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Importance of biophysical states in plants for physiology and pathology

J. BENADA

Agricultural Research Institute Kroměříž Ltd.,Havlíčkova 2787, CZ-76741 Kroměříž, Czech Republic.

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. Such a factor could be found in the biophysical states of plant organs (redox potential and pH). 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 oxidoreductase of the host plasmalemma. 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 organells which produce the electrons and the activity of oxidoreductase (the terminal oxidase) in the plasmalemma. 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.

Additional key words: redox potential, pH, gradients, powdery mildew, rusts, phenolics, respiration, oxygen, oxidoreductase, plant integrity, correlations

1. Introduction

In spite of the attention paid to looking for the principle of disease resistance by plant pathologists over many years (see the review by Fuchs (1976), there is still missing general explanation of its nature (Heitefuss 1992, Hartleb,Heitefuss,Hoppe 1997). The basic role is ascribed to the recognition of the host by the parasite and to the exploitation of the host as a substrate without declaring what the real mechanism is. Unconscious generalisation of results obtained with special objects lead to theories which were not generally applicable. Fuchs (l.c.) concluded that the problem was necessary to recede to new, unknown horizons. The older data on host-parasite relationships were summarized in compendia such as those by Heitefuss & Williams (1976), Vanderplank (1982) and Horsfall and Cowling (1980). In the last 20 years more success has been expected from molecular biology and molecular genetics (Heitefuss 1992) and summarized e.g. by Newton and Andrivon (1995).

In this article most author’s investigations on variable resistance as a starting point for considering the nature of disease resistance and some selected data of redox potential and pH will be presented. The variable resistance (sometimes termed field resistance, race non-specific resistance, partial resistance, horizontal resistance, general resistance) designates a plant resistance which changes during the ontogeny of the host and under the environment and which involves: 1. the disease gradients on a plant, 2. the change of susceptibility of organs during the ontogeny and growth, 3. the difference in resistance in individual cells of a plant, 4. relatively swift changes of resistance during a couple of hours.

The aim of this article is to formulate a hypothesis why the biophysical states may play such an important role in resistance and in plant physiology in general.

2. Materials and Methods

In the centre of investigations there was the variable resistance (VR) to obligate parasites such as powdery mildew of cereals and rusts (Benada 1964b,c). To limited extent other host-parasite couples were studied (Benada 1967a, 1974).

In elucidation of VR studies the attention was focused on redox potential (RP) and pH (Benada 1966a,1967c, 1968a), where the different methods of RP measurement were considered. The reasons for the selection of this direction of investigation are contained in the cited literature.

Bright foil platinum electrode and saturated calomel electrode was used for RP measurement in the tissues non-damaged by disintegration and in roots exudates (Benada 1995) similarly to that used in the soil (Flessa and Fischer 1992, Králová 1992). The prepolarisation of the Pt electrode by ferricyanide was necessary in some cases to obtain the distinct lower turn point. To simplify the RP interpretation the shown data do not regard the potential of saturated calomel electrode (+244 mV). Generally 10 leaves or other organs were taken for one series of measurement.

pH was measured using a glass electrode in a drop of disintegrated tissue (Benada 1965, 1966b,1967a), but other methods were tried too.

3. Results

3.1 Expression of VR

3.1.1 Gradients of disease

The lower leaves developed on the cereal plants during the stem elongation are susceptible to powdery mildew, the upper ones are resistant for some time (Benada 1966a). This is called the infection gradient (Tapke 1953 who reviews the literature on this topic). The outer side of coleoptiles and the young leaves in them are resistant, which was demonstrated by an inoculation test on isolated organs (Benada 1964a,c). Distinct differences in resistance were found within one leaf blade, between its top and base, between leaf sheaths and the stem.

Quantitative differences in resistance were found even in neighbouring individual epidermic cells (Benada 1969, 1970a,b,c, 1971) and it was found that the resistance could change within few hours, which could be demonstrated by haustoria formation.

