Models of water transport in the soil‐plant system: A review

Although the study of plants (botany) is one of the oldest sciences, relatively detailed quantitative theories of water transport in plant tissue have lagged behind those describing water transport in soils and other geologic materials which constitute the saturated and unsaturated zones. Many existing texts deal with various aspects of water transport in these earth materials, but little or nothing is devoted to the analogous transport of water in plant roots and tissue at a similar quantitative level. Yet the soil‐root‐stem water pathway is a major component of the subsurface hydrologic system. Evidently there is a need for both engineering and agricultural hydrologists to further develop their quantitative understanding of water movement in plant and soil‐plant systems. Modern quantitative theories of water transport in plants can be traced to concepts developed and disseminated effectively in landmark papers by Gradmann and van den Honert in 1928 and 1948 respectively. The material reviewed in this paper, while more advanced, is based on these concepts. Emphasis is placed on water movement in soil containing roots and on a general approach to water transport in living plant tissue. Detailed quantitative studies of water extraction by plant roots date back to studies by Gardner published in 1960. Many contemporary models are built around extraction functions in the Darcy‐Richards equation. Several such functions are listed in a table, and their applications, relative advantages, and limitations are discussed in the text. In a series of papers published in 1958, Philip developed the first detailed quantitative description of water transport in plant tissue. His approach resulted in a diffusion equation which could be written with water potential as the dependent variable. Philip's derivation assumed that water movement was primarily from vacuole to vacuole. Subsequent workers have refined and extended Philip's development to include water movement in cell walls and plasmodesmata. The development, interpretation, and application of these models over the past decade is presented in some detail. It can be argued that contemporary models of water transport in plant tissue are oversimplified. However, they have been subjected to some successful testing and they provide a framework within which to devise experiments. Moreover, the recent development of sophisticated experimental techniques should result in more detailed model testing during the 1980's.