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 (O-3) and/or water stress, integrating the gas exchange and bioche mical data with fluorescence OJIP polyphasic transient data. Plants submitt ed to only water stress (T-1) and with ozone (T-3) showed a strong decrease in stomatal conductance and gas exchange, coinciding with a reduction of m aximum yield of photochemistry (rho (po)) 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 r eaction centre were found. The reduction of photosynthesis in the O-3-treat ed plants (T-2) by a slowing down of the Calvin cycle was supported by the increase of related fluorescence parameters such as relative variable fluor escence, heat de-excitation constant, energy deexcitation by spillover, and the decrease of pp.. We suggest an antagonistic effect between the two str esses to explain the delayed ozone-induced decrease of stomatal conductance values for T-3 with respect to T-1 plants by an alteration of the physiolo gical mechanisms of stomatal opening, which involve the increase of intra-c ellular free-calcium induced by ABA under co-occurring water shortage. We e mphasise the importance of considering the intensity of the individual stre ss factor in studies concerning the interaction of stresses.