 | 5464篇 |
2020年 | 3583篇 |
2019年 | 4400篇 |
2018年 | 4367篇 |
2017年 | 3252篇 |
2016年 | 4616篇 |
2015年 | 6715篇 |
2014年 | 7898篇 |
2013年 | 8348篇 |
2012年 | 10013篇 |
2011年 | 8918篇 |
2010年 | 5470篇 |
2009年 | 4770篇 |
2008年 | 5610篇 |
2007年 | 4945篇 |
2006年 | 4389篇 |
2005年 | 3354篇 |
2004年 | 2947篇 |
2003年 | 2545篇 |
2002年 | 2216篇 |
2001年 | 2006篇 |
2000年 | 1860篇 |
1999年 | 1846篇 |
1998年 | 1020篇 |
1997年 | 1143篇 |
1996年 | 1023篇 |
1995年 | 920篇 |
1994年 | 944篇 |
1993年 | 668篇 |
1992年 | 994篇 |
1991年 | 838篇 |
1990年 | 613篇 |
1989年 | 559篇 |
1988年 | 485篇 |
1987年 | 411篇 |
1986年 | 388篇 |
1985年 | 390篇 |
1984年 | 211篇 |
1983年 | 197篇 |
1982年 | 137篇 |
1981年 | 114篇 |
1980年 | 107篇 |
1979年 | 115篇 |
1978年 | 78篇 |
1977年 | 60篇 |
1974年 | 74篇 |
1972年 | 62篇 |
Salinity impairs plant growth and development, thereby leading to low yield and inferior quality of crops. Nitric oxide (NO) has emerged as an essential signaling molecule that is involved in regulating various physiological and biochemical processes in plants. In this study, tomato seedlings of Lycopersicum esculentum L. “Micro-Tom” treated with 150 mM sodium chloride (NaCl) conducted decreased plant height, total root length, and leaf area by 25.43%, 24.87%, and 33.67%, respectively. While nitrosoglutathione (GSNO) pretreatment ameliorated salt toxicity in a dose-dependent manner and 10 µM GSNO exhibited the most significant mitigation effect. It increased the plant height, total root length, and leaf area of tomato seedlings, which was 31.44%, 20.56%, and 51.21% higher than NaCl treatment alone, respectively. However, NO scavenger 2-(4-carboxyphenyl)-4, 4, 5, 5-tetramethylimidazoline-1-oxyl-3-oxide potassium (cPTIO) treatment reversed the positive effect of NO under salt stress, implying that NO is essential for the enhancement of salt tolerance. Additionally, NaCl?+?GSNO treatment effectively decreased O2? production and H2O2 content, increased the levels of soluble sugar, glycinebetaine, proline, and chlorophyll, and enhanced the activities of antioxidant enzymes and the content of antioxidants in tomato seedlings in comparison with NaCl treatment, whereas NaCl?+?cPTIO treatment significantly reversed the effect of NO under salt stress. Moreover, we found that GSNO treatment increased endogenous NO content, S-nitrosoglutathione reductase (GSNOR) activity, GSNOR expression and total S-nitrosylated level, and decreased S-nitrosothiol (SNO) content under salt stress, implicating that S-nitrosylation might be involved in NO-enhanced salt tolerance in tomatoes. Altogether, these results suggest that NO confers salt tolerance in tomato seedlings probably by the promotion of photosynthesis and osmotic balance, the enhancement of antioxidant capability and the increase of protein S-nitrosylation levels.
相似文献This study examined the effects of nitrogen (N) fertilizer reduction on the carbon (C) metabolism and yield of Coreopsis tinctoria. A two-year (2020–2021) hydroponic experiment was conducted in accordance with a randomized complete group design with five N levels [0.875 mM Ca(NO3)2 (N1), 1.750 mM Ca(NO3)2 (N2), 3.500 mM Ca(NO3)2 (N3), 7.000 mM Ca(NO3)2 (N4), and 14.000 mM Ca(NO3)2 (N5)] and three replications. The results showed that low N significantly affected the functional leaf weight, C metabolism, and flower bud (or flower) numbers of C. tinctoria at harvest. Lower-N levels, especially those of the N2 treatment, significantly increased Rubisco, sucrose synthase (SS), sucrose phosphate synthase (SPS), soluble acid invertase (SAI), glucose 6-phosphate dehydrogenase (G6PDH), and 6-phosphogluconate dehydrogenase (6PGDH) activity and maintained the flower number of C. tinctoria. In addition, the balance of carbohydrates (sucrose, starch, glucose, and fructose) and ATP contents was more efficiently maintained under relatively low-N levels. These findings might suggest that reduced application of N fertilizer affects the C metabolism of leaves and maintains the number of flowers in Coreopsis tinctoria. Applying relatively low-N fertilizer levels is also a promising cultivation strategy for C. tinctoria.
