The role of the external mycelial network of vesicular-arbuscular mycorrhizal fungi: a study of carbon transfer between plants interconnected by a common mycelium

The transfer of ¹⁴C from Lolium perenne (the donor) to Plantago lanceolata (the receiver), mediated by vesicular-arbuscular (VA) mycorrhizal fungi, was examined when the two species were grown together or separately. The VA mycorrhizal infection led to a significant increase, relative to that in uninfected plants, in the ¹⁴C transferred from donor to receiver plants, not only when the roots of the two plants were growing in intimate mixture, but also when they were separated by a root-free zone of 2.33 cm. The majority of isotope transfer between the two plant species was along the direct pathway via VA mycelium.     

A comparison of phosphorus and nitrogen transfer between plants of different phosphorus status

Summary This study has two objections: (1) to compare transfers of phosphorus (³²P) with nitrogen (¹⁵N) from undefoliated and defoliated mycorrhizal P-rich plants to an adjacent mycorrhizal plant and (2) to determine whether the improved nutrient status of a plant growing with a nutrient-rich plant is due primarily to movement of nutrients from roots of its nutrient-rich neighbor (= nutrient transfer) or to reduced nutrient uptake by its nutrient-rich neighbor (=shift in competition). Two plants of Plantago lanceolata were grown in a three-pot unit in which each of their root systems were split, with part in the central shared pot and part by themselves in an outside pot. There were three treatments: (1) no added P; (2) P added in the outer pot to only plant, termed the donor plant, since it might provide P to the companion plant, acting as a receiver; and (3) as in the previous treatment but the P-fertilized donor plant was also clipped. To encourage the formation of hyphal links between roots of the different plants, transfers were determined when root length densities were high (90 to 130 cm cm^-3 soil) and when 56 to 85% of the root length was infected with vesicular-arbuscular mycorrhizae. Phosphorus fertilization enhanced P but not N movement within donor plants. Regardless of treatment, N transfer from donor to receiver plants was an order of magnitude greater than P transfer and in amounts that could potentially affect plant nutrition in very infertile soils. Phosphorus transfer was very small in any of the treatments. Although P fertilization and clipping improved P status of receiver plants, P transfer was not indicated as the main reason for the improved nutrition. A shift in competition between donor and receiver plants was likely the major factor in the shift in nutrition of the receiver plants.     

Cycling of nutrients from dying roots to living plants,including the role of mycorrhizas

This paper presents information about the release of nitrogen and phosphorus from dying grass roots and the capture of phosphorus by other, living plants. We have paid particular attention to the part played by mycorrhizas in this phosphorus capture, and the possible importance of mycorrhizal links between dying and living roots.WhenLolium perenne plants were grown with ample nutrients and their roots then detached and buried in soil, about half the nitrogen and two-thirds of the phosphorus was lost in three weeks, but only one-fifth of the dry weight. The C:N and C:P ratios suggest that microbial growth in the roots would at first be C-limited but would become N- and P-limited within three weeks.Rapid transfer of³²P can occur from dying roots to those of a living plant if the two root systems are intermingled. The amount transferred was substantially increased in two species-combinations that are known to form mycorrhizal links between their root systems. In contrast, in a species-combination where only the living (receiver) plant could become mycorrhizal no significant increase of³²P transfer occurred. This evidence, although far from conclusive, suggests that mycorrhizal links between dying and living roots can contribute to nutrient cycling. This research indicates a major difference in nutrient cycling processes between perennial and annual crops.     

COMPARISON OF STAGES OF VESICULAR-ARBUSCULAR MYCORRHIZA FORMATION IN SUDANGRASS GROWN AT TWO LEVELS OF PHOSPHORUS NUTRITION

