Vesicular-arbuscular Mycorrhizal Fungi Research Articles

Phosphorus amendment inhibits hyphal branching of the VAM fungus Gigaspora margarita directly and indirectly through its effect on root exudation

The effect of solution phosphorus (P) concentration upon growth of pregerminated spores of the vesicular-arbuscular mycorrhizal fungus Gigaspora margarita was examined in vitro. P at 1 mM significantly inhibited branching of the primary germ tube. The number of branches and the total hyphal length were both significantly inhibited at 10 mM P. In addition, germinated spores exposed to exudates produced by Ri T-DNA-transformed roots of Daucus carota L. grown in the presence of P showed significantly less hyphal branching than those exposed to exudates produced by P-stressed roots. These phenomena could contribute to the observed inhibition of mycorrhiza formation by high P.

Nitrogen fixation and CO2 exchange in soybeans (Glycine max L.) inoculated with mixed cultures of different microorganisms

N2 fixation, photosynthesis of whole plants and yield increases in soybeans inoculated with mixed cultures of Bradyrhizobium japonicum 110 and Pseudomonas fluorescens 20 or P. fluorescens 21 as well as Glomus mosseae were found in pot experiments in gray forest soil carried out in a growth chamber. The effects of pseudomonads and vesicular-arbuscular (VA) mycorrhizal fungus on these parameters were found to be the same. Dual inoculation of soybeans with mixed cultures of microorganisms stimulated nodulation, nitrogenase activity of nodules and enhanced the amount of “biological” nitrogen in plants as determined by the 15N dilution method in comparison to soybeans inoculated with nodule bacteria alone. An increased leaf area in dually infected soybeans was estimated to be the major factor increasing photosynthesis. P. fluorescens and G. mosseae stimulated plant growth, photosynthesis and nodulation probably due to the production of plant growth-promoting substances. Increasing phosphorus fertilizer rates within the range of 5–40 mg P 100 g-1 1:1 (v/v) soil: sand in a greenhouse experiment led to a subsequent improvement in nodulation, and an enhancement of N2 fixation and yield in soybeans dually inoculated with B. japonicum 110 and P. fluorescens 21. These indexes were considerably higher in P-treated plants inoculated with mixed bacterial culture than in plants inoculated with nodule bacteria alone.

Auxin interaction with the vesicular-arbuscular mycorrhizal fungus Glomus intraradices Schenck & Smith improves vegetative growth of two citrus rootstocks

The vesicular-arbuscular mycorrhizal (VAM) fungus Glomus intraradices Schenck & Smith increased root and shoot growth of the citrus rootstocks sour orange ( Citrus aurantium L.) and ‘Carrizo’ citrange ( Citrus sinensis (L.) Osb. X Poncirus trifoliata (L.) Raf.). The application of indolebutyric acid (IBA) at 2.0 g l −1 as a 10 s root dip to non-mycorrhizal seedlings was ineffective in increasing growth, but application to mycorrhizal seedlings had a positive VAM-IBA effect on growth.

Evaluation of Vesicular–Arbuscular Mycorrhizal Fungi in Commercial Production of Silver King (Aglaonema commutatum L.) Plants

In the commercial production of silver king plants and other ornate plants, the substrates are treated with fungicides, which affect the vesicular–arbuscular mycorrhizal (VAM) fungi and the plant growth negatively. The restoration of de VAM fungi to the substrate, after its disinfection, might improvement the development. The effectiveness and infectiveness of Glomus fasciculatum and Glomus aggregatum on silver king (Aglaonema commutatum) plants was evaluated in this work. Seedlings of 4-week-old, growing treated with mancozeb, were removed and planted in pots filled with a disinfected mixture of sand soil, cow manure and coconut powder (1:2:2), containing the inoculum of VAM fungi (soil with spores and colonized roots). After 3 and 4 months of the inoculation, plants were removed and dry weight of roots and shoot, number and length of leaves, and mycorrhizal colonization were evaluated. A better development was showed in plants inoculated, resulting highest values in number and length of leaves in relation to control plants. Both VAM fungi improvement the number and length of leaves. The percentage root length colonized (80%) and visual density of endophyte in roots was highest in plants inoculated with Glomus aggregatum in both sampling period.

Interaction of multiple VAM fungal species on root colonization, plant growth and nutrient status of tomato seedlings ( Lycopersicon esculentum Mill.)

