Influence of a vesicular-arbuscular mycorrhizal fungus on the competitive ability of Bradyrhizobium spp. for nodulation of cowpea Vigna unguiculata (L.) Walp in non-sterilized soil

Summary We examined the influence of a vesicular-arbuscular (VAM) fungus (Glomus pallidum Hall) on the competitive ability of introduced and native Bradyrhizobium strains to nodulate cowpeas [Vigna unguiculata (L) Walp]. Our experiments in non-sterilized soil revealed that in the presence of VAM fungus, introduced Bradyrhizobium spp. strains become more competitive than native rhizobia. For example, strain JRC29 occupied 59.2% of the total nodules when inoculated alone, but this figure increased to 71.2% when JRC29 was used in dual inoculations with VAM fungus. A similar pattern of enhanced competitiveness for nodule formation was observed with the two other strains in the presence of the VAM fungus. Our results suggest that the competitiveness of rhizobia can be enhanced by co-inoculating with a selected strain of a VAM fungus.     

Co-selection of compatible rhizobia and vesicular-arbuscular mycorrhizal fungi for cowpea in sterilized and non-sterilized soils

Summary We selected two isolates of Rhizobium for cowpea (Vigna unguiculata) with sterilized soil tests and two different isolates by non-sterilized soil testing. The four rhizobia were then paired individually with either Glomus pallidum, Glomus aggregatum, or Sclerocystis microcarpa in separate, sterilized, or non-sterilized soil experiments. The purpose of the experiments was to determine the effect of soil sterilization on the selection of effective cowpea rhizobia, and to see whether these rhizobia differed in their effects on cowpea growth when paired with various vesicular-arbuscular mycorrhizal (VAM) fungi. Our experiments showed that the rhizobia selected in sterilized soil tests produced few growth responses in the cowpea compared to the other introduced rhizobia, irrespective of pairing with VAM fungi in sterilized or non-sterilized soil. In contrast, the two rhizobia initially selected by non-sterilized soil testing significantly improved cowpea growth in non-sterilized soil, especially when paired with G. pallidum. Our results suggest that it is important to select for effective rhizobia in non-sterilized soil, and that pairing these rhizobia with specific, coselected VAM fungi can significantly improve the legume growth response.     

Sources of nitrogen and yield advantages for monocropping and mixed cropping with cowpeas (Vigna unguiculata L.) and upland rice (Oryza sativa L.)

Summary Sources of N used by cropped and intercropped cowpeas and rice were determined by the ¹⁵N isotope-dilution technique. The biological efficiency of intercropping cowpeas and rice was assessed by calculating the land equivalent ratio of dry matter yield, total N, and uptake of N. A reduced N uptake by both cowpeas and rice during mixed cropping was attributed to mutual competition, with both crops competing effectively for the scarce environmental resources. The lack of a significant difference in the uptake of fertilizer and soil N by mixed-crop rice and cowpeas is an indication that the soil N was sufficient and that the mixed cropping did not create any imbalance in soil and fertilizer N uptake. The land equivalent ratio ranged between 120% and 180% for shoot dry matter and total N, showing that biological efficiency was increased by intercropping cowpeas with rice. The proportion of N derived from the atmosphere by mono- and intercropped cowpeas was not significantly different, showing that the potential of cowpeas to fix N₂ was independent of the cropping system, since the rice did not stimulate the cowpeas to fix more N₂.     

Genetic diversity in cowpea [Vigna unguiculata (L.) Walp.] as revealed by RAPD markers

