O-3 and O-3+CO2 effects on a Mediterranean evergreen broadleaf tree, holm oak (Quercus ilex L.)

The aim of this work is to analyze the effects of a) short-term treatments with different concentrations of O3 (daily 6-hour fumigation with 0, 65, 175 and 300 ppb for 3-4 days); and b) a medium term treatment with O3 (150 ppb three times a week for thirty days on the whole) and CO2 (700 ppm) on gas exchange, chlorophyll fluorescence, and peroxidase activity in holm oak (Quercus ilex L.) leaves. The results show that net photosynthesis, transpiration, F(V)/F(M) ratio and POD activity were not influenced until an O3 concentration of 300 ppb was reached. At this threshold, significant alterations in these physiological and biochemical parameters were found. The treatment with +CO2 +O3 showed an increase of net photosynthesis suggesting thus an antagonistic effect of CO2 with regard to O3-induced injuries, while CO2 alone increased photosynthesis and decreased transpiration but induced no effects on F(V)/F(M) ratio or peroxidase activity. From our data, holm oak shows a resistance to episodes of photochemical O3 stress, that in the Mediterranean region can reach peaks of > 100 ppb. This response is likely to be due to the morpho-anatomic structure of holm oak leaves and their sclerophyllous adaptations. This might leads holm oak to adopt a stress tolerance strategy with regard to leaf response to O3, the effects of which cannot be ameliorated by an increase of atmospheric CO2.