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SEM analysis of Commelina communis L. leaves after exposure to SO2 and NO2 pollution
Environmental Pollution (Series A) 42 (1986) 353-360
SEM Analysis of Commelina communis L. Leaves After Exposure to S O 2 and N O 2 Pollution P. C. P a n d e * & K. Oates Department of Biological Sciences,Universityof Lancaster, Lancaster LAI 4YQ, Great Britain
ABSTRACT S E M studies were made of the effects of SO 2 and N O 2 pollution on the leaves of Commelina communis. Plants at the two-leaf stage were exposed to a mixture of 100 nl litre-1 SO 2 and 100 nl litre-1 N O 2 f o r 10 days in recycling-type fumigation chambers. The first visible symptoms of injury appeared after 4 days on the abaxial surface of the oldest leaf in the form of glazed light brown patches along the margins. These symptoms also became visible on the adaxial leaf surface by the seventh day of exposure. S E M analysis of leaf epidermis showed that control plants had evenly distributed surface waxes while fumigated plants had wax accumulated in small heaps with large denuded areas. The palisade cells in fumigated leaves became flaccid due to loss of turgidity and there was a complete collapse of spongy tissue. Commelina communis thus appears to be extremely sensitive to an environment containing SO 2 + N O 2.
INTRODUCTION Plants have been used to estimate the risk that pollution presents to the biological components of the affected environment (Guderian, 1977). The response of plants to pollutants usually depends upon the concentration and duration of exposure. Although growth reduction without visible injury has been reported in many species, especially grasses (Ashenden, 1979; Ashenden & Williams, 1980; Whitmore & Mansfield, 1983), marked visible injury due to air pollutants is also not uncommon * Present address: Department of Botany, Meerut College, Meerut 250001, India. 353 Environ. Pollut. Ser. A 0143-1471/86/$03.50 © ElsevierApplied SciencePublishers Ltd, England, 1986. Printed in Great Britain
354
P. C. Pande, K. Oates
(Jacobson & Hill, 1970; Cathey & Heggested, 1972; Elkiey & Ormrod, 1979). The injury symptoms act as a sensitive early warning system which may stimulate prophylactic measures to prevent or diminish the more disastrous effects of air pollution (Posthumus, 1982). SO 2 and NO 2 are the most widespread air pollutants in industrialised countries. The main source of SO 2 is the combustion of sulphurcontaining fuels and that of NO X automobile exhausts. Reinert et al. (1975) and Ormrod (1982) concluded that these two gases interact to affect the plant, and they may be additive, antagonistic or synergistic, depending upon the species and the environmental conditions. Commelina communis is a small herbaceous plant growing widely in tropical and sub-tropical parts of the world. This study was initiated to identify the most assessable tissue injury targets in this species.
MATERIALS AND METHODS
Plant material Plants of Commelina communis were raised from seed in 12-cm pots containing John Innes No. 2 compost. The glasshouse temperature was maintained at 20°C. Daylength was 14h and the photon flux density was 1 0 0 m o l m - 2 s -1 at pot level. At the two-leaf stage, twenty pots were selected, each containing one plant of more or less the same leaf size. They were randomly allocated into two groups of ten pots each and each group was then transferred to the fumigation chambers.
Fumigation The fumigation chambers were of the recycling-type as described by Whitmore (1982). The chambers received charcoal-filtered air at a speed to provide one complete air change per minute, which was sufficient to prevent condensation on the leaves of the plants and walls of the chambers. Air entering into one chamber was mixed with SO 2 and NO 2, supplied in cylinders by British Oxygen Co. The concentration of SO 2 and NO 2 was maintained at 100nl litre -1. Pollutant levels were monitored continuously using a Meloy SA 285 flame photometric analyser for SO 2 and a Meloy NA 520 chemiluminiser for NO 2. The
S E M analysis
ofCommelina communis after
exposure to SO 2 and N O 2
355
second chamber was supplied with clean air only and served as the control. Both control and fumigation chambers were illuminated with a horizontally fitted metal halide lamp to provide photon flux density similar to that in the glasshouse. A capillary watering system was used to keep the plants in the chambers well watered. Fumigation continued for 10 days.
