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Measurement of Redox Potential and pH in Plants and their Function in the Mechanism of Plant Resistance and in Plant Physiology

Abstract

The study of biophysical states in plants was initiated 35 years ago in connection with the looking for the mechanism of plant resistance to parasites. Here the attention was focused on the variable resistance of cereals to obligate parasites such as powdery mildew and rusts. It was necessary to find out a factor which changes during the ontogeny and through the environment and which involves: 1. The disease gradients on plant, 2. The change of susceptibility of organs during the ontogeny and growth, 3. The difference in resistance in individual plant cells, 4. Relatively swift changes of resistance during a couple of hours. This factor could be found in the biophysical states of plant organs (redox potential (RP) and pH).The method of RP measurement in plant tissues in aerobic conditions and measurement in hypoxia are presented. The redox state changes during the growth and development of organs and is influenced by the outer conditions such as light, temperature, moisture, growth regulators, nutrition, etc. and by the organ senescence.This is not a standard redox potential, as defined in physical chemistry, which is constant, but redox potential in plant is dependent on activity of respiration enzymes and it changes. The actual state can be red from the lower turn point of mV changes on indication instrument (milivoltmetre) with input resistance at least 10 MΩ. A hypothesis was formulated for further investigation: The principle of resistance consists in the ability of the parasite to gain the energy in the host cell. The parasite uses the terminal oxidase of the host plasma lemma. The specific phenolics are the substrate for this enzyme. The main features of this hypothesis are: 1. There is no free oxygen in the cell plasma. 2. The redox potentials are generated by the respiration of the cells by the sum of activities in cell organelles which produce the electrons and the activity of oxidoreductase (the terminal oxidase) in the plasma lemma. The electron carriers can permeate from the cell to the environment and they are soluble in water. 3. The redox potential is the basis of electric gradients in the plant which plays the main role in its integrity as well as for the life of the parasite. 4. The parasite respires through the terminal oxidase of the host. 5. The environmental conditions influence the enzyme activity of host and parasite cells differently by which different redox potentials may appear in the host and parasite cells resulting in unspecific oxidation or reduction of electron carriers. 6. The acidity of the host cells determinates the formation of conidia or cleistothecia (in powdery mildew) or formation of redia or telia (in rusts). 7. Depend on redox potential it is possible to explain some correlations in plants. Without knowledge of RP gradients it is not possible to explain the integrity of plants and related phenomena.

Benada J Kroměříž

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