利用代谢组技术研究南林895杨响应盐胁迫的生理代谢机制

为揭示林业树种在盐胁迫下的响应机制,以中等耐盐的美洲黑杨无性系后代南林895杨为材料,研究对照和盐胁迫(100 mmol·L^-1 NaCl)条件下杨树的生长情况,采用超高效液相色谱-四极杆-飞行时间质谱(UHPLC-QTOF-MS)技术分析叶片代谢物,利用实时荧光定量PCR(RT-qPCR)技术检测部分关键基因表达。结果表明,盐胁迫16 d导致杨树生长受抑制,但对其叶和根中的丙二醛含量无显著影响。正交偏最小二乘法判别分析(OPLS-DA)鉴定出两组的差异成分,筛选出5条代谢组分差异显著的代谢通路。盐胁迫下,南林895杨叶片中α-酮戊二酸、磷脂酰胆碱(PC)以及棉子糖家族低聚糖(RFOs)积累,而苹果酸和延胡索酸减少;核酸、细胞分裂素B、氨基酸和大部分有差异的酚类含量下降。同时,三羧酸循环、RFOs合成、果糖代谢,以及谷氨酸合成主途径等相关基因上调表达;而海藻糖激酶、酚类合成及谷氨酸合成支路等相关基因下调表达。本研究结果有利于深入了解林木树种耐盐机制,可为更好地发展杨树产业提供理论基础。     

盐敏感型与耐盐型高粱对盐胁迫反应的转录组差异分析

  【目的】  研究高粱盐胁迫的生理学差异及其分子机制,发掘高粱在盐胁迫过程中的关键调控基因,筛选高粱耐盐和盐敏感材料,探讨高粱耐盐胁迫的机制。  【方法】  本试验以耐盐材料“67B”及盐敏感材料“3560R”为研究对象,加入150 mmol/L NaCl溶液进行盐胁迫,测定叶片生长指标、进行转录组测序和生物信息学分析。  【结果】  盐分胁迫下,耐盐材料生长速率快,表现出较强的耐盐性,耐盐材料可以提高Na+的选择吸收及其在植株体内的积累与分配。盐胁迫下耐盐材料可以维持较高的过氧化氢酶活性,受到盐胁迫后该酶活性升高幅度相对较大,进而保持了较强的过氧化氢清除能力,能够及时清除过量积累的活性氧。盐胁迫下两个品系共有5040个差异表达基因。盐敏感材料和耐盐材料对盐胁迫的响应途径是相同的,两者差异表达基因在KEGG各pathway中的分布趋势差别很大,排名前五的基因数条目有3条相同,分别为苯丙烷类合成、植物激素信号转导和碳代谢通路,盐敏感材料中另外两条不同的条目为淀粉与蔗糖代谢及氨基酸生物合成通路,与基础代谢有关,盐敏感材料中差异基因主要集中在基础代谢和次生物质合成途径,是造成两个材料耐盐性差异的重要原因。  【结论】  高粱的耐盐机制调控是一个复杂的过程,是由不同通路一系列基因表达共同作用的结果,依赖于多个基因在复杂网络中的平衡表达。     

植物耐盐性机理及基因控制技术研究进展

该文对近年来国内外植物耐盐性方面的研究成果作了简单的综述,包括植物的耐盐机制、盐胁迫对植物光合作用、细胞膜透性、激素、矿质元素吸收、有机物质的积累、保护酶类的活性的影响,以及植物的耐盐性在基因工程上的应用。     

水稻生长、根系生理特性和ABA含量的基因型差异与耐盐性的关系

为进一步揭示不同基因型水稻耐盐性差异的生理机制,探明盐胁迫初期根部激素ABA对水稻耐盐性的调控机理。于2009年10月～2010年2月在严格控制水、温、光和营养元素供应的国际水稻研究所人工气候室进行水培试验。结果表明,盐胁迫条件下耐盐基因型（IR651）相对于敏感基因型（IR29）保持了更高的生物量,稀释了植株体内盐分离子的浓度而减轻盐胁迫。两水稻基因型盐胁迫条件下对盐分的总吸收量并无明显差异,IR651根部较强的耐盐性和较大的生物量可以储存更多的Na+,从而减少Na+向地上部的转运量。盐胁迫初期IR651根部ABA的大量合成是叶片蒸腾速率显著降低的主要原因,从而抑制了盐分离子的大量吸收,大大减轻了盐胁迫初期大量盐分离子吸收对植株造成的不可恢复性伤害。可见,盐胁迫条件下耐盐基因型较大的生物量、根的生理特性以及盐胁迫初期ABA的特有调控都大大增强了其耐盐胁迫性能,是耐盐基因型相对敏感基因型有更强耐盐胁迫能力的重要原因。     

