Influence Of Arbuscular Mycorrhizae Research Articles

Occurrence and influence of arbuscular mycorrhizae on chemical and enzymatic parameters of soil grown with different crops

In the present system of agriculture, chemical fertilizers are used indiscriminately throughout the world. However, the cost of these fertilizers has been increasing, exponentially and at the same time polluting soil and ground water. In this scenario, the use of eco-friendly bio fertilizers, particularly arbuscular mycorrhizae (AM) is one of the promising alternatives to increase the productivity of agricultural crops. An attempt was made to study the chemical and enzymatic parameters of soil with the occurrence and prevalence of AM fungi in different agricultural crops of Andhra Pradesh. Root and rhizosphere soils of four major crops such as cotton, sugarcane, rice and black gram were collected at different growth stages of the crops and assayed for AM fungi colonization and spore density. Also, the soil chemical and enzymatic parameters such as organic carbon, alkaline phosphatase, glomalin were assayed from these soil samples. Our results showed that the four crop roots colonized by AM fungi. The percent root colonization by AM fungi was maximum in cotton (82.3%) followed by black gram (70%) and rice (35.6%). Whereas the spore number was highest in black gram (42/100g of soil) followed by cotton (39.2/100g of soil) and rice (26.6/100 g of soil). The chemical and enzymatic parameters measured such as organic carbon was maximum in rice (0.79%), followed by sugarcane and cotton (0.73%). Alkaline phosphatase was highest in sugarcane (32.73 p-nitro phenol μg/g wt. of soil/hour) followed by black gram (23.93 p-nitro phenol μg/g wt. of soil/hour) and glomalin was highest in sugarcane (7.96 mg/g) followed by black gram (5.49 mg/g). Our results showed that there was a positive correlation between chemical and enzymatic factors measured with the AM fungi prevalence in the roots and rhizosphere soils of the crops evaluated.

Influence of arbuscular mycorrhizae on plant growth, essential oil production and phosphorus uptake of Salvia officinalis L.

Arbuscular mycorrhiza fungi (AMF) contribute to the secondary metabolism and production of active ingredients in aromatic and medicinal plants. This symbiotic association is particularly affected by the availability of phosphorus (P) in the soil. This study was conducted on Salvia officinalis L. using two inocula, commercial Symbivit and Septoglomus viscosum (syn. Glomus viscosum), alone or supplemented with two doses of actual P (0.03, 0.06gkg−1). The effects of these fungi and their combinations with P were determined in relation to the growth of sage plants (Regula variety), to the concentration of P in leaf tissues, and to the quantity and quality of essential oils (EOs). S. viscosum treated plants showed better growth with or without P-supply compared to non-mycorrhizal plants. The plants inoculated with S. viscosum presented the highest dry weight regardless of addition of P. Both AM fungi increased the leaf P content as more P was applied to the soil, whereas the EO content did not change with any of the treatments. Although the EO yield slightly increased with the Symbivit treatment, the chemical composition of the oil was drastically altered by S. viscosum in which the manool was the main component (28.13%), while α-thujone decreased (13.09%). These results suggest that AM symbiosis is a good candidate for promoting plant growth and essential oil composition and for improving P uptake in low fertility soils. Thus, mycorrhiza can be considered as a sustainable strategy based on natural resources in order to influence the manool and α-thujone content in sage EO composition. These compositions are very important to develop new classes of biocides and contribute to reducing risks to both human health and the environment.

Influence of arbuscular mycorrhizae and plant growth promoting rhizobacteria on proline, membrane permeability and growth of strawberry ( Fragaria x ananassa ) under salt stress

Salinity is one of the most important factors negatively effecting the yield in crop species. In this study the effect of plant growth promoting rhizobacteria (PGPR) and arbuscular mycorrhizae on proline content, membrane permeability and growth of strawberry cv. 'San Andreas' were studied under different salt treatments (0, 30 and 60 mM/L NaCl). The leaf area was measured 0, 15, 30, 45 and 60 days after saline solution applications on the plants. The results showed that increasing concentrations of NaCl decreased all growth parameters. Increased salt concentration led to increased proline level compared to the control. Bacterial application at 60 mM/L NaCl concentration provided the highest ameliorative effect and therefore determined the most effective protection of the plant against salt stress. It was observed that the anthocyanin content increased in line with the increasing salt concentration. In general, the salt applied on the plants causes an increase in membrane permeability and thus disrupts membrane stability and becomes a significant factor damaging the plant. Membrane permeability increased at applications with 30 mM/L and 60 mM/L NaCl. Our results revealed that bacteria application can have an ameliorative effect that helps the plant to tolerate the negative effects of salt stress by increasing proline and anthocyanin levels.

