Phosphate solubilizing bacteria and fungi in desert soils: species,limitations and mechanisms

Desert soils are infertile, and the ability to improve them by P-fertilization is limited by the solubility of phosphate. We aimed to understand the function of phosphate solubilizing bacteria and the mechanisms behind phosphate solubilization in desert soils. Vegetated and barren desert soils, mine spoil and a fertile temperate grassland loam were sampled. Bacteria and fungi were isolated and identified, and their phosphate-solubilizing abilities were measured in vitro. The release of plant available PO₄, SO₄, NO₃ and NH₄ from desert soils did not compare with that of a grassland soil. Desert soils had substantially lower solubilization than grassland, 162 and 99–121 µg PO₄-P g^?1 dry soil, respectively. Phosphate-solubilizing bacteria and fungi were inhabiting the soils. Si addition increased phosphate solubilization of fungi by 50%. The isolated microbes were shown, using ³¹P nuclear magnetic resonance (NMR) analysis, to rapidly take-up both intracellular and extracellular phosphate during the phosphate solubilizing process. Desert soil had potentially active microbial populations that are capable to solubilize inorganic phosphorus; S and Si as the limiting factors. Acidification as the main mechanism to solubilize mineral phosphate was not as evident in our desert soils as in former studies dealing more fertile soils.     

Characterization of phosphate solubilizing fluorescent pseudomonads from the rhizosphere of Aloe vera (L.)

Soil phosphorous (P) deficiency is a major constraint to plant production which is overcome by adding inorganic-phosphate as chemical fertilizers. Fluorescent pseudomonads are the diverse group of bacteria able to mobilize sparingly soluble phosphate form. Total three hundred seven fluorescent Pseudomonas isolates were obtained from the Aloe barbadensis (Miller) rhizosphere. These Pseudomonas strains were further evaluated in vitro for their ability to solubilize phosphate and to produce indole acetic acid (IAA), hydrogen cyanide (HCN), siderophore and 1-aminocyclopropane 1- carboxylate (ACC) deaminase. Fifty three (36.8%) isolates produced IAA and 52 (36.1%) isolates produced siderophores whereas 36 (25.0%) and 31 (21.5%) isolates produced HCN and ACC deaminase, respectively. A positive correlation existed between siderophore and ACC deaminase producers. Cluster analysis showed rhizosphere as the major factor influencing the ecological distribution and physiological characterization of phosphate solubilizing bacteria (PSB). Based on partial 16S rRNA gene sequencing PSB were identified as Pseudomonas putida, Pseudomonas sp. and Pseudomonas plecoglossicida with highest phosphate solubilization ability. In conclusion, these phosphate solubilizing fluorescent pseudomonads would help in understanding their role in phosphorus solubilization and identification of potent phosphorus solubilizers from the rhizosphere of commercially grown A. barbadensis.     

Phosphorus availability,root exudates,and microbial activity in the rhizosphere

Field and pot experiments showed that the P demand of wheat is highest in early stages of growth (up to 1.67 μg P per cm² root surface and day). The needed orthophosphate ions H₂PO₄? and HPO₄²-move from soil to the root by diffusion. This process is controlled by the concentration gradient of the diffusible phosphate and the effective diffusion coefficient according to Pick's first law. Root excretions (rhizodeposition) are able to affect both characteristics. The water soluble portion of rhizodeposition contains more than 50% of up to 8 different sugars, 10–40% carboxylic acids and 10–15 amino acids and amides. The composition varies in dependence on the age of the root parts and on nutrition (Zea mays L., Brassica napus L., Pisum sativum L.). Diffusion experiments using small soil blocks showed that 50–75% of the root exudates were decomposed by respiration within 3 days. The rest was largely chemically converted. Originally present sugars disappeared. Due to the biosynthesis of different organic acids from the individual sugars the mobilisation of Ca₃(PO₄)₂ by Pantoea agglomerans increased when the sugar mixture was derived from the rhizodeposition of P deficient plants with more pentoses instead of glucose and fructose (mainly effect of anions). In the rhizosphere therefore a mixture of rhizodeposition and its conversion products exists which affects the binding of phosphorus in soil and the P transport to the root. This should be considered both for the development of new soil extractants and for modelling the P supply to plants.     

