Direct Detection of C4H2 Photochemical Products: Possible Routes to Complex Hydrocarbons in Planetary Atmospheres

The photochemistry of diacetylene (C4H2), the largest hydrocarbon to be unambiguously identified in planetary atmospheres, is of considerable importance to understanding the mechanisms by which complex molecules are formed in the solar system. In this work, the primary products of C4H2's ultraviolet photochemistry were determined in a two-laser pump-probe scheme in which the products of C4H2 photoexcitation are detected by vacuum ultraviolet photoionization in a time-of-flight mass spectrometer. Three larger hydrocarbon primary products were observed with good yield in the C4H2* + C4H2 reaction: C6H2, C812, and C8H3. Neither C6H2 nor C8H3 is anticipated by current photochemical models of the atmospheres of Titan, Uranus, Neptune, Pluto, and Triton. The free hydrogen atoms that are released during the formation of the C8H3 and C8H2 products also may partially offset the role of C4H2 in catalysing the recombination of free hydrogen atoms in the planetary atmospheres.     

Yield-related agronomic traits evaluation for hybrid wheat and relations of ethylene and polyamines biosynthesis to filling at the mid-grain filling stage

Because of the yield increase of 3.5–15% compared to conventional wheat, hybrid wheat is considered to be one of the main ways to greatly improve the wheat yield in the future. In this study, we performed a principal component analysis(PCA) on two-line hybrids wheat and their parents using the grain weight(GW), the length of spike(LS), the kernel number of spike(KSN), and spike number(SPN) as variables. The results showed that the variables could be classified into three main factors, the weight factor(factor 1), the quantity factor 1(factor 2) and the quantity factor 2(factor 3), which accounted for 37.1, 22.6 and 18.5%, respectively of the total variance in different agronomic variables, suggesting that the GW is an important indicator for evaluating hybrid combinations, and the grain weight of restorer line(RGW) could be used as a reference for parents selection. The hybrid combination with a higher score in factor 1 direction and larger mid-parent heterosis(MPH) of the GW and its parents were used to carry out the analysis of grain filling, 1-aminocylopropane-1-carboxylicacid(ACC) and polyamine synthesis related genes. The results suggested that the GW of superior grain was significantly higher than that of inferior grains in BS1453×JS1(H) and its parents. Both grain types showed a weight of H between BS1453(M) and JS1(R), and a larger MPH, which may be caused by their differences in the active filling stage and the grain filling rate. The ADP-glucose pyrophosphorylase(AGPase), granule bound starch synthase Ⅰ(GBSSI), starch synthase Ⅲ(SSS), and starch branching enzyme-I(SBE-I) expression levels of H were intermediated between M and R, which might be closely related to MPH formation of the GW. Compared with R and H, the GW of M at maturity was the lowest. The expression levels of spermidine synthase 2(Spd2), ornithine decarboxylase(ODC) and S-adenosylmethionine decarboxylase(SAMDC) had significantly positive correlations with the grain filling rate(r=0.77*, 0.51*, 0.59*), suggesting their major roles in the grain filling and heterosis formation. These provide a theoretical basis for improving the GW of photo-thermo-sensitive male sterile lines(PTSMSL) by changing the endogenous polyamine synthesis in commercial applications.     

Glacial-interglacial Indian summer monsoon dynamics

The modern Indian summer monsoon (ISM) is characterized by exceptionally strong interhemispheric transport, indicating the importance of both Northern and Southern Hemisphere processes driving monsoon variability. Here, we present a high-resolution continental record from southwestern China that demonstrates the importance of interhemispheric forcing in driving ISM variability at the glacial-interglacial time scale as well. Interglacial ISM maxima are dominated by an enhanced Indian low associated with global ice volume minima. In contrast, the glacial ISM reaches a minimum, and actually begins to increase, before global ice volume reaches a maximum. We attribute this early strengthening to an increased cross-equatorial pressure gradient derived from Southern Hemisphere high-latitude cooling. This mechanism explains much of the nonorbital scale variance in the Pleistocene ISM record.     

