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The effect of mercury on some aquatic plants
j
The Effect of Mercury on Some Aquatic Plants
G. N. M h a t r e * & S. B. C h a p h e k a r Department of Botany, The Institute of Science, 15 Madam Cama Road, Bombay 400 032, India
ABSTRACT Three aquatic plants, Hydrilla verticillata Presl, Pistia stratiotes L. and Salvinia molesta D.S. Mitchell, were treated with different concentrations of mercury ranging from 1 to 1000 #g litre- 1 at three different exposure durations, i.e. 1, 3 and5 h. All were found to be severely affected by mercury. Foliar injury, chlorophyll content and phytomass showed perceptible effects with increasing exposure to the metal. In the case of .floating plants a positive relationship was obtained between Leaf Injury Index (LII) and doses of the metal. The possible use of aquatic plants in general, and floating plants in particular, as simple bioassay material in biomonitoring and toxicity studies is discussed with special reference to L I I as a simple biomonitoring parameter.
INTRODUCTION Many lower plants (Nieboer et al., 1972; Little & Martin, 1974; Wielgolaski, 1975; Blinn et al., 1977) and higher plants (Antonovics et al., 1971 ; Bradshaw, 1976; Ernst & Bast-Cramer, 1980) have been studied for effects of toxic environmental pollutants. Some of the easily observed effects are foliar injury and suppression of vegetative and reproductive growth of species exposed to pollutants. In extreme cases, suppression of * Present address: 3/25 Worli Gopchar, 128 Dr. Annie Besant Road, Worli, Bombay 400018, India. 207 Environ. Pollut. Ser. A. 0143-1471/85/$03.30 .c5)ElsevierApplied SciencePublishers Ltd, England, 1985. Printed in Great Britain
208
G. N. Mhatre, S. B. Chaphekar
growth leads to elimination of susceptible species. Many instances of species-poor vegetation prevalent in contaminated areas have been reported (Mhatre et al., 1980). Tolerant species in such areas have a high indicator value, for long employed in geoprospecting (see review by Antonovics et al., 1971). In short-term studies within the same generation of plants it is, however, safer to use plant injury as a simple indication of a polluted environment. Numerous attempts have been made to relate foliar injury and growth suppression to intensity of atmospheric pollution (Chaphekar et al., 1980), although the same has rarely been attempted in the case of water pollution. An effort in this direction is reported here, aiming to quantify plant injury in a simple way for convenient use as an index of the level of water pollution due to a heavy metal, mercury, reported in effluents around the city of Bombay (Maharashtra Prevention of Water Pollution Board, 1975).
MATERIALS AND METHODS Three aquatic plant species, Hydrilla verticillata Presl, Pistia stratiotes L. and Salvinia molesta D.S. Mitchell, were collected from a local, uncontaminated pond and kept in aquaria containing tap water. After 24 h the plants were transferred to aquaria containing known concentrations of Hg. A stock solution of Hg was prepared using HgCI z, as given by Sandell (1959), and from which 1, 10, 100 and 1000/~glitre -1 Hg concentrations were obtained through dilutions in distilled water. The plants were exposed to the test concentrations for 1,3 and 5 h each and then transferred to aquaria containing tap water. The aquaria were artificially aerated. Twenty-four hours after transfer to tap water the area of visible leaf injury was recorded on graph paper. The Leaf Injury Index (LII) values for P. stratiotes and S. molesta were calculated as a product of percentage leaf area injured and percentage of leaves showing injury (Mhatre & Chaphekar, 1984). Chlorophyll content of the plants was determined by the method of Arnon (1949). Some treated plant material was dried at 80 + 1 °C in an air-oven and its dry weight recorded. Comparison with the control showed a loss of weight as a result of the treatment. Significance of results was tested by applying Student's t-test.