Tomato seedlings were grown in sterile, phosphorus deficient soil and inoculated with four species of vesicular-arbuscular mycorrhizal (VAM) fungi in 15 possible combinations. Mycorrhizal plants exhibited a significantly higher shoot length and biomass, than non-mycorrhizal plants ( P > 0.05). VAM also increased host tissue N and P concentrations. Among the VAM combinations, inocula containing all four endophytes promoted markedly better shoot length and biomass than other combinations, although spore yields were lower in mixed inocula. Mycorrhizal dependency (MD), percentage increase of biomass compared to controls, was also more in quadruple combination than in either triple or double combinations. A negative interaction on plant growth and biomass was observed between Glomus aggregatum and G. fasciculatum. This effect was nullified when more species were added to this combination.

Correction of dual phosphorus and zinc deficiencies of linseed ( Linum usitatissimum L.) with cultures of vesicular-arbuscular mycorrhizal fungi

A glasshouse experiment demonstrated that cultures of vesicular-arbuscular mycorrhizal fungi (VAMF) can correct dual phosphorus and zinc deficiencies of linseed ( Linum usitatissimum L.) that are typical of long-fallow disorder. Linseed was grown in pots of steamed vertisolic soil from a field site with a history of long-fallow disorder. The soil was inoculated with cultured sppores of several VAMF species in factorial combinations with or without P and Zn fertilizers. Linseed plants without VAMF in steamed soil grew very poorly unless supplied with both P and Zn fertilizers. Inoculation of soil with cultured spores of local VAMF isolates of Glomus mosseae, or a mixture of isolates of G. mosseae and G. etunicatum, markedly improved P and Zn uptake and dry matter accumulation of linseed. Inoculation with spores of an isolate of G. macrocarpum resulted in slower VA-mycorrhizal colonization and poorer shoot growth and seed yield. Non-mycorrhizal plants responded poorly to either P or Zn alone but showed a marked synergistic response to the combination of both elements. My study demonstrates that VAMF are responsible for substantially improving uptake of both P and Zn and increasing growth and seed yield of linseed (99% mycorrhizal dependency). Long-fallowing, which markedly reduces VAMF populations in the soil, can result in dual P and Zn deficiencies in crops that are highly mycorrhizal-dependent. Farming practices that maintain VAMF populations will result in more efficient use of both P and Zn from soil.

Vesicular–arbuscular mycorrhizal associations of American ginseng (Panax quinquefolius) in commercial production

Panax quinquefolius L. (American ginseng) roots collected from 1st year seedlings and 3-year-old plants on three commercial farms were colonized by vesicular–arbuscular mycorrhizal (VAM) fungi. Roots collected from ginseng plants in a managed maple–beech woodlot were also colonized by VAM fungi. Fungal hyphae entered roots either directly through root hairs or by forming appressoria on the surface of epidermal cells from which penetration hyphae formed. Hyphae colonized roots by passing intracellularly through cortical cells. Hyphal coils, from which arbuscular branches formed, were typical of the Paris-type of VAM association. The extent of colonization by hyphal coils and by arbuscules varied between 1-year-old and 3-year-old plants and among farms. The seasonal maxima for colonization differed among combinations of farms and plant age-classes in such a way as to suggest that a significant degree of the variability was due to characteristics of the individual seed beds. Over the season, we saw earlier development of hyphal coils that was only later exceeded by production of arbuscules; this pattern is consistent with the Paris-type morphology observed, in that arbuscule branches develop from hyphal coils. However, the speed at which arbuscules developed at some points in the season was so great that at those times the transition from newly formed hyphal coils to arbuscules must be extremely fast. Vesicles were rarely found in any root samples examined. Keywords: ginseng, Panax quinquefolius, vesicular–arbuscular mycorrhiza, commercial farms.

Tomato Growth in Response to Salinity and Mycorrhizal Fungi from Saline or Nonsaline Soils

Tomato (Lycopersicon esculentum Mill. `Heinz 1350 VF 402') seedlings were inoculated with populations of vesicular–arbuscular mycorrhizal (VAM) fungi collected from saline or nonsaline soil or remained nonmycorrhizal as a control. Plants then were salinated for 8 weeks at 1.0, 2.0, 5.0, or 10.0 dS·m–1 produced by dilutions of 1 m NaCl: 1 m CaCl2 in deionized water. Inoculation with VAM fungi from nonsaline soil enhanced shoot growth, while VAM fungi from saline soil suppressed shoot and root growth. Plants inoculated with VAM fungi from nonsaline soil and non-VAM control plants showed a quadratic increase in leaf Cl– concentration in response to an increased salinity level, whereas plants inoculated with VAM fungi from saline soil showed a linear increase in leaf Cl– concentration. Mycorrhizal-induced growth responses and changes in leaf Cl– concentration were not associated with any apparent alterations in tomato plant P status. Although VAM fungi originating from saline soil did not promote plant growth, reduction in leaf Cl– concentration mediated by these VAM fungi at moderate salinity levels may have beneficial implications for plant survival in saline soil.

