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Stomatal responses to raised atmospheric CO2 concentrations during exposure of plants to SO2 pollution
STOMATAL RESPONSES TO RAISED ATMOSPHERIC CO2 CONCENTRATIONS DURING EXPOSURE OF PLANTS TO SO2 POLLUTION ONDRFJ MA"~.NIK & T. A. MANSFIELD
Department of Biological Sciences, University of Lancaster, Great Britain
ABSTRACT
Stomata provide the main routes for entry of pollutants into the leaves of higher plants, and it has been suggested that inhibitors of stomatal opening could be used to protect plants against pollution. Experimental evidence is presented indicating that the stomatal closing response to CO 2 still occurs in the presence of SO 2, even in conditions in which S02 alone stimulates stomatal opening. The increases in C02 concentration that occur in polluted atmospheres appear to be sufficient to cause partial stomatal closure. The variability between individual plants in the magnitude of the stomatal response to CO 2 is considerable, and the response could be one useful criterion to employ for selection purposes when crop varieties are produced for growing in polluted areas.
INTRODUCTION
Stomata provide the main route for entry of air pollutants into the leaves of higher plants, and Rich (1963) first suggested that protection against pollutants might be provided by chemical compounds that cause stomatal closure. He was interested in the application of substances in solution which, when applied to leaf surfaces, would enter the guard cells and disturb their metabolism. It is, however, known that increases in atmospheric CO2 concentration suppress stomatal opening in light (Heath & Russell, 1954; Meidner & Mansfield, 1968), and we recently suggested that this reaction to CO2 could be important in determining resistance to pollution (Mansfield & Majernik, 1970). Atmospheric CO 2 concentration usually increases appreciably when other pollutants reach toxic levels, indeed the correlation between CO2 and some pollutants is so good that CO2 concentrations, which are readily 1 Environ.Pollut. (3) (1972) pp. I-7--{~ Applied Science Publishers Ltd, England--Printed in Groat Britain
2
ONDREI MAJERNIK, T. A. MANSFIELD
measured, can be used as indicators of danger levels of other gases (Bush et al., 1967). We have previously reported (Majernik & Mansfield, 1970; Mansfield & Majernik, 1970) that when the relative humidity is greater than 4 0 ~ at 18°C, SO2 pollution causes a stimulation of stomatal opening. This would increase the penetration of the SO2 into the interior of the leaf. We have now investigated the question of whether stomata remain sensitive to CO2 in the presence of SO2, with the objective of discovering whether raised CO2 levels could inhibit SO2 penetration into the plant.
MATERIALS AND METHODS
Plants of broad bean (Vicia faba L. var. Windsor Harlington), 21-30 days old, were used in all the experiments. Observations of stomatal behaviour were made on leaflets at the second node from the base. Plants were raised in the greenhouse, and transferred for the experiments to growth cabinets with 50 % relative humidity at 20°C (water vapour pressure deficit = 8.75 mm Hg). The intensity of illumination was 8 Klux from water-cooled tungsten filament lamps. The air-flow containing CO2, or SO2 + CO2, was directed onto the surface of each leaf on which observations were being made, at a rate of 200 cm 3 min- 1. Control leaves were similarly treated with an air stream not containing the pollutants. The 'normal air' contained 320-330 ppm CO2. SO2 was applied at a concentration of 0.7 ppm, and CO2 concentrations were zero, 320-330, 550 or 1000 ppm, the last two being obtained from previously calibrated cylinders. Stomatal behaviour was recorded using an automated porometer (Heath & Mansfield, 1962) which enabled observations to be made simultaneously on four leaves of different plants.