Several stages of the infection process by the vesicular-arbuscular mycorrhizal fungus, Glomus fasciculatus, were compared in roots of sudangrass (Sorghum vulgare) grow in phosphorus-deficient or phosphorus-amended soil. Germination of fungal spores in soil and morphology of the fungus within the root were not affected by phosphorus amendment. There were no significant differences between phosphorus treatments in the percent of root infected or total length of root infected by the fungus until 25 days after inoculation. Between 25 and 35 days after inoculation, the percent of root infected increased 5-fold, and the total length of infected root increased nearly 30-fold in the phosphorus-deficient plants. Neither percent nor total length of infected root increased significantly over the same time period in the phosphorus amended plants. The increase in mycorrhiza formation in the phosphorus-deficient plants was associated with an increase in the number of penetrations into the root by the fungus, but not with an increase in the size of individual infections. Proliferation of external hyphae was greater in phosphorus-deficient than phosphorus-amended plants 25 days after inoculation. These data suggest that phosphorus nutrition of the host does not affect prepenetration stages of mycorrhiza formation by G. fasciculatus. However, external hyphal growth following initial penetration of the host is reduced with phosphorus amendment. The subsequent ability of the fungus to form secondary penetrations is thus decreased, ultimately resulting in the overall decrease in mycorrhiza formation commonly observed in plants receiving high levels of phosphorus.     

Effect of phosphate and the arbuscular mycorrhizal fungus Glomus intraradices on disease severity of root rot of peas (Pisum sativum) caused by Aphanomyces euteiches

 The effects of inorganic phosphate levels and the presence of arbuscular mycorrhiza on disease severity of Aphanomyces euteiches in pea roots were studied. Disease severity on roots and epicotyl as well as the oospore number within infected root tissue were correlated with the phosphorus (P) level in the growth medium. The arbuscular mycorrhizal fungus Glomus intraradices increased P uptake and the P concentration in the plant but reduced disease development in peas. Polyacrylamide gel electrophoresis followed by densitometry of glucose-6-phosphate dehydrogenase specific to A.euteiches was used to measure the activity of the pathogen in roots. The enzyme activity increased with disease severity and disease incidence, except in plants supplemented with P at the highest level, where a peak in activity was seen 12 days after inoculation with the pathogen, followed by a decrease in activity. The epicotyl of mycorrhizal plants showed a reduction in disease severity although this part of the plants was not mycorrhizal. Thus, an induced systemic factor may be responsible for increased resistance in mycorrhizal plants. Accepted: 21 August 1998     

Plant nitrogen acquisition and interactions under elevated carbon dioxide: impact of endophytes and mycorrhizae

Both endophytic and mycorrhizal fungi interact with plants to form symbiosis in which the fungal partners rely on, and sometimes compete for, carbon (C) sources from their hosts. Changes in photosynthesis in host plants caused by atmospheric carbon dioxide (CO₂) enrichment may, therefore, influence those mutualistic interactions, potentially modifying plant nutrient acquisition and interactions with other coexisting plant species. However, few studies have so far examined the interactive controls of endophytes and mycorrhizae over plant responses to atmospheric CO₂ enrichment. Using Festuca arundinacea Schreb and Plantago lanceolata L. as model plants, we examined the effects of elevated CO₂ on mycorrhizae and endophyte (Neotyphodium coenophialum) and plant nitrogen (N) acquisition in two microcosm experiments, and determined whether and how mycorrhizae and endophytes mediate interactions between their host plant species. Endophyte‐free and endophyte‐infected F. arundinacea varieties, P. lanceolata L., and their combination with or without mycorrhizal inocula were grown under ambient (400 μmol mol⁻¹) and elevated CO₂ (ambient + 330 μmol mol⁻¹). A ¹⁵N isotope tracer was used to quantify the mycorrhiza‐mediated plant acquisition of N from soil. Elevated CO₂ stimulated the growth of P. lanceolata greater than F. arundinacea, increasing the shoot biomass ratio of P. lanceolata to F. arundinacea in all the mixtures. Elevated CO₂ also increased mycorrhizal root colonization of P. lanceolata, but had no impact on that of F. arundinacea. Mycorrhizae increased the shoot biomass ratio of P. lanceolata to F. arundinacea under elevated CO₂. In the absence of endophytes, both elevated CO₂ and mycorrhizae enhanced ¹⁵N and total N uptake of P. lanceolata but had either no or even negative effects on N acquisition of F. arundinacea, altering N distribution between these two species in the mixture. The presence of endophytes in F. arundinacea, however, reduced the CO₂ effect on N acquisition in P. lanceolata, although it did not affect growth responses of their host plants to elevated CO₂. These results suggest that mycorrhizal fungi and endophytes might interactively affect the responses of their host plants and their coexisting species to elevated CO₂.     