Effect of mercury on aquatic plants
209
RESULTS A N D DISCUSSION
Foliar injury All three plant species were found to be susceptible to Hg. The injury was visible in the form of chlorosis during and after 24h of treatment. Intensity of injury increased with increasing metal concentration. Yellowish-red patches, visible at 10/aglitre -1, became intense reddish brown at 100 #g litre- 1. At 1000 #g litre- 1 concentration, total chlorosis of leaves, with a brownish tinge in between, resulted. The level of injury was also found to increase with increased exposure, with slight injury at 1 h, becoming progressively more severe at 3 h and 5 h duration. The pattern of injury varied with species. In H. verticillata the injury was perceptible within minutes after the plant came in contact with the toxicant, in the form of a chlorotic appearance, especially at concentrations of 100 and 1000 #g litre- 1. The injury was visible even at 1 h exposure and was irreversible at 3 h and 5 h. The stem eventually fragmented on touch, indicating collapse of the plant body. In P. stratiotes and S. molesta the pattern of injury was different. In both cases the submerged parts coming in direct contact with the toxic metal were affected first. At the two higher mercury concentrations the roots started showing change, even after 1 h, from off-white to yellowish red and reddish brown. At higher exposure durations the same concentrations proved lethal, the roots becoming detached from the rhizomes at the slightest touch. Whorls of leaves were the next to show injury. In P. stratiotes the outermost whorl was the first to show chlorosis, followed by the middle and the innermost whorls in that order. The ventral surface of the leaves in the outermost whorl became yellowish to yellowish red at higher concentrations. The symptoms intensified with increasing exposure. The damage then also spread to the dorsal surface. The veins showed a yellow tinge, initially at a slow rate but later on quickly changed to yellowish red with increasing exposure. The injury extended further towards the margins with a gradual change in leaf colour from dark green through pale yellow to yellowish brown. At higher concentrations the damage spread extensively in the intercostal regions of the leaves. The damaged margin showed inward curling. In S. molesta the floating leaves also showed a similar change from green to yellowish brown with increasing Hg concentrations and exposure
210
G. N. Mhatre, S. B. Chaphekar
periods. The midrib, the anastomosing veins and the branched hyaline hairs also showed similar effects. The severely damaged leaves in both plants shrivelled and fell off after the treatment was over, indicating the severity of the toxic effect. Similar phytotoxic symptoms of Hg poisoning were observed by Mhatre & Chaphekar (1984) in their studies on the response of cultivated plants such as Pennisetum typhoideum, Medicago sativa and Abelrnoschus esculentus to mercury under laboratory conditions. The LII was found to correspond with the Hg doses (Table 1). The subtle damage increased with the dose, as seen from the reduced chlorophyll contents, reduced phytomass, etc. As the LII of plants increased, there was a proportionate decrease in their chlorophyll contents and phytomass (Fig. 1).
Chlorophyll Total chlorophyll showed a significant decrease with increasing metal exposure in all three plants (Table 2). In an earlier investigation (Mhatre & Chaphekar, 1984) we recorded a similar decrease in total chlorophyll content with increasing concentrations of Hg in plants grown hydroponically. This probably appears to be a result of inhibition of biosynthesis of chlorophylls and lipids, especially galactolipids, as discovered in the freshwater algae Ankistrodesmus braunii and Euglena gracilis due to HgCI 2 and methyl mercuric chloride (Matson et al., 1972).
