Transformation of organic rhizodepositions by rhizosphere bacteria and its influence on the availability of tertiary calcium phosphate

This study assesses the influence of saccharides in the rhizodeposition on the phosphate solubilizing ability of rhizosphere bacteria. Water‐soluble rhizodeposits were analyzed of ¹⁴C‐labeled pea plants (Pisum sativum, cv. ‘Grapis’) which were grown at two different levels of P‐nutrition. The sugars produced were fed in vitro either as single compounds or as synthetic mixtures to three bacterial strains and the ability of the bacteria to mobilize Ca₃(PO₄)₂ was measured. The relative glucose proportion of pea exudates decreased under P deficiency while the content of galactose, ribose, xylose and fucose increased. In vitro feeding of single sugars and sugar mixtures showed that the ability of Pseudomonas fluorescens (PsIA12) to dissolve tertiary calcium phosphate was lower with pentoses and the mixed sugars of the P‐deficient plants than with glucose. On the other hand, the shifted sugar pattern observed under P deficiency increased the phosphate mobilization ability of Pantoea agglomerans (D5/23) and Azospirillum sp. (CC 322) considerably. This observation can only partly be explained by the acidification of the nutrient medium. Bacteria also produced different carboxylic anions depending on sugar supply. In addition to low‐molecular mono‐, di‐, and tricarboxylic acids which are known as P‐solubilizing substances, sugar acids also played an important role in cultures D 5/23 and CC 322.     

烟草根系分泌物酚酸类物质的鉴定及其对根际微生物的影响

【目的】烟草连作已导致土传病害发生、 烟株生长受抑制、 产量下降和品质恶化等问题。烟株对自身及土壤微生物产生的化感作用,是烟草产生连作障碍的一个重要原因,其中化感物质中的根系分泌物是烟株与土壤微生物间相互作用的重要物质,探索烟草根系分泌物对根际微生物生长的影响是生物防控烟草青枯病的理论依据。【方法】本文利用超高效液相色谱串联四级杆飞行时间质谱(UPLC-Q-TOF/MS)技术,分离、 鉴定烟草根系分泌物中主要酚酸类物质的种类和含量; 通过添加外源酚酸类物质,研究在液体培养基中烟草根系分泌物中的主要酚酸类物质对病原菌及拮抗菌的影响,并在土壤中添加鉴定出的主要酚酸类物质; 通过土壤培育试验,研究其对土壤微生物多样性和数量变化,特别是对烟草青枯病菌及其拮抗菌生长的影响。【结果】1)烟草根系分泌物粗提物对病原菌(茄科劳尔氏菌)生长的促进率为16.8％,对拮抗菌(短短芽孢杆菌)的生长抑制率达到29.4％; 2)UPLC-Q-TOF/MS检测根系分泌物中主要酚酸类物质为苯甲酸和3-苯丙酸,含量分别为0.25 μg/g干根重和1.15 μg/g干根重; 3)液体培养外源添加低浓度的苯甲酸(≤ 2 μg/L)和3-苯丙酸(≤ 3 μg/L)促进病原菌和拮抗菌的生长; 4 μg/L的苯甲酸对病原菌生长抑制作用不显著,对拮抗菌生长的抑制率达到90.2％,6 μg/L的 3-苯丙酸对病原菌的生长具有促进作用,对拮抗菌的生长抑制率达到81.1％,外源高浓度苯甲酸(≥4 μg/L)和3-苯丙酸(≥ 7 μg/L)抑制病原菌与拮抗菌的成长; 4)土壤中添加3 μg/kg土的苯甲酸时,土壤中病原菌的数量增加12.3％,而拮抗菌的数量减少21.0％,土壤细菌、 放线菌和真菌的数量分别降低37.5％,41.9％和55.6％; 3-苯丙酸浓度达到8 μg/kg土时,拮抗菌生长量减少14.5％,对病原菌没有显著影响,土壤细菌、 放线菌和真菌的数量分别降低69.9％,57.2％和80.7％; 5)土壤添加4 μg/kg的苯甲酸和7 μg/kg的3-苯丙酸后,土壤微生物Shannon指数、 Simpson指数、 McIntosh指数显著下降,分别仅为对照的57.7％、 94.1％、 88.1％和97.6％ 73.3％、 80.0％。【结论】烟草根系分泌物粗提物促进病原菌生长抑制拮抗菌生长,根系分泌物中酚酸类物质主要为苯甲酸和3-苯丙酸,液体培养中4 μg/L的苯甲酸或6 μg/L的 3-苯丙酸浓度是对病原菌生长抑制不明显但显著抑制拮抗菌生长的分界点,土壤外源添加3 μg/kg的苯甲酸或8 μg/kg的3-苯丙酸时,是土壤增加病原菌减少拮抗菌数量的分界点,同时土壤微生物功能多样性显著下降,病原菌对根系分泌物中苯甲酸和3-苯丙酸的利用优于拮抗菌,这也是烟草长期连作引起青枯病暴发流行的机理之一。     

