Improved Tolerance of <Emphasis Type="Italic">Acacia nilotica</Emphasis> to Salt Stress by Arbuscular Mycorrhiza, <Emphasis Type="Italic">Glomus fasciculatum</Emphasis> may be Partly Related to Elevated K/Na Ratios in Root and Shoot Tissues

A pot experiment was conducted to examine the effect of arbuscular mycorrhizal fungus, Glomus fasciculatum, and salinity on the growth of Acacia nilotica. Plants were grown in soil under different salinity levels (1.2, 4.0, 6.5, and 9.5 dS m⁻¹). In saline soil, mycorrhizal colonization was higher at 1.2, 4.0, and 6.5 dS m⁻¹ salinity levels in AM-inoculated plants, which decreased as salinity levels further increased (9.5 dS m⁻¹). Mycorrhizal plants maintained greater root and shoot biomass at all salinity levels compared to nonmycorrhizal plants. AM-inoculated plants had higher P, Zn, and Cu concentrations than uninoculated plants. In mycorrhizal plants, nutrient concentrations decreased with the increasing levels of salinity, but were higher than those of the nonmycorrhizal plants. Mycorrhizal plants had greater Na concentration at low salinity levels (1.2, 4.0 dS m⁻¹), which lowered as salinity levels increased (6.5, 9.5 dS m⁻¹), whereas Na concentration increased in control plants. Mycorrhizal plants accumulated a higher concentration of K at all salinity levels. Unlike Na, the uptake of K increased in shoot tissues of mycorrhizal plants with the increasing levels of salinity. Our results indicate that mycorrhizal fungus alleviates deleterious effects of saline soils on plant growth that could be primarily related to improved P nutrition. The improved K/Na ratios in root and shoot tissues of mycorrhizal plants may help in protecting disruption of K-mediated enzymatic processes under salt stress conditions.     

Effects of VA-mycorrhizal fungi on growth and nutrient uptake of cuttings ofRosa multiflora in two container media with three levels of fertilizer application

Rooted cuttings ofRosa multiflora ‘Brooks 56’ were grown in a medium of 1 mineral soil: 1 sand (v/v) or bark: 1 sand (v/v) inoculated with the VA-mycorrhizal (VAM) fungiGlomus mosseae (Nicol. and Gerd.) Gerd. and Trappe andG. fasciculatum (Thaxt. sensu Gerd.) Gerd. and Trappe or left as noninoculated controls. The slow release fertilizer osmocote was applied at rates of either 0, 1.2, or 4.2 kg/m³ (18N-6P-12K) and incorporated into the container medium. After 180 days plants were evaluated for growth, development and chemical leaf analysis. Greatest growth responses occurred after the highest fertilizer application of 4.2 kg/m³, and the poorest one after 0 kg/m³. Combination bark: sand medium was superior to mineral soil: sand medium for growth of mycorrhizal plants. At 0 kg/m³, mycorrhizal plants in bark: sand medium had longer shoots than other treatments. At 1.2 kg/m³, VAM plants compared to nonmycorrhizal plants in park:sand medium had greater effect on growth parameters. At the highest fertilizer application of 4.2 kg/m³, greatest growth responses occurred with VAM plants in bark:sand medium. Mycorrhizal plants compared to nonmycorrhizal plants in bark:sand medium had greater K and Zn uptake at 0 kg/m³, and greater K, Ca, S, Mn and Zn uptake at 1.2 kg/m³. Texas Agr. Expt. Sta. Journal Series No. TA-22264.     

