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In this study, water stress was applied by polyethylene glycol at a concentration of 20 perecentage. To improve the resistance of the plants, the samples were treated by potassium nitrate at concentrations 5, 10, and 15 mM within 9 days. Changes in proline, total protein, photosynthetic pigments, carotene, anthocyanin, malondialdehyde, phenols, flavonols, flavonoids, soluble sugars and potassium ion were examined. The results showed that tobacco plants which had been exposed to drought used the accumulation of osmolytes such as proline, soluble sugars and potassium in order to balance their osmotic pressure. Drought stress also caused oxidative stress and increased the production of active forms of oxygen. As a result, non-enzymatic antioxidant defense system of tobacco plants including anthocyanins, flavonoids, flavonols and beta-carotene increased, which could be considered to be a major step for resistance to drought. The results also showed that the concentration of 15 mM potassium nitrate in particular, could significantly improve some of the harmful effects of stress and reduced photosynthetic pigments and proteins. Potassium nitrate could also bring down the MDA and beta-carotene levels to equivalent levels in control plants. As a result, it seems that using potassium can affect plant resistance to drought and plays an important role to reduce some harmful effects of stress.

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Application of some chemical components including plant hormones such as methyl jasmonate causes resi-stance to increase in environmental stresses. In this study, the effect of metyl jasmonate in different concentrations (10, 20 and 30 uM) on the elevation of the plant resistance was investigated in drouht stress. Tobacco seedlings were studied under drought stress caused by polyethylene glycol (20%) during periods of 3, 6 and 9 days. The results revealed that the imposed stress significantly increased soluble sugar content, MDA and non-enzymatic defense factors such as ant-hocyanin and proline. In contrast, it is reduced the amount of photosynthetic pigments, carotenoids and flavonoids. On the other hand, the use of methyl jasmonate decreased malondialdehyde as a marker of lipid peroxidation and values of some detent factors such as proline, beta-carotene and MDA. Meanwhile, it increased the amount of soluble sugars and photosynthetic pigments, anthocyanins, flavonoids and flavonols. As a result of the simultaneous reduction of lipid pe-roxidation and some antioxidants, it seems that using exogenous methyl jasmonate can help the plant withstand against drought stress conditions.

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Melissa officinalis is a medicinal plant belong to  lamiacea family.  This plant has been cultivated in many parts of the world due to its therapeutic effects. This study was conducted to improve antioxidant activities of proliferated shoots from tissue culture. The seeds of plant   were cultured in MS hormone free medium in order to obtain   seedlings. Then the  shoots were transfer to MS medium supplemented with 2 mg/ L  BAP.After 45 days salicylic acid (SA) was added to the medium at concentrations of 0, 50, 100 and 200 µM. The antioxidant activity and phenolics, flavonoids and rosmarinic acid content in proliferated shoots were evaluated 4,7,10 and 14 days after treatment. As a result the highest free radical scavenging and reducing power activity, as well as the highest value of total phenolic   and rosmarinic acid were observed in shoots   after 14 days of treatment with 100 µM salicylic acid. The elevated level of superoxide anion scavenging activity      and total flavonoids        were obtained from the shoots treated with 50 µM salicylic acid for 14 days. Over all treatment with100 µM salicylic acid for 14 days was better in order to antioxidant activity and compounds with recognized antioxidant activity.

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One of the major abiotic stresses that negatively affects plants is the presence of heavy metals. Soil pollution with heavy metals, resulting from the industrial development and use of fertilizers containing heavy metals, has become a major environmental concern in human societies. Mercury is a toxic heavy metal that causes pollution in agricultural lands. Accumulation of Hg by plants may disrupt many cellular functions and block growth and development. Under such conditions, the enzymatic and non-enzymatic defense systems of plants are activated. Several defense systems are cooperating together in plants to cope with stressful situations. In this study, the effect of different concentrations of mercury on the photosynthetic pigments content and non-enzymatic defence systems in Nicotiana tabacum was studied. After planting the plants under the same conditions in the hydroponic medium and feeding the plants with Hoagland solution, treatments with different concentrations of mercury nitrate (0.5, 1 and 3 mM and a control group) were applied to the seedlings in three replications. Ten days after the application of the treatments, the plants were harvested and examined. The results showed that tobacco plants which had been exposed to heavy metal used the accumulation of osmolytes such as proline and soluble sugars in order to balance their osmotic pressure. The decrease in the amounts of photosynthetic pigments and increase in the levels of malondialdehyde in the leaves indicated the elevation of oxidative damage. Increased activity of non-enzymatic antioxidants in tobacco leaves, including anthocyanins, phenol, flavonols and flavonoids, can be interpreted as the mechanisms of resistance to heavy metal stress induced by mercury.