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Mycorrhizae: Implications for Enviro nmental Remediation and Resource Conservation1 ____________________________________________________________________________________________________________________ J. Sharma, A.V. Ogram, and A. Al-Agely2 Fungi are common in aquatic and terrestrial envir onments, absorb the nutrients they need, and occur as either free-livin g or in symbiotic forms. In terr estrial environments, fungi are of fundamental importance as decomposers, plant pathogens, symbionts, and in biogeochemical cycles. In soils, fungi can comprise the largest pool of biomass. To date, there are an estimated 1.5 million species of fungi on the planet, of wh ich fewer than 80,000 have been identified. Mycorrhizae (singular "mycorrhiza") are symbiotic rela tionships between plant roots and one or more fungi. The Greek words mycos, meaning "fungus," and rhiza, meaning "root," were combined to classify this relationship between organisms belonging to two different kingdoms. The mycorrhizal condition is a "norm" for most pl ants because up to 80% of the flowering plants (angiosperms) and up to 95% of all plants form mycorrhizal relationships. Photosynthetic plants support the fungi by providing fixed carbon (up to 20% of the photosynthate may be allocated to roots to support mycorrhizae) and nutrients; the fungi in return provide the main plant-growthlimiting nutrients, nitrogen and phosphorus. Myco rrhizal non-photosynthetic plants rely on the fungal partners for carbon in addition to the nutr ients. It is thought that the non-photosynthetic plants may be parasitizing the fungi because degr adation of fungal hyphae insi de the root cells is documented. Mycorrhizae are broadly classified into two f unctional categories: (1) ectomycorrhizae; and (2) endomycorrhizae. Ectomycorrhizal relationships involve col onization of space between the cortical cells of roots and formation of a fungal sheath on the out side of the root. Endomycorrhizal relationships involve colonization of sp ace within the cortical root cells. In addition, the mycelium (a network of fungal hyphae) extends into the soil in both cases, thereby vastly extending the area available for the abso rption of water and nutrient elements. The fungal partner(s) in mycorrhizae ma y account between 80% and 100% of phosphorus (P) taken up by a plant. There is now increasing evidence fo r nitrate and ammonium uptake by plants via mycorrhizal relationships. ENH-1086 1. This document is ENH1086, one of a series of the Environmental Horticulture Department, Florid a Cooperative Extension Service, Institute of Food and Agricultural Sciences, University of Florida. Original publication date November 2007. Visit the EDIS Web Site at http://edis.ifas.ufl.edu. 2. Dr. Jyotsna Sharma, Assistant Professor, Environmental Horticulture, University of Florida, Quincy, Florida, 32351. E-mail: email@example.com. Dr. Andrew V. Ogram, Pr ofessor, Soil and Water Science, Universi ty of Florida, Gainesville, Florida 32611. Dr Abid AlAgely, Senior Biologist, Soil and Water Science, University of Florida, Ga inesville, Florida 32611. The use of trade names in this publication is solely for the pu rpose of providing specific information. UF/IFAS does not guaran tee or warranty the products named, and references to them in this pub lication does not signify our approval to the exclusion of other products of suitable composition. The Institute of Food and Agricu ltural Sciences (IFAS) is an Equal Opportunity Institution authorized to provide research, educational information and other services only to individuals and institutions that f unction with non-discrimination with respect to race, creed, colo r, religion, age, disability, sex, sexual orientation, mar ital status, national origin, political opinions or affiliations. For more information on obtaining other extension publications, contact your county Cooperative Extension service. U.S. Depa rtment of Agriculture, Cooperative Extensi on Service, University of Florida, IFAS, Florida A. & M. University Coopera tive Extension Program, and Boards of County Commissioners Coope rating. Larry Arrington, Dean.
Beyond the two functional categories (ectoand endomycorrhizae), mycorrhizae are further classified into different groups based on the interacting organi sms and mycorrhizal morphology. For example, ericaceous plants (e.g., Calluna Erica Vaccinium and Rhododendron) form what are termed "ericoid mycorrhizae." Similarly, se veral other groups such as arbutoid mycorrhizae, monotropoid mycorrhizae, orchid my corrhizae, etc., are classified based on specific associations between certain types of plants and fungi. These associations involve ecto-, endo-, or a combination thereof in forming the various mycorrhizae. Ectomycorrhizae are commonly found in the boreal and temperate deciduous forest trees in the genera Pinus Picea Larix Betula Salix Fagus and Quercus These mycorrhizae exhibit a short, branched structure, and mycelia can often be seen on the outside of the root (Figure 1). Figure 1. Short, branched ectomycorrhizae of Pinus taeda (loblolly pine) growing in north Florida. The segment shown is 1.5 cm in length. Photograph: Jyotsna Sharma. Arbuscular mycorrhizae, or AM, are the most common type of endomycorrhizae, and known to have originated 460 Million Years Ago (MYA). These fungi are obligate symbionts occurring with most herbaceous and woody plants throughout the temperate deciduous forests, grasslands, and sub-tropical and trop ical regions in savannas and rainfore sts (Leake, 2007). The arbuscule, a highly branched hyphal structure that forms within th e cortical root cells, is of central importance in AM because nutrient exchange occurs acro ss the interface between the arbuscule and the cellular contents (Figure 2). Figure 2. The arrows on the left panel point to arbuscules, which are characteristic of arbuscular mycorrhizae (endomycorrhizae) and provide th e interface for nutrient transfer between the fungus and the plant cell. The arrows on the right panel show vesicles, used for storage of carbon by the fungus within th e plant cell. Images: Abid Al-Agely.