Mycorrhizae, biocides, and biocontrol. 1. Herbicide-mycorrhiza interactions in soybean and cocklebur treated with bentazon

Interactions between herbicides and vesicular-arbuscular mycorrhizal (VAM) fungi are little-known but may differentially affect the development of tolerant and susceptible plants. We conducted this greenhouse study to determine if VAM fungi modify the effects of the herbicide bentazon (3-isopropyl-1H-2,1,3-benzothidiazin-(4)3H-one 2,2-dioxide) on soybean ( Glycine max (L.) Men.) and common cocklebur ( Xanthium strumarium L.). The experiment was designed as an eight-treatment, 2 × 2 × 2 factorial. Individual potted soybean or cocklebur plants were grown in a high (28 mg kg −1) P-content soil. The plants were colonized by VAM fungi (a mixture of Glomus etunicatum Becker and Gerdemann, Glomus leptotichum Schenck and Smith, and Glomus mosseae (Nicol. and Gerd.) Gerd. and Trappe) and sprayed with bentazon. Alternatively, they were exposed only to VAM fungi or to the herbicide, or to neither of these factors. The effects of VAM fungi on plant dry mass were small, but larger in cocklebur than in soybean, especially for the herbicide-treated plants. Root/shoot ratios in cocklebur were larger and shoot dry matter content smaller than those in soybean. The VAM effect in the herbicide-treated plants was greater than in the nontreated ones for both parameters. Colonized root length in cocklebur was drastically reduced by the herbicide (43%), as were leaf dry weight and N, P, and K concentrations. Changes were small, and generally not significant in soybean. Gradual necrosis of treated cocklebur shoots was related to an accelerated loss of shoot dry matter, especially in VAM plants. The data suggest that changes in source-sink relations following herbicide treatment favor enhanced export of shoot dry matter to the roots in susceptible relative to tolerant plants. This biomass may be preferentially available for export from susceptible-plant roots to the soil system through the VAM mycelium.

Mycorrhizae, biocides, and biocontrol. 2. Mycorrhizal fungi enhance weed control and crop growth in a soybean-cocklebur association treated with the herbicide bentazon

The mediation of herbicide effects on weed-crop combinations by vesicular-arbuscular mycorrhizal (VAM) fungi is essentially unknown. We conducted a greenhouse study to determine if VAM fungi, whose hyphae interconnect the roots of adjacent plants, modify herbicide effects by enhancing nutrient fluxes between associated plants. Soybean ( Glycine max (L.) Merr.) and common cocklebur ( Xanthium strumarium L.) plants were grown together in pots (1.5 L) in a high-P (28 mgkg −1) soil. They were sprayed with the herbicide, bentazon (BEN, 3-isopropyl-1H-2,1,3-benzothidiazine-(4)3H-one 2,2-dioxide), at dose rates of 0, 1 3 , 2 3 , 3 3 , and 4 3 of field recommendation (FR, 1.12 kg a.i. ha −1) while in the 7- to 8-leaf stage. Labelled N (1 mL of 100 mM 15NH 4NO 3, 98 atom percent 15 N) was applied to the cocklebur leaves 4 days before spraying and then assayed in the soybean leaves at harvest. Growth and nutrient contents of +VAM soybean shoots were enhanced only at the intermediate FR levels, while shoot growth in adjacent cocklebur was inhibited beyond the extent measured in - VAM plants. Labelled N was at natural abundance in both + VAM and - VAM soybean leaves at 4/3-FR, but at 1/3-FR to 3/3-FR 15N abundance was significantly higher in + VAM than in -VAM plants. These results suggest that shifts in source-sink relations occurred both within each plant and between plants as a result of the selective stress imposed on cocklebur. Moreover, this shift in competitiveness permitted a VAM-mediated flux of nutrients from weed to crop.

