Reducing sugars: a potential factor for onion plant salinity adaptation

Salinity stress critically impairs agricultural productivity by disrupting plant growth, nutrient homeostasis, and oxidative balance. Although sugars contribute to osmoprotection, their specific role in onion (Allium cepa L.) salinity adaptation remains underexplored. This study evaluated the effects of foliar applications of reducing sugars (glucose, lactose, melibiose, and xylose at 10 mM) on the physiological, biochemical, and elemental responses of pot-grown onions exposed to salinity stress (0, 75, and 125 mM NaCl). Growth parameters, photosynthetic pigments, osmoprotectants, oxidative stress markers, antioxidant enzymes, elemental composition, and volatile compounds were analyzed. At 125 mM NaCl, salinity reduced plant height by 29.45% and fresh weight by 63.33%. Sugar treatments markedly alleviated these effects, with lactose showing the greatest improvement in fresh weight (189.09% increase). Photosynthetic pigments were preserved, notably chlorophyll a by xylose (558.04 mg 100 g⁻¹ FW) and chlorophyll b by melibiose (272.65 mg 100 g⁻¹ FW). Osmoprotectants increased under sugar application, with melibiose enhancing proline (8.93 mg 100 g⁻¹ FW) and xylose elevating reducing sugars (229.16 mg 100 g⁻¹ FW). Lactose lowered oxidative stress (H₂O₂: 198.19 mg 100 g⁻¹; MDA: 0.00075 µmol g⁻¹ FW) while enhancing PAL (9314 U mg⁻¹ protein) and PPO (26 U mg⁻¹ protein) activities. Sugar treatments also influenced ion homeostasis, with melibiose and lactose improving Ca²⁺/Na⁺ and K⁺/Na⁺ ratios. Elemental intake assessment showed that most essential elements remained within safe limits, although Fe and Mn slightly exceeded dietary thresholds in a few treatments. These findings identify lactose and melibiose as promising foliar treatments for enhancing onion salinity tolerance through improved osmoprotection, antioxidant defense, and mineral regulation.