Phytomass With increasing doses of Hg the phytomass of all three plants decreased significantly (Table 3). The reduction in phytomass was very severe in H. verticillata with a significant reduction at as low as 1.0/~glitre -1 concentration at 5h duration. At two higher concentrations, at all exposure durations, more than 2 5 ~ decrease was recorded. In P. stratiotes and S. molesta, however, the decrease at 100~glitre -1 concentration was between 10 and 20 ~o at exposures of 3 and 5 h, whereas it was more than 25 ~ at 1000/~g litre- 1 concentration at the same length of exposure. Photosynthesis and growth of phytoplankton has been observed to be significantly reduced at Hg concentrations as low as 1-0/~g litre - ~ (Jones, 1971). Blinn et al. (1977) recorded a reduction by at least 40 ~ . Mhatre & Chaphekar (1984) found 5 0 ~ reduction in overall phytomass of three
exposure
(pg litre l)
0 0 13.4 + 1,2
1 3 5
1 3 5
1 3 5
1 3 5
10°
101
10z
109
48.35+-2'7 50'4 + 4'6 60"9_+4'5
11 "5 + 1'7 19"2 + 1"8 36"5+__2'9
0 3"9 + 0"7 13"6 + 1"5
0 0 0
leaf area injured (1)
1 3 5
Control
of
ofttg
(h)
Duration
Concentration
100.0 + 0'0 100-0 + 0-0 100.0_+ 0'0
43.0 + 3"0 57.0 + 3"8 88.0+_4-3
0 14.0 + 1,5 75.0 + 4"0
0 0 30.0 + 2-5
0 0 0
~ leaves injured (2)
P. stratiotes
4835"0 5 040-0 6090"0
494"0 1 094-4 3212"0
0 54-6 1 020"0
0 0 402
0 0 0
Lll (= 1 × 2)
58-4+4.6 74"6 + 3' 1 93'2_+ 3'0
0 44.0 + 3"1 63"9+3"7
0 0 35"2 + 2'5
0 0 0
0 0 0
% leaf area injured (4)
100"0 + 0"0 100-0 + 0"0 100"0 + 0"0
0 53'8 + 5"5 80'0+-5'9
0 0 72-7 + 4.7
0 0 0
0 0 0
% leaves injured (5)
S. molesta
5840"0 7 460"0 9320"0
0 2367-2 5112"0
0 0 2 559'04
0 0 0
0 0 0
LII (= 4 x 5)
TABLE 1 Foliar Injury in Pistia stratiotes and Salvinia molesta Plants Treated with Different Concentrations of Hg at Three Different Exposure Periods (Mean of 13 _+ SE; the base data were transferred in ~o before analysis)
212
G. N. Mhatre, S. B. Chaphekar lh
5h Hydrtlla
5h ~
o
verticillata
Chlorophyll
z~ P h y t o m o s s
I 0 0 r-
50
o-
~
i
,
T
, $olvinio
molesto
50
5000
0
~
k-O
'°°I
r'-
'°'°°°
0
I0
I0
I0
3 I0 0
I0
I0
I0
3 I0
0
0 I0
I0
I
I0
2
3 I0
Hcj concentrotions in ppb
Fig. 1. Percentages of total chlorophyll and phytomass and Leaf Injury Index (LII) of Hydrilla verticillata, Salvinia molesta and Pistia stratiotes treated with different concentrations of Hg for 1, 3 and 5 h.
flowering plants exposed to 100 and 1000/~g litre - 1 Hg. It is possible that the reduction in standing phytomass was the net result of an Hg-induced reduction in photosynthesis and chlorophyll synthesis, as well as distortion of respiratory metabolism. Gradual chlorosis at lower concentrations and an instant chlorosis at higher concentrations of Hg, as well as reduction in the chlorophyll content and biomass, within 5 h exposure to Hg, suggests the possibility
1 3 5
1 3 5
1 3 5
H. t~erticillata
P. stratiotes
S. molesta 0 1 8 _+ 0 0 0 7 0-18-+0.007 0.18+_0.007
0.28 4- 0-01 0-28 _+ 0.01 0.28_+0.01
0.19 + 0.008 0194-0"008 0.19 4- 0.008
Control
0.18 4- 0"008 0-18_+0.007 0.164-0.007
0.27 _+ 0"009 0.26 _+ 0"009 0.244-0.008
0.19 4- 0"007 0.19_+0.009 0" 18 4- 0.005
10 °
0'16 -+0"006 0.14-+0"005" 0-13_+0.005 °
0.27 -+ 0-009 0.24 4- 0-006 0.21 _+0"005"
0.18 4- 0"006 0.18_+0.007 0.17 _+ 0"008
101
102
0.14 _+ 0-005 ~ 0.124-0.008" 0.10_+0.009 b
0.26 _+ 0'008 0"23 4- 0'005" 0-18_+0.005 b
0' 16 4- 0.007 0.14 4- 0'005" 0-26 _+ 0.005 b
Concentration q / H g (~g litre- 1)
Significant at ~ P < 0.05, b p < 0.02, " P < 0-01 as c o m p a r e d to control.
Duration of exposu re (h )
Plant species
0.07 _+ 0.009 b 0.05_+0.007 ~ 0.03 4- 0.005 ~
I).23 -+ 0.005 ~ 0.18 4- 0-003 b 0.15+0-005 ~
0- t 4 _+ 0.005 ~ 0.09+0"003 c 0.07 _+ 0.005 c
lO 3
TABLE 2 Total Chlorophyll C o n t e n t (rag g - ~ fresh weight) of the T h r e e Plant Species Tested at Different C o n c e n t r a t i o n s of Hg at Three Different Exposure D u r a t i o n s ( M e a n of 3 _+ SE)
%,a
,2
e~
TABLE 3
1 3 5
1 3 5
P. stratiotes
S. molesta 138.9 ± 4.5 138'9 ± 4"5 138.9 ± 4-5
119.4 + 3.1 119.4 ± 3.1 119.4 ± 3.1
251 "8 +_ 6-2 251.8 ± 6'2 251"8 ± 6"2
138.6 _+ 4'3 133-5 ± 4.5 131"0 ± 4'0
118.2 ± 3-0 116-4 + 3.2 114'2 ± 3.7
250'2 + 6"7 241.1 ± 6-6 225.1 ± 6'9 b
Significant at ~ P < 0.05, b p < 0,02, c p < 0-01 as c o m p a r e d to control.
