中国普通野生稻（Oryza rufipogon Griff.）群体遗传学及保护策略的研究

普通野生稻的基因资源对水稻的育种已经起过并将继续起至关重要的作用。所以，研究和保存其遗传多样性具有重要的意义。首先对普通野生稻在我国的分布现状作了较为全面的野外调查。采集了69个群体，近2000个个体的硅胶干燥叶片。研究了从硅胶干燥的小量叶片中高效率、高质量制备DNA的方法。并用此方法制备了44个代表群体，1168个个体的总DNA，建立了中国普通野生稻的总DNA库。随机选取19个引物对29个群体、651个个体的总DNA进行了RAPD扩增。统计出群体和地区内的多态性片段百分比（PPB），RAPDistance和WINAMOVA程序联用分析遗传变异在地区间、地区内群体间和群体内的组成。广西濠江流域5个野生群体的分析结果表明了中下游群体的PPB值显著高于中上游群体。从而从分子水平上证实了水流对无性繁殖体的传播影响了沿江分布群体的遗传多样性和遗传结构。6个现有原位保护群体的分析结果说明了5个群体值得进一步原位保护，而另一个群体适合异位保护。中国境内29个群体的统计分析揭示出群体间的遗传多样性差异很大（PPB值从23.29%到46.18%）。广东具有的PPB值最高（79.12%），其次是海南（77.11%）和广西（68.67%）。因而，华南是我国普通野生稻的遗传多样性中心。18个群体AMOVA分析结果均表明了遗传变异主要存在于群体内，但群体间业已有较大的遗传分化。从175个随机引物中筛选出9个引物，成功地鉴定出5个群体的克隆多样性和克隆结构。这二者与群体的生态环境，特别是水分状况密切相关。也同时受人类干扰强度的影响。而与群体所处的纬度没有明显的相关性。 根据以上结果，普通野生稻原位保护的策略是选取分布在不同地理区域、遗传多样性水平高的群体。分布在广东、海南和广西的6个群体，具有很高的遗传多样性，建议加以原位保护。此外，孤岛状隔离在我国普通野生稻分布区边缘的3个小群体也应立即进行原位保护。此外，原位保护的群体应给予适度的干扰以维系较高的克隆多样性和遗传多样性。在进行遗传多样性研究和异位保护时，每群体的取样个体数应不少于25株，相邻取样个体间的距离应大于12米。

