152 S. Bohler et al.
Visible symptoms of ozone and drought are quite characteristic and can allow
differentiation between both types of stress. Whereas ozone leads to the formation
of necrotic patches and irregular chlorotic discoloration of leaves, drought rather
induces a homogenous and gradual discoloration of leaves that in addition appear
stunted and droopy (Bohler et al. 2013 ). It has furthermore been shown that, in pop-
lar saplings, the combined effect of ozone and drought leads to an additive display
of both symptoms (Bohler et al. 2013 ). In contrast, a field survey by Showman
( 1991 ) determined that in 1988 (a year with particularly high ozone levels in com-
bination with drought), less ozone-related injuries were observed as compared to
1989 (a year with lower ozone concentrations and less drought). Besides, Matyssek
et al. ( 2010 ) discussed that in 2003 (an exceptionally dry summer), the impact of
ozone on beech trees at a test site in Kranzberg forest (Germany) was most likely
reduced by drought, and the detrimental effects on radial and whole-stem volume
increment were most likely due to the water deficit. These are further indications
that protection manifested by drought may be very dependent on specific environ-
mental conditions and that even if drought has a protective effect against ozone, the
aftermath of drought itself may be equally or more detrimental than ozone.
7.7 Carbon Metabolism
Physiological measurements of ozone-exposed plants have shown that net pho-
tosynthetic rate, maximum rate of RuBisCO-mediated carboxylation and carbox-
ylation efficiency can be decreased (Biswas and Jiang 2011 ). This can partly be
attributed to a decrease in stomatal conductance, but there are indubitably further
reasons for these effects. It has been clearly shown on multiple occasions, and for
many species, that the enzyme RuBisCO is affected by ozone stress. Studies have
shown reduced enzyme activity and abundance of RuBisCO subunits, but also of
RuBisCO activase (Bohler et al. 2007 , 2010 , 2013 ; Pelloux et al. 2001 ; Brendley
and Pell 1998 ). It is likely that the enzyme itself is damaged during ozone exposure,
as illustrated by the increase in abundance of a degradation fragment of RuBisCO
in poplar (Bohler et al. 2013 ). In addition, a number of enzymes of the Calvin
cycle have been shown to be decreasing in abundance in response to ozone (Bohler
et al. 2007 , 2010 , 2013 ). The fact that these are mostly redox-regulated enzymes
(RuBisCO activase, fructose-1,6-bisphosphatase, sedoheptulose-1,7-bisphospha-
tase and ribulose-5-phosphokinase) is understandable in the context of oxidative
stress, since the Calvin cycle is activated during the day by the reduction of key
enzymes. In an oxidative environment, these enzymes can become inhibited, which
might also be the cause for lower abundances.
During periods of water deficiency, various observations have been made for
RuBisCO. Parry et al. suggested that the reduction in activity of RuBisCO in
droughted tobacco was due to increases in the inhibition of the enzyme (Parry et al.
2002 ), while Pelloux et al. ( 2001 ) determined that the abundance of both RuBisCO
and RuBisCO activase transcripts and proteins in Aleppo pine hardly changed dur-
ing drought. Nevertheless, Sergeant et al. detected increases in the abundance of the