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Sorption of Ozone by ‘New Yorker’ tomato leaves
Environmental Pollution 58 (1989) 213-220
Sorption of Ozone by 'New Yorker' Tomato Leaves S. Graham & D. P. Ormrod* Department of Horticultural Science,University of Guelph, Guelph, Ontario, Canada N1G 2Wl (Received 5 May 1988; accepted 29 December 1988)
ABSTRACT Tomato (Lycopersicon esculentum Mill.) 'New Yorker'plants were exposed to 03 to compare leaf diffusive conductance ( LDC ) before exposure to 03 with 0 3 sorption rates and visible injury ratings. Two plant development stages and four or five leaf growth stages were examined. The LDC varied among leaf growth stages and between plant development stages and leaf surfaces; there was no continuity in the LDC pattern. Sorption rates differed among some leaf growth stages, and between plant development stages in expanding leaves (growth stage 1). For both development stages high sorption rates occurred in fully mature leaves; otherwise little similarity between corresponding leaf growth stages was evident. Total 03flux to the leaf was not well predicted by the LDC for water vapour; nor was visible injury well related to total flux. Differential mesophyll processes and leaf surface sorption capabilities may have accounted for some of the inconsistencies observed.
INTRODUCTION Several attempts have been made to explain the differential species sensitivity to 03 that is demonstrated by differences in leaf visual injury development after exposure to the same Oa treatment. Normally, increasing 0 3 levels are associated with greater per cent necrosis and chlorosis of leaf surfaces (Heck & Dunning, 1967). Gradients of severity of leaf injury within * To whom correspondence should be addressed. 213 Environ. Pollut. 0269-7491/89/$03-50 © 1989 Elsevier Science Publishers Ltd, England. Printed in Great Britain
214
S. Graham, D. P. Ormrod
a plant have been observed in various species and attributed to differences in 'leaf physiology'; in the evaluation to foliar injury, it is generally noted that intermediate-aged leaves which have just completed expansion are most sensitive to 0 3 (Glater et al., 1962; Ting & Dugger, 1971). The developmental stage at which a plant is exposed to 0 3 also may determine the plant's sensitivity. Ozone exposure of plants in the early vegetative stage is reflected by increased foliar injury and decreased growth in red maple (Townsend & Dochinger, 1974), turfgrasses (Richards et al., 1980) and petunia (Hanson et aL, 1975), compared with exposure at later development stages. Gaseous pollutant sorption studies have been conducted with several species (Craker & Starbuck, 1973; Roberts, 1974; Bennett, 1975; Taylor et al., 1982) to determine whether 0 3 uptake rate plays a significant role in determining a plant's 03 sensitivity. The uptake of gaseous pollutants has been described by the electrical analogue model (Bennett et al., 1973) and the chemical kinetics model (Rogers et al., 1977); the rate of pollutant uptake is affected by the plant surface area, boundary layer and stomatal resistance and metabolic activities of the plant. It has been determined that bean leaves sorb 03 in proportion to leaf area (Rich et al., 1970) and that younger leaves in soybean cultivars sorb more 0 3 on a leaf area basis than older leaves (Taylor et aL, 1982). Uptake of 0 3 was not correlated with stomatal closure in alfalfa (Hill, 1971) or tobacco (Craker and Starbuck, 1972). However, 03 uptake was associated with transpirational water loss in bean and petunia (Thorne & Hanson, 1972). There has been little research conducted on tomato sensitivity to 0 3 in terms of 0 3 sorption. Such studies may be useful in the explanation of sensitivity differences among plant development stages and leaf growth stages, that is, different leaf physiological ages or maturities. The objective of this research was to study the sorption of a moderately injurious level of 03 by tomato foliage of differing ages and to relate the sorption data to stomatal behaviour in terms of leaf diffusive conductance.