The present study, using RAPD analysis, was undertaken to characterize genetic variation in domesticated cowpea and its wild progenitor, as well as their relationships. The materials used consisted of 26 domesticated accessions, including accessions from each of the five cultivar-group, and 30 wild/weedy accessions, including accessions from West, East and southern Africa. A total of 28 primers generated 202 RAPD bands. One hundred and eight bands were polymorphic among the domesticated compared to 181 among wild/weedy cowpea accessions. Wild accessions were more diverse in East Africa, which is the likely area of origin of V. unguiculata var. spontanea. Var. spontanea is supposed to have spread westward and southward, with a loss of variability, loss counterbalanceed in southern Africa by introgressions with local perennial subspecies. Although the variabilty of domesticated cowpea was the highest ever recorded, cultivar-groups were poorly resolved, and several results obtained with isozyme data were not confirmed here. However primitive cultivars were more diverse than evolved cultivars, which still suggests two consecutive bottlenecks within domesticated cowpea evolution. As isozymes and AFLP markers, although with a larger number of markers, RAPD data confirmed the single domestication hypothesis, the gap between wild and domesticated cowpea, and the widespread introgression phenomena between wild and domesticated cowpea.     

Effect of vesicular-arbuscular mycorrhizae and zinc application on dry matter and zinc uptake of greengram (Vigna radiata L. Wilczek)

Summary In a greenhouse study we examined the effects of vesicular-arbuscular mycorrhizae (VAM) inoculation, using Glomus macrocarpum and of Zn application on dry matter production and Zn uptake by greengram in two mollisols. The VAM inoculation significantly increased the dry weight of different plant parts and the Zn uptake in both soils. Inoculated plants showed a greater response to the application of Zn at 2.5 and 5.0 mg kg^-1 soil in a Zn deficient clay loam soil. The inoculated plants also absorbed — more water than the uninoculated plants. Mass flow and diffusion were the principal processes by which Zn reached the plant roots; mass flow was particularly important in the absence of VAM in a sandy soil fertilized with higher Zn doses (5 and 10 mg kg^-1 soil). The greater supply of Zn to inoculated roots was attributed to an apparent diffusion process rather than to mass flow of Zn.     

Growth response of field-grown Siratro (Macroptilium atropurpureum Urb.) and Aeschynomene american L. to inoculation with selected vesicular-arbuscular mycorrhizal fungi

Summary The effect of inoculation with a selected isolate of Glomus etunicatum Becker and Gerdemann and one of G. intraradices Schenck and Smith on the growth and nutrient content of Macroptilium atropurpureum Urb. cv. Siratro and Aeschynomene americana L., at applied P levels of 10, 30, 60, and 120 kg ha^-1, was studied under field conditions. At all P levels and for all harvests, the shoot dry mass of Siratro and A. americana were greater for the plants inoculated with the vesicular-arbuscular mycorrhizal (VAM) fungi than the control plants. Differences between the VAM fungus-inoculated and the control plants were most marked between 30 and 90 kg ha^-1 of applied P and diminished at 120 kg ha^-1. At the first harvest of Siratro, the plants inoculated with G. etunicatum had a greater shoot dry mass than those inoculated with G. intraradices, for all levels of applied P. However, for subsequent harvest of Siratro and for the one harvest of A. americana the response of shoot dry mass to the two VAM fungi was equivocal. Fungal inoculation gave at least a 30% saving in the amount of P fertilizer required (40 kg ha^-1) for the maximum yield. The plants inoculated with VAM fungi had a greater tissue concentration and total content of P and N than the control plants at low and intermediate levels of applied P. The percentage of root colonized by VAM fungi for the inoculated plants of the two legumes increased linearly with P additions up to 60 kg ha^-1. The conclusion is that under amended (limed and fertilized) soil conditions, inoculation with selected VAM fungi can improve the establishement and growth of forage legumes in fields that contain ineffective populations of native VAM fungi.     