1 3 5
134-6 ± 4"1 128.7 +_ 3.9 122.8 ± 3'9
116.0 ± 3'0 112'9 _+ 3.5 104.4 ± 2-7 c
236'6 + 7"9 213"4 ± 7'9 b 205-0 ± 7"0c
10 2
128.9 + 2-7 118.3 + 2.1 c 112.3 ± 2.6 c
112.3 ± 2.9 109.5 ± 2.5 ~ 98-2 ± 2.6 ~
190.2 _+ 9.3 ¢ 184.4 _ 9.5 c 165.0 ± 9.8 ~
101
10 °
(h) Control
Concentration of H g (l~g litre- ~)
Duration of exposure
H. verticillata
Plant species
123.8 ± 2.2 b 102.6 ± 2,5 c 90-1 ± 2.1 c
107.8 ± 1.9 b 103.8 ___2.2 ~ 85.2 ± 2.0 c
165.0 ± 8.5 c 152.8 ± 8.8 c 100-0 ± 8.1 c
10 3
Phytomass (mg) o f the Three Plant Species Treated at Different C o n c e n t r a t i o n s o f Hg at Three Durations o f Exposure ( M e a n o f 10 ± SE)
l,,,a
Effect of mercury on aquatic plants
215
that these plant species could be used as biological monitors for toxicants in water. Since plant material is easy to culture, it can easily replace the present method of using fish as biomonitors in numerous water pollution testing laboratories. Use of Myriophyllum spathulatum to monitor concentrations of sewage in lake waters has been suggested by Salman et aL (1982). The method suggested here, on the other hand, prescribes the use of one of the three species of plants for a heavy metal at concentrations known to contaminate natural waterways. More experiments along the lines discussed in this paper will be needed to standardise the responses of these plant species to a large variety of aquatic pollutants found, singly and in combination, in many waterways. CONCLUSIONS All three aquatic plant species were found to be sensitive to Hg. Visible injury in the form of foliar damage, however, varied with the species in terms of Hg concentrations and exposure durations. Foliar injury was visible at as low as 1 #g litre-1 concentration within 5 h in P. stratiotes. Injury occurred at somewhat higher doses in the two other plant species tested. Subtle damage in the form of a reduction in chlorophyll content and standing phytomass was also observed. The total damage expressed as Leaf Injury Index (LII) values also increased with increasing doses of Hg. LII has been suggested as a reliable index for giving a quick and fairly accurate estimate of plant injury and hence as a reliable parameter in biomonitoring. ACKN O W L E D G E MENTS The authors are grateful to the Head of the Botany Department and the Director, Institute of Science, Bombay, for providing facilities for this study. A Daxina Fellowship awarded to G.N.M. is duly acknowledged. REFERENCES Antonovics, J., Bradshaw, A. D. & Turner, R. G. (1971). Heavy metal tolerance in plants. Adv. in Ecol. Res., 7, 1 85. Arnon, D. I. (1949). Copper enzymes in isolated chloroplasts, polyphenoloxidase in Beta vulgaris. Plant Physiol., Lancaster, 24, 1-15.
216
G. N. Mhatre, S. B. Chaphekar
Blinn, D. W., Tompkins, T. & Zaleski, L. (1977). Mercury inhibition on primary productivity using large volume plastic chambers in situ. J. Phycol., 13, 58-61. Bradshaw, A. D. (1976). Pollution and evolution. In Effects of air pollutants on plants, ed. by T.A. Mansfield, 134-59. London, Cambridge University Press. Chaphekar, S. B., Boralkar, D. B. & Shetye, R. P. (1980). Plants for air monitoring in industrial areas. In Tropical ecology and development, Proceedings of International Symposium on Tropical Ecology, 5th, ed. by J. Furtado, 2, 669 76. Kuala Lumpur, International Society for Tropical Ecology. Ernst, W. H. O. & Bast-Cramer, W. B. (1980). The effect of lead contamination of soils and air on its accumulation in pollen. Plant & Soil, 57, 491 6. Jones, H. R. (1971). Mercury pollution control~1971. New Jersey, Noyes Data Corporation. Little, P. & Martin, M. H. (1974). Biological monitoring of heavy metal pollution. Environ. Pollut., 6, 1 19. Maharashtra Prevention of Water Pollution Board (1975). Report of study group formed jor detection of mercury contamination in the environment and measures for its control. Bombay, Maharashtra State. Matson, R. S., Mustoo, G. E. & Chang, S. B. (1972). Mercury inhibition on lipid biosynthesis in freshwater algae. Environ. Sci. & Technol., 6, 158. M hatre, G. N. & Chaphekar, S. B. (1984). Response of young plants to mercury. Water, Air & Soil Pollut., 21, 1-8. Mhatre, G. N., Chaphekar, S. B., Rao, R. I. V., Patti, M. R. & Haldar, B. C. (1980). Effect of industrial pollution on the Kalu river ecosystem. Environ. Pollut., Ser. A, 23, 67 78. Nieboer, E., Ahmed, H. M., Puckett, K. J. &Richardson, D. H. S. (1972). Heavy metal content of lichens in relation to distance from a nickel smelter in Sudbury, Ontario. Lichenologist, 5, 292 304. Salman, S., Belsare, D. K. & Ahmed, A. M. (1982). A simple dynamic method to measure water pollution by plant growth. Indian Association Jor Water Pollution Control Tech. Annual. 9, 172 5. Sandell. E. B. (1959). Colorimetric determination of traces oJmetals. New York, Interscience Publishers. Wielgolaski, F. E. (1975). Biological indicators of pollution. Urban Ecol., 1, 63 79.
