Effects of introduced and indigenous VA mycorrhizal fungi on nodulation,growth and nutrition ofMedicago sativa in phosphate-fixing soils as affected by P fertilizers

Summary The legumeMedicago sativa was grown in two phosphate-fixing soils which received soluble or rock phosphate. The effects of the inoculation withGlomus mosseae on plant nutrition and nodulation were studied. The introduced VA fungi became successfully established and improved the degree of infection over level achieved by native endophytes. In all experimental conditions tested, plant dry weight, the total uptake of N and P and nodulation byRhizobium meliloti were increased by mycorrhizal inoculation. The size of the increase was inversely correlated with soluble P content in the soil. Mycorrhization, enhanced by introduction of suitable VA fungi, had similar effects to that of the dose of soluble phosphate tested. Indigenous and native endophytes cooperated in these effects. Results are discussed in terms of reducing the input of soluble P fertilizer to phosphate-fixing soils and the possibility of restoring the phosphate stock using a more rational supply of soluble P, that allows cooperation with VA fungi, or by the use of less soluble and expensive forms of P fertilizers.     

Effects of P-fertilization and inoculation by two vesicular-arbuscular mycorrhizal fungi on growth and nodulation ofCalopogonium caeruleum

The growth response ofCalopogonium caeruleum, a leguminous covercrop in plantation agriculture, to inoculation with two vesicular-arbuscular mycorrhizal (VAM) fungi was investigated in five phosphorus (P)-deficient soils supplied with various levels of rock phosphate. Significant shoot yield increases over the uninoculated controls were obtained in most sterilised or unsterilised soils at all applied P levels, although the inoculant VAM fungi differed in their effectiveness in the soils used. Responses in mycorrhizal root infections, P and nitrogen (N) concentrations in tops and plant nodulation varied. The results are discussed in relation to the edaphic environment of the mycorrhizal association.     

The interaction of rockphosphate,Bradyrhizobium, vesicular-arbuscular mycorrhizae and phosphate-solubilizing microbes on soybean grown in a sub-Himalayan mollisol

A factorial design 2³ × 4 with two levels of Mussorie rockphosphate (RP) with or without vesicular-arbuscular mycorrhizal (VAM) fungi and Bradyrhizobium japonicum, and four treatments of phosphate-solubilizing microbes (PSM) Pseudomonas striata, Bacillus polymyxa, Aspergillus awamori was employed using Patharchatta sandy loam soil (Typic Hapludoll). The observations included mycorrhization, nodulation, grain and straw yield, N and P uptake, available soil P and the PSM population in the soil after crop harvest. Inoculation with endophytes alone caused about 70% root colonization. Addition of rockphosphate or inoculation with PSM, except B. polymyxa, stimulated root infection of native as well as introduced VAM endophytes. Application of RP or inoculation with Bradyrhizobium japonicum, mycorrhizal fungi or phosphate-solubilizing microorganisms significantly increased nodulation, N uptake, available soil P and the PSM population in the soil after the crop harvest. The grain and straw yields did not increase following RP addition or mycorrhizal inoculation but increased significantly after inoculation wit Bradyrhizobium or PSM. In general, the application of RP, Bradyrhizobium, VAM and PSM in combinations of any two or three resulted in significant increases in nodulation, plant growth, grain yield and uptake of N and P. Among the four factor interactions, rockphosphate, Bradyrhizobium and P. striata in the absence of VAM resulted in maximal nodulation, grain and straw yields and N uptake by soybean. The highest P uptake by soybean grain was recorded with Bradyrhizobium and A. awamori in the absence of rockphosphate and VAM. Generally, available soil P and PSM population after crop harvest were not significantly increased by the treatment combinations giving the maximal uptake of nutrients. However, they increased significantly in response to PSM, which produced no significant increase in total uptake of nutrients.Research paper no. 7498     

