Perspectives in Biology and Medicine

SOIL-PLANT INTERFACE IN THE ROOT-HAIR ZONE AS A UNITY OF OPPOSITES* F. W. PAULI and]. C. DEELMANf On the molecular level, a relation between a system and an environment we may call the mapping ofthe environment by the system. [Roland Fischer, 1967] Little difficulty is usually encountered when attempting to distinguish between a living organism and a dead object. In a living being, countless physical and chemical processes are highly "ordered" to allow the living system to persist, to grow, to develop, to reproduce. The living organism conducts a continuous exchange of components. Metabolism is the very essence of living systems. Biochemistry has secured the knowledge of reaction paths of numerous individual metabolic processes. At present, attention is focused more and more on attempts to comprehend integrated metabolic systems as functional units. Technological advances in light microscopy and especially in electron microscopy have created a wealth of information on the morphological organization of living systems . But there is still another, nearly invisible organization in biophysical dynamics determined by rates of reaction, by uptake and transport, and by environmental defense mechanisms. Plants, like all other living organisms, are fundamentally what von Bertalanffy (1968) called "open systems," exchanging ions, molecules, and energy with the environment. The order of metabolic processes is such as to maintain the system itself. The reaction of plant roots to any change in their soil habitat consists of a regulatory biochemical process superimposed on automatic activities such as adaptation of stimuli response . The principal reason for the unique complexity of biochemical reactions is that some of the products of these reactions form physical boundaries, thereby allowing the various reactions to take place simul- ?Based on a lecture given at the Second International Symposium on Environmental Biogeochemistry at Burlington, Ont., April 8-11, 1975. tAddress: University of Heidelberg, D69 Heidelberg, Berlinerstrasse 19. IV., Federal Republic of Germany. This manuscript is dedicated to the memory of the late Professor Dr. Ludwig von Bertalanffy, founder of the general system theory and pioneer in promoting the organismic view in biology and related fields. Perspectives in Biology and Medicine · Summer 1976 | 493 taneously in different parts of the biological system (organism, cells, and cell structures). During the biochemical adaptation process of evolution, these boundaries came to be selective ones, permitting ingestion of required chemical components, enabling material to be excreted (waste stuffs and mobilizing agents), and preventing the ingestion of harmful components to a certain extent. Such boundary layers eventually developed the capacity to select the necessary external components and to place them into the transporting pathways inside the organism. The system as such gradually became able to store energy, to store chemical compounds, and to store newly created enzymes or catalysts, leading in the course of time to what is known as cellular life. It may therefore be concluded that the lowest level oforganization ofmetabolism necessarily corresponds to pathways developed as the earliest forms of life [I]. The roots of plants consist of the interaction interface between biosphere and lithosphère. The complex system known as "the soil" (minerals , organic residues, gaseous phase, and ions in solution) supplies, at present, all essential requirements for life of soil-inhabiting organisms, the edaphon. Moreover, the soil-root interface will be the region of maximum metabolic activity. Here, the whole plant concentrates a large portion of its biophysical processes. The energy exchange taking place between the plant's rhizosphere and various soil constituents seems to attract a multitude of microscopic life. Noninvading microorganisms such as bacteria, actinomycetes, and fungi as well as certain members of the microfauna are concentrated at the surface of these roots (rhizoplane) and in the immediate part of the adjoining soil (rhizosphere). Very recently, a diffuse network of electron-opaque fibrils and loosely assembled tufts of organic polymers (linear polygalacturonic acid) was detected at the rhizoplane of many plant roots [2]. Although these fibrils are extracellular products, they seem to facilitate contact between plant root and soil components and might also provide a welcome adherence surface for microorganisms. The presence and activity ofthese organisms interferes with the nutrition of plant roots and, at the same time, influences the exchange of a multitude ofsubstances, from simple organic compounds (sugars) up...