稻属Oryza rufipogon和Oryza nivara的遗传多样性和居群分化研究

普通野生稻(Oryza rufipogon Griff.)和尼瓦拉野生稻(O. nivara Sharma et Shastry)是亚洲栽培稻的最近缘野生种，是亚洲栽培稻育种和遗传改良的重要基因库。由于人类的破坏、生境片段化及亚洲栽培稻的遗传渐渗等因素，这两种野生稻处于濒危状态而急需保护。本文用微卫星标记研究中国普通野生稻居群的遗传多样性和居群分化，用核基因序列研究了全分布区的普通野生稻和尼瓦拉野生稻的遗传变异及模式，同时发现了巴布亚新几内亚普通野生稻的特殊分子变异。主要结果如下： 取自中国普通野生稻四个主要分布区的12个天然居群，共237份个体材料，利用10对微卫星引物对遗传多样性进行比较研究。结果表明这些居群的遗传多样性处于中高水平，其中每个位点的等位基因数之在2至18之间不等，平均10.6个，多态位点比率P从最低40.0%最高100%，平均为83.3%。观察杂合度在 (HO) 0.163-0.550变化，平均为0.332, 期望杂合度(HE) 在0.164- 0.648，平均 0.413，从地区来看，广西居群的遗传多样性水平是最高的，其次是广东、海南居群，江西的最低。这些结果与以前用其他分子标记得到的结果基本一致。但是本研究得到的居群间遗传分化却相对较高，分化系数RST为 0.5199，θ为 0.491，这表明一半左右的遗传变异存在于居群之间。两两居群之间的分化系数(pairwise θ)与地理距离呈正相关(r = 0.464)，这表明中国普通野生稻居群之间的遗传分化是由于地理的隔离造成的。造成目前这种遗传结构和分化模式的主要原因在于普通野生稻生境的破坏和生境的片段化。 选择6个普通野生稻居群，5个尼瓦拉野生稻居群共11个居群105个个体材料为研究对象，取样覆盖两个野生稻的主要分布区，用核基因片段Lhs1研究了这两种野生稻的序列水平的变异，探讨了它们的遗传结构和基因流方向。结果表明，普通野生稻的核苷酸多样性，无论在居群还是物种水平上，均显著高于尼瓦拉野生稻的。在居群水平上，普通野生稻的核苷酸多样性(Hd = 0.712; θsil = 0.0017)是尼瓦拉野生稻的2-3倍(Hd = 0.306; θsil = 0.0005)。AMOVA的结果表明，尼瓦拉野生稻居群之间的遗传分化(78.2%)显著高于普通野生稻居群间的分化(52.3%)。造成这种不同层次的遗传多样性和完全不同的遗传分化模式的原因在于这两种野生稻差异显著的生活史和交配系统。普通野生稻是多年生、异交的野生种，营养与有性繁殖混合，尼瓦拉野生稻为一年生的自交种，靠种子繁殖。模拟分析表明，二者之间的基因流方向是单向的，明显是从自交的尼瓦拉野生稻到异交的普通野生稻，基因流在这两个野生 稻遗传多样性模式的形成上，扮演至关重要的角色。 对于稻属A基因组物种的进化和多样性来说, 巴布亚新几内亚(PNG)是个很重要的地方。我们用两个核基因和一个叶绿体基因，对巴布亚新几内亚的普通野生稻样品进行了序列分析，结果发现来自巴布亚新几内亚北部Sepik河谷的普通野生稻样品在核基因Os0053位点和叶绿体基因序列上与其他地区的样品有很大的差异。巴布亚新几内亚北部的普通野生稻在Os0053位点上杂合的，其中一条等位基因与普通野生稻基本一致，另一条等位基因则与A基因组的另一个物种O. meridionalis基本一致，证明巴布亚新几内亚北部的普通野生稻实际上是亚洲典型普通野生稻与澳洲野生稻O. meridionalis种间进行天然渐渗杂交的后代。这个结果从分子序列上验证了前人有关巴布亚新几内亚北部的普通野生稻是比较特殊的观点。

