Study of timing of irrigation after foliar application of putrescine on phytochemical and physiological responses of sage

Abstract Salvia officinalis L. (sage) is a valuable medicinal plant with various secondary metabolites. These compounds are usually inducible by different stress conditions, for example, water‐deficit stress. Therefore, cost‐effective and eco‐friendly approaches for alleviating water‐deficit stress in medicinal plant crops are in high demand. Polyamines (PAs), such as putrescine, have a significant effect on improving crop yield under water‐deficit stress conditions. Therefore, a factorial experiment based on a completely randomized design using two factors and three replications was conducted. Treatments included irrigation regime (IR) and timing of IR after foliar application of putrescine (TIRFP) as follows: irrigation after depletion of 20%, 40%, 60%, and 80% available soil water (ASWD). TIRFP was performed at 0, 24, 48, and 72 h. The treatments affected morphological, physiological, molecular, and biochemical attributes. TIRFP of 72 h under IR of 80% ASWD improved the leaf area index, relative water content, antioxidant enzyme activities, and total radical scavenging capacity compared with the controlled treatment. The results of real‐time quantitative polymerase chain reaction (qRT‐PCR) and gas chromatography/mass spectrometry (GC/MS) analysis showed the highest level of sabinene synthase (SS) and bornyl diphosphate synthase (BPPS), the highest contents of 1,8‐cineole, β‐thujone, and camphor appeared in plants after 72 h of TIRFP under IR of 80% ASWD. The findings of this study confirm the protective role of putrescine against water stress. In conclusion, putrescine foliar application allows for balanced water retention. It preserves antioxidant capacity resulting in efficient photosynthesis and the restricted risk of oxidative damage under water‐deficit stress. It improves sage plants’ physiological attributes and valuable monoterpenes components of essential oil.