Effects of mycorrhizae and chitin-hydrolysing microbes on Vicia faba

The effect of Streptomyces albovinaceus (S-22) and Bacillus sp. (B1) on the growth response, nodulation, nutrition and nitrogenase activities of faba bean (Vicia faba) varieties infected with Glomus mosseae under pot conditions in sterile soil amended with chitin was studied. The growth, nodulation, nutrients content and nitrogenase activity of mycorrhiza-treated plants of Giza-667 were significantly increased compared to untreated ones. Such increases were related to the increase in mycorrhizal root infection. Amendment of soil with chitin alone reduced the growth, nodulation, total nitrogen contents and nitrogenase activities of mycorrhiza-treated faba bean plants (Giza-667) compared to untreated plants. Inoculation of plants with S. albovinaceus or Bacillus sp. significantly increased the level of mycorrhizal roots infection, but addition of chitin to the soil in combination with Bacillus sp. reduced the mycorrhizal infection of faba bean roots. Highest phosphorus contents of faba bean Giza-667 were recorded after G. mosseae inoculation in the presence of all treatments. Similar results were observed for the other varieties. The microbial populations were significantly increased in rhizospheres amended with chitin. Such increases were not in response to the mycorrhizal inoculation. Generally, the microflora of faba bean rhizospheres was increased after treatment with G. mosseae in the absence of chitin amendment alone compared with non-mycorrhizal rhizospheres.     

Enhancement of nitrogen fixation in lentil,faba bean,and soybean by dual inoculation with Rhizobia and mycorrhizae

The combined effect of Vesicular Arbuscular Mycorrhizae (VAM) and Rhizobium on the cold season legumes, lentil and faba bean, as well as on summer legume, soybean, were studied in soils with low indeginous VA mycorrhizal spores. Inoculation of the plant with VA mycorrhizal fungi increased the level of mycorrhizal root infection of lentil, faba bean and soybean. The inoculation with Rhizobium had no significant effect on VA mycorrhizal infection percent, but VA mycorrhizal inoculation increased nodulation of the three legumes. The inoculation with Rhizobium alone significantly increased plant dry weight and N content of lentil and faba bean as well as seed yield of soybean. VA mycorrhizal inoculation also significantly increased plant dry weight and phosphorus content of the plants as did fertilization with superphosphate. Rock phosphate fertilization, however, had no significant effect on plant growth or phosphorus uptake. The addition of rock phosphate in combination with VA mycorrhizal inoculation significantly increased plant dry weight and P uptake of the plants. The dual inoculation with both rhizobia and mycorrhizae induced more significant increases in plant dry weight, N and P content of lentil and faba bean as well as seed yield of soybean than inoculation with either VA mycorrhizae or Rhizobium alone.     

Growth and nutrient status of citrus plants as influenced by mycorrhiza and phosphorus application

To test the hypothesis that high levels of soluble phosphate applied in combination with VAM fungi, to citrus plants, can cause growth depression even in the absence of other limiting factors, and also to test if rock phosphate, under these conditions, may be a satisfactory P source, a greenhouse experiment was conducted using sterilized soil with four levels of phosphate (0, 50, 100 and 200 ppm P) supplied either as soluble P or as rock phosphate. Citrus seedlings were either inoculated with the vesicular-arbuscular mycorrhizal (VAM) fungus Glomus etunicatum or left uninoculated. Six months after the start of the experiment, the plants were harvested and shoot dry weight, P and K uptake, root colonization and the number of spores in 50 cm³ of soil were determined. Significant increases were found in dry matter yields and in P and K contents, due to VAM fungus inoculation, at the zero and 50 ppm soluble P levels and at all rock phosphate levels. At 100 ppm soluble P, the development of VAM plants was equilvalent to that of non-VAM plants, and at 200 ppm, growth was significantly less than that of non-VAM plants. Root colonization and sporulation were reduced at higher P levels. The absolute growth depression of VAM plants at the higher P level was likely due to P toxicity. In addition, high leaf P and K concentrations may have interfered with carbohydrate distribution and utilization in these symbioses. Rock phosphate may be used with VAM citrus to substitute for medium amounts of soluble phosphate.     

