Arbuscular mycorrhizal fungi from three genera induce two-phase plant growth responses on a high P-fixing soil

Plant growth enhancing effects of arbuscular mycorrhizal (AM) fungi are suitably quantified by comparisons of mycorrhizal and non-mycorrhizal plant growth responses to added phosphorus (P). The ratio between the amounts of added P required for the same yield of mycorrhizal and non-mycorrhizal plants is termed the relative effectiveness of the mycorrhiza. Variation in this relative effectiveness was examined for subterranean clover grown on a high P-fixing soil. Plants were either left non-mycorrhizal or inoculated with one of three AM fungal species with well-characterised differences in external hyphal spread. With no P added, plants from all treatments produced <10% of their maximum growth achieved at non-limiting P supply. The growth response of non-mycorrhizal plants was markedly sigmoid. Mycorrhizal growth responses were not sigmoid but their shape was two-phased. The first phase was an asymptotic approach to 25–30% of maximum growth, followed by a second asymptotic rise to maximum growth. Growth effects of Glomus invermaium and Acaulospora laevis were quite similar. Plants in these treatments produced up to four times greater shoot dry biomass than non-mycorrhizal plants. Scutellospora calospora was less effective. The relative effectiveness of AM fungi varied with the level of P application. This is expected to apply to all soils on which a sigmoid response is obtained for growth of non-mycorrhizal plants. In a simple approximation the relative effectiveness was calculated to range from 1.46 to 15.57. Shoot P contents were increased by up to 25 times by A. laevis, significantly more than by the other two fungi. The further mycelial spread of this fungus is thought to have contributed to its relatively greater effect on plant P content.     

Effect of vesicular arbuscular mycorrhiza and soluble phosphate onAbelmoscus esculentus (L.) Moench

Summary Effect of VA mycorrhiza and soluble phosphorus onAbelmoscus esculentus (L.) Moench was studied in a phosphorus deficient sandy loam soil with pH 5.5. The mycorrhizal infection and spore production were reduced by an increase of added soluble phosphorus. Root, shoot and total plant dry weight were significantly greater in mycorrhizal plants than in non-mycorrhizal controls, at all levels of added soluble P. Mycorrhizal dependency was found to decrease with increase in added soluble P. Depression of growth, as compared with growth at 100% was noticed in mycorrhizal plants when 200% of the recommended P was added.     

Influence of vesicular-arbuscular mycorrhizal infection and P addition on growth and P nutrition of Anthyllis cytisoides L. and Brachypodium retusum (Pers.) Beauv.

The effect of P applications and mycorrhizal inoculation on the growth and P nutrition of Anthyllis cytisoides L. (Fabaceae) and Brachypodium retusum (Pers.) Beauv. (Poaceae) was studied. Both plants are widely distributed and well adapted to semi-arid habitats in southern Spain. In all treatments, even with high P doses, mycorrhizal plants showed a higher concentration of phosphorus in their tissues than non-mycorrhizal plants. Mycorrhizal inoculation enhanced the growth of the plants when no P was applied. At high P addition, non-mycorrhizal plants showed higher growth than mycorrhizal plants. The response of each plant type to P application was somewhat different.     

Vesicular arbuscular mycorrhizal mediation of grass response to acidic and heavy metal depositions

Summary The perennial bunchgrassEhrharta calycina was grown with and without V.A.M. fungal infection (Glomus fasciculatum) in a sandy loam exposed to a range of acidic and heavy metal depositions. Heavy metals (Cu, Ni, Pb, Zn, Fe, and Co) were applied in simulated rain (pH 3.0, 4.0, and 5.6) at deposition rates approximating those observed to result from smelter efluents. Metal concentrations in the roots and shoots of mycorrhizal plants were greater than those of non-mycorrhizal plants. Mycorrhizal enhancement of plant metal uptake increased with greater acidity and higher heavy metal content of treatment. The growth of mycorrhizal plants was reduced compared to non-mycorrhizal plants when metal depostion was combined with simulated acid rain. We propose that mycorrhizal enhancement of heavy metal uptake caused reduced growth in plants exposed to acidic and heavy metal depositions.     

