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PHOTOSYNTHESIS AND TRANSPIRATION MEASUREMENTS AS BIOMARKERS OF AIR POLLUTION EFFECTS ON FORESTS Dr. William E. Winner Department of General Science Weniger Hall 355 Oregon State University Corvallis, OR 97331 ABSTRACT Gaseous air pollutants such as SO2 and Of are known to alter rates of CO2 fixation and water loss by leaves. In addition, acid deposition may increase the availability of nitrogen and other potential nutrients, thereby altering foliar nutrient content and metabolic capacity. Thus, many forms of air pollutants have the potential to affect photosynthesis, transpiration, and other important, derived features of leaves, such as stomata! conductance, CO2 internal, and water use efficiency Using these direct effects of air pollutants on leaf physiology as biomarkers of tree responses to air pollutants will be discussed. These physiological parameters may be useful as biomarkers because they change dynamically as pollutant exposures change, they are easy to measure with portable field equipment, they are related to long-term growth processes, and when taken together, they can provide an overview of how the leaf is linked to its environment. The fact that photosynthesis and transpiration change with season, leaf age, time of day, and with non-pollution environmental factors poses problems for using these parameters as biomarkers of air pollution. Also, the capacity for plants to compensate for stresses by reallocating resources between organs makes it difficult to extrapolate from leaf level measurements to an assessment of whole tree and canopy processes. Any attempt for such an application would require carefully established sampling protocols to define background rates of photosynthesis and transpiration, a method for defining control measurements, and an approach for putting single leaf measurements in perspective with the whole tree. Measurements of photosynthesis and transpiration may be most useful when included in a suite of parameters used in biomarker assessments. INTRODUCTION The idea of using physiological features of trees as markers of air pollution stress is attractive for a number of reasons. Most importantly, physiological features of trees are likely to respond in a dynamic way to air pollutants and, in so doing, indicate that air pollutants are causing important changes in the biological processes necessary for the tree to sustain itself. In addition, these physiological changes should be detectable before more overt symptoms of damage, such as foliar injury, are apparent. Another feature of physiological markers of air pollution response is that these markers can be 303

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304 viewed in the context of whole plant growth and survival, i.e., air pollution-caused changes in the physiological processes of plants can be linked to their productivity, longevity, and reproduction. Many physiological parameters could be used as markers of tree responses to air pollutants. A few of these parameters, such as foliar gas emission, foliar enzyme activity levels, and cuticular integrity, are discussed in other chapters. Successful application of these leaf characteristics to monitor air pollution impact requires a detailed understanding of the way by which the leaf feature relates to environment (Fig. 1~. For example, under optimal conditions, the status of a leaf parameter can be assessed. However, plants are almost always growing in the presence of naturally occurring environmental stresses, such as drought, inadequate water and nutrients, and low light. These natural stresses cause a deflection in the rate of the leaf process of interest and this deflection should be viewed as natural, normal, and with adaptive value over the longterm. Air pollutants typically cause a further deflection in the process of interest and represents the response which must be identified as the biomarker. Our capacity to see the marker depends upon our understanding of the process of interest, and the knowledge necessary to predict how the process will be influenced by naturally occurring stresses and by air pollutants. RELATIVE REDUCTION IN METABOLISM Optimal Natural Natural Stresses Plus Conditions Stresses Air Pollution 1 00%. ._ o - 0 ._ - a) - CL o% - Figure 1. The variable effects of natural stresses (a) combined with air pollutants (b) on depression of plant metabolism. The purpose of this chapter is to highlight the use of photosynthesis, transpiration, and stomata! conductance measurements as indicators of air pollution stress. There are several reasons why assessments of photosynthesis and stomata! status should be important candidates in the attempt to identify markers of air pollution stress. For example,

