Ozone quenching properties of isoprene and its antioxidant role in leaves

Isoprene is formed in and emitted by plants and the reason for this apparen t carbon waste is still unclear. It has been proposed that isoprene stabili zes cell and particularly chloroplast thylakoid membranes. We tested if mem brane stabilization or isoprene reactivity with ozone induces protection ag ainst acute ozone exposures. The reduction of visible, physiological, anato mical, and ultrastructural (chloroplast) damage shows that clones of plants sensitive to ozone and unable to emit isoprene become resistant to acute a nd short exposure to ozone if they are fumigated with exogenous isoprene, a nd that isoprene-emitting plants that are sensitive to ozone do not suffer damage when exposed to ozone. Isoprene-induced ozone resistance is associat ed with the maintenance of photochemical efficiency and with a low energy d issipation, as indicated by fluorescence quenching. This suggests that isop rene effectively stabilizes thylakoid membranes. However, when isoprene rea cts with ozone within the leaves or in a humid atmosphere, it quenches the ozone concentration to levels that are less or non-toxic for plants. Thus, protection from ozone in plants fumigated with isoprene may be due to a dir ect ozone quenching rather than to an induced resistance at membrane level. Irrespective of the mechanism, isoprene is one of the most effective antio xidants in plants.