Effects of drought and salinity stress on chemical constituents of essential oils from different plant organs of Ammi visnaga L.

Ammi visnaga L. is a medicinal and aromatic plant in the Apiaceae family. The essential oils of this plant have highly valuable aromatic compounds with valuable therapeutic and nutritional properties. Environmental stresses affect the quantity and quality of essential oils (EOs) in medicinal plants. This study was carried out to evaluate the independent and combined effects of salinity and drought stresses on EOs (%) and their chemical constituents in different organs of A. visnaga (seeds and head branches) under field conditions.

This study was carried out in the research farm of Isfahan University of Technology located in Lavark, Najafabad, using a split-plot design with three replications. Each replicate consisted of two rows (3 m long) of A. visnaga seeds. Different levels of salinity stress and drought stress were considered as main plots and subplots, respectively. Three levels of salinity stress [S1 (0 mM NaCl), S2 (80 mM NaCl), and S3 (80 mM NaCl)] and three drought stress levels based on soil depletion rate [D1: normal 50%, D2: moderate stress 60% and D3: severe stress 85%] were applied. Applying salt and drought stresses simultaneously started at the flowering stage (10%) and continued until the full maturity stage. After the full maturity stage of the plants, seed yield was measured. Then, the EOs extraction was done based on the Clevenger method in about 4 hours from head branches and seed samples for all studied treatments. The chemical constituents were identified using Gas Chromatography (GC) and Gas Chromatography/Mass Spectrometry (GC/MS) techniques.

The independent and combined stresses of drought and salinity caused a significant increase in EOs (%) of A.visnaga. The highest EOs in the head branch (1.26%) were detected under non-drought and medium salinity stress conditions (D1S1). The lowest EOs (0.25%) were observed under non-drought and non-salinity conditions (D1S1). The highest EOs (0.25%) in seed samples were observed under D2S2 conditions (moderate drought and salinity stress), but the least amount (0.12%) was found under D1S2 conditions (moderate salinity stress) and D1S3 (severe salinity stress). The analysis of essential EOs identified 25 compounds in both seeds and head branch samples. These compounds belong to six main chemical groups. The most and lowest compounds belonged to the groups of hydrocarbon monoterpenes and oxygenated diterpenes in both branches and seeds, respectively. Cembrene was identified as the main chemical compound in seed and head branch EOs. The lowest values for cembrene were reported as 23.32 % and 26.16% in seed and head branch EOs, respectively. The highest values for cembrene were reported as 33.31% and 36.62% in seed and head branch EOs under treatment. The compound 2-methyl-2-methylbutyl ester was identified as the second most abundant compound in the head branches and seeds of A. visnaga. The highest amount of this compound was observed in seed EOs (9.38%) under mild drought and mild salinity stress (D2S2), and the highest amount of this compound was observed in head branch EOs (18.5%) under mild drought stress (D2S1) conditions.

According to the study results, drought and salinity stress caused a quantitative and qualitative increase in A. visnaga EOs. Considering the climatic conditions of the arid and semi-arid regions of Iran and the consequent gradual salinization of soils, this plant can be considered a plant of choice in medicinal plant breeding programs, as well as goals related to the food and pharmaceutical industries.