喷管仰角和长度对负压反馈射流喷头水力性能的影响

负压反馈射流喷头(简称射流喷头)是中国自主研发的新型中程灌溉喷头。喷管是喷头的重要组成部分,对喷头水力性能影响重大。为针对性研究喷管参数(仰角、长度组合)对射流喷头水力性能的影响,寻找最优喷管参数,开展了不同喷管参数下射流喷头与PY_210摇臂式喷头水力性能对比试验。结果表明,在相同主喷嘴尺寸时,不同工作压力和不同喷管参数下,射流喷头均同比摇臂式喷头射程远1～2.5 m;射流喷头水量分布中近程呈现较好的"三角形"分布,远处出现水量"凸峰"。最后对试验数据采用综合评分法和熵权法进行分析,同时综合考虑实际喷头野外抗风性能和单一造价,确定最优综合评分下的主副喷管参数为:主副喷管长度组合4.2 cm×4.2 cm,工作压力为0.20～0.30 MPa时,主副喷管仰角40°×40°;工作压力为0.35 MPa时,主副喷管仰角30°×30°。     

掺气水射流应用于低压摇臂喷头的试验

为解决固定式旋转喷头低压喷灌时,水射流向末端集中形成水量分布不均匀的问题,提出水气两相射流进行喷灌的方法。在摇臂喷头结构的基础上,增加掺气结构,形成掺气射流喷头,以相同工作水压力、射流仰角、喷嘴出口流量相同为约束,以及不考虑副喷嘴对喷洒的影响,对比了掺气与不掺气2种情况下 PY20喷头的射程、径向水量分布、1倍射程间距的正方形组合喷灌均匀系数,雨滴粒径等参数。试验结果表明：原不掺气摇臂喷头出口直径7 mm,安装内径2 mm 的掺气管后出口直径改为8.3 mm,此时两者具有相同的出口流量,2种喷头在相同工作压力下具有近似相等的射程;在掺气喷头工作水压低至100 kPa 情况下,喷头仍具有76 mm 水银柱高差的掺气负压能力;掺气摇臂喷头改善了径向水量分布线射程中段的水量,使水量分布线发生了中段略微增高、末端略下降的变化,从而使1倍间距的正方形组合喷灌均匀系数在低于国家标准工作压力的200 kPa 情况下,从62.8%提高到68.8%;采用激光雨滴谱仪测量射程中部和末端2个地方的水滴粒径表明：掺气状态下射程中部的水量累积百分比中位直径 d50远大于不掺气状态,射流末端对比 d50则小于不掺气状态,说明掺气改变了喷头的雨滴粒径分布。该文试验结果证明掺气摇臂喷头在农业喷灌中应用具有可行性。     

负压反馈射流喷头主副喷嘴直径及流道结构参数的优化

负压反馈射流喷头是一种基于射流脉冲特性设计的全圆旋转灌溉喷头。喷头的主副喷嘴尺寸组合对该型喷头脉冲特性和水力性能影响较大。为确定主副喷嘴直径的设计区间及各流道结构参数对喷头性能的影响,首先利用单因素数值模拟方法确定具有良好脉冲效果的主副喷嘴直径设计区间,再以进口流量和射程为评价指标,进行了九因素四水平正交试验。采用极差分析法和相对影响指数评价法处理正交试验结果,得到了喷头主副喷嘴最佳尺寸组合与内部最优流道参数,加工优化后样机进行优化前后喷头水力性能对比试验和数值模拟验证试验。结果表明,在0.15～0.30 MPa工作压力范围内,主、副喷嘴直径的设计区间分别为4.0～4.5 mm、3.5～4.5 mm,最优流道结构具体参数为位差比0.475、侧壁倾角12.0°、劈距比9.0、深宽比2.5、相对曲率半径比3.0、主副喷管相对长度组合8 cm×6 cm、主副喷嘴组合4.5 mm×3.5 mm、仰角30°。优化后的样机比优化前的进口流量降低了12.1%～14.6%、射程增加了6.5%～9.4%,优化后喷头的中近程降水深较优化前减小,远处降水深增加。优化后试验值与模拟值的归一化均方根误差指标分别为4.2%和6.7%,说明优化效果较好,水体模型精度较高。研究提出的最优流道结构参数及研究过程可为喷头后续设计提供参考。     

