Ethylenediurea applied during grain filling alleviates physiological damages without yield recovery in wheat under elevated ozone.

Tropospheric ozone (O3) threatens crop growth and yield, particularly in China, necessitating the urgent exploration and optimization of effective mitigation strategies. Ethylenediurea (EDU) has been widely utilized as a common exogenous chemical protectant against O3 stress, while its specific mechanism of action remains unclear. In this study, wheat cultivar Lianmai 7 was subjected to ambient O3 and elevated O3 (1.5 × ambient) in an O3 free-air concentration enrichment (O3-FACE) system and sprayed with either water or 450 μmol/mol EDU exclusively during grain filling, at the interval of 7-10 days. EDU provided partial mitigation for photosynthetic impairment induced by O3 exposure and effectively improved the antioxidant defense capacity, by enhancing leaf ascorbic acid (AsA) contents, and altering antioxidant enzymes response patterns. Water-treated plants primarily relied on enhancing ascorbate peroxidase (APX) activities to mitigate O3 stress, while EDU-treated plants tended to maintain peroxidase (POD) and superoxide dismutase (SOD) activities. EDU-mediated protection became progressively insufficient to counteract cumulative O3 damage between 3 and 7 days post-application during mid-grain filling, as evidenced by attenuated correlations between physiological traits. EDU significantly improved transport and deposit of carbon assimilates from vegetative organs into grains under elevated O3 as indicated by harvest index, though non-significantly increasing grain yield under elevated O3. These results suggested that optimizing frequency of EDU application during critical growth stages is essential to improve its protective efficacy under elevated O3, particularly for the O3-sensitive cultivars. This study contributes to elucidating the EDU protection mechanism and informing its optimized application in high-O3 scenarios.