Salicylic Acid-Mediated Abiotic Stress Tolerance - Salicylic Acid: Plant Growth and Development - page 198

glutathione reductase (GR) and guaiacol peroxidase (G-POD) increased as a result
of preliminary treatment with SA. These changes might explain the increased cold
tolerance. Further studies proved that not only SA, but also related compounds,
such as BA, aspirin or coumaric acid, may have a protective role against chilling
stress in young maize plants (Janda et al. 1998 , 2000 ; Horváth et al. 2002 ).
It should also be mentioned, however, that these compounds may cause severe
damage to the leaves and roots of maize when added to the nutrition solution
above a certain concentration at normal growth temperature (Janda et al. 1998 ,
2000 ; Pál et al. 2002 ). SA treatment resulted in many changes in the ultrastructure
of banana cells, such as the separation of the cells from palisade parenchymas, the
appearance of crevices in the cell walls, the swelling of grana and stromal thy-
lakoids, and a reduction in the number of starch granules (Kang et al. 2007 ).These
results also demostrated that SA treatment at normal conditions could act as a
stress factor.
In contrast to this, the foliar application of SA or aspirin enhanced stomatal
conductance, transpiration and photosynthetic rates in both soybean and maize
(Khan et al. 2003 ). Other stimulating effects of SA were reported in embryogenic
cell suspension cultures of Coffea arabica L. treated with picomolar concentrations
of SA (Quiroz-Figueroa et al. 2001 ). Seed treatment with 100 lM SA significantly
improved the germination percentage, germination rate and seedling criteria,
compared with control seeds under optimal and low temperature stress conditions
(Gharib and Hegazi 2010 ). Furthermore, seed priming with SA improved seedling
emergence, root and shoot length, seedling fresh and dry weights, and leaf and root
scores compared to the control both at optimal and chilling temperatures in maize
(Farooq et al. 2008 ). The cell ultrastructure of banana seedlings pre-treated with
0.5 mM SA in the form of foliar spray or root irrigation showed less deterioration
than that of control seedlings after chilling stress (Kang et al. 2007 ).
The chilling tolerance of leaves or hypocotyls was significantly increased by the
application of 0.5 mM SA not only in maize, but also in cucumber and rice (Kang
and Saltveit 2002 ), but this improved chilling tolerance was only observed in the
leaves of rice and in the hypocotyl of cucumber, as exposure to SA caused a
significant reduction in the growth of the radicle. The inhibitory effect of SA on
radicle growth did not appear if SA application started at germination, suggesting
that seedlings may become acclimated to SA (Kang and Saltveit 2002 ). Pre-
treatment with 0.5, 1.0 and 2.0 mM SA for 24 h before chilling at 5 C for 1 d
decreased the chilling tolerance of rice (Wang et al. 2009b ), while a 2 mM con-
centration of SA was highly effective in reducing chilling injury in pomegranate
fruit (Sayyari et al. 2009 ). SA was able to regulate the leaf photosynthetic func-
tions of cucumber seedlings and enhance seedling resistance to low temperature
and light intensity. The optimum concentration of SA for foliar-spraying was
1 mM (Liu et al. 2009 ).These results clearly showed that there is a need to
investigate the best method of SA application and the dose giving maximum
protection, as a function of plant species, the growth stage and the plant organ,
treated.