Ozone stress in Melissa officinalis plants assessed by photosynthetic function

Photosynthetic functions have been investigated in ozone stressed (200ppb, 5h) Melissa officinalis plants at the end of fumigation and 24 and 48h after. Plants exhibited foliar injury and membrane permeability was significantly increased, indicating that there was membrane damage. After the end of treatment, CO2 fixation capacity decreased and this lasted during the recovery period (until a maximum of -63% when compared to controls). These strong negative effects on photosynthetic ability were observed to be due both to stomatal and mesophyllic limitations, since stomatal conductance decreased (-23%) and intercellular CO2 concentration significantly increased (+41%). Reduction in PSII efficiency is evidenced by (i) decrease of Fv/F0 (-11.4%), indicating a partial inhibition at PSII donor side; (ii) significant correlation between the apparent electron transport rate through PSII and photosynthetic activity, suggesting that the O3-induced effects are well established, as demonstrated by the development of leaf necrosis; (iii) increase in electrons required to fix one molecule of CO2, showing a decrease in activity of photosynthetic enzymes and their ability to fix CO2 in the presence of O3; (iv) decrease of qL, resulting in an increase in the PSII excitation pressure. On the other hand, a regulatory adjustment of PSII efficiency was highlighted by (i) higher value of qNP, abling to counteract the negative effects of O3 at chloroplast level because of their capacity to dissipate the excess of excitation energy; (ii) increase of the xanthophyll cycle pool size and DEPS index, showing a marked activation of photoprotective mechanisms. This represents an active response that M. officinalis initiates to cope with increased oxidative load.