烤烟根系分泌物对烤烟幼苗生长和养分吸收的影响

连作是烟草栽培中的普遍现象,已引起生长抑制、产量下降和品质恶化等问题。本试验采用溶液培养的方法,在培养液中分别加入未分组和分组后的烤烟根系分泌物,研究根系分泌物对烤烟幼苗生长和养分吸收的影响。结果表明,加入未分组的烤烟根系分泌物显著抑制幼苗的生长,降低根系活力,并随加入量的增加抑制作用增强;加入酸溶性、碱溶性、中性组分的根系分泌物,均降低幼苗根系活力,以中性组分的抑制作用较强。三种不同组分的根系分泌物均显著降低根系对NO3-、PO43-、K+离子的吸收,其中中性组分对NO3-吸收的影响最大,而酸溶性组分对K+的吸收抑制作用较强。推测在烤烟根系分泌物中,可能存在多种抑制烤烟生长和养分吸收的化学物质。     

Phosphorus solubilization in the rhizosphere and its possible importance to determine phosphate plant availability in soil. A review with main emphasis on German results

Limited fertilization adapted to plant demand is of high economical and ecological relevance. This requires a reliable analysis of plant available P, based on knowledge of phosphorus dynamics in soils and P mobilization by plants. On chernozem-like soils, as well as under dry conditions, the double-lactate (DL) phosphate extraction methods apparently do not adequately reflect the P uptake ability of plants. This paper summarizes rhizosphere processes that affect P availability partly by reference of selected own experiments. Root exudates increased the double-lactate (DL) extractable P amount of soils in sterile and non sterile cultures. Microbial colonisation increased both the exudate amount and the specific ability of exudates to solubilize P. In spite of rapid exudate turnover, DL-P solubility was increased. Sugars released from P-deficient plants increased the P solubilizing ability of a bacterial strain (Enterobacter radicincitans), perhaps by changing bacterial acid production. Root exudates solubilized more P from soil than lactate extracts did. An investigation of physiological processes in the rhizosphere could contribute to a better understanding of nutrient availability and perhaps lead to the development of extraction methods that better reflect the availability of soil phosphorus to plants. Connecting field experiments with basic studies offers the opportunity to better understand plant nutritional processes to realize an effective and sustainable agriculture.     

Soil sickness of peanuts is attributable to modifications in soil microbes induced by peanut root exudates rather than to direct allelopathy

The quantity and quality of peanut yields are seriously compromised by consecutive monoculture in the subtropical regions of China. Root exudates, which represent a growth regulator in peanut–soil feedback processes, play a principal role in soil sickness. The growth inhibition of a species in an in vitro bioassay enriched with root exudates and allelochemicals is commonly viewed as evidence of an allelopathic interaction. However, for some of these putative examples of allelopathy, the results have not been verified in more natural settings with plants continuously growing in soil. In this study, the phenolic acids in peanut root exudates, their retention characteristics in an Udic Ferrosol, and their effects on rhizosphere soil microbial communities and peanut seedling growth were studied. Phenolic acids from peanut root exudates were quickly metabolized by soil microorganisms and did not accumulate to high levels. The peanut root exudates selectively inhibited or stimulated certain communal bacterial and fungal species, with decreases in the relative abundance of the bacterial taxa Gelria glutamica, Mitsuaria chitosanitabida, and Burkholderia soli and the fungal taxa Mortierella sp. and Geminibasidium hirsutum and increases in the relative abundance of the bacterial taxon Desulfotomaculum ruminis and the fungal taxa Fusarium oxysporum, Bionectria ochroleuca and Phoma macrostoma. The experimental application of phenolic acids to non-sterile and sterile soil revealed that the poor performance of the peanut plants was attributed to changes in the soil microbial communities promoted by phenolic acids. These results suggest that pathogenic fungal accumulation at the expense of such beneficial microorganisms as plant growth promoting rhizobacteria, mycorrhizal fungi induced by root exudates, rather than direct autotoxicity induced by root exudates, might represent the principal cause underlying the soil sickness associated with peanut plants. We hope that our study will motivate researchers to integrate the role of soil microbial communities in allelopathic research, such that their observed significance in soil sickness during continuous monocropping of fields can be further explored.     