Influence of Arbuscular Mycorrhizae on Biomass Production and Nitrogen Fixation of Berseem Clover Plants Subjected to Water Stress

Several studies, performed mainly in pots, have shown that arbuscular mycorrhizal symbiosis can mitigate the negative effects of water stress on plant growth. No information is available about the effects of arbuscular mycorrhizal symbiosis on berseem clover growth and nitrogen (N) fixation under conditions of water shortage. A field experiment was conducted in a hilly area of inner Sicily, Italy, to determine whether symbiosis with AM fungi can mitigate the detrimental effects of drought stress (which in the Mediterranean often occurs during the late period of the growing season) on forage yield and symbiotic N2 fixation of berseem clover. Soil was either left under water stress (i.e., rain-fed conditions) or the crop was well-watered. Mycorrhization treatments consisted of inoculation of berseem clover seeds with arbuscular mycorrhizal spores or suppression of arbuscular mycorrhizal symbiosis by means of fungicide treatments. Nitrogen biological fixation was assessed using the 15N-isotope dilution technique. Arbuscular mycorrhizal symbiosis was able to mitigate the negative effect of water stress on berseem clover grown in a typical semiarid Mediterranean environment. In fact, under water stress conditions, arbuscular mycorrhizal symbiosis resulted in increases in total biomass, N content, and N fixation, whereas no effect of crop mycorrhization was observed in the well-watered treatment.

Influence of arbuscular mycorrhizae on root colonization, growth and productivity of two wheat cultivars under salt stress

Two pot experiments were conducted in the greenhouse of the National Research Center, Egypt during 2003/2004 and 2004/2005 to investigate the efficacy of arbuscular mycorrhizae (AM) on root colonization, growth and productivity in two wheat cultivars, Sakha 8 and Giza 167, under salt stress. The extent of the AM effect on wheat development varied with plant cultivar and salinity level. Maximum root colonization and spore production were observed with the Sakha 8 cultivar, which resulted in greater plant growth and productivity at all salinity levels. AM and plant development were adversely affected by increasing salinity. However, the presence of mycorrhizal fungi protected wheat against the detrimental effect of salinity, and stimulated growth, productivity, total crude protein concentration and nitrate reductase activity. The average enhancement in grain yield due to AM inoculation was 76 and 68% at 0.15 mS cm−1, 93 and 84% at 3.13 mS cm−1, 130 and 115% at 6.25 mS cm−1, and 154 and 120% at 9.38 mS cm−1 salinity for Sakha 8 and Giza 167, respectively. In general, mycorrhizal inoculation enhanced the ability of wheat to cope with saline conditions and using AM inoculants can help plants to thrive in degraded arid/semi-arid areas.

Influence of arbuscular mycorrhizae on biomass and root morphology of selected strawberry cultivars under salt stress

To investigate the influence of arbuscular mycorrhizal fungi (AMF) on biomass and root morphology, a greenhouse experiment was conducted using three elite strawberry ( Fragaria × ananassa Duch.) cultivars (‘Kent’, ‘Jewel’, and ‘Saint-Pierre’). They were subjected to three NaCl levels (0, 30, and 60 mmol/L) and were inoculated and noninoculated (control) with AMF Glomus irregulare . The presence of AMF significantly changed root morphology and increased root-length percentages of medium (0.5 mm < root diameter φ ≤ 1.5 mm) and coarse (φ > 1.5 mm) roots, shoot and root tissue biomass, root to shoot ratio (R/S ratio), and specific root length (SRL), regardless of cultivar and salinity. In contrast, salt alone changed root morphology and decreased shoot and root tissue biomass, R/S ratio, and SRL. The AMF colonization rates were reduced linearly and significantly with increasing salinity levels. Cultivars responded differently to AMF than to salt stress. ‘Saint-Pierre’ seemed to be the most tolerant cultivar to salinity, while ‘Kent’ was the most sensitive. Consequently, AMF symbiosis highly enhanced salt tolerance of strawberry plants, which confirmed the potential use of mycorrhizal biotechnology in sustainable horticulture in arid areas.