不同淀粉含量木薯品种块根淀粉粒结构观察比较

[目的]探讨不同淀粉含量木薯品种块根淀粉粒结构差异及其与淀粉含量的潜在关系,为研究木薯块根淀粉存储状态及品种选育提供理论依据.[方法]以高淀粉木薯品种GR891、辐选01和低淀粉木薯品种华南124为材料,通过块根形态解剖、石蜡切片观察、电镜扫描等方法比较不同淀粉含量木薯块根淀粉粒的结构特点.[结果]苗期时木薯块根淀粉主要存在于次生木质部.块根形成期淀粉粒大量存在;淀粉储藏区是木薯块根淀粉储藏的最主要区域,皮层部分少量分布,储藏区淀粉粒密度高于皮层;靠近中柱的次生木质部区域,其淀粉粒呈放射线状排列分布.不同淀粉含量木薯品种在淀粉粒形态上无明显差别,多数为一面不规则的半球形、椭球形.高淀粉品种的淀粉粒直径小于低淀粉品种,但差异不显著(P>0.05).[结论]木薯块根淀粉积累的主要位置是淀粉储藏区,导管内部及临近区域的淀粉粒数量多、直径小、填充程度高,且在块根膨大期高淀粉木薯品种淀粉粒聚集程度高于低淀粉木薯品种.     

Direct coupling between meiotic DNA replication and recombination initiation

During meiosis in Saccharomyces cerevisiae, DNA replication occurs 1. 5 to 2 hours before recombination initiates by DNA double-strand break formation. We show that replication and recombination initiation are directly linked. Blocking meiotic replication prevented double-strand break formation in a replication-checkpoint-independent manner, and delaying replication of a chromosome segment specifically delayed break formation in that segment. Consequently, the time between replication and break formation was held constant in all regions. We suggest that double-strand break formation occurs as part of a process initiated by DNA replication, which thus determines when meiotic recombination initiates on a regional rather than a cell-wide basis.     

根据花粉形态探讨中国栽培牡丹的起源

该文对中国栽培牡丹各品种群48个代表品种的花粉形态进行了扫描电镜观察,结果表明:中国栽培牡丹的花粉形态具有较明显的多样性.在此基础上,结合形态分析,提出了绝大多数栽培牡丹起源于多个野生原种的观点.矮牡丹、紫斑牡丹、杨山牡丹是最主要的起源种,卵叶牡丹的作用较小.杂交是中国栽培牡丹最重要的起源方式.少数栽培牡丹是单种起源的,它们直接起源于紫斑牡丹或杨山牡丹.四川牡丹未参与中国现有栽培牡丹的起源.最后讨论了花粉形态对品种分类和鉴定的价值及不同制样方法对花粉粒形状的影响.     

根据花粉形态探讨中国栽培牡丹的起源

该文对中国栽培牡丹各品种群 4 8个代表品种的花粉形态进行了扫描电镜观察 ,结果表明 :中国栽培牡丹的花粉形态具有较明显的多样性 .在此基础上 ,结合形态分析 ,提出了绝大多数栽培牡丹起源于多个野生原种的观点 .矮牡丹、紫斑牡丹、杨山牡丹是最主要的起源种 ,卵叶牡丹的作用较小 .杂交是中国栽培牡丹最重要的起源方式 .少数栽培牡丹是单种起源的 ,它们直接起源于紫斑牡丹或杨山牡丹 .四川牡丹未参与中国现有栽培牡丹的起源 .最后讨论了花粉形态对品种分类和鉴定的价值及不同制样方法对花粉粒形状的影响     