MATERIALS A N D METHODS Tomato plants (Lycopersicon esculentum Mill. cv. New Yorker) were grown from seed in 13cm diameter plastic pots containing Pro-mix BX (1"1:1 mixture of sphagnum peat moss, vermiculite, perlite, with added nutrients) and irrigated with complete nutrient solution (Hoagland & Arnon, 1950). Eight seeds were sown per pot and the germinated seedlings were thinned to one per pot 3 days after emergence. The plants were grown in a Conviron Model EY 15 growth chamber under the following environmental conditions: day/night temperatures, 24/20 ___I°C; relative humidity (RH),
Tomato sorption o f ozone
215
75+_5%; photosynthetic photon flux (PPF), 325 +_ 1 0 p m o l m - 2 s -1 at canopy level for 14 h per day. Irradiance was provided by a combination of 80% input wattage cool white fluorescent lamps and 20% input wattage incandescent lamps. Plants were exposed to 03 at two stages of development, based on plastochron index (PI). A PI of 6 indicated the early vegetative stage and a PI of 11 indicated the pre-floral stage. Within each plant development stage, four or five leaf growth stages were studied. In the early vegetative plant development stage these were two fully expanded leaf stages, an almost fully expanded leaf, and an expanding leaf. One of the fully expanded leaves was a day ahead of the other in reaching full expansion. At the pre-floral plant development stage, the same leaf growth stages were evaluated as well as a second almost fully expanded leaf. Expanding leaves differed in their date of appearance and degree of expansion. Foliar sorption of ozone relative to leaf growth stage and plant development stage was determined using a randomized complete block design, with four replicates (blocks). Each block was completed in 1 day, and consisted of a determination of ozone sorption for each leaf stage, each of which was taken from a different plant. In consequence, for early vegetative plants, there were four individuals measured each day (one block) resulting in one determination of ozone sorption for leaf stages 1, 3, 4 and 5. For prefloral plants, there were five individuals measured each day (one block) resulting in one determination of ozone sorption for leaf stages 1, 2, 3, 4 and 5. For both plant development stages, there were four independent replicate blocks. In total, 16 early vegetative and 20 pre-floral plants were examined. One day prior to 0 3 exposure, four or five uniform plants which were similar in PI and height, were removed from the growth chamber, irrigated with nutrient solution and placed in an exposure cabinet (Marie & Ormrod, 1984). The cabinet was illuminated by high pressure sodium lamps. Since the exposure cabinet was not situated within a growth chamber, temperature and humidity were more variable than in the growth chamber and were approximately 25°C and 70%, respectively. On the day of exposure, a 1-1itre cuvette (LiCor Model 6000-12), as is used in the Portable Photosynthesis System (LiCor 6200), was placed in a separate cabinet and exposed to 0.16/~1 litre- ~ 0 3 for 4 h. Preliminary experiments showed that after 4 h the inlet and outlet O 3 concentrations were similar, suggesting that saturation of the cuvette surfaces had occurred. The cuvette had been coated with Teflon and was modified to allow ozone uptake measurements. Fans within the cuvette were situated diagonally across from each other and were regulated to permit thorough mixing. Openings were drilled in the back of the cuvette to permit tubing to pass through to the upper (gas inlet) and lower (gas outlet) portions of the cuvette. Ozone was
216
S. Graham, D. P. Ormrod
generated by a Grace LG-2L2 Corona generator from filtered ambient air, cooled to 23°C and diluted with filtered ambient air in the plenum of the exposure cabinet to maintain a concentration of 0.16#1 litre-1 03 . Ozone was drawn into the cuvette from the cabinet and passed over the leaf. The outlet air stream was monitored with an 0 3 analyser (Dasibi model 1003-AH). The LDC of the abaxial and adaxial surfaces of the leaflet to be studied was measured prior to the 1-h ozone exposure, using a LiCor LI-700 transient porometer. The plant was then placed in the cabinet containing the ozone and the cuvette and the lower third portion (2 cm from base) of the tip leaflet of a leaf at a particular leaf growth stage was enclosed within the cuvette. Inlet and outlet concentrations of 0 3 were monitored prior to opening the cuvette and each 3 min for the 1-h exposure. The plants were not exposed to ozone except for the 1 h period of the cuvette study. Since only one plant could be studied at one time, the four or five leaf stages examined in one day were exposed at different times of the day. However, the order in which the leaf stages were examined was randomly assigned in each block, so that time of day effects were minimized. Ozone sorption was calculated as the difference between inlet and outlet concentrations. A thermocouple was attached to the leaf to monitor leaf temperature throughout the exposure. The CO 2 concentration was measured within the exposure cabinet and was found to be 430 #g litre- 1. The leaf area contained within the cuvette was measured after the 1-h exposure with a LiCor LI-300 Area Meter. Following exposure, the plants were returned to the growth chambers and kept for 3 days for signs of visual injury. On the third day after exposure, a relative injury index based on extent of necrosis and chlorosis was assigned to each leaf surface. The leaves were rated between 1 and 10 by visual assessment of per cent leaf area injured according to the Horsfall-Barratt Scale (Horsfall & Barratt, 1945).