Scanning electron microscopy Three samples were taken from each plant from apparently healthy areas of the oldest leaves. To obtain scanning electron micrographs, square pieces of approximately 5 mm 2 were taken from either side of the midrib, both from control and fumigated plants. Each piece of plant material was mounted onto an aluminium specimen stub with TissueTek II adhesive (Miles Laboratories, Inc.) and quench frozen in nitrogen slush. The stub with the attached material was transferred under vacuum on to a cold pedestal 84K (-189°C) in the SEM air lock. Here the material was coated with 50 nm of gold by high vacuum evaporation. The specimen was then moved under high vacuum inside the scanning electron microscope (SEM) to a low temperature stage maintained at 81 K (-192°C). Specimens were examined at an accelerating voltage of 15kV.
RESULTS
Visible symptoms of leaf injury The first visible injury symptoms appeared on the abaxial leaf surfaces after 4 days of continuous exposure to a mixture of 100 nl litre-1 SO 2 and 100 nl litre- 1 NO 2. The injury was in the form of glazed light brown areas along the margins of the oldest leaf, which gradually extended acropetally, as well as laterally, towards the midrib. On the seventh day of exposure the injury symptoms also became visible on the adaxial leaf surfaces. The glazed brown areas soon turned into dark brownish-green water-soaked lesions. At the end of the fumigation period (when the leaves were harvested for SEM studies) approximately 20% of the area was injured in the oldest leaf. The extent of injury gradually decreased in second and third leaves.
Figs 1-6. Scanning electron micrographs of cuticular wax and mesophyll tissue of the oldest leaves of Commelina communis grown in an environment containing charcoalfiltered air or 100nllitre 1 SO2+ 100nllitre-t NO2. l. Abaxial leaf surface from control plant grown in charcoal-filtered air (original magnification x 3000). 2. Abaxial leaf surface from plant exposed to SO 2 + N O 2 (original magnification x 1000); 3 and 4. Palisade and spongy tissue of leaf from control plant grown in charcoal-filtered air (original magnification × 600); 5 and 6. Palisade and spongy tissue of leaves exposed to SO 2 + NO 2 (original magnification x 600).
S E M analysis of Commelina communis after exposure to SO 2 and NO 2
357
SEM studies Cuticular wax
The leaf epidermis of C o m m e l i n a c o m m u n i s has a waxy outer covering, the wax being of the amorphous type. SEM analysis showed that the control plants had evenly distributed surface waxes (Fig. 1). In fumigated leaves small heaps of wax were left behind after the erosion of the surrounding areas (Fig. 2). M e s o p h y l l tissue C. c o m m u n i s leaves are dorsiventral with well differentiated palisade
and spongy tissue. On the adaxial surface there is a single layer of compactly arranged palisade cells with small intercellular spaces (Fig. 3). This is followed by several layers of spongy parenchyma with loosely arranged cells enclosing large air spaces (Fig. 4). In fumigated leaves the palisade cells lost their turgidity and became flaccid (Fig. 5). There was a complete collapse of the spongy tissue, the shape of the cells was completely lost and the intercellular spaces were more or less obliterated (Fig. 6). However, there was much less damage to the leaf epidermis compared with the mesophyll tissue.
