Salt eustress modulates physiological responses and enhances yield, nutritional quality, and antioxidant capacity in sunflower microgreens.

BACKGROUND: Microgreens have gained attention as nutritious superfoods and as model systems for studying plant stress responses. Salt stress can elicit hormetic effects by enhancing plant growth and quality at low concentrations (eustress) while inhibiting performance at higher levels (distress). METHODS: Sunflower (Helianthus annuus L.) microgreens were hydroponically cultivated in a completely randomized design with three vessel replicates per treatment. Seedlings were grown for ten days using root-dipping technique in five NaCl concentrations (0-100 mM) under controlled natural environmental conditions. Growth parameters, physiological status, stress biomarkers, proximate composition, mineral content, fatty acid profile, and antioxidant capacity were assessed. RESULTS: Low stress induced by 25 mM NaCl increased shoot fresh yield (58.7%), shoot dry yield (55.4%), shoot length (48.3%), root length (25.7%), leaf area (81.4%), and specific leaf area (19.7%). Mild stress by 50 mM NaCl also improved growth, but its stimulatory effects were less pronounced. Conversely, moderate and severe stress levels (75 and 100 mM) significantly reduced these growth parameters. These responses were associated with modified physiological status indicated by photosynthesis and transpiration rates, chlorophyll content, relative water content, and water use efficiency. At 25 mM NaCl, proline content increased (5.7%), while catalase activity, lipid peroxidation, and H2O2 content did not change significantly. Higher salt levels induced marked oxidative stress through elevated lipid peroxidation and H2O2 accumulation. Nutritionally, 25 mM NaCl increased total carbohydrates (27.2%), total proteins (27.6%), Ca (11.9%), Mg (13.3%), Mn (14.3%), Zn (12.9%), and linoleic acid (5.0%). However, this low stress decreased ash content (-27.3%), total fats (-69.8%), K (-12.0%), Fe (-15.2%), palmitic acid (-9.7%), stearic acid (-12.2%), and oleic acid (-4.8%). Antioxidant profiling showed that 25 mM NaCl maximized total antioxidant activity (38.1%), DPPH-scavenging activity (6.0%), total phenols (8.8%), ascorbic acid (37.5%), and carotenoids (11.5%), while moderate and severe stress levels had opposite effects. CONCLUSION: Low-dose salt eustress (25 mM NaCl) can be applied as a simple and cost-effective elicitor to improve growth, nutritional value, and antioxidant capacity of sunflower microgreens.