7 Interactive Effects Between Ozone and Drought: Sorrow or Joy? 149
tissue. The highly energetic ozone molecule reacts with cell wall components and
causes the formation of reactive oxygen species (ROS). These highly reactive mole-
cules in turn diffuse into the cells where they can damage proteins, genes, lipids and
other biomolecules. The high oxidative potential of ROS interferes with cell signal-
ling and regulation, including induction of programmed cell death (Kangasjärvi
et al. 2005 ), and may eventually lead to the death of exposed plants. Leaf chlorosis,
formation of necrotic patches and an increase in the number of senescing leaves are
the most visible symptoms of ozone stress (Bohler et al. 2007 , 2013 ).
7.3 Drought
Drought is the prolonged absence of rain that leads to a transient water deficit in the
soil and concomitantly a stress situation for plants. Not only does drought depend
on precipitation but also on the speed of water evaporation from the soil (Sherwood
and Fu 2014 ). The occurrence of drought is variable among the different regions
of the earth and dependent strongly on climatic regions. Droughts are common in
arid regions; but in recent years, occurrences have also become more frequent in
moderate climate, posing a threat for crops and forests (Kreuzwieser and Gessler
2010 ; Ciais et al. 2005 ). Recent considerations show that background dryness is
as important for hydrological changes as acute occurrences of drought, and needs
to be given more importance in the evaluation of the effect of climate change on
hydrological changes (Sherwood and Fu 2014 ).
In plants, drought induces a decrease in the internal water potential. The first
response is usually a reduction in stomatal conductance to reduce evaporation and
save water (Warren et al. 2007 ). Further effects involve an accumulation of os-
motically active solutes, to increase internal osmotic potential and improve water
retention and absorption (Evers et al. 2010 ). Visible symptoms of drought include
stunted leaves and an increase in leaf senescence (Bohler et al. 2013 ; Munn-Bosch
and Alegre 2004 ).
7.4 Co-Occurrence of Ozone and Drought
Due to the meteorological conditions favouring both ozone formation and drought
(i.e. a succession of warm days free of cloud cover), both are very likely to occur
simultaneously. This can have drastic consequences for vegetation, if the effects of
both stresses are synergistic. However, the effects of ozone and drought can also be
antagonistic, in which case a simultaneous occurrence might be beneficial to plants.
It has been postulated that a stomatal closure induced by drought may reduce the
flux of ozone into the plant and thus be protective. In this chapter, a closer look is
taken on the current state of understanding of the physiological, biochemical and
molecular effects that ozone and drought in combination have on plants.