Genotypic Differences In Tolerance To Lead (Pb) Toxicity In Tobacco(Nicotiana Tabacum), Associated Physiological Mechanisms And The Possible Alleviatory Role Of N-acetyl Cysteine (NAC)

Due to the heavy industrialisation taking place world over, agricultural lands have been plagued by heavy metal pollution. Heavy metals such as Pb have a negative impact on plant growth leading to yield losses. Lead emanates from various sources which may include pesticides, automobile exhaust fumes, mining and fertilisers and finds its way into agricultural soils and since it’s a non-essential element, it is very toxic to plants even at very low levels. Pb stress causes generation of ROS which results in oxidative stress as they oxidise various cellular components such as proteins disturbing metabolic processes such as photosynthesis resulting in stunted growth and a range of other effects. Plants ameliorate harmful effects of Pb toxicity through various mechanisms and such as action of antioxidant enzymes and increasing the content of non-protein antioxidants. Stress response is usually complex involving various inducer molecules triggering each step of the response mechanism from sensing of the heavy metal to transcription of the stress proteins. The efficiency of the genotype to carry out stress alleviatory mechanisms determines its ability to resist heavy metal stress and this is mainly determined by the genetic makeup.It is logical to find ways to improve performance of tobacco under Pb stress by assessing responses of different genotypes to Pb stress, having an understanding of the effect of Pb stress and finally finding possible methods of improving performance of the crop under Pb stress and such problems were tackled in the present study.1. Genotypic differences in tolerance to Lead (Pb) toxicity in different tobacco genotypes and associated physiological mechanisms.The effects of Lead (Pb) toxicity on seven tobacco genotypes was assessed by exposing the plants to varying levels of lead nitrate (Pb(NO3)2). The results showed that growth and photosynthetic parameters decreased with increase in lead toxicity. Activities of antioxidant enzymes increased significantly with increase in lead toxicity. However, CAT activity decreased at high Pb(NO3)2treatments. MDA and root and leaf Pb content increased significantly in all genotypes with increase in Pb concentration and genotype. The genotypes exhibited great variations in the measured parameters showing variations in tolerance to Pb toxicity.2. Alleviatory effects of N-acetyl cysteine (NAC) on Pb stressed tobacco plants.Ability of NAC to ameliorate the harmful effects of Pb stress on tobacco plants was tested by exposing plantlets to varying levels of lead nitrate (Pb(NO3)2) with or without200and300μM NAC. The results showed that increase in Pb toxicity inhibited growth but application of NAC improved growth especially in medium levels of Pb(NO3)2treatments,In the500μM Pb(NO3)2treatment, NAC application did not result in growth increase but significantly increased photosynthetic pigments content. Application of NAC in lower Pb treatments that is in the10μM Pb(NO3)2treatment had ubiquitous effects on various physiological parameters. Application of NAC also increased activity of SOD, APX and CAT but lowered that of APX. The uptake and translocation of Pb to the leaves was significantly reduced with application of NAC. Application of NAC also protected chloroplast integrity during Pb stress reducing ultrastructural damage.