旋转式喷头空间流道设计及低压水力性能试验

喷头在低压工况下存在均匀性较差、射程变短及工作不稳定等问题。为了扩宽喷头的工作压力范围,该研究设计了旋转式喷头空间流道结构。以空间流道仰角、中间截面偏置角、出口截面偏置角为因素,以喷头空间流道出口平均速度和旋转驱动力为指标,采用CFD数值模拟正交设计,分析了空间流道结构参数对喷头水力性能的影响,并对旋转式喷头与相同规格的NelsonR33喷头在不同压力工况下进行水力性能对比试验。结果表明：空间流道各参数对喷头出口平均速度影响顺序依次为：空间流道仰角、中间截面偏置角、出口截面偏置角;对喷头旋转驱动力的影响顺序依次为：中间截面偏置角、空间流道仰角、出口截面偏置角。空间流道结构最佳参数组合为：出口截面偏置角为3°、中间截面偏置角为3.5°和空间流道仰角为30°。在工作压力为150及200 kPa下,旋转式喷头的水量分布更为均匀,组合均匀性系数更高。为扩宽喷头工作压力范围以及提高喷头低压下水力性能提供依据。     

全射流喷头喷洒水滴动能分布规律

喷灌动能是评价喷头水力性能优劣的重要指标之一。该文在0.15、0.20、0.25、0.30和0.35 MPa工作压力下,采用激光雨滴谱仪测量技术(laser precipitation monitor,LPM)对全射流喷头的水滴直径、速度和水量分布等参数进行试验,研究了单个水滴动能、单位体积水滴动能、动能强度分布规律及动能强度均匀性系数与组合间距之间的关系。结果表明:全射流喷头的单个水滴动能分布与水滴直径之间的关系与该文所建立的模型拟合较好,呈幂函数关系;单位体积水滴动能沿径向呈一次函数关系增大,与压力的-0.556次方呈正比关系;动能强度沿径向逐渐增大,而在射程末端迅速减小至0,随压力增大而减小,且沿径向距离的增大而动能强度减小程度越大;在各工作压力下,全射流喷头的矩形最佳组合间距分别为1.2、1.0、1.1、1.0、1.1倍喷头射程;所对应的动能强度均匀性系数分别为56.6%、71.1%、76.2%、77.2%、72.9%。该结果对研究喷头外特性、优化喷头结构、喷灌系统优化配置提供了一定的理论价值。     

摇臂式喷头组合喷洒均匀性的改进

该文针对摇臂式喷头副喷嘴为圆形,径向水量分布中部较高不利于组合的问题,根据水射流原理,设计出一种新型结构的副喷嘴,其上端设有盖板,起到阻挡水流的作用,使副喷嘴水流射出后主要分散在近处,从而有利于提高组合喷洒的均匀性。分别对15PY、20PY、30PY摇臂式喷头副喷嘴改进前后的水量分布进行了对比试验,同时,以正方形布置方式为例,计算出不同组合间距下的喷洒匀性系数。结果表明：副喷嘴的改进消除了改进前喷头中间部分水量,使径向水量分布呈“三角形”。改进副喷嘴后提高了均匀性系数,不同组合间距下的值均为80%以上,平均喷灌强度符合喷灌规范中的要求。最后,提出副喷嘴改进后15PY,20PY,30PY摇臂式喷头的最佳组合间距分别为1.2R,1.3R,1.1R,计算出组合均匀性系数值分别为85.6%,86.3%,85.3%。改进后的副喷嘴对喷洒均匀性有较好的改善作用。     

