Flaxseed oil cake improves basil (Ocimum basilicum L.) yield under drought stress by increasing herb biomass and quality of essential oil

Oil cakes are agro-industrial wastes produced in large quantities, with still untapped potential in plant production. Basil is one of the most economical essential oil crops cultivated worldwide, requiring sustainable production methods. This greenhouse experiments evaluated the effects of flaxseed oil cake (FOC) added to peat substrate at the rates of 5, 10, and 20 g dm−3 on herbal yield, content of bioactive compounds and mineral elements, antioxidant activity, and essential oil composition of basil (Ocimum basilicum 'Dark Opal'), grown under well-watered and drought stressed conditions. Two-way ANOVA, principal components analysis (PCA), and correlation networks were applied to associate the data. PCA analysis indicated variability in basil responses to the different treatments, with each treatment forming distinct clusters. The study revealed the beneficial effect of FOC on basil herb yield under water stress. Applying 10 g dm−3 FOC was the most effective in mitigating drought stress in basil. Under drought stress conditions, FOC at the rate of 10 g dm−3 increased fresh weight by 63.9 % and herb dry weight by 59.9 % compared with the peat-only substrate plants. As FOC rates increased, the nitrogen level of plant tissues and substrates increased while reducing sugars, total polyphenols, and total flavonoids content decreased. GC-MS analysis of basil essential oil showed that the main compounds were τ-cadinol (10.79–12.54 %), linalool (4.93–10.01 %), methyl eugenol (4.11–6.71 %), α-bergamotene (3.11–5.48 %) and eugenol (2.10–5.05 %). Drought stress lowed the linalool, methyl eugenol and α-bergamotene concentrations compared with the well-watered plants. However, adding FOC increased the accumulation of linalool and methyl eugenol in all of the FOC treatments under drought stress. Analysis of the differential network showed that the fresh and dry weight and the methyl-eugenol content as “hub” parameters correlated with most traits of basil grown under well-watered and drought stress conditions. In conclusion, using FOC at appropriate levels can effectively reduce plant herb yield loss caused by water stress.