Physiological effects of five months exposure to low concentrations of O₃ and NH₃ on Douglas fir (Pseudotsuga menziesii)

Three years old seedlings of Douglas fir ( Pseudotsuga menziesii ) were exposed lo filtered air, O 3 (day and night concentrations of 78 and 30 μgm −3 : respectively). NH 3 (54 μg m −3 ) and to a mixture of NH 3 +O 3 (day and night concentrations of 49 + 83 and 49 + 44 μg m −3 respectively), for 5 months in fumigation chambers. Both gas exchange and chlorophyll fluorescence were measured on shoots which had sprouted at the beginning of the exposure period. After 4. 8, 10 and 20 weeks of exposure, light response curves of electron transport rate (J) were determined, in which J was deduced from chlorophyll fluorescence. Net CO 2 assimiialion was measured at maximum light intensity of 560) μmol m −2 S −1 (P n.560 ). After 8 and 10 weeks of exposure also light response curves of CO 2 assimilation were assessed. Shoots exposed to O 3 showed a reduction in net CO 2 assimilation as compared to the control shoots during the entire exposure period. The reduction was related lo a lower chlorophyll content and a lower electron transport rate, whereas no effect on quantum yield efficiency (qy) was observed. In contrast, shoots exposed to NH 3 showed a positive effect on photosynthesis. Shoots exposed to NH 3 . + O 3 showed a rapid increase in P n.560 , in the period between 4 and 8 weeks to a level equal of that of the NH 3 ‐treatment. After this period a decline in P n.560 was observed. After 10 weeks of exposure shoots exposed to O 3 showed an increased transpiration rate in the dark as compared to the control shoots. In addition, water use efficiency (WUE) declined as a result of an increase in leaf conductance. Both observations indicate that the stomatal apparatus was affected by O 3 . A high transpiration rate in the dark was also found for shoots esposed to NHX. However, shoots exposed to NH 3 + O 3 showed neither an effect on WUE, nor an effect on transpiration rate in the dark. The possibility that NH 3 delayed the O 3 induced effects on photosynthesis and stomatal conductance is discussed.