Effect of Ozone and Elevated Carbon Dioxide on Cuticular Membrane Ultrastructure of Yellow Poplar (Liriodendron tulipifera)

Abstract

Ozone causes damage to leaves of many tree species and has been shown to increase cuticular membrane thickness in a conifer (red spruce). It has been suggested that elevated carbon dioxide can protect plants against air pollutants by reducing pollutant flux into the leaves, but it is not known whether ozone-induced changes in the leaf cuticle will be altered by elevated carbon dioxide. In 1991, white pine (Pinus strobus) and yellow poplar (Liriodendron tulipifera) seedlings were planted in a previously forested area. In May 1992, seedlings were enclosed in open-top chambers and the following treatments were initiated: 1) charcoal-filtered (CF) air, 2) ambient air (1X ozone), 3) two times ambient ozone (2X ozone), and 4) 2X ozone plus 350 ppm carbon dioxide (2X plus CO2). First-year leaf samples were collected in August 1992. Midregions of white pine needles and yellow poplar leaves were prepared for electron microscopy by conventional methods and examined in a transmission electron microscope. The cuticular membrane of white pine grown in CF air was approximately 500 nm thick and consisted of an amorphous outer region and an inner reticulate region. Cuticular membrane thickness and wax tubule diameter were slightly greater in white pine needles grown in 2X plus CO2. The cuticular membrane of yellow poplar leaves grown in CF air was approximately 200 nm thick and consisted of an outer polylamellate region and a reticulate inner region. Cuticular membranes of leaves grown in 2X plus CO2 were also approximately 200 nm thick, but the outer membrane region had an amorphous structure. The cuticular membrane of yellow poplar leaves grown in 2X ozone were approximately 150 nm thick due to a thinner reticulate region. Second-year white pine and yellow poplar leaves will be collected in late August 1993, and cuticular membrane structure will be compared with the 1992 results.