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305 1. The relationship between photosynthesis and conductance is under biological control and is related to productivity; 2. Photosynthesis and conductance are known to change quickly with the onset of air pollution exposures, 3. Stomatal conductance values can be used to calculate air pollution absorption by plants; 4. Photosynthesis and conductance can be easily measured with non-destructive techniques as well as by analysis of collected tissue. Thus, one goal of this paper will be to expand on each of these points to clarify the unique advantages associated with the use of photosynthesis and conductance as markers of tree response to air pollutants. There are a number of reasons why photosynthesis and conductance measurements are not currently used as biomarkers. Most importantly, it is difficult to associate changes in these leaf-level processes to specific exposures to air pollutants. In addition, calibrating photosynthetic and stomata! responses to air pollutants will prove to be difficult. Thus another goal of this paper will be to identify the barriers to the use of these measurements of leaf physiology as air pollution markers. LINKS BETWEEN PHOTOSYNTHESIS, STOMATAL CONDUCTANCE, AND GROWTH Stomata are small pores on leaf surfaces which can open and close. The importance of these pores is that they represent the main route of gas exchange between the leaf and air. The degree of stomata! opening is under biological control (Fig. 2) and represents one of the ways by which plants relate to environmental change (Jones 1983~. The paradox for plants is that conductance should be as high as possible in order to maximize rates of carbon gain via photosynthesis, while at the same time, conductance should be such that transpiration rates are not so high that plants desiccate (Farquhar et al. 1980~. Plants are thought to maximize carbon gain while minimizing water loss by continually sensing the environment and adjusting- stomata to the appropriate pore size. For example, many plants close their stomata at night. This is adaptive because plants conserve water during dark periods when the light reactions of photosynthesis cannot occur. Thus, stomata respond to a CO2 sensor. Stomata also respond to a sensor linked to plant water relations. If transpiration rates are high due to low relative humidity or limited soil water availability, many plants will close stomata. During these periods, CO2 cannot diffuse from the air into the leaf, and plants sacrifice photosynthesis to conserve water. Although there are exceptions to the general rules above, stomata play a central role in regulating rates of carbon gain and water loss. The rate of photosynthesis is intimately linked to growth. More than 90% of the dry weight of a plant is in the form of carbohydrate which originates from photosynthesis. Thus, the source of plant carbohydrate is known. However, the links between rates of CO2 fixation and rates of growth are complex (Mooney, 1972~. For example, one useful measure of photosynthesis is "photosynthetic capacity," a measure made on a leaf-area basis when environmental conditions are near optimal for metabolism. Plants which have high photosynthetic capacity can be small. In fact, desert annuals have among- the highest photosynthetic rates but are typically small (Mooney et al., 1976~. Crop cultivars which have the highest photosynthesis rates may not have the highest yield. Finally, the largest trees are often among the plants with the lowest photosynthetic capacities.

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Direct piglet effect (feedforward) 306 _ _ , Environment I__ ,- .. , / ~ ~ \ CO2 H2O Guard cell ~ . ~ ~ 4K't ~ ` Direct humidity effect (fee(3forwar(i) ~~ _ - CO2 feedback /~71 Me~abo~ic l+~\ Flydroactive ~ '_~~ // \\feedbaci``, . . ~ Tissue ~ _ ~ I ~ '-tCO2 sensor | I. .. . tlydro passive feed back Figure 2. Active and passive feedback control systems for regulation stomata! conductance involving CO2 and water. Source: Reprinted with permission of Cambridge University Press from Plants and Microclimate 1983. Copyright 1983 by Cambridge University Press. Photosynthesis is linked to growth via leaf longevity (Mooney 1972~. Thus, leaves with low photosynthesis can contribute a great deal of carbohydrate to growth by remaining active for long periods. Conversely, leaves which are short-lived will not produce much carbohydrate regardless of their photosynthetic capacity. Photosynthesis is also linked to growth via the process of carbon allocation (Monsi 1968; Mooney 1972) (Fig. 3~. The product of leaf biomass and photosynthetic rate gives the carbon supply rate for the entire canopy. Some of this carbon is allocated to roots and some is allocated to shoots. The carbon allocated to shoots can be used to acquire more carbon for the plant whereas carbon allocated to roots, although essential for plant survival, does not acquire more carbon. Thus, plants with equal photosynthetic capacities, but different carbon allocation schemes will have different growth rates. Interestingly, many species differ in their patterns of carbon allocation between roots and shoots. In addition, environmental factors also cause changes in carbon allocation patterns. Atmospheric stresses which limit carbon gain shift allocation to favor the shoots whereas stresses which reduce uptake of nitrogen and water by roots shift allocation to roots. The potential for air pollutants to alter photosynthesis and growth of both roots and shoots increases the value of this physiological parameter as a biomarker of tree response to air pollution.