Influence of bacteria on growth and phosphorus nutrition of mycorrhizal corn

Com plants were grown in a non‐sterile soil in a greenhouse or in hydroponic culture in a growth chamber. We studied the influence of chitinolytic, pectinolytic, P‐solubilizing bacterial isolates, and a collection of bacterial strains on the development of native vesicular‐arbuscular mycorrhizal (VAM) populations, colonization of roots by the VAM fungus Glomus fasciculatum and their influence on the phosphorus (P) nutrition and growth of plants. As compared with VAM native control, the most potent stimulants for root colonization of soil‐grown plants by the VAM native population was a strain of Agrobacterium radiobacter and isolate H30. All bacteria used significantly supressed shoot fresh weight of mycorrhizal plants (‐13% up to ‐37%), with the exception of Agrobacterium. Under hydroponic conditions, the P‐solubilizing isolate F27 significantly stimulated the intensity of mycorrhiza, the number of arbuscules in roots, and increased both the P concentration and P content in corn shoots (+30% and +35%), than did the VAM fungus alone. Isolate F27 significantly increased shoot dry weight as compared with the mycorrhizal control. The other bacteria did not influence biomass production of corn.     

Effect of organic manure on the biological activities associated with insoluble phosphorus release in a blue purple paddy soil

Abstract

This study examined the effects of organic manure and chemical fertilizer on soil microflora, soil respiration, number of inorganic phosphate solubilizing bacteria, and organic phosphorus (P) mineralizing bacteria. The inorganic phosphate solubilizing rate, organic P mineralizing rate, and selected enzyme activities in a blue purple paddy soil were also studied. The results showed that organic manure significantly increased the total number of fungi, actinomyces, bacteria, P solubilizing bacteria, organic P mineralizing bacteria, P solubilizing rate and organic P mineralizing rate, soil respiration rate, and selected enzyme activities, whereas chemical fertilizer resulted in a smaller effect, and bacteria were affected more than fungi and actinomyces by organic manure. The enhancement of biological activities caused by organic manure might be due to the introduction of a large amount of living microorganisms and readily‐utilizable carbon source on which microorganisms live. This study showed that the augment of inorganic phosphate solubilizing bacteria and organic P mineralizing bacteria was one of the reasons that organic manure increased the avilability of P in a blue purple paddy soil.     

Changes in Nutrient Status During Preparation of Enriched Organomineral Fertilizers Using Rice Straw,Low‐Grade Rock Phosphate,Waste Mica,and Phosphate Solubilizing Microorganism

An experiment was carried out to study the changes in nutrient contents during preparation of enriched organomineral fertilizers using rice straw, low‐grade rock phosphate (RP), waste mica, and phosphate‐solubilizing microorganism (Aspergillus awamori). Composting reduced the total carbon (C) but increased total nitrogen (N) content with the progress of composting. This was reflected in the decrease of the C/N ratio. Significant increases in total phosphorus (P) and potassium (K) were also observed where both RP and waste mica was added along with Aspergillus awamori. Ammonium N (NH₄ ⁺‐N) decreased while nitrate N (NO₃ ^?‐N) increased at the end of composting. Olsen P content increased up to 90 days, thereafter decreased up to day 150, whereas ammonium acetate K (NH₄OAc‐K) increased gradually with the progress of composting. The study thus revealed that crop residue could be converted into a value‐added product through composting technology using low‐grade rock phosphate and waste mica along with phosphate‐solubilizing microorganisms.     

