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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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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.