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307 Initial al location sta te Environmental stress New allocation state Leaf X Photosynthetic `t Roof X Nutrients biomass rate =K Kbiomass uptake J \\ Carbon // Reduce carbon supply Low light SO2 / O3 / Leaf ~ Roof biomass biomass Harbor / \ Reduce nitrogen/water supply / \ Drought \ Low soil fertility Leaf ~ Roof biomass biomass carbon Figure 3. Effects of environmental stresses on carbon allocation patterns of plants (Winner and Atkinson, 1986~. EFFECTS OF AIR POLLUTANTS ON PHOTOSYNTHESIS AND STOMATAL FUNCTION Leaf-level Responses. Most forms of air pollutants have the potential to alter leaf metabolism. The effects of gaseous pollutants on CO2 and water vapor exchange rates has been studied most thoroughly in experiments with SO2 and O3 (see reviews in Winner et al. 1985a, Guderian 1985~. However, any gaseous pollutant known to alter plant growth will also alter both photosynthesis and conductance. The potential also exists for oxides of sulfur and nitrogen to affect leaves. More specifically, foliar nitrogen content should increase with deposition of nitrate from the atmosphere. Since photosynthetic capacity and conductance increase with foliar nitrogen content (Field and Mooney 1986), increased nutrient availability due to wet and dry deposition processes should be reflected in increased rates of leaf metabolism. Studies with SO: and O3 have shown that there are several mechanisms of stomata! response to gaseous pollutants (Fig. 4~. These mechanisms may operate both at the leaf level and at the whole plant level. Leaf level responses to SO2 and O3 gas exchange studies have clearly shown that mechanisms of leaf level responses can be fast and extreme. For example, a radish leaf exposed to 0.4 ppm SO2 decreased its photosynthetic rate by 75% within 10 minutes (Winner et al. 1988) (Fig. 5~. In addition, stomata also responded to this treatment within 30 minutes.

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308 MECHANISMS OF STOMATAL RESPONSES TO GASEOUS POLLUTANTS * Leaf level Direct effects on guard cells / epidermal cells Indirect effects on photosynthesis * Canopy / whole plant level Premature leaf loss Indirect effects on foliar N Figure 4. Some mechanisms of stomata! responses to gaseous air pollutants. This simple experiment (Fig. 5) is important because it shows that a gaseous pollutant can cause stomata! closure. Many other studies have also shown that SO2 (Winner and Mooney l980a,b; Kimmerer and Kozlowski, 1981; Olszyk and Tingey, 1986) and ON (Reich and Lassoie, 1984; Olszyk and Tingey, 1986; Temple 1986,) can cause stomata! closure. Thus, the idea that severe and untimely stomata! closure might be a marker of tree response to air pollution stress seems attractive. However, other studies have shown that SO2 (Mansfield and Majernik, 1970; Black and Unsworth, 1980) and Of (Evans and Ting, 1974) can also cause stomata! opening. Variation in stomata! response to SO2 and Of has been recently reviewed (Winner et al. 1988~. Thus, any attempts to use stomata! conductance as a marker of leaf level responses to gaseous pollutants must be framed within an understanding of factors which lead to either stomata! opening or closing. The radish fumigation experiment described above (Fig. 5) is also important because it provides a clue about why stomata can either open or close in response to air pollution exposure. SO2 and O3 have the potential of affecting stomata by acting directly on guard cells. In addition, it is possible for these pollutants to act first on photosynthesis which then activates the previously described CO: sensor (Fig. 2) and results in subsequent changes in conductance. The time sequence analysis of the radish experiment shows that SO: causer! changes in photosynthesis before changes in conductance suggesting, that in this experiment, stomata may be responding to air pollutants via changes in carbon metabolism.

