No abstract

Melatonin, as an indoleamine molecule, regulates a wide range of physiological functions during the growth, morphogenesis and response of plants to biotic and abiotic stresses. In this research, the effect of exogenous application of melatonin (0 (distilled water), 50, 100 and 150 µM) to the leaves of Moldavian balm plants grown under different levels of drought stress (100% (control), 80%, 60% and 40% of field water capacity) was investigated. The results indicate that plants which were treated with 100 µM melatonin showed the greatest leaf surface area, lateral branching, flower length and activities of antioxidant enzymes (superoxide dismutase, guaiacol peroxidase and ascorbate peroxidase). Foliar application of 100 µM melatonin had no significant difference in catalase activity in comparison with the control and other concentrations of melatonin under normal, moderate and severe drought stress conditions. The lowest H2O2 content and lipid peroxidation (electrolyte leakage, concentrations of malondialdehyde and other aldehydes) were obtained at the concentration of 100 µM melatonin under severe drought stress. This concentration also significantly increased the chlorophyll content and enhanced the relative water content; however, foliar application of 100 µM melatonin had no significant effect on leaf length and proline content compared with the control under normal and stress conditions. The obtained results suggested that foliar application of 100 µM melatonin was more effective than the concentrations of 50 and 150 µM melatonin in reducing the adverse effects of moderate and severe drought stress.

To study the effect of salicylic acid on photosynthetic pigments (chlorophyll a, b, total chlorophyll, and carotenoids), polyphenol compounds, anthocyanin, flavonoids, phenylalanine ammonia-lyase activity, malondialdehyde, lipoxygenase activity, electrolyte leakage, relative water content, soluble sugar contents, and protein content of black cumin (Nigella sativa) under drought stress in hydroponic culture, an experiment was conducted as a completely randomised design in a factorial arrangement with three replicates. Experimental treatments included salicylic acid at three levels (0, 5, and 10µM) and drought stress (induced by polyethylene glycol 6000) at four levels (0, –0.2, –0.4, and –0.6 MPa). Results showed that salicylic acid application through the root medium increased drought tolerance of black cumin seedlings. Plants pre-treated with salicylic acid exhibited slight injury symptoms whereas those not pre-treated with salicylic acid had moderate damage and lost considerable portions of their foliage. In conclusion, salicylic acid could protect the Nigella plant against drought stress through increasing of all the mentioned traits, and 10µM salicylic acid was the most effective level under both conditions.

Compounds which are able to reduce the damaging effects of various stresses such as drought should be of great importance. In this research we have used arginine pretreatment and the effect of this compound on alleviation of oxidative damages under drought stress has been investigated. Our findings showed that arginine pretreatment reduced the lipid peroxidation when water stress was imposed. In drought stressed plants, H 2 O 2 increased and the activity of antioxidative enzymes were elevated over the controls, while glutathione reductase (GR) activity decreased. When plants pretreated with arginine, activity of catalase and guaiacol peroxidase decreased while the activity of superoxide dismutase (SOD), ascorbate peroxidase, and GR increased. Drought stress decreased ascorbate and reduced glutathione and increased dehydroascorbate. Opposite results were obtained after arginine pretreatment. When arginine was used as a precursor of nitric oxide (NO), the amelioration of the drought effects which was observed could well be the indication that these effects may be related to NO production. To prove that, we applied arginine'Nw-nitro-l-arginine methyl ester (LNAM) and on many parameters, arginine and arginine'LNAM pretreatment had the same effects and it seems that in these situations other pathways of arginine metabolism rather than nitric oxide synthase may be activated

Environmental stresses, such as cold, heat, salinity, and drought, induce ethylene production and oxidative stress and cause damage in plants. On the other hand, studies have shown that salicylic acid (SA) induced resistance to environmental stresses in plants. In this research, the effects of ethylene on chlorophyll (Chl), carotenoid (Car), anthocyanin, flavonoids, ascorbic acid, dehydroascorbic acid, total ascorbate, lipid peroxidation, and ethylene production in leaves of canola pretreated with SA were studied. The plants were grown in pots until they have four leaves. Leaves were sprayed for two days with three different concentrations of SA (0, 0.5, and 1 mM). The plants were treated for three days with three concentrations of ethylene (0, 50, and 100 ppm). At the end of the ethylene treatments, all examined parameters were measured. The results showed that the ethylene treatments induced lipid peroxidation, while SA mitigated this effect. The ethylene treatment lowered significantly Chl and Car contents and anthocyanin accumulation, but SA alleviated these effects. SA induced an increase in ascorbic acid content in canola plants after the ethylene treatments. Therefore, we concluded that SA played an important role in the alleviation of damages caused by stress conditions.