Rock phosphate and vesicular-arbuscular mycorrhiza effects on growth and nutrient uptake of Faidherbia albida (Del.) seedlings in an alkaline sandy soil

Response of Faidherbia albida (Del.) to five levels of Kodjari rock phosphate (KRP) application (0, 310, 620, 1240 and 2480 ppm P, equivalent to 0, 775, 1550, 3100 and 6200 kg P/ha) and vesicular-arbuscular mycorrhizal fungi (VAMF) Glomus manihotis Howeler, Sieverding & Schenck or Glomus aggregatum Schenck & Smith emend. Koske was evaluated in an alkaline sandy soil. The F. albida seedlings grew poorly without mycorrhizal colonization and without KRP applications. For non-VAM Faidherbia, the maximum growth response and both P and N uptake in shoots was achieved with the 620 ppm P. However, even without KRP application, VAM plants achieved better results in terms of biomass. VAM plants with G. manihotis and G. aggregatum improved plant growth and increased nutrient contents at any KRP application rate. Although mycorrhizal colonization was comparable at all levels of KRP application, the impact of nutrient content of the shoot varied. Finally, VAM plants did not accumulate more biomass than non-VAM plants at 620 ppm P and above. Growth response and mycorrhizal dependency decreased as KRP applied levels increased. These results suggest that VAM Faidherbia seedlings take up more P from soil and KRP than non-VAM.

Vesicular-arbuscular mycorrhizal fungi in earthworm casts and surrounding soil in relation to soil management of a semi-arid tropical Alfisol

Spores and infective propagules of vesicular-arbuscular (VA) mycorrhiza were examined in earthworm casts and field soils collected from three different soil management treatments (zero tillage with straw amendment, deep tillage without amendment (bare), and zero tillage with previous perennial cropping of Stylosanthes hamata) on an Alfisol in the semi-arid tropics (SAT) of India. The average mean of spore counts and most probable number (MPN) of infective propagules of VA mycorrhiza (VAM) were significantly ( P < 0.05) higher in earthworm casts than in field soil across the three soil management treatments. There was no significant difference in the number of VAM spores or propagules among field soils from the three different soil management treatments, but the number of VAM spores and propagules in the earthworm casts from the deep tillage bare treatment was significantly higher ( P ≤ 0.05) than in the earthworm casts from the other two treatments. In the deep tillage bare treatment, the number of spores and MPN of infective propagules were significantly ( P < 0.05) higher in earthworm casts than in field soil. Therefore, it may be concluded that earthworms can concentrate VA mycorrhizal spores and propagules in their casts.

Relationships between soluble sugar concentrations in roots and ecosystem stress for first year sugar maple seedlings

Accumulation of reducing sugars (i.e., glucose and fructose) in plant roots has been consistently correlated with forest dieback and decline and, therefore, has potential as a biological indicator of ecosystem stress. In this study, the relationships between acidic deposition and “natural” (temperature, mycorrhizae, and nutrition) factors with first-year sugar maple seedling root sugar concentrations and growth were assessed in two sugar maple dominated forests in Michigan. Seedlings at the southern site (Wellston) had greater root growth, phosphorus, total sugar, and sucrose concentrations in roots, but lower reducing sugar concentration in roots. In addition, percent root length colonized by vesicular-arbuscular mycorrhizal fungi was less than that found for seedlings growing at the northern site (Alberta). Throughfall deposition of nitrate, sulfate, and hydrogen ions was not significantly correlated with seedling total or reducing sugar concentration. Total sugar concentration in seedling roots was positively correlated with air and soil temperatures at the southern site, but not at the northern site. Seedling tissue phosphorus concentration was correlated with total sugars at both sites, with sucrose at the southern site, and reducing sugars at the northern site. Mycorrhizal colonization rates at the Alberta site were positively correlated with reducing sugar concentration in seedling roots and negatively correlated with sucrose concentration. The results suggest that differences in seedling root sugar concentrations in these two forests are related to seedling root growth and are most likely due to ecological variables, such as available soil phosphorus, temperature, and growing season length through some complex interaction with mycorrhizae rather than acidic deposition stress.