Inoculation response in kidney beans (Phaseolus vulgaris,L.) to vesicular-arbuscular mycorrhizal fungi and rhizobia in non-sterilized soil

The effect of dual inoculation on three local cultivars (Miss Kelly, Portland Red, Round Red) of red kidney beans (Phaseolus vulgaris, L.) with four strains of Rhizobium leguminosarum bv. phaseoli and three species of vesicular-arbuscular mycorrhizal (VAM) fungi was examined in a clay loam soil. Rhizobial strains B 17 and B 36, each paired with Glomus pallidum or G. aggregatum, were the most effective pairings for cv. Miss Kelly. Inoculation of Miss Kelly with any of these pairings significantly (P=0.05) increased growth, number of nodules, nodule dry weight, mycorrhizal colonization, and shoot N and P content than other pairings. The growth response by cv. Portland Red was significantly improved by pairings of B 36 or B 17 with any of the three VAM fungi. For both cultivars (Miss Kelly and Portland Red), CIAT 652 or T 2 paired with VAM fungi did not give a positive growth response. In contrast, for cv Round Red the T 2 rhizobial strain in combination with any of the three VAM fungi showed a significant (P=0.05) growth improvement in all parameters. Our results suggest that while dual inoculation of VAM fungi and rhizobia significantly improves the growth response by red kidney beans, the best pairings of VAM fungus and rhizobia for each cultivar need to be carefully selected.     

Impact of simulated erosion on the abundance and activity of indigenous vesicular-arbuscular mycorrhizal endophytes in an Oxisol

Summary The influence of simulated erosion on the abundance and activity of indigenous vesiculararbuscular mycorrhizal (VAM) populations was evaluated in an Oxisol. Surface-soil losses in excess of 7.5 cm were generally associated with significant decreases in the numbers of total and active VAM propagules and in the symbiotic effectiveness of the active propagules. Surface-soil removal not exceeding 7.5 cm was associated with decreased propagule abundance without adverse effects on VAM colonization of roots and symbiotic effectiveness of the fungi. The extent of VAM colonization of roots and the degree of symbiotic effectiveness observed at this level of simulated erosion were significantly higher than those observed in the soil not subjected to simulated erosion. This stimulation is attributed to the removal of antagonistic biotic factors as the top 7.5 cm of soil was removed. It is concluded that propagules lost during erosional soil losses must be replaced before legumes grown on relatively highly weathered and severely eroded tropical soils could appreciably benefit from the VAM symbiosis.Hawaii Institute of Tropical Agriculture and Human Resources Journal Series No. 3234     

Reduction of bacterial growth by a vesicular-arbuscular mycorrhizal fungus in the rhizosphere of cucumber (Cucumis sativus L.)

Summary Cucumber was grown in a partially sterilized sand-soil mixture with the vesicular-arbuscular mycorrhizal (VAM) fungus Glomus fasciculatum or left uninoculated. Fresh soil extract was places in polyvinyl chloride tubes without propagules of mycorrhizal fungi. Root tips and root segments with adhering soil, bulk soil, and soil from unplanted tubes were sampled after 4 weeks. Samples were labelled with [³H]-thymidine and bacteria in different size classes were measured after staining by acridine orange. The presence of VAM decreased the rate of bacterial DNA synthesis, decreased the bacterial biomass, and changed the spatial pattern of bacterial growth compared to non-mycorrhizal cucumbers. The [³H]-thymidine incorporation was significantly higher on root tips in the top of tubes, and on root segments and bulk soil in the center of tubes on non-mycorrhizal plants compared to mycorrhizal plants. At the bottom of the tubes, the [³H]-thymidine incorporation was significantly higher on root tips of mycorrhizal plants. Correspondingly, the bacterial biovolumes of rods with dimension 0.28–0.40×1.1–1.6 m, from the bulk soil in the center of tubes and from root segments in the center and top of tubes, and of cocci with a diameter of 0.55–0.78 m in the bulk soil in the center of tubes, were significantly reduced by VAM fungi. The extremely high bacterial biomass (1–7 mg C g^-1 dry weight soil) was significant reduced by mycorrhizal colonization on root segments and in bulk soil. The incorporation of [³H]-thymidine was around one order of magnitude lower compared to other rhizosphere measurements, probably because pseudomonads that did not incorporate [³H]-thymidine dominated the bacterial population. The VAM probably decreased the amount of plant root-derived organic matter available for bacterial growth, and increased bacterial spatial variability by competition. Thus VAM plants seem to be better adapted to compete with the saprophytic soil microflora for common nutrients, e.g., N and P, compared to non-mycorrhizal plants.     