稻属A基因组的分子系统发育--兼论亚洲栽培稻的起源

稻属Oryza隶属禾本科Poaceae，包括20多个野生种和2个栽培种(亚洲栽培稻O. sativa L和非洲栽培稻O. glaberrima Steud) ，广泛分布于全球热带和亚热带。稻属物种可划分为10个基因组(又称染色体组)类型：A, B, C, BC, CD, E, F, G, HJ 和 HK。栽培稻所属的A基因组是稻属中物种数目最多、地理分布最广的基因组类型，由8个种组成。由于栽培稻属于A基因组，故A基因组物种是栽培稻遗传改良的巨大基因源。数十年来，国际上许多学者对A基因组类群开展了大量涉及形态、细胞、同工酶和分子标记方面的研究，但由于A基因组物种间遗传关系十分接近，形态上差异小且地理分布重叠，使得A基因组物种的系统发育、物种起源和生物地理学等方面存在诸多悬而未决的问题，是稻属中分类和鉴定困难较多的类群。本文利用核基因内含子序列，结合转座子插入分析，重建了A基因组的系统发育，估测了各类群的分化时间;与此同时，基于多克隆测序和基因谱系分析，探讨了O. rufipogon和O. nivara遗传关系以及亚洲栽培稻起源。主要研究结果如下： 1. A基因组的系统发育 在水稻全基因组数据库搜索的基础上，测定了4个单拷贝核基因（Adh1 及3个未注释基因）的内含子序列，构建了稻属A基因组8个种的系统发育关系。基于最大简约法和贝叶斯法的系统发育分析表明：1）澳大利亚的O. meridionalis为A基因组的基部类群;2）亚洲栽培稻两个亚种O. sativa ssp. japonica 和 O. sativa ssp. indica分别和不同的野生类群聚为独立的两个分支，支持japonica 和 indica为多次起源;3）O. rufipogon和O. nivara在系统发育树上完全混在一起，显示出二者间不存在遗传分化;4）非洲一年生野生种O. barthii是非洲栽培稻O. glaberrima的祖先，而非洲多年生野生种O. longistaminata与O. glaberrima/O. barthii.亲缘关系较远;5）分子钟方法估测A基因组类群约在2百万年前（2.0MYA）开始分化，亚洲栽培稻和非洲栽培稻，以及亚洲栽培稻的两个亚种则分别在0.7和 0.4 MYA左右开始分化。此外，通过核基因内含子序列与其它常用片段如ITS，matK等对比分析表明，进化速率相对较快的核基因内含子序列可以有效地用于近缘类群的系统发育研究。 2. Oryza rufipogon 和O. nivara群体遗传研究及亚洲栽培稻起源 对于亚洲野生类群O. rufipogon和O. nivara是合并为一个种还是处理为两个独立的种一直存在争议。在系统发育研究基础上，我们选取4个核基因内含子或5’-UTR区（Waxy, LHS，CatA和1个未注释基因），对采自整个分布区的群体样品进行了多克隆测序，结果表明：1）检测到O. rufipogon和O. nivara均有较高的核苷酸多态性，4个位点上π值和θw值平均分别为0.011和0.014;2）且二者在遗传上没有明显分化，两个类群在4个核基因位点上均检测到大量共享多态（shared polymorphism），未发现固有差异(fixed difference)，表明它们历史上可能属于一个大群体，支持将二者作为种内不同生态型或亚种处理;3）基因谱系树表明亚洲栽培稻的两个亚种indica和japonica分别和不同的O. rufipogon (包括O. nivara)群体聚在一起，进一步从基因谱系角度支持亚洲栽培稻多次起源假说。 3．转座子在群体遗传与系统发育研究中的应用 鉴于目前植物谱系地理学研究中缺乏具有足够信息量的分子标记用于检测种内遗传变异，我们选取3个核基因中的转座子，通过对取自O. rufipogon和O. nivara整个分布区的37份样品的克隆测序，探讨了进化速率快、信息含量丰富的转座子序列在群体遗传上的应用。结果表明：1）无论在物种水平还是群体水平，转座子能检测到比包括内含子在内的其它DNA区域高得多的遗传变异;2）在物种水平上，异交多年生的O. rufipogon和自交一年生的O. nivara多样性均较高，且2个种间相差很小，二者在3个位点上平均核苷酸多样性π值均为0.013，差别主要表现在O. rufipogon杂合位点比例（46.1%）明显高于O. nivara（9.1%），说明交配系统不同并不一定和物种多样性水平相关;3）是否发生转座子序列插入是有价值的系统发育信息，发生在不同染色体上3个基因中的转座子插入进一步证实A基因组基部类群是O. meridionalis;通过叶绿体中3个转座子的插入现象推断了稻族一些四倍体物种，如稻属BC基因组的一些类群的母本来源。