The Effect of Mercury on Some Aquatic Plants
G. N. M h a t r e * & S. B. C h a p h e k a r Department of Botany, The Institute of Science, 15 Madam Cama Road, Bombay 400 032, India
ABSTRACT Three aquatic plants, Hydrilla verticillata Presl, Pistia stratiotes L. and Salvinia molesta D.S. Mitchell, were treated with different concentrations of mercury ranging from 1 to 1000 #g litre- 1 at three different exposure durations, i.e. 1, 3 and5 h. All were found to be severely affected by mercury. Foliar injury, chlorophyll content and phytomass showed perceptible effects with increasing exposure to the metal. In the case of .floating plants a positive relationship was obtained between Leaf Injury Index (LII) and doses of the metal. The possible use of aquatic plants in general, and floating plants in particular, as simple bioassay material in biomonitoring and toxicity studies is discussed with special reference to L I I as a simple biomonitoring parameter.
INTRODUCTION Many lower plants (Nieboer et al., 1972; Little & Martin, 1974; Wielgolaski, 1975; Blinn et al., 1977) and higher plants (Antonovics et al., 1971 ; Bradshaw, 1976; Ernst & Bast-Cramer, 1980) have been studied for effects of toxic environmental pollutants. Some of the easily observed effects are foliar injury and suppression of vegetative and reproductive growth of species exposed to pollutants. In extreme cases, suppression of * Present address: 3/25 Worli Gopchar, 128 Dr. Annie Besant Road, Worli, Bombay 400018, India. 207 Environ. Pollut. Ser. A. 0143-1471/85/$03.30 .c5)ElsevierApplied SciencePublishers Ltd, England, 1985. Printed in Great Britain
208
G. N. Mhatre, S. B. Chaphekar
growth leads to elimination of susceptible species. Many instances of species-poor vegetation prevalent in contaminated areas have been reported (Mhatre et al., 1980). Tolerant species in such areas have a high indicator value, for long employed in geoprospecting (see review by Antonovics et al., 1971). In short-term studies within the same generation of plants it is, however, safer to use plant injury as a simple indication of a polluted environment. Numerous attempts have been made to relate foliar injury and growth suppression to intensity of atmospheric pollution (Chaphekar et al., 1980), although the same has rarely been attempted in the case of water pollution. An effort in this direction is reported here, aiming to quantify plant injury in a simple way for convenient use as an index of the level of water pollution due to a heavy metal, mercury, reported in effluents around the city of Bombay (Maharashtra Prevention of Water Pollution Board, 1975).
MATERIALS AND METHODS Three aquatic plant species, Hydrilla verticillata Presl, Pistia stratiotes L. and Salvinia molesta D.S. Mitchell, were collected from a local, uncontaminated pond and kept in aquaria containing tap water. After 24 h the plants were transferred to aquaria containing known concentrations of Hg. A stock solution of Hg was prepared using HgCI z, as given by Sandell (1959), and from which 1, 10, 100 and 1000/~glitre -1 Hg concentrations were obtained through dilutions in distilled water. The plants were exposed to the test concentrations for 1,3 and 5 h each and then transferred to aquaria containing tap water. The aquaria were artificially aerated. Twenty-four hours after transfer to tap water the area of visible leaf injury was recorded on graph paper. The Leaf Injury Index (LII) values for P. stratiotes and S. molesta were calculated as a product of percentage leaf area injured and percentage of leaves showing injury (Mhatre & Chaphekar, 1984). Chlorophyll content of the plants was determined by the method of Arnon (1949). Some treated plant material was dried at 80 + 1 °C in an air-oven and its dry weight recorded. Comparison with the control showed a loss of weight as a result of the treatment. Significance of results was tested by applying Student's t-test.
