Responses of two birch (Betula pendula Roth) clones to different ozone profiles with similar AOT40 exposure

Saplings of two clones of European white birch (Betula pendula Roth) were e xposed to three different ozone profiles resulting in same AOT40 value of 1 3-14 ppm h in a chamber experiment, The sensitive clone 5 and the more tole rant clone 2 were growing (1) under filtered air (= control), or (2) were e xposed to 70 ppb ozone for 24 h d(-1) (= profile 1), (3) to 100ppb ozone fo r 12 h d(-1) at 8:00 20:00 (= profile 2), or (4) to 200ppb ozone for 4.5 h d(-1) at 9:30-14:00 (= profile 3) for 20 d. The saplings were determined fo r growth, visible leaf injuries, stomatal conductance, and concentrations o f Rubisco, chlorophyll and carotenoids. Growth responses and induction of v isible foliar injuries under different ozone profiles were variable, result ing in 4-17% lower dry mass of shoot, 16-46% reduction in stern height incr ement and 11-43% increase in visible injuries in clone 5, which was accompa nied by higher leaf turnover rate under profile 3 indicating compensation g rowth. In clone 2, ozone-induced responses ranged from slight stimulation i n stem height growth to 13% decrease in dry mass of shoot and 2-16% increas e in visible injuries. Daytime stomatal conductance rates were lowered by 1 4-54% in clone 5 and 9-74% in clone 2, depending on profile. The additional power-weighted analyses revealed that high peak concentrations and exposur e shape were important for induction of visible injuries in both clones and reduction in stomatal conductance in clone 5, whereas growth reductions we re rather related to total cumulative exposure. The results indicate that p rofile of ozone exposure, night-time stomatal conductance (24h flux), and r ecovery time for defence and compensations reactions should not be ignored in plant response and ozone flux modelling. (C) 2001 Elsevier Science Ltd. All rights reserved.