Abstract Background Henna, a medicinal plant with a long history, is traditionally used as a dye for cosmetic purposes and also has the potential to be utilized as a hedge plant in tropical and subtropical landscapes. Putrescine (Put) is recognized as an important polyamine that has beneficial effects on plant growth and their responses to environmental stresses, including salinity, which is a key abiotic stress. An experiment was carried out to assess how Put (0 and 0.75 mM) and NaCl salinity (Control, 50, 100, 150 mM) affected the visual quality, growth, and physio-biochemical parameters of mature henna plants (two years old) over six weeks. Results The findings indicated that, although henna’s visual quality, growth parameters, RWC% and lawsone content decreased when exposed to 150 mM salinity levels, the most significant decrease (-64.02%) was noted in the leaf area. Conversely, salinity stress led to increases in EL% (19.4%), Na+ content (3.9-fold), proline accumulation (2.63-fold), total phenolic compounds (43.5%) and antioxidant activity. Meanwhile, henna’s response to Put pretreatment was complex and varied. While there were no changes in visual quality, leaf area, or shoot and root biomass, Put caused a minor reduction in plant height, the length of axillary shoots, the number of axillary shoots, the length of internodes, root system architecture (RSA) and the growth index, along with an increase in EL%, SOD, CAT, and POD activity in henna leaves under normal salinity conditions. Moreover, the interaction between Put and salinity demonstrated that Put enhanced plant height, the length of axillary shoots, the growth index, RSA, the leaf greenness index, photosynthetic pigments, K+ content, RWC and CAT and APX activity, while reducing Na+ content in leaves by -41.3%, the Na+/K+ ratio by -49.2%, EL% by -14.7%, and SOD and POD activity by -15.4% and − 44.98%, respectively, under 150 mM salinity stress. Additionally, correlation analysis revealed that the Na+/K+ ratio had strong negative associations with other henna parameters. Conclusions The present research indicates that Put foliar spraying can ameliorate the adverse effects of salinity stress on mature henna plants by reducing the Na+/K+ ratio, improving RSA, modification in antioxidant system activity and preserving photosynthetic effectiveness. Nonetheless, its use under non-saline conditions slightly decreased growth, which may be a positive factor for hedge plants.