Preinoculation of lettuce and onion with VA mycorrhizal fungi reduces deleterious effects of soil salinity

The hypothesis that inoculation of transplants with vesicular-arbuscular mycorrhizal (VAM) fungi before planting into saline soils alleviates salt effects on growth and yield was tested on lettuce (Lactuca sativa L.) and onion (Allium cepa L.). A second hypothesis was that fungi isolated from saline soil are more effective in counteracting salt effects than those from nonsaline soil. VAM fungi from high- and low-salt soils were trap-cultured, their propagules quantified and adjusted to a like number, and added to a pasteurized soil mix in which seedlings were grown for 3–4 weeks. Once the seedlings were colonized by VAM fungi, they were transplanted into salinized (NaCl) soil. Preinoculated lettuce transplants grown for 11 weeks in the saline soils had greater shoot mass compared with nonVAM plants at all salt levels [2 (control), 4, 8 and 12 dS m^–1] tested. Leaves of VAM lettuce at the highest salt level were significantly greener (more chlorophyll) than those of the nonVAM lettuce. NonVAM onions were stunted due to P deficiency in the soil, but inoculation with VAM fungi alleviated P deficiency and salinity effects; VAM onions were significantly larger at all salt levels than nonVAM onions. In a separate experiment, addition of P to salinized soil reduced the salt stress effect on nonVAM onions but to a lesser extent than by VAM inoculation. VAM fungi from the saline soil were not more effective in reducing growth inhibition by salt than those from the nonsaline site. Colonization of roots and length of soil hyphae produced by the VAM fungi decreased with increasing soil salt concentration. Results indicate that preinoculation of transplants with VAM fungi can help alleviate deleterious effects of saline soils on crop yield.     

Light effects in mycorrhizal soybeans

Soybean (Glycine max. L. Merr.) plants were grown in an experiment with a 3 × 3 factorial design using different levels of light (170, 350, and 700 μE·m⁻²·s⁻¹) and P as factors. Plants were grown in a greenhouse in pot cultures using a soil low in plant-available P under three P regimes: no additional P, P added as KH₂PO₄, or P uptake enhanced by colonization of the host plant with the vesicular-arbuscular mycorrhizal (VAM) fungus Glomus fasciculatum (Thaxt. sensu Gerd.) Gerd. and Trappe. Development of the VAM fungal endophyte and of plants under all three P regimes was depressed by limiting light. However, the growth response of VAM plants to increasing light relative to non-VAM plants in the absence of additional P increased while the response relative to non-VAM plants with additional P decreased slightly. The highly significant interaction between the factors (P < 0.001) of the experiment was due to differences in the magnitude and direction of simple effects of the factors. The implications of these differences in terms of source-sink relationships of the symbionts and the value of different non-VAM controls in interpreting VAM effects are discussed.     

Influence of the fungicide pentachloronitrobenzene on VA-mycorrhizal and total root length and phosphorus uptake of oats (Avena sativa)

The effect of application of the fungicide pentachloronitrobenzene (PCNB) at levels between 2 and 50 mg kg^–1 soil on root growth, mycorrhizal infection and P uptake was studied in pot culture with oats (Avena sativa cv. Alfred) growing in a rendzina soil low in available P. The soil had been partially sterilized by X-ray, and half of the pots were inoculated with spores of the VAM-fungusGlomus mosseae (indigenous species).Soil irradiation (0.5 Mrad) did not decrease the levels of infection by VAM. Application of PCNB decreased the VAM-infected root length, at 50 mg PCNB kg^–1 soil VAM-infected root length was about 12% of the controls. Total root length, however, increased to about 126% of control values at PCNB rates up to 20 mg kg^–1 soil, but decreased to 89% of the controls at 50 mg kg^–1 soil. Total P-uptake decreased with increasing levels of PCNB and was linearly correlated with infected root length (r=0.92).The stimulation of root growth by PCNB at rates up to 20 mg kg^–1 soil is regarded as an indirect effect, brought about by suboptimal P-supply due to inhibition of VA-mycorrhiza. Conversely, the reduction of total root length at 50 mg PCNB kg^–1 soil is most likely a direct effect. Due to the phytotoxicity of the fungicide, the contribution of the indigenous VA-mycorrhiza to plant P uptake under field conditions cannot be determined by soil application of PCNB at rates sufficient for complete inhibition of VAM.As inhibition or absence of VAM may lead to compensatory root growth, mycorrhizal dependency ought to be calculated from the amounts of P taken up per unit root length in mycorrhizal and nonmycorrhizal plants, respectively.     

