Effect of growth regulators on some physiological characteristics and essential oil of dragonhead (Dracocephalum moldavica L.) under drought stress

Introduction The dragonhead plant is herbaceous that originated in middle Asia and about 66 compound are identified in this plant which the most important compounds are Geranyl acetate, Geranial, Geraniol and Nearl. Application of growth regulator lead to increases of plant resistance against biotic and abiotic stress and is involved as a strategy to prevents adverse effects of environmental stress. According to semi-sensitivity of dragonhead plant to drought condition, its medicinal importance as well as, its industrial widespread use, this study was conducted to evaluate dragonhead essential oil yield and its compound under drought condition and also, to evaluate physiological aspect of drought resistance. Material and method A split plot experiment with two factors moisture levels (normal and stress) and hormone treatment (control, 600 and 1200 µm cycocel and 800 and 1600 µm salicylic acid) was conducted in based of completely randomized block design with four repeats. Two moisture regimes (normal and drought stress) were main factors. In drought stress, plants were irrigated before flowering stage and after that irrigation was stopped in drought stress treatment until end of experiment. Different levels of salicylic acid (800 and 1600 µm) and cycocel (600 and 1200 µm) and control were sub factors which was applied once, two days before drought treatment. Findings Membrane stability was negatively correlated to essential oil yield and chlorophyll content index and positively correlated to proline content. In normal condition, there were differences between hormone levels and essential oil yield, so that treatment with salicylic acid 1600 µM lead to highest amount of essential oil yield and treatment with cycocel 1200 µM had no effects on essential oil yield in comparison with untreated plants, moreover treatment with salicylic acid 800 µM and cycocel 600 µM adversely affect essential oil yield than untreated plants. The analysis of essential oil under stress conditions showed that most of the decomposed compounds under these conditions were less than that control and none of the growth regulators have not been able to increase the amount of compounds that have been reduced in stress conditions. Unlike other compounds Geraniol Influenced by growth regulators in stress conditions there was a significant increase compared to the control and Geranyl acetate Was increased influenced by Cycocele 1200 µm and Salicylic acid 1600 µm. And for compounds Neral and Geranial in stress conditions for all levels of growth regulators observation was not found.Under drought condition, Geraniol was significantly increased by application of all hormones treatment than untreated plants while Geranyl acetate was significantly increased only by application of salicylic acid 1600 µM and cycocel 1200 µM. Conclusion The results of this research showed that by increasing the amount of proline, which is a positive reaction to drought stress the percentage of essential oil also increases. Correlation results revealed that Essential oil yield has a positive and significant relationship with chlorophyll content index and with membrane stability has a negative and significant relationship. This indicates that Essential oil yield has a positive relationship with the plant's greenness. That is, the plant has the more leaves and limiting conditions affect the plant less the plant will have a higher essential oil yield. Under stress conditions, there is no difference between levels of growth regulators but there are differences between levels of growth regulators in normal conditions for essential oil yield. So that salicylic acid 1600 had the highest essential oil yield. Analysis of essential oil under drought condition indicated that, the most of compounds had higher content under normal condition and any hormonal treatment compensated this decrease. In contrast, Geraniol significantly increased by hormonal treatments under drought than normal condition; thus it could by concluded that in drought condition, application of higher dose of hormones will increase essential oil compounds.