RESULTS
The results of the main experiment are shown in Fig. 1. Stomatal behaviour in the control plants in 320-330 ppm COz followed the expected pattern, opening rising to a maximum aperture in early afternoon followed by the characteristic slow closure which is thought to be endogenously controlled (Meidner & Mansfield, 1968). When the CO2 concentration was increased to 550 ppm there was an appreciable suppression of stomatal opening, and increasing the concentration further to 1000 ppm suppressed opening only slightly more. In the presence of 0.7 ppm SO 2, both CO2 concentrations suppressed stomatal opening to mean apertures that were slightly below those in the absence of SO2. This further suppression of opening did not, however, reach the required level of statistical significance. The
3
STOMATAL RESPONSES TO RAISED ATMOSPHERIC CO 2 CONCENTRATIONS
==if o
^| /
o
/----- °----°~°-----°~-°~O~o
_/Y/A---------A~A~-~
lnOo.m. 11
12
~
1p.m. time
2
3
e
~
4
o ~
5
6
7
of day
Fig. 1. Course ofstomatal behaviour under the following treatments: Q, 550 ppm CO2 and zero SO2; ~ , 1000 ppm CO2 and zero SO2; A, 550 ppm CO2 and 0.7 ppm SO2; A, 1000 ppm CO2 and 0.7 ppm SO2; O, control in 320-330 ppm CO2 with zero SO2. The ordinate scale is in the arbitrary units of an automated resistance porometer.
reduced opening in the presence of SO2, at both concentrations, was significantly different from the controls at P < 0.001. Experiments were also performed, on plants drawn from the same batch, to discover whether, in normal air, the stomata showed the opening reaction to SO2 that we have previously reported (Majernik & Mansfield, 1970). The same stimulation of opening was found. Figure 2 presents data obtained at much lower light intensities than we have hitherto used. The considerable stimulation of opening demonstrates that SO2 can overrule the partial closure induced by low intensity light. 4
o
•
~2 ¢0 Un "10
--_ J I b a . m 11
,
12
,
i
i
I p.m.
2
3
time
J
4
,
J
I
5
O
7
o f clay
Fig. 2. Stomatal responses to 0-7 ppm 502 in low integsity light. ©, + SO2 in light of 3500 lux; O, zero SO2 in light of 3500 lux; A, + SO2 in light of' 1800 lux; A , zero SO2 in light of 1800 lux. CO2 concentration was 320-330 ppm.
ONDREJ MAJERNIK, T. A. MANSFIELD
It was of physiological interest to examine the stomatal response to SO2 in CO2-free air in light, Le. under conditions that normally induce very wide opening. The results are shown in Fig. 3. There was very little effect of SO2 under these conditions, probably because the opening due to lack of CO2 is nearly as wide as is mechanically possible. Hence no opening response to SO 2 could be expected.
o
~ o ~....._.--o~
~ " • u ~ o
• o
• o
°
.£ o •
3
,
Fig. 3.
I
i
I
I
1
2 time
3
4
(hours)
Stomatal behaviourin the presence of SO2 under CO2-freeconditions. O, with 0.7 ppm SO2; O, control with zero SO2.
DISCUSSION
Enrichment of the atmosphere with COz to protect plants from damage by air pollutants would only be feasible in enclosed spaces such as glasshouses. The increase in CO2 concentration during periods of heavy pollution in urban areas is, however, appreciable; we have previously noted that it cart reach 600 ppm on occasions (Mansfield & Majernik, 1970). The stomatal closure obtained in the present experiments during treatment with 550 ppm CO2 in light of 8 Klux is, therefore, a useful guide to the probable behaviour in the field under medium light intensities. The failure of SO2 to reverse the stomatal closure induced by high CO2 levels gives further encouragement to the idea of protection being available to the plant by this means. We have foui~d that the stomatal closing reaction to COz in Vicia faba is extremely variable in magnitude. A reduction in aperture was always induced and the effect was statistically significant at P < 0.001, but the variability was such that
STOMATAL RESPONSES TO RAISED ATMOSPHERIC CO s CONCENTRATIONS
5