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309 and en 20 o in 18 - In o I_ o AL ~ `~` i' _ _ 10 20 30 TIME (minutes) - `_ _ - _ _ 400 40 50 cat 500 ~ - - 450 '5 3 I: C' Figure 5. Time course for changes in photosynthesis (is- ~ ) and conductance (------) following the additon of SO2 and 0.4 ppm at time 0. Ambient CO2 concentration was 350 ppm (Winner et al., 1988~. A number of other studies have shown that SO2 can cause changes in photosynthesis prior to changes in conductance (see summary in Winner et al., 1988~. Data from many of these studies were not collected by investigators for the purpose of time series analysis, but the trends of response sequence can nonetheless be determined. Interestingly, reports indicating that stomata! responses to SO2 precede photosynthetic responses were not found. In addition, there are no experiments with Of that allow such time course analyses. If SO2-caused decreases in photosynthesis can lead to stomata! closure, then SO2- caused increases in photosynthesis may lead to stomata! opening. The CO2 sensor operates by causing stomata! closure when CO2 concentrations in the mesophyll increase. Such shifts in CO2 would occur if SO2 damaged the photosynthetic apparatus. Conversely, SO2 may enhance electron transport processes associated with photosynthesis leading to increased photosynthesis and decreased CO2 concentrations in leaves. Such a shift in CO2 concentrations would result in stomata! opening. Plant-level Responses. Air pollutants can also alter conductance of the entire canopy of trees by altering stomata! pore size. For example, O3 is known to cause premature senescence and abscission of foliage (Guderian et al., 1985~. One consequence of this leaf loss is a reduction in leaf surface area capable of exchanging gases between the canopy and air. Just as air pollution-caused stomata! closure is adaptive in that air pollution absorption rates are decreased, so too is the response of leaf loss a potentially adaptive response.

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310 -O. ~ I, Time o3 s 3 o o CHANG ES IN G ROWTH o AND FOLIAR NITROGEN ~ Figure 6. Hypothetical time course of root and shoot growth responses to O3 episodes. Another mechanism by which gaseous pollutants can potentially alter whole plant canopy conductance involves the relative synchrony of suppression of shoot and root growth. Gaseous pollutants are known to shift root/shoot ratios (Winner and Atkinson, 1986) and many studies show that root growth is more sensitive to exposures than is shoot growth (Tingey and Reinert, 1971~. These conclusions are typically based on studies in which plants are fumigated for periods ranging from 30 to 90 days and then harvested for whole-plant growth analysis. A strong possibility exists that shoot growth suppression and root growth suppression are not synchronous (Fig. 6~. Similarly, recovery of shoot growth and recovery of root growth when air pollution exposures terminate are apt to be out of phase. The importance of root and shoot growth rates from the perspective of conductance is manifest through foliar nitrogen content. As discussed earlier, stomata! conductance and photosynthetic capacity increase with foliar nitrogen content (Field and Mooney, 1986~. If shoot growth recovery precedes root growth recovery, then foliar nitrogen content may be reduced by dilution and photosynthetic capacity and conductance will also be lowered. However, if an air pollution episode results in a great deal of leaf loss, the recovery process might result in a large root system supplying nitrogen to relatively few leaves. In this case, the leaves may increase in N content resulting in higher levels of leaf metabolism. These patterns of whole-plant resource allocation might help explain why, in some studies, O3 or SO2 can bring apparent increases in leaf-level processes (Libera et al., 1973~.

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311 CALCULATIONS OF AIR POLLUTION ABSORPTION Stomatal conductance to water vapor not only is important as it relates to fluxes of CO: and H:O between the leaf and air but also as it relates to air pollution absorption (Winner and Mooney 1980 a,b; Winner and Atkinson, 1986~. Calculating air pollution flux rates through stomata requires knowing leaf conductance values for H2O, ambient concentrations of any gaseous pollutants, and the diffusion coefficient of that pollutant in air. Flux rates of pollutants can then be integrated over time to determine total quantity of pollutant absorbed during exposure. The strength of this approach is that physiological and growth responses of plants to air pollutants can be expressed on the basis of a quality of air pollution absorbed rather than on the basis of atmospheric concentrations. Calculating flux rates of air pollutants into foliage on the basis of stomata! conductance assumes that intracellular concentrations of the pollutant are zero and that stomata! conductance is lower than all other components of leaf level conductance (Winner et al., 1 985b). The extent to which these assumptions are untrue represents error in the method. Indications are that error from these assumptions are hard to measure but nonetheless small (Winner et al., 1 985b). The error for estimating air pollution uptake by foliage by chemical analysis of foliar samples before and after exposure is known to be large, is not dynamic, requires destructive sampling, and is not useful for pollutants such as Of because oxygen is present in foliage in high concentrations. Thus, estimation of gaseous air pollution absorption by foliage is best done by calculating flux rates of pollutants through stomata. MEASUREMENTS OF PHOTOSYNTHESIS AND CONDUCTANCE Non-destructive Techniques. A number of scientfic equipment manufacturing firms produce portable gas exchange systems useful for screening leaf populations for photosynthsis and conductance. For example, systems manufactured by LiCor, Inc. (Lincoln, NE) and by Analytical Design Corporation, Ltd. (Hodgested, England) are commonly available and are favorably compared (Parkinson et al., 1988~. Also, Data Design Group, Inc. (La Jolla, LA) makes a comparable system. These systems are typically battery powered and easily carried into the field. Another important feature of these systems is their capacity to acquire and store data. The systems can be operated to survey numbers of leaves quickly with each measurement taking about 1 minute, and can be used to measure photosynthesis and conductance for many (20-200) leaves, that remain attached to trees, within a single day. This capacity to measure gas exchange rates of numerous leaves in a short time is essential if photosynthesis and conductance measurements are to play a useful role as air pollution markers. Sample Collections. Leaves can also be collected from tree tops or other inaccessible places and then used in gas exchange measurements. This technique has been applied in a number of remote field sites (Lange et al., 1986~. Use of this method requires measuring gas exchange rates of leaves attached to a tree, followed by remeasurement at intervals after the leaf is detached. This protocol is followed to characterize the effects of leaf removal, handling, and storage on gas exchange parameters. Handling techniques that minimize artifacts include storing the leaf in cool places, retrimming the collected branch or petiole under water, and leaving the cut surface in water, and keeping the time period between leaf collection and gas exchange measurements as short as possible.