Mycorrhizal inoculant alleviates salt stress in<Emphasis Type="Italic"> Sesbania aegyptiaca</Emphasis> and<Emphasis Type="Italic"> Sesbania grandiflora</Emphasis> under field conditions: evidence for reduced sodium and improved magnesium uptake

A field experiment was conducted to examine the effect of the arbuscular mycorrhizal fungus Glomus macrocarpum and salinity on growth of Sesbania aegyptiaca and S. grandiflora. In the salt-stressed soil, mycorrhizal root colonisation and sporulation was significantly higher in AM-inoculated than in uninoculated plants. Mycorrhizal seedlings had significantly higher root and shoot dry biomass production than non-mycorrhizal seedlings grown in saline soil. The content of chlorophyll was greater in the leaves of mycorrhiza-inoculated as compared to uninoculated seedlings. The number of nodules was significantly higher in mycorrhizal than non-mycorrhizal plants. Mycorrhizal seedling tissue had significantly increased concentrations of P, N and Mg but lower Na concentration than non-mycorrhizal seedlings. Under salinity stress conditions both Sesbania sp. showed a high degree of dependence on mycorrhizae, increasing with the age of the plants. The reduction in Na uptake together with a concomitant increase in P, N and Mg absorption and high chlorophyll content in mycorrhizal plants may be important salt-alleviating mechanisms for plants growing in saline soil.     

Density-dependent response to mycorrhizal infection in Abutilon theophrasti Medic

Summary One purpose of this study was to determine whether an increase in plant density would result in a decrease in response to mycorrhizal infection (particularly as measured by phosphorus content). Increases in plant density generally result in increases in root density in the volume of soil occupied by the plants. Root density, in turn, largely determines phosphorus uptake. If mycorrhizal plants had significantly higher effective root densities than non-mycorrhizal plants due to the fungal hyphae and thus were more thorough in exploiting a given volume of soil for phosphorus, then a given increase in root density might result in a greater proportional increase in phosphorus uptake for non-mycorrhizal plants than for mycorrhizal plants. Two experiments were performed in which mycorrhizal infection and available soil volume per plant were manipulated; one in which the number of plants within a given pot size was varied (experiment 1), and another in which single plants were grown in pots of differing volume (experiment 2). The two experiments yielded similar results but for apparently different reasons. In the first experiment, for a given increase in root density, non-mycorrhizal plants had a greater proportional increase in phosphorus uptake than mycorrhizal plants. Thus, as predicted, response to mycorrhizal infection was greatest at the lowest planting density (highest available soil volume per plant, lowest root density). In experiment 2, response to infection was also greatest at the highest available soil volume per plant (largest pot), but pot size did not influence root density. These results show that the benefit from mycorrhizal infection may be partly determined by root density and they suggest that plants either occurring in patches of contrasting root density in a given community, or occurring in different communities with inherently different root densities may differ in their reliance upon mycorrhizal fungi for phosphorus uptake.     

Effects of elevated nickel and cadmium concentrations on growth and nutrient uptake of mycorrhizal and non-mycorrhizal Pinus sylvestris seedlings

