Báo cáo lâm nghiệp: "Physiological responses to air pollutants"

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Physiological responses to air pollutants G. Halbwachs Osterreich Zentrum fur Umwelt- und Naturschutz, Universitit fur Bodenkultur, Wien, trees, bushes, herbs, animals, the investigating the phenomenon of When tween the organisms that live in scale ’forest decline’, particularly its soil large including it and the special climatic condi- appearance in so-called clean air regions, and on tions. In the forest ecosystem with its plant physiology has gained considerable diversity in vegetation and animal life, a importance. Especially physiology, tree near equilibrium is reached between de- which deals with the life processes of trees, has again become interesting not composition and synthesizing processes. only for scientists, but also for the ecologi- Even though this equilibrium is rather labile because of permanent natural cally conscious public (Eschrich, 1987). changes, it still works very efficiently to Trees are long-lived organisms which maintain nutrients in the system. An addi- many decades pass through various over tional attribute is the ability of trees, whose stages of development (seedling, sapling, tops are strongly coupled with the atmo- young growth and old growth), each with sphere, to filter out dust and trace ele- its own distinctive physiological charac- ments, which ;are then integrated into the teristics. In addition to these variations, nutrient cycle. It is precisely this large fil- sensitivity varies during the daily and tering capacity that appears increasingly annual rhythm. Since trees tower over all to be a disadvantage for the forest in light other forms of vegetation, they have a of the present atmosphere load of pol- definite advantage in the struggle for light. lutants. Furthermore, they have evolved a system of compartmentalization which allows Because of of the attributes alrea- some them the loss of larger plant parts without dy discussed, it is understandable that substantially affecting their chances for trees have not often been studied by plant survival. Because of these attributes they physiologists. Some of the difficulties in possess a dominant position in a forest. investigating trees range from the carrying Nevertheless, the term forest not only experiments on tall trees and forest out of includes all closely interacting trees locat- stands to the interpretation of the gathered data. Small trees, which are easy to han- ed in a defined area, but it also includes dle as test objects, are usually only a few the complex structure of interactions be-
pleasant consequence is not only the years old and, therefore, are not com- parable in their physiological reactions to increased appreciation of tree physiology, but also the increase in funds for research. mature trees in forest stands. Large trees, The latter aspect has even allured some however, are practically impossible to place in an experimental situation. This is physiologists away from peas though - perhaps only for a short period of time. especially true from an aboveground The unpleasant consequence manifests microclimatic perspective. James Bonner itself in the growing impatience of politi- once said, &dquo;everything that can be done, can be done better with peas&dquo;, but, unfor- cians and the general public. They expect tree physiologists to bring forth prompt tunately, this does not apply to the study of and clear statements about the causes of woody, long-lived plants. the present forest damage. From what has The central problem of experimental already been said about research prob- forest research lies in the decision wheth- lems with trees, it is evident that, in tree er one carries out the experiments in labs physiology, it seems to be almost impos- or chambers with artificial but controlled sible to get quickly, universally applicable conditions or undertakes field studies with research results. ’Forest decline’ is a com- realistic conditions, but with the influence plex phenomenon which has only surfaced of many uncontrollable environmental fac- as a major research focus in the past few tors. Whenever the clarification of special questions or specific mechanisms concer- years. ning tree damage is desired, the first type Without delving into a discussion about of experiments would be chosen. The the causes of ’forest decline’, most scien- results of fumigation experiments on tists agree that diverse air pollutants of the young plants could be utilized for inter- acidic or oxidative type play a significant preting some effects when air pollution is role in this problem. the dominant stress factor. This experi- These air pollutants along with other mental approach is not adequate for more abiotic and biotic stresses account for precise analysis of the interaction between those conditions which could inhibit phy- air pollution and the forest ecosystem, siological processes. Since these physio- where not only emission stress is at hand, logical processes determine the quantity but a complex system involving many and quality of tree growth limited by gene- stress factors (Lefohn and Krupa, 1988). tic potential and directed by environmental