Practical aspects of mycorrhizal technology in some tropical crops and pastures

Summary Greenhouse and field experiments were conducted on the effect of VA mycorrhiza (VAM) on the growth of cassava, various tropical grass and legume species, as well as beans, coffee and tea. A large number of VAM fungal species were evaluated for effectivity in increasing cassava growth and P uptake in acid low-P soils. The effectivity of VAM species and isolates was highly variable and dependent on soil pH and fertilizer applications, as well as on soil temperature and humidity. Two species,Glomus manihotis andEntrophospora colombiana were found to be most effective for a range of crops and pastures, at low pH and at a wide range of N, P, and K levels. At very low P levels nearly all crops and pasture species were highly mycorrhizal dependent, but at higher soil P levels cassava and several pasture legumes were more dependent than grass species. Mycorrhizal inoculation significantly increased cassava and bean yields in those soils with low or ineffective indigenous mycorrhizal populations. In these soils cassava root yields increased on the average 20–25% by VAM inoculation, both at the experiment station and in farmers’ fields. VAM inoculation of various pasture legumes and grasses, in combination with rock phosphate applications, increased their early growth and establishment. Agronomic practices such as fertilization, crop rotations, intercropping and pesticide applications were found to affect both the total VAM population as well as its species composition. While there is no doubt about the importance of VA mycorrhiza in enhancing P uptake and growth of many tropical crops and pastures grown on low-P soils, much more research is required to elucidate the complicated soil-plant-VAM interactions and to increase yields through improved mycorrhizal efficiency.     

The management of populations of vesicular-arbuscular mycorrhizal fungi in acid-infertile soils of a savanna ecosystem

A field trial was conducted at two sites in the savanna ecosystem of eastern Colombia to compare the effects of inoculation with vesicular-arbuscular mycorrhizal fungi (VAMF) ofBrachiaria dictyoneura (a tropical grass), cassava (Manihot esculenta), the tropical forage legume kudzu (Pueraria phaseoloides) andSorghum sp., and two phosphate sources. The second stage of the trial studied the effect of these pre-crop treatments on the subsequent growth, nutrition and VAM status of cowpea (Vigna unguiculata) andStylosanthes capitata in the following season, compared with both crops sown in native savanna. Inoculation significantly increased the levels of VAM and plant yields in the early growth stages of all crops during the first season, particularly with the rock phosphate (RP) source. The most significant increases were observed in the mycorrhiza-dependent cassava and kudzu crops up to 15 weeks after sowing, and were associated with increased foliar uptake of P and Mg. The effectiveness of the introduced inoculum was greater at the field site with a sandier soil. In the second season the levels of VAM in roots of cowpea andS. capitata were all increased significantly in pre-cropped plots compared with a savanna control. The increased presence of VAM was associated with significantly increased yields on plots previously sown to cassava, kudzu andSorghum sp. The data support the idea that increasing the VAMF inoculum potential of these acid-infertile soils by inoculation or pre-crops can greatly increase the rate of establishment of mycorrhiza-dependent host plants.     

Response ofLeucaena leucocephala to vesicular-arbuscular mycorrhizal colonization and rock phosphate fertilization in an Oxisol

Response ofLeucaena leucocephala (Lam) de Wit to rock phosphate application and inoculation with the vesicular-arbuscular mycorrhizal (VAM) fungusGlomus aggregatum (Schenck and Smith emend Koske) was evaluated in a pot experiment. VAM colonization increased as rock phosphate application increased. Using phosphorus concentration in pinnules as an indicator of VAM activity, significant VAM activity occurred at 25 days after planting at the lower levels of rock phosphate application (0, 0.34 and 0.68 g P kg^–1). The time required for significant VAM activity was shortened by 5 days at the higher P levels (1.36, 2.72 and 5.44 g P kg^–1). The highest VAM activity was associated with the highest rate of rock phosphate application.Inoculation withG. aggregatum significantly increased the uptake of Cu, P and Zn and dry-matter yield at all levels of rock phosphate applied. Copper concentrations in roots of mycorrhizal Leucaena were significantly higher than that of shoots. The results indicated that Leucaena in symbiotic association with VAM fungi effectively utilized P from rock phosphate. However, high rates of rock phosphate are required to attain growth comparable to that obtained with the application of water-soluble phosphate.Contribution from Hawaii Institute of Tropical Agriculture and Human Resources, Journal Series No. 3243.     

