How sensitive is Melissa officinalis to realistic ozone concentrations?

Lemon balm (Melissa officinalis, L.; Lamiaceae) was exposed to realistic ozone (O-3) dosages (80 ppb for 5 h), because high background levels of O-3 are considered to be as harmful as episodic O-3 regimes. Temporal alterations of different ecophysiological, biochemical and structural parameters were investigated in order to test if this species can be considered as an O-3-bioindicator regarding changes in background concentrations. At the end of ozone exposure, the plants did not exhibit any visible foliar symptoms, as only at microscopic level a small number of dead cells were found. Photosynthetic processes, however, were significantly affected. During and after the treatment, ozone induced a reduction in CO2 fixation capacity (up to 52% after 12 h from the beginning of the treatment) due to mesophyllic limitations. Intercellular CO2 concentration significantly increased in comparison to controls (+90% at the end of the post-fumigation period). Furthermore impairment of carboxylation efficiency (-71% at the end of the post-fumigation period compared to controls in filtered air) and membrane damage in terms of integrity (as demonstrated by a significant rise in solute leakage) were observed. A regulatory adjustment of photosynthetic processes was highlighted during the post-fumigation period by the higher values of qNP and (1-qp) and therefore suggests a tendency to reduce the light energy used in photochemistry at the expense of the capacity to dissipate the excess as excitation energy. In addition, the chlorophyll a/b ratio and the de-epoxidation index increased, showing a rearrangement of the pigment composition of the photosynthetic apparatus and a marked activation of photoprotective mechanisms. (C) 2013 Elsevier Masson SAS. All rights reserved.