The Effect of Fluorine-Containing Emissions on Conifers

Anatoly S. Rozhkov & Tatyana A. Mikhailova
Siberian Institute of Plant Physiology and Biochemistry
Siberian Branch of the Russian Academy of Sciences

Translated by L. Kashhenko

Publisher: Springer-Verlag
Published: 1993
ISBN: 3-540-54735-5

Preface

Preservation of nature and the environment has become one of the most important issues of the end of the twentieth century. It has become evident that the methods used for industrial and agricultural production in many countries produce pollutants that cannot undergo natural neutralization by entering the atmosphere, soil or water. Ecosystems that have been developing for centuries are undergoing degradation and what is even more regrettable is that there is an actual threat of profound disorder in the biosphere which could lead to heavy and irreversible changes.

Fluorine derivatives are the most aggressive among toxic compounds polluting the atmosphere. Moreover, the percentage of fluorides in industrial emissions is constantly increasing with the bulk of fluorides being emitted by aluminium smelters. Fluorine is poorly detoxified by both plants and animals and the accumulation of even relatively low concentrations over a long period causes a cumulative toxic effect. Among woody plants conifers are less resistant to fluorine. Fluorine derivatives as phytopollutants have been studied less than sulphur compounds, nitrogen oxides, chlorine and hydrogen chloride. It was not until the late 1960s when there was a rapid decline of coniferous forests that researchers directed their attention towards phytotoxic properties of fluorides.

This book is the result of many years' study on the impact of fluorine on conifers. The work has been performed in Eastern Siberia where rapid development of the aluminium industry, which has arisen from the availability of electric power provided by hydroelectric plants, has rapidly become detrimental to coniferous forests in polluted areas.

The investigations have been carried out in the Laboratory of Pathology of Woody Plants in the Siberian Institute of Plant Physiology and Biochemistry, Siberian Branch of the USSR Academy of Sciences (Irkutsk, USSR). The laboratory first initiated the study as a single project. In 1973 the work was expanded and included in the projects of the Institute; some of the investigations have been further supported by the State Committee of Science and Engineering of the Council of Ministers of the USSR and a contract was signed with the Ministry of Forestry of the Russian Federation. The experiments were carried out at the laboratory field station in the Tunka valley (southern Baikal area) between 1976 and 1987. The station has experimental plots, permanent and transportable gas chambers with controlled levels of fluorine concentration and laboratory equipment suitable for conducting biochemical and toxicological assays...

The ambient atmosphere around many industrial plants, including aluminium smelters, is characterized by the presence of a complex of pollutants acting simultaneously. Under such circumstances one can expect variation in the phytotoxic activity of emissions. The effect of a mixture of pollutants on plants is a relatively new field of research and not much data are yet available to characterize the effect of combined pollutants. However, such research is needed to assess MPCs of air pollutants and to advise businesses within a certain region of allowable levels of emission. This would reduce the damage suffered by vegetation, especially sensitive coniferous forests. There are many examples where factories emitting pollutants of the same type in one place caused massive decline of the vegetation and resulted in the formation of vast industrial deserts.

Four types of action of pollutant mixtures can be distinguished; addition, synergism, domination and antagonism. Addition describes the situation where the extent of damage caused by a pollutant mixture corresponds to the sum of damages caused by each substance. Synergism implies a greater overall damage than the sum of single damages, and antagonism implies less damage by substances acting together than by each substance acting separately. The domination of any one pollutant is the case when the extent of damage by a mixture containing this pollutant remains the same as under exposure to this pollutant alone.

The influence of any pollutant mixture on the plant organism largely depends on the concentration, duration and sequence of the mixture as well-as on physico-geographical conditions. In addition, gas resistance of plants is important as is their age and stage of development. For example, the same mixture composed of low concentrations of sulphur dioxide, nitrogen dioxide and ozone had differing effects on eight clones of eastern white pine with different gas resistances. Even within a group of clones of similar sensitivity three types of effect were noted: additive, synergetic and antagonistic (Yang et al. 1982).

Analysis of available data suggests that synergism is more common when plants are exposed to high concentrations of pollutant mixtures. Addition and domination effects are possible with the low concentration of acid gases in the atmosphere. Combinations of pollutants resulting in decreasing harmful activity are extremely rare. Antagonism might be expressed when acid and alkaline gases, or oxidants and reducing agents are simultaneously present in the air.

