Changes in total phenol content, photosynthetic pigments and gas exchange of dragonhead (Dracocephalum moldavica L.) in response to different concentrations of abscisic acid and three moisture regimes

Drought is the major cause of yield reduction in the arid and semi-arid regions. Abscisic acid (ABA) plays a crucial role in the responses of plants to environmental stresses such as drought. This experiment was carried out as split plot based on complete random block design at research farm of University of Zanjan, during 2016 growth season to evaluate the effect of drought and ABA on total phenol content and some physiological traits of dragonhead (Dracocephalum moldavica L.). Three moisture regimes (well- watered, moderate drought with delay irrigation up to -0.8MPa and severe drought with delay irrigation up to -1.5MPa) were the main plots and five ABA concentrations (0, 5, 10, 20 and 40 µM) were subplots. The effect of moisture regimes, ABA and their interaction were significant for the studied traits. Without ABA application, drought stress reduced the plant total chlorophyll content, photosynthesis, stomatal conductance, mesophilic conductivity and transpiration. ABA application reduced the studied gas exchange traits. Overall, ABA decreased physiological parameters and seed yield by intensifying the drought stress effect. According to the results, without ABA application, moderate and severe drought conditions increased the total phenol content compared to the irrigation condition and application of ABA increased the total phenol content under moderate and severe drought conditions. Since the most active substance in dragonhead is phenolic compounds, it seems that the plant total phenol content can be increased by applying drought condition and ABA application.