植物耐盐性机理及基因控制技术研究进展

该文对近年来国内外植物耐盐性方面的研究成果作了简单的综述，包括植物的耐盐机制、盐胁迫对植物光合作用、细胞膜透性、激素、矿质元素吸收、有机物质的积累、保护酶类的活性的影响，以及植物的耐盐性在基因工程上的应用。     

大豆耐盐性研究进展

大豆是重要的粮食和经济作物。盐害作为主要非生物胁迫之一,对大豆的生长、结瘤、农艺性状、种子品质和籽粒数造成负面影响,最终会降低大豆的产量。了解盐害对大豆生产造成的具体影响,能够帮助育种工作者在耐盐育种时进行针对性精确耐盐表型选择。分子标记辅助选择育种是一种高效的育种方法,可促进大豆耐盐品种育种进程;而从基因组水平上了解大豆耐盐相关机制,则为"组装"耐盐基因,培育耐盐大豆新品种提供参考依据。本文总结了盐害对大豆生长发育造成的具体影响,介绍了大豆耐盐相关性状的分子遗传研究以及大豆耐盐相关离子转运蛋白基因及其耐盐机制,以期为耐盐大豆新品种的选育及加速大豆耐盐精确育种进程提供一定的参考。     

高盐胁迫下小麦幼苗离子吸收动态及耐盐性筛选

耐盐能力评价是小麦引种、筛选和育种的研究基础。为利用离子流检测技术快速筛选耐盐小麦品种提供依据,本文以普通小麦耐盐品种‘德抗961’和‘薛早’、中度耐盐材料3D232、盐敏感品种‘辽春10号’和‘京411'为试验材料,利用动态离子流检测技术对250 mmol·L~(-1) NaCl胁迫下小麦苗期根部对K~+、Na~+、C~-的吸收情况进行检测,并对小麦生长性状及离子浓度变化进行测定,以确立离子吸收与小麦耐盐性的关系。研究结果表明:1)与无盐胁迫(CK)相比,250 mmol·L~(-1) NaCl胁迫24 h后,盐敏感小麦品种‘辽春10’和‘京411’的K~+由内流转变为外流,中等耐盐材料3D232表现出K~+外流速度减少,耐盐品种‘德抗961’和‘薛早’则表现出维持K~+的内流或K~+外流变为内流;Na~+均表现为胁迫后外排速度增大,速度值区间由13.86～46.88 pmol·cm~(-2)·s~(-1)变为61～150 pmol·cm~(-2)·s~(-1):相较Na~+,Cl~-外排速度升高幅度较大,其中‘辽春10号’外排量变化最大,外排速度是胁迫前的10倍,Na~+、Cl~-外排速度变化与品种耐盐性无明显相关性。2)高盐胁迫下,盐敏感小麦的根苗比降低,耐盐小麦根苗比升高;盐敏感小麦品种鲜重较CK显著下降,耐盐小麦品种变化不显著。3)盐胁迫条件下,耐盐及中等耐盐小麦品种,根部及地上部K~+含量较CK分别增加57%～88%和18%～112%,盐敏感小麦则分别降低40%～44%和24%～42%;耐盐小麦地上部Na~+增加倍数小于盐敏感材料,将更多的Na~+阻隔在根部,表现出了较好的区隔Na~+能力。4)盐胁迫后k~+流速与耐盐性评价指标根冠比变化量、鲜重变化率均呈高度的相关,其拟合度分别为0.972和0.832。250 mmol·L~(-1) NaCl胁迫24 h后小麦根部成熟区K~+流速可以作为小麦耐盐性筛选的重要生物标记。     