Improved growth of Cajanus cajan (L.) Millsp. in an unsterile tropical soil by three mycorrhizal fungi

Under glasshouse conditions Cajanus cajan plants grown in a dark red latosol were fertilized with soluble simple superphosphate and hardly soluble rock phosphate and inoculated with three VA mycorrhizal fungi (M₁, Gigaspora margarita; M₂, Scutellospora verrucosa; M₃, Acaulospora rehmii) from the Cerrado ecosystem, Brazil. Only with rock phosphate plant growth was significantly increased by all fungi. Enhanced P uptake corresponded with higher yields and proved to be a characteristic of the VA myccorhizae. A definite relationship between infection intensity and efficiency of VA mycorrhizae was not detected. Spore production was generally more pronounced in the treatment with rock phosphate, especially with M₁ and M₂. Nodulation of Cajanus cajan was greatly improved by all fungi in the treatment with rock phosphate. It is suggested that the increased plant development and nodulation was due to improved uptake of P by mycorrhiza.     

Effect of mycorrhizal inoculation and soluble phosphorus fertilizer on growth and phosphorus uptake of pearl millet

Summary Six mycorrhizal fungi were tested as inoculants for pearl millet (Pennisetum americanum Leeke) grown in pots maintained in a greenhouse. VAM fungi varied in their ability to stimulate plant growth and phosphorus uptake. Inoculation withGigaspora margarita, G. calospora andGlomus fasciculatum increased shoot drymatter 1.3 fold over uninoculated control. In another pot trial, inoculation withGigaspora calospora andGlomus fasciculatum resulted in dry matter and phosphorus uptake equivalent to that produced by adding phosphorus at 8 kg/ha.The influence of inoculatingGigaspora calospora on pearl millet at different levels of phosphorus fertilizer (0 to 60 kg P/ha) as triple superphosphate in sterile and unsterile alfisol soil was also studied. In sterile soil, mycorrhizal inoculation increased dry matter and phosphorus uptake at levels less than 20 kg/ha. At higher P levels the mycorrhizal effect was decreased. These studies performed in sterilized soil suggest that inoculation of pearl millet with efficient VAM fungi could be extremely useful in P deficient soils. However, its practical utility depends on screening and isolation of fungal strains which perform efficiently in natural (unsterilized) field conditions.     

Potentials and limitations of mycorrhizal inoculation illustrated by experiments with field-grown cassava

Summary Field inoculation trials with cassava (Manihot esculenta Crantz) were conducted in Quilichao (typic Dystropept soil) and Carimagua (Haplustox soil). In Quilichao, with a large and effective native VA-mycorrhizal (VAM) population, inoculation withGlomus manihotis did not increase cassava yields significantly, neither when different sources and levels of inoculum material were used, nor with different cassava cultivars, or after stabilizing soil temperature through mulching. Field inoculation did result in a decrease of the coefficient of variation with respect to yield. The high dependency of cassava on an effective VAM association was indicated by a marked decrease in yield after eradication of native VAM by soil sterilization. In Carimagua, with a lower native VAM population, mycorrhizal inoculation withG. manihotis increased yields significantly at intermediate levels of 100 kg/ha of applied P, using either inoculum of cassava orPanicum maximum roots or inoculum of a soil-root mixture of maize or tropical kudzu. Higher or lower levels of P decreased the effect of inoculation on yield. There were no significant differences among P sources, ranging from highly soluble triple superphosphate to low solubility rock phosphates. Inoculation with different VAM isolates had a variable effect on cassava yields, and showed that there may be an interaction between P fertilizer level and isolate efficiency. It is concluded that there may be a potential to increase yields or decrease the fertilizer P requirements of cassava through field inoculation with effective VAM isolates, in the vast areas of acid infertile Oxisols and Ultisols with low native VAM fungal populations, represented by Carimagua.     