根际微生态系统中的碳循环

本文综述了根际微生态系统中碳素循环的研究动态。着重讨论了碳在根际微生态系统中的分配和去向 ,分析了根际沉积及其影响因素与根际微生态系统中碳素平衡之间的关系 ,并提出了今后有关研究的若干重点     

根系分泌物抑制连作障碍线虫病的根际调控机制及其应用

作物长期连作极易出现连作障碍(再生病害),本文围绕因传统耕作模式和种植习惯而诱发的新疆棉花、黑龙江大豆、河南花生、山东设施蔬菜、两广香蕉等连作障碍问题,对全国连作现象进行系统分析,发现全国连作现象普遍。按照耕地面积将连作障碍划分为五个等级,其中华北三省和东北的黑龙江省连作障碍等级最高,且各省连作现象均以大田经济作物为主。以香蕉、大豆(大田经济作物)、黄瓜(设施园艺作物)和三七(中草药)为代表,比对最低产量和正常产量在连作年份的变化,表明连作障碍发生规律成抛物线式,防控连作障碍需找到问题关键时期。单一根系分泌物介导的微生物多样性降低、病原菌富集,植物寄生线虫危害和土壤弱化是导致连作障碍的主要原因,其中线虫对植物的侵染危害作为土传病害是防治连作障碍中最难解决的问题之一,尤其是在设施蔬菜上。线虫在长期进化过程中形成了具有识别、寻找和侵染寄主的生物学功能,而不同植物根系分泌物对线虫发育和对宿主的识别侵染能力有不同的调控作用。针对根系分泌物–线虫互作为诱因的线虫病害,深入探讨易感作物和抗性/非寄主植物根系分泌物对线虫发育和对植物侵染的生物学机制,提出根际调控措施。在J2时期利用抗性/非寄主植物根系分泌物,调控根结线虫Mi-16D10、Mi-flp-18等基因和孢囊线虫的Hg-rbp-2等基因的表达控制线虫的发育、侵染和迁移,通过生物源功能物质定向防控线虫侵染作物。这些结果加深了我们对生物活性物质调控植物寄生线虫机制的认识。未来,以筛选和鉴定抗性或非寄主作物特异根系分泌物对线虫侵染的调控为依据,配置生物功能型肥料,利用植物源活性物质替代传统农药控制线虫病害的根际调控措施,定向调控植物根际生物学过程将成为国内外研究热点。此文将为未来深入系统地研究根系分泌物–线虫的相互作用及克服连作障碍提供理论基础,进而促进土壤健康和作物优质高产高效,对实现绿色农业和可持续发展具有重要的理论和实践指导意义。     

不同品种小麦与蚕豆间作对蚕豆枯萎病发生、根系分泌物和根际微生物群落功能多样性的影响

通过田间小区试验和盆栽试验,研究了3个不同品种小麦［云麦42（YM42）,云麦47（YM47）和绵阳29（MY29）］与蚕豆间作对蚕豆枯萎病发生、 根系分泌物和根际微生物群落功能多样性的影响。结果表明,云麦42与蚕豆间作处理（YM42//B）和云麦47与蚕豆间作处理（YM47//B）使蚕豆枯萎病病情指数分别降低47.6%和33.3%,而绵阳29与蚕豆间作处理（MY29//B）对蚕豆枯萎病病情指数无显著影响。与蚕豆单作相比,YM42//B和YM47//B处理显著增加了根系分泌物中有机酸的含量,显著降低了根系分泌物中可溶性总糖和游离氨基酸含量; 显著提高了蚕豆根际微生物对碳源的利用强度,明显改变了蚕豆根际微生物群落结构,并且YM42//B处理的影响大于YM47//B; 而MY29//B处理对碳源利用强度、 根际微生物群落结构和根系分泌物中有机酸、 可溶性总糖和游离氨基酸含量均无显著影响。 YM42//B处理根际微生物利用碳源的种类比YM47//B处理多,同时YM42//B和YM47//B处理利用的糖类、 氨基酸类和羧酸类碳源种类完全不同。表明云麦42和云麦47与蚕豆间作通过增加有机酸含量,从而提高根际微生物活性和多样性,促进了根际微生物利用更多的碳源,同时云麦42和云麦47与蚕豆间作抑制了氨基酸和总糖的分泌,而最终控制了蚕豆枯萎病的发生。表明不同品种小麦与蚕豆间作系统根系分泌物-根际微生物的互作是影响蚕豆枯萎病抗性的重要原因。小麦与蚕豆间作控病效果受小麦品种的影响,以云麦42与蚕豆间作效果最好,其次为云麦47与蚕豆间作,而绵阳29与蚕豆间作无显著控病效果。     