The effects of Ni and Cd on growth and nutrient uptake of mycorrhizal and non-mycorrhizal Pinus sylvestris L. seedlings were investigated in a pot experiment. Seedlings were either inoculated with Laccaria bicolor (Maire) Orton or left uninoculated before being planted in pots containing a mixture of sandy soil from the B-horizon of a coniferous forest, small stones and pure quartz sand. The pots were supplied with small amounts of a balanced nutrient solution every 24 h using peristaltic pumps. Nickel or Cd were added as chlorides to the nutrient solution at levels of 85 M Ni (Ni 1), 170 M Ni (Ni 2), or 8.9 M Cd. Mycorrhizal colonisation of the roots was nearly 100% in the mycorrhizal treatments. The mycorrhizal seedlings grew significantly better than the non-mycorrhizal ones. The weight of mycorrhizal seedlings in the Ni 2 treatment was 29% lower than that of the mycorrhizal controls, but still 34% greater than that of the non-mycorrhizal seedlings not exposed to metals. There was an overall, statistically significant, negative effect of metals on plant yield. Mycorrhizal plants had lower root:shoot (R:S) ratios than non-mycorrhizal plants and the R:S ratio was increased by metal exposure, particularly in the non-mycorrhizal seedlings. Plant concentrations of Cd or Ni were not affected by mycorrhizal colonisation, but total uptake of Cd and Ni was higher in bigger mycorrhizal seedlings. Nickel decreased P concentration in all seedlings and Cd decreased P concentration in the non-mycorrhizal seedlings. Generally, the mycorrhizal seedlings grew better than non-mycorrhizal ones and had better P, K, Mg and S status. Root growth was not significantly affected by the metal treatments. The reduction in mean shoot growth of non-mycorrhizal plants, relative to the metal-free control, appeared higher than in mycorrhizal plants but was not statistically significant due to high variation in the non-mycorrhizal plants not exposed to metals. The main mycorrhizal effect was thus increased nutrient uptake and growth of the seedlings.     

Increased drought tolerance of mycorrhizal onion plants caused by improved phosphorus nutrition

Onion plants (Allium cepa L, cv. Downing Yellow Globe) grown in pots and infected by the mycorrhizal fungusGlomus etunicatus Becker and Gerdemann were more drought tolerant than were non-mycorrhizal individials when exposed to several periods of soil water stress separated by periods of high water supply, as shown by greater fresh and dry weights and higher tissue phosphorus levels in the mycorrhizal plants. The tissues of stressed, non-mycorrhizal plants were deficient in P, despite the fact that only non-mycorrhizal plants were fertilized with high levels of P (26 mg P per 440 g soil). Differences in plant water relations (leaf water potentials or transpiration rates) and changes in soil P levels which may have affected plant growth were investigated, and discounted as factors important for the results. The P nutrition of plants has been implicated in the ability of plants to tolerate drought and it was concluded that the ability of the mycorrhizal fungus to maintain adequate P nutrition in the onions during soil water stress was a major factor in the improved drought tolerance. Infection of the root by the fungus was found not to be affected by water stress or P fertilization but fungal reproduction, as determined by spore numbers in the soil, was decreased by water stress and by P fertilization.Michigan Agricultural Experiment Station Article No. 10050     

Density dependent interactions between VA mycorrhizal fungi and even-aged seedlings of two perennial Fabaceae species

Summary The interaction of density and mycorrhizal effects on the growth, mineral nutrition and size distribution of seedlings of two perennial members of the Fabaceae was investigated in pot culture. Seedlings of Otholobium hirtum and Aspalathus linearis were grown at densities of 1, 4, 8 and 16 plants per 13-cm pot with or without vesicular-arbuscular (VA) mycorrhizal inoculum for 120 days. Plant mass, relative growth rates, height and leaf number all decreased with increasing plant density. This was ascribed to the decreasing availability of phosphorus per plant as density increased. O. hirtum was highly dependent on mycorrhizas for P uptake but both mycorrhizal and non-mycorrhizal A. linearis seedlings were able to extract soil P with equal ease. Plant size distribution as measured by the coefficient of variation (CV) of shoot mass was greater at higher densities. CVs of mycorrhizal O. hirtum plants were higher than those of non-mycorrhizal plants. CVs of the facultatively mycorrhizal A. linearis were similar for both mycorrhizal and non-mycorrhizal plants. Higher CVs are attributed to resource preemption by larger individuals. Individuals in populations with high CVs will probably survive stress which would result in the extinction of populations with low CVs. Mass of mycorrhizal plants of both species decreased more rapidly with increasing density than did non-mycorrhizal plant mass. It is concluded that the cost of being mycorrhizal increases as plant density increases, while the benefit decreases. The results suggest that mycorrhizas will influence density-dependent population processes of faculative and obligate mycorrhizal species.     