隙控式全射流喷头性能特点及与摇臂式喷头的比较研究

研制了一种新型旋转式喷头——隙控式全射流喷头，介绍了其结构形式及工作原理。首次对二种PX10型号的全射流喷头及一种PY210摇臂式喷头的水力性能进行了对比试验及分析。研究表明：隙控式全射流喷头的流量大于试验用PY2摇臂式喷头8％左右，射程比摇臂式喷头的射程明显加大，最大增幅达到31％。全射流喷头水量分布均匀性与摇臂式喷头相当，但末端雨滴直径小于摇臂式喷头。由于其独特的射流元件及反向机构，全射流喷头结构简单，性能优良，价格低廉，具有显著的节能效果及良好的市场前景。     

摇臂式喷头副喷嘴仰角及位置参数的优化

摇臂式喷头副喷嘴的主要作用是增加喷头近处的喷水量,从而提高喷头的喷灌均匀度,因此对摇臂式喷头副喷嘴的研究具有重要意义。该文采用流场模拟和试验相结合的方法对摇臂式喷头的副喷嘴结构参数进行了优化设计。首先采用逆向工程的设计方法,运用Pro/E建立了摇臂式喷头的三维结构,选择副喷嘴的仰角和位置为待优化参数,设计出9种副喷嘴结构的摇臂式喷头。运用Pro/E和Hyper Mesh软件建立了9种副喷嘴结构摇臂式喷头的三维内流道模型,利用CFD软件FLUENT对9种喷头的内流道进行三维模拟,运用快速成型技术加工出喷头样品进行试验验证,在矩形组合方式下运用4种插值方法(距离插值法、线性插值法、立方插值法和三次样条插值法)计算喷灌组合均匀度,并且利用Sprinkler3D软件通过二维插值的方法建立了9种喷头的压力水量分布模型,对喷头喷洒均匀性进行评价。结果表明,6号喷头(副喷嘴位置参数为19.8 mm、仰角参数为18°)的模拟流量和速率较大,试验水量分布在2~12 m之间能够保持较好均匀性,且射程达到14 m以上,喷灌均匀度在压力为250和300 k Pa下都达到80%以上。因此,副喷嘴位置参数19.8 mm、仰角参数18°为摇臂式喷头副喷嘴的最佳结构参数。该研究为摇臂式喷头结构设计和喷灌系统优化等提供参考。     

摇臂式喷头与全射流喷头水滴分布对比试验

摇臂式喷头(impact sprinkler,PY)出口流体为单相水,全射流喷头(complete fluidic sprinkler,PXH)出口流体为气液两相流,为了深入探索2种类型喷头水滴分布的存在规律及差别,该文采用激光雨滴谱仪测量技术对PY及PXH喷头的水滴分布进行试验研究,采用体积加权法分析了这2种喷头在工作压力为150、200、250、300和350 k Pa情况下,距喷头不同距离处的水滴频率分布、水滴累计频率及中数直径的变化规律。结果表明:1)PXH喷头水滴频率普遍小于PY喷头。PXH喷头和PY喷头水滴频率分布分别符合对数正态分布和正态分布;2)PXH喷头水滴累计频率变化更加均匀,2种喷头的水滴直径分布均符合指数函数分布规律,在距离喷头距离较小时,PXH喷头比PY喷头的拟合精度更高,在距喷头距离为4 m下,PXH喷头拟合函数的R2值较PY喷头高3.5%;3)在低压条件下距喷头不同距离时,PXH喷头的水滴分布更加连续及均匀。建立了2种喷头中数直径与工作压力及距喷头距离的函数。该结果完善了多类型喷头喷洒水力学特性,对研究射流运动模型及喷洒的外特性提供了参考。     