Influence of arbuscular mycorrhizae on yield, nutrients, organic solutes, and antioxidant enzymes of two wheat cultivars under salt stress

AbstractThe efficacy of arbuscular mycorrhizae (AM) on nutrients, organic solutes, and antioxidant enzymes of wheat under salt stress was investigated and related to root colonization and plant productivity. The mycorrhizal inoculation increased N, P, K, Ca, and Mg uptake, soluble sugars, free amino acids, and proline accumulation, as well as peroxidase and catalase activities under saline conditions as compared to nonmycorrhizal plants. On the other hand, Na concentration was lower in mycorrhizal than in nonmycorrhizal plants grown under saline conditions. Arbuscular mycorrhizae protected wheat against the detrimental effects of salinity and stimulated its productivity. Hence, mycorrhizal colonization can play a vital role in the mitigation of the adverse effects of salinity by improving the wheat osmotic adjustment response, enhancing its defense system, and alleviating oxidative damage to cells. Arbuscular mycorrhizae are able to alter plant physiology in a way that empowers the plant to grow more efficiently on salt‐affected lands.

Influence of arbuscular mycorrhizae on the root system of maize plants under salt stress

Salt stress has become a severe global problem, and salinity is one of the most important abiotic factors limiting plant growth and yield. It is known that arbuscular mycorrhizal (AM) fungi decrease plant yield losses under salinity. With the aim of determining whether AM inoculation would give an advantage to root development under salt stress, a greenhouse experiment was carried out with AM or without AM fungi. Maize plants were grown in a sand and soil mixture with 5 NaCl levels (0, 0.5, 1.0, 1.5, and 2.0 g/kg dry substrate) for 55 days, following 15 days of nonsaline pretreatment. At all salt levels, mycorrhizal plants had higher dry shoot and root mass, higher root activity, and lower root to shoot ratios than non-mycorrhizal plants. In salt-free soil, root length, root surface area, root volume, and number of root tips and forks were significantly larger in mycorrhizal plants than in non-mycorrhizal plants, whereas, under salt stress, average root diameter and root volume of mycorrhizal plants were larger than those of non-mycorrhizal plants. Regardless of the NaCl level, mycorrhizal plants had lower specific root length, lower percentage of root length in the 0-0.2 mm diameter class, and higher percentage of root length in both the 0.2-0.4 mm and 0.4-0.6 mm diameter classes, which suggests that the root system shows a significant shift towards a thicker root system when maize plants were inoculated with Glomus mosseae (Nicolson & Gerdemann). The results presented here indicate that the improvements in root activity and the coarse root system of mycorrhizal maize may help in alleviating salt stress on the plant.

Influence of arbuscular mycorrhizae on photosynthesis and water status of maize plants under salt stress

The influence of arbuscular mycorrhizal (AM) fungus Glomus mosseae on characteristics of the growth, water status, chlorophyll concentration, gas exchange, and chlorophyll fluorescence of maize plants under salt stress was studied in the greenhouse. Maize plants were grown in sand and soil mixture with five NaCl levels (0, 0.5, 1.0, 1.5, and 2.0 g/kg dry substrate) for 55 days, following 15 days of non-saline pretreatment. Under salt stress, mycorrhizal maize plants had higher dry weight of shoot and root, higher relative chlorophyll content, better water status (decreased water saturation deficit, increased water use efficiency, and relative water content), higher gas exchange capacity (increased photosynthetic rate, stomatal conductance and transpiration rate, and decreased intercellular CO(2) concentration), higher non-photochemistry efficiency [increased non-photochemical quenching values (NPQ)], and higher photochemistry efficiency [increased the maximum quantum yield in the dark-adapted state (Fv/Fm), the maximum quantum yield in the light-adapted sate (Fv'/Fm'), the actual quantum yield in the light-adapted steady state (phiPSII) and the photochemical quenching values (qP)], compared with non-mycorrhizal maize plants. In addition, AM symbiosis could trigger the regulation of the energy biturcation between photochemical and non-photochemical events reflected in the deexcitation rate constants (kN, kN', kP, and kP'). All the results show that G. mosseae alleviates the deleterious effect of salt stress on plant growth, through improving plant water status, chlorophyll concentration, and photosynthetic capacity, while the influence of AM symbiosis on photosynthetic capacity of maize plants can be indirectly affected by soil salinity and mycorrhizae-mediated enhancement of water status, but not by the mycorrhizae-mediated enhancement of chlorophyll concentration and plant biomass.