黄牡丹花粉和植株形态性状关联性的研究

通过对6个不同居群的黄牡丹花粉形态进行扫描。结果显示,花粉形态具有多样性,花粉外壁纹饰有脑纹网状、皱波网纹状、粗网状、网状和穴状细网5种;花粉粒的变幅为(32.94-48.87)μm×(21.57-29.70)μm,居群花粉在极轴、赤道轴和极赤比都存在显著差异。通过对6个花粉形态指标和植株11个外部形态指标进行相关性分析。结果表明,花粉极轴与叶长宽比、极赤比与叶长宽比呈明显正相关,花粉极轴与叶长柄比、极赤比与叶长柄比呈明显负相关;花粉外壁纹饰变化与花瓣数量、宿存花萼和苞片数量呈显著相关。依据花粉外壁纹饰的演化规律可推测,花瓣数少、宿存萼片少,苞片数多、叶长宽比越大、叶长柄比越小应是植株较进化的特征。     

Adaptation in a plant-hummingbird association

Sexual dimorphism in bill morphology and body size of the Caribbean purple-throated carib hummingbird is associated with a reversal in floral dimorphism of its Heliconia food plants. This hummingbird is the sole pollinator of H. caribaea and H. bihai, with flowers of the former corresponding to the short, straight bills of males, the larger sex, and flowers of the latter corresponding to the long, curved bills of females. On St. Lucia, H. bihai compensates for the rarity of H. caribaea by evolving a second color morph with flowers that match the bills of males, whereas on Dominica, H. caribaea evolves a second color morph with flowers that match the bills of females. The nectar rewards of all Heliconia morphs are consistent with each sex's choice of the morph that corresponds to its bill morphology and energy requirements, supporting the hypothesis that feeding preferences have driven their coadaptation.     

花后干旱胁迫对大麦子粒碳、氮化合物积累的影响

以防雨池栽的方式研究了干旱胁迫对大麦花后子粒碳、氮化合物积累的影响,结果表明,淀粉和蛋白质沉积生理机制的明显差异是导致干旱胁迫下子粒淀粉沉积降低、蛋白质积累增加的内在因素;子粒形成后期的碳氮代谢互作在竞争和能量方面的差异也对淀粉沉积减少、蛋白质含量增加起一定的作用.     

不同氮效率水稻根系形态和氮素吸收利用与产量的关系

【目的】探究不同氮效率水稻根系形态和氮素吸收利用与产量关系的规律,明确水稻高产根系形态特征,提出相应的水氮优化管理措施。【方法】2013年以氮高效品种川农优498和氮低效品种川优6203为试验材料,进行不同穗肥运筹比例的裂区试验;2014年以相同试验材料进行穴苗数和促花肥、保花肥配比的裂裂区试验;2015年以氮高效品种德香4103和氮低效品种宜香3724为试验材料,进行水分管理方式和氮肥施用模式的裂裂区试验;分别以上述3个大田试验获得的产量、根系形态及氮素吸收利用相关指标为样本,通过计算方差膨胀因子诊断变量间存在的多重共线性关系。在多重共线性严重的情况下,运用岭回归分析研究根系形态、氮素吸收利用和产量三者之间的关系。基于这些关系在年度间的重演性,确定其中普遍存在的共性规律。【结果】(1)氮高效品种拔节期、抽穗期及成熟期的氮素积累量、氮素干物质生产效率和干物重与产量岭回归方程的决定系数范围分别为0.0219—0.3961、0.0452—0.1379和0.0914—0.6694,氮低效品种分别为0.0084—0.6190、0.1224—0.4341和0.0818—0.4881,产量与氮素吸收利用的关系年度间重演性较差,无明显共性规律;以根干重、不定根数量、长度、表面积、体积、粗分枝根长度、表面积、体积以及细分枝根长度、表面积、体积11项根系形态指标为自变量,氮素积累量、氮素干物质生产效率和干物重三者分别为因变量进行岭回归分析,氮高效品种的回归方程决定系数范围分别为0.0527—0.2728、0.0653—0.3139和0.0714—0.3158,氮低效品种分别为0.0607—0.5040、0.0555—0.4411和0.0724—0.5449,氮素吸收利用与根系形态的关系规律年度间存在较大差异;2013—2015年,氮高效和氮低效品种抽穗期根系形态与产量岭回归方程的决定系数均超过0.8,P0.001,表明抽穗期根系形态对产量具有显著影响是共性规律。(2)在以抽穗期根系形态指标为自变量,产量为因变量的岭回归方程中,氮高效品种的粗分枝根长度标准回归系数最高,对产量影响最大;对氮低效品种而言,细分枝根表面积的增加对产量提高最有利。(3)常规施氮量(150 kg·hm~(-2))下,优化施肥模式为穗肥占比40%,且保花肥的比例达到或超过50%,其结合交替灌溉,有利于优化根系形态;SPAD指导施肥模式下,施氮量较优化施肥减少(由150 kg·hm~(-2)降为120 kg·hm~(-2)),采用常规灌溉比交替灌溉更有利于根系形态优化。【结论】水稻抽穗期根系形态与产量关系极为密切,合理的水氮管理措施能够优化根系形态提高产量。常规灌溉结合SPAD指导施肥或交替灌溉结合优化施肥均有利于氮高效品种抽穗期粗分枝根长度的增长和氮低效品种细分枝根表面积的增大,最终提高产量。     