相似文献Low-temperature and high humidity are typical environmental factors in the plastic tunnel and solar greenhouse during the cold season that restricts plant growth and development. Herein, we investigated the impact of different combinations of low-temperature and high humidity (day/night: T1 15/10 °C?+?95%, T2 12/8 °C?+?95%, and T3 9/5 °C?+?95%) along with a control (CK 25/18 °C?+?80%) on cucumber cultivars viz: Zhongnong37 (ZN37: resistant) and Shuyanbailv (SYB: sensitive). The low-temperature and high humidity stresses increased electrolyte leakage (EL), malondialdehyde (MDA), hydrogen peroxide (H2O2) and intercellular concentration of carbon dioxide (Ci), and reduced morphological indices, relative water content (RWC), net photosynthesis rate (Pn), stomatal conductance (Gs), transpiration rate (E) and leaf pigments in both cultivars as compared to control (CK). Superoxide dismutase (SOD), peroxidase (POD), catalase (CAT), ascorbate peroxidase (APX), monodehydroascorbate reductase (MDHAR), dehydroascorbate reductase (DHAR), and glutathione reductase (GR) were decreased in cv. SYB under stress conditions as compared to cv. ZN37. Low-temperature and high humidity treatments showed an increase in proline and soluble protein content in cv. ZN37 as compared to cv. SYB. Abscisic acid (ABA) and jasmonic acid (JA) were augmented while indole-3-acetic acid (IAA), zeatin (ZT), zeatin riboside (ZR), and gibberellic acid (GA) were decreased in both cultivars. Under T3 (9/5 °C?+?95%), Pn, protoporphyrin, and ZT were extremely decreased by 71.3%, 74.3%, and 82.4%, respectively, in cv. SYB compared to control. Moreover, principal component analysis (PCA) based on physiochemical traits confirmed that cv. ZN37 had the strongest correlation with antioxidant enzymes, proline, and soluble protein content than cv. SYB under low-temperature and high humidity treatments. Our results suggest that a stress-tolerant cultivar mitigates stress damage in cucumber transplants by regulating photosynthetic efficiency, antioxidant capacity and hormonal profile when compared to a stress-sensitive cultivar.
相似文献In this study, two herbaceous peony cultivars with different heat tolerances (‘Fenyunu’ FYN low sensitivity and ‘Qiaoling’ QL high sensitivity) were used as research materials. An integrated view of the factors underlying the decrease in photosynthetic rate under high-temperature (HT) stress was provided by analyzing the biochemical parameters, chloroplast ultrastructure, gas-exchange parameters, chlorophyll fluorescence, and modulated 820 nm reflection of herbaceous peony leaves. The results showed that hydrogen peroxide, superoxide anion, malondialdehyde, and electrical conductivity increased significantly, while the photosynthetic pigments content and photosynthetic capacity decreased significantly in QL than in FYN under HT. The contents of soluble sugars and proline increased greatly in FYN than in QL, while the activity of SOD decreased markedly in QL than in FYN after HT. Compared with FYN, the ultrastructure of QL was more seriously disrupted under HT. Chlorophyll fluorescence analysis showed that HT changed the shapes of OJIP curve, resulting in the increase of K phase and J phase. The PSII acceptor side was more damaged than the donor side, and the electron transfer was seriously blocked. The energy flow in the process of light energy absorption, capture, and electron transfer were significantly changed after HT stress. Meanwhile, PSI was also significantly inhibited, and the coordination of both photosystems decreased. The variation of these parameters in FYN was less than that in QL. These results suggested that FYN featured a more heat-tolerance ability as evidenced by the good performances on the antioxidant system, osmoregulatory capacity, and the thermostability of membranes and photosystems.
相似文献Water stress is one of the most important factors limiting sustainable crop production. Therefore, the effects of the plant growth regulators (PGRs) fulvic acid (FA), brassinolide (BR), and uniconazole (Uni) on seedling growth and physiology of two maize (Zea mays L.) varieties were evaluated under???0.7 MPa water stress induced by polyethylene glycol-6000. Under drought stress, the PGRs promoted seedling growth, altered the root-to-shoot ratio, and significantly increased root biomass, length, surface area, diameter, and volume. In addition, depending on the PGR, net photosynthesis rate, SPAD value (indicating chlorophyll content), and water use efficiency increased significantly, under drought stress, whereas transpiration rate decreased. The PGRs also significantly increased antioxidant enzyme activities and significantly decreased malondialdehyde accumulation in leaves and roots under drought stress. Zhengdan958 showed greater variation in physiological responses and stronger drought resistance than Xundan20. In alleviating drought stress in maize seedlings, FA had the greatest effects on shoot growth and leaf physiology; Uni exerted its effects by regulating root structure, and BR effects were intermediate. Under drought stress, the three PGRs increased maize seedling growth, which reduced drought stress-induced damage and improved plant ability to resist the adversity. Based on a comprehensive analysis of physiological indices of drought resistance, Uni is recommended as the best PGR to improve maize seedlings resistance to drought.
相似文献