External phosphorus requirement of mycorrhizal and non-mycorrhizal barley and soybean plants

An experiment was conducted under greenhouse conditions to evaluate the effects of vesicular arbuscular mycorrhizal (VAM) fungi on the external P requirements of barley and soybeans. The plants were grown in pots containing a P-deficient soil. A range of 10 P levels was obtained by adding 0, 20, 30, 40, 50, 60, 70, 110, 160, or 310 mg P kg-1 as NaH2PO4 2H2O. Half of the pots were inoculated with the VAM fungus Glomus intraradices. The P concentration in the soil solution was determined using an adsorption isotherm and plotted against the relative yield. Barley did not respond to mycorrhizal inoculation and we concluded that P nutrition was not the limiting factor on the growth of this lowmycotrophic plant. In contrast, mycorrhizal inoculation stimulated the growth of soybeans. The external P requirements were 0.110 μg ml-1 for mycorrhizal and 0.148 μg ml-1 for non-mycorrhizal soybeans to obtain 80% of the maximum yield. In terms of P fertilization this corresponds to a saving of 222 kg P2O5 ha-1. The mycorrhizal dependency of the soybean was highly correlated with the P concentration in the soil solution and it is proposed that both values should be displayed together.

Vesicular-arbuscular mycorrhizal spore populations in sugar maple (Acer saccharum marsh. L.) forests

The numbers and types of spores of vesicular-arbuscular mycorrhizal fungi occurring in the top 15 cm of the soil in three maple forests in Eastern Canada were investigated using traditional wet-sieving/decanting methods. In the most acid site, at St. Hippolyte, Quebec, where the soil had been amended with base cations, after 1 year there was no effect on the numbers of spores present. Vesicular-arbuscular mycorrhizal spores present at St. Hippolyte consisted of Glomus rubiforme, other Glomus spp. and Acaulospora spp. Although the sporocarpic species, G. aggregatum, G. macrocarpum and G. rubiforme occurred at St. Hippolyte, they were not found at the two less acid sites (Waterloo, Ontario and Lacolle, Quebec) . Spores of Acaulospora spp. were found at all three sites, but were most abundant at St. Hippolyte. At St. Hippolyte the total number of spores was much higher than at the other two sites; at Waterloo numbers were an order of magnitude lower than at St. Hippolyte. It is suggested that G. rubiforme and Acaulospora species may be adapted to acid conditions. Seasonal patterns of spore abundance suggested that Acaulospora spp. may sporulate during the spring, whereas G. rubiforme may sporulate during the fall.

Vesicular-arbuscular mycorrhiza effects on Gliricidia sepium and Senna siamea in a fallowed alley cropping system

Nitrogen fixing and non-N2 fixing legumes such as Gliricidia speium and Senna siamea have been used in alley cropping systems for soil improvement and source of N for food crops. However their establishments could be limited by P and moisture deficiencies in degraded soils. Vesicular-arbuscular mycorrhizal fungi can help to overcome these deficiencies. We examined the effects of a vesicular-arbuscular mycorrhizal (VAM) fungus, Glomus deserticola, on the performance of sole hedgerow trees of Gliricidia sepium and Senna siamea and their mixtures (interplanted) in a fallowed alley cropping experiment on a degraded Alfisol in southwestern Nigeria. Percentage root infection by VAM fungi was higher in inoculated plants than in uninoculated ones irrespective of whether they were interplanted or non-interplanted. Inoculation with G. deserticola increased dry matter accumulation and nutrient uptake (N. P, Mg and K) but there was no significant interaction between mycorrhizal inoculation and interplanting for growth and nutrient uptake except for the uptake of P, Mg and K in G. sepium. Inoculation with G. deserticola reduced leaf shedding of G. sepium by 50% but did not have the same effect for S. siamea. For both tree species inoculated plants extracted more water from 0–30 cm depth than the uninoculated ones.

Response of Acacia mangium to vesicular – arbuscular mycorrhizal inoculation, soil pH, and soil P concentration in an oxisol