Response of 11 eucalyptus species to inoculation with three arbuscular mycorrhizal fungi

 Numerous publications have reported growth stimulation of Eucalyptus following ectomycorrhizal inoculation in nursery or field conditions. Although Eucalyptus species can also form arbuscular mycorrhiza, their dependency on this type of mycorrhiza is still debatable. This paper presents information on the effect of inoculation of arbuscular mycorrhizal fungi on eucalypt growth. Twenty weeks after mycorrhizal inoculation, Eucalyptus seedlings' stem dry weight could be increased up to 49% compared to non-inoculated control plants. Intensity of root colonization by a given fungus depended on the host species, but it was not related to a plant growth response. Leaf phosphorus concentration of non-inoculated Eucalyptus seedlings varied greatly between species. Increases in leaf phosphorus concentration following mycorrhizal infection were not necessarily associated with plant growth stimulation. The most mycorrhiza-dependent Eucalyptus species tended to be those having the highest leaf phosphorus concentration in the absence of a fungal symbiont. These mycorrhiza-dependent Eucalyptus species seem to have greater phosphorus requirements and consequently to rely more on the symbiotic association. Accepted: 1 September 1995     

Specificity of interplant cycling of phosphorus: The role of mycorrhizas

The cycling of nutrients from dying roots of one plant (the donor) to other intact plants (the receivers) was examined in a series of pot experiments. In each pot receiver plants formed either the same or a different type of mycorrhiza as the donor plant and was therefore respectively either capable or incapable of forming mycorrhizal hyphal links to the donor. There was a preferential transfer of ³²P from the dying roots of the vesicular-arbuscular mycorrhizal (VAM) Lolium perenne to VAM-infected trees Acer pseudoplatanus and Fraxinus excelsior compared to the ectomycorrhizal (ECM) Larix eurolepis, this despite an apparently greater competitive ability of L. eurolepis to obtain ³²P from the soil. Following the death of L. perenne roots there was also an increase in total P in the VAM tree receiver. These findings could not be explained by similarities in rooting distribution of the VAM-infected plants.In a similar study of the transfer of ³²P between heathland plants there was a preferential cycling of ³²P from one ericoid mycorrhizal Calluna vulgaris to another rather than to the VAM Molinia caerulea. In contrast, when ³²P was supplied directly to the soil, M. caerulea obtained significantly more ³²P than C. vulgaris. These results are discussed in relation to the potential role of interplant mycorrhizal links in the cycling of nutrients within partially closed cycles and the implications that this might have for species balance in plant communities.     

Mycorrhizal feedback is not associated with the outcome of competition in old‐field perennial plants

The symbiosis between plants and arbuscular mycorrhizal (AM) fungi is hypothesized to be an important contributor to plant–soil feedbacks, which can influence the outcome of inter‐specific competition. Mycorrhizal feedbacks can be conspecific, which affects individuals of the same species, or heterospecific, which affects individuals of a different species. When heterospecific feedbacks are more positive than conspecific feedbacks, heterospecific individuals are expected to outcompete conspecific individuals. To test this hypothesis, we quantified conspecific mycorrhizal feedback for Plantago lanceolata as a focal species, and heterospecific mycorrhizal feedbacks for 21 competitor old‐field species using mycorrhizae cultured with P. lanceolata. We quantified inter‐specific competition against the focal species by growing the 21 old‐field species with and without P. lanceolata in the presence of mycorrhizae cultured with P. lanceolata. Heterospecific and conspecific feedbacks were both positive, and average heterospecific feedbacks exceeded conspecific feedback by 75%. Competition suppressed P. lanceolata biomass by 14% and average competitor biomass was reduced by 44% in the presence of P. lanceolata, and these effects varied with competitor species identity. Contrary to predictions, the magnitude of heterospecific feedbacks did not predict the ability of competitor species to either suppress or resist suppression by P. lanceolata. Instead, the outcome of competition was significantly and positively correlated with intrinsic growth rate, measured as biomass of competitor species five weeks after germination in non‐inoculated conditions. Our findings suggest that species experiencing more positive mycorrhizal feedbacks than a competitor do not necessarily have a competitive advantage. Mycorrhizal mediated soil feedbacks may be less important than intrinsic differences in growth rate in determining competitive outcomes.     