Growth of mycorrhizal tomato and mineral acquisition under salt stress

 High salt levels in soil and water can limit agricultural production and land development in arid and semiarid regions. Arbuscular mycorrhizal fungi (AMF) have been shown to decrease plant yield losses in saline soils. The objective of this study was to examine the growth and mineral acquisition responses of greenhouse-grown tomato to colonization by the AMF Glomus mosseae [(Nicol. And Gerd.) Gerd. and Trappe] under varied levels of salt. NaCl was added to soil in the irrigation water to give an EC_e of 1.4 (control), 4.7 (medium) and 7.4 dS m^–1 (high salt stress). Plants were grown in a sterilized, low P (silty clay) soil-sand mix. Mycorrhizal colonization was higher in the control than in saline soil conditions. Shoot and root dry matter yields and leaf area were higher in mycorrhizal than in nonmycorrhizal plants. Total accumulation of P, Zn, Cu, and Fe was higher in mycorrhizal than in nonmycorrhizal plants under both control and medium salt stress conditions. Shoot Na concentrations were lower in mycorrhizal than in nonmycorrhizal plants grown under saline soil conditions. The improved growth and nutrient acquisition in tomato demonstrate the potential of AMF colonization for protecting plants against salt stress in arid and semiarid areas. Accepted: 21 February 2000     

Dependency of cassava (Manihot esculanta Crantz) on vesicular-arbuscular mycorrhizal fungi

Cassava plants were started in the greenhouse either from small cuttings (2.0 mg P/cutting) or large cuttings (20.2 mg P/cutting) in a subsurface Oxisol not inoculated or inoculated with Glomus aggregatum at target soil solution P concentrations of 0.003–0.2 mg l^-1. Vesicular-arbuscular mycorrhizal (VAM) fungal colonization levels in excess of 60% were attained on cassava roots irrespective of the size of cutting material used or target soil solution P status. However, plants started from large cuttings grew faster and better than those started from smaller ones. Cassava was found to be very highly dependent on VAM fungi if grown from small cutting but only marginally dependent if grown from large cuttings. The lower dependence of cassava on VAM fungi when started from larger cuttings appears to be related to the high P reserve in these cuttings and hence the low requirement of the plants for soil P until the P reserve in the cuttings is significantly depleted.A contribution from the Hawaii Institute of Tropical Agriculture and Human Resources Journal Series No. 3896     

Contribution of the VA mycorrhizal hyphae in acquisition of phosphorus and zinc by maize grown in a calcareous soil

An investigation was carried out to test whether the mechanism of increased zinc (Zn) uptake by mycorrhizal plants is similar to that of increased phosphorus (P) acquisition. Maize (Zea mays L.) was grown in pots containing sterilised calcareous soil either inoculated with a mycorrhizal fungus Glomus mosseae (Nicol. and Gerd.) Gerdemann and Trappe or with a mixture of mycorrhizal fungi, or remaining non-inoculated as non-mycorrhizal control. The pots had three compartments, a central one for root growth and two outer ones for hyphal growth. The compartmentalization was done using a 30-m nylon net. The root compartment received low or high levels of P (50 or 100 mg kg^–1 soil) in combination with low or high levels of P and micronutrients (2 or 10 mg kg^–1 Fe, Zn and Cu) in the hyphal compartments.Mycorrhizal fungus inoculation did not influence shoot dry weight, but reduced root dry weight when low P levels were supplied to the root compartment. Irrespective of the P levels in the root compartment, shoots and roots of mycorrhizal plants had on average 95 and 115% higher P concentrations, and 164 and 22% higher Zn concentrations, respectively, compared to non-mycorrhizal plants. These higher concentrations could be attributed to a substantial translocation of P and Zn from hyphal compartments to the plant via the mycorrhizal hyphae. Mycorrhizal inoculation also enhanced copper concentration in roots (135%) but not in shoots. In contrast, manganese (Mn) concentrations in shoots and roots of mycorrhizal plants were distinctly lower, especially in plants inoculated with the mixture of mycorrhizal fungi.The results demonstrate that VA mycorrhizal hyphae uptake and translocation to the host is an important component of increased acquisition of P and Zn by mycorrhizal plants. The minimal hyphae contribution (delivery by the hyphae from the outer compartments) to the total plant acquisition ranged from 13 to 20% for P and from 16 to 25% for Zn.     