Antagonism of mixtures of sulphur dioxide with ozone and of nitrogen oxide with ozone has been reported by some authors (Landolt and Keller 1985). The addition of sulphur dioxide caused a reduction of the adverse effect of the mixture of ozone and products of petrol consumption (Haagen Smit et al. 1952). The decline in phytotoxicity of sulphur dioxide and phenol following the addition of a low concentration of pyridine has been reported (Tarabrin and Bashkatov 1986).

However, in the majority of cases the action of various combinations of similar pollutants results in development of additive and synergetic effects. When oats, barley and lucerne were fumigated with acid gases and mixtures of two pollutants no antagonism was recorded; most common was the additive effect (Bennett and Hill 1974). Pronounced synergism was reported in the case of mixing sulphur dioxide with hydrogen chloride (Guderian. 1979), oxides of nitrogen (Bennett and Hill 1974) and ozone (Menser and Heggestand 1966; Boyer et al. 1986).

Bearing in mind the strong aggression of gaseous fluorides, one can theoretically predict the synergetic effect of fluorine in combination with other pollutants. The prediction is corroborated by data showing a marked increase in damage suffered by agricultural crops on exposure to a mixture of hydrogen fluoride and sulphur dioxide (Ten Houten 1974; Morel and Chaouard 1967).

One of the aims of our work was to evaluate comparatively the toxicity of the most common acid gases and their mixtures upon coniferous plants (see Table 1). We also followed the effects of combinations of pollutants, i.e. the demonstration of synergetic or additive effects as well as those of antagonism and domination.

Table 1. Toxicity of most commonly met acid gases for coniferous plans*
Gas	Toxicity, points	Concentration of pollutants in needles (% DW)
		Threshold	During decline
HF	100	0.004-0.006	0.020
Cl2	0.3-30	0.10	0.25-0.40
SO2	0.1-10	0.11-0.14	0.3-0.5
NO2	0.03-3	---	---
* The toxicity of hydrogen fluoride was taken as 100 points (without regard for the high cumulative effect typical of fluorides) and a threshold concentration of fluorides was taken as the maximum content at which there was no visible sign of damage. The table is compiled from our own results and from a literature analysis.

The experiments were run in field chambers with 10- to 12-year-old Siberian larch trees as the objects of the study. The following gases were used for fumigation: (1) hydrogen fluoride; (2) chlorine; (3) sulphur dioxide; (4) carbon monoxide; (5) nitrogen oxides (NO and N02); (6) hydrogen fluoride and chlorine; (7) hydrogen fluoride and sulphur dioxide; (8) hydrogen fluoride and carbon monoxide; (9) hydrogen fluoride and nitrogen oxides; (10) chlorine, sulphur dioxide, nitrogen oxides and carbon monoxide; and (11) hydrogen fluoride, chlorine, sulphur dioxide, nitrogen oxides and carbon monoxide.

The gases were obtained by reaction of weighed amounts with an excess amount of a certain acid. Hydrofluoric acid was a source of hydrogen fluoride. The concentrations of gases, except for hydrogen fluoride, were determined in chambers with a universal field gas analyser UG-2 (USSR). The content of hydrogen fluoride was assayed photometrically. The following concentrations of pollutants were used in the experiments: 0.1-0.2 mg/m3 HF; 1.0 mg/m3 SO2; 1.0 mg/m3 Cl2; 2.0 mg/m3 NO(2); and up to 30 to 35 mg/m3 CO. The experiments were repeated five times. It was predetermined that a given concentration of a gas was maintained for 2-3h in the chambers, which were 2 m3 in volume, and than the concentration began to drop; therefore, reacting substances were regularly renewed in a reaction vessel.

The plants were fumigated for 18h/day. In the warmest part of the day, from 12 a.m. to 6 p.m., fumigation was stopped. The fumigation continued for 20 days. During this period the larch needles treated with 0.1-0.2 mg/m3 hydrogen fluoride suffered total damage. The damage to plants was estimated visually by assessing the number of necrotic needles in the tree crown (as a percentage of the total number of needles) (Table 2). Gases tested could be arranged in the following order in terms of declining toxicity to plants: HF, C12, S02, NO(2) and CO.