The dependence of resistance on the ontogeny and on the environment is known for a long time in many parasite-host couples and was shown in the case of powdery mildew (Benada 1966a where some fundamental papers are reviewed).

3.1.2 Comparison of non-host resistance and VR

In experiments and observations of plants grown in the field it was noticed that during the stem elongation of cereals the new leaves were temporarily fully resistant to powdery mildew and behaved similarly to those in the case of non-host resistance (Benada 1966a) in respect to haustoria formation.

3.2 Method of RP measurement

Based on the conducted experiments it can be concluded that the tissue must not be crushed or disintegrated before measurement and that the measurement must be done under aerobic conditions ( with the exception of experiments in anaerobiosis). It must be taken into consideration that RP is dependent on the enzyme activity and that it changes during a time period. Since the RP is dependent on enzyme activity, the use of redox dyes are limited.

3.3 RP values

The redox state changes during the growth and development of organs and it is influenced by outer conditions such as light, temperature, moisture, growth regulators, nutrition, etc.(Benada 1966c, 1967e, Benada and Váňová 1972).

3.3.1 Limits

RP of cereal leaf blades during their growth ranges in most cases from +100 to -100 mV. In the exudates of cereal roots during 40 hrs RP reached values of -550 mV under anaerobic conditions (Benada 1995). In the exudates of germinated cereal seeds in the same conditions the above RP value was obtained during 30 minutes.

3.3.2 Light

Light is the most important factor in influencing the RP values in leaves. Low values of RP were obtained under intensive light in the field only. Those parts of leaves, which were covered by other parts, had high RP, e. g. basal parts of leaf blades covered by the sheaths during their development. The influence of light on the decrease in RP is shown in Table 1.

3.3.3 Temperature

The influence of the temperature is complicated by the fact that RP is simultaneously dependent on the light. In the laboratory trials the increased temperature caused the increase in RP and the lower temperature had a reverse effect. On the contrary, the temperature below +5 oC resulted in the increase in RP.

3.3.4 Water

It was found that the wilting of young plant leaves in the glasshouse which had the high RP values at the beginning of experiment caused an increased RP. On the contrary, in leaves of plants growing in the field with initial low RP the wilting resulted in the decrease in RP values (Benada 1967e).

3.3.5 Nutrition

The first leaves of cereals growing in the hydroponic culture with full nutrition had lower RP than plants growing in pure water (Table 2).

3.3.6 Ontogeny

In cereal leaves growing in the field RP sank from approx. +50 mV to -100 mV or lower during the ontogenetic development. In dicotyledon plants such as the sunflower RP in hypocotyls were approx. +260 mV, in the leaves on the stem it sank to the area of -20 mV. The decrease in RP seems to be of general validity during the ontogeny in all plants.

There are gradients of RP in the whole plant. In cereal leaves during the stem elongation the lowest RP value was in the second upper leaf (Table 3), whereas when the ear appeared, then in wheat the lowest value was in the top leaf (Benada 1967c). The RP gradients were not so regular in some cases.

There are differences of RP in leaves on the main and side tillers (Table 4), the main tiller having lower values (Benada 1966a). More data are obtained in the author’s cited publications. There are gradients in other plants, too (Benada 1967b).

3.3.7 Senescence

During the senescence the RP of leaves increases as can be seen in Table 3.

3.3.8 Anaerobic conditions

It was assessed that RP state was very sensitive to aerobic conditions of measured organs (Benada 1965, 1966a, 1967d, 1968a,c). The most expressive effect of anaerobic conditions was observed in the inundated roots of cereals where in the root exudates RP sank to -560 mV after 40 hrs, whereas in the aerobic conditions it was +160 mV (Benada 1995).Some plant organs (e.g. sunflower hypocotyls) are very sensitive to anaerobic conditions, too. During one hour, RP sank from +200 mV to +60 mV in them. The cereal leaves are not so sensitive.

