Auxin and hydrogen peroxide (H2O2) interaction differentially regulate seedling growth, Na+/K+ ratio and H2S homeostasis accompanying NaCl stress in etiolated sunflower (Helianthus annuus L. cv. Microgreen) seedling roots and cotyledons

•Exogenous IAA (25 µM) and/or H2O2 (350 µM) regulates seedling growth, Na+/K+ homeostasis, and H2S metabolism in etiolated sunflower seedlings.•Exogenous auxin (IAA) and/or H2O2 treatment recovered both root length and lateral root proliferation under NaCl stress (120 mM).•IAA and H2O2 modules H2S content in seedling roots and cotyledons and impact l-DES activity.•TIBA disrupts the internal auxin gradient in roots and reverse the positive effects of exogenous IAA.