R E S U L T S A N D DISCUSSION Prior to exposure to ozone, there were some differences in leaf diffusive conductance among leaf growth stages, within plant development stages and leaf surfaces (Table 1). For early vegetative plants, there were no differences among leaf growth for abaxial leaf diffusive conductance. However, the adaxial surface of the oldest expanded leaf (stage 5) and youngest expanding leaf (stage 1) had a lower leaf diffusive conductance than did the adaxial surface of fully expanded and almost fully expanded leaves (stages 4 and 3). For pre-floral plants, the abaxial surface of the expanding leaves (stages 2 and 1) had a higher leaf diffusive conductance than stage 3 leaves (almost
Tomato sorption o f ozone
217
TABLE 1
Leaf DiffusiveConductance (cm s-1) of Tomato Leaves of Various Growth Stages before Exposure to 03 at Early Vegetativeand Pre-floral Plant DevelopmentStages Plant development stage
Leaf growth stage a 1
Early vegetative Abaxial Adaxial Pre-floral Abaxial Adaxial
68"56a b'c ___0.8 2.75b + 0.4 13.60a ___2.0 3.22a +__0-6
2
--13.60a ___0.8 4-14a + 0.6
3
4
5
10.80a + 0.9 4.57a ___0.7
8-46a + 0.8 4-28a + 0.4
6.75a ___0.9 3.44b _+ 0.6
6-60b __+2.0 2.60a __+0-6
8-74ab __+2.4 4.12a + 0.8
8.00ab __+2.4 5.21 + 0 . 8
a 1 - - Y o u n g e x p a n d i n g leaf. 2 - - 8 0 % E x p a n d e d leaf. 3 - - 9 5 % E x p a n d e d leaf. 4---Fully e x p a n d e d leaf. 5 - - F u l l y e x p a n d e d leaf plus l day. b M e a n s in s a m e row followed by the s a m e letter are n o t significantly different ( P = 0.05). c E a c h entry is the m e a n o f four i n d e p e n d e n t replicates.
fully expanded (95 %)). The adaxial surface leaf diffusive conductance of prefloral plants did not differ among leaf growth stages. Ozone uptake was quantified on a leaf area basis for leaves at various growth stages within the two plant development stages (Table 2). There were some significant differences among leaf growth stages but little difference between plant development stages. For early vegetative plants, the oldest fully expanded leaf sorbed less 03 than did leaves of other ages. Ozone sorption was also different between the expanding leaf stages. At the prefloral plant development stage, the leaf stages segregated into two groups on the basis of their uptake rates: 03 sorption by the oldest fully expanded leaves and 80% expanded leaves was higher than by leaves at the other three growth stages. There was no continuum of 03 sorption rate in relation to leaf growth stage in pre-floral plants. Expanding (stage 1) leaves of early vegetative plants sorbed more ozone than expanding leaves of pre-floral plants (0.05 < P < 0"10). No visual injury was observed on plants at the early vegetative development stage whereas visual injury occurred on the pre-floral development stage plants. Some leaf growth stages developed more injury than others, although the relationship between adaxial and abaxial injury indices varied with leaf growth stage (Table 2). The amount of O3 sorbed by the tomato foliage varied both between the two plant development stages and among various leaf growth stages. The LDC before exposure varied among leaf growth stages, but the pattern was not consistent within either leaf surface or plant development stage. Ozone
S. Graham, D. P. Ormrod
218
TABLE 2 Ozone Sorption ( # g m - 2 s - 1 ) and Visible Injury Indices of Tomato Leaves of Various Growth Stages Exposed to 0 . 1 6 # 1 - 1 0 3 at Early Vegetative and Pre-floral Plant Development Stages Plant development stage
Early vegetative sorption Pre-floral sorption ta
Early vegetative injury index Abaxial Adaxial Pre-floral injury index Abaxial Adaxial
Leaf growth stage ~ 1
2
3
4
5
2.21c ~'c _ 0.2
--
1.47b _+ 0'3
0.61a _+ 0.2
1.76bc _+ 0-4
2.50b + 0.3 --
0"93a _+ 0.3 0.73
1.17a + 0.4 0.79
2.49b + 0.4 0.82
0 0
---
0 0
0 0
0 0
1.3ab 3.0b
0.6a 1.8a
1.6a 2.5ab
0.4a 1.3a
1.8b 1.3a
0-74a _ 0'3 2'10 e
a 1--Young expanding leaf. 2--80% Expanded leaf. 3--95% Expanded leaf. 4 - - F u l l y expanded leaf. 5--Fully expanded leaf plus 1 day. Means in the same row followed by the same letter are not significantly different ( P = 0-05). c Each entry is the mean of four independent replications. a Studentized t-test comparing ozone sorption between plant development stages within leaf growth stages. e 0-05 < P < 0"10 for t-test.