3.4 pH

Plant organs showed gradients in pH, too: young organs being acidic, older ones having pH near the neutral point. In dependence on pH values the symptoms of powdery mildew as well as production of conidia and cleistothecia in powdery mildew or uredia and telia of rusts were changed (Benada 1965, 1966b, 1967a, 1970a). The formation of conidial stage of powdery mildew or uredial stage of rusts was linked with tissues of pH value 6 and lower , the cleistothecia of powdery mildew and telia of rusts were formed at tissue pH around 7 and higher.

3. 5 Resistance

In special experiments on detached leaves of cereals (Benada 1971) we tried to find exact limits of RP and pH values for resistance or susceptibility against powdery mildew. Because measurement of RP and pH in individual cells was not possible such exact limits could not be found. Nevertheless, organs with a low RP (lower than 0 mV) were resistant against powdery mildew, organs with a higher value were susceptible (Benada, 1971). In the case of tomatoes the resistant leaves and fruits against Phytophthora infestans had RP higher than +100 mV, the susceptible ones had lower RP (not published values).So, the leaves of cucumber were resistant against Pseudoperonospora cubensis at RP higher than 0 mV. The RP of older leaves on the stem was falling under 0 mV and they grew susceptible.

4. Discussion

4.1 Variable resistance

The variable resistance in some characteristics is close to race non-specific resistance.The meaning of race non-specific resistance (called field resistance, general, non specific, partial resistance ) have been formulated by many authors and sometimes with different contents. A recent example is given by Jorgensen (1987): 1. it reduces the amount of powdery mildew , irrespective of the pathogen genotype, 2. it is governed by apparently many additive acting genes, and 3. it is expressed (and measured) in quantitative terms. The differences in the definition of different kinds of resistance reflect the present unsufficient knowledge of the mechanism of it.

Already earlier investigations (Benada 1966a) suggested that the biophysical states in plants could accomplish the requirements for expressing the VR: 1. the disease gradients on a plant, 2. the change of susceptibility of organs during the ontogeny and growth, 3. the difference in resistance in individual cells of a plant, 4. relatively swift changes of resistance during a couple of hours.

The experiments with VR have advantage in investigation of resistance nature because we can use one variety only, take different organs and parts of them and in this way to eliminate the chemical differences given by varieties.

4.2 Redox

It was shown that the resistance in examined plants was linked with areas of distinct RP and that powdery mildew of cereals was a very suitable object of resistance investigation because the development of haustoria could be relatively simply observed. The strategy of looking for the mechanism of resistance on the basis of molecular biology in a substance seems to be very complicated, because there is a great number of substances at stake. Moreover, the valid mechanism of resistance must be applicable in the other areas of plant physiology.

During 30 years of investigation it has been shown that the RP values could be measured and that under similar conditions the respective organs had rather the same RP.

Up to that time biophysical states have not included in the consideration the mechanism of plant resistance at least as it will be formulated in the next part. The electric potential in plants is not a redox potential in a strict sense of physical chemistry because it has no steady state and it only reflects the oxidative and reductive processes in plant cells. Therefore, the measurement of RP in plant is not accepted by the researchers in physical chemistry and it is not mentioned in Physical methods in plant sciences by Linskens and Jackson (1990).

4.3 An outline of resistance hypothesis

4.3.1 Phenolics

There are many redox systems that operate in the plant cell. With respect to the method of measurement and that the redox system exudates from roots (Benada 1995) and from other plant organs (unpublished results) the attention was concentrated on phenolic compounds (Vaugham and Ord 1991). Moreover, the phenols are supposed to play an important role in resistance. The exudation of phenol compounds takes place from both powdery mildew conidia (Vizarová and Janitor 1968) and uredospores of rust (Summere et al. 1957). Nevertheless, I suppose that the target redox systems are not only phenols but other aromatic (heterocyclic) hydroxy compounds, too. The generally present ascorbic acid redox system is not specific enough for electron exchange (see later) with the exception that it would be combined with phenols. Further on the designation "phenolics" will be used for all these electron carriers.

Expected properties of "phenolics" as carriers of electrons in respiration:

1.Solubility in water.

2.Formation of the redox system in cytoplasma connecting all cell organelles in demand and supply of electrons.