Phosphorus and Nitrogen Regulate Arbuscular Mycorrhizal Symbiosis in Petunia hybrida

Phosphorus and nitrogen are essential nutrient elements that are needed by plants in large amounts. The arbuscular mycorrhizal symbiosis between plants and soil fungi improves phosphorus and nitrogen acquisition under limiting conditions. On the other hand, these nutrients influence root colonization by mycorrhizal fungi and symbiotic functioning. This represents a feedback mechanism that allows plants to control the fungal symbiont depending on nutrient requirements and supply. Elevated phosphorus supply has previously been shown to exert strong inhibition of arbuscular mycorrhizal development. Here, we address to what extent inhibition by phosphorus is influenced by other nutritional pathways in the interaction between Petunia hybrida and R. irregularis. We show that phosphorus and nitrogen are the major nutritional determinants of the interaction. Interestingly, the symbiosis-promoting effect of nitrogen starvation dominantly overruled the suppressive effect of high phosphorus nutrition onto arbuscular mycorrhiza, suggesting that plants promote the symbiosis as long as they are limited by one of the two major nutrients. Our results also show that in a given pair of symbiotic partners (Petunia hybrida and R. irregularis), the entire range from mutually symbiotic to parasitic can be observed depending on the nutritional conditions. Taken together, these results reveal complex nutritional feedback mechanisms in the control of root colonization by arbuscular mycorrhizal fungi.     

Alien vs. native plants in a Patagonian wetland: elemental ratios and ecosystem stoichiometric impacts

Wetlands are subject to invasion by exotic plant species, especially during the dry season when they resemble terrestrial systems; therefore, terrestrial plants could exploit this situation to colonize this environment. We analyzed P. anserina invading Patagonian wetlands in terms of elemental ratios that would modify wetland stoichiometry due to organic matter inputs. We studied the elemental relationship (carbon/nitrogen/phosphorus) of P. anserina in comparison with native emergent macrophytes (Eleocharis pachicarpa and Carex aematorrhyncha). These plant species are common and dominant in the wetland. Additionally, we analyzed the presence of mycorrhizal fungi in the roots and their proportion of root infection. Our study reveals that the invasive species presented nutrient (especially phosphorus) allocation in roots and differences in mycorrhizal infection, with a predominance of arbuscular mycorrhiza, compared with native species. During flooded periods with the decay of aerial parts, P. anserina stores phosphorus in the roots and releases dissolved organic matter of high molecular weight molecules, high color, and a high C-to-nutrient ratio in comparison with native macrophytes. These results show the strategy of an invasive terrestrial plant in temporary aquatic systems, and how the elemental relationships of the invasive plant can modify the stoichiometry of the environment.     

Mycorrhizal responses of barley cultivars differing in P efficiency

The purpose of this study was to investigate how barley cultivars which are different in dry matter yield at low phosphorus (P) supply (i.e. they differ in agronomic P efficiency) respond to mycorrhizal infection. In a preliminary experiment, six mycorrhizal fungi were tested for their ability to colonize barley (Hordeum vulgare L.) roots at a soil temperature of 15°C.Glomus etunicatum was the most effective species and was used in the main experiment. The main experiment was conducted under glasshouse conditions in which soil temperature was maintained at 15°C. Treatments consisted of a factorial arrangement of 8 barley cultivars, 2 mycorrhiza (inoculated and non-inoculated), and 3 rates of P (0, 10 and 20 mg kg^-1). P utilization efficiency (dry matter yield per unit of P taken up) and agronomic P efficiency among the barley cultivars was significantly negatively correlated with mycorrhizal responses. However, the response to mycorrhizal infection was positively correlated with response to P application. Poor correlation was observed between P concentration when neither mycorrhiza nor P were supplied and the percentage of root length infected. The extent of mycorrhizal infection among the barley cultivars in soil without P amendment varied from 8.6 to 28.6%. Significant interactions between cultivar and P addition, and between mycorrhiza and P addition were observed for shoot dry weight but not root dry weight.     

