Essential oils and volatile compounds in Mentha spicata plants exposed to phytotoxic ozone

In the Mediterranean basin, ambient ozone concentrations have been reported to cause visible injury on several crops and wild plants. Studies under controlled conditions showed a range of physiological responses, including reductions in above- and below-ground biomass, seed production, photosynthetic rate and stomatal conductance, altered patterns of assimilate allocation, accelerated senescence, and modification of antioxidant levels. Quantitative dose/response relationships have been detected on major crops, but few studies have dealt with the effects of ozone on the quality of plant products. The aim of this study was to investigate the response of a piperitenone oxide chemotype in Mentha spicata subsp. glabrata exposed to a realistic ozone regime (100 ppb for 21 days, 5 h d-1) in fumigation chambers. Analyses were carried out at the end of the treatment, which corresponded to an AOT40 (i.e. accumulated exposure over a threshold of 40 ppb) of 6300 ppbh. Significant inhibition of photosynthetic activity, mainly due to stomatal closure was observed in ozonated individuals in comparison to filtered-air grown controls. Leaf cell membrane permeability, chlorophyll content and electron transport were unaffected. No visible injury symptoms were detected. From this point of view, this species should be regarded as ozone-resistant. A 4% decrease in piperitenone oxide and a doubling of germacrene-D content was observed in the leaves of ozonated plants, which also showed traces of essential oil components not detected in untreated plants. The main impact of ozone on volatile compounds was a 2.5-fold increase in piperitenone oxide; several organic compounds, detectable in percentage lower than 0.1% in control plants, were released by the ozonated ones. Ecological implications of these findings, as well as the impact of ozone on the quality of mint yield deserve attention