Influence of arbuscular mycorrhizae on soil P dynamics, corn P-nutrition and growth in a ridge-tilled commercial field

Ridge-tilled corn (Zea mays L.) could benefit from arbuscular mycorrhizal (AM) fungi. Under low soil disturbance, AM hyphal networks are preserved and can contribute to corn nutrition. A 2-yr study was conducted in the St. Lawrence Lowlands (Quebec, Canada) to test the effects of indigenous AM fungi on corn P nutrition, growth, and soil P in field cropped for 8 yr under ridge-tillage. Phosphorus treatments (0, 17, 35 kg P ha-1) were applied to AM-inhibited (AMI) (fungicide treated) and AM non-inhibited (AMNI) plots. Plant tissue and soil were sampled 22, 48 and 72 days after seeding (DAS). P dynamics was monitored in situ with anionic exchange membranes (PAEM) from seeding to the end of July. AMNI plants showed extensive AM colonization at all P rates. At 22 DAS, AMI plants had decreased growth in the absence of P inputs, while AMNI plants had higher dry mass (DM) and P uptake in unfertilized plots. The PAEM was lower in the AMNI unfertilized soils in 1998 and at all P rates in 1999, indicating an inverse relationship between P uptake and PAEM. At harvest, grain P content of AMNI plants was greater than that of AMI plants. In 1998, only AMI plants had decreased yield in the absence of P fertilization. In 1999, AMNI plants produced greater grain yield than AMI plants at all P rates. AM fungi improve the exploitation of soil P by corn thereby maintaining high yields while reducing crop reliance on P inputs in RT. Key words: Arbuscular mycorrhizae, ridge-tillage, soil P dynamics, corn, P nutrition

The influence of arbuscular mycorrhizae and light on Wisconsin (USA) sand savanna understories 2. Plant competition

Wisconsin (USA) oak savannas are endangered plant communities that have remarkably high plant species diversity. To investigate factors underlying this richness, we experimentally investigated the potentially interacting effects of light gradients and arbuscular mycorrhizal fungi (AMF) on plant competition in the greenhouse, using a fully randomized block design. We used four plant species, soil, and AMF from a remnant sand savanna, under two light and five AMF treatments. Plants were grown four per pot under two competition treatments (either one or four species per pot) for 20 weeks. Using ANOVA, we found that all species showed significant treatment effects on total and shoot biomass, primarily due to differences in competition and light, less to AMF. However, effects were the opposite of predictions. Putatively mycorrhizal plants showed neutral to negative responses to AMF, and a nonmycorrhizal species outcompeted AMF species in infected pots. We concluded that our experimental setup of small pots, sandy soil, and long growing period had induced parasitism by the AMF on susceptible hosts. This unexpected result is consistent with field data from the sand savanna, and may help explain how nonmycorrhizal plants can compete successfully with AMF species in established, species-rich communities.

The influence of arbuscular mycorrhizae and light on Wisconsin (USA) sand savanna understories 1

To explain the complex community composition found in Wisconsin (USA) oak savannas, we investigated potentially interacting effects of light gradients and arbuscular mycorrhizal fungi (AMF) on community composition in the greenhouse, using a fully randomized block experimental design. We used plant species, soil, and AMF from a remnant sand savanna in setting up the experiment, using two light and five AMF treatments. Eleven plant species were seeded into 80 microcosms, and they were grown together for 20 weeks. Plant numbers and biomass were measured, and Simpson's index was calculated for both. Data were analyzed using ANOVA and nonparametric ANOVA. We found significant light effects on biomass and on numbers of four species. There were no treatment effects on Simpson's index, and only Schizachyrium numbers showed a significant AMF effect. These findings are consistent with results from other studies of the sand savanna, and, collectively, these data suggest that plant community composition in this species-rich savanna is not strongly influenced by arbuscular mycorrhizae. This is a novel finding with important implications for understanding interactions between plant and AMF diversity in wild communities.

