The control of ozone uptake by Picea abies (L.) Karst. and P. sitchensis (Bong.) Carr. during drought and interacting effects on shoot water relations

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Abstract
Exposure to O3 alone has not yet been shown to reproduce the symptoms of the various types of spruce decline which have been identified in Europe. However, there is increasing evidence that this pollutant has physiological effects which interact with those of other environmental factors in ways which may be important in determining tree condition and growth. The effects of O3 episodes and drought on O3 uptake, gas exchange and water relations of Picea abies (L.) Karst. and P. sitchensis (Bong.) Carr. were investigated in two experiments. In the first a rapidly drying soil mixture was used, and seedlings of P. abies were exposed to short (2 h) daily episodes of O3 at 80 nl 1−1 on each day of a 5 d drought. Photosynthesis (A) and stomatal conductance (gs) were significantly decreased (P= 0.01) by water deficit and as a consequence, uptake of O3 by the plants was also significantly decreased. Exposure to O3 did not affect A or gs for this species. In the second experiment a soil mixture designed to give a slower development of water deficit was used and 1 + 1 transplants of P. sitchensis were exposed to a single O3 episode (up to 100 nl 1−1 for 3 h) after water had been withheld for 7 or 14 d. Hofler diagrams showed that mild water deficits did not affect shoot water relations. However, O3 significantly increased solute potential (Ψs) after 7 d of drought, an effect which was lost after 14 d of drought. Flux of O3 to the watered plants was greater than to the unwatered plants at all concentrations, the effect being more marked at higher concentrations. This effect was partly attributable to the greater stomatal conductances recorded for the well watered plants, but was also partly due to stomatal opening caused by O3, an effect which was diminished or reversed for unwatered plants.