3.Absence of oxidation by the air oxygen at all or of specific oxidation. The same is valid for hydrogen peroxide and other oxidizing factors.

4. Formation of exudates.

5. The phenolics can be obtained in oxidized or reduced states by inducing aerobic or anaerobic conditions.

Point 1.,3.,4. and 5. could be demonstrated by growing the cereal plants in hydroponic culture (Benada 1995). The exudation of phenolics and other substances from roots is known (Přikryl and Vančura 1990).

4.3.2 Redox values

Having found that the plant organs are resistant/susceptible within certain limits of RP and pH , it was necessary to ask a question why RP played such an important role. RP was supposed to play a significant role in respiration of both host and parasite and in the ability of the parasite to gain the energy in the host (Benada 1964a,1991). The influence of the environment on the activity of reducing enzymes in cell organelles and on the activity of terminal oxidase in cell wall (plasmalemma) is expected to be different, therefore low or high RP results.

It remains to explain why the parasite cannot live in the cell at a high, or more often, low RP. It is known that the redox system with a high RP oxidizes other redox systems with a lower RP and vice versa. If the parasite produces its own redox system in the cell which is oxidized by the host at a proper site, but this system interferes with the redox system of the host with much lower RP, then non-specific reduction takes place which cannot be oxidized by the terminal oxidase. It is similar at a high RP of the host cell,too. Thus, the importance of RP of the host cell or tissue consists in the fact that the redox system of the host and parasite interfere mutually through oxidation or reduction, but non-specifically, i.e. in non-correct sites of OH/O groups. Also, reverse reaction would be possible. Here the amount of the redox system of the parasite would surpass the amount of the redox system of the host. However, the host cell would die.

Up to now, I do not have the explanation of the pH function for the transformation of the conidium stage to ascomycete one, and similarly uredia to telia. I am of the opinion that this change is also associated with the oxidation of phenolic substances as carriers of electrons. On the other hand, an appropriate value of pH is necessary for correct function or oxidation-reduction enzymes.

4.3.3 Electron transport

The hypothesis has been formulated that there is no free oxygen in the cytoplasma and the transport of electrons among cell organelles is mediated by a system of "phenolics" forming redox couples in combination with highly specific oxido-reductases being fixed in cell walls (plasmalemma). The specificity consists in oxidation or reduction of OH/O groups of "phenolics" in distinct positions only. The electron exchange must take place in the same position by the enzyme in the walls of inner organelles: chloroplasts, mitochondria, nucleus, etc. I suppose it is the same enzyme as that in outer cell wall which can work as oxidase or reductase in dependence on the supply or demand of oxygen/electrons. The specificity is implemented by this mechanism. The parasite in host cell cannot use its own enzyme (terminal oxidase, oxido-reductase), because there is no free oxygen in the cell cytoplasma, it must use the enzyme in outer membrane of the host cell. The oxidation of its "phenolics" must be so specific that the reductase of inner membrane of its own organelles converts the "phenolics" to the original state. In other cases unspecifically oxidized "phenolics" accumulate in the host cell and the parasite dies for the lack of energy. Because these "phenolics" of the parasite are "strange" for the enzyme of the host, the specific oxidation is very sensitive to redox and pH stage of the host. The analysis of free O2 in the cell is difficult and up to this time the importance of such investigations has not been put in foreground (e.g. Linskens and Jackson (1990). Most plant pathologists presume that O2 is present in the cell. Nevertheless, when the RP in plant tissues should be dependent directly on O2 pressure, then there would not be any gradient of RP. But the gradients of RP within different plant organs do exist.

4.3.4 Obligate parasites

An obligate parasite without the host has not its terminal oxidase (phenol oxido-reductase) active enough to ensure its energy requirement for the growth and multiplication. For example, the conidia of powdery mildew germinate on leaf surface and the infection hypha grow mainly above the septa of cells where the highest accumulation of host oxidase may be expected (Benada 1970b). Haustoria of powdery mildew are formed in the host epidermal cell only when its redox and pH stage are suitable for the specific oxidation/reduction and in this way satisfying enough energy by respiration. Moreover, IAA (indole acetic acid) is translocated to mycelium areas with high RP and therefore the mycelium grows in the direction of the highest oxidation.