Mitochondrial dynamics caused by QoIs and SDHIs fungicides depended on FgDnm1 in Fusarium graminearum

Fusarium head blight (FHB) caused by Fusarium graminearum is a devastating fungal disease on small grain cereal crops, because it reduces yield and quality and causes the mycotoxin contamination to the grain. Dynamins and dynamin-related proteins (DRPs) are large GTPase superfamily members, which are typically involved in the budding and division of vesicles in eukaryotic cells, but their roles in Fusarium spp. remain unexplored. Here, we found that FgDnm1, a DRP and homolog to Dnm1 in Saccharomyces cerevisiae, contributes to the normal fungal growth, sexual reproduction and sensitivity to fungicides. In addition, we found FgDnm1 co-localizes with mitochondria and is involved in toxisome formation and deoxynivalenol (DON) production. Several quinone outside inhibitors (QoIs) and succinate dehydrogenase inhibitors (SDHIs) cause fragmentated morphology of mitochondria. Importantly, the deletion of FgDnm1 displays filamentous mitochondria and blocks the mitochondrial fragmentation induced by QoIs and SDHIs. Taken together, our studies uncover the effect of mitochondrial dynamics in fungal normal growth and how such events link to fungicides sensitivity and toxisome formation. Thus, we concluded that altered mitochondrial morphology induced by QoIs and SDHIs depends on FgDnm1.     

A role for histone acetylation in the developmental regulation of VDJ recombination

VDJ recombination is developmentally regulated in vivo by enhancer-dependent changes in the accessibility of chromosomal recombination signal sequences to the recombinase, but the molecular nature of these changes is unknown. Here histone H3 acetylation was measured along versions of a transgenic VDJ recombination reporter and the endogenous T cell receptor alpha/delta locus. Enhancer activity was shown to impart long-range, developmentally regulated changes in H3 acetylation, and H3 acetylation status was tightly linked to VDJ recombination. H3 hyperacetylation is proposed as a molecular mechanism coupling enhancer activity to accessibility for VDJ recombination.     

Evidence for reduced recombination on the nondisjoined chromosomes 21 in Down syndrome

Trisomy 21 usually results from nondisjunction during meiosis I. In order to determine whether nondisjunction results from failure of normal chromosome pairing or premature unpairing, recombination frequencies were estimated between DNA polymorphic markers on the long arm of chromosome 21 in families containing one individual with trisomy 21. The recombination frequencies on chromosomes 21 that had undergone nondisjunction were then compared to those on chromosomes 21 that had disjoined normally. The data indicate that recombination is reduced between DNA markers on nondisjoined chromosomes 21. These results are consistent with the hypothesis that reduced chiasma formation predisposes to nondisjunction, resulting in trisomy 21 in humans.     

