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Background: Photosystem II (PSII) is susceptible to heat stress. Plants naturally accumulate trehalose to improve stress tolerance. However, the mechanism by which trehalose affects PSII during heat stress is still unknown. Questions: How does trehalose affect PSII during heat stress? Studied species: Triticum aestivum L. Study site and dates: Shanghai, China. 2019-2021. Methods: Trehalose -pretreated wheat seedlings suffered from heat stress and their photosynthetic parameters were measured. Results: Heat stress caused a reduction in the photochemical efficiency of PSII, the electron transfer rate (ETR(II)), the quantum yield of regulated energy dissipationY(NPQ) and the coefficient of photochemical quenching (qP), but increased the quantum yield of non-regulated energy dissipation of PSII (Y[NO]). The shape of the fast chlorophyll fluorescence induction kinetics (OJIP) curve in the heat-stressed wheat was altered and the primary photochemistry maximum yield of PSII (Fv/Fo) and the PSII performance indicator PIabs were reduced. Accordingly, the activities of PSII and electron transport chain, the amount of ordered α-helix structures and the content of D1 protein also decreased. However, in trehalose-pretreated wheat, D1 protein and protein secondary structures of PSII were both protected, the electron transport activities of PSII and the whole chain were improved and greater fluorescence parameters values were maintained. Lower Y(NO) and more stable OJIP were obtained. Conclusions: Exogenous trehalose acted a vital role in the protection of the function of PSII, resulting in higherphotosynthetic capacity under heat stress.