Root products

In this chapter I have taken a somewhat skeptical view of the currently accepted rosy picture so readily painted. The role of the devil s advocate has been adopted to raise issues that are too readily disregarded or assumed glibly to be true. Although some attention is given to the quantitative aspects of the release of root products, the main purpose is to classify types of root products on the basis of their known properties and perceived roles in the rhizosphere. 

This chapter considers the various types of root products with a potential functional role in the usually tough environment of soil. Only direct effects of immediate benefit to plant growth—e.g., an increase in nutrient solubility—are considered here. Although root products of a plant species may have a direct effect on important groups of soil organisms, such as rhizobia and mycorrhizae. their effect on the plant is not immediate these and aspects related to microbial activity in the rhizosphere are not considered here (see Chaps. 4, 7, and 10). For an extensive and recent review of the microorganisms in the rhizosphere, the reader is referred to Bowen and Rovira (23). 

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. 

The bulk of root products are C compounds derived from products of photosynthesis. The root products that are not C compounds are few (H", inorganic ions, water, electrons) but nevertheless are deemed to be highly significant. Both H and electrons may be secreted as C compounds in the form of undissociated acids and reducing agents, respectively. 

IV. TYPES OF ROOT PRODUCTS, PARTICULARLY SECRETIONS, AND THEIR ROLES... 

Most root products are by-products that represent some of the costs of... 

Root products may be classified into types on the basis of their (1) chemical properties, such as composition, solubility, stability (e.g., hydrolysis, oxidation), volatility, molecular weight etc. (2) site of origin and (3) e.stablished, not just perceived, functions. The chemical properties determine in turn their biological activity and how the compounds will behave in soils their persistence in soil is very much an outcome of their chemical behavior, particularly sorption and their biodegradability. 

The growth of roots through soil is perceived often as improving soil structure for plant growth. In the context of this review, the question is one of whether a plant s root products directly improve the soil structure for the growth of that plant. However, it is a difficult question to answer because, in addition to the... 

Root products represent a vast array of predominantly organic compounds. Of these, secretions represent a small proportion, but they are deemed the most likely of all root products to have a direct effect on the growth of the plant that produced them. When a secretion is released by a root, all the following are likely to affect its behavior. 

The quantification of gross root production, rhizodeposition, microbial assimilation, and the production of organic materials in soil has made increasing progress ever since stable ( C) and radioactive ( C) carbon isotopes have been used (see Chap. 12). Measurements of soil organic matter dynamics without these isotopes are difficult due to the large amount present as compared to the smaller rates of input. 

A. H. Fitter, G. K. Self, J. Wolfenden, M. M. I. van Vuuren, T. K. Brown, L. Williamson, J, D. Graves, and D. Robinson, Root production and mortality under elevated atmospheric carbon dioxide. Plant Soil 187 299 (1996). 

GA3 caused elongation in sago pondweed, even in the presence of solstitialin (Table IV). At 1 to 50 ppm, solstitialin reduced the number of leaves per plant even in the presence of GA3. At 50 ppm solstitialin caused a reduction in the number of daughter plants, and seemed to have slightly inhibited root production as well. 

Fig. 6.1 Different plant strategies for dealing with patchy distribution of P in soil (a) initial situation random distribution of roots, (b) increased root production, (c) production of cluster roots in high-P environments, (d) production of cluster roots at random. For discussion, see text...

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. 

Because of the obligate outcrossing nature of alfalfa, all experiments were carried out using vegetative cuttings of primary transformants. Nodal stem sections were placed in sterile, moist vermiculite for approx 14 d to stimulate adventitious root production, then transferred to a mix of sand and soil (1 1 [v/v]) in 3.8 x 21 cm plastic cone-tainers (Stuewe Sons, Corvallis, OR) and fertilized monthly with soluble 10 10 10 (N P K) fertilizer. 

The machinery of tapioca processing is highly varied. in there are well-equipped factories that utilize local, custom-built d lai anc and Brazil roots, product streams, by-products and effluent. In addition sonigVlCes for Process nS successfully utilized equipment that is more common to potato and manufacturers have basic process of screening and density separation remains commonprocessing. The... 

Some components of NPP, such as root production, are particularly difficult to measure and have sometimes been assumed to be some constant ratio (e.g., 1 1) of aboveground production (Fahey et al., 1998). Fewer than 10% of the studies that report total ecosystem NPP actually measure components of belowground production (Clark et al., 2001). Estimates of aboveground NPP sometimes include only large plants (e.g., trees in forests) and exclude understory shrubs or mosses, which can account for a... 

Fahey T., Bledsoe C., Day R., Ruess R., and Smucker A. (1998) Fine Root Production and Demography. CRC Press, Boca Raton, FL. 

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