Liriodendron tulipifera (known as the tulip tree) is a woody species that has been previously classified as sensitive to ozone (O3) in terms of visible leaf injuries and photosynthetic primary reactions. The objective of this work is to give a thorough description of the detoxification mechanisms that are at the basis of O3 sensitivity. Biochemical and molecular markers were used to characterize the response of 1-year-old saplings exposed to O3 (120 ppb, 5 h day-1, for 45 consecutive days) under controlled conditions. O3 effects resulted in a less efficient metabolism of Halliwell-Asada cycle as confirmed by the diminished capacity to convert the oxidized forms of ascorbate and glutathione in the reduced ones (AsA and GSH, respectively). The reduced activity of AsA and GSH regenerating enzymes indicates that de novo AsA biosynthesis occurred. This compound could be a cofactor of several plant-specific enzymes that are involved in the early part of the phenylpropanoid and flavonoid biosynthesis pathway, as confirmed by the significant rise of PAL activity (+75%). The induction of the defence-related secondary metabolites (in particular, rutin and caffeic acid were about threefold higher) and the concomitant increase in transcript levels of PAL and CHS genes (+120 and 30%, respectively) suggest that L. tulipifera utilized this route in order to partially counteract the O3-induced oxidative damage.
Keywords: Gene expression; Halliwell-Asada cycle; Ornamental plant; Oxidative stress; Phenylpropanoids; Reactive oxygen species.