DISCUSSION C o m m e l i n a c o m m u n i s appears to be extremely sensitive to an environment containing SO 2 + NO 2. Injury symptoms on the abaxial leaf surface appeared at least 2 days earlier than on the adaxial surface. This differential sensitivity of two surfaces may be due to higher abaxial conductance coupled with the greater intercellular volume of the spongy parenchyma. The adaxial surface has poor conductance due to the compactness of the palisade tissue. This reduces the flux of gaseous pollutants into the leaf interior. The higher sensitivity of old and mature leaves observed in the present study is similar to the situation described for many other plant species (Barrett & Benedict, 1970; Stern et al., 1973; Elkiey & Ormrod, 1979). This is probably due to the increase in intercellular spaces at maturity, and other histological changes that facilitate more rapid gaseous diffusion. Cuticular waxes regulate the diffusion of water and gases and serve
358
P. C. Pande, K. Oates
as a barrier to air pollutants (Sch6nherr, 1976). A thicker cuticle provides a much higher tolerance, because the diffusion time through the cuticle increases with the square of the distance (Nobel, 1970). The pollutioninduced erosion of cuticular wax of the Commelina leaves used here exposed a major portion of the epidermis and the diffusion of pollutants through the epidermis was probably accelerated. Damage to the cuticle may also increase cuticular transpiration, and this might lead to water stress in a polluted environment. Stomata are the main portals by which gaseous pollutants enter the leaf, and one of the factors determining tolerance of a species is stomatal conductance (Mansfield, 1973; Thorne & Hanson, 1976). Within the leaves, pollutants may accumulate in the intercellular spaces of the mesophyll tissue, which will be damaged when the threshold of tolerance is exceeded (Solberg & Adams, 1956; Thomas, 1956). The preferential damage to the spongy parenchyma is evidently due to the presence of large intercellular Spaces which accumulate pollutants in amounts higher than the metabolic capacity of the tissue. 5032 and N O 3 2 i o n s thus accumulated are toxic to the cell in various ways; for example, they inhibit photosynthetic capacity (Ziegler, 1975) and electron flow within the chloroplasts (Nieboer et al., 1976). The collapse of the mesophyll tissue is presumably due to plasmolysis. The internal leaf atmosphere is thought to be at nearly 100% relative humidity, and a thin layer of water exists around each cell (Heath, 1975). The acidic solution formed by the dissolution of SO 2 and N O 2 in moist cell layers promotes plasmolysis. Spongy parenchyma has a weak architecture due to the presence of large air spaces, and the plasmolysis of the cells causes the complete collapse of this tissue, after which hardly any air space is left for the exchange of gases for important metabolic processes like photosynthesis and respiration. The supply of energy is thus further reduced at a time when it is urgently required for the repair of injured tissue. Flowering plants such as the tobacco variety Bel-W 3, Gladiolus, Medicago sativa and Fagopyrum have been widely used as bioindicators for gaseous pollutants in various parts of the world (Ashmore et al., 1978; Posthumus, 1982; Laurence et al., 1985). Posthumus (1982) has outlined the advantages of using plants as pollutant indicators. The ambient concentration of primary pollutants such as SO 2 and N O 2 in many industrialised areas is above 200nllitre-1. Commelina communis was found to be sensitive to these pollutants at even lower
SEM analysis of Commelina communis after exposure to SO 2 and NO 2
359
concentrations. Severe lesions appeared on the leaves after only 4 days o f exposure, thus making it a potential bioindicator. ACKNOWLEDGEMENTS The authors would like to thank Professor T. A. Mansfield for his advice and critical review o f this work. Thanks are also due to D r V. Singh o f Meerut University for helpful discussion during the preparation o f this paper.
