Lead, as a toxic heavy metal is considered as one of the most common and dangerous environmental contaminants. In the present work, the physiological and ultra-structural effects of Pb on 14-day old Vigna unguiculata seedlings were assessed and the alleviating role of salicylic acid (SA) in Pb stressed seedlings was evaluated. The results showed that Pb treatment (150 mM) resulted in a significant decrease in some growth criteria, photosynthetic pigments and activity. Furthermore, Pb stress led to oxidative stress and increased cell membrane leakage in the treated seedlings. In the ultra-structural study, Pb treatment led to distorted chloroplasts with compact grana, small starch grains, disrupted thylakoid membranes, deposition of Pb on the inner surface of the cell wall, unilateral blocking of bordered pits and aggregation of chromatin and its adhesion to the inner surface of the nuclear membrane instead of dispersion within the nucleus. Pretreatment with SA led to alleviation of the above mentioned damaging effects of Pb on Vigna unguiculata seedlings. It improved the growth criteria, photosynthetic pigments, alleviated the inhibitory effects of oxidative stress of Pb and reduced membrane damage. SA also minimized the deleterious ultra-structural effects of Pb. Interestingly; SA pretreatment reduced the absorbed level of Pb from soil. Depending on the data presented here, it is suggested that SA antagonizes the oxidative damaging effects of Pb not only directly through activating the antioxidative system such as catalase, peroxidase and ascorbate, but also indirectly through lowering the uptake of Pb from soil. Thus, it might be concluded that SA is a powerful tool for alleviating lead stress in plants growing in Pb-contaminated environments.
Key words: Ascorbate, Catalase, Growth criteria, Lead, Peroxidase, Photosynthetic pigments, Salicylic acid, Stress, Ultra-Structure, Vigna unguiculata
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