Iron-modified carbon nitride nanostructures (Fe–N–O/C3N4) mitigate salinity stress in grapefruit mint (Mentha suaveolens × piperita) via boosted antioxidant defense and essential oil content

Salinity stress constrains medicinal plant productivity, especially in arid regions of the world. Grapefruit mint ( Mentha suaveolens × piperita ) is economically valued for its essential oils rich in linalool and linalyl acetate. Eco-friendly nanotechnologies offer novel strategies for enhancing crop stress tolerance. This study evaluated foliar-applied Fe–N–O/C 3 N 4 nanostructures for mitigating salinity stress in grapefruit mint under four NaCl levels (0–120 mM). Under severe salinity (120 mM NaCl), the Fe–N–O/C 3 N 4 treatment significantly increased chlorophyll a (+ 253%) and b (+ 369%), fresh weight (+ 10.7%), and dry weight (+ 8.9%) compared with control conditions at the same stress level. Remarkably, application of C 3 N 4 alone showed superior biomass enhancement (+ 64.9% fresh weight), suggesting carbon nitride-specific mechanisms on plant growth, while Fe–N–O alone demonstrated stronger reductions in oxidative stress markers (MDA: − 34.5%; proline: − 57.1%), highlighting the critical role of iron in membrane protection and osmotic adjustment. The combined Fe–N–O/C 3 N 4 treatment provided complementary benefits by enhancing both antioxidant enzyme activities (SOD: +12.1%; APX: +4.3%) and chlorophyll protection, indicating synergistic effects of the two components. On the other hands, salinity significantly influenced essential oil composition, with maximum linalool (51.55%) at 80 mM NaCl. Importantly, nanostructure treatments did not alter individual compound percentages, preserving the characteristic essential oil profile, although total essential oil percentage was enhanced under moderate stress with C 3 N 4 treatment (0.742%) by 96.3% over control. Fe–N–O/C 3 N 4 nanostructures represent an effective, biocompatible nano-priming strategy for improving grapefruit mint performance under saline conditions through enhanced iron bioavailability, antioxidant defense activation, and optimized secondary metabolism. This approach holds promise for sustainable cultivation of medicinal plants in salt-affected agricultural systems.