we could not claim that differences between the four different treatments in Fig. 1 were real. This was discouraging at the time of the experiments but in retrospect we should perhaps feel encouraged. If varieties having a greater stomatal response to CO2 are to be produced for growing in polluted areas, then wide variability in the population is desirable for selection purposes. The cellular mechanism behind the stomatal reaction to CO2 is not understood (Meidner & Mansfield, 1968), and hence it would be futile to attempt to explain fully the interaction with SO2 which has been revealed by our experiments. It may be useful to summarise the complicated situation as it now appears from the present data, and from those reported in our earlier papers. (a) In moist air (water vapour pressure deficit less than 7 mm Hg) containing 320-330 ppm CO2, the stomata normally open more widely in the presence of SO2 at concentrations of 0.25 ppm and above (Majernik & Mansfield, 1970; 1971). (b) When relative humidity is low (water vapour pressure deficit greater than 7 mm Hg) the stomata close in response to the same range of SOs concentrations (Mansfield & Majernik, 1970). (c) The opening reaction to SO2 in moist air can be counteracted by raised ambient CO2 levels. (d) The very wide stomatal opening in moist CO2-free air is only slightly enhanced by SOs. These conclusions are based on our findings for one species, Vicia faba. The stomata of this plant are well-known for their great reactivity, particularly to changes in water status (Stalfelt, 1961 ; Glinka, 1971). It is clearly desirable that these studies should be extended in the future to cover other species and other pollutants, so that more general conclusions can be drawn. The question of SOs damage to plants has been intensively investigated in the past, especially by a group of workers in the United States whose work was reviewed by Katz (1949), Thomas et al. (1950) and Thomas (1951). Apart from establishing the degree of exposure to SO2 required to cause leaf damage, these workers took great pains to determine the environmental conditions under which damage was most likely to occur. At night the sensitivity was usually low, which they attributed to the closure of the stomata. During the day an appreciable effect of humidity was found. Katz states that the relative humidity of the atmosphere below 7 0 ~ is one of the most important external factors influencing the susceptibility of plants to SO 2, and Thomas summarises work which demonstrated an eight-fold decrease in susceptibility between 100 and 10~ relative humidity. They supposed that this effect was also due to the closure of stomata, induced by low humidity, but Wilson's (1948) extensive investigation into the role of humidity in determining stomatal aperture suggested that dry air alone would not induce sufficient closure to explain the effect entirely. Raschke & Kuhl (1969) have recently shown that a change from
ONDREJ MAJERNIK,T. A. MANSFIELD moist to dry air did not close the stomata of maize, even when it was forced through the stomata and into the intercellular spaces. Our data strongly suggest that it is the stimulatory effect of SO2 on stomatal opening that is the factor determining the susceptibility of plants in moist air, and the stomatal closure induced by SO2 in dry air is responsible for the greater resistance under these conditions. We can thus take a step forward in explaining one aspect of the findings of the earlier workers, but there is still the important question of determining the nature of the effects at the cellular level. A reaction of the stomata to CO2 has beert established for many different species, and it is known to be one part of the normal physiological mechanism that is responsible for the diurnal pattern of stomatal movements (Meidner & Mansfield, 1968). SO 2, on the other hand, cannot be expected to evoke a 'normal' physiological reaction. The fact that the response to SOz is so complex, and can be quite different upon a change in external conditions, may merely be a result of its action on different cells in the epidermis. Stomatal opening and closing are caused by changes in the turgor difference between the guard cells and their immediate neighbours. Opening can result either from an increase in guard cell turgor, or a fall in the turgor of the adjacent cells; similarly, closure cart be caused by a fall in guard cell turgor, or an increase in turgor of the adjacent cells. It will be important in future work to discover on which part of this system SOn operates.
ACKNOWLEDGEMENT The financial support of the Natural Environment Research Council is gratefully acknowledged.
