ABSTRACT
Root systems are congregates of several individual components that together constitute the functional “hidden half of plants (cf. Bohm, 1979; Feldman, 1984; Eshel and Waisel, 1996). This attribute is of special importance for roots of terrestrial plants that occupy heterogeneous environments that vary spatially and temporally (Caldwell and Pearcy, 1994). The ability of a plant to produce different types of roots is an inherent aspect of its plasticity which has an important adaptive characteristic (Barlow, 1993; Bell and Lechowicz, 1994). Variation in traits among the various components of plant root systems affects the capability of these plants to cope with their complex environments (Caldwell, 1994; Fitter, 1996; see also Chapter 53 by Nobel in this volume.)
Very few of the numerous roots that constitute one root system are exposed to the same conditions. Usually, different roots within each root system are exposed to different physical or chemical conditions that prevail in various microsites within the rooting volume. Moreover, the roots themselves increase the environmental variability by depleting certain zones of the soil of minerals and water (Grime, 1994; Stark, 1994), by secreting organic compounds that are utilized by microorganisms and by changing the ionic composition of their immediate rhizosphere (see Chapter 36 by Neumann and Roemheld in this volume).