Effect of mercury on aquatic plants
209
RESULTS A N D DISCUSSION
Foliar injury All three plant species were found to be susceptible to Hg. The injury was visible in the form of chlorosis during and after 24h of treatment. Intensity of injury increased with increasing metal concentration. Yellowish-red patches, visible at 10/aglitre -1, became intense reddish brown at 100 #g litre- 1. At 1000 #g litre- 1 concentration, total chlorosis of leaves, with a brownish tinge in between, resulted. The level of injury was also found to increase with increased exposure, with slight injury at 1 h, becoming progressively more severe at 3 h and 5 h duration. The pattern of injury varied with species. In H. verticillata the injury was perceptible within minutes after the plant came in contact with the toxicant, in the form of a chlorotic appearance, especially at concentrations of 100 and 1000 #g litre- 1. The injury was visible even at 1 h exposure and was irreversible at 3 h and 5 h. The stem eventually fragmented on touch, indicating collapse of the plant body. In P. stratiotes and S. molesta the pattern of injury was different. In both cases the submerged parts coming in direct contact with the toxic metal were affected first. At the two higher mercury concentrations the roots started showing change, even after 1 h, from off-white to yellowish red and reddish brown. At higher exposure durations the same concentrations proved lethal, the roots becoming detached from the rhizomes at the slightest touch. Whorls of leaves were the next to show injury. In P. stratiotes the outermost whorl was the first to show chlorosis, followed by the middle and the innermost whorls in that order. The ventral surface of the leaves in the outermost whorl became yellowish to yellowish red at higher concentrations. The symptoms intensified with increasing exposure. The damage then also spread to the dorsal surface. The veins showed a yellow tinge, initially at a slow rate but later on quickly changed to yellowish red with increasing exposure. The injury extended further towards the margins with a gradual change in leaf colour from dark green through pale yellow to yellowish brown. At higher concentrations the damage spread extensively in the intercostal regions of the leaves. The damaged margin showed inward curling. In S. molesta the floating leaves also showed a similar change from green to yellowish brown with increasing Hg concentrations and exposure
210
G. N. Mhatre, S. B. Chaphekar
periods. The midrib, the anastomosing veins and the branched hyaline hairs also showed similar effects. The severely damaged leaves in both plants shrivelled and fell off after the treatment was over, indicating the severity of the toxic effect. Similar phytotoxic symptoms of Hg poisoning were observed by Mhatre & Chaphekar (1984) in their studies on the response of cultivated plants such as Pennisetum typhoideum, Medicago sativa and Abelrnoschus esculentus to mercury under laboratory conditions. The LII was found to correspond with the Hg doses (Table 1). The subtle damage increased with the dose, as seen from the reduced chlorophyll contents, reduced phytomass, etc. As the LII of plants increased, there was a proportionate decrease in their chlorophyll contents and phytomass (Fig. 1).
Chlorophyll Total chlorophyll showed a significant decrease with increasing metal exposure in all three plants (Table 2). In an earlier investigation (Mhatre & Chaphekar, 1984) we recorded a similar decrease in total chlorophyll content with increasing concentrations of Hg in plants grown hydroponically. This probably appears to be a result of inhibition of biosynthesis of chlorophylls and lipids, especially galactolipids, as discovered in the freshwater algae Ankistrodesmus braunii and Euglena gracilis due to HgCI 2 and methyl mercuric chloride (Matson et al., 1972).