水稻抗旱性与光合特征的资源评价及其根系抗旱生理基础的研究

为了挖掘和利用野生资源抗旱和高光效的有益基因性状，本研究针对野生种的抗旱生理性状、光合作用和抗旱关系进行了测定，同时利用野生稻和栽培稻远缘杂交获得不同光合特点的后代材料进行了胁迫反应试验，并以栽培稻（陆稻和水稻）为材料研究了抗旱反应中根系的激素信号变化。旨在明确水分胁迫条件下，水稻不同野生种对干旱反应的差异及其机理，以及进一步鉴定有效的抗旱指标，为今后育种和生产实践提供理论依据和指导。主要结果如下： 1．通过对具有不同抗旱性状的四个野生种O.Granulata、O.Alta、O,Officinalis、O.Latifolia水分胁迫处理的抗旱相关生理性状的变化特点的研究表明：(1)在胁迫早期，野生种间黎明前叶片水势没有明显的差别，随着胁迫加剧，种间差异显示明显。耐旱性弱的野生种（O.Granulata、O.Latifolia）黎明前叶片水势和中午水势下降幅度大于耐旱性强的野生种( O.Alta、O.Officinalis)。这表明在一定的干旱处理程度下，黎明前叶片水势和中午水势可以反映出不同种间植株忍受干旱胁迫的能力。(2)四个野生种的植株在轻微胁迫时，膜稳定性呈现出增强的趋势，这可能与干旱胁迫的适应性相关。随着胁迫时间的延长，胁迫程度的加重，叶片膜稳定性破坏，植株叶片渗漏率增加。(3)在胁迫条件下，不同野生种表现出茎杆中糖分增加，叶片的光合酶和光合速率下降，且抗旱性弱的品种这种变化比抗旱性强的品种更加明显。这可能是由于干旱处理降低了同化物的运转和淀粉合成，导致了糖分积累。(4)在水分胁迫条件下，植株中的ABA浓度增加而IAA浓度下降，其中ABA的浓度增加与种间的抗旱性相关。抗旱性差(O.Granulata、O.Latifolia)的野生种叶片ABA浓度增加幅度高于抗旱性强的野生种（O.Alta、O.Officinalis）。Officinalis和Alta在水分胁迫下，仍比Granulata 和Latifolia具有较高的分蘖，这可能与其在胁迫下具有较高GA3浓度有关。 2．通过对20个野生种叶片膜电解质泄漏率的比较，研究其叶片细胞膜稳定性的特点表明，46℃的温度，水浴时间24小时是较适宜的条件。在适宜的处理条件下，Longistaminata、Punctata渗漏率较低，表明其具有良好的膜稳定性：O. Officinalis、O.Glumaepatula、O.Glaberrima，O.Latifolia、Meridionalis、Rufipogon (105697)、100889、Nivara (80683)、Sativa( IR-36)电解质泄漏率居中在60-80%;O.Alta、Rampur6、Azucena、Rufipogon (105599)、Bartlic、Rufipogon(104640)、105429、Minuta (101099)、HP4的渗漏值均较高，表明其膜稳定性较差，尤其是Minuta。此外，叶片光合速率和叶片膜稳定性的关系并不完全一致，在育种时应选择光合速率较高而叶片膜稳定性好的品种，O.Longistaminata因其具有较高的光合速率和较好的膜稳定性，是一个值得关注的材料。 3．研究了栽培稻（Oryza sativa）和普通野生稻（Oryza rufipogon）的杂种后代F3，即：Azucena×Rampur6杂交得到的Fi植株(25007-10)，通过自交得到其F3代植株，在水分胁迫下不同光合速率类型植株以及相关的抗旱生理特点。结果表明， (1)光合速率高的株系黎明前叶片水势下降较大，而中午叶片水势下降却较小：光合速率低的株系黎明前叶片水势下降幅度较小，而中午叶片水势下降幅度却较大，这表明水分胁迫下，植株中午叶片水势与植株的光合速率密切相关。(2)在轻微水分胁迫下（缓慢干旱0-40天），所有株系经受抗旱锻炼的植株，其叶片渗漏率均下降，膜稳定性增强;随着胁迫时间延长（胁迫后40-80天）其叶片泄漏值上升，这可能是由于：水分胁迫下通过植株的渗透调节能力，使得叶片中累积的有机溶质增加：且干旱胁迫使得叶片的质膜破坏，电解质外渗，相对电导率提高。 (3)水分胁迫下，从不同光合速率类型的植株，其光合速率的变化看：高光合速率类型的植株，在水分胁迫下光合速率的下降幅度，要大于光合速率较低类型的植株。在水分胁迫下，SHPl-2株系后代比SHPl-1株系具有较强的抗旱性，能维持较高的光合速率。 (4)在水分胁迫下，不同光合速率的株系，其气孔阻抗均增加：低光合速率的株系，气孔阻抗上升幅度大于高光合速率类型的株系，并且其气孔阻抗上升的时间要早于高光合速率的株系。 4．研究了两个陆稻品种Azucena、IRAT104和两个水稻品种IR64、Salumpikit，在干旱胁迫下根系木质部汁液中内源激素的变化。(1)干旱复水后不同时间进程中，各品种木质部汁液内源激素的变化看，IRAT104和Salumpikit木质部汁液中ABA含量迅速降低，其下降幅度较大。说明ABA作为胁迫信号，通过木质部汁液传递干旱信息;在干旱胁迫解除后，ABA的浓度亦发生相应的变化。(2)在对根系施加不同压力后，测定其木质部汁液内源激素变化，结果表明：在不同的根系压力下，不同品种间（IR64、Salumpikit、IRAT104和Azucena） 木质部汁液中GA3含量变化较大，其中两个陆稻品种IRAT104和Azucena均较高，两个水稻品种IR64和Salumpikit均较低。这表明，在不同的压力下，陆稻品种（Azucena和IRAT104）比水稻品种（IR64和Salumpikit）具有较高的GA3含量;在胁迫条件下，维持较高的GA3含量，这有利于其维持正常的生长发育。(3)在干旱胁迫及复水过程中，根据木质部汁液中ABA和GA3含量变化的结果表明，虽然Azucena和IRAT104均属于陆稻品种，但二者的抗旱性不同。(4)水稻品种木质部汁液的pH值高于陆稻品种。木质部汁液中pH在干旱胁迫下升高，复水后又降低，其可作为干旱胁迫的一个信号，它与木质部汁液中ABA联合调节植株对干旱的生理生化反应。 上述结果表明，叶片水势与水分胁迫密切相关，可作为植株抗旱性的鉴定指标;抗旱性强的品种，叶片水势下降幅度小，有利于维持较高的水势，保证植株生长发育的需要。严重干旱使得所有品种的质膜稳定性降低，电解质外渗，相对电导率提高;抗旱性强的品种的质膜伤害程度小，电导率上升幅度小，表明其质膜稳定性好，对干旱的忍耐能力强。质膜稳定性的变化实际上反映了品种的耐旱性，所以是一种综合而又比较准确的抗旱鉴定指标。对于酶和激素的测定方法，由于操作比较烦琐，且数据分析和产量比较复杂等原因，不太适合于常规的大田选育工作中。水稻的抗旱性是多种生理生化变化综合作用的结果，因此进行水稻抗旱性鉴定时，不能使用单一的生理生化指标，而应对多个指标进行综合分析，依据综合值对水稻品种的抗旱性进行评价较为科学。