REFERENCES Ashenden, T. W. (1979). The effects of long-term exposures to SO 2 and N O 2 pollution on the growth of Dactylis glomerata L. and Poa pratensis L., Environ. Pollut., 18, 249-58. Ashenden, T. W. & Williams, I. A. D. (1980). Growth reductions in Lolium multiflorum Lam. and Phleum pratense L. as a result of SO 2 and NO 2 pollution. Environ Pollut., Ser. A, 21, 131-9. Ashmore, M. R., Bell, J. N. B. & Reily, C. L. (1978). A survey of ozone levels in the British Isles using indicator plants. Nature, Lond., 276, 814-15. Barrett, T. W. & Benedict, H. M. (1970). Sulfur dioxide. In Recognition of air pollution injury to vegetation: A pictorial atlas. Ed. by J. S. Jacobson and A. C. Hill, CI-CI 7, Pittsburgh, PA, Air Pollution Control Association. Cathey, H. M. & Heggested, H. E. (1972). Reduction of ozone damage to Petunia hybrida Vilm. by use of growth regulating chemicals and tolerant cultivars. J. Am. Soc. hort. Sci., 97, 695-700. Elkiey, T. & Ormrod, D. P. (1979). Petunia cultivar sensitivity to ozone and sulphur dioxide. Scientia Hort., 11, 269-80. Guderian, R. (1977). Discussion of the suitability of plant responses as a basis for air pollution control measures. In Air pollution: Phytotoxicity of acidic gases and its significance in air pollution control, ed. by R. Guderian, 75112. Berlin, Springer-Verlag. Heath, R. L. (1975). Ozone. In Responses of plants to air pollution, ed. by J. B. Mudd and T. T. Kozlowski, 23-55, Academic Press, New York. Jacobson, J. S. & Hill, A. C. (1970). Recognition of air pollution injury to vegetation: A pictorial atlas, Pittsburgh, PA, Air Pollution Control Association. Laurence, J. A., Reynolds, K. L. & Greitner, C. S. (1985). Bioindicators of SO2: Response of three plant species to variation in dosage-kinetics of SO 2. Environ. Pollut., Ser. A, 37, 43-52. Mansfield, T. A. (1973). The role of stomata in determining the responses of plants to air pollutants. Current Adv. Plant Sci., 2, 11-20.
360
P. C. Pande, K. Oates
Nieboer, E., Richardson, D. H. S., Puckett, K. J. & Tomassini, F. D. (1976). The phytotoxicity of sulfur dioxide in relation to measurable responses in lichens. In Effects of air pollution on plants, ed. by T. A. Mansfield, 6685, Cambridge, Cambridge University Press. Nobel, P. S. (1970). Biophysical plant physiology. San Francisco, W. H. Freeman. Ormrod, D. P. (1982). Air pollutant interactions in mixture. In Effects of gaseous air pollution in agriculture and horticulture. Proc. School in Agricultural Science, University of Nottingham, 32nd, ed. by M. H. Unsworth and D. P. Ormrod, 307-31. London, Butterworth. Posthumus, A. C. (1982). Biological indicators of air pollution. In Effects of gaseous air pollution in agriculture and horticulture. Proc. School in Agric. Sci., University of Nottingham, 32nd, ed. by M. H. Unsworth and D. P. Ormrod, 27-42. London, Butterworth. Reinert, R. A., Heagle, A. S. & Heck, W. W. (1975). Plant response to pollutant combinations. In Responses of plants to air pollutants, ed. by J. B. Mudd & T. T. Kozlowski, 159-78, New York, Academic Press. Sch6nherr, J. (1976). Water permeability of isolated cuticular membranes: The effect of cuticular waxes on diffusion of water. Planta, Berl., 131, 159-64. Solberg, R. A. & Adams, D. F. (1956). Histological responses of some plant leaves to hydrogen fluoride and sulfur dioxide. Am. J. Bot., 43, 755-60. Stern, A. C.,Wohlers, H. C., Boubel, R. W. & Lowry, W. P. (1973). Fundamentals of air pollution. New York, Academic Press. Thomas, M. D. (1956). The invisible injury theory of plant damage. J. Air Pollut. Control Ass., 5, 205-6. Thorne, L. & Hanson, G. R. (1976). Relationship between genetically controlled o z o n e sensitivity and gas exchange rate in Petunia hybrida Vilm. J. Am. Soc. hort. Sci., 101, 60-3. Whitmore, M. E. (1982). A study of the effects of SO z and NO 2 pollution on grasses with special reference to Poa pratensis. L. PhD thesis, Lancaster University. Whitmore, M. E. & Mansfield, T. A. (1983). Effects of long-term exposure to SO 2 and NO 2 on Poa pratensis and other grasses. Environ. Pollut., 31, Ser A, 217-35. Ziegler, I. (1975). The effect of SO: pollution on plant metabolism. Residue Rev., 56, 79-105.
