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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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A laboratory experiment was carried out to investigate the effect of salinity on some morphological and phys-iological parameters in four Canola (Brassica napus L.) cultivars (Talaye, Sarigol, Zarfam and Opera) under salinity stress. Plants were grown in hydroponic solution (Hoagland 1/4 strength) under greenhouse conditions and on five-leaf stage, treated with different concentrations of NaCl: 0 (control), 50, 75 and 100 mM. After 14 days of treatment, plants were harvested and the length of shoots and roots, photosynthesis, chlorophylls and carotenoids contents of leaves were measured on four studied cultivars. In general the results showed that increasing salinity reduced leaf relative water co-ntent, shoot and root lengths and leaf area. Photosynthetic rate was declined in Talaye and Sarigol, but it was elevated in Opera and Zarfam. It should be noted that the changes induced by salinity on photosynthetic pigments was not regu-lar. The results of this study showed that among the investigated cultivars, in vegetative phase, Opera and Zarfam had higher capacity and function to salt stress tolerate than other cultivars. It also seems that Sarigol may be more vuln-erable than other cultivars under salinity 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.
 

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Seaweed extract can improve the physicochemical properties of the soil and has a favorable effect on the plant growth and development due to having high nutrient content, high water holding capacity, plant growth regulators and beneficial microorganisms. The aim of this experiment was to investigate the effect of Ascophyllum nodosum extract, a brown alga, on the morphophysiological characteristics of bean plant (Phaseolus vulgaris) under water stress. The experiment was performed in a Completely Randomized Factorial Design with three replications. Experimental treatments included 4 concentrations of foliar application of seaweed extract (0, 0.2, 0.4 and 0.6%) and 3 levels of water stress including no stress, mild stress and severe stress (irrigation at 75, 50 and 25% of field capacity, respectively). The results showed that the interaction effects of stress and algal extract on the evaluated traits were not significant. However, the seaweed extract significantly increased the leaf area, number of the leaves, leaf length, surface and length of the roots, root dry weight, the content of chlorophyll a and b and the total chlorophyll content. Application of algal extract at levels of 0.2 and 0.4% caused a significant increase in the evaluated parameters as compared with the control plants. The evaluation of water scarcity stress effects showed a significant reduction in the all studied characteristics in accordance with increasing water stress level from 75% to 25% of field capacity. The results of this study showed that foliar application of seaweed extract significantly increased some morphological and photosynthetic properties, while water stress significantly reduced the investigated parameters.