The above principle should be the same for the race-specific and race non-specific resistance (Benada 1997). Each race has its own substrate and oxidation/reduction enzyme. When the enzyme of the host is not suitable, then there is no energy supply and no growth of the parasite. This fits with the "gene-for-gene" theory formulated by Flor (1942) very well.

The host for mono- and dicaryotic states of some rusts are different. Therefore, the respiratory substrate must be different at these two stages and the parasite grows in host with suitable oxidase for the specific substrate only.

Since cereal powdery mildew infects epidermal cells only and rust does mesophyl cells, therefore the different oxidation-reduction enzymes are expected in them. It is not clear if there are different substrates for them, too, or if there is the same substrate, but only the oxidation and reduction go in other places of the same molecule. What may be the fate of this substrate when it comes to the epidermal cell and vice versa is open for further investigation.

4.3.5 Support of hypothesis

The above-mentioned hypothesis is supported by some previous findings of other authors (cited from Heitefuss and Williams 1976):

Control of spore germination and infection structure (Allen, p.78):...endogenous inhibitors are diffusible, readily reversible dormancy agents, small molecular compounds, the target of their action is the spore wall and they are characterized by their mobility from cell to cell.

Protein metabolism (Uritani, p.521): "The injury or death of parasitized cells often induce oxidation of polyphenols or the formation of lignin in the infected cells and in the non-infected cells adjacent to the infected cells."

Endogenous auxins in healthy and diseased plants (Pegg, p.575): "Particular interest centres around the role of phenolic compounds and auxin metabolism. The production of phenolic compouds and phenol oxidases and hydroxylases is an almost universal feature of disease involving facultative pathogens."

Oxidative enzymes (Frič, p.623): "Increased phenolase activity in diseased or wounded areas of plant tissues is generally accompanied by increased concentration of phenolic substances."

Phytoalexins (Kuc, p.646): "It appears that the key to the timing of the response is determined by the plant`s ability to react to components in or produced by the infection agent (recognition). It is suggested a surface phenomenon based on components of cell walls or membranes."

Increased respiration connected with infection (Daly, p.541). My own explanation: when the "phenolics" of the parasite is not specifically oxidized, the product cannot be used for energy gain of the parasite. Also, the "phenolics" from the parasite enter the host cell,they cause damage by competition with indigenous phenolic substances and increased respiration results.

The observed different susceptibility of neighbouring epidermal cells to powdery mildew and rapid changes of susceptibility and resistance (Benada 1970b) can be understood by different biophysical states in them.

4.3.6 Expected objections against the hypothesis

A general opinion of plant physiologists on electron transport in the cell counts that cytochrome system takes the function of terminal oxidase. Because the most parts of respective experiments were done in disintegrated cells or isolated cell components, it would be necessary to make experiments with intact cells.

There are several reviews dealing with a possible role of phenolic compounds in diseased plants (Farkas & Király 1962, Rohringer and Samborski 1967, Kosuge 1969, and others). Other literature cannot be listed here. The general opinion is that phenol oxidations are always connected with cell disintegration (Frič,l.c., p.623.). Nevertheless, Frič (l.c., p.627) is aware of that there are no data on their physiological role in intact plants.

The researchers in molecular plant pathology (e.g. Patil et al.1991) will claim that the genes which are responsible for production of toxins, enzymes, etc. are the cause of resistance. From the point of view of my hypothesis this may be correct in a special case, nevertheless this does not cover all aspects of VR.

4.4 The role of pH

The telia of cereal rusts can scarcely be found on cereal seedlings grown in the greenhouse and they occur in uredial stage mostly on younger tissues in the field, too. Similarly, the powdery mildew on the seedlings in the greenhouse occurs only in the conidial form. The cleistothecia can be found abundantly on the mature plants in the field. This phenomenon is generally known and several attempts were made in the past to explain it (Benada 1966b).

