150 S. Bohler et al.
7.5 Stomatal Conductance: Protection Against
Ozone by Drought
One of the common characteristics between ozone and drought exposure is the sig-
nificant involvement of stomata. While ozone enters plants through the stomata,
water vapour escapes through them; therefore, stomatal closure would theoretically
protect plants against both stresses (Fig. 7.2). While observations have shown that
stomatal conductance is an excellent marker for the severity of drought (Medrano
et al. 2002 ), the situation is less evident for ozone. Reports have concluded that
stomatal behaviour is not consistent, but dependant on many factors (Wittig et al.
2007 ). This has been mainly attributed to stomatal sluggishness in multiple publica-
tions (Hoshika et al. 2012 , 2014 ; Paoletti and Grulke 2010 ; Dumont et al. 2013 ).
It has been shown that this delayed response time of stomata varies among species
(Hoshika et al. 2012 ; Paoletti and Grulke 2010 ), severity of stress (Hoshika et al.
2012 ) and on seasonal changes (Hoshika et al. 2014 ). It has furthermore been con-
cluded that the sluggish behaviour of stomata under ozone exposure can lead to
perturbations in the response to water deficit (Hoshika et al. 2014 ).
Stomatal closure during drought has been proposed as a protective measure
against ozone exposure if both stresses are present simultaneously. However, this
phenomenon has not been consistently observed. It has been shown that the interac-
tive effect between ozone and drought is dependent on many factors, e.g. species
(Wagg et al. 2012 ; Ribas et al. 2005 ; Biswas and Jiang 2011 ; Pell et al. 1993 ), se-
quence of appearance (Bohler et al. 2013 ; Le Thiec et al. 1994 ), severity (Le Thiec
et al. 1994 ), time of day (Le Thiec et al. 1994 ), developmental stage (Alonso et al.
2001 ; Skärby et al. 1998 ) or season (Pell et al. 1993 ). Biswas and Jiang ( 2011 )
showed, for instance, that, under conditions of combined ozone and drought stress,
the ozone-sensitive modern winter wheat cultivar ( Triticum aestivum L. cv. Xiaoyan
22) improved its tolerance against ozone, while the ozone-tolerant primitive wheat
( Turgidum ssp. durum) lost ozone tolerance. Le Thiec et al. ( 1994 ) and Bohler et al.
( 2013 ) hypothesized that the order of occurrence could play an important role in the
combined effect. An early drought could lead to a decrease in stomatal conductance
Fig. 7.2 Stomata forming a physical barrier against ozone absorption and water vapour loss