Investigating the effects of salicylic acid and cycocel on gas exchange process in Moldavian balm (Dracocephalum moldavica L.) under drought stress conditions

The Moldavian balm is herbaceous that originated in middle Asia and about 66 compound are identified in this plant. 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. In order to investigate the effects of salicylic acid and cycocel on the gas exchange on dragonhead in drought stress conditions a research was conducted in the spring of 2014 and 2015 at research farm of agricultural college, Zanjan University. The experiment was split plot with two factors of moisture levels and growth regulators in a randomized complete block design with four replications. The main factor is the moisture levels at two levels without stress (Complete irrigation until the end of the growth period), drought stress (Complete irrigation before start flowering and discontinuation of irrigation until the end of the growth period). Sub-substrate of different levels of salicylic acid (800 and 1600 μm), cycocel (600 and 1200 μm), and control (dissolve with distilled water), which applied for a single application time for 10 days before start flowering. The results showed that the interactions of the sampling stage in the year in hormone in moisture for intercellular CO2 concentration, transpiration, stomatal conductance and photosynthesis water use efficiency at probability level of 1%, it was quite significant. Intercellular CO2 concentration at the start flowering, full flowering and the end of flowering has a growing trend. And the rate of transpiration, stomatal conductance, photosynthesis and photosynthesis water use efficiency are decreasing. Cycocel treatment (600 μm) at the end of flowering stage increased intercellular CO2 concentration compared to other treatments. Salicylic acid (800 μm) treatment increased transpiration, stomatal conduction and photosynthesis compared to other treatments. The results of this study showed that with the application of drought stress, the gas exchange of the plant will change meaningfully. By closing the stomata, stomatal resistance increases and intercellular CO2 concentration increases and leads to a decrease in photosynthesis rate Reducing transpiration in drought stress is due to stomatal closure and stomatal conductance reduction. So that with intensity of draught stress, transpiration was decreased. The results showed that salicylic acid treatment can increase chlorophyll content by reducing the Chlorophyllase enzyme and increase the rate of photosynthesis and through increased stomatal conductance increases transpiration and it mitigates the negative effects of drought stress.The rate of photosynthesis had a decreasing trend from the start flowering, full flowering and flowering end. Salicylic acid treatment is likely to increase photosynthesis in the plant by increasing the leaf area, increasing chlorophyll content, preventing biosynthesis of ethylene and reducing the chlorophyllase enzyme. Cycocel treatments through increase stomatal resistance and stomatal conductance reduction, thereby reducing transpiration and decreasing the rate of photosynthesis, resulting in increased photosynthetic water use efficiency.