Genetic relationships within Vigna unguiculata (L.) Walp. based on isozyme analyses

Summary Isozyme analyses of genetic diversity in Vigna unguiculata were performed to determine genetic relationships and level of genetic diversity between wild and cultivated cowpea. Thirty-four cultivated accessions of V. unguiculata, 56 wild accessions of V. unguiculata, and six accessions representing five related wild Vigna species were analyzed. Ten enzyme systems were polymorphic within Vigna unguiculata: AAT, ACO, G6PDH, DIAP, LAP, MUE, ME MDH, PRX, and SOD. Fourteen of 24 putative loci (58%) were polymorphic within wild V. unguiculata, but only one locus (4%) was polymorphic within cultivated cowpea; when five related Vigna species were examined, 21 of the 24 bands of activity showed polymorphisms (88%) adding 33 alleles to the 48 identified within V. unguiculata. In one F₂ population of 68 plants (UCDVg 36 × UCDVg 21) a loose linkage was indicated between Diap-2 and G6pd-1 (² = 15.39; p = 0.004) with an estimated distance of 36.0 cM ± 5.02 (recombination (r) = 0.31). Also in another F₂ population of 38 plants (CB 88 × UCDVg 21) a loose linkage was indicated between Lap-1 and Prx (\gC² = 9.62; p = 0.047) with an estimated distance of 39.8 cM ± 7.0 (r = 0.33). Total genetic diversity (H_T) was 0.085 over all of the accessions including the one classified as V. nervosa. Within accession diversity (H_s) approached zero and between accession diversity (D_si) was responsible for all of the genetic diversity present. Therefore the coefficient of gene differentiation (G_ST = D_STII_T) approached 1. Absolute gene differentiation (D_m) was 0.087. Two of the nine segregations in this study were skewed. In general, results of this study concurred with the taxonomic classification within V. unguiculata and provided a strong indication that a severe genetic bottleneck occurred during the domestication process of cowpea.Abbreviations AAT aspartate amino-transferase - ACO aconitase - ALD aldolase - AUS Australia - BDI Burundi - BWA Botswana - CHN China - CMR Cameroon - DIAP diaphorase - DZA Algeria - ETH Ethiopia - G6PDH glucose-6-phosphate dehydrogenase - GDH glutamate dehydrogenase - GHA Ghana - GUY Guyana - IDH isocitrate dehydrogenase - IND India - KEN Kenya - LAO Laos - LAP leucine aminopeptidase - MDH malate dehydrogenase - ME malic enzyme - MEX Mexico - MOZ Mozambique - MUE methylumelliferyl-esterase - MWI Malawi - MYS Malaysia - NER Niger - NGA Nigeria - PRX peroxidase - RBSC ribulose-bisphosphate carboxylase - SEN Senegal - SLE Sierra Leone - SOD superoxide dismutase - TGO Togo - TZA Tanzania - USA United States of America - XDH xanthine dehydrogenase - ZAF South Africa - ZAR Zaire - ZIM Zimbabwe - ZMB Zambia     

Synergistic effects of vesicular-arbuscular mycorrhizal fungi and diazotrophic bacteria on nutrition and growth of sweet potato (Ipomoea batatas)

