National Ambient Air Quality Objectives For Ground-Level Ozone- Summary - Science Assessment Document

6. Vegetation Effects

The discussion in this document on the effects of ozone on plants has been based upon the prior report of the Vegetation Objective Working Group (VOWG) of the Multistakeholder NO_X/VOC Science Program. A review of more recent literature indicated that the information base on the effects of ozone on plants had not progressed sufficiently to warrant any change to the discussion or conclusions of the report by the VOWG.

6.1 Effects Summary

Ozone injury was first observed and documented under field conditions in the Los Angeles area. The majority of research that followed through the 1950s to the 1970s was conducted with pot-grown plants under greenhouse or controlled environment conditions. It is only since the mid-1980s that research activities have attempted to mimic field conditions more closely through the use of open top chambers and fumigation systems.

Acute symptoms on broad-leaved plants consist of chlorosis, fleck, stipple and uni- or bifacial necrosis. On conifers, acute responses consist of mottle, banding and chlorosis. Chronic symptoms are related to frequent, relatively low hourly ozone concentrations, with periodic, intermittent peaks of relatively high hourly concentrations. Chronic effects can lead to changes in plant growth, productivity and quality, and these effects may occur without visible symptoms. When symptoms do develop, they can include chlorosis, delayed early season growth, premature senescence and leaf abscission. In the case of acute effects, plants can compensate for stress during respite periods; therefore, the frequency of ozone episodes and the time interval between such episodes are critical in evaluating and modeling plant response.

It is well recognized that foliage is the primary site of plant response to ozone exposure. It is also known that ozone exerts a phytotoxic effect only if a sufficient amount reaches sensitive sites within the leaf. Thus, ozone injury will not occur if the rate of uptake is low enough that the plant can detoxify the ozone or repair or compensate for the effects. Ozone can be destroyed at the leaf surface through interactions with surface waxes. Oxidation or cleavage of surface waxes can lead to changes in composition and physical properties of the leaf surface (e.g. decreased water repellence) that may subsequently affect the uptake of ozone. Once ozone enters the leaf via open stomata, it has the potential to impair cellular function. Because oxidants are also produced within the cell as a result of normal photosynthetic processes, and are injurious to cell constituents, plants have evolved enzymatic mechanisms to transform oxidants to less toxic forms. The detoxifying enzymes are saturable, however, thus cellular systems may be overwhelmed by the presence of extra oxidants from ambient ozone exposure, resulting in plant damage.

The role of exclusion or detoxifying mechanisms in determining ozone sensitivity among species or cultivars is not well understood, as there is not at present a conceptual model describing plant resistance to oxidants. Scientific understanding of resistance remains uncertain. It seems clear that the detoxification of ozone and its products would consume energy, although whether this additional energy burden would significantly decrease plant productivity, relative to the direct effects of ozone on photosynthesis for example, is not known.

In summary, it is the integrated cellular system that confers and determines plant sensitivity to ozone. Effects at the cellular level are ultimately expressed as visible injury to the leaf or as secondary effects that can be expressed as reduced root growth, reduced yield of fruits or seeds, or both. These responses only appear after initial defence mechanisms are overrun. Biochemical and physiological changes can occur without visible injury symptoms appearing.

In Ontario, studies of the impact of ozone on crop yield have identified the following crops to be at greatest risk: dry bean, potato, onion, hay, turnip, winter wheat, soybean, spinach, green bean, flue-cured tobacco, tomato and sweet corn. Crops estimated to be marginally at risk (insufficient data did not permit more accurate quantification of loss) included cucumber, squash, pumpkin, melon, grape, burley tobacco and beet. In Alberta, the crop yield analysis consisted of a review of the available literature for ozone response based on crops grown. This information was then compared with a limited amount of urban ozone-monitoring data, and it was concluded that there were no identifiable risks to sensitive crops at that time. Other agricultural crops commonly grown in Canada but not mentioned above should not be considered resistant to the impact of ozone--their response is simply not known at this time.

Tree species common to Canada that have demonstrated ozone sensitivity with respect to a variety of endpoints (e.g. biomass, height, photosynthesis) under controlled ozone exposure conditions include: maples (sugar, silver, red), ash (white, green), spruce (white), white pine, poplar (hybrid), cottonwood, cherry, walnut, sycamore, white birch and red oak. Although ozone impacts varied significantly, and included both positive (growth stimulus) and negative (growth reduction) responses in many of the experimental studies, the response to seasonal mean exposures of 40-60 ppb for over half of the studies was reported as at least a marginal growth reduction. There is also considerable evidence that ozone can injure many annual and perennial grass species commonly used in turfgrass production in parts of Canada.