Mycorrhizal infection of wild oats: maternal effects on offspring growth and reproduction

Summary The objective of this study was to determine whether infection of Avena fatua L. plants by the mycorrhizal fungus Glomus intraradices Schenck & Smith could influence the vigor of the offspring generation. Two experiments demonstrated that mycorrhizal infection of the maternal generation had slight but persistent positive effects on offspring leaf expansion in the early stages of growth. In two other experiments, mycorrhizal infection of mother plants had several long lasting effects on their offspring. Offspring produced by mycorrhizal mother plants had greater leaf areas, shoot and root nutrient contents and root:shoot ratios compared to those produced by non-mycorrhizal mother plants. Moreover, mycorrhizal infection of mother plants significantly reduced the weight of individual seeds produced by offspring plants while it increased the P concentrations of the seeds and the number of seeds per spikelet produced by offspring plants. The effects of mycorrhizal infections of maternal plants on the vigor and performance of offspring plants were associated with higher seed phosphorus contents but generally lighter seeds. The results suggest that mycorrhizal infection may influence plant fitness by increasing offspring vigor and offspring reproductive success in addition to previously reported increases in maternal fecundity.     

Arbuscular mycorrhiza reduces susceptibility of tomato to Alternaria solani

Mycorrhiza frequently leads to the control of root pathogens, but appears to have the opposite effect on leaf pathogens. In this study, we studied mycorrhizal effects on the development of early blight in tomato (Solanum lycopersicum) caused by the necrotrophic fungus Alternaria solani. Alternaria-induced necrosis and chlorosis of all leaves were studied in mycorrhizal and non-mycorrhizal plants over time course and at different soil P levels. Mycorrhizal tomato plants had significantly less A. solani symptoms than non-mycorrhizal plants, but neither plant growth nor phosphate uptake was enhanced by mycorrhizas. An increased P supply had no effect on disease severity in non-mycorrhizal plants, but led to a higher disease severity in mycorrhizal plants. This was parallel to a P-supply-induced reduction in mycorrhiza formation. The protective effect of mycorrhizas towards development of A. solani has some parallels to induced systemic resistance, mediated by rhizobacteria: both biocontrol agents are root-associated organisms and both are effective against necrotrophic pathogens. The possible mechanisms involved are discussed.     

Mycorrhizal symbiosis increases growth,reproduction and recruitment of Abutilon theophrasti Medic. in the field

We examined in the field the effect of the vesicular-arbuscular (VA) mycorhizal symbiosis on the reproductive success of Abutilon theophrasti Medic., an early successional annual member of the Malvaceae. Mycorrhizal infection greatly enhanced vegetative growth, and flower, fruit and seed production, resulting in significantly greater recruitment the following year. In addition, the seeds produced by mycorrhizal plants were significantly larger and contained significantly more phosphorus than seeds from non-mycorrhizal plants, an effect which may improve offspring vigor. Infection by mycorrhizal fungi may thus contribute to the overall fitness of a host plant and strongly influence long-term plant population dynamics.     

Seed phosphorus (P) content affects growth,and P uptake of wheat plants and their association with arbuscular mycorrhizal (AM) fungi

Two experiments were carried out in a growth chamber and a naturally lit glasshouse to investigate the influence of seed phosphorus (P) reserves on growth and P uptake by wheat plants (Triticum aestivum cv Krichauff), and their association with arbuscular mycorrhizal (AM) fungi. Increased seed P reserves improved plant growth at a range of P supply up to over 100 mg P kg^–1 soil. Plants grown from seeds with high P reserves tended to accumulate more P from soil, which was mainly attributed to better root system development. Mycorrhizal colonisation did not significantly affect P uptake of plants grown with low irradiance (in growth chamber). However, in the naturally lit glasshouse, mycorrhizal plants had significantly higher P concentrations than non-mycorrhizal plants. Furthermore, mycorrhizal plants grown from seeds low in P accumulated similar amounts of P compared with those grown from seeds with high P, indicating that mycorrhizal colonisation may overcome the disadvantage of having low seed P reserves in the field.     