Root exudation of sugars,amino acids,and organic acids by maize as affected by nitrogen,phosphorus, potassium,and iron deficiency

Root exudates play a major role in the mobilization of sparingly soluble nutrients in the rhizosphere. Since the amount and composition of major metabolites in root exudates from one plant species have not yet been systematically compared under different nutrient deficiencies, relations between exudation patterns and the type of nutrient being deficient remain poorly understood. Comparing root exudates from axenically grown maize plants exposed to N, K, P, or Fe deficiency showed a higher release of glutamate, glucose, ribitol, and citrate from Fe‐deficient plants, while P deficiency stimulated the release of γ‐aminobutyric acid and carbohydrates. Potassium‐starved plants released less sugars, in particular glycerol, ribitol, fructose, and maltose, while under N deficiency lower amounts of amino acids were found in root exudates. Principal‐component analysis revealed a clear separation in the variation of the root‐exudate composition between Fe or P deficiency versus N or K deficiency in the first principal component, which explained 46% of the variation in the data. In addition, a negative correlation was found between the amounts of sugars, organic and amino acids released under deficiency of a certain nutrient and the diffusion coefficient of the respective nutrient in soils. We thus hypothesize that the release of dominant root exudates such as sugars, amino acids, and organic acids by roots may reflect an ancient strategy to cope with limiting nutrient supply.     

C、N源及C/N比对微生物溶磷的影响

以不同的氮源 (NH4+、NO3- 、尿素 )、不同的碳源 (葡萄糖、蔗糖、糖蜜和淀粉 )及碳氮比 (34∶1、20∶1、5∶1)为培养基研究不同C、N源和C/N比对微生物溶磷的影响。结果发现 ,曲霉 2TCiF2和 4TCiF6在以NO3-为氮源的培养基中表现出强的解磷活力 ,而节杆菌 1TCRi7和 1TCRi14的溶磷活性则在NO3-存在时降低 ,青霉 1TCRiF5、2TCRiF4、肠杆菌 1TCRi15和欧文氏菌 4TCRi2 2则只有在供给NH4+时 ,才具有溶解磷矿粉的能力。加入少量可溶性磷对大多数微生物的溶磷能力没有显著的影响。曲霉 2TCiF2在蔗糖为碳源时溶磷活力最高 ,节杆菌 1TCRi7只有在葡萄糖为碳源时才具有溶磷能力。培养基的C/N比越高 ,曲霉和欧文氏杆菌的溶磷活力越高 ,而青霉和肠杆菌则在C/N比最低时 ,其溶磷活力最强。这些微生物之所以具有溶解磷矿粉的能力 ,主要是由于分泌有机酸 ,但非有机酸物质的络合和螯合作用 ,可能在肠杆菌和欧文氏菌溶磷中起重要作用。氮源、碳源和碳氮比极大地影响微生物的代谢 ,尤其对分泌有机酸等物质的种类可能产生很大的影响。     

Effects of Plant Growth Promoting Rhizobacteria on Yield and Some Fruit Properties of Strawberry

Effects of plant growth promoting rhizobacteria (PGPR) [(Pseudomonas BA-8 (biological control agent), Bacillus OSU-142 (N₂-fixing), and Bacillus M-3 (N₂-fixing and phosphate solubilizing)] on yield and some fruit properties of strawberry cultivar ‘Selva’ in the province of Erzurum, Turkey in 2002–2003. Foliar + root application of PGPR strains significantly increased yield per plant as compared with the control. Root application of PGPR strains significantly increased total soluble solids, total sugar and reduced sugar, but decreased titratable acidity. It was also determined that bacteria applications have no important effect on the average fruit weight and pH. The results of this study suggested that Pseudomonas BA-8, Bacillus OSU-142 and Bacillus M-3 have potential for increasing yield in strawberry plant.     

