Experimental fumigation studies on the effects of fluoride on Australian native vegetation

Abstract

Experimental fumigation studies were conducted to investigate the effects induced by fluoride in plants and to rank some Australian native plant species according to their susceptibility to injury by this pollutant. Because of the numerous environmental, edaphic and biological factors which influence plant response to fluoride, the results of these experimental fumigations cannot be applied directly to a particular field situation.Two types of fumigation were conducted. The first involved exposure of single attached leaves for short time periods, usually 30 minutes, in the laboratory. High fluoride concentrations were used in order to produce a response within this time, and continuous measurements of the rates of net photosynthesis and transpiration were made during and following exposure. The second type of fumigation involved exposure of whole plants for periods of up to 8 weeks in naturally-illuminated, air-conditioned growth chambers specially constructed for this purpose. Fluoride concentrations of 2 to 15 µg m-3 were used in these studies, with plant response assessed from visible injury, effects on rates of gas exchange and/or effects of plant growth. Most published work on fluoride-induced effects in plants have concentrated on visible symptoms, and in the research described in this thesis, these effects were correlated with metabolic effects, measured through rates of gas exchange, and effects on growth.Thirty-nine species, including 31 native to Australia, were classed according to their susceptibility to fluoride pollution. The Australian species, none of which had previously been ranked in this way, fell within the general range of susceptibility of exotic species Physiological evidence of injury measured as a reduction in the rate of net CO2 uptake was compared with visible symptoms of injury as a criterion for assessing relative susceptibility. The former proved a more sensitive criterion although the relative species groupings were usually independent of the assessment used.Net photosynthesis appeared to be the first and most severely affected metabolic process during fluoride fumigation and evidence is provided that this was due to a direct effect on the photosynthetic apparatus. Ultrastructural and histological comparisons of fumigated and control leaves indicated that chloroplasts were an early site of fluoride attack. Analysis of photosynthetic response supported this evidence. At low fluoride dosages, the rate of net photosynthesis was stimulated, with the degree and duration of this phase dependent both on fluoride concentration and plant sensitivity.Chronic fluoride exposure reduced the growth rate of plants when the concentration was greater than the threshold for the particular species. The threshold concentration or time for effects on growth was normally higher than for altered rates of net photosynthesis.Investigations were conducted into the effects on both foliar fluoride concentration and plant response of the pattern of exposure to fluoride, whether continuous or intermittent, as well as ambient concentration and total duration of exposure. In general, a greater foliar concentration of fluoride could be accumulated from the same ambient level with less injury when exposure was continuous than when intermittent. During intermittent exposures, the amount of fluoride accumulated depended on both the length of individual exposure episodes and the time between successive episodes. Plant response to intermittent exposure was dependent on species, and evidence is provided that this may be due to differences in stomatal response to fluoride. Even when fumigation was continuous, there was not a direct relationship between fluoride dose and plant response.Time-concentration relationships were developed for representative species to enable prediction of the severity of injury as visible symptoms or depression of net photosynthesis. The confidence limits for these relationships are largely determined by the modification of plant response by different environmental conditions or patterns of exposure as occur in the field.Measurements showed that significant disruption of normal plant metabolism could occur without the appearance of visible symptoms of injury, the latter criterion being that commonly used for assessment of fluoride-induced effects in plants. It is advocated that air quality standards be described in terms of the probable effects of the specified concentrations on different levels of biological organisation. The effect on plants should also be related to their role in a commercial, aesthetic and/or ecological sense.