Teejet雾化喷头的水力性能试验及工作参数优选

为探究Teejet雾化喷头工作参数对单喷头及组合喷洒水力性能的影响,进而得到雾化喷头的较优配置工况,该研究通过试验分析、理论计算和综合评价相结合的方法,探究等效直径、安装高度、工作压力对雾化喷头水力性能的影响规律;定量分析得到安装高度、工作压力、等效直径、组合间距与组合均匀性系数之间的关系;构建综合评价指标体系,采用主成分分析法确定雾化喷头最优配置方案。结果表明：1）随着工作压力的升高,喷头喷灌强度峰值逐渐增长;随着安装高度的升高,喷头的喷洒范围增大,水量分布更加均匀;相同安装高度和工作压力条件下,喷嘴的喷灌强度峰值与等效直径呈正比。2）3种等效直径喷头组合均匀性系数随工作压力和组合间距的变化趋势基本一致,组合间距对组合均匀系数的影响最为明显,随着喷头组合间距的增加,喷头的组合均匀系数大多呈先减小后增加再减小趋势。3）雾化喷头最优配置工况为喷嘴等效直径1.81 mm、安装高度0.6 m、组合间距0.2 m、工作压力400 kPa。该研究可为适用于温室的移动式微喷灌机组的参数配置提供依据,并为该机组在全国范围内的推广应用提供理论支撑。     

Soil-profile distribution of primary and secondary plant-available nutrients under conventional and no tillage

Nutrient distributions under no tillage (NT) compared with conventional disk-and-bed tillage (CT) management in the warm, humid region of the southeastern USA need to be assessed so that future placement, quantity, and type of fertilizers can be altered, if necessary, to efficiently match crop demands. We determined soil-profile distributions of pH, N, P, S, K, Ca, Mg, Na, Zn, Fe, Mn, and Cu to a depth of 0.9 m at the end of 8.5 years of continuous CT and NT management on a Weswood silty clay loam (fine, mixed, thermic Fluventic Ustochrept) in southcentral Texas. Most dramatic changes occurred within the 0–0.05 m depth, where soil under NT had lower pH, Fe, and Cu than under CT, but greater P, K, Zn, and Mn. Greater P and K under NT than under CT also occurred below the till-zone (0.15–0.3 m). At a depth of 0–0.3 m, soil under NT contained greater amounts of extractable P, K, Zn, Fe, Mn, and Cu than under CT. Nitrogen fertilization had little effect on nutrient distributions, except resulting in greater extractable K at 0–0.05 m and greater nitrate at 0–0.15 m. Few changes in soil-profile distributions were observed for extractable S, Ca, Mg, and Na. Long-term continuous use of NT on this fine-textured, high-fertility (except for N) soil had no apparent adverse effects on nutrient distributions relative to CT, but enhanced conservation and availability of P, K, Zn, Fe, Mn, and Cu near the soil surface where crop roots proliferate.     

Assessing the effectiveness of conservation measures: Resolving the “wicked” problem of the Steller sea lion

“Wicked” problems are those that are complex and that change when solutions are applied. Many conflicts in conservation fall into this category. The study approached the problem of how to constrain the apparent wickedness of a problem in the conservation management of a species by using simple empirical indicators to carry out iterative assessment of the risk to a population and to document how this risk evolves in relation to the addition of new data and the implementation of management actions. Effects of high levels of uncertainty within data and also concerning population structure were examined through stochastic simulation and by exploration of scenarios. Historical trends in the example used, the Steller sea lion, showed rapid declines in abundance in some regions during the 1980s. The current total population is 130,000-150,000 Steller sea lions through Alaska and British Columbia and this number has been stable since about 1990 in spite of regional differences in population dynamics. Regional differences in the sequence of changes in the number of pups and non-pups, suggested that an internal re-distribution of juveniles could have happened between 1980 and 1990. Current productivity also appears close to the long-term mean. Stochastic population projection using various scenarios showed that, based upon this history, the risk of extinction for the population has declined and is below reasonable thresholds for considering the population to be endangered. The trends in risk suggest that management actions taken since 1990 have probably been effective. Consequently, the conservation management objectives for the Steller sea lion are probably being met. The approach provides a mechanism, based upon experience and scenario analysis, for exploring future policy options and may help to constrain the debate amongst stakeholders about the cost-benefit trade-offs associated with different options.     

