Physiological response of Pinus halepensis needles under ozone and water stress conditions.

The aim of this study was to evaluate how physiological processes of potted Pinus halepensis plants, grown under controlled conditions, were affected by ozone (O3) and/or water stress, integrating the gas exchange and biochemical data with fluorescence OJIP polyphasic transient data. Plants submitted to only water stress (T1) and with ozone (T3) showed a strong decrease in stomatal conductance and gas exchange, coinciding with a reduction of maximum yield of photochemistry (varphipo) and very negative values of leaf water potential. Simultaneously, a great increase of both PSII antenna size, indicated by absorption per reaction centre, and electron transport per reaction centre were found. The reduction of photosynthesis in the O3-treated plants (T2) by a slowing down of the Calvin cycle was supported by the increase of related fluorescence parameters such as relative variable fluorescence, heat de-excitation constant, energy de-excitation by spillover, and the decrease of varphipo. We suggest an antagonistic effect between the two stresses to explain the delayed ozone-induced decrease of stomatal conductance values for T3 with respect to T1 plants, by an alteration of the physiological mechanisms of stomatal opening, which involve the increase of intra-cellular free-calcium induced by ABA under co-occurring water shortage. We emphasise the importance of considering the intensity of the individual stress factor in studies concerning the interaction of stresses.