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are important for increasing tolerance to short-term heat stress, this same con-
clusion is not applicable to gaining tolerance to low temperature stress. Rather, SA
accumulation was associated with increased susceptibility to low temperature
stress. Further, based on studies with ABA and SA biosynthesis inhibitors, it seems
that the heat-induced increases in endogenous SA levels are not the result of a
direct interaction. Rather, there is an indirect interaction, one where heat stress
increases ABA accumulation, which then acts to protect the plant from heat shock.
Coincidental with elevated ABA, an accumulation of SA also occurs, cause
unknown. A similar scenario also seems likely for the apparent ABA-SA inter-
action in plants subjected to low temperature stress.
5 Water Stress and Endogenous Salicylic Acid Levels
There is good evidence that exogenously applied SA can improve the growth and
productivity of plants subjected to drought stress. For example, pre-treatment of
bean (Phaseolus vulgaris L.; Senaratna et al.
2000
), chickpea (Cicer arietinum L.;
Kumar Patel et al.
2011
), rice (Farooq et al.
2010
), tomato (Senaratna et al.
2000
;
Hayat et al.
2008
) and wheat (Triticum aestivum L.; Singh and Usha
2003
) plants
with SA by seed imbibition, soil drenching or foliar spray improved the ability of
these plants to tolerate drought stress. These SA-treated plants accumulated
increased biomass and exhibited a higher photosynthesis rate relative to drought-
stressed plants which did not receive SA pre-treatment. Currently, the decreases in
plant growth and photosynthesis that occur in response to drought stress are gen-
erally attributed to increases in stress-induced biosynthesis of ABA which then
functions to reduce water stress by enhancing stomatal closure (Assmann
2010
).
However, Rai et al. (
1986
) demonstrated that subsequent application of SA actually
antagonizes the stomatal closure that is induced by applied ABA. While this may be
correct when both of ABA and SA are applied exogenously, we would postulate that
an antagonism between endogenous ABA and endogenous SA, in the regulation of
stomatal closure, is unlikely. This conclusion is supported by studies where appli-
cation of SA at low (optimal) concentrations increased endogenous ABA levels
(Shakirova et al.
2003
; Bandurska and Stroinski
2005
). Further, as presented below
for SA (Munne-Bosch and Penuelas
2003
; Abreu and Munne-Bosch
2008
; Bechtold
et al.
2010
; Fig.
1
) and from a vast literature on ABA (Assmann
2010
), moderate
drought stress increases endogenous levels of both ABA and SA. Finally, numerous
studies using a range of plant species have observed a closure of stomata in response
to the exogenous application of SA (Manthe et al.
1992
; Chen et al.
1993
; Rao et al.
1997
; Shirasu et al.
1997
; Mateo et al.
2004
).
Munne-Bosch and Penuelas (
2003
) have worked with field-grown Phillyrea
angustifolia (L.), a member of the Oleaceae, which is an evergreen bush or a small
tree and native to the Mediterranean region. They reported that drought stress of
these plants in their natural habitat resulted in an increased accumulation of
endogenous SA in leaves, relative to SA levels seen for well-irrigated plants.
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