Root Distribution and Its Representation for Quantifying Plant Water Uptake: An Applied Perspective

Plant roots are a central figure regulating various processes in the rhizosphere. The function of roots and its ecological consequences pervade a wide range of disciplines including forage, horticultural and crop sciences. A perennial obstacle to understanding comes from the hidden nature of plant roots. This may to some extent be alleviated by new development in root imaging technologies. But it is extremely difficult to fully validate some of the intriguing and huge information provided by today’s phenotyping tools. Yet, accurate knowledge of roots and their function in the rhizosphere is central to the solution of today’s leading agricultural problems, such as feeding a growing world population while minimizing pollution to soils and waters. This poses tremendous pressures for scientists to come up with approximation solutions based on incomplete information and knowledge gaps. In soil-plant water relations problems, this naturally requires integrating simulation models with experimental measurements. Despite the rapid standardization of the leading methods describing soil-plant system water follow, one aspect that has subtle and practical consequences but is still not sufficiently appreciated in applications, is the representation of root distribution and its linkage with the soil water flow mechanisms. In this paper, I will review several major forms of root functions as used widely in soil-plant water flow problems and compare the consequences of their respective uses for sensitively describing root water uptake response to local soil dryness, which is important for plant breeding and effective field testing. As a case study, four types of root distribution function (linear, exponential, asymptotic and polynomial), with and without uptake compensation, will be discussed in a native prairie (central North Dakota) and a winter wheat crop (southwest Texas). The case study will demonstrate the usefulness of different root distribution functions in varying soil types and their effectiveness in characterizing plant water use patterns for drought stress adaptation.