RESPONSE OF HONEYLOCUST CULTIVARS TO AIR POLLUTION STRESS IN AN URBAN ENVIRONMENT1

Five honeylocust cultivars were evaluated for tolerance to O 3 pollution over a 3-year period. Symptom development was monitored on a monthly basis during the growing season and changes in diameter growth measured once each year. Measurements of diffusive resistance and transpiration were used to determine if cultivar differences in Og-tolerance were associated with differences in gas ex- change rates through the stomates. 'Imperial' was the most O3 sensitive and 'Majestic' the most O3 resistant cultivar based on foliar response. The average yearly increase in diameter was greatest in the cultivars more resistant to O3. Ozone sensitivity was negatively correlated with leaf diffusive resistance and positively correlated with transpiration rate. When making cultivar selections for urban tree plantings on the basis of O3-tolerance 'Majestic' should be considered first, followed by 'Sunburst,' 'Skyline,' and 'Shademaster.' 'Imperial' should not be used in polluted environments.