The volume starts with an overview of the form and function of roots and the many problems that they encounter by life in soil (Chap. 1), introducing many of the topics that are discussed in more detail in the chapters that follow. Chapter 2 describes the spatial distribution of roots, including the responses to heterogeneous soils that are interpreted in terms of foraging for nutrients and water. Chapter 3 deals with root distribution in time, by reviewing the knowledge on turnover of roots in various ecosystems and their implications for ecosystem processes. The following five chapters provide physiological background to the basic functions of roots, including carbon in- and output (Chap. 4) and water and solute movements (Chap. 5), and the physiological and morphological solutions that roots have developed to cope with three major abiotic stresses, i.e. hard soil structure (Chap. 6), drought (Chap. 7) and flooding (Chap. 8). The carbon balance in roots largely determines the growth of a root system, and is therefore curcial not only for our understanding of root proliferation, but also for the role of roots as carbon source for the soil ecosystem (Chap. 4). The allocation of such carbon compounds and the transport of nutrients taken up by the roots depend on a carefully controlled hydraulic balance (discussed in Chap. 5). Nutrient and water uptake, two prime tasks of a root system, may be severely hampered if a root cannot penetrate the bulk soil (Chap. 6), encounters low soil water potential (Chap. 7) or low oxygen concentrations (Chap. 8). The regulatory control of the anatomical and morphological changes that enable roots to overcome such adverse conditions is greatly similar among stresses, and based on key plant hormones such as ethylene and abscisic acid. Additionally, specific biochemical pathways add to the resistance of the roots to these extreme habitats (Chap. 7 and 8). The volume concludes with six chapters on biotic interactions emphasising the complex soil ecosystem that roots influence and, vice versa, influences the roots. Roots have evolved symbiotic interactions with mycorrhiza (Chap. 11), rhizobia (Chap. 12) and soil bacteria (Chaps. 12 and 13) that assist in the capture of soil resources such as nitrogen and phosphorus that are often in short supply. Roots compete for these resources with other roots (Chap. 9) and with soil microorganisms (Chap. 12) and are an important food source for a variety of soil herbivores (Chap. 14). Many of the biotic interactions involve the exudatin of organic substances (Chap. 10) and release of gases, such as oxygen in flooded soils (Chap. 13). In the way, roots possess an array of intriguing mechanisms by which they manipulate the soil environment and its biota, facilitating the growth of soil bacteria that promote plant growth or suppress diseases (Chap. 12), stimulate microbial processes that accelerate soil nutrient cycling (Chap. 13), or provide a chemically hostile environment for competitor plants (Chap. 10.).