The impact of sugarcane filter cake on the availability of P in the rhizosphere and associated microbial community structure

This study aimed to evaluate the interaction between filter cake (FC), and phosphorus fertilizers with differing solubility on the growth and P nutrition of sugarcane. Effects of soil amendment with FC on different soil P fractions and influence on microbial community structure in the rhizosphere were also assessed. Two glasshouse experiments were conducted with completely randomized block designs. The first experiment evaluated rates of FC using a factorial design (5 × 2): 0, 2.5, 5, 10 and 15 g FC kg^?1 soil applied as either broadcast in bulk soil or in the planting furrow. The second experiment used a factorial design (2 × 3): without and with FC (5 g kg^?1 soil, dry basis), both without P (NP) and with P supplied as either triple superphosphate (TSP) or as rock phosphate (RP), both at the rate of 78.4 mg kg^?1 based on total P. Microbial community structure was determined using TRFLP and dynamics of soil P by Hedley fractionation. Filter cake applied at increasing rates in the absence of P was effective in increasing shoot growth and P uptake by plant, particularly when applied to bulk soil as compared to furrow application. Also, FC improved P uptake and increased the availability of labile inorganic P in the rhizosphere and modified the structure of fungal and bacterial communities, whereas only bacterial and archaea communities were influenced by P fertilizer use. Filter cake was more effective when combined with RP, with increased growth and P utilization and thus can be considered as a feasible and practical option for farmer use in substitution to TSP, a more expensive source.     

植物根际微生物组的研究进展

根际微生物组(rhizosphere microbiome),是植物从其种子库土壤微生物组中有选择性地招募在根际聚集的动态微生物集群.随着近年来高通量测序技术、宏基因组学等的飞速发展,根际微生物组与植物宿主及土壤微生物组间的紧密联系引起了全球关注和研究热潮.根际微生物组被视作植物第二基因组,其与植物间的互作极为复杂,有...     

Trichoderma harzianum T-E5 significantly affects cucumber root exudates and fungal community in the cucumber rhizosphere

The Fusarium wilt in cucumbers, caused by the pathogenic fungus Fusarium oxysporum f. sp. cucumerinum, is a serious and destructive disease worldwide. An effort was made to explore the role of Trichoderma harzianum T-E5 in reducing the incidence of Fusarium wilt. Three treatments (Control, T1, and T2) were established in the greenhouse experiment. The effects of T-E5 on the composition of root exudates and fungal community in the cucumber rhizosphere were measured. Compared with the control, the application of a bio-organic fertilizer (BIO) enriched with T-E5 was found to decrease the incidence of Fusarium wilt notably and promote the growth of cucumber plants. Based on real-time PCR, the population of F. oxysporum in the control without T-E5 increased from 10³ to 10⁴ ITS copies g⁻¹ soil, whereas the population decreased from 10³ to 10² ITS copies g⁻¹ soil in the T1 and T2 treatment groups when T-E5 was included. Significant difference in fungal community was also found among the treatment groups. HPLC analysis showed that the detected levels of phenolic compounds in control were significantly higher than the levels in the samples subjected to T1 and T2 treatments. The root exudates from the control group significantly increased the numbers of germinating spores of the pathogen compared with those from the samples treated with T1 and T2. In conclusion, the modification of root exudates and the fungal community by the application of BIO might account for the effective suppression of Fusarium wilt disease in cucumbers.     