Phosphate solubilizing microorganisms isolated from rhizosphere of maize cultivated in an oxisol of the Brazilian Cerrado Biome

Many soil microorganisms are able to transform insoluble forms of phosphorus to an accessible soluble form, contributing to plant nutrition as plant growth-promoting microorganisms (PGPM). The objective of this work was to isolate, screen and evaluate the phosphate solubilization activity of microorganisms in maize rhizosphere soil to manage soil microbial communities and to select potential microbial inoculants. Forty-five of the best isolates from 371 colonies were isolated from rhizosphere soil of maize grown in an oxisol of the Cerrado Biome with P deficiency. These microorganisms were selected based on the solubilization efficiency of inorganic and organic phosphate sources in a modified Pikovskaya's liquid medium culture containing sodium phytate (phytic acid), soybean lecithin, aluminum phosphate (AlPO₄), and tricalcium phosphate (Ca₃(PO₄)₂). The isolates were identified based on nucleotide sequence data from the 16S ribosomal DNA (rDNA) for bacteria and actinobacteria and internal transcribed spacer (ITS) rDNA for fungi. Bacteria produced the greatest solubilization in medium containing tricalcium phosphate. Strains B17 and B5, identified as Bacillus sp. and Burkholderia sp., respectively, were the most effective, mobilizing 67% and 58.5% of the total P (Ca₃(PO₄)₂) after 10 days, and were isolated from the rhizosphere of the P efficient L3 maize genotype, under P stress. The fungal population was the most effective in solubilizing P sources of aluminum, phytate, and lecithin. A greater diversity of P-solubilizing microorganisms was observed in the rhizosphere of the P efficient maize genotypes suggesting that the P efficiency in these cultivars may be related to the potential to enhance microbial interactions of P-solubilizing microorganisms.     

Utilization of organic phosphorus sources by oilseed rape,sunflower, and soybean

We evaluated the ability of Brassica napus L. (oilseed rape), Helianthus annus L. (sunflower), and Glycine max L. (soybean) plants grown inoculated with or without bacteria to utilize organic P sources. Plants were supplied with inorganic (dibasic sodium phosphate) and organic P sources (phytate and glucose phosphate) at three concentrations and grown for 40 d under sterile conditions. Three inoculation treatments were compared: control (non‐inoculated plants), inoculation with Bacillus amyloliquefaciens BNM340, and inoculation with Pseudomonas fluorescens BNM296 (two bacteria with proven phytase activity). Oilseed rape, sunflower and soybean could utilize organic P sources. For example, when phytate (0.5 mM) P was used as the external P source, the increase factors over the no‐P treatments were 4.5, 1.4, and 1.4 for oilseed rape, sunflower, and soybean P uptake, respectively. When glucose 1‐phosphate disodium salt (G1P, 0.5 mM) was the P source, the increase factors were 8.8, 1.7, and 1.9 respectively. Positive responses to the organic P sources were found for the biomass accumulation of oilseed rape and soybean but not for sunflower. The inoculation with bacteria did not exert a promoting effect on P uptake. We demonstrate that the three species can effectively use organic P sources. The existence of crop plants that are more efficient in the utilization of different soil P sources would be particularly beneficial to improve P recycling and use of P fertilizers in agriculture.     

The influence of plant growth–promoting rhizobacteria on growth and enzyme activities in wheat and spinach plants

A pot experiment in a greenhouse was conducted in order to investigate the effect of different N₂‐fixing, phytohormone‐producing, and P‐solubilizing bacterial species on wheat and spinach growth and enzyme activities. Growth parameters and the activities of four enzymes, glucose‐6‐phosphate dehydrogenase (G6PD; EC 1.1.1.49), 6‐phosphogluconate dehydrogenase (6PGD; EC 1.1.1.44), glutathione reductase (GR; EC 1.8.1.7), and glutathione S‐transferase (GST; EC 2.5.1.18) were determined in the leaves of wheat (Triticum aestivum L., Konya) and spinach (Spinacia oleracea L.), noninoculated and inoculated with nine plant growth–promoting rhizobacteria (PGPR: Bacillus cereus RC18, Bacillus licheniformis RC08, Bacillus megaterium RC07, Bacillus subtilis RC11, Bacillus OSU‐142, Bacillus M‐13, Pseudomonas putida RC06, Paenibacillus polymyxa RC05 and RC14). Among the strains used in the present study, six PGPR exhibited nitrogenase activity and four were efficient in phosphate solubilization; all bacterial strains were efficient in indole acetic acid (IAA) production and significantly increased growth of wheat and spinach. Inoculation with PGPR increased wheat shoot fresh weight by 16.2%–53.8% and spinach shoot fresh weight by 2.2%–53.4% over control. PGPR inoculation gave leaf area increases by 6.0%–47.0% in wheat and 5.3%–49.3% in spinach. Inoculation increased plant height by 2.2%–24.6% and 1.9%–36.8% in wheat and spinach, respectively. A close relationship between plant growth and enzyme activities such as G6PD, 6PGD, GR, and GST was demonstrated. Plant‐growth response was variable and dependent on the inoculant strain, enzyme activity, plant species, and growth parameter evaluated. In particular, the N₂‐fixing bacterial strains RC05, RC06, RC14, and OSU‐142 and the P‐solubilizing strains RC07 and RC08 have great potential in being formulated and used as biofertilizers.     