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312 The radio isotope 14C has been used to generate 14CO2 ~ which is then used to assess the effects of air pollutants on photosynthesis (e.g., McLaughlin et al., 1982~. In these studies, leaves are enclosed in a chamber and exposed to 14CO2 for specific periods of time. The 14CO2 provides a tag which can be used to document photosynthesis rate and the metabolic fate of photosynthate. Air pollution effects are determined by comparing the 14CO2 fixation rates and fates for air pollution treated plants and controls. Photosynthesis and conductance can also be indirectly assessed by collecting leaf samples for a-nalysis of stable carbonisotope ratios, or more specifically, their 1 C/ 1 3C ratios. The o l 3c values of C3 plants, typically ranges from -26/oo to 32/oo (Fig. 7) (Troughton, 1979~. This is because the CO2 fixing enzyme of C3 plants (RUBISCO) preferentially fixes the 1 2C-containing CO2 over the heavier form. Thus, carbohydrate of C3 plants is depleted in 13C relative to the atmosphere. o: 1 3C VALUES FOR COT C3 PLANTS, AND C4 PLANTS C 3 PLANTS 51 3C = -32 5 %o 81 3 C = -26.3%o ~, 1 ppm 't_ ~_ ~ ~- ppm ? CO2 = 350 ppm o$1 3C = -7%O C4 PLANTS ~ 1 3C = -9%O =120/ ,_ J ppm -`'_ _ . ~ Figure 7. 13C values for C in atmospheric CO2 and in foliage. The range of ol3c values for C3 plants is in large part dependent upon the CO2 concentrations in the leaf mesophyll (Farquhar et al., 1982) (Fig. 7). These CO2 concentrations reflect both the rate of photosynthesis and the degree of stomata! opening. Under some conditions, CO2 internal concentrations for C3 plants can be as low as 220 ppm. This would be under high light with closed stomata. Under these circumstances, RUBISCO is less descriminating and less fractionation of the two stable C isotopes occurs. C stable isotope fractionation is highest when CO2 internal concentrations are high which occurs when stomata are open.

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313 The idea that any environmental factor which changes CO2 internally also changes the 513C ratio has been shown for drought and other stresses (Guy et al. 1980; Winter et al., 1982). Recent studies show that ambient O3 levels can also alter o l 3C values for leaves, stems and roots (Greitner and Winner, 1988) (Fig. S). This is not surprising since this O3 treatment was al-so found to deplete CO2 internal concentrations. This study raises the possibility that o l 3c values of plants in the field may be useful in identifying plants growing with air pollution stress. However, this will require developing the capacity for partitioning 13C shifts due to air pollutants from shifts due to other environmental stresses. '~ _ A , , , ~ ~ . ~ _ _ ~1 ~0 - _ _ _ ^29 lo; %_ =1 O Cat) 20 _ Cal C 27 Zen 1 _ _ ~ ~- ~ 1 control treatment ozone, LEGEN D radish leaven radish hypocotyl~ ~3 soybean leaven =~ soybean root" radish roots Figure 8. O3-caused shifts in ol3c values of leaves, stems, and roots (Greitner and Winner, 1988~. PROBLEMS USING PHOTOSYNTHESIS AND CONDUCTANCE AS AIR POLLUTION MARKERS Pollutant Specificity. One difficulty with using physiological properties of leaves as markers of air pollution is that responses may not be specific to pollution. As mentioned above, most physiological processes of leaves, including photosynthesis and conductance,

