Root Extension and Water Absorption

Abstract

This chapter is not intended to be an extensive review of root -soil water relationships. Balanced coverage of this field has been most adequately undertaken by texts such as Slatyer (1967) and Kramer (1969). Furthermore, various facets of the subject have been well covered in recent reviews by Weatherley (1970), Gardner (1968), Cowan and Milthorpe (1968), and particularly by Newman (1974). Characteristics of root growth, development, and senescence have been reviewed by Head (1973). Instead, it is intended to address the specific topic of the role of root extension in water absorption. Extension of roots into previously unexplored areas of the soil by annual plants or incompletely established perennial species is readily envisaged as an often necessary venture in order to obtain moisture, as will be discussed. However, the more intriguing question of the significance of continual re-exploration of the same soil mass by established perennial plant root systems in relation to water absorption will receive particular emphasis. The energetic cost of new root extension in established perennial plant root systems becomes apparent when belowground productivity is assessed. For example, reports recently emanat- ing from the International Biological Programme attribute over half of the annual primary production of an established deciduous forest and a mesic fescue meadow and about 75 /r) of the annual production of shortgrass prairie and shrub steppe communities to be in the form of new root production (Harris and Coleman, personal communication; Speidel and Weiss, 1972; Caldwell and Camp, 1974). Yet is this annual investment of energy in re-exploration of this same soil mass particularly necessary for water uptake?