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quantified by the reference to the stable isotope-labeled internal standard using
equations for isotope dilution analysis using the 124/120 ion pair (see Gaskin and
MacMillan
1991
; Jacobsen et al.
2002
).
As with other classes of plant hormones, large variations in endogenous SA
levels between species, ecotypes, tissues, and, more importantly in response to
various environmental signals are not uncommon. Species-related difference in
endogenous SA levels can be very significant. In two-week old A. thaliana and
sunflower (Helianthus annuus L.) shoots grown under exactly the same conditions,
they can vary from a few ng per gDW in A. thaliana to several thousand ng per
gDW in sunflower (Kurepin et al.
2010a
). Ecotype-related differences can also be
large. For example, alpine plants of Stellaria longipes [(L.) Goldie] have ca. half
the amount of endogenous SA, relative to plants of a prairie ecotype of the same
species, both ecotypes being grown under the same conditions and also being of
the same age (Kurepin et al.
2012a
). Tissue-related difference in SA can be
highlighted by comparing sunflower internodes (which contain several hundred ng
per gDW) with leaves (which have several thousand ng per gDW) above that
internode. Although species-, ecotype- and tissue-related differences are ''inter-
esting'', environment-related differences in endogenous SA will have the most
substantial impact on plant performance and, even survival (Scott et al.
2004
;
Abreu and Munne-Bosch
2008
; Kurepin et al.
2010a
)—see also evidence from
numerous SA application studies (Hayat et al.
2010
).
3 Light Signaling and Endogenous Salicylic Acid Levels
Among the many environmental factors, light plays a key role in plant growth and
development. The irradiance levels of ''visible'' light (ca. 400-800 nm) received
by a plant can control both photosynthesis and growth, including etiolation and de-
etiolation. In contrast, it is the quality or specific wavelengths of visible light that
regulates many growth and developmental events (Smith
2000
). Additionally,
invisible light, such as ultraviolet (UV) light (ca. 100-400 nm), can both influence
growth and cause damage to plants (Jenkins
2009
). For example, UV-C irradiance
of tobacco (Nicotiana tabacum L.) plants caused a significant accumulation of SA
(Yalpani et al.
1994
). Silencing the isochorismate (a SA biosynthetic precursor)
synthase (ICS) gene using a virus-induced technique prevented an accumulation in
endogenous SA in tobacco plants inoculated with a pathogen (Catinot et al.
2008
).
Exposure of these transgenic plants to UV-C light also significantly decreased the
accumulation of endogenous SA (Catinot et al.
2008
). This implies that a de novo
synthesis of SA may occurr in response to UV light stress. Additionally, exoge-
nously applied SA is reported to alleviate the UV-B irradiance-related stress in
swards of both Kentuky bluegrass (Poa pratensis L.) and tall fescue (Festuca
arundinacea Schreb.) (Ervin et al.
2004
).
In A. thaliana, a plant that is often grown for experimental use at light irra-
diance levels well below those of natural sun light, endogenous SA levels were
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