Soil fungal isolates produce different organic acid patterns involved in phosphate salts solubilization

Phosphorus availability is a major limiting factor for yield of most crop species. The objective of this study was to compare the solubilization of three sources of phosphorus (P) by different fungal isolates and to determine the possible mechanisms involved in the process. Talaromyces flavus (S73), T. flavus var flavus (TM), Talaromyces helicus (L7b) and T. helicus (N24), Penicillium janthinellum (PJ), and Penicillium purpurogenum (POP), fungal strains isolated from the rhizosphere of crops, are known to be biocontrol agents against pathogenic fungi. The P solubilization efficiency of these fungal strains in liquid media supplemented either with tricalcium phosphate (Ca₃(PO₄)₂; PC), aluminum phosphate (AlPO₄; AP), or phosphorite (PP) depended on the source of P and the fungal species. The type and concentration of organic acids produced by each species varied according to the source of available P. In the medium supplemented with PC, the highest proportion was that of gluconic acid, whereas in the media supplemented with the other P sources, the highest proportion was that of citric and valeric acids. This suggests that the release of these organic compounds in the rhizosphere by these microorganisms may be important in the solubilization of various inorganic P compounds. Results also support the hypothesis that the simultaneous production of different organic acids by fungi may enhance their potential for solubilizing insoluble phosphate.     

K3解磷菌的解磷机理及其对缓冲容量的响应

以对磷酸三钙具有高效溶解作用且对玉米苗生长有促生效果的假单胞菌K3为模式菌株,采用NBRIP液体培养基研究了解磷菌K3的解磷机制及缓冲容量对其解磷量的影响。结果表明,解磷菌K3液体摇瓶培养7 d后,培养液中水溶性磷从6.54 μg/mL增加至655.23 μg/mL,pH从7.00降至3.99。高效液相色谱测定发现,K3菌液中的主要代谢产物为苹果酸、乳酸和草酸,浓度分别为47.39 mmol/L、25.67 mmol/L和1.89 mmol/L。人工模拟K3菌株产生的有机酸及调节培养基不同pH值对磷酸三钙溶解度影响的试验表明,有机酸的螯合作用是解磷细菌K3菌株解磷的主要机理,而调节培养基pH对解磷的作用有限。液体摇瓶和土培试验结果显示,土壤缓冲容量对K3解磷菌的解磷效应有显著的抑制作用。     

Biosolubilization of phosphorus from rock phosphate and other P fertilizers in response to phosphate solubilizing bacteria and poultry manure in a silt loam calcareous soil