不同盐胁迫条件下微生物菌剂对土壤盐分及苜蓿生长的影响

为研究微生物菌剂对盐碱土的改良效果以及筛选较为耐盐的苜蓿品种,通过室外盆栽试验,研究了不同盐胁迫下微生物菌剂对土壤盐分和苜蓿生长状况的影响,并探讨了盐胁迫微生物菌剂苜蓿品种三者之间的交互作用。结果表明:微生物菌剂对020cm土壤脱盐率有显著影响,与不加菌剂处理相比脱盐率提高9.04%;盐胁迫对不同苜蓿品种生物量有显著影响,表现为随盐胁迫的增加生物量下降;不同苜蓿品种之间的耐盐性差异显著,其中劳博品种最为耐盐,保定次之,三得利最差。因此,盐胁迫下微生物菌剂、耐盐苜蓿品种对于降低土壤盐分以及促进苜蓿生长具有良好调控效果。     

转碱蓬SsNHX1基因烟草的耐盐抗旱性研究

烟草是重要的模式植物和经济作物,盐害和干旱两种环境因子对其生长发育、产量和品质都危害很大。为了提高烟草的耐盐抗旱性,本研究利用农杆菌介导的遗传转化法在烟草中过量表达了碱蓬液泡膜Na~+/H~+逆向转运基因SsNHX1,对转基因烟草的耐盐及抗旱性进行表型鉴定和各项生化指标的检测,以期得到耐盐抗旱表性良好的SsNHX1转基因烟草。表型分析发现,SsNHX1基因过表达株系L1和L5的抗盐能力比野生型显著提高,表现为盐胁迫条件下仍能保持旺盛的生长且根系的伸长未受抑制。SsNHX1过表达株系在叶片和根系中积累了更多的Na~+和K~+,同时Na~+含量增长速率较快,而K~+含量降低速率较缓,并可维持较高的叶片相对含水量和叶绿素含量,及较低的丙二醛含量和相对电导率。干旱胁迫发现,过表达株系受干旱胁迫程度更小,并在复水后迅速恢复正常生长。同时,过表达株系的丙二醛含量和相对电导率显著低于野生型,且维持了较高的叶片相对含水量及叶绿素含量。这些结果说明SsNHX1基因在烟草中过量表达后,降低了盐胁迫和干旱胁迫对烟草根系及细胞膜的损伤,并通过调节离子含量、降低细胞的渗透势,维持了叶片较高的相对含水量和叶绿素含量,最终提高了烟草的抗盐和抗旱性。     

氯化钠胁迫对不同耐盐黄瓜品种的损伤效应及反射光谱特性的影响

研究了氯化钠胁迫对不同耐盐黄瓜品种的损伤效应及反射光谱特性的影响。结果表明,黄瓜品种新泰密刺比津优1号具有更强的耐盐性;盐处理明显增加了黄瓜叶片在可见光区的光谱反射率,并且盐敏感品种津优1号在盐胁迫下光谱反射率增加的幅度明显高于耐盐品种新泰密刺,尤其在绿光区差异更为明显。利用光谱反射指标得出的叶绿素含量、叶黄素循环库的大小与黄瓜的盐害程度及不同品种的耐盐程度有密切关系,反射光谱特性可以用来作为筛选黄瓜耐盐的生理指标。     

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.     

The abundance of heterocystous blue-green algae in rice soils and inocula used for application in rice fields

Summary Algal populations were quantified (as colony-forming units [CFU] per square centimetre) in 102 samples of rice soils from the Philippines, India, Malaysia and Portugal, and in 22 samples of soil-based inocula from four countries. Heterocystous blue-green algae (BGA) were present in all samples. Nostoc was the dominant genus in most samples, followed by Anabaena and Calothrix. In soils, heterocystous BGA occurred at densities ranging from 1.0 × 10² to 8.0 × 10⁶ CFU/cm² (median 6.4 × 10⁴) and comprised, on average, 9% of the total CFU of algae. Their abundance was positively correlated with the pH and the available P content of the soils. In soil-based inocula, heterocystous BGA occurred at densities ranging from 4.6 × 10⁴ to 2.8 × 10⁷ CFU/g dw (dry weight), comprising only a moderate fraction (average 13%) of the total algae. In most soils, the density of indigenous N₂-fixing BGA was usually higher than that attained by applying recommended rates of soil-based inoculum. Whereas research on the practical utilization of BGA has been mostly directed towards inoculation with foreign strains, our results suggest that attention should also be given to agricultural practices that enhance the growth of indigenous strains already adapted to local environmental conditions.Maître de Recherches ORSTOM (France), Visiting Scientist at IRRI