The effect of soil compaction on growth and P uptake by Trifolium subterraneum: interactions with mycorrhizal colonisation

The effects of vesicular-arbuscular mycorrhizal (VAM) colonisation on phosphorus (P) uptake and growth of clover (Trifolium subterraneum L.) in response to soil compaction were studied in three pot experiments. P uptake and growth of the plants decreased as the bulk density of the soil increased from 1.0 to 1.6 Mg m^-3. The strongest effects of soil compaction on P uptake and plant growth were observed at the highest P application (60 mg kg^-1 soil). The main observation of this study was that at low P application (15 mg kg^-1 soil), P uptake and shoot dry weight of the plants colonised by Glomus intraradices were greater than those of non-mycorrhizal plants at similar levels of compaction of the soil. However, the mycorrhizal growth response decreased proportionately as soil compaction was increased. Decreased total P uptake and shoot dry weight of mycorrhizal clover in compacted soil were attributed to the reduction in the root length. Soil compaction had no significant effect on the percentage of root length colonised. However, total root length colonised was lower (6.6 m pot^-1) in highly compacted soil than in slightly compacted soil (27.8 m pot^-1). The oxygen content of the soil atmosphere measured shortly before the plants were harvested varied from 0.18 m³m^-3 in slightly compacted soil (1.0 Mg m^-3) to 0.10 m³m^-3 in highly compacted soil (1.6 Mg m^-3).     

Response of citronella Java (Cymbopogon winterianus Jowitt) to VA mycorrhizal fungi and soil compaction in relation to P supply

Nutrient acquisition and growth of citronella Java (Cymbopogon winterianus Jowitt) was studied in a P-deficient sandy soil to determine the effects of mycorrhizal symbiosis and soil compaction. A pasteurized sandy loam soil was inoculated either with rhizosphere microorganisms excluding VAM fungi (non-mycorrhizal) or with the VAM fungus, Glomus intraradices Schenck and Smith (mycorrhizal) and supplied with 0, 50 or 100 mg P kg^-1 soil. The soil was compacted to a bulk density of 1.2 and 1.4 Mg m^-3 (dry soil basis). G. intraradices substantially increased root and shoot biomass, root length, nutrient (P, Zn and Cu) uptake per unit root length and nutrient concentrations in the plant, compared to inoculation with rhizosphere microorganisms when the soil was at the low bulk density and not amended with P. Little or no plant response to the VAM fungus was observed when the soil was supplied with 50 or 100 mg P kg^-1 soil and/or compacted to the highest bulk density. At higher soil compaction and P supply the VAM fungus significantly reduced root length. Non-mycorrhizal plants at higher soil compaction produced relatively thinner roots and had higher concentrations and uptake of P, Zn and Cu than at lower soil compaction, particularly under conditions of P deficiency. The quality of citronella Java oil measured in terms citronellal and d-citronellol concentration did not vary appreciably due to various soil treatments.     

