Douglas A. Findley, Gary J. Keever, Arthur H. Chappelka, Charles H. Gilliam and D. Joseph Eakes

Ozone (O₃) was identified as a significant phytotoxic air pollutant during the 1950s and has progressively become a major air pollutant across the United States. Normally associated with urban areas with large numbers of automobiles, tropospheric O₃ is readily transported long distances to non-urban or rural areas. The major effects of O₃ on terrestrial vegetation include visible injury and reductions in growth, productivity, and plant quality. Visible injury from acute O₃ exposures (i.e. exposure to high concentrations for short periods of time) has been observed on a number of landscape plants in the northeastern United States. In a screening of landscape plants common in the southeastern United States to chronic ozone exposure, cultivar differences were observed in buddleia. Chronic O₃ effects are caused by exposure to frequent, relatively low hourly concentrations, with periodic random, intermittent peaks of relatively high hourly concentrations on one or more days. Sensitivity to chronic and acute O₃ exposures is not necessarily correlated. Based on these results and the fact that even minor foliar injury can make a plant undesirable or unmarketable, the objective of this study was to determine differences among buddleia cultivars in sensitivity to acute concentrations of O₃ .

METHODS
Liners of eight buddleia cultivars were exposed to four O₃ treatments: 0, 125, 250, or 375 parts per billion (ppb) for 4 hours on 2 consecutive days in continuously stirred tank reactors located within a walk-in growth chamber. Plants were evaluated 2 and 7 days later using a severity index (SI) which is an estimated percentage of the leaves injured (PLI) and leaf area injured. A second experiment investigated the effects of both O₃ concentration and number of exposures. ‘Black Knight’ was exposed to O₃ concentrations of 0, 125, 250, or 375 ppb for 1 to 5 days for 4 hours daily.

RESULTS
The most common O₃ injury symptom was stippling of the upper leaf surface, which consisted of numerous small, reddish-purple, discrete spots. At O₃ concentrations
of 250 and 375 ppb, visible injury was observed on both the oldest and most recently matured leaves. Visible injury was observed under all O₃ concentrations except the control for all cultivars except ‘Charming Summer’ and ‘Lochinch’ in which visible injury was only observed under the two higher concentrations (250 and 375 ppb) (see table).

When exposed to 125 ppb O₃ , ‘Black Knight’, ‘Opera’, and ‘Royal Red’ had the highest SI, while the other five cultivars had much lower SI value. Although injury at 125 ppb was minor, these concentrations of O₃ have been recorded in urban areas of the southeastern United States during the summer, indicating a potential for injury under ambient conditions.

The SI ranged from 0.20 for ‘Charming Summer’ to 1.23 for ‘Royal Red’ when plants were exposed to an O₃ concentration of 250 ppb. Visible injury detected at this concentration was more severe than at lower concentrations with larger stipples and more leaves affected. This resulted in an increased SI for all cultivars. The SI ranged from 1.28 for ‘Opera’ to 4.56 for ‘Pink Delight’ when exposed to 375 ppb O₃. Foliar injury was extensive and was visible within 24 hours of the first exposure on the most severely injured cultivars, ‘Black Knight’, ‘Nanho Blue’, ‘Pink Delight’, and ‘Royal Red’. One week after the final exposure the most severely injured leaves in all cultivars were senescing.

The effects of a 4-hour exposure to O₃ for 1 to 5 days was evaluated using ‘Black Knight’, an O₃-sensitive cultivar. The SI increased as both O₃ concentration and number of exposures increased with the highest SI (6.28) observed for plants given the most exposures of the highest O₃ concentration. No visible injury was observed on control plants, and only minor visible injury was observed on plants exposed to 125 ppb of O₃. The SI ratings were similar for plants exposed to 250 ppb of O₃ for 1 or 2 days, as well as for plants exposed to 0 or 125 ppb of O₃ for 1 to 5 days. Plants exposed to 375 ppb of O₃ for 1 day or 250 ppb of O₃ for 2 or 3 days had similar SI ratings (SI of 0.60, 0.34, and 0.73, respectively). The SI was also similar for plants exposed to 2 days of 375 ppb of O₃ or 4 days of 250 ppb of O₃ (1.47 and 1.22, respectively). For a specific ozone exposure, which is the product of O₃ concentration and exposure duration, concentration was more important in inducing visible injury than number of exposures.

This study demonstrates that acute exposure to O₃ can cause visible injury to buddleia cultivars. Visible foliar injury is particularly detrimental to the value of landscape plants since visual appearance is a primary attribute in selection and use. Cultivars that appear most tolerant to elevated ozone concentrations include ‘Empire Blue’, and ‘Opera’. ‘Black Knight’, ‘Nanho Blue’, ‘Pink Delight’, and Royal Red’ appear more sensitive. Data collected in the O₃ exposure-response study indicate the importance of both concentration and number of exposures on visible injury. At concentrations currently found in urban areas of the southeastern United States, injury from acute exposure to O₃ would be relatively minor, but after extended periods of time (chronic exposure) injury could become extensive.