Comparison of microsatellites and isozymes in genetic diversity studies of Oryza glumaepatula (Poaceae) populations

The study of the genetic structure of wild plant populations is essential for their management and conservation. Several DNA markers have been used in such studies, as well as isozyme markers. In order to provide a better comprehension of the results obtained and a comparison between markers which will help choose tools for future studies in natural populations of Oryza glumaepatula, a predominantly autogamous species, this study used both isozymes and microsatellites to assess the genetic diversity and genetic structure of 13 populations, pointing to similarities and divergences of each marker, and evaluating the relative importance of the results for studies of population genetics and conservation. A bulk sample for each population was obtained, by sampling two to three seeds of each plant, up to a set of 50 seeds. Amplified products of eight SSR loci were electrophoresed on non-denaturing polyacrylamide gels, and the fragments were visualized using silver staining procedure. Isozyme analyses were conducted in polyacrylamide gels, under a discontinuous system, using six enzymatic loci. SSR loci showed higher mean levels of genetic diversity (A=2.83, p=0.71, A(P)=3.17, H-o=0.081, H-e=0.351) than isozyme loci (A=1.20, p=0.20, A(P)=1.38, H-o=0.006, H-e=0.056). Interpopulation genetic differentiation detected by SSR loci (R-ST=0.631, equivalent to F-ST=0.533) was lower than that obtained with isozymes (F-ST=0.772). However, both markers showed high deviation from Hardy-Weinberg expectations (F-IS=0.744 and 0.899, respectively for SSR and isozymes). The mean apparent outcrossing rate for SSR ((t) over bar (a)=0.14) was higher than that obtained using isozymes ((t) over bar (a)=0.043), although both markers detected lower levels of outcrossing in Amazonia compared to the Pantanal. The migrant number estimation was also higher for SSR (Nm=0.219) than isozymes (Nm=0.074), although a small number for both markers was expected due to the mode of reproduction of this species, defined as mixed with predominance of self fertilization. No correlation was obtained between genetic and geographic distances with SSR, but a positive correlation was found between genetic and geographic distances with isozymes. We conclude that these markers are divergent in detecting genetic diversity parameters in O. glumaepatula and that microsatellites are powerful for detecting information at the intra-population level, while isozymes are more powerful for inter-population diversity, since clustering of populations agreed with the expectations based on the geographic distribution of the populations using this marker. Rev. Biol. Trop. 60 (4): 1463-1478. Epub 2012 December 01.