土壤水分对花生根际分泌物中低分子量有机酸的释放及根质外体中铁的积累的影响

A three-compartments rhizobox was designed and used to study the low-molecular-weight organic acids in root exudates and the root apoplastic iron of “lime-induced chlorosis“ peanut grown on a clacareous soil in realtion to different soil moistrue conditions.Results showed that chlorosis of peanuts developed under condition of high soil mositure level(250 g kg^-1),while peanuts grew well and chlorosis did not develop when soil moisture was managed to a normal level(150 g kg^-1).The malic acid maleic acid and succinic acid contents of chlorotic peanut increased by 108.723,0.029,and 22.446ug cm^-1 ,respectively,compared with healthy peanuts.The content of citric acid and fumaric acid also increased in root exudates of chlorotic peanuts.On Days 28 and 42 of peanut growth,the accumulation of root apoplastic iron in chlorotic peanuts was higher than that of healthy peanuts.From Day 28 to Day 42,the mobilization percentages of chlorotic peanuts and healthy peanuts to root apoplastic iron were almost the smae,being 52.4% and 52.8%,respectively,indicating that the chlorosis might be caused by the inactivation of iron within peanut plant grown on a calcareous soil under soil moisture conditions.     

氮素对不同大豆品种根系分泌物中有机酸的影响

采用室内溶液培养方法,分别研究了接种根瘤菌处理下,两种氮源和两种氮浓度对两个大豆品种根系分泌物中有机酸的影响。结果表明,合丰25号根系分泌的有机酸种类和数量无论苗期或花期,接种或不接种根瘤菌,均表现为硝态氮处理高于酰胺态氮处理,表明合丰25号大豆更喜硝态氮,硝态氮促进了有机酸的分泌。绥农10号在酰胺态氮下的有机酸种类和数量均高于硝态氮处理,表明其更喜酰胺态氮,酰胺态氮下根瘤菌的存在增加其根系分泌物中有机酸种类和数量。可见,大豆根系分泌物中有机酸的种类和数量因品种而异,因品种对氮源的喜好而变化;根瘤菌在不同程度上增强或减弱根系有机酸的分泌作用。柠檬酸受氮素供应浓度影响很大,当氮素供应浓度较低时,大豆根系分泌物中可检测到柠檬酸,供氮浓度升高则检测不到。     

The influence of root hairs on the uptake of phosphate

Abstract

The uptake of phosphate from stirred solution by roots was not affected by root hairs. In contrast to this, root hairs appreciably increased the uptake of phosphate from a clay soil.     

Substrate inputs and pH as factors controlling microbial biomass, activity and community structure in an arable soil

Our aim was to determine whether the smaller biomasses generally found in low pH compared to high pH arable soils under similar management are due principally to the decreased inputs of substrate or whether some factor(s) associated with pH are also important. This was tested in a soil incubation experiment using wheat straw as substrate and soils of different pHs (8.09, 6.61, 4.65 and 4.17). Microbial biomass ninhydrin-N, and microbial community structure evaluated by phospholipid fatty acids (PLFAs), were measured at 0 (control soil only), 5, 25 and 50 days and CO₂ evolution up to 100 days. Straw addition increased biomass ninhydrin-N, CO₂ evolution and total PLFA concentrations at all soil pH values. The positive effect of straw addition on biomass ninhydrin-N was less in soils of pH 4.17 and 4.65. Similarly total PLFA concentrations were smallest at the lowest pH. This indicated that there is a direct pH effect as well as effects related to different substrate availabilities on microbial biomass and community structure. In the control soils, the fatty acids 16:1ω5, 16:1ω7c, 18:1ω7c&9t and i17:0 had significant and positive linear relationships with soil pH. In contrast, the fatty acids i15:0, a15:0, i16:0 and br17:0, 16:02OH, 18:2ω6,9, 17:0, 19:0, 17:0c9,10 and 19:0c9,10 were greatest in control soils at the lowest pHs. In soils given straw, the fatty acids 16:1ω5, 16:1ω7c, 15:0 and 18:0 had significant and positive linear relationships with pH, but the concentration of the monounsaturated 18:1ω9 PLFA decreased at the highest pHs. The PLFA profiles indicative of Gram-positive bacteria were more abundant than Gram-negative ones at the lowest pH in control soils, but in soils given straw these trends were reversed. In contrast, straw addition changed the microbial community structures least at pH 6.61. The ratio: [fungal PLFA 18:2w6,9]/[total PLFAs indicative of bacteria] indicated that fungal PLFAs were more dominant in the microbial communities of the lowest pH soil. In summary, this work shows that soil pH has marked effects on microbial biomass, community structure, and response to substrate addition.     