sorption was greatest in the youngest expanding and the oldest expanded leaves at the early vegetative plant development stage, although these leaves had a similar upper (abaxial) or lower (adaxial) LDC in comparison with leaves at stages 4 (fully expanded leaf) and 3 (95% expanded leaf). At the pre-floral plant development stage, sorption was greatest in the oldest fully expanded leaves and in the expanding (80%) leaves; although initial LDC was high in these leaf growth stages, it was not different from that observed in the pre-floral youngest expanding leaves which had a lower sorption rate. It is clear that initial LDC for water vapour does not correlate well with either ozone sorption or injury. These results do not support the hypothesis that recently expanded leaves are more sensitive to Oa, nor that they may be capable of increased 03 uptake (Glater et al., 1962). At low concentrations, younger soybean leaves absorbed more 03 per unit area than older leaves, and this relationship between leaf growth stage and Oa uptake was cultivar dependent (Taylor et al., 1982). In addition,
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219
foliar injury has not been observed to be consistently related to either the magnitude of the total 03 flux to the leaf(Evans & Ting, 1974) or 03 flux into the leaf interior (Taylor et al., 1982). This leads to speculation that leaf age associated differences in mesophyll processes, such as resistance to or detoxification of ozone, may be responsible for differential leaf flux and sensitivity. In this study, total 03 flux was not differentiated into adsorption and absorption: adsorption of 0 3 by the leaf surface may account for some o f the differences observed in sorption rates among leaf growth stages. Surface adsorption differences have been significantly correlated with cultivar sensitivity to 0 3 in petunia (Elkiey & Ormrod, 1980). The development of surface features such as pubescence, cuticular waxes and trichomes with leaf age may result in increased adsorption rates in older leaves. This study with tomato demonstrates that L D C for water vapour is not a particularly good predictor of total 0 3 flux to a leaf, nor is total 0 3 flux to the leaf a good predictor of visible injury. It also indicates that the separation of absorption from adsorption is probably a necessary element of any study which examines different leaf ages. ACKNOWLEDGEMENTS This research was funded by an operating grant from the Natural Sciences and Engineering Research Council of Canada. The assistance ofB. A. Marie and L. Pyear is gratefully acknowledged. REFERENCES Bennett, J. H. (1975). Interactions of air pollutants with canopies of vegetation. In Responses of Plants to Air Pollution, eds J. B. Mudd & T. T. Kozlowski. Academic Press, New York. Bennett, J. H., Hill, A. C. & Gates, D. M. (1973). A model for gaseous pollutant sorption by leaves. J. Air Pollut. Control Assn., 23, 957-62. Craker, L. E. & Starbuck, J. S. (1972). Metabolic changes associated with ozone injury of bean leaves. Can. J. Plant Sci., 52, 589-97. Craker, L. E. & Starbuck, J. S. (1973). Leaf age and air pollutant susceptibility: uptake of ozone and sulphur dioxide. Environ. Res., 6, 91~,. Elkiey, T. & Ormrod, D. P. (1980). Absorption of 03, SO2 and NO2 by petunia plants. Environ. Exp. Bot., 21, 63-70. Evans, L. S. & Ting, I. P. (1974). Ozone sensitivity of leaves: relationship to leaf water content, gas transfer resistance and anatomical characteristics. Am. J. Bot., 61, 592-7. Glater, R. A., Solberg, R. A. & Scott, F. M. (1962). A developmental study of the leaves of Nicotiana glutinosa as related to their smog sensitivity. Am. J. Bot., 49, 954-70.
