Assessing ascorbic and salicylic acid-induced drought tolerance in marjoram (Origanum vulgare L.) using the MGIDI index.

Water deficit stress, exacerbated by climate change, severely limits crop and medicinal plant productivity. This study evaluated the effects of foliar-applied ascorbic acid (AsA) and salicylic acid (SA) (0, 1, and 2 mM) under varying water deficit levels (90%, 60%, and 30% field capacity (FC)) on antioxidant responses, growth, and essential oil content of marjoram. The treatment ranking results, using the multivariate MGIDI index, showed a statistically significant difference between the treatments. The four selected treatments, were T13, T16, T17, and T18. Mean comparisons revealed that increased stress severity significantly reduced shoot fresh and dry weight (FW and DW), photosynthetic pigment content, and stem height (SH). Under drought stress, essential oil percentage (EO (%)) and the activities of total superoxide dismutase (T-SOD), and its isozymes (Fe-SOD, Cu/Zn-SOD, and Mn-SOD), catalase (CAT), guaiacol peroxidase (GPX), and ascorbate peroxidase (APX) exhibited an initial increase, followed by a decline at higher stress intensities. Concurrently, increasing stress led to a reduction in essential oil yield (EOY), whereas hydrogen peroxide (H2O2) and malondialdehyde (MDA) levels increased. Conversely, the combined application of SA and AsA mitigated these adverse effects, particularly under drought conditions. Notably, treatments including 1 and 2 mM AsA, as well as combinations of 2 mM AsA with 1 and 2 mM SA, at 60% FC, enhanced FW, carotenoid content, antioxidant enzyme activities, EO (%), and EOY. These same treatments also resulted in a significant reduction in H2O2 and MDA levels. Overall, this study highlights the effectiveness of AsA and SA in mitigating water deficit stress and demonstrates the utility of the MGIDI index as a powerful multivariate tool for treatment optimization.