SEM Analysis of Commelina communis L. Leaves After Exposure to S O 2 and N O 2 Pollution P. C. P a n d e * & K. Oates Department of Biological Sciences,Universityof Lancaster, Lancaster LAI 4YQ, Great Britain
ABSTRACT S E M studies were made of the effects of SO 2 and N O 2 pollution on the leaves of Commelina communis. Plants at the two-leaf stage were exposed to a mixture of 100 nl litre-1 SO 2 and 100 nl litre-1 N O 2 f o r 10 days in recycling-type fumigation chambers. The first visible symptoms of injury appeared after 4 days on the abaxial surface of the oldest leaf in the form of glazed light brown patches along the margins. These symptoms also became visible on the adaxial leaf surface by the seventh day of exposure. S E M analysis of leaf epidermis showed that control plants had evenly distributed surface waxes while fumigated plants had wax accumulated in small heaps with large denuded areas. The palisade cells in fumigated leaves became flaccid due to loss of turgidity and there was a complete collapse of spongy tissue. Commelina communis thus appears to be extremely sensitive to an environment containing SO 2 + N O 2.
INTRODUCTION Plants have been used to estimate the risk that pollution presents to the biological components of the affected environment (Guderian, 1977). The response of plants to pollutants usually depends upon the concentration and duration of exposure. Although growth reduction without visible injury has been reported in many species, especially grasses (Ashenden, 1979; Ashenden & Williams, 1980; Whitmore & Mansfield, 1983), marked visible injury due to air pollutants is also not uncommon * Present address: Department of Botany, Meerut College, Meerut 250001, India. 353 Environ. Pollut. Ser. A 0143-1471/86/$03.50 © ElsevierApplied SciencePublishers Ltd, England, 1986. Printed in Great Britain
354
P. C. Pande, K. Oates
(Jacobson & Hill, 1970; Cathey & Heggested, 1972; Elkiey & Ormrod, 1979). The injury symptoms act as a sensitive early warning system which may stimulate prophylactic measures to prevent or diminish the more disastrous effects of air pollution (Posthumus, 1982). SO 2 and NO 2 are the most widespread air pollutants in industrialised countries. The main source of SO 2 is the combustion of sulphurcontaining fuels and that of NO X automobile exhausts. Reinert et al. (1975) and Ormrod (1982) concluded that these two gases interact to affect the plant, and they may be additive, antagonistic or synergistic, depending upon the species and the environmental conditions. Commelina communis is a small herbaceous plant growing widely in tropical and sub-tropical parts of the world. This study was initiated to identify the most assessable tissue injury targets in this species.
MATERIALS AND METHODS
Plant material Plants of Commelina communis were raised from seed in 12-cm pots containing John Innes No. 2 compost. The glasshouse temperature was maintained at 20°C. Daylength was 14h and the photon flux density was 1 0 0 m o l m - 2 s -1 at pot level. At the two-leaf stage, twenty pots were selected, each containing one plant of more or less the same leaf size. They were randomly allocated into two groups of ten pots each and each group was then transferred to the fumigation chambers.
Fumigation The fumigation chambers were of the recycling-type as described by Whitmore (1982). The chambers received charcoal-filtered air at a speed to provide one complete air change per minute, which was sufficient to prevent condensation on the leaves of the plants and walls of the chambers. Air entering into one chamber was mixed with SO 2 and NO 2, supplied in cylinders by British Oxygen Co. The concentration of SO 2 and NO 2 was maintained at 100nl litre -1. Pollutant levels were monitored continuously using a Meloy SA 285 flame photometric analyser for SO 2 and a Meloy NA 520 chemiluminiser for NO 2. The
S E M analysis
ofCommelina communis after
exposure to SO 2 and N O 2
355
second chamber was supplied with clean air only and served as the control. Both control and fumigation chambers were illuminated with a horizontally fitted metal halide lamp to provide photon flux density similar to that in the glasshouse. A capillary watering system was used to keep the plants in the chambers well watered. Fumigation continued for 10 days.
