This study aimed to investigate the interaction effects of iron oxide nanoparticle (nα-Fe 2 O 3) and its bulk counterpart (bα-Fe 2 O 3) with and/or without citrate interaction on germination and seedling growth of Oenothera biennis L. We prepared buffered suspensions of nα-Fe 2 O 3 and bα-Fe 2 O 3 at 4 levels (0, 0.2, 0.5 and 1 g.L −1). The nα-Fe 2 O 3 increased 89.17% germination percentage, 53.36% germination tolerance index, 82.21% root tolerance index, and 79.42% ferric reducing antioxidant power. The nα-Fe 2 O 3 +citrate improved α-Fe 2 O 3 function with a 47% relative hypocotyl length, and 97.32% vigor index2 increase. Leaf area, tolerance index of seedling fresh weight increased at bα-Fe 2 O 3 +citrate. Antioxidant enzyme activities are increased in α-Fe 2 O 3 +citrate, through which increased biomass and tolerance indices. Oxidative stress control was an important component of this plant's ability to withstand environmental stress and to succeed in germination and seedling growth. This finding emphasizes that if nα-Fe 2 O 3 were used at low and intermediate concentrations, it would not have toxic effects on this plant, and potentially would result in successful stress tolerance and could act as iron fertilizers for the growth of Oenothera biennis L. Abbreviations: AC (allometric coefficient); bα-Fe 2 O 3 (bulk α-Fe 2 O 3); CAT (catalase); Cit (citrate); Control (deionized water); FRAP (ferric reducing antioxidant power); FGP (final germination percentage); GP (germination percentage); GR (germination rate); GTI (germination tolerance index); HL (hypocotyl length); IONP (iron oxide nanoparticle); LA (leaf area); MDA (malondialdehyde); nα-Fe 2 O 3 (α-Fe 2 O 3 nanoparticle); POD (peroxidase); RHL (relative hypocotyl length); RL (root length); RSG (relative seed germination); RTI (root tolerance index); SOD (superoxide dismutase); TIFW (tolerance index of seedling fresh weight); TISL(tolerance index of seedling length), and VI (vigor index).

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