Effect of cinnamic acid on soil microbial characteristics in the cucumber rhizosphere

In order to elucidate the effect of allelochemicals on soil microbial characteristics in the cucumber rhizosphere, the soil microbial biomass and respiration, community functional diversity and RAPD marker diversity as affected by exogenous cinnamic acid were studied. Exogenous cinnamic acid increased soil microbial respiration and the metabolic quotient, but decreased soil microbial biomass-C. Soil microbial community functional diversity and genetic diversity (as indicated by RAPD markers) were also significantly altered by exogenous cinnamic acid. These results suggest that allelochemicals can change soil microbial genetic diversity, biological activity and microbial metabolic activity, which alter soil microbial ecology and accordingly affect the growth of cucumber with accumulation in the soil of allelochemicals.     

籽粒苋不同富钾基因型根系分泌物中有机酸和氨基酸的变化特点

采用溶液培养试验研究了籽粒苋(Amaranthus.spp.)不同富钾基因型在不同供钾水平条件下3个不同生长时期内根系分泌物中氨基酸和有机酸的种类及含量变化情况。结果表明,1)籽粒苋根系分泌物中氨基酸和有机酸含量随着供钾水平的升高而降低,且富钾基因型始终大于一般基因型。在低钾胁迫时,根系分泌物中有10种氨基酸和5种有机酸出现,而在供钾正常时则缺少谷氨酸、甘氨酸、苯丙氨酸、草酸和柠檬酸等;酪氨酸、-氨基丁酸和丝氨酸的含量约占氨基酸分泌总量的一半以上;苹果酸含量约占有机酸分泌总量的60%以上。2)籽粒苋生长50d时,一般基因型氨基酸和有机酸的分泌总量较生长40d时迅速降低,而富钾基因型降低速度则相对较为平稳。在3个生长期内,酪氨酸、-氨基丁酸和丝氨酸均为两类基因型根系分泌物中的主要氨基酸种类,苹果酸则是主要的有机酸类型,其在氨基酸和有机酸分泌总量中所占相对比例均随生长期的延长而升高。3)籽粒苋根系分泌物处理后的土壤速效钾含量均高于清水对照处理,富钾基因型在低钾胁迫时的根系分泌物对土壤钾的活化作用明显大于一般基因型。     

外源BT基因在棉花中的过量表达对根系分泌物中低分子量成分的影响

Most research in the past using genetically modified crops （GM crops） has focused on the ecological safety of foreign gene （i.e., the gene flow）, gene products （for example, Bt （Bacillus thuringiensis） protein）, and the safety of transgenic food for humans. In this study, changes in both the species and amounts of low-molecular-weight components in cotton （Gossypium hirsutum L.） root exudates after foreign Bt gene overexpression were investigated under different nutritional conditions. Transgenic cotton containing Bt （Bt-cotton）, supplemented with all the mineral nutrients, secreted more organic acids than the wild-type cotton （WT）. When nitrogen was removed from the full-nutrient solution, the amount of organic acids secretion of Bt-cotton was lesser than that of WT. The roots of the transgenic cotton secreted lesser amounts of amino acids and soluble sugars than the WT roots in the full-nutrient solution. Deficiencies of P and K caused a large increase in the total amino acid and soluble sugar secretions of both Bt-cotton and WT, with larger increases observed in Bt-cotton. Because transferring the foreign Bt gene into cotton can result in alterations in the components of the root exudates, with the effect varying depending on the nutritional status, the cultivation of genetically modified crops, such as Bt-cotton, in soil environments should be more carefully assessed, and the possible effects as a result of the alterations in the root exudate components should be considered.     

Phosphorus (P) Uptake mechanisms of crops grown in soils with low P status

In our previous studies, pigeonpea (Cajanus cajan L.), groundnut (Arachis hypogaea L.), and rice (Oryza sativa L.) were found to have a higher ability to take up Fe- or Al-bound phosphorus (P) than soybean (Glycine max L.) and sorghum (Sorghum bicolor L.). Phosphorus absorption characteristics like I _max, K _m, C _min, and Fe^III reduction activity of roots, and root exudates in various crops were examined with a view to analyzing the mechanisms of P uptake. Phosphorus uptake ability was largely unrelated to variations in I _max, K _m, C _min, and Fe^III reduction activity of roots. Phosphorus-solubilizing activity in anionic fractions of root exudates was detected in pigeonpea but not in rice or groundnut. Malonic acid was the major component followed by oxalic and piscidic acid. These organic acids were able to release P from FePO₄ and A1PO₄. The higher P uptake ability of pigeonpea in soils with low P fertility presumably depends on the secretion of such organic acids from roots.