Qualitative and quantitative effects of acid misting and two air pollutants on foliar structures of Liriodendron tulipifera

Abstract

One-year-old seedlings of Liriodendron tulipifera (yellow poplar) were exposed to a daily misting regime at pH levels of 5.6, 4.6, 3.6, or 2.6 for a period of 8 weeks. Identical misting conditions supplemented with either a 0.1 ppm fumigation of ozone or NO2 were established for additional treatments. Leaf samples were prepared for light and transmission electron microscopy and light micrographs were used for quantitative analyses of transectional cellular and air-space areas. Leaves exposed to acid misting alone exhibited a cellular collapse at pH 2.6. With increasing acidity, a steady decline in mesophyll air space, an increase in dense inclusion bodies, and a decline in plastid starch content were observed. Plants that received supplemental ozone fumigation during misting exhibited cellular swelling at all pH levels and an increase in total mesophyll area from pH 5.6 to 3.6. While mesophyll cell transectional areas were virtually unchanged from pH 3.6 to 2.6, the air space was significantly reduced. Distinctive differences in apparent cell wall thickness were noted between palisade and spongy cells at pH 3.6 and 2.6. Leaves exposed to NO2 fumigation plus mist macroscopically appeared vigorous and generally showed increased blade thickness with increasing acidity primarily because of changes in mesophyll cell content. Chloroplasts showed prominent stroma protrusions into the cytoplasm. Thus, this study reveals distinctively different quantitative and morphological foliar responses to the misting and fumigation regimes.