6.2 Quantification of Impacts

An absolute threshold ozone concentration above or below which vegetation injury will or will not occur has not been identified in the scientific literature. A threshold exposure level for plant biochemical response to ozone is largely conceptual in nature. Theoretically, biochemical systems could reach a saturation level above which they can no longer compensate for injury caused by ozone. A threshold dose response for ozone may exist, but the threshold may be so subtle that it cannot be detected, given current methods of investigation.

There are several endpoints that may be considered in establishing concentration-response relationships for vegetation. The two most common ones are biomass (or biomass losses) and visible foliar injury. (Note that "biomass" with respect to agricultural crops is measured as the yield of the relevant crop part). Biomass (or yield) losses are related to chronic exposures and visible injury to acute exposures. Both biomass loss and foliar injury have been investigated in crop, ornamental and tree species.

Exposure Index

For adequately quantifying acute and chronic effects, it is necessary to identify both short- and long-term exposure indices. A number of different exposure indices were reviewed in terms of their efficacy in describing ozone exposure - plant response relationships and their suitability as management tools. This was done for both acute and chronic exposure indices. The evaluation concluded that the form of an index to protect vegetation should be cumulative (summation of hourly values) and should emphasize peak concentrations. The SUM60, the AOT40 and the W126 are three such indices. The W126 index was dismissed from consideration on the basis that it was too complex an index to administer. The review of the SUM60 and the AOT40 showed there was no compelling scientific reason to favour one or the other. Furthermore, a regression of these two indices performed using air quality data from the years 1980-1993 confirmed a high degree of similarity (r² = 0.97). Therefore, in terms of assessing the areas in Canada where vegetation is impacted by ozone, use of either the SUM60 or the AOT40 would yield similar results. Consequently, on the basis of other factors, predominantly the access to databases in the United States, where the SUM60 has been used, a decision was made to recommend use of the SUM60 in the assessment of chronic effects on vegetation in Canada. The SUM60 index was also selected for assessment of acute effects on vegetation based on studies in Ontario of white bean and radish.

The SUM60 index is the sum of hourly ozone concentrations equal to or greater than 60 ppb over the daylight period 08:00 - 19:59. The daily sums are then added over a specified time period; a 3-day SUM60 is used for the assessment of acute effects, whereas a seasonal (3-month) SUM60 is used for the assessment of chronic effects. Although the SUM60 clearly encapsulates some aspects of plant exposure that are important in the plant response (i.e. cumulative exposure over a time period and the relative importance of peak concentrations), there are other factors demonstrably important in determining plant response (e.g. phenology, time of day etc.) that are not accounted for in the SUM60 index. Furthermore, there is no biological basis for assuming that concentrations below 60 ppb ozone are not significant in the plant response. In the future it may be possible to develop a more biologically relevant index.

LOAEL Determination

In terms of quantifying the impacts on vegetation associated with exposure to ozone, this report has focused on identifying LOAELs, that is, the lowest ozone concentrations that have been shown to induce an adverse response in plants under experimental conditions. In this regard, a minimum yield loss level must be identified that can be directly attributed to ozone exposure. Losses below this amount are within the range of experimental uncertainty. Similarly, for acute effects, a trace level of foliar injury (defined as a foliar injury index score of 1-20) is the lowest level of injury that can be reliably quantified and is therefore the recommended endpoint of assessment of acute effects on vegetation.

Given the paucity of Canadian data on ozone exposure - crop yield relationships, particularly the lack of data amenable to LOAEL determination using the SUM60 index, the U.S. NCLAN (National Crop Loss Assessment Network) database was relied upon. In order to use the data to develop LOAELs in a Canadian context, a subset of the data was analyzed by removing data for crops not grown in Canada as well as those grown in California, where growing conditions differ markedly from those in Canada. The 3 month, 12 hr. SUM60 values corresponding to 10% yield loss levels from this suite of crops are shown in Table 1 (SAD Table 8.9). From these data, turnip and wheat are identified as the most sensitive crops. Given experimental uncertainties, the limitations of the NCLAN protocol for LOAEL determination, and the amount of both inter- and intra-specific variability in the response of crops to ozone exposure, it was considered inappropriate to identify the single lowest effect level as the LOAEL. Instead, a more conservative approach was adopted, and a LOAEL range of 5900 to 7400 ppb-h was identified. This range excludes a SUM60 level of 2900 ppb-h identified for one particularly sensitive wheat cultivar.

Similarly, based upon the results of studies conducted by the U.S. EPA in the late 1980s on the impact of ozone on forest trees, a 3 month, 12 hr SUM60 LOAEL range of 4,400 to 6,600 ppb-h was identified for 10% biomass loss. This LOAEL range was based upon the response of black cherry and aspen (Table 2 (SAD Table 8.11)).

Table 1 Summary of NCLAN SUM60 index values resulting in a 10% yield loss in NCLAN studies (excluding cotton and crops assessed in California).