‘Phosphate solubilizing bacteria' (PSBs) are able to release unavailable P from native and applied P sources into plant‐available soil pool through their solubilizing and acidifying effects. The effects of three indigenous and one exotic PSBs on P solubilization from different P sources, plant biomass production, and P‐uptake efficiency of maize (Zea mays L.) were examined in an incubation and greenhouse study. For incubation study, surface (0–15 cm) soil was collected from an arable field (Inceptisols) and amended with rock phosphate (RP), single superphosphate (SSP), poultry manure (PM), and RP+PM with and without PSBs. The amended soil was incubated in the control environment at 25 ± 2°C for a total of a 100‐d period to establish relative potential rate of P solubilization of added P sources. A complementary greenhouse experiment was conducted in pots by growing maize as a test crop. Growth characteristics, P‐uptake, and P‐utilization efficiency (PUE) were determined. Phosphate solubilizing bacteria generated a solubilization effect on different P sources by releasing more P into plant‐available soil pool, i.e., 14.0–18.3 µg g^?1 in RP, 5.0–9.9 µg g^?1 in SSP, 1.4–4.4 µg g^?1 in PM, and 4.5–7.8 µg g^?1 in RP+PM compared to their sole application without PSBs. The available P from inorganic SSP declined continuously from the mineral pool (after day 30) and at the end 40% of applied P was unaccounted for. However, P losses were reduced to 28 and 27% when PSBs (PSB₁ and PSB₃) were applied with superphosphate treatments. In the absence of PSBs, the recoveries of applied P (in soil) from RP, SSP, PM and RP+PM were 4, 25, 9, and 12%, respectively, those had been increased to 14, 30, 12 and 15% in the presence of PSBs. Similarly, the plant biomass in RP+PSBs treatments compared to the RP without PSBs increased between 12–30% in first sampling (30 DAG) and 13–30% in the second sampling (60 DAG). The P utilization efficiency (PUE) in plants supplemented with PSBs was 20–73% higher compared to those without PSBs. The detection of oxalic and gluconic acids in culture medium treated with PSBs (7.8–25.0 and 25–90 mg L^?1, respectively) confirmed the production of organic acids by the indigenous bacterial isolates. This study indicate that low P recovery both in plant and soil can likely be improved by using indigenous PSBs and organic amendment poultry manure, which allowed a more efficient capture of P released due to P solubilization.     

黑土区高效溶磷真菌筛选及其溶解磷矿粉效果的研究

黑土区高效溶P真菌筛选及其溶解磷矿粉效果的试验结果表明 ,溶P真菌溶P效果高于溶P细菌 ,且其溶P性状稳定。曲霉菌“P39”、“P37”和青霉菌“P6 6”、“P1”溶P效果高于其他供试菌 ,菌株之间溶P活性与培养液pH值和有机酸含量间不存在必然相关性 ,推测不同菌株间溶P活性差异与菌株产生的有机酸种类和数量有关     

Analysis of wild sunflower (Helianthus annuus L.) root exudates using gas chromatography‐mass spectrometry

Plant root systems mediate ecological processes in the rhizosphere through the exudation of organic compounds. Although exudate composition is thought to depend strongly on plant nutrient status, little is known about the influence of multi‐nutrient stresses. In this study, we examined responses to short‐term (3 d) nutrient limitation in Helianthus annuus (common sunflower), and root exudates were collected for 2, 4, or 6 h with the trap‐solution method. Root exudates, analyzed by means of gas chromatography‐mass spectrometry, consisted of over 60 sugars, sugar alcohols, amino acids, organic acids, and phosphates, with sugars and organic acids generally detected in the highest quantities. Twenty‐five of the detected metabolites, including half of the organic acids, sugars, and sugar alcohols, differed in relative abundance among the three sampling intervals, exhibiting higher abundance in sampling intervals greater than 2 h. Similarly, 24 of the detected metabolites, including half of the amino acids, phosphates, and sugar alcohols, were affected by nutrient supply, with 20 exhibiting higher abundance in the high‐nutrient treatment. Fumaric acid, quinic acid, and glucose were detected at significantly higher levels in the low‐nutrient treatment, potentially representing an adaptive response to nutrient limitation in sunflower. However, as sampling interval exerted a strong influence on the apparent effects of nutrient supply, future studies should consider the potential impacts of sampling‐interval length in comparative analyses of genotypes or treatments.     