Influence of restoration on arbuscular mycorrhiza of Biscutella laevigata L. (Brassicaceae) and Plantago lanceolata L. (Plantaginaceae) from calamine spoil mounds

The arbuscular mycorrhizal status of two plant species, Biscutella laevigata L. and Plantago lanceolata L., was investigated on calamine spoil mounds in Boles^aw (southern Poland). Although B. laevigata is a member of the Brassicaceae, a family generally accepted as non-mycorrhizal, this species formed AM symbioses on both heavy metal-contaminated and non-contaminated sites. Besides vesicles and coils, arbuscules were also observed, especially in roots collected prior to seed maturity. Relative mycorrhizal root length and relative arbuscular richness were usually much higher in P. lanceolata than in B. laevigata but not absolute arbuscule richness. Roots of P. lanceolata showed higher colonisation than B. laevigata. Although roots were collected from plants in close proximity, no correlation in mycorrhizal parameters was found between the two species.     

Endophytic fungus Serendipita indica increased nutrition absorption and biomass accumulation in Cunninghamia lanceolata seedlings under low phosphate

Cunninghamia lanceolata is important forest tree species in southern China, and its successive plantations resulted in degradation of soil fertility in pure stands, causing decline in forest productivity. How to improve productivity in C. lanceolata pure stands is a tough task. Usage of mycorrhizal fungi might be a plausible access to the task. The objective is to study the possibility of the endophytic fungus Serendipita indica (named formerly as Piriformospora indica) in culture of C. lanceolata. Seeds were sowed in plastic pots with river sand. When seedlings had two true leaves, hyphae suspension solution of S. indica was added to near the roots of seedlings in each plastic pot. Such pots with seedlings were placed in a greenhouse and normal management was carried out for the seedlings. Symbiosis effects on root development, nutrition uptake and allocation, and biomass accumulation of C. lanceolata seedlings under low phosphate were investigated. The results showed that S. indica could symbiose with C. lanceolata. The symbiosis did not result in significant changes in root system architecture under low phosphate, but significantly increased nitrogen and phosphorus levels in leaves under low phosphate. Although the symbiosis did not significantly increased nitrogen allocation in leaves under low phosphate, it significantly increased phosphorus allocation in leaves. The interaction between S. indica and C. lanceolata resulted in increase in total biomass under low phosphate and changes in biomass allocation between shoots and roots. The results suggested that S. indica helps host plants to absorb more nutrients under low phosphate and to allocate more nitrogen and phosphate to leaves, promoting plant growth; the fungus might be used in pure stands of C. lanceolata because of its large-scaled axenic culture.     

Effects of soil fertility and mycorrhizal infection on pollen production and pollen grain size of Cucurbita pepo (Cucurbitaceae)

The effects of soil fertility (two levels of soil nitrogen and two levels of soil phosphorus) and mycorrhizal infection on pollen production and pollen grain size were studied in two cultivars of the common zucchini (Cucurbita pepo). Overall, soil fertility and mycorrhizal infection had significant effects on traits affecting the male function of plants (staminate flower production, pollen production per flower and pollen grain size). There were also differences between the cultivars for these male traits in all three experiments. In addition, pollen grain size decreased toward the end of the growing season. In the mycorrhiza experiment, both phosphate concentration per pollen grain and total phosphate content per anther were greater but not significantly greater in the mycorrhizal plants than in the non-mycorrhizal plants. A significant negative relationship between pollen production and pollen grain size was found in the mycorrhiza and soil phosphorus experiments, indicating that there was a trade-off between pollen production and pollen size. This study is the first to show that mycorrhizal infection has an effect on male function (pollen production and size) in addition to the well-documented effects on female function (fruit/seed production and size).     