Scanning electron microscopy Three samples were taken from each plant from apparently healthy areas of the oldest leaves. To obtain scanning electron micrographs, square pieces of approximately 5 mm 2 were taken from either side of the midrib, both from control and fumigated plants. Each piece of plant material was mounted onto an aluminium specimen stub with TissueTek II adhesive (Miles Laboratories, Inc.) and quench frozen in nitrogen slush. The stub with the attached material was transferred under vacuum on to a cold pedestal 84K (-189°C) in the SEM air lock. Here the material was coated with 50 nm of gold by high vacuum evaporation. The specimen was then moved under high vacuum inside the scanning electron microscope (SEM) to a low temperature stage maintained at 81 K (-192°C). Specimens were examined at an accelerating voltage of 15kV.
RESULTS
Visible symptoms of leaf injury The first visible injury symptoms appeared on the abaxial leaf surfaces after 4 days of continuous exposure to a mixture of 100 nl litre-1 SO 2 and 100 nl litre- 1 NO 2. The injury was in the form of glazed light brown areas along the margins of the oldest leaf, which gradually extended acropetally, as well as laterally, towards the midrib. On the seventh day of exposure the injury symptoms also became visible on the adaxial leaf surfaces. The glazed brown areas soon turned into dark brownish-green water-soaked lesions. At the end of the fumigation period (when the leaves were harvested for SEM studies) approximately 20% of the area was injured in the oldest leaf. The extent of injury gradually decreased in second and third leaves.
Figs 1-6. Scanning electron micrographs of cuticular wax and mesophyll tissue of the oldest leaves of Commelina communis grown in an environment containing charcoalfiltered air or 100nllitre 1 SO2+ 100nllitre-t NO2. l. Abaxial leaf surface from control plant grown in charcoal-filtered air (original magnification x 3000). 2. Abaxial leaf surface from plant exposed to SO 2 + N O 2 (original magnification x 1000); 3 and 4. Palisade and spongy tissue of leaf from control plant grown in charcoal-filtered air (original magnification × 600); 5 and 6. Palisade and spongy tissue of leaves exposed to SO 2 + NO 2 (original magnification x 600).
S E M analysis of Commelina communis after exposure to SO 2 and NO 2
357
SEM studies Cuticular wax
The leaf epidermis of C o m m e l i n a c o m m u n i s has a waxy outer covering, the wax being of the amorphous type. SEM analysis showed that the control plants had evenly distributed surface waxes (Fig. 1). In fumigated leaves small heaps of wax were left behind after the erosion of the surrounding areas (Fig. 2). M e s o p h y l l tissue C. c o m m u n i s leaves are dorsiventral with well differentiated palisade
and spongy tissue. On the adaxial surface there is a single layer of compactly arranged palisade cells with small intercellular spaces (Fig. 3). This is followed by several layers of spongy parenchyma with loosely arranged cells enclosing large air spaces (Fig. 4). In fumigated leaves the palisade cells lost their turgidity and became flaccid (Fig. 5). There was a complete collapse of the spongy tissue, the shape of the cells was completely lost and the intercellular spaces were more or less obliterated (Fig. 6). However, there was much less damage to the leaf epidermis compared with the mesophyll tissue.