Summary Sweet potatoes were micropropagated and then transplanted from axnic conditions to fumigated soil in pots in the greenhouse. Spores of Glomus clarum were obtained from Brachiaria decumbens or from sweet potatoes grown in soil infected with this fungus and with an enrichment culture of Acetobacter diazotrophicus. Three experiments were carried out to measure the beneficial effects of vesicular-arbuscular mycorrhizal (VAM) fungi-diazotroph interactions on growth, nutrition, and infection of sweet potato by A. diazotrophicus and other diazotrophs obtained from sweet potato roots. In two of these experiments the soils had been mixed with ¹⁵N-containing organic matter. The greatest effects of mycorrhizal inoculation were observed with co-inoculation of A. diazotrophicus and/or mixed cultures of diazotrophs containing A. diazotrophicus and Klebsiella sp. The tuber production was dependent on mycorrhization, and total N and P accumulation were increased when diazotrophs and G. clarum were applied together with VAM fungal spores. A. diazotrophicus infected aerial plant parts only when inoculated together with VAM fungi or when present within G. clarum spores. More pronounced effects on root colonization and intraradical sporulation of G. clarum were observed when A. diazotrophicus was co-inoculated. In non-fumigated soil, dual inoculation effects, however, were of lower magnitude. ¹⁵N analysis of the aerial parts and roots and tubers at the early growth stage (70 days) showed no statistical differences between treatments except for the VAM+Klebsiella sp. treatment. This indicates that the effects of A. diazotrophicus and other diazotrophs on sweet potato growth were caused by enhanced mycorrhization and, consequently, a more efficient assimilation of nutrients from the soil than by N₂ fixation. The possible interactions between these effects are discussed.     

Dual inoculation of pretransplant stage Oryza sativa L. plants with indigenous vesicular-arbuscular mycorrhizal fungi and fluorescent Pseudomonas spp.

Summary This study examined the response of rice (Oryza sativa L.) plants at the pretransplant/nursery stage to inoculation with vesicular-arbuscular mycorrhizal (VAM) fungi and fluorescent Pseudomonas spp., singly or in combination. The VAM fungi and fluorescent Pseudomonas spp. were isolated from the rhizosphere of rice plants. In the plants grown in soil inoculated with fluorescent Pseudomonas spp. alone, I found increases in shoot growth, and in root length and fine roots, and decreases in root growth, and P and N concentrations. In contrast, in the plants colonized by VAM fungi alone, the results were the reverse of those of the pseudomonad treatment. Dual inoculation of soil with VAM fungi and fluorescent Pseudomonas spp. yielded plants with the highest biomass and nutrient acquisition. In contrast, the plants of the control treatment had the lowest biomass and nutrient levels. The dual-inoculated plants had intermediate root and specific root lengths. The precentages of mycorrhizal colonization and colonized root lengths were significantly lower in the dual-inoculated treatment than the VAM fungal treatment. Inoculation of plants with fluorescent Pseudomonas spp. suppressed VAM fungal colonization and apparently reduced photosynthate loss to the mycorrhizal associates, which led to greater biomass and nutrient levels in dual-inoculated plants compared with plants inoculated with VAM fungi alone. Dual inoculation of seedlings with fluorescent Pseudomonas spp. and VAM fungi may be preferable to inoculation with VAM alone and may contribute to the successful establishment of these plants in the field.     

Influence of soluble phosphorus fertilizer on the interaction between a vesicular-arbuscular mycorrhizal fungus and Azospirillum brasilense in barley (Herdeum vulgare L.)

Summary Pot-culture studies were carried out to examine the response of barley (Hordeum vulgare L.) to inoculation with Azospirillum brasilense and Glomus versiforme, singly and/or in combination, under varying levels of nitrogenous [(¹⁵NH₄)₂SO₄] and soluble phosphatic (single superphosphate) fertilizers. The interaction between both the endophytes led to increased growth and nutrition of the barley plants. Roots from plants inoculated with Azospirillum brasilense and Glomus versiforme exhibited very low acetylene reduction activity. N₂ fixation in the plants increased with the increase in plant growth but the mycorrhiza alone gave a low level of N₂ fixation in the plants compared to combined inoculation with both the endophytes.     

