Ozone-elicited secondary metabolites in shoot cultures of Melissa officinalis L.

The effects of ozone treatment (200 ppb, 3 h) on the accumulation of main secondary metabolites have been investigated in Melissa officinalis (lemon balm) aseptic shoot cultures in order to evaluate the biotechnological application of this gas for improving the yield of secondary metabolites of medicinal plants. During the treatment, we found (i) an activation of enzymes involved in phenolic metabolism [as confirmed by the increase of shikimate dehydrogenase, phenylalanine ammonia-lyase and cinnamyl alcohol dehydrogenase activities (about twofold higher than controls)], (ii) a development of cellular barriers with a higher degree of polymerization of monolignols [as indicated by the increase of lignin (+23% compared to controls)], (iii) an accumulation of phenolic compounds, in particular rosmarinic acid (about fourfold compared to control plants cultivated in filtered air) and (iv) an increase of antioxidant capacity [as documented by the improved 1,1-diphenyl-2-picrylhydrazyl radical (DPPH) scavenging activity]. The effect of ozone as elicitor of the production of secondary metabolites in lemon balm shoot cultures was dependent on the specific regime, the time of exposure and the concentration of the stressor. After the end of the treatment, we found cell death and hydrogen peroxide (H2O2) deposition concomitant with a prolonged superoxide anion-generation suggesting that a transient oxidative burst had occurred