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Hanson, G. P., Thorne, L. & Addis, D. H. (1975). The ozone sensitivity of Petunia hybridia Vilm. as related to physiological age. J. Am. Soc. Hort. Sci., 100, 188-90. Heck, W. W. & Dunning, J. A. (1967). The effect of ozone on tobacco and Pinto bean as conditioned by several ecological factors. J. Air. Pollut. Control. Assn., 17, 112-14. Hill, A. C. (1971). Vegetation: A sink for atmospheric pollutants. J. Air Pollut. Control Assn., 21, 341-6. Hoagland, D. R. & Arnon, D. I. (1950). The water culture method for growing plants without soil. Calif. Agr. Expt. Sta. Cir. 347, Berkeley. Horsfall, J. G. & Barratt, R. W. (1945). An improved grading system for measuring plant diseases. Phytopath., 35, 655. Marie, B. A. & Ormrod, D. P. (1984). Tomato plant growth and continuous exposure to sulphur dioxide and nitrogen dioxide. Environ. Pollut., 33, 257-65. Rich, S., Waggoner, P. E. & Tomlinson, H. (1970). Ozone uptake by bean leaves. Science, 169, 79-80. Richards, G. A., Mulchi, C. L. & Hall, J. R. (1980). Influence of plant maturity on the sensitivity of turfgrass species. J. Environ. Qual., 9, 49-53. Roberts, B. R. (1974). Foliar sorption of atmospheric sulphur dioxide by woody plants. Environ. Pollut., 7, 133-40. Rogers, H. H., Jeffries, H. E., Stahel, E. P., Heck, W. W., Ripperton, C. A. & Witherspoon, A. M. (1977). Measuring air pollutant uptake by plants: a direct kinetic technique. J. Air Pollut. Control Assn., 27, 1192-7. Taylor, G. E., Tingey, D. T. & Ratsch, H. C. (1982). Ozone flux in Glycine max (L.) Merr.: Sites of regulation and relationship to leaf injury. Oecologia (Berl.), 53, 179-86. Thorne, L. & Hanson, G. (1972). Species differences in rates of vegetal 0 3 absorption. Environ. Pollut., 3, 303-12. Ting, I. P. & Dugger, W. M. (1971). Ozone resistance in tobacco plants: A possible relationship to water balance. Atmos. Environ., 5, 147-50. Townsend, A. M. & Dochinger, L. S. (1974). Relationship of seed source and developmental stage to the 03 tolerance of Acer rubrum seedlings. Atmos. Environ., 8, 957-64.
Sorption of Ozone by 'New Yorker' Tomato Leaves S. Graham & D. P. Ormrod* Department of Horticultural Science,University of Guelph, Guelph, Ontario, Canada N1G 2Wl (Received 5 May 1988; accepted 29 December 1988)
ABSTRACT Tomato (Lycopersicon esculentum Mill.) 'New Yorker'plants were exposed to 03 to compare leaf diffusive conductance ( LDC ) before exposure to 03 with 0 3 sorption rates and visible injury ratings. Two plant development stages and four or five leaf growth stages were examined. The LDC varied among leaf growth stages and between plant development stages and leaf surfaces; there was no continuity in the LDC pattern. Sorption rates differed among some leaf growth stages, and between plant development stages in expanding leaves (growth stage 1). For both development stages high sorption rates occurred in fully mature leaves; otherwise little similarity between corresponding leaf growth stages was evident. Total 03flux to the leaf was not well predicted by the LDC for water vapour; nor was visible injury well related to total flux. Differential mesophyll processes and leaf surface sorption capabilities may have accounted for some of the inconsistencies observed.
INTRODUCTION Several attempts have been made to explain the differential species sensitivity to 03 that is demonstrated by differences in leaf visual injury development after exposure to the same Oa treatment. Normally, increasing 0 3 levels are associated with greater per cent necrosis and chlorosis of leaf surfaces (Heck & Dunning, 1967). Gradients of severity of leaf injury within * To whom correspondence should be addressed. 213 Environ. Pollut. 0269-7491/89/$03-50 © 1989 Elsevier Science Publishers Ltd, England. Printed in Great Britain
214
S. Graham, D. P. Ormrod
a plant have been observed in various species and attributed to differences in 'leaf physiology'; in the evaluation to foliar injury, it is generally noted that intermediate-aged leaves which have just completed expansion are most sensitive to 0 3 (Glater et al., 1962; Ting & Dugger, 1971). The developmental stage at which a plant is exposed to 0 3 also may determine the plant's sensitivity. Ozone exposure of plants in the early vegetative stage is reflected by increased foliar injury and decreased growth in red maple (Townsend & Dochinger, 1974), turfgrasses (Richards et al., 1980) and petunia (Hanson et aL, 1975), compared with exposure at later development stages. Gaseous pollutant sorption studies have been conducted with several species (Craker & Starbuck, 1973; Roberts, 1974; Bennett, 1975; Taylor et al., 1982) to determine whether 0 3 uptake rate plays a significant role in determining a plant's 03 sensitivity. The uptake of gaseous pollutants has been described by the electrical analogue model (Bennett et al., 1973) and the chemical kinetics model (Rogers et al., 1977); the rate of pollutant uptake is affected by the plant surface area, boundary layer and stomatal resistance and metabolic activities of the plant. It has been determined that bean leaves sorb 03 in proportion to leaf area (Rich et al., 1970) and that younger leaves in soybean cultivars sorb more 0 3 on a leaf area basis than older leaves (Taylor et aL, 1982). Uptake of 0 3 was not correlated with stomatal closure in alfalfa (Hill, 1971) or tobacco (Craker and Starbuck, 1972). However, 03 uptake was associated with transpirational water loss in bean and petunia (Thorne & Hanson, 1972). There has been little research conducted on tomato sensitivity to 0 3 in terms of 0 3 sorption. Such studies may be useful in the explanation of sensitivity differences among plant development stages and leaf growth stages, that is, different leaf physiological ages or maturities. The objective of this research was to study the sorption of a moderately injurious level of 03 by tomato foliage of differing ages and to relate the sorption data to stomatal behaviour in terms of leaf diffusive conductance.