Enhanced establishment and growth of giant cardon cactus in an eroded field in the Sonoran Desert using native legume trees as nurse plants aided by plant growth-promoting microorganisms and compost

To evaluate the feasibility of long-term desert reforestation technology of mixed vegetation, cardon cactus (Pachycereus pringlei) seedlings from indoor and outdoor nurseries were planted in the field adjacent to one seedling of potential legume nurse trees: mesquite amargo (Prosopis articulata), yellow palo verde (Parkinsonia microphylla), and blue palo verde (Parkinsonia florida). Some of the planting holes were also supplemented with common dairy compost. Additionally, the combinations of legume tree–cactus were inoculated with either a consortium of desert arbuscular mycorrhizal (AM) fungi, plant growth promoting bacteria (PGPB; the diazotroph Azospirillum brasilense Cd, and the phosphate solubilizer Paenibacillus sp.), or a mixture of all. The field experiments were evaluated periodically during 30 months for survival and growth. Cardons reared in an outdoor screen house survived better in the field than those reared in a controlled growth chamber and hardened later outdoors. Association with any legume nurse tree increased survival and enhanced growth of untreated cardons. For cardons growing alone, application of either compost, AM fungi, and all the treatments combined increased survival. For these plants, no treatment affected plant growth during the first 3 months after transplanting. Later, all treatments, except for AM fungi, enhanced plant growth. However, only 2 years after transplanting the enhanced growth effect of AM fungi was also significant. In the presence of the legume nurse trees, transient positive effects on cardon growth were recorded. General evaluation after 30 months of cultivation showed that the treatments positively affected cardon growth when growing alone or in combination only with mesquite amargo but not with the other two legume trees. This study proposes that young legume trees have the capacity to enhance survival and growth of cardon cactus, depending on the legume cactus combination. Additional treatments such as compost or PGPB can either amplify the effect or else attenuate it.     

Karyotypic and genome size variation in Cajanus cajan (L.) Millsp. (pigeonpea) and some wild relatives

Detailed karyotypes and 4C DNA amounts have been studied in five cultivars of Cajanus cajan and 20 species belonging to Cajanus, Rhynchosia, Dunbaria, Flemingia and Paracalyx. C. cajan shows intraspecific variability and its karyotype is most similar to that of C. cajanifolius (sect. Cajanus) in the morphology and number of satellite chromosomes and the lack of any correlation between chromosome size within the complement and asymmetry. Karyotypes of C. lineatus and C. sericeus belonging to sect. Atylia are similar with respect to maximum r-index and the ratio of longest and shortest chromosomes in their respective complements. C. acutifolius (sect. Frutocosa) is distinct in having a very low ratio (1.44) between the longest and the shortest chromosomes, while in C. albicans, C. goensis, C. scarabaeoides (sect. Cantharospermum) there is no chromosome pair with r-index >2.0. C. mollis and C. volubilis (sect. Volubilis) show similarity with regard to ratio between the longest and the shortest chromosomes and a distinct chromosome pair with an arm ratio of 2.7 and C. platycarpus is distinct (sect. Rhynchosoides) in having the smallest ratio (1.36) between the longest and the shortest chromosomes. 4C DNA amounts in Cajanus and Rhynchosia vary between 3.28 pg to 11.69 pg and 4.92 pg to 11.13 pg respectively, while in Dunbaria, Flemingia and Paracalyx these vary between 4.70 and 7:06 pg. In the genus Cajanus karyotypic features and 4C DNA amounts agree with the sectional classification and bring out a close relationship between C. cajan and C. cajanifolius. This is supported by studies based on seed protein patterns, isozyme analysis, trypsin and chymotrypsin inhibitor patterns, RFLP and RAPD data and crossability relationships. A clear relation between DNA amount and abnormalities effecting the extent of pairing and recombination in interspecific hybrids shows the importance of this study in developing future breeding programme involving C. cajan and its wild relatives that are of potential value.     

