Salicylic Acid-Mediated Abiotic Stress Tolerance - Salicylic Acid: Plant Growth and Development - page 186

2.1.2 Effect of Exogenous Salicylic Acid During Heavy Metal Stress
One of the earliest works to report on the protective effect of SA against abiotic
stress factors dealt with heavy metals. SA at a concentration of 0.1 or 0.2 mM was
found to reduce the inhibitory effect of Pb 2+ and Hg 2+ on seed germination and
seedling growth, and its membrane-damaging effect in rice (Oryza sativa L.) seeds
placed in Petri plates containing filter paper discs moistened with SA and heavy
metal solutions (Mishra and Choudhuri 1997 , 1999 ). The higher concentration of
SA was more effective, as evident from the better recovery from metal-induced
growth inhibition. SA also moderated the inhibitory effect of lead on the activity of
the nitrate reductase enzyme in maize (Zea mays L.) plants (Sinha et al. 1994 ). SA-
induced aluminium tolerance was also reported in Cassia tora L. plants, where the
increased citrate efflux induced by 5 lM SA treatment was associated with a
decrease in the inhibition of root growth and in the Al content of the root tips
(Yang et al. 2003 ). SA (at concentrations of 0.1-200 lM) also had a protective
effect in soybean against cadmium stress (Dra ˇ i ´ and Mihailovi ´ 2005 ), against
lead stress in Brassica napus var. Okapi (Jazi et al. 2011 ) and against nickel stress
in Brassica napus L. (Kazemi et al. 2010 ) when added to the nutrient solution.
By contrast, 10 lM SA treatment stimulated the accumulation of cadmium at
the beginning of germination in Medicago sativa L. seedlings and was unable to
inhibit the damaging effects of Cd treatment on the shoot and root growth (Draˇi´
et al. 2006 ). The same concentration of SA, applied for 3 h at the beginning of
imbibition, stimulated Cd accumulation in a tolerant soybean genotype, while
accumulation was inhibited by SA in a susceptible genotype. This could indicate
that the Cd-tolerant genotype was better able to regulate the oxidative stress
induced by Cd, so mechanisms designed to prevent the further uptake of the heavy
metal were not triggered (Draˇi´ and Mihailovi´ 2009 ). These results suggest that
the effect of SA cannot be fully generalised, and that it depends greatly on the
genotype. In seedling of the same plant species 200 lM SA for 12 h was found to
alleviate mercury toxicity by inducing the antioxidant defence system (Zhou et al.
2009 ). In sunflower the exogenous application of SA appeared to induce an
adaptive response to Cu stress, including the accumulation of organic solutes
leading to the protection of photosynthetic pigments and membrane integrity (El-
Tayeb et al. 2006 ). The results obtained when soaking the seeds of Linum usita-
tissimum L. in SA suggested that it could be used as a growth regulator and a
stabilizer of membrane integrity to improve plant resistance to Cd stress (Belkhadi
et al. 2010 ).
Other results indicated that, although 500 lM SA reduced the Cd uptake of
maize roots and the inhibitory effect of Cd treatment on photosynthesis, the
compound itself stressed the seedlings, so preliminary treatment with SA could
aggravate the damaging effect of Cd (Pál et al. 2002 ). However, soaking maize
seeds in 500 lM SA for 6 h was able to reduce the inhibitory effect of 10, 15 and
25 lM Cd on growth, chlorophyll content and the RUBISCO and PEPC enzymes,
while also decreasing the proline production, lipid peroxidation and membrane
leakage induced by Cd (Krantev et al. 2008 ). Although the soaking of seeds in SA