Influence of arbuscular mycorrhizae and a genetically modified strain of Sinorhizobium on growth, nitrate reductase activity and protein content in shoots and roots of Medicago sativa as affected by nitrogen concentrations

This study examined the effect of nitrogen fertilization (0, 4 or 8 mmol N added kg −1 soil) on plant growth, nitrate reductase activity (NR) (EC 1.6.6.1) and protein content in Medicago sativa. Plants were inoculated with two strains of Sinorhizobium meliloti [the wild type (WT) strain GR4 and its genetically modified (GM) derivative GR4(pCK3)]. In combination with these inoculations, the arbuscular mycorrhizal (AM) fungus, Glomus mosseae, was compared to a non-mycorrhizal control supplemented with phosphate. The effects of AM on plant growth were greatest when no nitrogen was added to the soil. Nitrogen fertilization reduced these effects according to the S. meliloti strain involved. Growth responses of mycorrhizal plants coinoculated with the GM strain were affected less negatively than those of mycorrhizal plants associated with the WT strain. These results were not related to differential colonization by AM. Increasing nitrogen concentrations reduced mycorrhizal infection and nodule formation was drastically inhibited in mycorrhizal plants by the addition of 4 and 8 mmol N kg −1 soil. Nevertheless, AM symbiosis increased nodule formation in the absence of N fertilization. In non-mycorrhizal plants, however, N fertilizer application did not significantly affect shoot and root growth or nodule formation. Although the P content was higher in P-fertilized, non-mycorrhizal plants than in mycorrhizal ones AM colonization significantly improved P use efficiency. The N content and use efficiency were also highest in mycorrhizal plants. However, the most relevant result regarding NR activity was the varying distributions in mycorrhizal plant shoots and roots as affected by the particular Sinorhizobium strain. The root portion was enhanced by the GM strain, while the WT strain increased the shoot portion. This change in the distribution pattern of root and shoot NR activities was unaffected by N concentrations in the soil. Protein content was substantially higher in mycorrhizal plants. The protein distribution pattern in shoot and root was also highly influenced by mycorrhizal colonization, which enhanced the root portion. Increased nitrogen supply lowered the protein content in both shoot and root tissues. This effect was greatest in AM colonized plants. These results suggest that high N fertilization levels are detrimental to mycorrhizal legume plants. However, under stress conditions (e.g. high N levels) mycorrhizal legumes nodulated by the GM Sinorhizobium strain displayed a physiological response which was better than those nodulated by the WT strain.

The influence of arbuscular mycorrhizae on the relationship between plant diversity and productivity

Ecological theory predicts a positive and asymptotic relationship between plant diversity and ecosystem productivity based on the ability of more diverse plant communities to use limiting resources more fully. This is supported by recent empirical evidence. Additionally, in natural ecosystems, plant productivity is often a function of the presence and composition of mycorrhizal associations. Yet, the effect of mycorrhizal fungi on the relationship between plant diversity and productivity has not been investigated. We predict that in the presence of AMF, productivity will saturate at lower levels of species richness because AMF increase the ability of plant species to utilize nutrient resources. In this study we manipulated old‐field plant species richness in the presence and absence of two species of AMF. We found that in the absence of AMF, the relationship between plant species richness and productivity is positive and linear. However, in the presence of AMF, the relationship is positive but asymptotic, even though the maximum plant biomass was significantly different between the two AMF treatments. This is consistent with the hypothesis that AMF increase the redundancy of plant species in the productivity of plant communities, and indicates that these symbionts must be considered in future investigations of plant biodiversity and ecosystem function.

Influence of Arbuscular Mycorrhizae on Heavy Metal (Pb and Cd) Uptake, Growth, and Chemical Composition ofVitis viniferaL. (cv. Razaki)