Mycorrhizal inoculation effect on nodulation and N accumulation in Cajanus cajan at soil P concentrations sufficient or inadequate for mycorrhiza-free growth

The interaction of Cajanus cajan with Rhizobium and vesicular-arbuscular mycorrhizal fungi (VAMF) was investigated in a greenhouse experiment. C. cajan was planted in soil that had been inoculated with Glomus aggregatum or treated with benlate to suppress VAMF activity. Initial soil solution P concentrations of 0.06, 0.2, 0.4, and 0.8 mg l^-1 were established to test the interaction at external P levels that ranged from inadequate to nonlimiting for the host plant. At 0.06 and 0.2 mg P l^-1, mycorrhizal inoculation significantly increased plant P concentrations as well as nodule numbers and shoot dry weight. Mycorrhizal inoculation also significantly increased nodule dry weight at a soil P concentration of 0.4 mg l^-1 but did not significantly influence any of the other variables. The mycorrhizal inoculation effect observed at this soil solution P concentration could not be explained by any of the measures of plant P status. At 0.8 mg P l^-1, none of the measured variables were affected significantly by mycorrhizal inoculation. The results indicate that the enhanced nodulation associated with mycorrhizal inoculation at soil P concentrations lower than 0.4 mg l^-1 was explainable by mycorrhizal-mediated P uptake. The small but significant increase in nodule mass due to VAMF inoculation at 0.4 mg P l^-1 suggests that factors not related to plant P nutrition may be involved. On the other hand, the lack of a VAMF inoculation effect at 0.8 mg P l^-1 despite VAMF colonization at a level comparable to that observed at the former P concentration appear to discount this hypothesis. This observation is also supported by the lack of response of plant N status and nodule number to VAMF inoculation at this soil P concentration.Contribution from the Hawaii Institute of Tropical Agriculture and Human Resources Journal Series No.4066     

Response of lettuce to pre- and post-transplant phosphorus and pre-transplant inoculation with a VA-mycorrhizal fungus

Pre-transplant inoculation of lettuce (Lactuca sativa L.) seedlings with the vesicular-arbuscular mycorrhizal fungusGlomus aggregatum (Smith and Schenck emend. Koske) increased P uptake and dry matter yields after transplanting into soil when the concentration of P in the soil solution was 0.02 mg L^–1 but had little affect in soil with 0.30 mg L^–1 solution P. Tissue P concentrations and dry matter yields after transplanting were increased by supplying adequate P prior to transplanting. Adequate levels of pre-transplant P appeared to be more important than maximum mycorrhizal infection of transplants for promoting post-transplant growth of the fast maturing lettuce crop.Journal Series No. 0000 of the Hawaii Institute of Tropical Agriculture and Human Resources.     

Indigenous and introduced arbuscular mycorrhizal fungi contribute to plant growth in two agricultural soils from south-western Australia

Arbuscular mycorrhizal (AM) fungi occur in all agricultural soils but it is not easy to assess the contribution they make to plant growth under field conditions. Several approaches have been used to investigate this, including the comparison of plant growth in the presence or absence of naturally occurring AM fungi following soil fumigation or application of fungicides. However, treatments such as these may change soil characteristics other than factors directly involving AM fungi and lead to difficulties in identifying the reason for changes in plant growth. In a glasshouse experiment, we assessed the contribution of indigenous AM fungi to growth of subterranean clover in undisturbed cores of soil from two agricultural field sites (a cropped agricultural field at South Carrabin and a low input pasture at Westdale). We used the approach of estimating the benefit of AM fungi by comparing the curvature coefficients ( C) of the Mitscherlich equation for subterranean clover grown in untreated field soil, in field soil into which inoculum of Glomus invermaium was added and in soil fumigated with methyl bromide. It was only possible to estimate the benefit of mycorrhizas using this approach for one soil (Westdale) because it was the only soil for which a Mitscherlich response to the application of a range of P levels was obtained. The mycorrhizal benefit ( C of mycorrhizal vs. non-mycorrhizal plants or C of inoculated vs. uninoculated plants) of the indigenous fungi corresponded with a requirement for phosphate by plants that were colonised by AM fungi already present in the soil equivalent to half that required by non-mycorrhizal plants. This benefit was independent of the plant-available P in the soil. There was no additional benefit of inoculation on plant growth other than that due to increased P uptake. Indigenous AM fungi were present in both soils and colonised a high proportion of roots in both soils. There was a higher diversity of morphotypes of mycorrhizal fungi in roots of plants grown in the Westdale soil than in the South Carrabin soil that had a history of high phosphate fertilizer use in the field. Inoculation with G. invermaium did not increase the level of colonisation of roots by mycorrhizal fungi in either soil, but it replaced approximately 20% of the root length colonised by the indigenous fungi in Westdale soil at all levels of applied P. The proportion of colonised root length replaced by G. invermaium in South Carrabin soil varied with the level of application of P to the soil; it was higher at intermediate levels of recently added soil P.     

