共查询到20条相似文献，搜索用时 15 毫秒
1.
小麦雄性不育系细胞质效应的比较研究 总被引：2，自引：0，他引：2
通过对Ｋ、Ｖｅｏ、Ｑ３种新型不育系在同一保持系条件下的生长势及灌浆速度的测定，结合单倍体频率、恢复性等比较研究，认为：粘果山羊草和偏凸山羊草对普通小麦细胞质的不良效应是植株生长势弱和易产生单倍体，在配制杂交种时，应注意选用对上述异质效应有修复能力和对Ｐｔｇ基因的抑制作用的恢复系，同时强调适时人工辅助授粉；野生燕麦细胞质的不良效应是籽粒灌浆速度较慢，配制杂交种时，应注意选配灌浆速度快的恢复系；Ｋ、Ｖｅｎ型不育系恢复源虽广，但高恢材料不多，尤其要注意恢复性的稳定性培育；Ｑ型不育系的恢复系虽少，但其恢复度高而稳定，应进一步研究。 相似文献
2.
Since 1995, more than 150 extrasolar planets have been discovered, most of them in orbits quite different from those of the giant planets in our own solar system. The number of discovered extrasolar planets demonstrates that planetary systems are common but also that they may possess a large variety of properties. As the number of detections grows, statistical studies of the properties of exoplanets and their host stars can be conducted to unravel some of the key physical and chemical processes leading to the formation of planetary systems. 相似文献
3.
磷肥施用与镉污染的研究现状及防治对策 总被引：31，自引：0，他引：31
总结了近年来国内外部分磷肥含镉量以及磷肥投入对土壤含镉量的影响，作物减产的机理，镉在土壤中转移和转化机制，通过对现行土壤污染修复方法的修复效果及修复用的讨论，提出了适合含镉磷肥低强度，非点源污染特点的具体土壤修复技术。 相似文献
4.
Climate forcing by anthropogenic aerosols 总被引：14，自引：0，他引：14
Charlson RJ Schwartz SE Hales JM Cess RD Coakley JA Hansen JE Hofmann DJ 《Science (New York, N.Y.)》1992,255(5043):423-430
Although long considered to be of marginal importance to global climate change, tropospheric aerosol contributes substantially to radiative forcing, and anthropogenic sulfate aerosol in particular has imposed a major perturbation to this forcing. Both the direct scattering of shortwavelength solar radiation and the modification of the shortwave reflective properties of clouds by sulfate aerosol particles increase planetary albedo, thereby exerting a cooling influence on the planet. Current climate forcing due to anthropogenic sulfate is estimated to be -1 to -2 watts per square meter, globally averaged. This perturbation is comparable in magnitude to current anthropogenic greenhouse gas forcing but opposite in sign. Thus, the aerosol forcing has likely offset global greenhouse warming to a substantial degree. However, differences in geographical and seasonal distributions of these forcings preclude any simple compensation. Aerosol effects must be taken into account in evaluating anthropogenic influences on past, current, and projected future climate and in formulating policy regarding controls on emission of greenhouse gases and sulfur dioxide. Resolution of such policy issues requires integrated research on the magnitude and geographical distribution of aerosol climate forcing and on the controlling chemical and physical processes. 相似文献
5.
Wetherill GW 《Science (New York, N.Y.)》1991,253(5019):535-538
Present theories of terrestrial planet formation predict the rapid ;;runaway formation' of planetary embryos. The sizes of the embryos increase with heliocentric distance. These embryos then merge to form planets. In earlier Monte Carlo simulations of the merger of these embryos it was assumed that embryos did not form in the asteroid belt, but this assumption may not be valid. Simulations in which runaways were allowed to form in the asteroid belt show that, although the initial distributions of mass, energy, and angular momentum are different from those observed today, during the growth of the planets these distributions spontaneously evolve toward those observed, simply as a result of known solar system processes. Even when a large planet analogous to ;;Jupiter' does not form, an Earth-sized planet is almost always found near Earth's heliocentric distance. These results suggest that occurrence of Earth-like planets may be a common feature of planetary systems. 相似文献
6.
7.