Phytomass With increasing doses of Hg the phytomass of all three plants decreased significantly (Table 3). The reduction in phytomass was very severe in H. verticillata with a significant reduction at as low as 1.0/~glitre -1 concentration at 5h duration. At two higher concentrations, at all exposure durations, more than 2 5 ~ decrease was recorded. In P. stratiotes and S. molesta, however, the decrease at 100~glitre -1 concentration was between 10 and 20 ~o at exposures of 3 and 5 h, whereas it was more than 25 ~ at 1000/~g litre- 1 concentration at the same length of exposure. Photosynthesis and growth of phytoplankton has been observed to be significantly reduced at Hg concentrations as low as 1-0/~g litre - ~ (Jones, 1971). Blinn et al. (1977) recorded a reduction by at least 40 ~ . Mhatre & Chaphekar (1984) found 5 0 ~ reduction in overall phytomass of three
exposure
(pg litre l)
0 0 13.4 + 1,2
1 3 5
1 3 5
1 3 5
1 3 5
10°
101
10z
109
48.35+-2'7 50'4 + 4'6 60"9_+4'5
11 "5 + 1'7 19"2 + 1"8 36"5+__2'9
0 3"9 + 0"7 13"6 + 1"5
0 0 0
leaf area injured (1)
1 3 5
Control
of
ofttg
(h)
Duration
Concentration
100.0 + 0'0 100-0 + 0-0 100.0_+ 0'0
43.0 + 3"0 57.0 + 3"8 88.0+_4-3
0 14.0 + 1,5 75.0 + 4"0
0 0 30.0 + 2-5
0 0 0
~ leaves injured (2)
P. stratiotes
4835"0 5 040-0 6090"0
494"0 1 094-4 3212"0
0 54-6 1 020"0
0 0 402
0 0 0
Lll (= 1 × 2)
58-4+4.6 74"6 + 3' 1 93'2_+ 3'0
0 44.0 + 3"1 63"9+3"7
0 0 35"2 + 2'5
0 0 0
0 0 0
% leaf area injured (4)
100"0 + 0"0 100-0 + 0"0 100"0 + 0"0
0 53'8 + 5"5 80'0+-5'9
0 0 72-7 + 4.7
0 0 0
0 0 0
% leaves injured (5)
S. molesta
5840"0 7 460"0 9320"0
0 2367-2 5112"0
0 0 2 559'04
0 0 0
0 0 0
LII (= 4 x 5)
TABLE 1 Foliar Injury in Pistia stratiotes and Salvinia molesta Plants Treated with Different Concentrations of Hg at Three Different Exposure Periods (Mean of 13 _+ SE; the base data were transferred in ~o before analysis)
212
G. N. Mhatre, S. B. Chaphekar lh
5h Hydrtlla
5h ~
o
verticillata
Chlorophyll
z~ P h y t o m o s s
I 0 0 r-
50
o-
~
i
,
T
, $olvinio
molesto
50
5000
0
~
k-O
'°°I
r'-
'°'°°°
0
I0
I0
I0
3 I0 0
I0
I0
I0
3 I0
0
0 I0
I0
I
I0
2
3 I0
Hcj concentrotions in ppb
Fig. 1. Percentages of total chlorophyll and phytomass and Leaf Injury Index (LII) of Hydrilla verticillata, Salvinia molesta and Pistia stratiotes treated with different concentrations of Hg for 1, 3 and 5 h.
flowering plants exposed to 100 and 1000/~g litre - 1 Hg. It is possible that the reduction in standing phytomass was the net result of an Hg-induced reduction in photosynthesis and chlorophyll synthesis, as well as distortion of respiratory metabolism. Gradual chlorosis at lower concentrations and an instant chlorosis at higher concentrations of Hg, as well as reduction in the chlorophyll content and biomass, within 5 h exposure to Hg, suggests the possibility
1 3 5
1 3 5
1 3 5
H. t~erticillata
P. stratiotes
S. molesta 0 1 8 _+ 0 0 0 7 0-18-+0.007 0.18+_0.007
0.28 4- 0-01 0-28 _+ 0.01 0.28_+0.01
0.19 + 0.008 0194-0"008 0.19 4- 0.008
Control
0.18 4- 0"008 0-18_+0.007 0.164-0.007
0.27 _+ 0"009 0.26 _+ 0"009 0.244-0.008
0.19 4- 0"007 0.19_+0.009 0" 18 4- 0.005
10 °
0'16 -+0"006 0.14-+0"005" 0-13_+0.005 °
0.27 -+ 0-009 0.24 4- 0-006 0.21 _+0"005"
0.18 4- 0"006 0.18_+0.007 0.17 _+ 0"008
101
102
0.14 _+ 0-005 ~ 0.124-0.008" 0.10_+0.009 b
0.26 _+ 0'008 0"23 4- 0'005" 0-18_+0.005 b
0' 16 4- 0.007 0.14 4- 0'005" 0-26 _+ 0.005 b
Concentration q / H g (~g litre- 1)
Significant at ~ P < 0.05, b p < 0.02, " P < 0-01 as c o m p a r e d to control.