Regulation of reproduction in wild and cultivatedLycopersicon esculentum Mill. by vesicular-arbuscular mycorrhizal infection

Summary We examined how mycorrhizal infection byGlomus etunicatum Becker and Gerd. affected flowering phenology and components of reproduction in eight wild accessions and two cultivars ofLycopersicon esculentum Mill. We did this by performing a detailed demographic study of flower, fruit and seed production. Mycorrhizal infection had variable effects on the ten accessions. Infection significantly decreased the time taken to initiate flowering in some accessions. In addition, infection increased flowering duration in some accessions. In many accessions, infection significantly increased seed production, primarily by increasing the number of inflorescences and infructescences. In some accessions, mycorrhizal infection also increased the proportion of flowers that produced mature fruits or the number of seeds per fruit. Among accessions, shoot phosphorus content was correlated with seed productivity for both mycorrhizal and non-mycorrhizal plants. However, non-mycorrhizal plants produced more seed biomass per mg of shoot phosphorus than mycorrhizal plants.     

Influence of arbuscular mycorrhizal fungi and kinetin on the response of mungbean plants to irrigation with seawater

Increasing use of saline water in irrigation can markedly change the physical and chemical properties of soil. An experiment was carried out to investigate the interaction between the mycorrhizal fungus Glomus clarum, isolated from a saline soil, and kinetin on the growth and physiology of mungbean plants irrigated with different dilutions of seawater (0, 10, 20, and 30%). The growth, chlorophyll concentration and sugar content of mycorrhizal plants was greater than that of non-mycorrhizal plants under all conditions (with or without seawater). The dry weight of both mycorrhizal and non-mycorrhizal mungbean plants irrigated with 10% seawater was significantly increased by treatment with kinetin. The mycorrhizal symbiosis increased root:shoot dry weight ratio, concentrations of N, P, K, Ca and Mg, plant height, protein content, nitrogen or phosphorus-use efficiencies, and root nitrogenase, acid or alkaline phosphatase activities of seawater-irrigated mungbean plants, with little or no effect of kinetin. Kinetin treatment generally decreased chlorophyll concentration and sugar content in mycorrhizal plants as well as Na/N, Na/P Na/K, Na/Ca and Na/Mg ratios. Root colonization by G. clarum was increased by irrigation with seawater, and kinetin had no consistent effect on fungal development in roots. This study provides evidence that arbuscular mycorrhiza can be much more effective than kinetin applications in protecting mungbean plants against the detrimental effects of salt water.     

Arbuscular mycorrhizal fungi can contribute to maintain antioxidant and carbon metabolism in nodules of Anthyllis cytisoides L. subjected to drought

The symbiosis legume-arbuscular mycorrhizal fungi-nitrogen fixing bacteria is of relevant interest in Mediterranean regions where Anthyllis cytisoides L. grows. In these areas, nitrogen is one of the nutrients that most limits plant growth. In addition, the long periods of water deficit decrease the diffusion rate of phosphorus and, consequently, also decrease the biological nitrogen fixation. It is well known that mycorrhizal fungi can improve phosphorus uptake and, recently, some authors have found that antioxidant activities in mycorrhizal plants can delay drought-induced nodule senescence. The objective of our work was to evaluate weather mycorrhizal fungi could preserve the nodule metabolism in A. cytisoides subjected to drought. Results showed that a low soil water content associated with an enhancement of soil compaction accelerated the senescence of nodules in both non-mycorrhizal and mycorrhizal plants. However, while total soluble protein, leghaemoglobin (Lb) content, as well as carbon and antioxidant metabolism significantly decreased in nodules from non-mycorrhizal A. cytisoides subjected to drought, nodules from stressed mycorrhizal plants maintained Lb levels, showed greater rates of carbon metabolism, and exhibited higher enzymatic activities related to the removal of reactive oxygen species. In addition to the greater activity of antioxidant enzymes, other mechanisms related or unrelated to enhanced nodule water status could also be implied in the better nodule functioning observed in mycorrhizal plants under stressful conditions.     

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.     