The vesicular–arbuscular mycorrhizal (VAM) fungus Glomus aggregatum and the legume species Acacia mangium were grown together in a manganese-rich oxisol at pH 4.3 to 6.0 and at soil P concentrations favorable for VAM-host growth (0.02 mg ∙ L−1) and sufficient for non-VAM host growth (0.8 mg ∙ L−1). At the lower P concentration, vesicular–arbuscular mycorrhizal fungal (VAMF) colonization of roots increased as soil pH increased from 4.3 to 5.0. However, colonization of roots was not significantly influenced by further increases in pH. Growth of A. mangium at 0.02 mg P/L in the presence of G. aggregatum was inferior to that observed at 0.8 mg P/L, suggesting that there was some degree of host–endophyte incompatibility. Increasing P concentration from 0.02 to 0.8 mg P/L increased target soil pH in the unlimed soil from 4.3 to 4.8 and reduced the concentration of available soil Mn from 13.2 to 2.1 mg ∙ L−1. Thus, the better plant growth observed at the higher P concentration at pH &lt; 5 was mainly due to the alleviation of Mn toxicity due to the precipitation of the cation directly by excess phosphate and (or) phosphate-induced elevation of soil pH. The poor VAMF inoculation effect observed at the lower soil P level in the unlimed soil was thus largely due to the toxicity of high concentrations of Mn2+ and H+ ions. Keywords: hydrogen ion, calcium, manganese-rich, manganese toxicity, pinnule P, soil acidity, VAMF colonization, VAMF effectiveness.

Iron acquisition by mycorrhizal maize grown on alkaline soil

Abstract Vesicular‐arbuscular mycorrhizal fungi (VAMF) are known to improve the mineral nutritional status of plants. Since information on effects of VAMF on plant iron (Fe) nutrition is limited, an Fe deficient‐susceptible maize (Zea mays L., ysi/ysi) was grown on alkaline Quinlan (Typic Ustochrept, pH 8.0) and Ulysses (Aridic Haplustoll, pH 7.8) soils with Glomus VAMF isolates G. etunicatum WV579A (Ge), G. diaphanum WV579B (Gd), and G. intraradices WV894 (Gi) to determine plant growth responses to VAMF associations and efficacy of these VAMF isolates to enhance Fe acquisition. Shoot and root dry matter (DM) were higher in mycorrhizal (+VAM) then in nonmycorrhizal (‐VAM) plants, and followed a sequence of Ge > Gd ≥ Gi > ‐VAM. Root DM of Ge plants was higher than other +VAM plants. Even though relatively low, leaf chlorophyll (SFAD unit) readings were higher in +VAM than in ‐VAM plants and followed a sequence similar to DM. Plants colonized with Gi had higher leaf Fe, phosphorus (P), and zinc (Zn) concent...

Persistence of introduced Glomus intraradices in the field as influenced by repeated inoculation and cropping system

We studied the persistence of an introduced vesicular-arbuscular mycorrhizal (VAM) fungus in the field under the influence of a mycorrhizal host, a non-mycorrhizal host, and when the field was left fallow. There was a significantly greater, build-up of the mycorrhizal fungus in plots cropped with mycorrhizal hosts (finger millet or field beans) compared to plots cropped with a nonmycorrhizal host (mustard) or left fallow. Glomus intraradices, the introduced fungus, was further monitored by the electrophoretic mobility of the isozymes of malate dehydrogenase extracted from the resting spores of the fungus. The zymogram from G. intraradices spores showed three distinct isozyme bands: 1,3 and 7. A zymogram of malate dehydrogenase spores isolated from plots inoculated with G. intraradices for three seasons exhibited similar bands. Spores isolated from plots inoculated with G. intraradices for one or two seasons showed only one weak band, corresponding to isozyme band 7. The results indicated that the introduced fungus persisted in the field for only one season.

Colonisation of wheat in southern New South Wales by vesicular-arbuscular mycorrhizal fungi is significantly reduced by drought

Colonisation of wheat crops by vesicular-arbuscular mycorrhizal (VAM) fungi was examined on a pair of adjacent organic and conventionally managed farms in southern New South Wales during 1993-94. Although intended to be part of a larger study on the roles of VAM fungi in organic and conventional farming systems, the severe drought of 1994 allowed an examination of the effects of drought on VAM fungi. In 1993, rainfall was above average, crop growth was good and VAM fungi colonised between 40 and 70% of crop root length. In 1994, low rainfall resulted in poor crop growth, and colonisation by VAM fungi was significantly lower than in 1993, ranging between 5 and 16% in the crops most affected by the drought. Wheat plants adjacent to tree lines exhibited particularly poor growth and low VAM colonisation, presumably due to the trees competing with the crop for water. The lower colonisation of crops by VAM fungi in 1994 resulted in reduced inoculum levels in the soil which could affect growth of a subsequent VAM-dependent crop. In contrast to the results of research carried out under conditions of milder drought stress, the low level of VAM colonisation found in this study suggests that the VAM fungi had no significant role in alleviating the drought stress experienced by the crop.