REFERENCES BUSH,A. F., ASCE,M. & NOTTAGE,H. B. (1967). Carbon dioxide as an indicator of air pollution. J. sanit. Engng Div. Am. Soc. civ. Engrs, 93, 211-53. GLINKA, Z. (1971). The effect of epidermal cells water potential on stomatal response to illumination on leaf discs of Vicia.faba. Physiologia PI., 24, 476-79. HEATh,O. V. S. & MANsFn~LD,T. A. (1962). A recording porometer with detachable cups operating on four separate leaves. Proc. R. Soc. B, 156, 1-13. HEATh,O. V. S. & RUSSELL,J. (1954). Studies in stomatal behaviour. VI. An investigation of the light responses of wheat stomata with the attempted elimination control by the mesophyll. Part 2. J. exp. Bot., 5, 269-92. KATZ,M. (1949). Sulfur dioxide in the atmosphere and its relation to plant life. Ind. Engng Chem. ind. Edn, 41, 2450-65. MAJERNIK,O. & MANSFIELD,T. A. (1970). Direct effect of SOn pollution on the degree of opening of stomata. Nature, Lond., 227, 377-8. MAJERr~IK,O. & MANSFmLD,T. A. (1971). Effects of SOz pollution on stomatal movementsin Vicia faba. Phytopath. Z., 71, 123-8. MANSrIELO,T. A. & MAJmtNIK,O. (1970). Can stomata play a part in protecting plants against air pollutants? Environ. Pollut., 1, 149-54. MEIDNER,H. & MANS~LD,T. A. (1968). Physiology of stomata. London, McGraw-Hill.
STOMATAL RESPONSES TO RAISED ATMOSPHERIC CO 2 CONCENTRATIONS
7
RASCHKE, K. & KUHL, U. (1969). Stomatal responses to changes in atmospheric humidity and water supply: experiments with leaf sections of Zea mays in CO 2-free air. Planta, 67, 224--41. RICH, S. (1963). The role of stomata in plant disease. Bull. Conn. agrie, exp. Sin, 664, 102-16. STALFELT,i . G. (1961). The effect of water deficit on stomatal movements in a carbon dioxidefree atmosphere. PhyMologia PI., 14, 826--43. THOMAS, M. D. (1951). Gas damage to plants. A. Rev. Pl. Physiol., 2, 293-322. THOMAS, M. D., HENDRICKS,R. H. & HILL, G. R. (1950). Sulfur metabolism of plants: effect of sulfur dioxide on vegetation. Ind. Engng Chem. ind. Edn, 42, 2231-5. WILSON,C. C. (1948). The effect of some environmental factors on the movements of guard cells. Pl. Physiol., Lancaster, 23, 5-37.
Department of Biological Sciences, University of Lancaster, Great Britain
ABSTRACT
Stomata provide the main routes for entry of pollutants into the leaves of higher plants, and it has been suggested that inhibitors of stomatal opening could be used to protect plants against pollution. Experimental evidence is presented indicating that the stomatal closing response to CO 2 still occurs in the presence of SO 2, even in conditions in which S02 alone stimulates stomatal opening. The increases in C02 concentration that occur in polluted atmospheres appear to be sufficient to cause partial stomatal closure. The variability between individual plants in the magnitude of the stomatal response to CO 2 is considerable, and the response could be one useful criterion to employ for selection purposes when crop varieties are produced for growing in polluted areas.
INTRODUCTION
Stomata provide the main route for entry of air pollutants into the leaves of higher plants, and Rich (1963) first suggested that protection against pollutants might be provided by chemical compounds that cause stomatal closure. He was interested in the application of substances in solution which, when applied to leaf surfaces, would enter the guard cells and disturb their metabolism. It is, however, known that increases in atmospheric CO2 concentration suppress stomatal opening in light (Heath & Russell, 1954; Meidner & Mansfield, 1968), and we recently suggested that this reaction to CO2 could be important in determining resistance to pollution (Mansfield & Majernik, 1970). Atmospheric CO 2 concentration usually increases appreciably when other pollutants reach toxic levels, indeed the correlation between CO2 and some pollutants is so good that CO2 concentrations, which are readily 1 Environ.Pollut. (3) (1972) pp. I-7--{~ Applied Science Publishers Ltd, England--Printed in Groat Britain
2
ONDREI MAJERNIK, T. A. MANSFIELD
measured, can be used as indicators of danger levels of other gases (Bush et al., 1967). We have previously reported (Majernik & Mansfield, 1970; Mansfield & Majernik, 1970) that when the relative humidity is greater than 4 0 ~ at 18°C, SO2 pollution causes a stimulation of stomatal opening. This would increase the penetration of the SO2 into the interior of the leaf. We have now investigated the question of whether stomata remain sensitive to CO2 in the presence of SO2, with the objective of discovering whether raised CO2 levels could inhibit SO2 penetration into the plant.
