Reactive oxygen species and antioxidant machinery in Liriodendrum tulipifera plants exposed to ozone

Trees are essential in the urban environment not only because of their aesthetic and social values, but also for their effects on air quality. Data of the present work show some of the integrated mechanisms that may confer sensitivity/tolerance in Liriodendron tulipifera (known as the tulip tree) saplings exposed to ozone (O3) (120 ppb, 5 h day-1 for 45 consecutive days) in order to improve the management of green spaces responding to oxidative stress. At the end of fumigation, visible injury due to O3 was observed: symptoms were minute (Ø 1-2 mm) roundish dark-blackish necrosis, mainly located in the interveinal areas of recently fully expanded leaves. The injured leaf area was about 40% of the total surface. Reactive oxygen species levels, membrane injury, enzymes/metabolites linked to ascorbate-glutathione (AsA-GSH) cycle and to the synthesis of phenylpropanoids show modifications caused by O3 in terms of: (i) increase in the superoxide radical production (+41%, in comparison to air filtered controls); (ii) reduction in accumulation of hydrogen peroxide (-55%) and of the enzymes involved in its metabolism [as confirmed by the decrease in superoxide dismutase and catalase activities (-33 and -82%, respectively)]; (iii) increase of antioxidant capacity [as documented by the improved 1,1-diphenyl-2-picrylhydrazyl radical (DPPH) scavenging activity]; (iv) alteration in the solute reactions of the membrane cells [as showed by the production of thiobarbituric acid reactive substances (+34%)]; (v) inhibition of enzymes linked to AsA-GSH cycle [as confirmed by the decrease of ascorbate peroxidase (48%), monodehydroascorbate, dehydroascorbate and glutathione reductase activities (-44, -56 and -80%, respectively)]; (vi) changes of the normal reduced state of cells [as evidenced by the decrease in the reduced ascorbate/dehydroascorbate and in the reduced/oxidized glutathione ratios (-37 and -60%, respectively)]; (vii) activation of enzymes involved in phenolic metabolism [as highlighted by the increase of phenylalanine ammonia-lyase and cinnamyl alcohol dehydrogenase activities (+75 and +67%, respectively)]; (viii) development of cellular barriers with a higher degree of polymerization of monolignols [as indicated by the increase of lignin (+47%)] and (ix) accumulation of phenolic and anthocyanin compounds (+68 and +43%, respectively). These results indicate that O3 exposure modifies reactive oxygen species metabolism rates, but the enzymatic and non-enzymatic antioxidant systems (that work in co-operation) could not provide a defence against free radicals, not preventing the oxidative damage. In conclusion, under these circumstances, L. tulipifera should be regarded as sensitive to this pollutant