MATERIALS A N D METHODS Tomato plants (Lycopersicon esculentum Mill. cv. New Yorker) were grown from seed in 13cm diameter plastic pots containing Pro-mix BX (1"1:1 mixture of sphagnum peat moss, vermiculite, perlite, with added nutrients) and irrigated with complete nutrient solution (Hoagland & Arnon, 1950). Eight seeds were sown per pot and the germinated seedlings were thinned to one per pot 3 days after emergence. The plants were grown in a Conviron Model EY 15 growth chamber under the following environmental conditions: day/night temperatures, 24/20 ___I°C; relative humidity (RH),
Tomato sorption o f ozone
215
75+_5%; photosynthetic photon flux (PPF), 325 +_ 1 0 p m o l m - 2 s -1 at canopy level for 14 h per day. Irradiance was provided by a combination of 80% input wattage cool white fluorescent lamps and 20% input wattage incandescent lamps. Plants were exposed to 03 at two stages of development, based on plastochron index (PI). A PI of 6 indicated the early vegetative stage and a PI of 11 indicated the pre-floral stage. Within each plant development stage, four or five leaf growth stages were studied. In the early vegetative plant development stage these were two fully expanded leaf stages, an almost fully expanded leaf, and an expanding leaf. One of the fully expanded leaves was a day ahead of the other in reaching full expansion. At the pre-floral plant development stage, the same leaf growth stages were evaluated as well as a second almost fully expanded leaf. Expanding leaves differed in their date of appearance and degree of expansion. Foliar sorption of ozone relative to leaf growth stage and plant development stage was determined using a randomized complete block design, with four replicates (blocks). Each block was completed in 1 day, and consisted of a determination of ozone sorption for each leaf stage, each of which was taken from a different plant. In consequence, for early vegetative plants, there were four individuals measured each day (one block) resulting in one determination of ozone sorption for leaf stages 1, 3, 4 and 5. For prefloral plants, there were five individuals measured each day (one block) resulting in one determination of ozone sorption for leaf stages 1, 2, 3, 4 and 5. For both plant development stages, there were four independent replicate blocks. In total, 16 early vegetative and 20 pre-floral plants were examined. One day prior to 0 3 exposure, four or five uniform plants which were similar in PI and height, were removed from the growth chamber, irrigated with nutrient solution and placed in an exposure cabinet (Marie & Ormrod, 1984). The cabinet was illuminated by high pressure sodium lamps. Since the exposure cabinet was not situated within a growth chamber, temperature and humidity were more variable than in the growth chamber and were approximately 25°C and 70%, respectively. On the day of exposure, a 1-1itre cuvette (LiCor Model 6000-12), as is used in the Portable Photosynthesis System (LiCor 6200), was placed in a separate cabinet and exposed to 0.16/~1 litre- ~ 0 3 for 4 h. Preliminary experiments showed that after 4 h the inlet and outlet O 3 concentrations were similar, suggesting that saturation of the cuvette surfaces had occurred. The cuvette had been coated with Teflon and was modified to allow ozone uptake measurements. Fans within the cuvette were situated diagonally across from each other and were regulated to permit thorough mixing. Openings were drilled in the back of the cuvette to permit tubing to pass through to the upper (gas inlet) and lower (gas outlet) portions of the cuvette. Ozone was
216
S. Graham, D. P. Ormrod
generated by a Grace LG-2L2 Corona generator from filtered ambient air, cooled to 23°C and diluted with filtered ambient air in the plenum of the exposure cabinet to maintain a concentration of 0.16#1 litre-1 03 . Ozone was drawn into the cuvette from the cabinet and passed over the leaf. The outlet air stream was monitored with an 0 3 analyser (Dasibi model 1003-AH). The LDC of the abaxial and adaxial surfaces of the leaflet to be studied was measured prior to the 1-h ozone exposure, using a LiCor LI-700 transient porometer. The plant was then placed in the cabinet containing the ozone and the cuvette and the lower third portion (2 cm from base) of the tip leaflet of a leaf at a particular leaf growth stage was enclosed within the cuvette. Inlet and outlet concentrations of 0 3 were monitored prior to opening the cuvette and each 3 min for the 1-h exposure. The plants were not exposed to ozone except for the 1 h period of the cuvette study. Since only one plant could be studied at one time, the four or five leaf stages examined in one day were exposed at different times of the day. However, the order in which the leaf stages were examined was randomly assigned in each block, so that time of day effects were minimized. Ozone sorption was calculated as the difference between inlet and outlet concentrations. A thermocouple was attached to the leaf to monitor leaf temperature throughout the exposure. The CO 2 concentration was measured within the exposure cabinet and was found to be 430 #g litre- 1. The leaf area contained within the cuvette was measured after the 1-h exposure with a LiCor LI-300 Area Meter. Following exposure, the plants were returned to the growth chambers and kept for 3 days for signs of visual injury. On the third day after exposure, a relative injury index based on extent of necrosis and chlorosis was assigned to each leaf surface. The leaves were rated between 1 and 10 by visual assessment of per cent leaf area injured according to the Horsfall-Barratt Scale (Horsfall & Barratt, 1945).