The influence of arbuscular mycorrhizae (AM) on plant growth, mineral uptake as well as on soluble solids and acid content of the Razaki grapevine variety grafted onto 110R, in experiments with different amounts of Pb and Cd heavy metals, was studied. Mycorrhizal and non-mycorrhizal plants were grown outdoors in pots containing soil, supplemented with 0, 50, 100, and 300 mg of Pb/kg or 0, 5, 10, and 50 mg of Cd/kg soil. After 3.5 years mycorrhizal colonization was varied from 26.3% to 58.7% in Pb treatments and from 44.7% to 53.3% in Cd treatments. In the same time spore population was varied from 126 to 266 spores per 100 g soil and from 186.7 to 264.0 spores in Pb and Cd treatments, respectively. Treatments receiving 300 mg Pb/kg and 50 mg Cd/kg in the soil showed a reduction in pruning weight compared with the control (0 mg Pb or Cd/kg). Mycorrhizal colonization significantly increased pruning weight in the control and the low Pb and Cd treatments (0, 50, and 100 mg/kg and 0, 5, and 10 mg/kg respectively). In elevated soil Pb supplements, the fruits had decreased total soluble solids and increased total acidity. With the changes to Cd level, berry soluble solids and acids were not affected. No significant differences of leaf N, K, Ca, Mg, and B nutrient concentrations were found between the heavy metal Pb and Cd treatments nor between mycorrhizal and non-mycorrhizal plants. The M/NM (mycorrhizal/non-mycorrhizal) ratio for N, K, Ca, and Mg concentrations varied from 1.0 to 1.2 and for B from 1.07 to 1,5. Leaf Mn, Zn, Fe, Cu, Pb, and Cd concentrations were lower in mycorrhizal plants compared to non-mycorrhizal plants and the M/NM ratios were less than 1.0. Berry Pb concentrations in the mycorrhizal plants were low (up to 20 mg/kg) in control and increased in Pb treatments (up to 463 mg/kg and 1315 mg/kg in M and NM plants, respectively). Appreciable Cd was detected only in the high Cd treatments, in which berry concentrations were 0.8 mg/kg and 2.5 mg/kg in the M and NM plants, respectively (50 mg of Cd/ kg).

Influence of arbuscular mycorrhizae on growth, leaf yield and phosphorus uptake in mulberry ( Morus alba L.) under rainfed, lateritic soil conditions

A field experiment was conducted to study the effect of arbuscular mycorrhizal association in mulberry saplings var. S1, together with two levels of phosphate, on growth characters, leaf yield and phosphorus uptake. The experiment was conducted in the period 1995 to 1997 at the Technical Service Centre, Kashipur, Purulia (West Bengal) under rainfed, lateritic soil conditions. Pooled data analysis revealed that there was no significant variation among the treatments in most of the characters except height of plant and leaf moisture percentage. Plant height in both treatments with Glomus mosseae was significantly less than in the others, while there was significant reduction in moisture percentage in G. fasciculatum with 25 kg of phosphate and G. mosseae with 10 kg of phosphate ha–1 year–1 than in other treatments. There was no significant season–treatment effect on any of the parameters. Considering all the quantitative and qualitative characters, G. fasciculatum together with the most economic dose of phosphate showed more promising performance than G. etunicatum and G. mosseae with different doses of phosphate fertilizer. At 80% of the recommended dose of inorganic phosphate fertilizer, Rs 534.0 were earned as net profit.

Influence of arbuscular mycorrhizae and Rhizobium on free polyamines and proline levels in water-stressed alfalfa

Summary The objective of this research was to study the effect of drought on polyamine and proline levels in alfalfa ( Medicago sativa L. cv. Aragon) plants inoculated with a mycorrhizal fungus and/or Rhizobium compared with non-inoculated ones. The four treatments were: a) plants inoculated with Glomus fasciculatum (Taxter sensu Gerd.) Gerdemann and Trappe and Rhizobium meliloti 102 F51 strain (MR), b) plants inoculated with Rhizobium only (R), c) plants inoculated with Glomus only (M), and d) non-inoculated plants (N). Plants were drought stressed during two cycles of moisture stress and recovery. Although proline concentrations increased and free polyamine (spermidine and spermine) contents decreased in leaves and roots of alfalfa under water stress, symbiotic R, M and MR plants maintained higher free polyamine concentrations than non-symbiotic N ones. Results suggest that symbiotic alfalfa plants are better adapted than non-symbiotic ones to cope with water deficit.