Effect of some fungicide seed treatments on the survival of Bradyrhizobium japonicum and on the nodulation and yield of soybean [Glycine max. (L) Merr.]

Several commercial fungicide seed treatments were evaluated for their possible effect on the survival of Bradyrhizobium japonicum on seeds and on the nodulation and yield of soybeans in a greenhouse and a field experiment. quinolate Pro (carbendazim and oxine copper), Vitavax 200FF (carboxin and thiram), and Monceren (pencycuron) had a small effect or no effect on the survival of B. japonicum and on the nodulation and yield of soybeans. They can thus be considered compatible with soybean seed inoculation. Germipro UFB (carbendazim and iprodione), Apron 35J (metalaxyl), and Tachigaren (hymexazol) decreased B. japonicum survival and the nodulation and yield of soybeans and thus cannot be considered compatible with soybean seed inoculation.     

Einfluss der düngungsmethoden auf die entwicklung von zuckerrübenkrankheiten

Während neunjähriger (1994-2002) Untersuchungen in der Versuchsanstalt Ba?cyny (Fahlerde, Fall-Gleyboden, ausgebildet aus leichtem Lehmboden) wurde der Befall mit Krankheiten an der Zuckerrübe in Abhängigkeit von der Düngungsmethode bewertet. Folgende Methoden wurden angewendet: A - Mineraldünger, B - Stalldung und Mineraldünger, C - Stroh-, Gründung und Mineraldünger, D - Kompostdünger und Biopräparate (ökologische Düngung). Auf den Objekten A, B, C wurden bei konstanter Phosphor-Kalium-Düngung die Stickstoffgaben von 0 bis 180 kg.ha^?1 differenziert. Auf dem ökologischen Objekt D ohne Mineraldünger wurden statt Stickstoff die Biopräparate P₅₀₀ und P₅₀₁ verabreicht. Der geringste Befall von Krankheiten wurde an der Zuckerrübe auf jenem Objekt festgestellt, das mit Kompost und den Biopräparaten gedüngt worden war. Im Rahmen der konventionellen Düngungsmethoden wurde allgemein die höchste Intensität der Krankheiten an der mit Stroh, Gründung und Mineraldünger gedüngten Zuckerrübe beobachtet. Ausnahmen von dieser Regel bildete das Vorkommen von echtem Mehltau und Keimlingsfäule. Auf den Objekten A, B, C nahm die Intensität der Krankheiten mit der Steigerung der Stickstoffgabe zu. Auch die auf Objekt D verabreichten Biopräparate stimulierten die Entwicklung der Krankheiten. The effect of various fertilization methods on the diseases of sugar beet was estimated on the basis of nine-year studies (1994-2002) conducted in the Experimental Production Farm in Ba?cyny on loess-based, sedimentary-gley, light-clay soil. The following fertilization methods were applied: A - mineral fertilization, B - mineral and manure fertilization, C - straw, green manure and mineral fertilization, D - ecological compost and biodynamic preparations fertilization. In the objects A, B and C fertilized with phosphorus and potassium, various doses of nitrogen ranging from 0 to 180 kg/ha^?1 were used. In the ecological object D, where mineral fertilization was not used, biodynamic preparations P₅₀₀ and P₅₀₁ were applied instead of nitrogen. The lowest ratio of the diseases of sugar beet was observed in the object fertilized with the compost and biodynamic preparations. In view of the conventional fertilization methods, generally the highest incidence of diseases was noted in the sugar beet fertilized with straw, green manure and mineral fertilizers. The exception was the occurrence of powdery mildew and the black leg. In objects A, B and C the incidence of the diseases increased with the increase in nitrogen doses. The applied biodynamical preparations in the object D stimulated the development of diseases too.     