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314 change with many environmental parameters. air pollutants will be a difficult task. Identifying changes in metabolism due to Another problem is that physiological responses may not be specific for specific pollutants. For example, O3 and SO2 may both cause stomata! closure and decreases in photosynthesis. If an air pollution-caused decrease in leaf metabolism were identified, how would the causal agent be identified? Thus, the marker would reveal the presence of an air pollutant but the air pollutant which caused the effect would remain unknown. Photosynthesis and conductance changes in response to air pollutants will be difficult to use as air pollution markers because even once the pollution response is known, it will not be quantified. The degree of stomata! response to SO2 or O3 is apt to change with temperature, humidity, age, season, and the presence of other air pollutants. Current understanding is not sufficient to predict the degree of physiological responses to well defined air pollution exposures in controlled environments. Therefore, it will be some time before that capacity is sufficiently developed for application to field sites. One of the challenges to those intent on using photosynthesis and conductance measurements as air pollution markers for trees will be to use these data with other carefully selected data. Using only gas exchange measurements will be insufficient for assessing trees in controlled- studies or in uncontrolled field surveys. Gas exchange measurements in both mechanistic experiments and surveys are most useful when combined with other data, including biochemical analysis, growth analysis, and community analysis, and thereby put into a broad biological perspective. SUMMARY Photosynthesis, transpiration, and conductance have potential air pollution biomarker applications. Leaf-level metabolism, alone, is insufficient as an air pollution biomarker. Leaf-level metabolism, used with other measurements, might be used to develop an air pollution biomarker index. REFERENCES Black, V.J., and M.H. Unsworth. 1980. Stomatal responses to sulfur dioxide and vapor pressure deficit. Exp. Bot., 31:667-677. Evans, L.S., and I.P. Ting. 1974. Ozone sensitivity of leaves: relationship to leaf water potential, gas transfer resistance, and anatomical characteristics. Am. I. Bot., 61:592-597. Farquhar, G., E.-D. Schulze, and M. Kuppers. 1980. Responses to humidity by stomata of Nicotiana Glauca L. and Corylus avellana L. are consistent with the optimization of carbon dioxide uptake with respect to water loss. Australian I. Plant Physiol., 7:3 1 5-327. Farquhar, G.D., M.H. O'Leary, and J.A. Berry. 1982. On the relationship between carbon isotope discrimination and the intracellular carbon dioxide concentration in leaves. Australian J. Plant Physiol., 9:121-137.

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315 Field, C. and H.A. Mooney. 1986. The photosynthesis-nitrogen relationship in wild plants. Pp. 25-55 in On the Economy of Plant Form and Function, Givnish, T. (ed.~. Cambridge University Press, Cambridge, England. Greitner, C.S. and W.E. Winner. ~ van. increases In ~ ~ =c values of radish and soybean plants caused by ozone. New Phytologist, 108:489-494. ^1 1_ Guderian, R. (ed.~. 1985. Air Pollution by Photochemical Oxidents: Formation, Transport, Control and Effects on Plants. Springer Verlay, Berlin. 346 p. Guderian, R., D.T. Tingey, and R. Rabe. 1985. Effects of photochemical oxidants on plants. P. 346 in Air Pollution by Photochemical Oxidants: Formation, Transport, Control and Effects on Plants. Guderian, R. (ebb. Springer Verlag, Berlin. 346 p. Guy, R.D., D.M. Reid, and H.R. Krouse. 1980. Shifts in carbon isotope ratios of two C3 halophytes under natural and artificial conditions. Oecologia, 44:241-247. Jones, H. 1983. Plants and Microclimate. England. 323 p. Cambridge University Press, Cambridge, Kimmerer, T.W., and T.T. Kozlowski. 1981. Stomatal conductance and sulfur uptake of five clones of Populus tremuloides exposed to sulfur dioxide. Plant Physiol., 67:990-995. Lange, O.L., G. Fahrar, and J. Gaebel. 1986. Rapid field determination of photosynthetic capacity of cut spruce twigs (Picea abies) at saturating ambient CO2. Trees, 1:70-77. Libera, W., H. Ziegler, and I. Ziegler. 1973. Forderung der Hill-reaktion und der CO2 Fixierung in isolierten spinatchchloroplasten durch niedere sulfitkonzentraionen. Planta, 190:269-279. Mansfield, T.A., and O. Majernik. 1970. Can stomata play a part in protecting plants against air pollutants. Environ. Pollution, 1:149-154. McLaughlin, S.B., R.K. McConathy, D. Duvick, and K.L. Mann. 1982. Effects of chronic air-pollution stress on photosynthesis, carbon allocation, and growth of white pine. Forest Sci., 28:60-70. Monsi, M. 1968. Mathematical models of plant communities. Pp. 131 - 150 in Functioning of Terrestrial Ecosystems at the Primary Production Level, Eckarcit, F. (ed.~. UNESCO, Paris. Mooney, H.A. 1972. The carbon balance of plants. Ann. Rev. Ecol. Syst., 3: 315-346. Mooney, H.A., Ehleringer, J., and J. A. Berry. 1976. High photosynthetic capacity of a winter annual in Death Valley. Science, 194:322-324. Olszyk, D.M., and D.T. Tingey. 1986. Joint action of O3 and SO2 in modifying plant gas exchange. Plant Physiol., 82:401-405.