Also, fumigation experiments using open- conditions, physiology as a science should topped chambers may not correctly model be strongly anchored in forestry. Unfortu- the coupling between forest trees and the nately, the role of physiology in this branch atmosphere (as reported by Dr. Jarvis in of science as Kramer (1986) regrettably - these proceedings). determined was often not correctly - This is surely a reason why today not understood. This has turned out to be a enough tree-specific physiological infor- disadvantage because it is difficult to dis- mation is available which is needed to cuss changes when one does not possess explain the intricate phenomenon of ’forest sufficient information about the original decline’ in its varied manifestations. conditions. For example, the first signs of The fact that knowledge about physio- injury from ’forest decline’ have been behavior of trees has become of logical found at the macroscopic level, even great importance today, leads to two though the of these disturbances causes consequences for tree physiologists found the cellular and subcellular are on a - pleasant and an unpleasant one. The important task for the plant phy- levels. An
is to determine the mechanisms discussed mainly in these papers topics siologist responsible for such damage and, if pos- I. listed in Table are The various test parameters listed in the sible, also the primary cause of it. Physio- logical criteria, however, should also help table changedl in the presence of air pollu- to quantify and differentiate the damage to tants, however, the mechanism of change trees. Above that, they should be capable has not been specified. Many of these of following the course of destruction and parameters also behave in a similar way its effects from the primary injury, which when exposed to other abiotic or biotic should be detected as early as possible, stresses, such as frost, heat, light, drought as well as fungus infection and insect until the death of the tree. Reports have attack. The isolated observation of these only recently been released concerning physiological and biochemical reactions of parameters is not useful when trying to place the reaction on a whole tree basis. trees and shrubs to different air pollutants For example, it is unrealistic to determine (Kozlowski and Constantinidou, 1986) as the vitality of ;a whole tree or canopy from well as physiological and biochemical changes in chlorophyll fluorescence in a changes within damaged trees (Lange and Zellner, 1986; Ziegler, 1986) and few needles. In order to be able to apply plant physiological criteria as an effective about the effects of gaseous air pollutants on forest trees from a plant physiological determinant, the suggestions from Weigel and Jager (1985) to compile and combine point of view (Weigel et aL, 1989). The
Tranquillini’s (1976) observations in the various physiological, biochemical and Alps, the thickness of the cuticle from chemical parameters to build a chain of spruce and stone pine needles decreases evidence should be tried. In this manner, the elevation and wind-exposure at least an indication of overlying toxicity as increase, which is at the same time principles can be achieved, such as the general acid effect, the formation of radi- connected with a higher transpiration rate. These factors determine not only the tim- cals and the role they play as well as the ber line in temperate zones, but could also destruction of membrane systems. be used to explain the often observed Unalterable assumptions for the investi- exceptional sensitivity of trees in the ridge gation of pollution effects using physiologi- areas of mountains. The ozone concentra- cal and biochemical parameters must tions which generally increase with the include the local emission situation and elevation (Smidt, 1983; Bucher et al., the consideration of the climatic condi- 1986) are correlated with a reduced quan- tions. tity and poorer quality of cuticle. Therefore, The knowledge of reactions which take these ozone concentrations can lead to re- place on the plant’s surface and inside it latively severe damage to trees, especially allows inferences about the various resis- under unfavorable weather conditions and tance mechanisms of trees in contact with shortening of the vegetation period. air pollutants. According to Levitt (1980), The stomata can play a role similar to two strategies can be distinguished: avoid- the cuticle with respect to the avoidance of ance and tolerance. While avoidance stra- absorption of gaseous substances, when tegies include the cuticle and the stomata, the absorbed substance causes the sto- tolerance plays a part whenever gaseous mata to close. Indeed, from the studies of air pollutants penetrate into the leaves or Black (1982) and Mansfield and Freer- needles. A few examples will demonstrate Smith (1984), it has been shown that sto- this. mata can open or close as a result of the The cuticular wax layers of the leaves penetration of pollutants. Considering the from trees present themselves as the pri- complexity of stomatal function, it is hard mary target for air-borne pollutants (Huttu- to make general statements about the nen and Soikkeli, 1984). These