Ozone exposure results in various carry-over effects and prolonged reduction in biomass in birch (Betula pendula Roth)

In the first experiment, saplings of ozone-sensitive and a more tolerant clone of Betula pendula Roth were exposed to ambient ozone (control treatment, accumulated exposure over a threshold 40 mmol mol(-1) (AOT40) exposure of 1.0 mu mol mol(-1) h) and 1.5 x ambient ozone (elevated-ozone treatment, AOT40 of 17.3 mu mol mol(-1) h) over one growing season, 1996, After over-wintering, the dormant elevated-ozone saplings were transferred to the control blocks and assessed for short-term carry-over effects during the following growing season. In the second experiment, three sensitive, four intermediate and three tolerant clones were grown under ambient ozone (control treatment, AOT40 of 0.5-0.8 mu mol mol(-1) h per growing season) and 1.6-1.7 x ambient ozone (elevated-ozone treatment, AOT40 of 18.3-18.6 mu mol mol(-1) h per growing season) from May 1994 until May 1996, and were assessed for long-term carry-over effects during growing season 1997, after a 12-16 months recovery period. Deleterious short-term carry-over effects of ozone exposure included reduced contents of Rubisco, chlorophyll, carotenoids, starch and nutrients in lea,es, lower stomatal conductance, and decreased new shoot growth and net assimilation rate, followed by a 7.5% (shoot dry weight (DW)), 15.2% (root DW) and 23.2% (foliage area) decreased biomass accumulation and yield over the long term, including a reduced root:shoot ratio. However, a slow recovery of relative growth rates during the following two seasons without elevated ozone was apparent. Several long-lasting structural, biochemical and stomatal acclimation, stress-defence and compensation reactions were observed in the ozone-tolerant clone, whereas in the sensitive clone allocation shifted from growth towards defensive phenolics such as chlorogenic acid. The results provide evidence of persistent deleterious effects of ozone which remain long after the ozone episode.