Crop Evaluated	Cultivar	Moisture Status	12-hour SUM60 (ppb-h)
Corn (L)	PIOPAG		41,600 55,800
Kidney Bean Kidney Bean (L)	CAL LT RED CAL LT RED		15,200 17,200
Peanut (L)	NC-6		36,200.
Potato	NORCHIP NORCHIP		9,900 20,300
Sorghum	DELALB		67,600
Soybean	CORSOY CORSOY AMSOY PELLA WILLIAMS CORSOY CORSOY	Dry Wet	15,300 42,200 32,800 18,200 15,500 71,200 70,000
Soybean	RSOY CORSOY CORSOY CORSOY WILLIANS WILLIAMS HODGSON DAVIS DAVIS DAVIS DAVIS DAVIS DAVIS YOUNG YOUNG	Dry Wet Dry Wet Dry Wet Dry Wet Dry Wet Dry Wet	89,100 62,200 10,200 11,800 21,100 14,800, 8,400 13,800 23,400 57,100 35,200 45,900 24,100 38,800 25,000
Tobacco (L)	MCNAIR		24,400
Turnip (T)	JUST RIGHT PURPLE TOP SHOGOIN TOKYO CROSS		7,400 5,900 6,600 9,300
Wheat	ABE ARTHUR ROLAND ABE ARTHUR VONA VONA		25,100 21,300 7,400 34,800 27,700 2,900 7,700

Table 2: Exposure-response data for 10% level of biomass loss, for trees exposed in OTCs to ozone

Tree Species Evaluated	12-hour SUM60 (ppb-h)
Aspen - wild	19,100
	15,800
	43,700
	55,900
	55,400
	18,700
Aspen 216	14,700
Aspen 253	8,100
Aspen 259	4,700
Aspen 271	13,300
Aspen 216	9,500
Aspen 259	5,200
Aspen 271	29,600
Aspen - Wild	15,000
Douglas Fir	89,300
	250,000
	90,800
	94,400
	72,000
	70,800
	63,000
Ponderosa pine	17,900
	26,300
	18,500
	27,100
	11,300
	21,600
	19,500
	14,900
	27,900
	55,200
	43,400
Red Alder	32,100
	17,900
	79,000
	3,8008
	250,000
	21,800
Black Cherry	6,600
	4,400
Red Maple	71,700
Tulip Poplar	23,400
	19,900
	14,700
Loblolly GADR 15-91	71,000
Loblolly GAKR 15-23	212,100
Sugar Maple	25,300
	23,800
E. White Pine	21,600
	31,500
Virginia Pine	191,200

Although the history of studies on the acute phytotoxic effects of ozone on plants is a long one, there are, unfortunately, very few studies available that can be evaluated retrospectively to quantify ozone exposure - foliar injury relationships using a SUM60 exposure index. The best data available were from studies carried out in Ontario on white bean over the period 1985-1995. These studies identified a LOAEL range for trace injury in white bean of 500-700 ppb-h. Although the recommended form of the short term exposure index was based on analysis of only two crops (white bean and radish), and the LOAEL range for acute effects was derived from only the white bean studies, both these plants are known to be sensitive to foliar injury following exposure to ozone. Clearly though, there is inadequate information currently to fully characterize the risk of acute foliar injury to crops, trees and native vegetation across Canada.

6.3 Co-Occurring Pollutants

The interaction of ozone and nitrogen oxides or the three- or four-way interaction of ozone, sulphur dioxide, nitrogen oxide and acid rain has not been specifically addressed in any of the field oriented, crop yield response research to date. Although photochemical smog and other forms of atmospheric pollution involve numerous pollutants in addition to ozone, the limited amount of information available on their combined effects on vegetation precludes any specific estimate of the magnitude of these effects in relation to the effects of ozone alone. This finding should be considered in the light of observed patterns of co-occurrence of ozone, SO₂, and NO₂ in urban, rural and remote sites. In the U.S., during 1978-82, co-occurrences were found to be infrequent and of short duration. When they did occur, they were usually sequential or a combination of sequential and overlapping exposures of short duration.

Because of its phytotoxic potential, peroxyacetylnitrate (PAN) could be the most relevant co-occurring pollutant and would not be expected to exhibit short-duration type of co-occurrence. Although PAN has been documented as acting synergistically with ozone in causing increased foliar injury to some species under some conditions, this combined impact cannot yet be generalized, as considerable variability has been demonstrated in the experimental findings published to date (synergistic, antagonistic and additive responses). Given the relatively low levels of PAN reported in the Canadian atmosphere, an evaluation of combined impacts with PAN was not undertaken for this review. However, this aspect of the nature of ozone effects on vegetation also warrants further investigation.