Salicylic Acid-Mediated Abiotic Stress Tolerance - Salicylic Acid: Plant Growth and Development - page 200

Fig. 1 Changes induced in one-week-old pea plants by soaking seeds in salicylic acid (SA)
before sowing. The applied SA remained mainly in the seeds but in conjugated form (a). An
increase was found in the free SA fraction in the epicotyl but the use of labelled SA and gene
expression measurements showed that this increase was derived from de novo synthesis (b).
Furthermore, altered antioxidant activity was detected in the whole plant (Szalai et al. 2011 )

leaves. These results show that SA can increase freezing tolerance by affecting
apoplastic proteins (Tas gín et al. 2003 ).TheSA-induced increase in the activity of
certain antioxidant enzymes during the cold hardening period has also been
described in wheat plants (Sasheva et al. 2010 ). In rye, however, the apoplastic
proteins induced by salicylic acid did not have antifreeze activity (Yu et al. 2001 ),
and only treatment with ethylene induced antifreeze activity in winter rye leaves.
Cryo preservation is actually a more valuable technique for the long-term in vitro
conservation of plant germplasm, since, when frozen in liquid nitrogen, the
metabolism ceases to function, tissues are maintained without growth and genetic
alterations do not take place even during a very long period of storage. SA also
significantly enhanced the viability percentage of encapsulated embryonic axes of
Persian lilac (Melia azedarach L.) when they were subjected either to the cryo
preservation technique, involving dehydration and freezing in liquid nitrogen, or to
cold preservation by storing the alginate beads in empty petri dishes at 4 C
(Bernard et al. 2002 ).
A SA analogue 4-hydroxybenzoic acid (4-HBA) ameliorated the freezing tol-
erance of the spring wheat Chinese Spring (Horváth et al. 2007 ). It is hypothesized
that 4-HBA increases the impermeability of the cell wall, leading to increased
resistance to pathogen infection. The reinforcement of the cell wall by 4-HBA may
also contribute to increased tolerance against freezing stress.

2.5.3 Role of Endogenous SA (Free and Bound Forms) During Cold
Stress

Chilling-induced SA accumulation, particularly that of conjugated SA was
reported in both the leaves and roots of rice cultivars. When the endogenous SA
content was monitored after SA treatment in the leaves and crowns of two wheat
cultivars

differing in cold tolerance, SA was found to accumulate in a