Root products exudate

Root products are all the substances produced by roots and released into the rhizo.sphere (Table 2) (17). Although most root products are C compounds, they include ions, sometimes O, and even water. Root products may also be classified on the basis of whether they have either a perceived functional role (excretions and secretions) or a nonfunctional role (diffusates and root debris). Excretions are deemed to facilitate internal metabolism, such as respiration, while secretions are deemed to facilitate external proces.ses, such as nutrient acquisition. Both excretion and secretion require energy, and some exudates may act as either. For example, protons derived from CO2 production in respiration are deemed excretions, while those derived from an organic acid involved in nutrient acquisition are deemed secretions. 

Root products, as defined by Uren and Reisenauer (17), represent a wide range of compounds. Only secretions are deemed to have a direct and immediate functional role in the rhizosphere. Carbon dioxide, although labeled an excretion, may play a role in rhizosphere processes such as hyphal elongation of vesicular-arbuscular mycorrhiza (39). Also, root-derived CO2 may have an effect on nonphotosynthetic fixation of CO2 by roots subject to P deficiency and thus contribute to exudation of large amounts of citrate and malate, as observed in white lupins (40). The amounts utilized are very small and, in any case, are extremely difficult to distinguish from endogenous CO2 derived from soil and rhizosphere respiration. 

Example 1 Citrus oil or citrus root products can Indeed kill slugs if they are applied as a contact agent or act as a slug repellent. Although some patents have been issued on the use of citrus oil as a slug control agent, such control with a contact agent is difficult and the chemical compositions of citrus oil and root exudate are ill-defined. Other obstacles for commercialization of citrus oil are (1) market size is too small to justify the development cost, and (2) an effective delivery method has yet to be developed. 

The early literature on naturally occurring plant growth inhibitors and the influence one plant might exert on another by chemical means (allelopathy) is considered comprehensively in various reviews (12, 15, 36, 37, 48, 49, 61, 67, 94, 121, 162). Reviews of studies concerned with the influence of plant exudates on root-infecting fungi (130) and the effects of phytotoxins which arise as decomposition products (113) have appeared recently. Because of the excellent coverage of the topics by others, no attempt is made here to review the early literature exhaustively. Instead, consideration is restricted essentially to specific compounds and to some of the more recent literature. 

E. Benizri, A. Courtade, C. Picard, and A. Guckert, Role of maize root exudates in the production of auxins by Pseudomonas fluorescens M.3.I. Soil Biol. Biochem. 30 1481 (1998). 

Extraction of rhizosphere soil (22,34,51,52) is an approach that can provide information about long-term accumulation of rhizosphere products (root exudates and microbial metabolites) in the soil. Culture systems, which separate root compartments from adjacent bulk soil compartments by steel or nylon nets (52-54) have been employed to study radial gradients of rhizosphere products in the root environment. The use of different extraction media can account for different adsorption characteristics of rhizosphere products to the soil matrix (22,34). However, even extraction with distilled water for extended periods (>10 min) may... 

A new approach to study root exudation of distinct compounds in soil-grown plants uses inoculation of roots with genetically engineered reporter bacteria, which are able to indicate the presence of particular compounds by indicator reactions, such as production of ice-nucleation proteins. This technique has been employed to detect the release of amino acids from roots of soil-grown A vena harbata (56). 

In many plants, P deficiency also enhances production and root exudation of phenolic compounds (Fig. 5) (27,31,128-130). Increased bio.synthesis of pheno-... 

As already noted by Campbell and Greaves (16), the rhizosphere lacks physically precise delimitations and its boundary is hard to demarcate. Dimensions may vary with plant species and cullivar, stage of development, and type of soil. Soil moisture may affect the measurable size of the rhizosphere as well wetter soils may stick better to roots than drier soils (Fig, 1). This will change the volume of soil regarded as rhizosphere soil upon separation of rhizosphere from bulk soil and thus alter the measured concentration in rhizosphere and non-rhizosphere soil of a response variable in exudate concentration or microbial production. 

Root exudates A wide variety of chemicals, such as sugars, amino acids, and aromatics, is excreted by roots of plants. Very little information is available on the allelopathic interaction of root exudates with the higher plants, except for the identification of a few products in isolated cases (46). 

In preparation for the Isolation of allelopathlcs from root exudates of A. artemlsllfolla using the method by Tang and Young (30) we wished to learn first about the major root constituents and possibly establish their structures. We describe here our preliminary findings on the root natural products of Louisiana A. artemlsllfolla populations. Freshly obtained roots of the ragweed,... 

Soybean nodulation. The results of this bioassay, shown in Table HI, are essentially the same as the dry matter production bioassay. Xanthlum root exudates reduced soybean nodule dry weight Glycine and Sesbanla root exudates reduced nodule, leaf and total dry weight. The percentage reduction is not quite as great as in the dry matter production bloassay. 

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