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Growth Responses of Origanum majorana L. to Methyl Jasmonic Acid under Limited Irrigation Conditions

Abstract Water deficit is one of the most important factors in reducing medicinal plants’ yield. Origanum majorana L. grows in the some regions of world and it is cultivated for consumption and pharmaceutical purposes. In order to study the effects of methyl jasmonic acid (MeJA) on O. majorana L. under limited irrigation conditions, a factorial experiment was performed as a complete randomized design with three replications. Experimental factors including different limited irrigation conditions such as keeping the pots at field capacity during the experiment (control), irrigation pots in 75 % field capacity (mild limited irrigation), and irrigation plants in 50 % field capacity (moderate limited irrigation), and spraying treatments include 100 μM MeJA and control with water. At flowering stage, fresh weight, dry weight, and essential oil percentage (EO %) and yield, and essential oil composition were measured. Results indicated that moderate limited irrigation decreased plant weight and essential oil yield compared to the control. However, both mild and moderate limited irrigations increased EO % compared to the control. Foliar spray of MeJA improved growth parameters such as O. majorana L. fresh and dry weight just under mild limited irrigation conditions, while under moderate limited irrigation application of MeJA could not significantly change these parameters. Under mild limited irrigation conditions, MeJA causes significant reductions of 16.9 and 12.0 % in EO % and yield, respectively. Although limited irrigation did not have a significant effect on all of the O. majorana L. essential oil components, MeJA application changed O. majorana L. essential oil composition. With an increase in the limited irrigation level, MeJA application diminished the content of monoterpene hydrocarbons components from 20.56 to 18.21 % and increased oxygenated monoterpenes from 62.73 to 65.44 %. Results of this study revealed that MeJA could alleviate drought stress damages by stimulating oxygenated monoterpenes production and, therefore, could improve essential oil production under water stress conditions.