Rapid cycling of nitrogen and phosphorus from dying roots of Lolium perenne

Summary Previous experiments, using ³²P pulse labelling, showed that when roots of Lolium perenne were detached from the shoot, a substantial proportion of the phosphorus in the roots could within a few weeks be released and be captured by another, living plant. This paper describes experiments designed to confirm and further investigate this rapid nutrient transfer. Roots from plants grown with ample N and P were detached and placed in litter bags in soil. They lost up to 60% of their initial N and up to 70% of their P in three weeks. Even when roots were grown with deficient P supply, resulting in C:P ratios of 300–400, they lost 20–30% of their initial P. Time-courses of ³²P loss from roots suspended in solution gave results which agreed with these figures. The initially rapid rate of ³²P loss had declined greatly within three weeks. In a pot experiment small L. perenne plants showed a marked increase in their N and P content during 30 days after a neighbouring large plant's shoot was removed, supporting rapid capture of nutrients lost from the detached roots. To investigate P loss from roots while attached to the shoot, L. perenne shoots were clipped every four days and ³²P loss from the roots measured. After the third clip the rate of loss increased, eventually to more than four times that from the control plants.     

The best for the guest: high Andean nurse cushions of Azorella madreporica enhance arbuscular mycorrhizal status in associated plant species

Positive interactions between cushion plant and associated plants species in the high Andes of central Chile should also include the effects of fungal root symbionts. We hypothesized that higher colonization by arbuscular mycorrhizal (AM) fungi exists in cushion-associated (nursling) plants compared with conspecific individuals growing on bare ground. We assessed the AM status of Andean plants at two sites at different altitudes (3,200 and 3,600 m a.s.l.) in 23 species, particularly in cushions of Azorella madreporica and five associated plants; additionally, AM fungal spores were retrieved from soil outside and beneath cushions. 18 of the 23 examined plant species presented diagnostic structures of arbuscular mycorrhiza; most of them were also colonized by dark-septate endophytes. Mycorrhization of A. madreporica cushions showed differences between both sites (68% and 32%, respectively). In the native species Hordeum comosum, Nastanthus agglomeratus, and Phacelia secunda associated to A. madreporica, mycorrhization was six times higher than in the same species growing dispersed on bare ground at 3,600 m a.s.l., but mycorrhiza development was less cushion dependent in the alien plants Cerastium arvense and Taraxacum officinale at both sites. The ratio of AM fungal spores beneath versus outside cushions was also 6:1. The common and abundant presence of AM in cushion communities at high altitudes emphasizes the importance of the fungal root symbionts in such situations where plant species benefit from the microclimatic conditions generated by the cushion and also from well-developed mycorrhizal networks.     

Membrane-mediated decrease in root exudation responsible for phorphorus inhibition of vesicular-arbuscular mycorrhiza formation

The mechanism responsible for phosphorus inhibition of vesicular-arbuscular mycorrhiza formation in sudangrass (Sorghum vulgare Pers.) was investigated in a phosphorus-deficient sandy soil (0.5 micrograms phosphorus per gram soil) amended with increasing levels of phosphorus as superphosphate (0, 28, 56, 228 micrograms per gram soil). The root phosphorus content of 4-week-old plants was correlated with the amount of phosphorus added to the soil. Root exudation of amino acids and reducing sugars was greater for plants grown in phosphorus-deficient soil than for those grown in the phosphorus-treated soils. The increase in exudation corresponded with changes in membrane permeability of phosphorus-deficient roots, as measured by K⁺ (⁸⁶Rb) efflux, rather than with changes in root content of reducing sugars and amino acids. The roots of phosphorus-deficient plants inoculated at 4 weeks with Glomus fasciculatus were 88% infected after 9 weeks as compared to less than 25% infection in phosphorus-sufficient roots; these differences were correlated with root exudation at the time of inoculation. For plants grown in phosphorus-deficient soil, infection by vesicular-arbuscular mycorrhizae increased root phosphorus which resulted in a decrease in root membrane permeability and exudation compared to nonmycorrhizal plants. It is proposed that, under low phosphorus nutrition, increased root membrane permeability leads to net loss of metabolites at sufficient levels to sustain the germination and growth of the mycorrhizal fungus during pre- and postinfection. Subsequently, mycorrhizal infection leads to improvement of root phosphorus nutrition and a reduction in membrane-mediated loss of root metabolites.