This investigation was conducted to compare the photosynthetic indices in ironwood trees infected by Viscum album L. and healthy trees located both in stand and out of stand in plain forest of Tamishan, Nour city. In each position, five healthy and five infected trees were selected and photosynthetic parameters, stomatal conductance, transpiration and internal CO₂ were examined. Results showed that the amount of photosynthesis and stomatal conductance in healthy and infected branches were varied depending on the position of trees, individual trees and branch type. The amount of transpiration and internal CO₂ were significantly different in healthy and infected branches. The highest amount of photosynthesis, stomatal conductance, transpiration and internal CO₂ both within and out of stand, were measured in healthy trees (except for stomatal conductance in out of stand). Both within and out of stand, healthy branch of infected tree indicates higher level of photosynthetic activity compared with infected branch. Also, twofold comparison of the four parameters investigated showed that photosynthesis, stomatal conductance, transpiration and internal CO₂ in healthy branches within stand were higher than those out of stand. It was also discovered that Viscum album caused disruption in photosynthetic activities of the host plant that, is why conducting supplementary studies in this regard is suggested to future researchers.

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.