MATERIALS AND METHODS
Plants of broad bean (Vicia faba L. var. Windsor Harlington), 21-30 days old, were used in all the experiments. Observations of stomatal behaviour were made on leaflets at the second node from the base. Plants were raised in the greenhouse, and transferred for the experiments to growth cabinets with 50 % relative humidity at 20°C (water vapour pressure deficit = 8.75 mm Hg). The intensity of illumination was 8 Klux from water-cooled tungsten filament lamps. The air-flow containing CO2, or SO2 + CO2, was directed onto the surface of each leaf on which observations were being made, at a rate of 200 cm 3 min- 1. Control leaves were similarly treated with an air stream not containing the pollutants. The 'normal air' contained 320-330 ppm CO2. SO2 was applied at a concentration of 0.7 ppm, and CO2 concentrations were zero, 320-330, 550 or 1000 ppm, the last two being obtained from previously calibrated cylinders. Stomatal behaviour was recorded using an automated porometer (Heath & Mansfield, 1962) which enabled observations to be made simultaneously on four leaves of different plants.
RESULTS
The results of the main experiment are shown in Fig. 1. Stomatal behaviour in the control plants in 320-330 ppm COz followed the expected pattern, opening rising to a maximum aperture in early afternoon followed by the characteristic slow closure which is thought to be endogenously controlled (Meidner & Mansfield, 1968). When the CO2 concentration was increased to 550 ppm there was an appreciable suppression of stomatal opening, and increasing the concentration further to 1000 ppm suppressed opening only slightly more. In the presence of 0.7 ppm SO 2, both CO2 concentrations suppressed stomatal opening to mean apertures that were slightly below those in the absence of SO2. This further suppression of opening did not, however, reach the required level of statistical significance. The
3
STOMATAL RESPONSES TO RAISED ATMOSPHERIC CO 2 CONCENTRATIONS
==if o
^| /
o
/----- °----°~°-----°~-°~O~o
_/Y/A---------A~A~-~
lnOo.m. 11
12
~
1p.m. time
2
3
e
~
4
o ~
5
6
7
of day
Fig. 1. Course ofstomatal behaviour under the following treatments: Q, 550 ppm CO2 and zero SO2; ~ , 1000 ppm CO2 and zero SO2; A, 550 ppm CO2 and 0.7 ppm SO2; A, 1000 ppm CO2 and 0.7 ppm SO2; O, control in 320-330 ppm CO2 with zero SO2. The ordinate scale is in the arbitrary units of an automated resistance porometer.
reduced opening in the presence of SO2, at both concentrations, was significantly different from the controls at P < 0.001. Experiments were also performed, on plants drawn from the same batch, to discover whether, in normal air, the stomata showed the opening reaction to SO2 that we have previously reported (Majernik & Mansfield, 1970). The same stimulation of opening was found. Figure 2 presents data obtained at much lower light intensities than we have hitherto used. The considerable stimulation of opening demonstrates that SO2 can overrule the partial closure induced by low intensity light. 4
o
•
~2 ¢0 Un "10
--_ J I b a . m 11
,
12
,
i
i
I p.m.
2
3
time
J
4
,
J
I
5
O
7
o f clay
Fig. 2. Stomatal responses to 0-7 ppm 502 in low integsity light. ©, + SO2 in light of 3500 lux; O, zero SO2 in light of 3500 lux; A, + SO2 in light of' 1800 lux; A , zero SO2 in light of 1800 lux. CO2 concentration was 320-330 ppm.