Importance of Mycorrhizae for Environmental Remediation and Resource Conservation Mycorrhizae and Enviro nmental Remediation Environmental remediation is defined as the removal of pollution or contaminants from environmental media such as soil, groundwater sediment, or surface water for the general protection of human health and the environment. In the plant rhizosphere (the zone of soil under the direct influence of a plan t root), biodegradation or tran sformation of pollutants by rootassociated bacteria and fungi under the influence of select plan t species occurs. Plants can increase the total numbers of be neficial fungi and bacteria in contaminated soil from a general rhizosphere effect. This is substantiated by the observation of higher microbial biomass and activity in the rhiz osphere (Olson et al., 2003). From the su ccessive use of select vegetation and sound plant management practices, rhizosphere ac tivity can lead to transformation and removal of compounds of interest. Mycorrhizae have been suggested to improve biod egradation of recalcitrant (persistent) organic pollutants because of the immense size and very high surface inte rface with soil. These fungi have enzymes which are known to metabolize an d degrade compounds such as polychlorinated biphenyls (PCBs) and polycyclic aromatic hydrocarbons (PAHs) (Olsen et al., 2003). Inoculation of plant roots with arbuscular mycorrhizal fungi has also been reported to enhance the phytoaccumulation of heavy metals zinc (Z n), cadmium (Cd), arsenic (As), and selenium (Se) (Khan et al., 2000; Al-Agely et al., 2005). It has been show n that water-insoluble carbonate forms of Zn and Cd can be changed to water-soluble forms by the activities of endomycorrhizal hyphae (Giasson et al., 2005). Spores of ar buscular mycorrhizal fungi in the genera Glomus and Gigaspora have been isolated from most of the plants growing in heavy-metal-polluted sites. It is also known that fungi from metal-contaminated sites are typically mo re tolerant to heavy metals compared to the reference strains from unc ontaminated soils. Most of the amelioration of contaminated soils is believed to result from the protective effect of the fungal hyphae, which create a physical or chemical barrier against the uptake by plants of heavy metals. These compounds bind to the fungal cell wall components such as chitin, cellulose derivatives, and melanin. However, heavy metal removal by mycorrh izal plants is variable, ranging from highly effective to limited efficacy. In general, th e combination of the f ungus and plant species determines the efficacy of the uptake and removal of elements. The concentration of the contaminant element in the substrate also in fluences the efficiency of removal by the mycorrhizal plants. Mycorrhizae also improve the remediation po tential of plants by producing plant-growthstimulating substances and by encouraging mine ral nutrition, better ge neral growth, and high biomass necessary for plant-based remediation. Plant growth also benefits because these fungi, along with other microorganisms, improve soil structure. Currently, a limitation for the application of specifi c mycorrhizal associations for remediation is that the beneficial effects depend on the combined efficacy of plants and the fungi involved, and also on their ecological interactions within the sy stem (soil, water, sediment, etc.) in which the contaminant is present. While there can be a ge neral beneficial effect of microbial communities, individual plant-fungus combinati ons can vary in their efficacy in removal of pollutants from the
environment. Selection of the most effective co mbination of plants and fungi is very important for achieving the desired benefits. Mycorrhizae and Resource Conservation in Commercial Plant Production In agricultural and horticultura l crop production, application of la rge amounts of fertilizers and pesticides is accepted, and is normal practice (Smith and Read, 1997). While there are many examples of improved crop productivity in mycorrh izal plants, there are few examples of largescale inoculation or management carried ou t in mainstream commercial production. The possible benefits of mycorrhizae in plant producti on are: (1) increased cr op yield; (2) reduced fertilizer and pesticide inputs; and (3) mainte nance of a healthier soil system and resulting benefits such as improved water relations and reduced severity of so me plant diseases. Commercial sources of mycorrhizal inoculum are available, howev er the efficacy of particular fungal strains or of fungal mixes in improving nutrient uptake and plant growth is known only for few crops. In horticultural, containerized production of ornamental plants, mycorrhizal colonization has been shown to increase the numbe r of buds and flowers, as well as shoot P and potassium (K) concentrations in Pelargonium peltatum (geranium). In other ornamental crops such as Ipomoea carnea ssp. fistulosa (bush morning glory), AM plants receiving 50% of recommended rate of controlled-release fertilizer exhibited comparable or better growth, higher N, P, and K and marketability than non-AM plan ts at 100% fertilizer rate (Carpio et al., 2005). Several other plants, such as Acacia greggii (cat claws), Plumbago auriculata 'Hullabaloo' (blue plumbago), Platanus occidentalis (sycamore), and Diospyros virginiana (common persimmon) showed similar increases in growth responses when inoculated with mycorrhizal fungi (Carpio et al., 2003). Because mycorrhizal plants have greater access to nutrients and water in the substrate, the benefits of this symbiotic association in resour ce conservation in agriculture are important to consider. The network of fungal hyphae vastly extends the area availa ble for absorption of substances required for plant grow th, and thereby can help in re ducing the inputs of fertilizers and water in agricultural and horticultural syst ems. For example, arbuscular mycorrhizae are very effective in helping plan ts absorb phosphorus from the soil, and phosphorus runoff is known to lead to eutrophication (= undesired bi ological growth and pr oductivity in aquatic systems). Because agriculture is a source of phosphate pollution in the environment, increased phosphorus uptake by mycorrhizal plants can help reduce the quantity of this nutrient to be added to the soil, and decrease the accumulated P in soil and water. The reduction in the occurrence of disease in mycorrhizal plants is a factor which needs consideration in integrated systems for pest ma nagement in agricultural systems. Mycorrhizae protect plants by competing with disease organi sms for colonization sites, and by improving the nutrient status of the plant and thereby increasi ng the resistance of the plants to attack by pathogens. Mycorrhizal plants al so may have increased tolerance of disease symptoms (Smith and Read, 1997). In summary, mycorrhizae improve plant growth, he lp in contaminant removal, reduce the need for fertilizer application in commercial plan t production, and improve the soil structure and
health. Although relatively few sp ecific plant-fungus combinations have been studied for their efficacy and application in remediation and re source conservation, the existing data on the benefits for mycorrhizae are promising. Literature Cited Al-Agely, A., D.M. Sylvia, and L.Q. Ma. 2005. Mycorrhizae increase arsenic uptake by the hyperaccumulator Chinese brake fern (Pteris vittata L.). Journal of Environmental Quality 34: 21812186. Carpio, L.A., F.T. Davies, Jr., and M.A. Arnold. 2003. Effect of commercial arbuscular mycorrhizal fungi on growth, survivability, and subsequent landscape performance of selected container grown nursery crops. Journal of Environmental Horticulture 21: 190-195. Carpio, L. A., F.T. Davies, Jr., M.A. Arnold. 2005. Arbuscular mycorrhizal fungi, organic and inorganic controlled-release fertilizers: Effect on growth and leachate of container-grown Bush Morning Glory [ Ipomoea carnea subsp. fistulosa] under high production temperatures. Journal of American Society for Horticultural Sciences 130(1): 131-139. Giasson, P., A. Jaouich, S. Gagne, and P. Moutoglis. 2005. Arbuscular mycorrizal fungi involvement in zinc and cadmium speciation change and phytoaccumulation. Remediation 15: 75-81. Khan, A.G., C. Kuek, T.M. Chaudhry, C.S. Khoo, and W.J. Hayes. 2000. Role of plants, mycorrhizae and phytochelators in heavy metal contaminated land remediati on. Chemosphere 41: 197-207. Leake, J.R. 2007. Mycorrhizas and the terrestrial carbon cycle: roles in global carbon sequestration and plant community composition. Chapter 8 In Fungi in the Environment. Gadd, G.M., S.C. Watkinson, and P.S. Dyer (eds.). pp. 161-184. Cambridge University Press, Cambridge, UK. Muchovej, R.M. 2004. Importance of mycorrhizae for agricultural crops. Document SS-AGR-170, Agronomy Department, Florida Cooperative Extension Service, Institute of Food and Agricultural Sciences, University of Florida, Ga inesville. http://edis.ifas.ufl.edu/AG116 Olson, P.E., K.F. Reardon, and E.A.H. Pilon-Smits. 2003. Ecology of rhizosphere bioremediation. Chapter 10 In Phytoremediation: Transformation and contro l of Contaminants. S.C. McCutcheon and J.L. Schnoor (eds.). pp. 317-353. Wiley-Interscience, John Wiley and Sons, Inc. Hoboken, New Jersey. Perner, H., D. Schwarz, C. Bruns, P. Mader, and E. George. 2006. Effect of arbuscular mycorrhizal colonization and two levels of com post supply on nutrient uptake and fl owering of pelargonium plants. Mycorrhiza 17(5): 469-474. Smith, S.E. and D.J. Read. 1997. Vesicular-arbu scular mycorrhizas in agriculture and horticulture. Chapter 16 In Mycorrhizal Symbiosis. Second edition. Smith, S.E. and D.J. Read (eds.). pp. 453-69. Academic Press, London, UK.