The accompanying article of Horo witz et al. concluded with the view that the COSPAR recommendations re garding Mars should be adjusted to re flect new environmental information. Specifically, it was concluded that viable terrestrial microorganisms which are transported to Mars inside solid components in sealed spaces have a low probability of being released to the sur face or atmosphere, and that, if any are released, they are not likely to in fect the planet. We suggest, in addition, that both the COSPAR recommenda tions and U.S. planetary quarantine policy should be altered to take into account past and continuing Soviet prac tice regarding the. exploration of Mars and Venus. No amount of analysis by COSPAR, or of costly, self-imposed restrictions by the U.S. on its own planetary exploration program, can reduce the probability of contamination of either Venus or Mars below what the Soviets have already made it, or will make it as they continue their large planetary effort. All that U.S. policy can accomplish is to insure that U.S. efforts do not significantly increase the probability above that level. Any rec ommended policy which would require the U.S. to apply significantly more stringent restrictions is illogical in that, in effect, the U.S. would be asked to increase greatly the cost and complexity of its planetary program without achieving any significant reduction in the probability of actual contamination. There exists some parallelism be tween the problem of planetary quaran tine and that of radioactive fallout from atmospheric nuclear testing, al though the desirable solution to the quarantine problem is not merely to stop all activity. Both are multilateral problems, and individual national policy necessarily must reflect the policy of other nations. Thus, the real questions that must be faced by COSPAR, and by the U.S., are, (i) What is the prob able number of viable terrestrial micro organisms alreadyr transported to Venus and to Mars? and (ii) What is the to tal number to be expected in the next decade or so from foreseeable Soviet efforts alone? Then COSPAR can rec ommend, and the U.S. can decide, that the total U.S. contribution should be equal to some specified fraction of the total present and future Soviet contribu tion. This approach in turn suggests that every effort should be made to induce the Soviets to supply additional de tails on the Zond 2 and Venus 3 mis sion and trajectory and, particularly, on the procedure used for sterilizing the components and assembly of both space craft. With such information, the proba ble number of viable terrestrial microor ganisms deposited on Venus and Mars could be estimated well enough to per mit a. realistic quantitative analysis of what U.S. policy and practice should be. However, if more complete informa tion on Soviet practice cannot be ob tained, then, it seems to us, the U.S. has no logical alternative but to per mit greater engineering freedom in lander delivery technique and to ac cept gaseous and other nonthermal sterilization procedures, where neces sary, in its own program. By relying on the demonstrated U.S. spacecraft reliability to insure that the U.S. con tribution to planetary contamination will remain significantly less than the Soviet contribution, we could reduce significantly the cost and time required to carry out serious scientific investiga tions of the surfaces of Venus and Mars. 相似文献
8.
Because of dynamical constraints in a rotating system, the component of gravity perpendicular to the axis of rotation is the dominant driving force of convection in liquid planetary cores and in stars. Except for the sign, the centrifugal force closely resembles the perpendicular component of gravity. Convection processes in stars and planets can therefore be modeled in laboratory experiments by using the centrifugal force with a reversed temperature gradient. 相似文献
9.
Gladman B 《Science (New York, N.Y.)》2005,307(5706):71-75
Our planetary system is embedded in a small-body disk of asteroids and comets, vestigial remnants of the original planetesimal population that formed the planets. Once formed, those planets dispersed most of the remaining small bodies. Outside of Neptune, this process has left our Kuiper belt and built the Oort cloud, as well as emplacing comets into several other identifiable structures. The orbits in these structures indicate that our outer solar system's comet disk was shaped by a variety of different physical processes, which teach us about how the giant planets formed. Recent work has shown that the scattered disk is the most likely source of short-period comets. Moreover, a growing body of evidence indicates that the sculpting of the Kuiper belt region may have involved large-scale planetary migration, the presence of other rogue planetary objects in the disk, and/or the close passage of other stars in the Sun's birth cluster. 相似文献
10.
Connolly HC 《Science (New York, N.Y.)》2005,307(5706):75-76
One of the most fundamental questions in planetary science is, How did the solar system form? In this special issue, astronomical observations and theories constraining circumstellar disks, their lifetimes, and the formation of planetary to subplanetary objects are reviewed. At present, it is difficult to observe what is happening within disks and to determine if another disk environment is comparable to the early solar system disk environment (called the protoplanetary disk). Fortunately, we have chondritic meteorites, which provide a record of the processes that operated and materials present within the protoplanetary disk. 相似文献
11.
Becker RH 《Science (New York, N.Y.)》2000,290(5494):1110-1111
The nitrogen isotopic compositions seen in lunar soils have long been a mystery to planetary scientists. As Becker discusses in his Perspective, new techniques, such as the depth profiling of mineral grains reported by Hashizume et al., are now shedding some light on the matter, allowing some theories to be excluded. Nevertheless, the relative role of the solar wind and other processes remains hotly debated. 相似文献
12.
稻米垩白直感遗传和杂交稻垩白米遗传分析 总被引：24，自引：0，他引：24
垩白是一胚乳性状,表现直感遗传(xenia);F_2的分离可在 F_1稻株上的米粒观察到。要选育无垩白米的杂交稻,必需不育系、恢复系均无垩白。观察了 F_1、F_2、B_1和 F_3的垩白分离,认为垩白由二对隐性主效基因控制。 相似文献
13.
Mueller RF 《Science (New York, N.Y.)》1963,141(3585):1046-1047
Recent measuremnents of the temperature of Venus suggest that a chemical interaction between the atmosphere and lithosphere may occur: the possible consequences of such interaction are examined. Results indicate that metallic iron, free carbon, and magnesium carbonates should not occur on the surface of Venus. Calcium carbonate, iron oxides and certain hydrous silicates may be present, but other hydrous silicates seem to be excluded. The effects of the physicochemical conditions on rock types and the various planetary processes are discussed. 相似文献
14.