R E S U L T S A N D DISCUSSION Prior to exposure to ozone, there were some differences in leaf diffusive conductance among leaf growth stages, within plant development stages and leaf surfaces (Table 1). For early vegetative plants, there were no differences among leaf growth for abaxial leaf diffusive conductance. However, the adaxial surface of the oldest expanded leaf (stage 5) and youngest expanding leaf (stage 1) had a lower leaf diffusive conductance than did the adaxial surface of fully expanded and almost fully expanded leaves (stages 4 and 3). For pre-floral plants, the abaxial surface of the expanding leaves (stages 2 and 1) had a higher leaf diffusive conductance than stage 3 leaves (almost
Tomato sorption o f ozone
217
TABLE 1
Leaf DiffusiveConductance (cm s-1) of Tomato Leaves of Various Growth Stages before Exposure to 03 at Early Vegetativeand Pre-floral Plant DevelopmentStages Plant development stage
Leaf growth stage a 1
Early vegetative Abaxial Adaxial Pre-floral Abaxial Adaxial
68"56a b'c ___0.8 2.75b + 0.4 13.60a ___2.0 3.22a +__0-6
2
--13.60a ___0.8 4-14a + 0.6
3
4
5
10.80a + 0.9 4.57a ___0.7
8-46a + 0.8 4-28a + 0.4
6.75a ___0.9 3.44b _+ 0.6
6-60b __+2.0 2.60a __+0-6
8-74ab __+2.4 4.12a + 0.8
8.00ab __+2.4 5.21 + 0 . 8
a 1 - - Y o u n g e x p a n d i n g leaf. 2 - - 8 0 % E x p a n d e d leaf. 3 - - 9 5 % E x p a n d e d leaf. 4---Fully e x p a n d e d leaf. 5 - - F u l l y e x p a n d e d leaf plus l day. b M e a n s in s a m e row followed by the s a m e letter are n o t significantly different ( P = 0.05). c E a c h entry is the m e a n o f four i n d e p e n d e n t replicates.
fully expanded (95 %)). The adaxial surface leaf diffusive conductance of prefloral plants did not differ among leaf growth stages. Ozone uptake was quantified on a leaf area basis for leaves at various growth stages within the two plant development stages (Table 2). There were some significant differences among leaf growth stages but little difference between plant development stages. For early vegetative plants, the oldest fully expanded leaf sorbed less 03 than did leaves of other ages. Ozone sorption was also different between the expanding leaf stages. At the prefloral plant development stage, the leaf stages segregated into two groups on the basis of their uptake rates: 03 sorption by the oldest fully expanded leaves and 80% expanded leaves was higher than by leaves at the other three growth stages. There was no continuum of 03 sorption rate in relation to leaf growth stage in pre-floral plants. Expanding (stage 1) leaves of early vegetative plants sorbed more ozone than expanding leaves of pre-floral plants (0.05 < P < 0"10). No visual injury was observed on plants at the early vegetative development stage whereas visual injury occurred on the pre-floral development stage plants. Some leaf growth stages developed more injury than others, although the relationship between adaxial and abaxial injury indices varied with leaf growth stage (Table 2). The amount of O3 sorbed by the tomato foliage varied both between the two plant development stages and among various leaf growth stages. The LDC before exposure varied among leaf growth stages, but the pattern was not consistent within either leaf surface or plant development stage. Ozone
S. Graham, D. P. Ormrod
218
TABLE 2 Ozone Sorption ( # g m - 2 s - 1 ) and Visible Injury Indices of Tomato Leaves of Various Growth Stages Exposed to 0 . 1 6 # 1 - 1 0 3 at Early Vegetative and Pre-floral Plant Development Stages Plant development stage
Early vegetative sorption Pre-floral sorption ta
Early vegetative injury index Abaxial Adaxial Pre-floral injury index Abaxial Adaxial
Leaf growth stage ~ 1
2
3
4
5
2.21c ~'c _ 0.2
--
1.47b _+ 0'3
0.61a _+ 0.2
1.76bc _+ 0-4
2.50b + 0.3 --
0"93a _+ 0.3 0.73
1.17a + 0.4 0.79
2.49b + 0.4 0.82
0 0
---
0 0
0 0
0 0
1.3ab 3.0b
0.6a 1.8a
1.6a 2.5ab
0.4a 1.3a
1.8b 1.3a
0-74a _ 0'3 2'10 e
a 1--Young expanding leaf. 2--80% Expanded leaf. 3--95% Expanded leaf. 4 - - F u l l y expanded leaf. 5--Fully expanded leaf plus 1 day. Means in the same row followed by the same letter are not significantly different ( P = 0-05). c Each entry is the mean of four independent replications. a Studentized t-test comparing ozone sorption between plant development stages within leaf growth stages. e 0-05 < P < 0"10 for t-test.