ONDREJ MAJERNIK, T. A. MANSFIELD
It was of physiological interest to examine the stomatal response to SO2 in CO2-free air in light, Le. under conditions that normally induce very wide opening. The results are shown in Fig. 3. There was very little effect of SO2 under these conditions, probably because the opening due to lack of CO2 is nearly as wide as is mechanically possible. Hence no opening response to SO 2 could be expected.
o
~ o ~....._.--o~
~ " • u ~ o
• o
• o
°
.£ o •
3
,
Fig. 3.
I
i
I
I
1
2 time
3
4
(hours)
Stomatal behaviourin the presence of SO2 under CO2-freeconditions. O, with 0.7 ppm SO2; O, control with zero SO2.
DISCUSSION
Enrichment of the atmosphere with COz to protect plants from damage by air pollutants would only be feasible in enclosed spaces such as glasshouses. The increase in CO2 concentration during periods of heavy pollution in urban areas is, however, appreciable; we have previously noted that it cart reach 600 ppm on occasions (Mansfield & Majernik, 1970). The stomatal closure obtained in the present experiments during treatment with 550 ppm CO2 in light of 8 Klux is, therefore, a useful guide to the probable behaviour in the field under medium light intensities. The failure of SO2 to reverse the stomatal closure induced by high CO2 levels gives further encouragement to the idea of protection being available to the plant by this means. We have foui~d that the stomatal closing reaction to COz in Vicia faba is extremely variable in magnitude. A reduction in aperture was always induced and the effect was statistically significant at P < 0.001, but the variability was such that
STOMATAL RESPONSES TO RAISED ATMOSPHERIC CO s CONCENTRATIONS
5
we could not claim that differences between the four different treatments in Fig. 1 were real. This was discouraging at the time of the experiments but in retrospect we should perhaps feel encouraged. If varieties having a greater stomatal response to CO2 are to be produced for growing in polluted areas, then wide variability in the population is desirable for selection purposes. The cellular mechanism behind the stomatal reaction to CO2 is not understood (Meidner & Mansfield, 1968), and hence it would be futile to attempt to explain fully the interaction with SO2 which has been revealed by our experiments. It may be useful to summarise the complicated situation as it now appears from the present data, and from those reported in our earlier papers. (a) In moist air (water vapour pressure deficit less than 7 mm Hg) containing 320-330 ppm CO2, the stomata normally open more widely in the presence of SO2 at concentrations of 0.25 ppm and above (Majernik & Mansfield, 1970; 1971). (b) When relative humidity is low (water vapour pressure deficit greater than 7 mm Hg) the stomata close in response to the same range of SOs concentrations (Mansfield & Majernik, 1970). (c) The opening reaction to SO2 in moist air can be counteracted by raised ambient CO2 levels. (d) The very wide stomatal opening in moist CO2-free air is only slightly enhanced by SOs. These conclusions are based on our findings for one species, Vicia faba. The stomata of this plant are well-known for their great reactivity, particularly to changes in water status (Stalfelt, 1961 ; Glinka, 1971). It is clearly desirable that these studies should be extended in the future to cover other species and other pollutants, so that more general conclusions can be drawn. The question of SOs damage to plants has been intensively investigated in the past, especially by a group of workers in the United States whose work was reviewed by Katz (1949), Thomas et al. (1950) and Thomas (1951). Apart from establishing the degree of exposure to SO2 required to cause leaf damage, these workers took great pains to determine the environmental conditions under which damage was most likely to occur. At night the sensitivity was usually low, which they attributed to the closure of the stomata. During the day an appreciable effect of humidity was found. Katz states that the relative humidity of the atmosphere below 7 0 ~ is one of the most important external factors influencing the susceptibility of plants to SO 2, and Thomas summarises work which demonstrated an eight-fold decrease in susceptibility between 100 and 10~ relative humidity. They supposed that this effect was also due to the closure of stomata, induced by low humidity, but Wilson's (1948) extensive investigation into the role of humidity in determining stomatal aperture suggested that dry air alone would not induce sufficient closure to explain the effect entirely. Raschke & Kuhl (1969) have recently shown that a change from
