154 S. Bohler et al.
apoplast is the first location of ozone attack, and consequently apoplastic ascorbate
and ascorbate peroxidase are the primary defence against ozone (Sanmartin et al.
2003 ; Luwe et al. 1993 ). In drought, cytosolic and chloroplastic ascorbate-depen-
dent detoxification is of more importance, but appears to be dependent on species
(Mittler and Zilinskas 1994 ; Zhang and Kirkham 1996 ). Nevertheless, Kronfuß
et al. showed that in Norway spruce, total needle ascorbate was increased signifi-
cantly by ozone, while apoplastic ascorbate was increased significantly by drought
and a combined exposure led to a significant increase in both. Combined stress may
therefore increase the reduction potential and improve protection against oxidative
stress (Kronfuß et al. 1998 ). Reduction potential is considerably dependent on plant
species, and it has been proposed that resistance to ozone is associated with both
ozone flux and reduction potential (Dizengremel et al. 2008 ). Similarly, the interac-
tive effect of ozone and drought may differ, depending on how much antioxidant
molecules and enzymes are induced by either of the stresses.
7.9 Conclusions
Since tropospheric ozone accumulation and soil drying are caused by similar me-
teorological conditions, both situations are likely to emerge in parallel in nature.
Even though the combination of two stress conditions very often causes cumulative
effects, it was proposed that the stomatal closure induced by drought may be able to
protect plants against the influx, and hence the detrimental effects of ozone. How-
ever, as is commonly the case, observations do not consistently corroborate these
expectations. It appears that the response caused by the combination of both stresses
is determined by many environmental and phenotypical factors.
One of the main relevant factors appears to be the sequence of events. The pri-
mary appearance of ozone is likely to cause disturbances in the reactivity of stoma-
ta. A subsequent drought will cause delayed and limited stomatal closure, allowing
continuous entry of ozone into the plant. Drought preceding ozone, on the other
hand, will cause stomatal closure early in the sequence and cause a natural barrier
against ozone absorption. The individual and combinatorial consequences of ozone
and drought exposure can be affected by a number of additional factors such as spe-
cies, ozone flux and antioxidant capacity, sensitivity to ozone and drought, time of
day and vegetative season.
Only few studies have investigated the effects of combined ozone and drought
exposure on plant metabolism (Bohler et al. 2013 ; Pelloux et al. 2001 ). Neither of
them (Bohler et al. 2013 ; Pelloux et al. 2001 ) identified any major synergistic or
antagonistic effects. In addition, the use of high throughput molecular approaches
is quite rare for this topic. This is regrettable, since high throughput techniques
like transcriptomics, proteomics and metabolomics present some considerable ad-
vantages versus targeted experiments. Where specific studies rely on prior knowl-
edge and a clearly stated hypothesis, high throughput techniques approach a sub-
ject without any prior bias. This approach may easily lead to new discoveries that
were previously unpredicted and therefore unconfirmed. In addition to the wealth