Duration of exposu re (h )
Plant species
0.07 _+ 0.009 b 0.05_+0.007 ~ 0.03 4- 0.005 ~
I).23 -+ 0.005 ~ 0.18 4- 0-003 b 0.15+0-005 ~
0- t 4 _+ 0.005 ~ 0.09+0"003 c 0.07 _+ 0.005 c
lO 3
TABLE 2 Total Chlorophyll C o n t e n t (rag g - ~ fresh weight) of the T h r e e Plant Species Tested at Different C o n c e n t r a t i o n s of Hg at Three Different Exposure D u r a t i o n s ( M e a n of 3 _+ SE)
%,a
,2
e~
TABLE 3
1 3 5
1 3 5
P. stratiotes
S. molesta 138.9 ± 4.5 138'9 ± 4"5 138.9 ± 4-5
119.4 + 3.1 119.4 ± 3.1 119.4 ± 3.1
251 "8 +_ 6-2 251.8 ± 6'2 251"8 ± 6"2
138.6 _+ 4'3 133-5 ± 4.5 131"0 ± 4'0
118.2 ± 3-0 116-4 + 3.2 114'2 ± 3.7
250'2 + 6"7 241.1 ± 6-6 225.1 ± 6'9 b
Significant at ~ P < 0.05, b p < 0,02, c p < 0-01 as c o m p a r e d to control.
1 3 5
134-6 ± 4"1 128.7 +_ 3.9 122.8 ± 3'9
116.0 ± 3'0 112'9 _+ 3.5 104.4 ± 2-7 c
236'6 + 7"9 213"4 ± 7'9 b 205-0 ± 7"0c
10 2
128.9 + 2-7 118.3 + 2.1 c 112.3 ± 2.6 c
112.3 ± 2.9 109.5 ± 2.5 ~ 98-2 ± 2.6 ~
190.2 _+ 9.3 ¢ 184.4 _ 9.5 c 165.0 ± 9.8 ~
101
10 °
(h) Control
Concentration of H g (l~g litre- ~)
Duration of exposure
H. verticillata
Plant species
123.8 ± 2.2 b 102.6 ± 2,5 c 90-1 ± 2.1 c
107.8 ± 1.9 b 103.8 ___2.2 ~ 85.2 ± 2.0 c
165.0 ± 8.5 c 152.8 ± 8.8 c 100-0 ± 8.1 c
10 3
Phytomass (mg) o f the Three Plant Species Treated at Different C o n c e n t r a t i o n s o f Hg at Three Durations o f Exposure ( M e a n o f 10 ± SE)
l,,,a
Effect of mercury on aquatic plants
215
that these plant species could be used as biological monitors for toxicants in water. Since plant material is easy to culture, it can easily replace the present method of using fish as biomonitors in numerous water pollution testing laboratories. Use of Myriophyllum spathulatum to monitor concentrations of sewage in lake waters has been suggested by Salman et aL (1982). The method suggested here, on the other hand, prescribes the use of one of the three species of plants for a heavy metal at concentrations known to contaminate natural waterways. More experiments along the lines discussed in this paper will be needed to standardise the responses of these plant species to a large variety of aquatic pollutants found, singly and in combination, in many waterways. CONCLUSIONS All three aquatic plant species were found to be sensitive to Hg. Visible injury in the form of foliar damage, however, varied with the species in terms of Hg concentrations and exposure durations. Foliar injury was visible at as low as 1 #g litre-1 concentration within 5 h in P. stratiotes. Injury occurred at somewhat higher doses in the two other plant species tested. Subtle damage in the form of a reduction in chlorophyll content and standing phytomass was also observed. The total damage expressed as Leaf Injury Index (LII) values also increased with increasing doses of Hg. LII has been suggested as a reliable index for giving a quick and fairly accurate estimate of plant injury and hence as a reliable parameter in biomonitoring. ACKN O W L E D G E MENTS The authors are grateful to the Head of the Botany Department and the Director, Institute of Science, Bombay, for providing facilities for this study. A Daxina Fellowship awarded to G.N.M. is duly acknowledged. REFERENCES Antonovics, J., Bradshaw, A. D. & Turner, R. G. (1971). Heavy metal tolerance in plants. Adv. in Ecol. Res., 7, 1 85. Arnon, D. I. (1949). Copper enzymes in isolated chloroplasts, polyphenoloxidase in Beta vulgaris. Plant Physiol., Lancaster, 24, 1-15.
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G. N. Mhatre, S. B. Chaphekar
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