氮肥施用模式对中籼超级稻产量及辐射利用率的影响

以中籼超级稻两优293为材料,研究了3种氮肥施用模式(实地养分管理模式:施氮量120kg/hm2;实时养分管理模式:施氮量120和210kg/hm2;农民模式:施氮量210kg/hm2)下,水稻群体产量、辐射利用率及纹枯病病情指数的差异.结果表明,相同施氮量水平下,采用实地养分管理模式对中籼超级稻产量的提高效果好于实时模式,提高了辐射利用率,在施氮量120kg/hm2水平下,产量增加主要是因为群体的有效穗数、中后期群体叶面积指数、干物质积累总量的显著增加及辐射利用率的提高和群体纹枯病发病率的降低,产量、有效穗、干物质总量和辐射利用率与实时养分管理模式相比,分别提高1.61%、10.3%、13.3%和12.0%,纹枯病病情指数下降17.2%.     

土壤耕作技术对小麦出苗质量、根系功能及粒重的影响

【目的】 针对黄淮平原农作区一年两熟条件下玉米秸秆还田严重影响麦苗质量的突出问题,探索适宜的土壤耕作技术以提高小麦幼苗质量,最终提高产量。【方法】 2016—2018年连续2年,在河南省新郑市辛店镇黄岗村开展田间试验。采用随机区组设计,将翻耕、耙、镇压3个因素组合配套实施8个处理,分别为深翻耕+旋耕（DT+RT;DT：30 cm,RT：15 cm）、深翻耕+耙（DT+H）、深翻耕+旋耕+镇压（DT+RT+C）、深翻耕+耙+镇压（DT+H+C）、旋耕（RT）、旋耕+耙（RT+H）、旋耕+镇压（RT+C）、旋耕+耙+镇压（RT+H+C）。对小麦出苗率及幼苗质量进行调查,并在越冬期、返青期、拔节期、抽穗期、灌浆期、蜡熟期对根系进行调查分析,分别在灌浆期对小麦籽粒性状、收获后对小麦产量及其构成因素进行调查分析。【结果】 小麦播种后20 d,不同土壤耕作处理间幼苗质量差异显著。旋耕后出苗整齐度高于深翻耕,而深翻耕后出苗率、基本苗数和株高高于旋耕。相同耕、镇压因素处理下,耙后出苗率增幅为1.0%—5.7%,相同耕、耙因素处理下,镇压后出苗率增幅为0.06%—8.3%;同时深翻耕后,极少出现缺苗、断垄,RT处理缺苗、断垄的累计长度最高,两年平均为55 cm。从越冬期到蜡熟期,不同土壤耕作处理的根系活力均呈现“高-低-高-低”的变化趋势,DT+H+C处理最高;在越冬期和拔节期,镇压和耙处理后,与无镇压、耙处理相比,根系活力均提高。单株次生根数目在抽穗期达到最大,DT+H+C处理最高,两年最高值分别为45.2条与40.2条;与无耙处理相比,耙处理后,单株次生根数目最高增加14.8%,与无镇压处理相比,镇压处理后,单株次生根数目最高增加12.2%。花后5—10 d,DT+H+C和RT+H+C处理的籽粒灌浆速率增长幅度显著高于其他处理,开花后20 d达到峰值,其中DT+H+C处理籽粒灌浆速率比其他处理高1.0%—19.4%,达显著水平。灌浆期籽粒千粒重,在花后0—15 d,DT+H+C处理增长最快,DT+RT处理增长最慢,花后25—30 d,DT+H+C处理千粒重最高,较RT处理提高20.8%。从不同土壤耕作技术对籽粒产量及其构成因素的影响来看,DT+H+C处理的籽粒产量最高;耙和镇压处理的单位面积穗数、穗粒数和千粒重的变化并不规律,籽粒产量均有明显提升,幅度为1.4%—12.2%。经济效益方面,与当地以往耕作方式RT相比,RT+H、RT+H+C、RT+C、DT+H+C、DT+H处理所得效益均高于RT处理,其中DT+H+C处理产生经济效益最高,两年平均比RT处理高12.3%。【结论】 黄淮平原农作区当前一年两熟制条件下,不同土壤耕作技术影响幼苗质量,旋耕有利于出苗的整齐度提高,而深翻耕则有利于出苗率及幼苗均匀度提高,株高增高,为冬前形成壮苗奠定基础;深翻耕将耕层加深,利于根系下扎,促进次生根数目的增加以及耕层根系活力的提高,间接影响籽粒产量。综合考虑植株根系生长发育、生育后期籽粒灌浆速率、粒重形成和产量表现等,研究认为黄淮农作区DT+H+C处理土壤耕作技术是当前的最佳选择。     