Hyphal transport by a vesicular-arbuscular mycorrhizal fungus of N applied to the soil as ammonium or nitrate

Summary Transport of N by hyphae of a vesicular-arbuscular mycorrhizal fungus was studied under controlled experimental conditions. The N source was applied to the soil as ¹⁵NH _inf4 ^sup+ or ¹⁵NO _inf3 ^sup- . Cucumis sativus was grown for 25 days, either alone or in symbiosis with Glomus intraradices, in containers with a hyphal compartment separated from the root compartment by a fine nylon mesh. Mineral N was then applied to the hyphal compartment as ¹⁵NH _inf4 ^sup+ or ¹⁵NO _inf3 ^sup- at 5 cm distance from the root compartment. Soil samples were taken from the hyphal compartment at 1, 3 and 5 cm distance from the root compartment at 7 and 12 days after labelling, and the concentration of mineral N in the samples was measured from 2 M KCl extracts. Mycorrhizal colonization did not affect plant dry weight. The recovery of ¹⁵N in mycorrhizal plants was 38 or 40%, respectively, when ¹⁵NH _inf4 ^sup+ or ¹⁵NO _inf3 ^sup- was applied. The corresponding values for non-mycorrhizal plants were 7 and 16%. The higher ¹⁵N recovery observed in mycorrhizal plants than in non-mycorrhizal plants suggests that hyphal transport of N from the applied ¹⁵N sources towards the host plant had occurred. The concentration of mineral N in the soil of hyphal compartments was considerably less in mycorrhizal treatments than in controls, indicating that the hyphae were able to deplete the soil for mineral N.     

Influence of vesicular-arbuscular mycorrhizae on phosphate fertilizer efficiency in two tropical acid soils planted with micropropagated oil palm (Elaeis guineensis jacq.)

Summary The influence of vesicular-arbuscular mycorrhizae on the efficiency of triple superphosphate and rock phosphate fertilizers was compared in two tropical, acid, P-fixing soils (Ivory Coast) in which the available P was labelled with ³²PO _inf4 ^sup3- . Both soils were planted with micropropagated oil palms. The growth reponses to the fertilizer applications were low unless accompanied by VAM inoculation, but both fertilizers were equally available to plants. Isotopic-dilution kinetics analyses indicated that the rock phosphate was solubilized in both soils and there was an enrichment of the labile pool of plant-available P, similar to that with superphosphate. The specific activity and the fraction of P derived from either fertilizer was similar in both mycorrhizal and non-mycorrhizal plants, showing that both absorbed P from the same labile pool of P in the fertilized soils. However, VAM inoculation increased the fertilizer utilization coefficient of plants 2.7- to 5.6-fold, depending on the soil and fertilizer. We conclude that VAM inoculation increases fertilizer efficiency, as much of rock phosphate as of superphosphate, for plants growing in acid, P-fixing soils, and the processes involved are not different for the two fertilizers.     

Inoculation with phosphate-solubilizing microorganisms and a vesicular-arbuscular mycorrhizal fungus improves dry matter yield and nutrient uptake by wheat grown in a sandy soil

 The effect of inoculating wheat (Triticum aestivum L.) with the PO₄ ^3–-solubilizing microorganisms (PSM) Bacillus circulans and Cladosporium herbarum and the vesicular-arbuscular mycorrhizal (VAM) fungus Glomus sp. 88 with or without Mussoorie rock phosphate (MRP) amendment in a nutrient-deficient natural sandy soil was studied. In the sandy soil of low fertility root colonization by VAM fungi was low. Inoculation with Glomus sp. 88 improved root colonization. At maturity, grain and straw yields as well as N and P uptake improved significantly following inoculation with PSM or the VAM fungus. These increases were higher on combined inoculation of PSM and the VAM fungus with MRP amendment. In general, a larger population of PSM was maintained in the rhizosphere of wheat in treatments with VAM fungal inoculation and MRP amendment. The results suggest that combined inoculation with PSM and a VAM fungus along with MRP amendment can improve crop yields in nutrient-deficient soils. Received: 4 September 1997     

The influence of inorganic nutrient fertilization on the growth,nutrient composition and vesicular-arbuscular mycorrhizal colonization of pretransplant rice (Oryza sativa L.) plants

