| Abstract: | Tropospheric ozone is a widespread and phytotoxic air pollutant in the industrialized world and causes reduced growth in many tree species. It is therefore important that, for example, the responses of the economically important loblolly pine to ozone are determined thoroughly. The objective of the study was to determine changes in ultrastructure, the vanadate-sensitive ATPase activity of the plasma membrane, the fatty acids of plasma membrane phospholipids, visible injury, and growth in loblolly pine (Pinus taeda L.) needles exposed to different concentrations of ozone in open-top chambers. The treatments were charcoal filtered air (CF), nonfiltered air (NF), or NF-air with ozone added for 12 h daily at 1.5- or 2-fold ambient ozone concentrations from May to October, 1993. Visible injury was more severe in the high than in the low ozone treatments. Growth of needles of the first flush of 1993 was significantly reduced in the highest ozone treatment. Two types of ultrastructural injury, characterized as either acute or chronic, were observed in mesophyll cells under elevated ozone. The acute injury lead to cell collapse and death. The chronic injury, characterized by several symptoms, e.g. decreased chloroplast size and increased density of the stroma, was also found in the NF ozone treatment. Increased density of chloroplast stroma and swelling of thylakoids were transient symptoms, suggesting partial recovery as ozone concentrations decreased in fall. Ozone induced decreases in the specific activity of vanadate-sensitive ATPase of plasmalemma and in the degree of unsaturation in phospholipid fatty acids. The detected reduced needle growth, ultrastructural injury and perturbations in the function and composition of the plasma membrane indicate susceptibility of loblolly pine to ozone. Changes in the plasma membrane phospholipids may have contributed to the decrease in ATPase activity. Injury to the key enzyme of the plasma membrane can directly affect intracellular processes. In the long-term, decreased viability of needles can lead to reductions in loblolly pine productivity. |