Tracking Cooper pairs in a cuprate superconductor by ultrafast angle-resolved photoemission

In high-temperature superconductivity, the process that leads to the formation of Cooper pairs, the fundamental charge carriers in any superconductor, remains mysterious. We used a femtosecond laser pump pulse to perturb superconducting Bi(2)Sr(2)CaCu(2)O(8+δ) and studied subsequent dynamics using time- and angle-resolved photoemission and infrared reflectivity probes. Gap and quasiparticle population dynamics revealed marked dependencies on both excitation density and crystal momentum. Close to the d-wave nodes, the superconducting gap was sensitive to the pump intensity, and Cooper pairs recombined slowly. Far from the nodes, pumping affected the gap only weakly, and recombination processes were faster. These results demonstrate a new window into the dynamical processes that govern quasiparticle recombination and gap formation in cuprates.     

收获期对玉米籽粒含水率及籽粒机收质量的影响

籽粒机收是我国玉米生产发展的趋势和方向,选用熟期适宜、抗倒性好、脱水快的玉米品种并农机农艺结合、良种良法配套是实现玉米籽粒机收的技术途径。选用以耐密抗倒伏、早熟脱水快的玉米骨干自交系京2416为父本组配选育并通过国审的3个早熟耐密玉米新品种京农科728、MC812、MC121,以及郑单958、先玉335两大主导品种,在黄淮北部夏播区北京通州开展籽粒机收试验,设置生理成熟期（H1）及生理成熟后5（H2）、10（H3）、15（H4）、20（H5）d共5个机收籽粒收获期处理,研究并明确了收获期对不同玉米品种籽粒含水率及籽粒机收质量的影响,为黄淮北部夏播区机收籽粒玉米品种选择和适期机收提供指导。结果表明：①参试玉米品种京农科728、MC812、MC121、先玉335和郑单958夏播出苗至成熟分别为101、106、105、111、113 d。②H1、H2、H3、H4、H5五个不同收获期条件下,籽粒含水率平均为31.2%、29.4%、27.6%、26.0%、244%;不同品种生理成熟后的籽粒平均脱水速率差异显著,表现为京农科728 [0.039%·(℃·d) ^-1]＞MC812 [0.037%·(℃·d) ^-1]＞MC121 [0.032%·(℃·d) ^-1]＞先玉335 [0.031%·(℃·d) ^-1]＞郑单958 [0.026%·(℃·d) ^-1]。③H1、H2、H3、H4、H5五个收获期,籽粒破碎率平均为7.4%、6.8%、6.2%、5.4%、5.0%,杂质率平均为1.2%、1.0%、0.8%、06%、0.6%,落粒率平均为3.7%、3.4%、3.3%、3.3%、3.1%;在H4收获期,早熟品种京农科728、MC812和MC121的籽粒破碎率、杂质率和落粒率均达到国家机收标准,而郑单958和先玉335因籽粒破碎率高不适宜机收籽粒。④相关性分析表明,籽粒含水率与破碎率、杂质率和落粒率均极显著正相关,落粒率与破损率和杂质率极显著正相关。由此说明,收获期是影响玉米籽粒含水率和籽粒机收质量特别是籽粒破碎率的重要因素,在黄淮北部夏播区选择早熟、脱水快、收获时籽粒含水率低的品种以及适期收获是实现玉米籽粒机收的关键;京农科728、MC812和MC121熟期早、脱水快,在黄淮北部6月15日夏播条件下生理成熟后15 d即可机收籽粒,而郑单958和先玉335熟期偏长且后期脱水相对较慢,在黄淮北部夏播区不宜机收籽粒。     