Summary The effects of P, N and Ca+Mg fertilization on biomass production, leaf area, root length, vesiculararbuscular mycorrhizal (VAM) colonization, and shoot and root nutrient concentrations of pretransplant rice (Oryza sativa L.) plants were investigated. Mycorrhizal plants generally had a higher biomass and P, N, K, Ca, Mn, Fe, Cu, Na, B, Zn, Al, Mg, and S shoot-tissue nutrient concentrations than non-mycorrhizal plants. Although mycorrhizal plants always had higher root-tissue nutrient concentrations than non-mycorrhizal plants, they were not significantly different, except for Mn. N fertilization stimulated colonization of the root system (colonized root length), and increased biomass production and nutrient concentrations of mycorrhizal plants. Biomass increases due to N were larger when the plants were not fertilized with additional P. P fertilization reduced the colonized root length and biomass production of mycorrhizal plants. The base treatment (Ca+Mg) did not significantly affect biomass production but increased the colonized root length. These results stress the importance of evaluating the VAM rice symbiosis under various fertilization regimes. The results of this study suggest that pretransplant mycorrhizal rice plants may have a potential for better field establishment than non-mycorrhizal plants.     

Variable effectivity of three vesicular-arbuscular mycorrhizal endophytes in Hedysarum coronarium and Medicago sativa

Summary Plants of Hedysarum coronarium L. and Medicago sativa L., inoculated with Glomus caledonium, were grown for 6 weeks in soils with increasing amounts of available phosphorus (P). H. coronarium showed no or very low levels of infection even at very low soil P content (4 ppm), while in M. sativa the highest soil P level (20 ppm) was associated with the lowest percentage of infection. Medicago sativa and H. coronarium grown for 16 weeks in a sandy soil showed clear effects of Glomus inoculation on shoot growth and P uptake. In M. sativa inoculation increased shoot growth and P uptake in all treatments considerably, while the P concentration in the shoot was depressed by G. caledonium. In H. coronarium inoculation with G. mosseae and G. occultum had only a modest impact on shoot growth; G. caledonium even depressed shoot growth significantly. Glomus inoculation had a positive impact on the P concentrations in the shoots of H. coronarium. The growth-depressing effect of G. caledonium on H. coronarium is discussed.     

Leucaena leucocephala seedling response to vesicular-arbuscular mycorrhizal inoculation in soils with varying levels of inherent mycorrhizal effectiveness

Summary The symbiotic effectiveness of vesicular-arbuscular mycorrhizal (VAM) fungi present in widely differring tropical soils was evaluated in a greenhouse experiment. Small volumes of field soil, a standard inoculum (Glomus aggregatum) or both were introduced into a fumigated sand-soil medium amended with nutrients for optimum VAM activity. Leucaena leucocephala (Lam.) de Wit var. K8 was grown in the medium as an indicator plant. VAM effectiveness was monitored as a function of time by determining the P status of pinnules. The soils differed from each other with respect to the time their endophytes required for the expression of initial and maximum effectiveness and in the level of maximum effectiveness they exhibited. The effect of mycorrhizal inoculation, calculated as the ratio of the areas enclosed by the effectiveness curve of G. aggregatum to that enclosed by the effectiveness curves of test soils, was found to be a good indicator of the response of L. leucocephala to inoculation of soils with G. aggregatum Hawaii Institute of Tropical Agriculture and Human Resources Journal series No. 3285     

Influence of humic acids on laurel growth,associated rhizospheric microorganisms,and mycorrhizal fungi

Summary Increasing concentrations of humic acids were tested in order to determine their effects on the microbial rhizosphere and the growth of laurel (Laurus nobilis L.). Plants that were treated with 300 mg kg^-1 of humic acids had the heaviest weights of both fresh and dry shoots; however, doses of 3000 mg kg^-1 were highly phytotoxic and inhibited the growth of laurel. Total aerobic bacteria and actinomycetes were stimulated by doses of 1500 and 3000 mg kg^-1 of humic acids at the first harvest. Nevertheless, at the end of the experiment no significant differences were found among the different doses. The number of fungi living in the laurel rhizosphere was not affected by any concentration of humic acids. Vesicular-arbuscular mycorrhizal (VAM) colonization was only slightly affected by the addition of increasing concentrations of humic substances to the soil, while the hyphal growth of Glomus mosseae was reduced.