Changes in essential oil-content and composition of Lavandula angustifolia ‘Hidcote’ in response to abscisic acid under irrigation regimes

Drought, as the most catastrophic abiotic stress, poses a significant threat to the growth and development of plants. Among the mechanisms employed by plants to cope with drought-induced stress, abscisic acid (ABA) which is the sesquiterpene hormone, occupies a pivotal role. A hypothesis has emerged that the exogenous application of ABA can positively influence the terpenoid content of Lavandula angustifolia cv Hidcote essential oil (EO), thereby conferring enhanced resilience to drought stress. A randomized complete block design experiment was conducted with three replications and four irrigation regimes, including I4 [30% - 40% of field capacity (FC)], I3 (50% - 60% FC), I2 (70% - 80% FC), and I1 (90% - 100% FC) as control. Application of ABA spraying included three concentrations, A3 (30 μmol · L -1 ABA), A2 (15 μmol · L -1 ABA), and A1 as control (distilled water). Results revealed that drought significantly affected all studied traits except for relative water content (RWC) and shoot dry mass. The ABA impact application on the observed traits was found to be dependent upon the level of drought to which the plants were exposed. Specifically, the highest levels of flavonoid content, total antioxidant activity, peroxidase (POX) activity, and EO percentage were observed under I4A2 conditions. Conversely, the highest levels of superoxide dismutase (SOD) and catalase (CAT) activity, and proline were recorded under I4A3 conditions, while the highest EO yield was obtained under I3A2 conditions. Analysis of the EO revealed that there were common indicative compounds across the varying levels of drought and ABA application, including linalool, camphor, borneol, bornyl formate, and caryophyllene oxide. The production pattern of monoterpene and sesquiterpene compounds demonstrated a distinct trend, with the highest concentration of monoterpene hydrocarbon compounds (average of 12.92%) being observed in the I2A3 treatment group, and the highest concentration of oxygenated monoterpene compounds (average of 64.76%) being recorded in the I1A1 group. Conversely, the most significant levels of sesquiterpene hydrocarbon compounds (14.98%) and oxygenated sesquiterpene compounds (10.46%) were observed in the I4A3 and I4A1 groups, respectively, showing the efficacy of monoterpenes and sesquiterpenes from the action of ABA under drought conditions. The observed results indicated that the concentration of oxygenated monoterpene compounds decreases with an increase in drought level. Conversely, the application of ABA at any given drought level appears to resulted in increased concentrations of oxygenated monoterpene compounds in the same conditions. It may be concluded that plants under high-stress drought conditions allocate more terpene precursors to the production of sesquiterpene hydrocarbon compounds, aided by ABA with the same properties.