Ozone uptake and its effect on photosynthetic parameters of two tobacco cultivars with contrasting ozone sensitivity.

Leaf discs of the ozone tolerant tobacco (Nicotiana tabacum L.) cv. Bel B and of the ozone sensitive cv. Bel W3, were exposed to an acute ozone fumigation (300 ppb) for 3 h. We measured ozone uptake by leaves and physiological characteristics before, during and after the treatment, in order to determine if the different O3 sensitivity was correlated to the leaf uptake. In the tolerant cv. Bel B, O3 uptake was high during the first 2 h of ozone exposure and then decreased. In the sensitive cv. Bel W3, the rate of O3 uptake decreased constantly during ozone fumigation. The estimated cumulative uptake over the treatment time was higher (200 ± 30 ?mol m-2) in Bel B than in Bel W3 (130 ± 12 ?mol m-2). Thus, the ozone sensitivity was not correlated with ozone uptake. Stomatal conductance and photosynthesis weresignificantly inhibited during the fumigation in both cultivars. However, these reductions were strong and irreversible in the cv. Bel W3, while in the cv. Bel B both parameters recovered in the post-fumigation period. Thus, ozone tolerance may be related to a sustained capacity of recovery. There was no linear correlation between ozone uptake and photosynthesis reduction, but a threshold of ozone uptake was found after which photosynthesis was substantially impaired. This threshold may or may not be reached under the same external ozone level, indicating that the AOT40 may not be a sufficiently accurate index for the detection of ozone damage in plants.