Up to this time it is not clear why pH of host tissue plays such an important role in the change of the conidial or uredial stage of parasites. It is very conspicuous that simultaneously symptoms caused by the parasite in the host change , too (e.g. brown patches or chlorotic spots developing in connection with powdery mildew infection) (Benada 1969).

4.5 Application of redox measurements in other areas of plant physiology

All deductions originate in the findings that transport of IAA (probably other hormone regulators too) depends on the electric gradient in the plant (Benada 1968b). I will present examples on peas and flax.

Peas and flax are two typical plants which were used for studying correlations and apical dominances. Basic data achieved in this study are reported by Dostál (1959) and later in several other contributions (Šebánek et al. 1983, Procházka et al. 1997, and others).

Conclusions were as follows: epigeic cotyledons of flax stimulate the growth of their axils. By contrast, hypogeic cotyledons of peas suppress their growth. Until now, explanations of these correlations have consisted in interferences of various growth substances.

The situation from the RP point of view is as follows: RP in cotyledon leaves of flax which grew in the light was on the average +237 mV. In cotyledons of pea, decreasing values from -27 to -140 mV with further decrease were measured. As far as roots are concerned, my technique enabled only to measure in part RP in roots (particularly in cereals) because I use common metal sheet electrodes which are too rough to measure RP. However, based on experiments with root exudates, I suppose that a very high RP (more than +100 mV) is in roots under aerobic conditions. Similarly to flax, cotyledons of sunflower also exhibit high RP. The experiments with sunflower showed that IAA was translocated in the field of high RP. A high (supraoptimum) concentration of auxin which inhibits growth is assumed in axile buds. The concentration is lowered by auxin transfer in the field of high RP. By contrast, organs with low RP increase the concentration in axile buds.

The presented examples of correlations in dicotyledonous plants are completed with correlations in cereals: tillering plants show high RP in leaves in the beginning. Under these conditions, buds are formed in axils. (A precondition: again supraoptimum concentration of IAA; it decreases by translocation in the field of high RP in blades.) As the leaf grows up, RP goes down and causes reverse reaction, IAA is translocated to the bud that grows to a tiller (shoot). Why do cereals not branch higher on the stem: Blades on the stem show low RP relatively soon, which induces translocation of IAA in an elongation zone of the internodium with high RP, and internodia are elongated. In the case of low RP in blades during stem extension of elongating plants the bud is not formed, so it cannot be elongated in the branch. Generally, I incline to the opinion that the bud is not formed at all in cotyledon axils with low RP in pea from the beginning, but it is formed later, after increasing RP. The development of buds should be studied with regard to dynamics of changes in RP.

It remains to explain a common phenomenon why plants in the glasshouse growing in the insufficient light and higher temperatures have long thin leaves, whereas plants in the field with the same number of leaves have shorter and wider leaves. Plants in the glasshouse have high RP, IAA is transferred in the upper part of blades which lengthen. In the field, RP is considerably lower, IAA is pressed in the growth part of blade bases, therefore, wider and shorter leaves are formed.

The explanation of correlation phenomena based on different concentration and translocation of IAA is simple and has a uniform basis, but there is a problem to verify what is the actual concentration of IAA at a certain place. This concentration depends not only on production of natural IAA in the plant at a certain place, but as well as on its translocation and reduction which could be caused by auxin oxidase. The activity of this enzyme can be different at different places and can change depending on environmental conditions and the age of organs. Therefore, I am of the opinion that the relationships between correlations and RP are necessary to study. If such measurements are not carried out, a great number of experiments aiming at experimental morphology from this new point of view are not possible to explain. However, that is a subject to explore. I would like to cite the conclusion of the 17th chapter Plant hormones and apical dominance, page 229, by Procházka (1997): "Auxin affects the polarity of cells and thus is likely to control the growth of axile buds. It is not yet known how auxin controls the cell polarity, but it can be an important precondition to understand the mechanism of apical dominance". Considering RP, I would add that the polarity in the plant and most likely in the cell too is conditioned by a RP gradient which originates through such a basic process as respiration. A secondary process is the translocation of auxin and other growth substances with all implications for the plant integrity.