Genetic Analysis of Tolerance to Rice Tungro Bacilliform Virus in Rice (Oryza sativa L.) Through Agroinoculation

Balimau Putih [an Indonesian cultivar tolerant to rice tungro bacilliform virus (RTBV)] was crossed with IR64 (RTBV, susceptible variety) to produce the three filial generations F1, F2 and F3. Agroinoculation was used to introduce RTBV into the test plants. RTBV tolerance was based on the RTBV level in plants by analysis of coat protein using enzyme-linked immunosorbent assay. The level of RTBV in cv. Balimau Putih was significantly lower than that of IR64 and the susceptible control, Taichung Native 1. Mean RTBV levels of the F1, F2 and F3 populations were comparable with one another and with the average of the parents. Results indicate that there was no dominance and an additive gene action may control the expression of tolerance to RTBV. Tolerance based on the level of RTBV coat protein was highly heritable (0.67) as estimated using the mean values of F3 lines, suggesting that selection for tolerance to RTBV can be performed in the early selfing generations using the technique employed in this study. The RTBV level had a negative correlation with plant height, but positive relationship with disease index value

Biolistic transformation of elite indica rice (Oryza sativa L.) cultivars through semi-solid and liquid medium selection systems

Following microprojectile mediated delivery of a plasmid construct (pAHC-25) encoding bar (bialophos resistance) gene into five-day-old scutellar calli derived from mature embryos, the effectiveness of selection procedure for bar-gene expressing tissue was compared for two indica rice cultivars (IR-64 and Karnal Local). While IR-64 transformants could be selected through the generally used semi-solid selection medium, the same procedure was not effective in the basmati cultivar Karnal Local. In the latter case, while lower concentrations (2–4 mg 1−1) of the selective agent phosphinothricin (PPT) yielded only escapes, higher concentrations (6–8 mg l−1) inhibited proliferation of transformed as well as untransformed sectors. For Karnal Local, a liquid medium based selection system was successfully utilized for recovering transformed sectors and, eventually, regenerants. The study demonstrates the generation of transformants of two elite indica cultivars using the environment-independent system of mature embryos from seeds.

Testing the simulation capability of APSIM-ORYZA under different levels of nitrogen fertiliser and transplanting time regimes in Korea

APSIM-ORYZA is a new functionality developed in the APSIM framework to simulate rice production while addressing management issues such as fertilisation and transplanting, which are particularly important in Korean agriculture. To validate the model for Korean rice varieties and field conditions, the measured yields and flowering times from three field experiments conducted by the Gyeonggi Agricultural Research and Extension Services (GARES) in Korea were compared against the simulated outputs for different management practices and rice varieties. Simulated yields of early-, mid- and mid-to-late-maturing varieties of rice grown in a continuous rice cropping system from 1997 to 2004 showed close agreement with the measured data. Similar results were also found for yields simulated under seven levels of nitrogen application. When different transplanting times were modelled, simulated flowering times ranged from within 3 days of the measured values for the early-maturing varieties, to up to 9 days after the measured dates for the mid- and especially mid-to-late-maturing varieties. This was associated with highly variable simulated yields which correlated poorly with the measured data. This suggests the need to accurately calibrate the photoperiod sensitivity parameters of the model for the photoperiod-sensitive rice varieties in Korea.

Responses of Super Rice (Oryza sativa L.) to Different Planting Methods for Grain Yield and Nitrogen-Use Efficiency in the Single Cropping Season

To break the yield ceiling of rice production, a super rice project was developed in 1996 to breed rice varieties with super high yield. A two-year experiment was conducted to evaluate yield and nitrogen (N)-use response of super rice to different planting methods in the single cropping season. A total of 17 rice varieties, including 13 super rice and four non-super checks (CK), were grown under three N levels [0 (N0), 150 (N150), and 225 (N225) kg ha−1] and two planting methods [transplanting (TP) and direct-seeding in wet conditions (WDS)]. Grain yield under WDS (7.69 t ha−1) was generally lower than TP (8.58 t ha−1). However, grain yield under different planting methods was affected by N rates as well as variety groups. In both years, there was no difference in grain yield between super and CK varieties at N150, irrespective of planting methods. However, grain yield difference was dramatic in japonica groups at N225, that is, there was an 11.3% and 14.1% average increase in super rice than in CK varieties in WDS and TP, respectively. This suggests that high N input contributes to narrowing the yield gap in super rice varieties, which also indicates that super rice was bred for high fertility conditions. In the japonica group, more N was accumulated in super rice than in CK at N225, but no difference was found between super and CK varieties at N0 and N150. Similar results were also found for N agronomic efficiency. The results suggest that super rice varieties have an advantage for N-use efficiency when high N is applied. The response of super rice was greater under TP than under WDS. The results suggest that the need to further improve agronomic and other management practices to achieve high yield and N-use efficiency for super rice varieties in WDS.

