Effects of ascorbic acid on some morpho-physiological traits of basil (Ocimum basilicum L.) under drought stress

Basil (Ocimum basilicum L.) is a plant from the Mint family and the genus Ocimum, which has many species, among which O. basilicum is the most useful economic species, and like other plants of the Mint family, it is a source of cyclic compounds and essential oil. It is also known as a rich source of phenolic compounds (especially rosmarinic acid and caffeic acid) and flavonoids. The plant's vegetative body contains essential oils. Linalool, methyl chavicol, citral, euginol, cineol, geraniol, camphor, and methyl cinnamate are important components of basil essential oil. As the most critical abiotic stress, drought stress plays a crucial role in reducing the yield of medicinal plants. It causes numerous morpho-physiological and biochemical changes in the plant. The performance of the effective substance is influenced by the plant's economic performance and environmental conditions, including drought stress. Stress effects are different at different stages of growth and development.

To investigate the effect of ascorbic acid on native O.basilicum the native population "Shahre-ray," under drought stress conditions, a factorial experiment in the form of a randomized complete block design was performed in four replications on the research farm of Horticulture Department of Mohaghegh Ardabili University in 2017. Treatments included drought stress (complete irrigation and cessation of irrigation at the beginning of reproductive growth and cessation of irrigation at 50% flowering) and ascorbic acid foliar application at four concentrations of 0.0, 0.5, 1, and 2 mM. This experiment was field operated in May 2016. Before the experiment, the bed preparation operation included plowing, disking, and leveling the ground. The plots of one meter in length and width were created. The seeds were planted in three lines with 15 cm plant spacing. 3-4 cm of rotted manure was used to cover the seeds. After determining the soil texture by the hydrometric method, the mentioned treatments were applied to stress. On the first day, irrigation was a flood, and after germination, irrigation was reduced. All traits were measured in the Physiology Laboratory after the Horticulture Science Department of the Faculty of Agricultural Sciences analyzed the data with SAS software. Also, the treatment averages were compared with Duncan's multi-range test at a probability level of 5%. 

The results showed that with increasing drought stress intensity, total plant height, stem length, number of leaves, and plant dry weight decreased by 36.08%, 39.75%, 51.26%, and 53.80% compared to the control treatment, respectively. If ascorbic acid spraying could improve these values, the highest root length was obtained under severe stress conditions (beginning of flowering) and treatment with ascorbic acid (15.83 cm), which showed an increase of about 31.91% compared to the control treatment. Drought stress conditions increased proline and essential oil components. Chlorophyll content also decreased under severe stress. Also, by applying drought stress, in all studied traits, the highest amount of ascorbic acid treatment (concentrations of 0.5 and 1 mM) increased, and higher concentrations decreased the content of studied traits. By analyzing the essential oil of plant samples, the main compounds obtained from basil essential oil included geraniol, methyl chavicol, alphapinene, caryophyllene, citral, coupon, carvacrol, neral, linalool, and limonene. In most compounds, essential oil compounds and components were enhanced by increasing drought stress and spraying. The highest amount of essential oil in the stress condition up to the 50% flowering stage and sprayed with 2 mM ascorbic acid was obtained for methyl chavicol compound (33.76%).

Applying ascorbic acid increased basil growth under drought stress and created resistance in the plant. Therefore, due to the lack of water resources and the great need of plants for water, it is possible to create resistance to stress in plants by spraying non-toxic and essential elements on them.