Root Exudates: an Overview
Roots of many weed and crop species contribute biologically active chemicals into the environment known as root exudates. Root exudates are known to influence growth and establishment of crop and weed species, and these are released from living root syste
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and L.A. WESTON
10.1 Introduction Roots of many weed and crop species contribute biologically active chemieals into the environment known as root exudates. Root exudates are known to influence growth and establishment of crop and weed species, and these are released from living root systems. Many perennial woody and herbaceous plants have deep and extensive root/rhizome subterranean systems, which can produce prolific amounts of root exudates over long periods of time. Root exudates contribute many types of organic compounds to the rhizosphere. In addition to simple and complex sugars and growth regulators, root exudates contain different classes of primary and secondary compounds including amino acids, organic acids, phenolic acids, flavonoids, enzymes, fatty acids, nucleotides, tannins, steroids, terpenoids, alkaloids, polyacetylenes, and vitamins (Table 10.1; Rovira 1969; Schönwitz and Ziegler 1982; Rice 1984; Uren 2000). Uren (2000) suggested that the amount of root exudates produced varies with the plant species, cultivar, the age of the plant, and substrate and stress factors. In particular, exudates influence growth and establishment of other plant species (allelopathy), microflora and microfauna, availability of inorganic ions, and may have other less well-documented ecophysiological functions. In studies performed with cereals, a portion of plant photosynthates (between 5-21 %) is typically released via root exudation (Haller and Stolp 1985; Flores et al. 1996). Depending on the size of a plant's root system and its response to biotic and abiotic factors, sizeable quantities of root exudates may be released over time into the soil rhizosphere. Root exudates can directly influence the soil rhizosphere and its composition through (1) their effects on soil-borne pests and pathogens, and other microflora and microfauna, (2) their influence on soil nutrients and resulting microbial ecology, and (3) their allelopathic activities on other plant species. Root exudation can often be modified by abiotic and biotic factors, as weIl as physical, chemical and biological soil factors. Ecological Studies, Vol. 168 H. de Kroon, E.J.W. Visser (Eds.) Root Ecology © Springer-Verlag Berlin Heidelberg 2003
Extracellular root proteins Uronic acid, deoxy sugars (rhamnose and fructose)
Sorgoleone (quinone) DIBOA (2,4-dihydroxy-l. 4-benzoxazin-3-one) (hydroxamic acid) DIMBOA (2,4-dihydroxy-7-methoxy-l, 4-benzoxazin-3-one) (hydroxamic acid) Quercitrin (flavonoid)
p-Benzoquinone (quinone)
Umali-Garcia et al. (1980)
Inderjit and Dakshini (1994) Flores et al. (1996)
Inhibits seedling growth of mustard Adaptation to nutrient-limitation (positive effects) Provide favorable environment for the growth of beneficial microorganisms such as the free-living N2-fixing bacterium Azospirillium lipoferum
Wheat allelochemical
Perez and Ormeno-Nunez (1991) Wu et al. (2000)
Inhibits root growth of wild oat
Hartwig et al. (1991); Poulin et al. (1993) Netzley et al. (1988) Einhellig and Souza (1992)
Moghimi et al. (1978)
Replace phosphat
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