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316 Parkinson, K.J., D. McDermitt, S.C., Roemer, S.P. Cline, and W.E. Winner. 1988. Measurement and comparison of gas-exchange of Pinus halepensis and Flaveria brownii using commercial photosynthesis systems. Pp. 130-164 in Responses of Trees to Air Pollution, Winner, W.E. and L.B. Phelps (eds.~. Proc. West. Conifer Res. Coop. Workshop, Nov., l9X7, Boulder, CO. Reich, P.B., and J.P. Lassoie. 1984. Effects of low level O3 exposure on leaf diffusive conductance and water-use efficiency in hybrid poplar. Plant, Cell, and Environ., 7:661-668. Temple, P.J. 1986. Stomatal conductance and transpirational responses of field-grown cotton to ozone. Plant, Cell, and Environ., 9:315-321. Tingey, D.T., and R.A. Reinert. 1971. Effect of low concentrations of ozone and sulfur dioxide on foliage, growth, and yield of radish. J. Amer. Soc. Hort. Sci., 96:369- 371. Troughton, J.H. 1979. 13C as an indicator of carboxylation reactions. Pp. 140-149 in Photosynthesis II. Gibbs, M. and E. Latzko (eds.~. Encyclopedia of Plant Physiology, New Series, Vol. 6, Springer Verlog, Berlin. pp.l40-149. Winner, W.E., and C.J. Atkinson. 1986. Absorption of air pollutants by plants and consequences for growth. Trends in Ecol. and Evol., 1:15-18. Winner, W.E., C. Gillespie, and W. Sheng. 1988. Stomatal responses to SO2 and O3. In: Physiological and Biochemical Responses of Plants to Air Pollutants. Welburn, A., and Darrall, N. (eds.~. DeGruyter, Inc., Berlin. In Press. Winner, W.E., and H.A. Mooney. 1 980a. Ecology of SO2 resistance: I. Effects of fumigations on gas exchange of deciduous and evergreen shrubs. Oecologia, 44: 290- 295. Winner, W.E., and H.A. Mooney. 1 980b. Ecology of SO2 resistance: II. Photosynthetic changes of shrubs in relation to SO2 absorption and stomata! behavior. Oecologia, 44:296-302. Winner, W.E., H.A. Mooney, and R.A. Goldstein. (eds.~. 1 985a. Sulfur Dioxide and Vegetation: Physiology, Ecology, and Policy Issues. Stanford, California. 593 p. Stanford University Press, Winner, W.E., H.A. Mooney, K. Williams, and S. Von Caemmerer. 1985b. Measuring and Assessing SO2 effects on photosynthesis and plant growth. Pp. 1 18-132 in Sulfur Dioxide and Vegetation: Physiology, Ecology, and Policy Issues. Winner, W.E., H.A. Mooney, and R.A. Goldstein (eds.~. Stanford University Press, Stanford, CA. Winter, K., J.A.M. Holtum, G.E. Edwards, and M. O'Leary. 1982. Effect of low relative humidity on o l 3C values of radish and soybean plants caused by ozone. New Phytologist, 108:489-494.