丛枝菌根真菌对枳根净离子流及锌污染下枳苗矿质营养的影响

盆栽实验研究了不同施Zn水平(0、300 mg/kg和600 mg/kg)下,接种丛枝菌根真菌Glomus intraradices对枳苗生长、Zn、Cu、P、K、Ca、Mg分布的影响,并采用非损伤微测技术测定分析了菌根化与非菌根化枳根净Ca²⁺、H⁺、NO₃^-离子流动态。结果表明:(1)在不同施Zn水平下,接种菌根真菌显著提高了枳苗地上部及根部鲜重;随着施Zn水平的提高,菌根侵染率呈降低趋势,枳苗地上部与根部Zn含量呈增加趋势,且接种株根部Zn含量显著高于未接种株。(2)接种株未施Zn处理的地上部Cu、P、K、Mg和根部Cu含量、施600 mg/kg Zn处理的根部Cu及施300 mg/kg Zn处理的根部P含量均显著高于对照,而菌根真菌侵染对枳苗Ca含量并无显著性影响。(3)接种株未施Zn处理的根部距根尖端0 μm和600 μm处净Ca²⁺流出速率、600 μm处净H⁺流入速率、2400 μm处净NO₃^-流入速率均显著高于未接种株。     

Interaction of vesicular-arbuscular mycorrhizal fungi and root knot nematode on cultivars of tomato and white clover susceptible to Meloidogyne hapla

The interactive effects of vesicular-arbuscular mycorrhizal (VAM) fungi and root-knot nematode (Meloidogyne hapla) were studied on nematode-susceptible cultivars of tomato (cv. Scoresby) and white clover (cv. Huia) at four levels of applied phosphate. The relative merits of simultaneous inoculation with mycorrhizal fungi and nematodes and of inoculation with mycorrhizal fungi prior to nematode inoculation were evaluated. Mycorrhizal plants were more resistant than non-mycorrhizal plants to root-knot nematode at all phosphate levels and growth benefits were generally greater in plants preinfected with mycorrhizal fungi. Nematode numbers increased with increasing levels of applied phosphate. In mycorrhizal root systems, nematode numbers increased in the lower phosphate soils; at higher phosphate levels nematode numbers were either unaffected or reduced. The numbers of juveniles and adults per gram of root were always lower in mycorrhizal treatments. Mycorrhizal root length remained unaffected by nematode inoculation. Mycorrhizal inoculation thus increased the plants' resistance to infection by M. hapla. This was probably due to some alteration in the physiology of the root system but was not entirely a result of better host nutrition and improved phosphorus uptake by mycorrhizal plants.     

Improved tolerance of maize plants to salt stress by arbuscular mycorrhiza is related to higher accumulation of soluble sugars in roots

The effect of colonization with the arbuscular mycorrhizal (AM) fungus Glomus mosseae (Nicol. & Gerd.) Gerdemann & Trappe on the growth and physiology of NaCl-stressed maize plants ( Zea mays L. cv. Yedan 13) was examined in the greenhouse. Maize plants were grown in sand with 0 or 100 mM NaCl and at two phosphorus (P) (0.05 and 0.1 mM) levels for 34 days, following 34 days of non-saline pre-treatment. Mycorrhizal plants maintained higher root and shoot dry weights. Concentrations of chlorophyll, P and soluble sugars were higher than in non-mycorrhizal plants under given NaCl and P levels. Sodium concentration in roots or shoots was similar in mycorrhizal and non-mycorrhizal plants. Mycorrhizal plants had higher electrolyte concentrations in roots and lower electrolyte leakage from roots than non-mycorrhizal plants under given NaCl and P levels. Although plants in the low P plus AM fungus treatment and those with high P minus AM fungus had similar P concentrations, the mycorrhizal plants still had higher dry weights, soluble sugars and electrolyte concentrations in roots. Similar relationships were observed regardless of the presence or absence of salt stress. Higher soluble sugars and electrolyte concentrations in mycorrhizal plants suggested a higher osmoregulating capacity of these plants. Alleviation of salt stress of a host plant by AM colonization appears not to be a specific effect. Furthermore, higher requirement for carbohydrates by AM fungi induces higher soluble sugar accumulation in host root tissues, which is independent of improvement in plant P status and enhances resistance to salt-induced osmotic stress in the mycorrhizal plant.