Submitted to: American Society of Agronomy Monograph Series
Publication Type: Book / Chapter
Publication Acceptance Date: July 30, 2004
Publication Date: November 7, 2005
Citation: Zobel, R.W. 2005. Primary and Secondary Root Systems. In: Zobel, R.W. and Wright S.A. American Society of Agronomy Monograph Series 48. Agronomy Society of America, Madison, WI, p. 3-14. Technical Abstract: The currently active root research paradigm is: Differences in root function are due to differences in surface area to volume (biomass) ratios. Root research undertaken with this paradigm in mind has been only marginally successful. A new working hypothesis is proposed: 'A plant root system is composed of a coordinated set of genetically and functionally distinct classes of root'. This hypothesis is supported by three demonstrable postulates, two of which are discussed in this paper: a) The primary and secondary root system consist of four classes of root that provide the plant with the ability to explore the available soil volume and retrieve nutrients and water extracted from bulk soil. b) Growth and function of developmentally different roots is, in whole or in part, conditioned by different genetic and physiological pathways. This paper summarizes the existing evidence for these postulates relative to primary and secondary roots, and concludes that the ultimate function of the Primary and Secondary root classes are to anchor the plant, to extend the size of the root system and to act as passive pipes to move water and nutrients from the tertiary roots to the shoot and carbohydrates back down to the tertiary roots. This being the case, the critical area of research is on the third class of root - Tertiary Roots.