layers behavior of stomata in a certain emission function as a protection against wind, non- situation, particularly for field studies. The stomatal transpiration, frost, pathogenic results of changes in stomatal aperture or and insect attack as well as against the regulation - for instance, reduction of pol- penetration of air pollutants. Their erosion lution intake coupled with a reduced C0 2 and destruction introduce, on the one absorption or an increase in transpiration hand, a loss of the barrier which prohibits which leads to an excessive water loss - the intake of pollutants and, on the other both could greatly affect the plant’s hand, facilitates the leaching of essential metabolism. nutrients, leading to an increase in the effects of the damage already done by Only after the penetration of wet or dry pollutants. Destruction of the cuticle has the cuticle or deposited pollutants through been observed after the impact of various the stomata are metabolic processes acidic air pollutants (Ulrich, 1980; Huttu- affected both physically and biochemically. nen and Laine, 1983; Godzik and Halb- These processes considered together wachs, 1986), even though this has form the internal resistance (H511gren, often been discussed in connection with 1984; Unsworth, 1981The magnitude of ozone penetration. According to Baig and the internal resistance is responsible for
the tolerance of a plant species with re- Since radicals also found in normal are metabolism, cells have developed spect to air pollutants. This internal resis- a me- tance is determined among other things by chanism to >eliminate them. Enzymes, the solubility of each pollutant in the water such as superoxide dismutase (SOD), of the cell wall, which, when considered catalase and peroxidase, or molecules singly, could be used to rank the internal produced by the cell itself, such as ascor- toxicity of the various pollutants. Also vital bate, which acts as an anti-oxidant, play a decisive role in the plant’s detoxification for the plant’s tolerance strategy is its abili- ty either to degrade the penetrated pol- system and, therefore, also in its toler- ance. The increase in SOD found in poplar lutant or to inactivate it through chemical binding or to metabolize it into non-toxic leaves as well as pine and spruce needles reaction products. The latter case is likely after fumigation and also in ’forest decline’ areas points to a participation of the oxy- with those pollutants containing essential gen radical in the damaging of trees. elements, such as S0 or NO,. 2 Fluoride-con!taining air pollutants serve Ozone is an example of a pollutant as an example of how penetrated toxic which degrades inside the plant. Even ions in the cell are taken out of the plant’s though, when compared to S0 N0 or 22 , metabolism by chemical binding, for HF, ozone demonstrates less solubility, its example, with Ca and Mg cations. high chemical reactivity with unsaturated fatty acids, aromatic compounds and sulf- The tolerance of forest trees with re- hydril groups necessitates the mainte- spect to sulfur- and nitrogen-containing air nance of a steep concentration gradient pollutants is dependent upon their ability between the outside air and the inside of to transform these compounds, so that the plant. Various radicals also take part in they can be utilized in their own metabo- the phytotoxic effect of ozone (Tingey and lism. The oxidation of sulfur to sulfate Taylor, 1982; Elstner, 1984). They not are occurs either c!nzymatically or in a radical only a result of the reaction of mole- ozone chain reaction. The increases in nitrite- cules with sulfhydril groups or aromatic and nitrate-reductase activities after N02 and olefinic compounds, but also stem impact also indicate a change in metabo- from reactions of ozone with the water in lism, as in the increase of sulfur-containing the cell wall. Equally possible is the forma- glutathione after S0 impact (Wellburn, 2 tion of hydroxyl-, hydroperoxy- and super- 1982; Grill et a,L, 1982). oxide anion radicals. The reactions of The synergistic effects observed with ozone and radical oxygen compounds with pollutant combinations are more many the unsaturated fatty acids of the biomem- understandable when considering that the brane lead to the formation of lipid radicals detoxification mechanism for one of the and, in the presence of oxygen, lipid pollutants may be blocked in its function peroxides and lipid hydroperoxides (Bus by the other one. and Gibson, 1979; Halliwell and Gutte- ridge, 1985). Elstner (1984) takes the pro- The demand on scientists from the cess of lipid peroxidation as the initial of forestry to contribute applied areas reaction for destruction of the membrane more to the solution of real problems system, which is responsible for the life concerning forests cannot be quickly ful- preserving compartmentalization of the filled by tree physiologists because of the cell. The process of the destruction of the difficulties in experimentation, as demon- membrane promotes both the damaging of strated at the beginning of this paper. A cuticular wax and the leaching of nutrients. step in the right direction has been the
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