Prospects for control of Pythium damping-off of lettuce with Trichoderma,Gliocladium and Enterobacter spp.

Summary Damping-off of lettuce (Lactuca sativa) caused by Pythium ultimum was studied in pots containing a non-sterile potting mix in the glasshouse. Fifty P. ultimum sporangia g^-1 compost reduced the plant stand to 15% and shoot dry weight to 18%, but this reduction was totally prevented by applying Trichoderma harzianum at 2×10⁵ viable propagules g^-1 potting mix. Gliocladium virens and Enterobacter cloacae also alleviated the damping-off. E. cloacae did not adversely affect the action of the fungal antagonists. The co-existence of the bacterial and fungal antagonists was revealed on the root surface and inner surface of the testa by scanning electron microscopy, indicating the compatibility of the biocontrol agents.     

细菌、真菌及植物氮营养信号研究进展

氮素吸收代谢是所有生物生命活动的核心部分,因此,各种生物对氮的吸收与同化过程都有精细的调节。这种调节依赖于生物对体内氮素状况信号及体外（生长介质中）氮素信号的感受。过去几十年中,在细菌、真菌中已经对氮素营养信号有了较好的研究,而植物中则相对缓慢,但也有了一定的认识。本文重点比较细菌、真菌及植物中氮信号系统的组成,以期为进一步认识植物中的氮信号接受与转导系统提供启示。     

Aircraft operations near concentrations of birds in Antarctica: The development of practical guidelines

Aircraft operations have the potential to disturb and to impact negatively on bird life. A gradient of increasing behavioural response is evident in birds when exposed to increasing aircraft stimulus. The most major disturbance is likely to lead to impacts on the health, breeding performance and survival of individual birds, and perhaps bird colonies. A process of revision to policies on aircraft operations contained in management plans for a number of specially protected areas in Antarctica by the United Kingdom, accompanied by consultations made within the scientific community through the Scientific Committee on Antarctic Research (SCAR) and with operational interests through the Council of Managers of National Antarctic Programmes (COMNAP) resulted in new guidelines being adopted by the Antarctic Treaty Consultative Parties in June 2004. The principal recommendations of the guidelines are that bird colonies should not be overflown below 2000 ft (∼610 m) above ground level and landings within 1/2 nautical mile (∼930 m) of bird colonies should be avoided wherever possible. These guidelines are less stringent and less specific than those that were recommended by the SCAR specialist group on birds, and represent a compromise to accommodate operational needs. While the adoption of clear and consistent guidelines for the operation of aircraft in Antarctica is welcome in that this provides practical advice that is likely to reduce incidences of close aircraft/bird encounters, there remains insufficient knowledge of the interactions between aircraft and birds in Antarctica, and the consequent impacts on individual birds and on bird populations. It is important, therefore, that the guidelines adopted are considered interim, and should be kept under scrutiny with revisions made as new and improved research results appear.     

Potential of wild species for genetic enhancement of some semi-arid food crops

Discovery and incorporation of genes from wild species provide means to sustain crop improvement, particularly when levels of resistance in the cultigens are low and virulent strains of pests and pathogens overcome the host plant resistance. The extent of utilization and the potential of the wild genepool for genetic enhancement were reviewed in five important food crops viz. sorghum, pearl millet, chickpea, pigeonpea and groundnut grown in the semi-arid tropics. Introgression from compatible wild germplasm in the primary gene pool resulted in transfer of new cytoplasmic male sterility systems in pearl millet and pigeonpea, development of high protein, cleistogamous flower and dwarf pigeonpea lines and foliar disease resistant groundnut cultivars. Utilization of wild species in secondary and tertiary gene pools has been generally limited due to sterility, restricted recombination or cross incompatibility. Nevertheless, these species are extremely important as they contain high levels of resistance to several important biotic and abiotic stresses. Several of them, like those belonging to the Parasorghum section and the rhizomatous Arachis species are sources of multiple resistances and hold great promise to sustain crop productivity.