Effects of a vesicular-arbuscular mycorrhizal fungus and other soil microorganisms on growth,mineral nutrient acquisition and root exudation of soil-grown maize plants

Maize (Zea mays L. cv. Alize) plants were grown in a calcareous soil in pots divided by 30-m nylon nets into three compartments, the central one for root growth and the outer ones for hyphal growth. Sterle soil was inoculated with either (1) rhizosphere microorganisms other than vesicular-arbuscular mycorrhizal (VAM) fungi, (2) rhizosphere microorganisms together with a VAM fungus [Glomus mosseae (Nicol. and Gerd.) Gerdemann and Trappel], or (3) with a gamma-irradiated inoculum as control. Plants were grown under controlled-climate conditions and harvested after 3 or 6 weeks. VAM plants had higher shootroot ratios than non-VAM plants. After 6 weeks, the concentrations of P, Zn and Cu in roots and shoots had significantly increased with VAM colonization, whereas Mn concentrations had significantly decreased. Root exudates were collected on agar sheets placed on the interface between root and hyphal compartments. Six-week-old VAM and non-VAM plants had similar root exudate compositions of 72–73% reducing sugars, 17–18% phenolics, 7% organic acids and 3% amino acids. In another experiment in which root exudates were collected on agar sheets with or without antibiotics, the amounts of amino acids and carbohydrates recovered were similar in VAM and non-VAM plants. However, threeto sixfold higher amounts of carbohydrates, amino acids and phenolics were recovered when antibiotics were added to the agar sheets. Thus, the high microbial activity in the rhizosphere and on the rhizoplane limits the exudates recovered from roots.     

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.     

Influence of the arbuscular mycorrhizal fungus Glomus mosseae on uptake of arsenate by the As hyperaccumulator fern Pteris vittata L.

We report for the first time some effects of colonization by an arbuscular mycorrhizal (AM) fungus (Glomus mosseae) on the biomass and arsenate uptake of an As hyperaccumulator, Pteris vittata. Two arsenic levels (0 and 300 mg As kg^–1) were applied to an already contaminated soil in pots with two compartments for plant and hyphal growth in a glasshouse experiment. Arsenic application had little or no effect on mycorrhizal colonization, which was about 50% of root length. Mycorrhizal colonization increased frond dry matter yield, lowered the root/frond weight ratio, and decreased frond As concentration by 33–38%. Nevertheless, transfer of As to fronds showed a 43% increase with mycorrhizal colonization at the higher soil As level. Frond As concentrations reached about 1.6 g kg^–1 (dry matter basis) in non-mycorrhizal plants in the As-amended soil. Mycorrhizal colonization elevated root P concentration at both soil As levels and mycorrhizal plants had higher P/As ratios in both fronds and roots than did non-mycorrhizal controls.     

Effects of vesicular-arbuscular mycorrhizal inoculation at different soil P availabilities on growth and nutrient uptake of in vitro propagated coffee (Coffea arabica L.) plants

 In a pot experiment, the growth and the nutrient status of in vitro propagated coffee (Coffea arabica L.) microcuttings were investigated for 5 months following vesicular-arbuscular mycorrhizal (VAM) inoculation with either Acaulospora melleae or Glomus clarum at four soil P availabilities. Control plants remained P-deficient even at the highest soil P availability while mycorrhizal plants were P-sufficient at all soil P availabilities. Growth of control plants was only improved at the highest soil P availability. In P-deficient soil, neither of the two VAM species improved plant growth. Plant growth increased by 50% following inoculation with either A. melleae or G. clarum when P availability went from deficient to low. No further plant growth improvement was induced by either VAM species at intermediate and high soil P levels. Nevertheless, growth of plants inoculated with G. clarum was still significantly greater than that of non-mycorrhizal plants at the highest soil P availability. Root colonization by G. clarum increased with increasing soil P availability while root colonization by A. mellea decreased with soil P level increasing above low P availability. Soil P availability also affected Zn nutrition through its influence on VAM symbiosis. With increasing soil P availability, foliar Zn status increased with G. clarum or decreased with A. mellea in parallel to root colonization by VAM. This study demonstrates the beneficial effects of VAM inoculation on in vitro propagated Arabica coffee microcuttings, as shown previously for seedlings. This study also demonstrates differences in tolerance to soil P availability between VAM species, most likely resulting from their differing abilities to enhance coffee foliar P status. Accepted: 14 November 1996     

Glycine-Glomus-Rhizobium Symbiosis: V. Effects of Mycorrhiza on Nodule Activity and Transpiration in Soybeans under Drought Stress