Effects of rhizosphere soil,vesicular-arbuscular mycorrhizal fungi and phosphate onPlantago major L. ssp.pleiosperma Pilger

Biotic factors in the rhizosphere and their effect on the growth ofPlantago major L. ssp.pleiosperma Pilger (Great plantain) were studied. In a pot experiment the effect on shoot growth of the addition of 2.5% rhizosphere soil at four levels of phosphate was highly dependent on the availability of phosphate: a promoting effect at low phosphate levels was observed while a reducing effect occurred at higher phosphate levels. As the roots were infected with vesicular-arbuscular mycorrhizal (VAM) fungi in the treatment with rhizosphere soil, two other experiments were set up to separate effects of the indigenous VAM fungi from effects of the total rhizosphere population. The uptake of phosphate and shoot growth was not decreased at higher phosphate availability when VAM inoculum was added alone or in combination with rhizosphere soil. The growth reducing effect of the rhizosphere soil could therefore not be ascribed only to mycorrhizal infection. The results suggest that biotic factors in the rhizosphere soil affect the phosphate uptake ofPlantago major ssp.pleiosperma. This may, under conditions of phosphate limitation, lead to an increase of phosphate stress and, subsequently, a growth reduction. Futhermore, it is concluded that VAM fungi, as part of the rhizosphere population, may compensate this phosphate stress by enhancing the phosphate uptake.Grassland Species Research Group Publication No. 148.     

Coinoculation of Bacillus thuringeinsis-KR1 with Rhizobium leguminosarum enhances plant growth and nodulation of pea (Pisum sativum L.) and lentil (Lens culinaris L.)

Nodulation and the subsequent nitrogen fixation are important factors that determine the productivity of legumes. The beneficial effects of nodulation can be enhanced when rhizobial inoculation is combined with plant-growth-promoting bacteria (PGPB). The PGPB strain Bacillus thuringiensis-KR1, originally isolated from the nodules of Kudzu vine (Pueraria thunbergiana), was found to promote plant growth of field pea (Pisum sativum L.) and lentil (Lens culinaris L.) under Jensen’s tube, growth pouch and non-sterile soil, respectively, when co-inoculated with Rhizobium leguminosarum-PR1. Coinoculation with B. thuringiensis-KR1 (at a cell density of 10⁶ c.f.u. ml⁻¹) provided the highest and most consistent increase in nodule number, shoot weight, root weight, and total biomass, over rhizobial inoculation alone. The enhancement in nodulation due to coinoculation was 84.6 and 73.3% in pea and lentil respectively compared to R. leguminosarum-PR1 treatment alone. The shoot dry-weight gains on coinoculation with variable cell populations of B. thuringiensis-KR1 varied from 1.04 to 1.15 times and 1.03 to 1.06 times in pea and lentil respectively, while root dry weight ratios of coinoculated treatments varied from 0.98 to 1.14 times and 1.08 to 1.33 times in pea and lentil respectively, those of R. leguminosarum-PR1 inoculated treatment at 42 days of plant growth. While cell densities higher than 10⁶ c.f.u. ml⁻¹ had an inhibitory effect on nodulation and plant growth, lower inoculum levels resulted in decreased cell recovery and plant growth performance. The results of this study indicate the potential of harnessing endophytic bacteria of wild legumes for improving the nodulation and growth of cultivated legumes.     