ONDREJ MAJERNIK,T. A. MANSFIELD moist to dry air did not close the stomata of maize, even when it was forced through the stomata and into the intercellular spaces. Our data strongly suggest that it is the stimulatory effect of SO2 on stomatal opening that is the factor determining the susceptibility of plants in moist air, and the stomatal closure induced by SO2 in dry air is responsible for the greater resistance under these conditions. We can thus take a step forward in explaining one aspect of the findings of the earlier workers, but there is still the important question of determining the nature of the effects at the cellular level. A reaction of the stomata to CO2 has beert established for many different species, and it is known to be one part of the normal physiological mechanism that is responsible for the diurnal pattern of stomatal movements (Meidner & Mansfield, 1968). SO 2, on the other hand, cannot be expected to evoke a 'normal' physiological reaction. The fact that the response to SOz is so complex, and can be quite different upon a change in external conditions, may merely be a result of its action on different cells in the epidermis. Stomatal opening and closing are caused by changes in the turgor difference between the guard cells and their immediate neighbours. Opening can result either from an increase in guard cell turgor, or a fall in the turgor of the adjacent cells; similarly, closure cart be caused by a fall in guard cell turgor, or an increase in turgor of the adjacent cells. It will be important in future work to discover on which part of this system SOn operates.
ACKNOWLEDGEMENT The financial support of the Natural Environment Research Council is gratefully acknowledged.
REFERENCES BUSH,A. F., ASCE,M. & NOTTAGE,H. B. (1967). Carbon dioxide as an indicator of air pollution. J. sanit. Engng Div. Am. Soc. civ. Engrs, 93, 211-53. GLINKA, Z. (1971). The effect of epidermal cells water potential on stomatal response to illumination on leaf discs of Vicia.faba. Physiologia PI., 24, 476-79. HEATh,O. V. S. & MANsFn~LD,T. A. (1962). A recording porometer with detachable cups operating on four separate leaves. Proc. R. Soc. B, 156, 1-13. HEATh,O. V. S. & RUSSELL,J. (1954). Studies in stomatal behaviour. VI. An investigation of the light responses of wheat stomata with the attempted elimination control by the mesophyll. Part 2. J. exp. Bot., 5, 269-92. KATZ,M. (1949). Sulfur dioxide in the atmosphere and its relation to plant life. Ind. Engng Chem. ind. Edn, 41, 2450-65. MAJERNIK,O. & MANSFIELD,T. A. (1970). Direct effect of SOn pollution on the degree of opening of stomata. Nature, Lond., 227, 377-8. MAJERr~IK,O. & MANSFmLD,T. A. (1971). Effects of SOz pollution on stomatal movementsin Vicia faba. Phytopath. Z., 71, 123-8. MANSrIELO,T. A. & MAJmtNIK,O. (1970). Can stomata play a part in protecting plants against air pollutants? Environ. Pollut., 1, 149-54. MEIDNER,H. & MANS~LD,T. A. (1968). Physiology of stomata. London, McGraw-Hill.
STOMATAL RESPONSES TO RAISED ATMOSPHERIC CO 2 CONCENTRATIONS
7
RASCHKE, K. & KUHL, U. (1969). Stomatal responses to changes in atmospheric humidity and water supply: experiments with leaf sections of Zea mays in CO 2-free air. Planta, 67, 224--41. RICH, S. (1963). The role of stomata in plant disease. Bull. Conn. agrie, exp. Sin, 664, 102-16. STALFELT,i . G. (1961). The effect of water deficit on stomatal movements in a carbon dioxidefree atmosphere. PhyMologia PI., 14, 826--43. THOMAS, M. D. (1951). Gas damage to plants. A. Rev. Pl. Physiol., 2, 293-322. THOMAS, M. D., HENDRICKS,R. H. & HILL, G. R. (1950). Sulfur metabolism of plants: effect of sulfur dioxide on vegetation. Ind. Engng Chem. ind. Edn, 42, 2231-5. WILSON,C. C. (1948). The effect of some environmental factors on the movements of guard cells. Pl. Physiol., Lancaster, 23, 5-37.