Influence of arbuscular mycorrhizae and phosphorus fertilization on growth, nodulation and N2 fixation (15N) inMedicago sativa at four salinity levels

The rose of an isolate of the arbuscular mycorrhizal (AM) fungusGlomus mosseae in the protection ofMedicago sativa (+Rhizobium meliloti) against salt stress induced by the addition of increasing levels of soluble salts was studied. The interactions between soluble P in soil (four levels), mycorrhizal inoculum and degree of salinity in relation to plant growth, nutrition and infective parameters were evaluated. Salt stress was induced by sequential irrigation with saline water having four concentrations of three salts (NaCl, CaCl2, and MgCl2).15N-labelled ammonium sulphate was added to provide a quantitative estimate of N2 fixation under moderate to high salinity levels. N and P concentration and nodule formation increased with the amount of plant-available P or mycorrhizal inoculum in the soil and generally declined as the salinity in the solution culture increased from a moderate to a high level. The mycorrhizal inoculation protected the plants from salt stress more efficiently than any amount of plant-available P in soil, particularly at the highest salinity level applied (43.5 dS m−1). Mycorrhizal inoculation matched the effect on dry matter and nutrition of the addition in the soil of 150 mg P kg−1. Nevertheless the highest saline solution assayed (43.5 dS m−1) affected more severely plants supplemented with phosphorus than those with the addition of mycorrhizal inoculum. Such a saline-depressing effect was 1.5 (biomass), 1.4 (N) and 1.5 (P) times higher in plants supplied with soluble phosphate than with AM inoculum. Mechanisms beyond those mediated by P must be involved in the AM-protectioe effect against salinity. The15N methodology used allowed the determination of N2 fixation as influenced by different P applications compared to mycorrhizal inoculation. A lack of correlation between nodule formation and function (N2 fixation) was evidenced in mycorrhizal-inoculated plants. In spite of the reduced activity per nodule in mycorrhizal-inoculated In spite of the reduced activity per nodule in mycorrhizal-inoculated plants, the N contents determined indicated the highest acquisition of N occurred in plants with the symbiotic status. Moreover, N and P uptake increased while Ca and Mg decreased in AM-inoculated plants. Thus P/Ca ratios and cation/anion balance in general were altered in mycorrhizal treatments. This study therefore confirms previous findings that AM-colonized plants have optional and alternative mechanisms available to satisfy their nutritive requirements and to maintain their physiological status in stress situations and in disturbed ecosystems.

Influence of arbuscular mycorrhizae and Rhizobium on nutrient content and water relations in drought stressed alfalfa

The objective of this research was to study the effect of drought on nutrient content and leaf water status in alfalfa (Medicago sativa L. cv Aragon) plants inoculated with a mycorrhizal fungus and/or Rhizobium compared with noninoculated ones. The four treatments were: a) plants inoculated with Glomus fasciculatum and Rhizobium meliloti 102 F51 strain, (MR); b) plants inoculated with R. meliloti only (R); c) plants with G. fasciculatum only (M); and d) noninoculated plants (N). Nonmycorrhizal plants were supplemented with phosphorus and nonnodulated ones with nitrogen to achieve similar size and nutrient content in all treatments. Plants were drought stressed using two cycles of moisture stress and recovery. The components of total leaf water potential (osmotic and pressure potentials at full turgor), percentage of apoplastic water volume and the bulk modulus of elasticity of leaf tissue were determined. Macronutrient (N, P, K, Ca, S and Mg) and micronutrient (Co, Mo, Zn, Mn, Cu, Na, Fe and B) content per plant were also measured. Leaves of N and R plants had decreased osmotic potentials and increased pressure potentials at full turgor, with no changes either in the bulk modulus of elasticity or the percentage of apoplastic water upon drought conditions. By contrast, M and MR leaves did not vary in osmotic and turgor potentials under drought stress but had increased apoplastic water volume and cell elasticity (lowering bulk modulus). Drought stress decreased nutrient content of leaves and roots of noninoculated plants. R plants showed a decrease in nutrient content of leaves but maintained some micronutrients in roots. Leaves of M plants were similar in content of nutrients to N plants. However, roots of M and MR plants had significantly lower nutrient content. Results indicate an enhancement of nutrient content in mycorrhizal alfalfa plants during drought that affected leaf water relations during drought stress.

Influence of arbuscular mycorrhizae and phosphorus fertilization on growth, nodulation and N 2 fixation ( 15 N) in Medicago sativa at four salinity levels

Influence of arbuscular mycorrhizae and phosphorus fertilization on growth, nodulation and N 2 fixation ( 15 N) in Medicago sativa at four salinity levels