sorption was greatest in the youngest expanding and the oldest expanded leaves at the early vegetative plant development stage, although these leaves had a similar upper (abaxial) or lower (adaxial) LDC in comparison with leaves at stages 4 (fully expanded leaf) and 3 (95% expanded leaf). At the pre-floral plant development stage, sorption was greatest in the oldest fully expanded leaves and in the expanding (80%) leaves; although initial LDC was high in these leaf growth stages, it was not different from that observed in the pre-floral youngest expanding leaves which had a lower sorption rate. It is clear that initial LDC for water vapour does not correlate well with either ozone sorption or injury. These results do not support the hypothesis that recently expanded leaves are more sensitive to Oa, nor that they may be capable of increased 03 uptake (Glater et al., 1962). At low concentrations, younger soybean leaves absorbed more 03 per unit area than older leaves, and this relationship between leaf growth stage and Oa uptake was cultivar dependent (Taylor et al., 1982). In addition,
Tomato sorption of ozone
219
foliar injury has not been observed to be consistently related to either the magnitude of the total 03 flux to the leaf(Evans & Ting, 1974) or 03 flux into the leaf interior (Taylor et al., 1982). This leads to speculation that leaf age associated differences in mesophyll processes, such as resistance to or detoxification of ozone, may be responsible for differential leaf flux and sensitivity. In this study, total 03 flux was not differentiated into adsorption and absorption: adsorption of 0 3 by the leaf surface may account for some o f the differences observed in sorption rates among leaf growth stages. Surface adsorption differences have been significantly correlated with cultivar sensitivity to 0 3 in petunia (Elkiey & Ormrod, 1980). The development of surface features such as pubescence, cuticular waxes and trichomes with leaf age may result in increased adsorption rates in older leaves. This study with tomato demonstrates that L D C for water vapour is not a particularly good predictor of total 0 3 flux to a leaf, nor is total 0 3 flux to the leaf a good predictor of visible injury. It also indicates that the separation of absorption from adsorption is probably a necessary element of any study which examines different leaf ages. ACKNOWLEDGEMENTS This research was funded by an operating grant from the Natural Sciences and Engineering Research Council of Canada. The assistance ofB. A. Marie and L. Pyear is gratefully acknowledged. REFERENCES Bennett, J. H. (1975). Interactions of air pollutants with canopies of vegetation. In Responses of Plants to Air Pollution, eds J. B. Mudd & T. T. Kozlowski. Academic Press, New York. Bennett, J. H., Hill, A. C. & Gates, D. M. (1973). A model for gaseous pollutant sorption by leaves. J. Air Pollut. Control Assn., 23, 957-62. Craker, L. E. & Starbuck, J. S. (1972). Metabolic changes associated with ozone injury of bean leaves. Can. J. Plant Sci., 52, 589-97. Craker, L. E. & Starbuck, J. S. (1973). Leaf age and air pollutant susceptibility: uptake of ozone and sulphur dioxide. Environ. Res., 6, 91~,. Elkiey, T. & Ormrod, D. P. (1980). Absorption of 03, SO2 and NO2 by petunia plants. Environ. Exp. Bot., 21, 63-70. Evans, L. S. & Ting, I. P. (1974). Ozone sensitivity of leaves: relationship to leaf water content, gas transfer resistance and anatomical characteristics. Am. J. Bot., 61, 592-7. Glater, R. A., Solberg, R. A. & Scott, F. M. (1962). A developmental study of the leaves of Nicotiana glutinosa as related to their smog sensitivity. Am. J. Bot., 49, 954-70.
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