Table 2. The degree of visual damage of needles of 10- to 12-year-old Siberian larch trees by acid gases and gas mixtures.
Gas (mixture)	Concentration (mg/m3)	Duration of fumigation (days)	Amount of necrotic needles (%)
HF	0.1-0.2	18-20	90-100
Cl2	1.0	20	Up to 60
SO2	1.0	20	Up to 40
NO + NO2	2.0	20	No damage
CO	Up to 30-35	20	No damage
HF + SO2	0.1-0.2 + 1.0	3	80-100
HF + Cl2	0.1-0.2 + 1.0	2	80-100
HF + NO(2)	0.1-0.2 + 2.0	20	90-100
HF + CO	0.1-0.2 + 30.0	20	90-100
Cl2 + SO2 + NO(2) + CO	1.0 + 1.0 + 2.0 + 30.0	7-8	80-100
HF + Cl2 + SO2 + NO(2) + CO	0.1-0.2 + 1.0 + 1.0 + 2.0 + 30.0	0.5	80-100

No visible signs of damage were observed in larch fumigated with nitrogen oxides and carbon monoxide. It is known that carbon monoxide can be oxidized to carbon dioxide and then incorporated in the photosynthetic cycle. Nitrogen oxide is oxidized in air to become nitrogen dioxide or can be converted into gaseous nitrogen by means of photochemical reactions. Nitrogen oxide dissolves in water to form nitrate and nitrate ions which can be reduced to ammonia in leaf cells (Bennett and Hill, cited by Smith 1985). Ammonia interacts with ketonic cells to form amino acids (Kretovich 1980). The plant is also able to detoxify other acid gases. Thus, sulphur dioxide can be either oxidized to sulphate, which reduces its toxicity 30-fold (Tomas 1962), or reduced to hydrogen sulphide and incorporated into amino acids. We failed to obtain evidence for specific mechanisms of detoxification of chlorine and fluorine in plants. These elements are apparently neutralized by buffer systems in the cell (see Chapter 3).

When hydrogen fluoride is added with chlorine or sulphur dioxide plants suffer greater damage, even during a shorter period of fumigation. In other words, pronounced synergism is observed with exposure to mixtures of hydrogen fluoride and chlorine, or hydrogen fluoride and sulphur dioxide. When the plants were fumigated with mixtures of hydrogen fluoride and nitrogen oxides, or hydrogen fluoride and carbon monoxide there was no increase in damage and the extent of damage remained the same as for exposure to hydrogen fluoride alone. Hence, in this case, hydrogen fluoride has a dominating toxic effect. The fumigation of plants with a mixture of chlorine, sulphur dioxide, nitrogen oxides and carbon monoxide showed this mixture to be several times more toxic than chlorine and sulphur dioxide acting separately. A mixture of all the gases (hydrogen fluoride, chlorine, sulphur dioxide, nitrogen oxides and carbon monoxide) turned out to be the most destructive to plants. Needles exposed to this mixture die during 10-15h. In this case several synergetic effects, produced by mixtures of hydrogen fluoride and chlorine, hydrogen fluoride and sulphur dioxide, and chlorine and sulphur dioxide, become superimposed.

Thus, the fluorine-containing emission is the most dangerous to plants in the presence of atmospheric chlorine and sulphur dioxide. Hydrogen fluoride is a dominant pollutant in the presence of nitrogen oxides or carbon monoxide.

During experimental fumigation of trees with acid gases and gas mixtures we also found certain differences in the development of visible damage to the needles. The effect of hydrogen fluoride and sulphur dioxide on larch initially inducts development of chlorosis on apices of needles of auxiblasts. Chlorosis spreads through the needles, whose apices become brown or greyish-brown because of cell death. Then necrosis rapidly extends to all the needles of auxiblasts. A little later the needles of brachyblasts are damaged in the same manner and the damage spreads to all needles of the tree. Dead needles gradually fall from the tree. This process proceeds more rapidly in the presence of hydrogen fluoride and gas mixtures containing hydrogen fluoride. Fumigation with sulphur dioxide results in a longer period between the first appearance of chlorotic mottling and the death of needles; damaged needles are more lightly coloured than under exposure to hydrogen fluoride.

Other symptoms appear following the exposure of larch to chlorine. Needle colour also changes though there is no intense discoloration. First the needles of auxiblasts and then those of brachyblasts become grey-green and later on light grey. Owing to extensive dehydration the needles dry out and break readily. They gradually turn brown and finally become greyish brown. Initial symptoms of the damage caused by mixture of chlorine with sulphur dioxide, nitrogen oxide and carbon monoxide are the same as for chlorine alone, though in the last stage of dieback the needles turn dark brown.

The elucidation of specific symptoms of plant damage by pollutants is of considerable interest to researchers as it can be useful for diagnostic purposes. Our experiments revealed that certain specific visual symptoms of plant damage by acid gases only become apparent with exposure to high concentration of the pollutant. It is harder to characterize symptoms that result from long-term exposure to relatively low concentrations of acid gases, where is is difficult to pinpoint an injurious factor.

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