INFLUENCE OF OZONE AND SOIL MAGNESIUM STATUS ON THE COLD HARDINESS OF LOBLOLLY-PINE (PINUS-TAEDA L) SEEDLINGS

One‐year‐old half‐sib loblolly pine (Pinus taeda L.) seedlings were planted in a sandy loam soil having approximately 15 or 35 mg kg−1 of exchangeable Mg and were exposed to subambient, ambient, or twice ambient (0.02, 0.05, or 0.09 μl O3 1−1, respectively) concentrations of O3 in open‐top chambers from April to October 1988. The cold hardiness of seedlings was tested on October 24, January 24, and March 21 by subjecting needle samples from the first and last growth flush to temperatures of ‐5, ‐10, ‐15, and ‐ 20 °C and determining relative electrical conductivity of the diffusate. Seedlings exposed to twice ambient O3 levels during the growing season were less hardened in early autumn and spring than seedlings exposed to ambient or subambient levels of O3, suggesting that O3 affects the plant's hardening and dehardening processes. Ozone and soil Mg treatments had no significant effects on visible injury of shoots following freezing treatments, nor were any visible symptoms of injury due to O3 evident during the study. Water potential components of excised shoots were estimated using pressure‐volume analysis. Osmotic potentials became more negative over the dormant period, suggesting that solutes had accumulated in the cells as seedlings hardened. Seedlings grown in high Mg soil had less negative osmotic potentials in January and March than those grown in low Mg soil, suggesting that seedlings in high Mg soil were more susceptible to cold temperatures, probably due to greater physiological activity. Copyright © 1990, Wiley Blackwell. All rights reserved