7 Interactive Effects Between Ozone and Drought: Sorrow or Joy? 151
and a subsequent protection against a later ozone exposure, while the appearance of
drought during pre-existing ozone stress would suffer under the appearing sluggish-
ness of stomata, initially caused by ozone.
The sluggishness of stomata under ozone stress is most likely due to a perturba-
tion of the abscisic acid (ABA)-induced stomatal regulation by ethylene (Fig. 7.3).
Wilkinson et al. (Wilkinson and Davies 2009 ) showed that ozone-treated Leontodon
hispidus present a reduced sensitivity to exogenously applied ABA and that stomata
display a decreased response to a gradual drought. They furthermore measured an
increase in ethylene production in ozone-exposed L. hispidus, while observing no
change in ABA concentrations. Most importantly, it was determined that the appli-
cation of 1-Methylcyclopropene (1-MCP), which prevents ethylene from binding
to its receptors, restored the sensitivity of stomata to externally applied ABA and
to soil drying (Wilkinson and Davies 2009 ). This shows that ozone-induced emis-
sion of ethylene is responsible for the sluggish behaviour of stomata, leading to
increased effects of drought, rather than protective effects against ozone. Ethylene-
mediated inhibition of ABA-induced stomatal closure was also shown by Tanaka et
al., independently of ozone exposure (Tanaka et al. 2005 ).
7.6 Biomass Changes and Visible Symptoms
Estimations predict that ozone may cause up to 30 % loss in biomass of crop plants,
and up to 10 % in forest species (Fuhrer 2009 ; Broadmeadow 1998 ). Drought may
lead to yield loss as well, as was shown by a 30 % decrease in plant productivity af-
ter the 2003 summer drought in Europe (Ciais et al. 2005 ). Decreases in biomass are
indeed to be expected as a consequence of both ozone and drought exposure, since
both phenomena may lead to a decrease in net photosynthetic rate (A) and thus in
the net CO 2 fixation (Wittig et al. 2007 ; Biswas and Jiang 2011 ; Flexas et al. 2002 ).
A decrease in biomass production can include reduced seed weight and number
(Biswas and Jiang 2011 ; Flexas et al. 2002 ). Coinciding appearance of ozone and
drought has been shown to have a cumulative effect on the decrease of seed biomass
(Biswas and Jiang 2011 ).
Fig. 7.3 Simplified representation of the interactive effects of ethylene and abscisic acid on sto-
matal closure, drawn in PathVisio (van Iersel et al. 2008 )