Coordinated N metabolism and NO signaling underlie allantoin-citrulline synergy in salt-stressed cucumber

Salinity stress severely limits cucumber (Cucumis sativus L.) productivity by disrupting growth, photosynthesis, and ion homeostasis. This study investigated the potential of foliar-applied allantoin (1 mM) and citrulline (1 mM) to enhance salinity tolerance in hairy cucumber, a salt-sensitive Iranian landrace. A completely randomized design factorial experiment with three replications tested salinity levels (0, 50, 100 mM NaCl) and foliar treatments (control, allantoin, citrulline, and their combination). The results showed that salinity reduced plant height, dry weight, chlorophyll content (SPAD, Fv/Fm), and relative water content (RWC) while increasing electrolyte leakage (EL), oxidative markers (MDA, H₂O₂), and Na⁺ accumulation. Allantoin mitigated these effects by enhancing nitrogen (N) metabolism, improving K⁺/Na⁺ homeostasis, and upregulating osmolyte (proline, sugars) and antioxidant (phenolics, ascorbic acid) accumulation. Citrulline boosted nitric oxide (NO) production, which reduced Na⁺ toxicity, improved stomatal conductance, and activated enzymatic antioxidants (SOD, APX). Their combined application synergistically improved growth (44.24% in yield), photosynthesis (5.12% in Fv/Fm), and RWC (6.45%) while reducing H₂O₂ (61.53%) and oxidative stress (43.74% MDA). Notably, salt-stressed plants treated with both compounds exhibited elevated fruit levels of health-promoting metabolites (cucurbitacin, citrulline). Moreover, allantoin enhanced N assimilation and polyamine-mediated membrane stability, while citrulline-derived NO optimized ion transport and ROS scavenging. These findings highlight the dual role of allantoin (N metabolism) and citrulline (NO signaling) in conferring salt tolerance and propose their combined foliar application as a sustainable strategy for improving cucumber resilience in saline agriculture.