Ion homeostasis and redox stability are associated with cultivar-dependent preservation of essential oil quality in salt-stressed Pelargonium graveolens

Abstract

Soil salinization increasingly constrains the productivity and quality of aromatic and medicinal crops, yet the mechanisms underlying cultivar-dependent salt tolerance in Pelargonium graveolens remain insufficiently understood. This study evaluated three contrasting cultivars (CIM Pawan, CIM Bio-171, and CIM Bharat) exposed to 0, 50, 100, 150, and 200 mM NaCl to identify the physiological, biochemical, anatomical, and metabolic traits associated with salinity tolerance and the preservation of essential oil yield and quality. At 200 mM NaCl, salinity reduced total biomass by 26.8%, 32.4%, and 37.3% in CIM Pawan, CIM Bio-171, and CIM Bharat, respectively, and progressively decreased photosynthetic pigments, photochemical efficiency, and essential oil yield while increasing Na⁺ accumulation, ROS production, and lipid peroxidation. However, responses were strongly cultivar dependent. CIM Pawan exhibited the greatest tolerance, maintaining a higher K⁺/Na⁺ ratio (0.68) than CIM Bio-171 (0.56) and CIM Bharat (0.38), together with better growth, photosynthetic performance, osmolyte accumulation, membrane stability, and preservation of stomatal, trichome, and internal tissue integrity. Under severe salinity, superoxide dismutase, catalase, peroxidase, and ascorbate peroxidase activities increased by 73.4%, 77.4%, 68.8%, and 60.7%, respectively, in CIM Pawan. In contrast, CIM Bharat was the most sensitive cultivar, exhibiting severe ionic imbalance, oxidative injury, anatomical disruption, and a 73.2% reduction in essential oil yield, whereas CIM Bio-171 showed an intermediate response. Mild salinity (50 mM NaCl) increased essential oil percentage by 4.7% and selected monoterpenes by 2.1–4.3% in the tolerant cultivars, whereas severe salinity caused marked metabolic suppression. Salinity tolerance in P. graveolens was associated with coordinated maintenance of ion homeostasis, osmotic adjustment, redox balance, membrane stability, and secretory tissue integrity, which collectively supported the preservation of essential oil yield and quality. Among the three cultivars, CIM Pawan showed the greatest resilience under greenhouse conditions, indicating its potential for cultivation in salt-affected environments and providing a basis for cultivar selection and future improvement of salinity tolerance in geranium. Salinity severely limits P. graveolens productivity, but CIM Pawan’s superior K⁺/Na⁺ homeostasis and sustained citronellol, geraniol, and citronellyl formate yield position it as a valuable cultivar for saline agriculture.