Soybean (Glycine max [L.] Merr.) plants were nodulated (Bradyrhizobium japonicum) and either inoculated with the vesicular-arbuscular mycorrhizal (VAM) fungus Glomus mosseae (Nicol. & Gerd.) Gerd. and Trappe or left uncolonized. All plants were grown unstressed for 21 days initially. After this period, some VAM and non-VAM plants were exposed to four 8-day drought cycles while others were kept well watered. Drought cycles were terminated by rewatering when soil moisture potentials reached −1.2 megapascal. Nodule development and activity, transpiration, leaf conductance, leaf and root parameters including fresh and dry weight, and N and P nutrition of VAM plants and of non-VAM, P-fed plants grown under the same controlled conditions were compared. All parameters, except N content, were greater in VAM plants than in P-fed, non-VAM plants when under stress. The opposite was generally true in the unstressed comparisons. Transpiration and leaf conductance were significantly greater in stressed VAM than in non-VAM plants during the first half of the final stress cycle. Values for both VAM and non-VAM plants decreased linearly with time during the cycle and converged at a high level of stress (−1.2 megapascal). Effects of VAM fungi on the consequences of drought stress relative to P nutrition and leaf gas exchange are discussed in the light of these findings and those reported in the literature.     

Mycorrhizal and nonmycorrhizal host growth in response to changes in pH and P concentration in a manganiferous oxisol

Glomus aggregatum and Leucaena leucocephala were allowed to interact in a manganese-rich oxisol at pH 4.3–6.0 and at soil P concentrations considered optimal for mycorrhizal host growth and sufficient for nonmycorrhizal host growth. At 0.02 mg P l^-1, vesicular-arbuscular mycorrhizal fungal (VAMF) colonization of roots increased as soil pH increased from 4.3 to 5.0. However, VAMF colonization of roots did not respond to further increases in pH. At pH 6.0, growth of mycorrhizal Leucaena observed at 0.02 mg P was comparable with that observed at 0.8 mg P l^-1. Increasing P concentration from 0.02 to 0.8 mg P 1^-1 increased target soil pH from 4.3 to 4.7 and reduced the concentration of available soil Mn from 15.1 to 1.9 mg 1^-1. Thus, the normal plant growth observed at the higher P concentration at pH<5 was mainly due to the alleviation of Mn toxicity as a result of its precipitation by excess P. VAMF colonization levels observed at pH 5.0–6.0 were similar, but maximal plant growth occurred at pH 6.0, suggesting that the optimal pH for mycorrhizal formation was substantially lower than for VAMF effectiveness. The poor growth of Leucaena at the lower P concentration in the unlimed soil was largely due to high concentrations of Mn²⁺ and H⁺ ions.Contribution from the Hawaii Institute of Tropical Agriculture and Human Resources Journal Series No. 3910     

Glycine-Glomus-Rhizobium Symbiosis : VI. Photosynthesis in Nodulated, Mycorrhizal, or N- and P-Fertilized Soybean Plants

Soybean (Glycine max [L.] Merr. cv Hobbit) plants were grown in a growth chamber for 56 days in a phosphorus- and nitrogen-deficient soil and were colonized by the vesicular-arbuscular mycorrhizal (VAM) fungus Glomus mosseae (Nicol. & Gerd) Gerd. and Trappe and Rhizobium japonicum strain USDA 136, or by either organism alone, or by neither. Non-VAM plants received supplemental phosphorus and nonnodulated plants supplemental nitrogen to achieve the same rate of growth in all treatments. Plants of all four treatments had the same (P > 0.05) dry weights at harvest, but VAM plants had higher rates of CO₂ exchange (CER, P < 0.05) and lower leaf P concentrations (P < 0.01). Leaf nitrogen concentrations were lower in nodulated than in nitrogen-supplemented plants (P < 0.01) while starch concentrations were higher (P < 0.01). There was a significant negative relationship between nitrogen and starch (r = −0.989). Statistical evaluation of the data showed that some parameters (CER, leaf area and phosphorus content) were associated with phosphorus nutrition (or the presence of the VAM fungus), others (leaf fresh weight and root dry weight) with nitrogen nutrition (or the presence of Rhizobium), and some (leaf nitrogen and starch content) by both factors. The development of microsymbiont structures and nodule activity were significantly lower in the tripartite association than in plants colonized by one endophyte only. The findings suggest that endophyte effects go beyond those of simple nutrition and associated source-sink relationships.     