Comparison of in vitro solubilization activity of diverse phosphate-solubilizing bacteria native to acid soil and their ability to promote Phaseolus vulgaris growth

To identify plant growth promotion ability of phosphorus-solubilizing native bacteria, we have examined a collection of isolates representing the diversity of culturable phosphate-solubilizing bacteria from acid soils of the northeast of Argentina. Assays in growth medium supplemented with tricalcium phosphate revealed different phosphorus solubilization activity and temporal patterns of solubilization. Acidification of the broth medium coincided with phosphorus solubilization. The isolates were grouped according to their Rep fingerprinting profiles and phylogenetically classified by 16S rDNA and biochemical analyses. These isolates were assigned to the genera Enterobacter, Pantoea, Pseudomonas, Acinetobacter, Burkholderia, and Exiguobacterium. Four isolates showing high phosphorus solubilizing activity in in vitro assays were inoculated on common beans (Phaseolus vulgaris); some of them promoted plant growth and increased photosynthesis and the P and N content of leaves. The results indicated that the ability to in vitro solubilize P is not necessarily associated to the promotion of plant growth.     

Plant Growth and Sugars Content of Agave americana L. Cultivated with Vermicompost and Rock Phosphate and Inoculated with Penicillium sp. and Glomus fasciculatum

The objective of this study was to investigate the effect of vermicompost, rock phosphate, Glomus fasciculatum (arbuscular mycorrhizal fungi), and Penicillium sp. (phosphate solubilizing fungi) on growth; mycorrhization percent; fructan, sucrose, glucose, and fructose contents in leaves and stem of Agave americana L. Experiments were done according to an orthogonal experimental design L₈ with four variables at two levels: vermicompost (0 or 10 g per plant), rock phosphate (0 or 1 g per plant), G. fasciculatum (0 or 1 × 10⁶ spores per plant), and Penicillium sp. (0 or 1 × 10⁹ spores per plant). Vermicompost affected stem dry weight; the fructan, glucose, and fructose in stem; and mycorrhization percent in roots. Rock phosphate and Penicillium sp. increased the mycorrhization percent. Co-inoculation of Penicillium sp. and G. fasciculatum increased A. americana plant growth. The results obtained suggested a synergistic interaction between the arbuscular mycorrhizic fungi and the phosphate solubilizing fungi.     

Effect of various microorganisms on phosphorus uptake from insoluble Ca‐phosphates by cucumber plants

Rhizospheric microorganisms can increase P availability to plants. The objective of this work was to elucidate the effects of two plant growth promoting rhizobacteria and biocontrol agents (Bacillus subtilis QST713 and B. Amyloliquefaciens FZB24), a biocontrol agent (the fungus Trichoderma asperellum T34), and Aspergillus niger CBS513.88 on P uptake from insoluble Ca‐phosphates by plants. An experiment involving microbial cultivation in liquid media and three involving cultivation of cucumber plants in a siliceous growing medium fertilized with 40 mg P kg^?1 as phosphate rock (PR), a calcareous medium supplied with the same fertilizer, and one fertilized with KH₂PO₄ or PR at 200 mg P kg^?1 were conducted. In spite of the observed PR solubilization in liquid culture, not all the microorganisms improved P uptake by plants from this P source. The effect of each microorganism also differed depending on the plant‐growing medium, revealing that its P‐solubilizing activity was affected by pH and P concentration in the medium. Overall, best results were obtained with Bacillus subtilis QST713 which increased P uptake from the siliceous growing medium and the calcareous medium fertilized with 200 mg P kg^?1. Improved P nutrition of plants was the result not only of increased P solubilization, but also of enhanced ability of plants to absorb P. The other microorganisms studied provided less promising results despite the P mobilizing strategies they exhibited in the liquid culture (acidification and organic anion exudation). Therefore, these cannot be the only mechanisms contributing to P uptake by plants. Our results support the ability of B. subtilis QST713 to enhance the use of PR as a P source for calcareous soils or to improve uptake of residual P in the form of sparingly soluble Ca‐phosphates.