DISCUSSION C o m m e l i n a c o m m u n i s appears to be extremely sensitive to an environment containing SO 2 + NO 2. Injury symptoms on the abaxial leaf surface appeared at least 2 days earlier than on the adaxial surface. This differential sensitivity of two surfaces may be due to higher abaxial conductance coupled with the greater intercellular volume of the spongy parenchyma. The adaxial surface has poor conductance due to the compactness of the palisade tissue. This reduces the flux of gaseous pollutants into the leaf interior. The higher sensitivity of old and mature leaves observed in the present study is similar to the situation described for many other plant species (Barrett & Benedict, 1970; Stern et al., 1973; Elkiey & Ormrod, 1979). This is probably due to the increase in intercellular spaces at maturity, and other histological changes that facilitate more rapid gaseous diffusion. Cuticular waxes regulate the diffusion of water and gases and serve
358
P. C. Pande, K. Oates
as a barrier to air pollutants (Sch6nherr, 1976). A thicker cuticle provides a much higher tolerance, because the diffusion time through the cuticle increases with the square of the distance (Nobel, 1970). The pollutioninduced erosion of cuticular wax of the Commelina leaves used here exposed a major portion of the epidermis and the diffusion of pollutants through the epidermis was probably accelerated. Damage to the cuticle may also increase cuticular transpiration, and this might lead to water stress in a polluted environment. Stomata are the main portals by which gaseous pollutants enter the leaf, and one of the factors determining tolerance of a species is stomatal conductance (Mansfield, 1973; Thorne & Hanson, 1976). Within the leaves, pollutants may accumulate in the intercellular spaces of the mesophyll tissue, which will be damaged when the threshold of tolerance is exceeded (Solberg & Adams, 1956; Thomas, 1956). The preferential damage to the spongy parenchyma is evidently due to the presence of large intercellular Spaces which accumulate pollutants in amounts higher than the metabolic capacity of the tissue. 5032 and N O 3 2 i o n s thus accumulated are toxic to the cell in various ways; for example, they inhibit photosynthetic capacity (Ziegler, 1975) and electron flow within the chloroplasts (Nieboer et al., 1976). The collapse of the mesophyll tissue is presumably due to plasmolysis. The internal leaf atmosphere is thought to be at nearly 100% relative humidity, and a thin layer of water exists around each cell (Heath, 1975). The acidic solution formed by the dissolution of SO 2 and N O 2 in moist cell layers promotes plasmolysis. Spongy parenchyma has a weak architecture due to the presence of large air spaces, and the plasmolysis of the cells causes the complete collapse of this tissue, after which hardly any air space is left for the exchange of gases for important metabolic processes like photosynthesis and respiration. The supply of energy is thus further reduced at a time when it is urgently required for the repair of injured tissue. Flowering plants such as the tobacco variety Bel-W 3, Gladiolus, Medicago sativa and Fagopyrum have been widely used as bioindicators for gaseous pollutants in various parts of the world (Ashmore et al., 1978; Posthumus, 1982; Laurence et al., 1985). Posthumus (1982) has outlined the advantages of using plants as pollutant indicators. The ambient concentration of primary pollutants such as SO 2 and N O 2 in many industrialised areas is above 200nllitre-1. Commelina communis was found to be sensitive to these pollutants at even lower
SEM analysis of Commelina communis after exposure to SO 2 and NO 2
359
concentrations. Severe lesions appeared on the leaves after only 4 days o f exposure, thus making it a potential bioindicator. ACKNOWLEDGEMENTS The authors would like to thank Professor T. A. Mansfield for his advice and critical review o f this work. Thanks are also due to D r V. Singh o f Meerut University for helpful discussion during the preparation o f this paper.