Infection by the VA-mycorrhizal fungusGlomus caledonium inHedysarum coronarium as influenced by host plant and P content of soil

The low degree of infection ofHedysarum coronarium L. (sulla) exposed to inoculum of the VAM endophyteGlomus caledonium was investigated. Infection began after a prolonged lag phase and remained at very low levels even after three months’ growth. Neither very high rates of inoculum, nor very low P content of the soil raised the low infection level of the sulla plants. There appeared to be some differences in rate of infection among ten different ecotypes of sulla but the level of infection remained low in all cases. In all tested populations some plants remained uninfected. The low infection rate of sulla may therefore have a genetic basis. It was shown that the growth ofH. coronarium is hardly improved by phosphate fertilization. This may explain the poor response of this plant species, adapted to grow in nutrient-deficient soil, to VAM. Programmes aimed at increasing the productivity in marginal soils through the introduction of efficient VAM endophytes should take into account the fact that certain plant species growing in marginal soils may not always benefit from mycorrhizal inoculation, due to their inherently low mycorrhizal dependency.     

Effects of liming and mycorrhizal colonization on soil phosphate depletion and phosphate uptake by maize (Zea mays L.) and soybean (Glycine max L.) grown in two tropical acid soils

The effects of liming and inoculation with the arbuscular mycorrhizal fungus, Glomus intraradices Schenck and Smith on the uptake of phosphate (P) by maize (Zea mays L.) and soybean (Glycine max [L.] Merr.) and on depletion of inorganic phosphate fractions in rhizosphere soil (Al-P, Fe-P, and Ca-P) were studied in flat plastic containers using two acid soils, an Oxisol and an Ultisol, from Indonesia. The bulk soil pH was adjusted in both soils to 4.7, 5.6, and 6.4 by liming with different amounts of CaCO₃.In both soils, liming increased shoot dry weight, total root length, and mycorrhizal colonization of roots in the two plant species. Mycorrhizal inoculation significantly increased root dry weight in some cases, but much more markedly increased shoot dry weight and P concentration in shoot and roots, and also the calculated P uptake per unit root length. In the rhizosphere soil of mycorrhizal and non-mycorrhizal plants, the depletion of Al-P, Fe-P, and Ca-P depended in some cases on the soil pH. At all pH levels, the extent of P depletion in the rhizosphere soil was greater in mycorrhizal than in non-mycorrhizal plants. Despite these quantitative differences in exploitation of soil P, mycorrhizal roots used the same inorganic P sources as non-mycorrhizal roots. These results do not suggest that mycorrhizal roots have specific properties for P solubilization. Rather, the efficient P uptake from soil solution by the roots determines the effectiveness of the use of the different soil P sources. The results indicate also that both liming and mycorrhizal colonization are important for enhancing P uptake and plant growth in tropical acid soils.     

The effect of VA mycorrhizal fungi,phosphorus and drought stress on the growth of Acacia nilotica and Leucaena leucocephala seedlings

The effect of vesicular-arbuscular mycorrhizal (VAM) fungi on growth and drought resistance of Acacia nilotica and Leucaena leucocephala seedlings was studied in a glasshouse experiment. The experimental design was a 2·2·2 factorial: ± mycorrhizal inoculation, ± application of phosphorus fertilizer and ± repeated drought treatment. The growth promoting effect of VAM fungi equalled the effect of phosphorus fertilization after 12 weeks. The drought treatment reduced seedling biomass and nodulation. Differences between the plant species were found with respect to growth improvements due to VAM inoculation and/or phosphorus fertilization under drought stress conditions. The results are discussed in relation to plant drought resistance and reforestation in the subhumid to arid tropics.