Growth response and phosphorus uptake of rye with long and short root hairs: Interactions with mycorrhizal infection

Plant growth and phosphorus (P) uptake of two selections of rye (Secale cereale L.) differing in length of root hairs, in response to mycorrhizal infection were investigated. Rye plants with short root hairs (SRH) had a greater length of root infected by Glomus intraradices (up to 32 m pot^–1) than those with long root hairs (LRH) (up to 10 m pot^–1). Application of P decreased the percentage of root length infected in both selections. In low-P soil, mycorrhizal infection increased shoot and root P concentration, especially in LRH plants. Generally, LRH had higher shoot dry weight than SRH plants. P uptake was increased both by LRH and by mycorrhizal infection. Differences in specific P uptake and P utilization efficiency between SRH and LRH plants were observed in non-mycorrhizal plants. With low P supply, P utilization efficiency (dry matter yield per unit of P taken up) of LRH plants increased with time. However, mycorrhizal infection reduced P utilization efficiency, particularly of SRH plants. SRH plants, which were agronomically less efficient (i.e. low dry matter yield at low P supply) were more responsive to either mycorrhizal infection or P addition than the LRH plants. No interaction was observed between mycorrhizal infection and root hair length.     

Effects of the root-lesion nematode Pratylenchus vulnus and the vesicular-arbuscular mycorrhizal fungus Glomus mosseae on the growth of three plum rootstocks

The effects of Pratylenchus vulnus and the endomycorrhizal fungus Glomus mosseae on growth of Myrobalan 605, Marianna 2624 and San Julian 655-2 plum rootstocks were measured under shadehouse conditions in the field for two growing seasons (1990–91). Shoot dry weights were higher in the majority of the vesicular-arbuscular mycorrhizal (VAM) alone inoculated plants after both growing seasons. Root weights of mycorrhizal Myrobalan and Marianna were higher than root weights of the same rootstocks lacking mycorrhizae, inoculated with P. vulnus, and VAM in combination with the nematode. Mycorrhizal Marianna inoculated with the nematode showed a considerably higher final nematode population in relation to non-inoculated VAM treatments. No correlation was found in the number of nematodes per gram of root between mycorrhizal and non-mycorrhizal treatments. P. vulnus adversely affected the mycorrhizal colonization in Marianna, but not in Myrobalan and San Julian. Marianna appears to be more mycorrhizal dependent than the two other rootstocks.     

VA-mycorrhiza mediated P effect on growth and yield of sunflower (Helianthus annuus L.) at different P levels

A field trial was conducted to study the response of sunflower (Helianthus annuus L.) to different phosphorus levels (16, 24 or 32 kg P ha^–1) and inoculation with vesicular-arbuscular mycorrhizal fungus, Glomus fasciculatum on vertisol during summer 1993. At the vegetative stage of sunflower, percent mycorrhizal root colonization, spore count, dry biomass and P uptake did not differ significantly between inoculated and uninoculated control plants. However, at later stages (flowering and maturity) percent root colonization, spore count, total dry biomass and total P uptake were significantly higher in inoculated plants than in uninoculated control plants. The total dry biomass, P content and seed yield increased with increasing P level in uninoculated plants, whereas no significant difference was observed between 16 and 32 kg P ha^–1 in inoculated plants. The positive effect of mycorrhizal inoculation decreased with increasing P level above 16 kg P ha^–1, due to decreased percent root colonization and spore count at higher P levels.