收获期对玉米籽粒含水率及籽粒机收质量的影响

籽粒机收是我国玉米生产发展的趋势和方向,选用熟期适宜、抗倒性好、脱水快的玉米品种并农机农艺结合、良种良法配套是实现玉米籽粒机收的技术途径。选用以耐密抗倒伏、早熟脱水快的玉米骨干自交系京2416为父本组配选育并通过国审的3个早熟耐密玉米新品种京农科728、MC812、MC121,以及郑单958、先玉335两大主导品种,在黄淮北部夏播区北京通州开展籽粒机收试验,设置生理成熟期（H1）及生理成熟后5（H2）、10（H3）、15（H4）、20（H5）d共5个机收籽粒收获期处理,研究并明确了收获期对不同玉米品种籽粒含水率及籽粒机收质量的影响,为黄淮北部夏播区机收籽粒玉米品种选择和适期机收提供指导。结果表明：①参试玉米品种京农科728、MC812、MC121、先玉335和郑单958夏播出苗至成熟分别为101、106、105、111、113 d。②H1、H2、H3、H4、H5五个不同收获期条件下,籽粒含水率平均为31.2%、29.4%、27.6%、26.0%、244%;不同品种生理成熟后的籽粒平均脱水速率差异显著,表现为京农科728 [0.039%·(℃·d) ^-1]＞MC812 [0.037%·(℃·d) ^-1]＞MC121 [0.032%·(℃·d) ^-1]＞先玉335 [0.031%·(℃·d) ^-1]＞郑单958 [0.026%·(℃·d) ^-1]。③H1、H2、H3、H4、H5五个收获期,籽粒破碎率平均为7.4%、6.8%、6.2%、5.4%、5.0%,杂质率平均为1.2%、1.0%、0.8%、06%、0.6%,落粒率平均为3.7%、3.4%、3.3%、3.3%、3.1%;在H4收获期,早熟品种京农科728、MC812和MC121的籽粒破碎率、杂质率和落粒率均达到国家机收标准,而郑单958和先玉335因籽粒破碎率高不适宜机收籽粒。④相关性分析表明,籽粒含水率与破碎率、杂质率和落粒率均极显著正相关,落粒率与破损率和杂质率极显著正相关。由此说明,收获期是影响玉米籽粒含水率和籽粒机收质量特别是籽粒破碎率的重要因素,在黄淮北部夏播区选择早熟、脱水快、收获时籽粒含水率低的品种以及适期收获是实现玉米籽粒机收的关键;京农科728、MC812和MC121熟期早、脱水快,在黄淮北部6月15日夏播条件下生理成熟后15 d即可机收籽粒,而郑单958和先玉335熟期偏长且后期脱水相对较慢,在黄淮北部夏播区不宜机收籽粒。     

Stable RNA/DNA hybrids in the mammalian genome: inducible intermediates in immunoglobulin class switch recombination

Although it is well established that mammalian class switch recombination is responsible for altering the class of immunoglobulins, the mechanistic details of the process have remained unclear. Here, we show that stable RNA/DNA hybrids form at class switch sequences in the mouse genome upon cytokine-specific stimulation of class switch in primary splenic B cells. The RNA hybridized to the switch DNA is transcribed in the physiological orientation. Mice that constitutively express an Escherichia coli ribonuclease H transgene show a marked reduction in RNA/DNA hybrid formation, an impaired ability to generate serum immunoglobulin G antibodies, and significant inhibition of class switch recombination in their splenic B cells. These data provide evidence that stable RNA/DNA hybrids exist in the mammalian nuclear genome, can serve as intermediates for physiologic processes, and are mechanistically important for efficient class switching in vivo.