Ecological significance of rice (Oryza sativa) planting density and nitrogen rates in managing the growth and competitive ability of itchgrass (Rottboellia cochinchinensis) in direct-seeded rice systems

Current understanding is that high planting density has the potential to suppress weeds and crop-weed interactions can be exploited by adjusting fertilizer rates. We hypothesized that (a) high planting density can be used to suppress Rottboellia cochinchinensis growth and (b) rice competitiveness against this weed can be enhanced by increasing nitrogen (N) rates. We tested these hypotheses by growing R. cochinchinensis alone and in competition with four rice planting densities (0, 100, 200, and 400 plants m-2) at four N rates (0, 50, 100, and 150 kg ha-1). At 56 days after sowing (DAS), R. cochinchinensis plant height decreased by 27-50 %, tiller number by 55-76 %, leaf number by 68-84 %, leaf area by 70-83 %, leaf biomass by 26-90 %, and inflorescence biomass by 60-84 %, with rice densities ranging from 100 to 400 plants m-2. All these parameters increased with an increase in N rate. Without the addition of N, R. cochinchinensis plants were 174 % taller than rice; whereas, with added N, they were 233 % taller. Added N favored more weed biomass production relative to rice. R. cochinchinensis grew taller than rice (at all N rates) to avoid shade, which suggests that it is a "shade-avoiding" plant. R. cochinchinensis showed this ability to reduce the effect of rice interference through increased leaf weight ratio, specific stem length, and decreased root-shoot weight ratio. This weed is more responsive to N fertilizer than rice. Therefore, farmers should give special consideration to the application timing of N fertilizer when more N-responsive weeds are present in their field. Results suggest that the growth and seed production of R. cochinchinensis can be decreased considerably by increasing rice density to 400 plants m-2. There is a need to integrate different weed control measures to achieve complete control of this noxious weed.

Efficacy and phytotoxicity of different rates of oxadiargyl and pendimethalin in dry-seeded rice (Oryza sativa L.) in Bangladesh

A field study was established to evaluate oxadiargyl and pendimethalin during the wet seasons in Bangladesh in 2012 and 2013. The study evaluated the following treatments: oxadiargyl applied at 80, 120, and 160 g ai ha−1; pendimethalin at 800, 1200, and 1600 g ai ha−1; partial weedy; and weed-free. Rice plant density was greatly affected by weed control treatment. Lower density and lower uniformity of the rice plant stand occurred as a result of increased rates of herbicides. Increased rates of pendimethalin were more toxic than increased rates of oxadiargyl. Both herbicides effectively controlled Digitaria ciliaris, Echinochloa colona, and Phyllanthus niruri; however, they were unable to control Murdannia nudiflora. Oxadiargyl controlled Cyperus rotundus across rates by 31–55%, but pendimethalin was completely ineffective on it, and higher rates of both herbicides had no effect in controlling this weed. Both herbicides at higher rates reduced total weed biomass significantly. Among herbicide treatments, the highest yield (3.7–4.0 t ha−1) was recorded in plots treated with oxadiargyl at 160 g ai ha−1 and the lowest yield (2.4–2.8 t ha−1) was in plots treated with pendimethalin at 1600 g ai ha−1. Results from our study suggest that a higher rate of oxadiargyl can increase yield by suppressing weeds in dry-seeded rice systems. Similar to the results of oxadiargyl, pendimethalin at higher rates also greatly suppressed weeds; however, yield decreased due to phytotoxicity to rice seedlings.

Nitrogenase activity (acetylene reduction) associated with leaves of different cultivars of paddy (Oryza sativa L.)

An investigation was conducted to study the levels of nitrogen fixation on the leaf or sheath surfaces of four cultivars of paddy plants by using acetylene reduction technique. Varying levels of positive nitrogenase activity were observed on all the leaf surfaces. Sheath of IET 1991 cultivar showed a higher rate of fixation than the leaf surface. All the nitrogen-fixing organisms on the leaf or sheath surfaces belonged to the genus Beijerinckia. There was no correlation between the bacterial density and the level of fixation. Scanning electron microscopic data revealed that the upper surface of IET 1991 leaf was highly silicified and the microflora was either scanty or nil while the lower surface appeared quite different and harboured more micro-organisms. Similarly, the inner surface of sheath was devoid of silicification and showed the presence of micro-organisms.