Interplay Between Environmental Signals and Endogenous Salicylic Acid Concentration - Salicylic Acid: Plant Growth and Development - page 68

(Farooq et al. 2008 ). Finally, exogenously applied SA alleviated the cold stress
effect on growth of cucumber seedlings (Lei et al. 2010 ). Thus, there is a plethora
of examples where a pre-treatment with exogenous SA has reduced the negative
effects of cold stress on plant growth and metabolism.
A. thaliana Col-0 plants grown at 23 8C and then transferred to 5 8C for several
weeks, had significantly increased endogenous SA levels when compared to plants
that remained at 23 8C for the same time period (Scott et al. 2004 ). A. thaliana
mutant lines with reduced SA levels (NahG; Larkindale and Knight 2002 ), defi-
cient in SA levels (eds5; Nawrath and Metraux 1999 ) and possessing elevated SA
levels (cpr1; Bowling et al. 1994 , 1997 ) were compared to wild type Col-0 plants
at both 23 and 5 8C. Under cold stress conditions, the plants of NahG line showed
no increase in endogenous SA levels and accumulated considerably more dry
weight than did Col-0 plants (though at 23 8C the NahG plants accumulate less dry
weight than Col-0 plants) (Scott et al. 2004 ). Further, the cold-stressed NahG
plants also had larger leaves as a result of increases in cell size, but there was no
change to leaf number. Although there was no cold stress-related damage in
photosystem II in either Col-0 or NahG plants, net assimilation rate after the cold
stress treatment was higher for NahG plants (Scott et al. 2004 ). Cold-stressed eds5
plants had a phenotype similar to NahG plants, but possessed even lower
endogenous SA levels. Another mutant, cpr1, whose plants have a dwarf pheno-
type at 23 8C relative to Col-0, showed even higher dwarfism at low temperatures,
and this dwarfism change was associated with ca. 2-fold higher levels of SA,
relative to cold-stressed Col-0 plants (Scott et al. 2004 ). Finally, canola (Brassica
napus L.) plants grown at 5 8C for four weeks after germination had significantly
higher endogenous SA levels than plants grown at 20 8C.
The application of low concentrations of SA can also influence (increase) the
tolerance of plant tissues to short-term heat stress, whereas at higher concentrations
SA had either the opposite or nil effect. There are numerous examples of this
phenomenon, i.e. mustard (Sinapis alba L.; Dat et al. 1998a ), tobacco (Nicotiana
tabacum L.; Dat et al. 2000 ), A. thaliana (Larkindale and Knight 2002 ; Clarke et al.
2004 ), Kentucky bluegrass (Poa pratensis L.; He et al. 2005 ), creeping bentgrass
(Agrostis stolonifera L.; Larkindale and Huang 2005 ) and pea (Pisum sativum L.;
Pan et al. 2006 ). Exogenous SA pre-treatment also improves net photosynthetic rate
of leaves of heat-stressed grape (Vitis vinifera L.) plants, apparently by maintaining
a higher Rubisco activation state and accelerating the recovery of PSII (Wang et al.
2010 ). Short-term heat stress caused increases in endogenous SA levels, likely as a
result of its de novo synthesis (Pan et al. 2006 ), in mustard (Dat et al. 1998b ), A.
thaliana (Kaplan et al. 2004 ) and pea (Liu et al. 2006 ) plants during the first 30 min
of the stress. The heat-stressed pea leaves also had parallel (with SA) increases in the
activities of phenylalanine ammonia lyase (PAL) and benzoic acid 2-hydroxylase
(BA2H) (Pan et al. 2006 ). However, as occurred with grape plants, the initial and
substantial increase in endogenous SA following the administration of short-term
heat stress diminishes over time. Thus, after 24 h the endogenous SA levels in
control and heat-stressed plants were essentially the same (Wang et al. 2004 , 2005 ).
Interestingly, it appears that the source of this increased SA in heat-stressed grape