REFERENCES Ashenden, T. W. (1979). The effects of long-term exposures to SO 2 and N O 2 pollution on the growth of Dactylis glomerata L. and Poa pratensis L., Environ. Pollut., 18, 249-58. Ashenden, T. W. & Williams, I. A. D. (1980). Growth reductions in Lolium multiflorum Lam. and Phleum pratense L. as a result of SO 2 and NO 2 pollution. Environ Pollut., Ser. A, 21, 131-9. Ashmore, M. R., Bell, J. N. B. & Reily, C. L. (1978). A survey of ozone levels in the British Isles using indicator plants. Nature, Lond., 276, 814-15. Barrett, T. W. & Benedict, H. M. (1970). Sulfur dioxide. In Recognition of air pollution injury to vegetation: A pictorial atlas. Ed. by J. S. Jacobson and A. C. Hill, CI-CI 7, Pittsburgh, PA, Air Pollution Control Association. Cathey, H. M. & Heggested, H. E. (1972). Reduction of ozone damage to Petunia hybrida Vilm. by use of growth regulating chemicals and tolerant cultivars. J. Am. Soc. hort. Sci., 97, 695-700. Elkiey, T. & Ormrod, D. P. (1979). Petunia cultivar sensitivity to ozone and sulphur dioxide. Scientia Hort., 11, 269-80. Guderian, R. (1977). Discussion of the suitability of plant responses as a basis for air pollution control measures. In Air pollution: Phytotoxicity of acidic gases and its significance in air pollution control, ed. by R. Guderian, 75112. Berlin, Springer-Verlag. Heath, R. L. (1975). Ozone. In Responses of plants to air pollution, ed. by J. B. Mudd and T. T. Kozlowski, 23-55, Academic Press, New York. Jacobson, J. S. & Hill, A. C. (1970). Recognition of air pollution injury to vegetation: A pictorial atlas, Pittsburgh, PA, Air Pollution Control Association. Laurence, J. A., Reynolds, K. L. & Greitner, C. S. (1985). Bioindicators of SO2: Response of three plant species to variation in dosage-kinetics of SO 2. Environ. Pollut., Ser. A, 37, 43-52. Mansfield, T. A. (1973). The role of stomata in determining the responses of plants to air pollutants. Current Adv. Plant Sci., 2, 11-20.
360
P. C. Pande, K. Oates
Nieboer, E., Richardson, D. H. S., Puckett, K. J. & Tomassini, F. D. (1976). The phytotoxicity of sulfur dioxide in relation to measurable responses in lichens. In Effects of air pollution on plants, ed. by T. A. Mansfield, 6685, Cambridge, Cambridge University Press. Nobel, P. S. (1970). Biophysical plant physiology. San Francisco, W. H. Freeman. Ormrod, D. P. (1982). Air pollutant interactions in mixture. In Effects of gaseous air pollution in agriculture and horticulture. Proc. School in Agricultural Science, University of Nottingham, 32nd, ed. by M. H. Unsworth and D. P. Ormrod, 307-31. London, Butterworth. Posthumus, A. C. (1982). Biological indicators of air pollution. In Effects of gaseous air pollution in agriculture and horticulture. Proc. School in Agric. Sci., University of Nottingham, 32nd, ed. by M. H. Unsworth and D. P. Ormrod, 27-42. London, Butterworth. Reinert, R. A., Heagle, A. S. & Heck, W. W. (1975). Plant response to pollutant combinations. In Responses of plants to air pollutants, ed. by J. B. Mudd & T. T. Kozlowski, 159-78, New York, Academic Press. Sch6nherr, J. (1976). Water permeability of isolated cuticular membranes: The effect of cuticular waxes on diffusion of water. Planta, Berl., 131, 159-64. Solberg, R. A. & Adams, D. F. (1956). Histological responses of some plant leaves to hydrogen fluoride and sulfur dioxide. Am. J. Bot., 43, 755-60. Stern, A. C.,Wohlers, H. C., Boubel, R. W. & Lowry, W. P. (1973). Fundamentals of air pollution. New York, Academic Press. Thomas, M. D. (1956). The invisible injury theory of plant damage. J. Air Pollut. Control Ass., 5, 205-6. Thorne, L. & Hanson, G. R. (1976). Relationship between genetically controlled o z o n e sensitivity and gas exchange rate in Petunia hybrida Vilm. J. Am. Soc. hort. Sci., 101, 60-3. Whitmore, M. E. (1982). A study of the effects of SO z and NO 2 pollution on grasses with special reference to Poa pratensis. L. PhD thesis, Lancaster University. Whitmore, M. E. & Mansfield, T. A. (1983). Effects of long-term exposure to SO 2 and NO 2 on Poa pratensis and other grasses. Environ. Pollut., 31, Ser A, 217-35. Ziegler, I. (1975). The effect of SO: pollution on plant metabolism. Residue Rev., 56, 79-105.