In this context, I would only mention the study by Adamec, Macháčková et al. (1989). Unfortunately, I do not have any experience with lambsquarters goosefoot in relation to RP. However, generally, light decreases RP, darkness increases RP and as well as the translocation of IAA in leaves with high RP. If the authors connected electric current (cathode, i.e. a negative pole) to prepared leaves in the darkness (I suppose the leaves with high RP), anthesis was inhibited. Plants which were grown in the light (low RP in leaves), did not flower at all. Thus, the stimulation to induce flowering is reduction of an IAA level in the apex through its translocation in the field of organs (leaves) with high RP.

The new hypothesis will also be probably used in the field of plant allelopathy. (One plant species exudates phenolic substances from roots or other organs which inhibit terminal oxidation in another species.) It can also be uncoupling when a strange phenol is oxidized non-specifically in such a way that reductase cannot return it back to the initial state, the phenolic substance begins to accumulate, respiration increases but without energy gain (i.e. without formation of macro-ergic bounds).

The distribution of RP in plant organs can explain the morphology and ontogeny of plants (Benada, 1986). The expanding fruits of plants with dividing cells have very low RP (Benada 1967f). The knowledge of RP is a useful tool in cereal ecology (Benada 1973). Compatibility in pistil-pollen relationship is probably of the same principle as in host-parasite relationship.

4.6 Offer for discussion

The above hypothesis is offered to general discussion and to verification with other host parasite couples and with the other phenomena connected with resistance. It was not possible to present all results obtained during 30 years of RP measurements and published mostly in Phytopath. Zeitschrift or Flora (D) in this article. This hypothesis opens a new field of investigations in identification of "phenolic" compounds playing the specific role in electron transport, in identification of specific oxidation- reduction enzymes and their localisation, in the recognition of host/parasite pairs from this point of view, in the effect of fungicides as inhibitors of above enzymes in terminal oxidation, and so on. It would be necessary to put in concordance the previous knowledge of transport of electrons in the cell with these findings of RPs in plants.

5. Conclusions

1. The principle of resistance is the ability of the parasite to gain the energy in the host cell.

2. The parasite must use the oxido-reductase in the cell wall (plasmalemma) of the host, because there is no free oxygen in the cell plasma.The specific phenolics are the substrate for this enzyme.

3. The function of this enzyme is dependent on redox potential and pH of the host cell, mainly in the case of "strange" (non-specific) phenolic substances of the parasite.

4.The redox potentials are generated by the respiration of the cells by the sum of activities in cell organells which produce the electrons and the activity of oxidoreductase (the terminal oxidase) in the plasmalemma.

5. 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.

6. 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. Increased respiration results.

Table 1. The effect of light on redox in the first leaf of the wheat and barley in the glasshouse (Benada 1966a)

 

light

 

dark

 

 

x

sx

x

sx

wheat

+ 4.6

3.2

+30.8

2.1

barley

+19.9

2.2

+60.7

3.3

 

Table 2. The effect of nutrition on redox of the first leaf of cereals (Benada 1973)

 

full nutrient

 

without nutrients

 

 

x

sx

x

sx

barley var. Valtický

+38.1

2.5

+71.4

2.7

wheat var. Zlatka

+10.6

4.3

+45.0

3.1

 

Table 3. Gradient of redox in the cereal leaves during the stem elongation.

 

The leaf range from top

 

1

2

3

4

5

6

wheat

-18

-47

-37

-15

+2

+2

barley

-48

-71

-67

-38

-16

+32

 

Table 4. Differences in redox values in the leaves of main and side tiller in rye

leaf range

main tiller

 

side tiller

 

from top

 

 

 

 

 

x

sx

x

sx

1

-76.1

4.5

-35.6

4.2

2

-67.2

4.1

-51.8

2.2

3

-57.8

1.0

-27.2

4.7

 

REFERENCES

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