Rhizosphere defined

Rhizobacteria, deleterious, 13 349 Rhizobitoxine, 13 300 Rhizobium-legume associations, in nitrogen fixation, 17 296-298 Rhizofiltration, 3 783-784 defined, 3 759t Rhizopus nigricans, 11 4, 9 Rhizosphere defined, 3 1 bit RHO A-alumina, 2 394-395 Rhodacarborane catalysts, 4 217-218 Rhodamine B... 

Heavy metal and radionuclide concentrations in soils increase due to man-made pollution. One of the first entry points of such elements into plant ecosystems is the rhizosphere, defined as the soil under the biological, physical and chemical influence of roots. Arbuscular mycorrhizal (AM) fungi, symbiotic microorganisms associated with the roots of many plant species, provide a direct link between soil and roots and affect metal transfer to plants. The present chapter includes recent laboratory work and some research aspects stiU to be adressed on the contribution of AM fungi to plant metal uptake. The necessity to develop new and adapted approaches, such as compartment devices and root-organ cultures, to separate AM to root contribution to metal uptake is emphasized. Available data may be difficult to compare because they were obtained under different experimental conditions. However, they suggest that the transfer of heavy metals from AM fungi to plants may be metal specific. Further research should focus on the mechanisms involved in reduced or improved uptake of metals by mycorrhizal plants, on AM tolerance to metals and radionuclides and on AM functional diversity in polluted soils. AM contribution to metal uptake should also be quantified to include data in models of plant uptake. 

Heavy metal and radionuclide concentrations in soils increase owing to man-made pollution related to industrial, agricultural or urban activities. Such concentrations can reach toxic levels and create major environmental and health problems. One of the first entry points of metals into plant ecosystems is the rhizosphere, defined as the soil under the biological, physical and chemical influence of roots. In the rhizosphere, the plant releases root exudates that soil microorganisms feed on, and... 

It is well known that chemical compo.sition of rhizosphere solution can affect plant growth. Particularly, uptake of nutrients may be considerably influenced by the ionic concentration of the rhizosphere solution (40). Despite the difficulty of defining the exact concentration of ions in the rhizosphere surrounding each root (or even root portion), it has been unequivocally demonstrated that plants have evolved mechanisms to cope with the uneven distribution of ions in the root surrounding in order to provide adequate supply of each essential nutrient (41). These mechanisms include expression of transporter genes in specific root zones or cells and synthesis of enzymes involved in the uptake and assimilation of nutrients (40,43). Interestingly, it has been shown that specific isoforms of the H -ATPase are expressed in the plasma membrane of cell roots it has been proposed that the expression of specific isoforms in specific tissues is relevant to nutrient (nitrate) acquisition (44) and salt tolerance (45). 

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. 

However, relatively few studies have included growing plants in their experimental systems. In order to mechanistically understand the effects of pine roots on microbial N transformations rates under conditions of N limitation, l-year-old pine seedlings were transplanted into Plexiglas microcosms (121) and grown for 10-12 months. Seedlings were labeled continuously for 5 days with ambient CO concentration (350 iL L ) with a specific activity of 15.8 MBq g C. Then, soils at 0-2 mm (operationally defined as rhizosphere soil) and >5 mm from surface of pine roots (bulk soil) of different morphology and functional type (coarse woody roots of >2 mm diameter fine roots of <2 mm diameter ... 

Architectural models explicitly specify the di.stribution of roots in space. An alternative approach, which is also useful for rhizosphere studies, is the continuum approach where only the amount of roots per unit soil volume is specified. Rules are defined that specify how roots propagate in the vertical and horizontal dimensions, and root propagation is u.sually viewed as a diffusive phenomenon (i.e., root proliferation favors unexploited soil). This defines the exploitation intensity per unit volume of soil and, under the assumption of even di.stribution, provides the necessary information for the integration step above. Acock and Pachepsky (68) provide an excellent review of the different assumptions made in the various continuum models formulated and show how such models can explain root distribution data relating to chrysanthemum. 

The first chapter defines the spatial and functional features of the rhizosphere, which make this environment the primary site of interaction between soil, plant, and microorganisms. Among the multitude of organic compounds present in the rhizosphere tho.se released by plant roots are the most important from a qualitative and quantitative point of view furthermore, the relationships with soil components of any released compound need to be considered (Chapter 2). The release of these compounds strongly depends on the physiological status of the plants and is related to the ability of plant roots to modify the rhizosphere in order to cope with unfavorable stress-reducing conditions. These aspects are dis-... 

The term rhizosphere was first used by Hiltner (1904) to indicate the area of the soil where root exudates released from plant roots can stimulate, inhibit, or have no effect on activities of soil microorganisms nowadays it is generally used to define the field of action or influence of a root. ... 

The findings in this chapter bring into question the ability to predict the influence of citrate, if not other rhizosphere products, on the soil chemistry of metals. Clearly, additional studies are needed to validate an existing model or to establish an Fe(III)-or Al-citrate aqueous speciation model that may be applied to the soil environment. Equally problematic are thermodynamic data compilations relative to the Fe(III)-or Al-citrate species that are included. Chemical models, as defined by various studies in Tables 10.4 and 10.5, cannot be mixed. They are constructed specifically to describe a given series of chemical systems. Presumably, the number of species, their stoichiometric characteristics, and their... 

Ectorhizosphere The soil layer surrounding roots and affected by the activity of roots themselves, and the microorganisms. The thickness of this soil portion usually ranges from one to a few millimetres. The ectorhizosphere was initially defined by Hiltner (1904), who referred to it simply as the rhizosphere. The same definition and term were reported by Curl and Truelove (1986). 

Many protocols for collecting rhizosphere soil have been developed, although the lack of a precise delimitation within a continuum means that subdivision has been performed as a function of the research aims. Soil scientists of different disciplines may use different procedures, and the rhizosphere soil so obtained has to be considered as an operationally defined fraction. According to Lynch (1990), this may indicate different things to different researchers. The procedure followed to obtain rhizosphere and bulk soils obviously depends on the type of plants investigated and whether they are cultivated (in containers or in the field) or are growing naturally. 

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