森林植被恢复重建的理论基础 总被引：21，自引：2，他引：19
作者认为 :恢复生态学是研究如何修复由于人类活动引起的原生生态系统生物多样性和动态损害的一门学科 ,它是森林植被恢复重建的重要理论基础 .就森林植被而言 ,它主要研究退化森林生态系统的成因、特征、恢复可行性评价、恢复技术、恢复生态系统的生态学过程、恢复过程中的生物安全以及恢复理论 .其具体理论包括 :植被的发生与气候及气候的变迁耦合或生物的发生与环境耦合理论、植物多样性是生态系统稳定性的基础理论、生态演替理论和作为系统的生态系统的结构和功能相适应理论等 .从国内大量的生态恢复重建实践来看 ,森林植被的恢复重建研究需要解决以下一些问题 :注重探讨森林植被退化的进化与历史决定因素、注意研究恢复森林生态系统在恢复中的生态学过程、重视森林植被恢复重建中的生物安全问题以及大尺度的森林植被恢复重建问题 相似文献
15.
丹江口水源涵养区退耕还草土壤线虫群落变化特征 总被引：1，自引：1，他引：0
为评估丹江口水源涵养区退耕还草的土壤生态效应,2017年9月,选取3块同一土壤类型、退耕3年的紫花苜蓿（Medicago sativa）草地作为退耕还草的代表样地,以相邻未退耕的玉米（Zea Mays）田为对照,比较分析退耕还草土壤线虫群落的变化特征。结果表明：退耕还草样地0~10 cm土层含水量显著提高15.86%（P<0.05）,pH值显著降低5.06%（P<0.05）;10~20 cm土层pH值显著降低4.57%（P<0.05）,有效磷含量显著降低26.83%（P<0.05）。退耕种植紫花苜蓿草地共鉴定出土壤线虫18 307条,49个属,未退耕玉米田共鉴定出土壤线虫10 706条,45个属,退耕还草后土壤线虫c-p类群由短世代型向长世代型转移,土壤环境受到扰动降低,土壤线虫群落的多样性和稳定性增加,结构指数（SI）和富集指数（EI）表明退耕还草提高了土壤健康程度,同时降低了土壤所受的干扰程度。土壤线虫数量与土壤pH值、含水量、全氮和硝态氮含量呈极显著相关（P<0.01）;食细菌线虫和食真菌线虫相对丰度与土壤有效磷含量呈极显著相关（P<0.01）,植物寄生性线虫和捕/杂食线虫相对丰度与有机碳和铵态氮含量呈极显著相关（P<0.01）,与全氮含量呈显著相关（P<0.05）。研究表明,退耕还草改变了土壤pH值及全氮、有效磷养分含量,改变了土壤线虫群落结构和多样性,提升了土壤健康水平。 相似文献
16.
4种园林植物对土壤重金属Mn的吸收及修复研究 总被引：3，自引：0，他引：3
为筛选适宜Mn矿废弃地生态恢复及污染土壤修复的植物种类,利用土培法研究了棕竹、宛田红花油茶、红背桂和地瓜榕4种园林植物对土壤Mn的吸收及修复能力。结果表明：500～1500mg/kgMn处理45d后,棕竹未出现中毒症状,宛田红花油茶、红背桂和地瓜榕均表现出不同程度的中毒症状;随着Mn处理浓度增大,棕竹根、叶Mn含量呈增高趋势,但地上部与地下部Mn含量比值及Mn迁移率略有下降;500mg/kgMn处理下,宛田红花油茶叶片Mn含量分别是其根、茎Mn含量的2.78、3.68倍,而红背桂根系Mn含量是其地上部Mn含量的2.56倍,但该浓度下红背桂地上部与地下部Mn含量比值及Mn迁移率均较低。可见,棕竹可作为废矿区生态恢复及污染土壤修复备选树种之一。 相似文献
17.
18.
Self-assembly at all scales 总被引：1，自引：0，他引：1
Self-assembly is the autonomous organization of components into patterns or structures without human intervention. Self-assembling processes are common throughout nature and technology. They involve components from the molecular (crystals) to the planetary (weather systems) scale and many different kinds of interactions. The concept of self-assembly is used increasingly in many disciplines, with a different flavor and emphasis in each. 相似文献
19.
黄土高原沟壑区林地土壤水分特征的研究（I）——土壤水分的垂直变化和季节变化特征 总被引：10，自引：2，他引：10
对黄土高原沟壑区南缘的油松(Pinus tabulaeformis)、刺槐(Robinia psedudoacacia)人工林及荒坡的土壤水分动态的研究分析，认为林地土壤水分的垂直变化可以分为：土壤水分微弱利用层、土壤水分利用层、土壤水分调节层和土壤水分微弱调节层，荒坡土壤水分的垂直变化可分为：速变层、活跃层、次活跃层和相对稳定层。各立地土壤水分的季节变化可以分为：土壤水分积累期、土壤水分消退期、土壤水分恢复期、土壤水分稳定期和土壤水分消耗期。 相似文献
20.
Chondrules have been observed in several breccia samples returned by the Apollo 14 mission. These lunar chondrules are believed to have formed during a large impact event, perhaps the one that formed the Imbrian Basin. This suggests that some meteoritic chondrules are also formed by impact processes such as crystallization after shock melting and abrasion and diffusion in base-surge and fall-back deposits generated by impacts on planetary surfaces. 相似文献