7 Interactive Effects Between Ozone and Drought: Sorrow or Joy? 153
previously discussed fragment of RuBisCO in droughted oak (Sergeant et al. 2011 ).
Similar results were found in the combined stress experiment from Bohler et al.,
although it was shown that the increase in fragment abundance was less strong for
drought than for ozone (Bohler et al. 2013 ).
Not many publications discuss the primary carbon metabolism of plants exposed
to both ozone and drought. Pelloux et al. detected that RuBisCO and RuBisCO acti-
vase abundance were not changed during double stress (Pelloux et al. 2001 ). Bohler
et al. ( 2013 ) reported that in poplar, a very similar set of proteins were differen-
tially abundant in combined stress compared to ozone alone, but that the differences
were less severe. This clearly shows the existence of an interactive effect between
both stresses, but not necessarily a protective effect, since visual and morphological
symptoms were reported to be either similar or cumulative.
On the level of the chloroplast electron transport chain, many subunits of pho-
tosystems and ATPase have been reported to be significantly lower in abundance
after ozone stress (Bohler et al. 2007 , 2011 ) while drought appears to have an effect
only on ATPase (Flexas et al. 2002 ; Tezara et al. 1999 ). Furthermore, ozone ap-
pears to induce an early increase in ferredoxin–NADP+–oxidoreductase, indicating
a need for reducing power that is consistent with the appearance of oxidative stress
(Bohler et al. 2013 ). Nevertheless, this is only transient, since after longer exposure
to ozone levels of ferredoxin–NADP+–oxidoreductase decrease, possibly due to an
overwhelmed system. The consistent reduction in photosystem subunits is most
likely caused by the accumulation of ATP and NADPH in the chloroplast, in conse-
quence of the decrease in Calvin cycle activity. These observations have not been
made in drought; and according to the results of Bohler et al. ( 2013 ), the combined
stress causes a very similar response, albeit to a lesser extent.
7.8 Antioxidant Metabolism
One of the main differences between ozone and drought is the induction of oxida-
tive stress, which is predominant during ozone exposure but less characteristic of
drought. Whereas ozone itself fragments into ROS and leads to a strong accumula-
tion (Langebartels et al. 2002 ; Pellinen 1999 ), drought response mostly uses ROS as
internally produced signalling molecules (Yao et al. 2013 ), although severe drought
may lead to photo-oxidative stress as well (Foyer and Noctor 2000 ). Consequently,
accumulation of ROS is likely to be considerably higher during ozone stress, and
more closely located to chloroplasts in drought. Experiments show an increase in
activity and/or abundance of antioxidant enzymes like peroxidases, catalases and
superoxide dismutases in plants exposed to ozone (Alonso et al. 2001 ) and of glu-
tathione reductase and superoxide dismutase during drought (Alonso et al. 2001 ;
Huseynova et al. 2014 ). Alonso et al. ( 2001 ) detected decreases in antioxidant en-
zyme activities in the combined stress compared to ozone or drought separately,
deducing that the cumulative effects of both stressors may overwhelm defence
systems. Similar observations were made by Wellburn et al. ( 1996 ). Among anti-
oxidant molecules, ascorbate is particularly important during ozone response. The