Brassinosteroids(BRs) are potent regulators of photosynthesis and crop yield in agricultural crops;however,the mechanism by which BRs increase photosynthesis is not fully understood.Here,we show that foliar application of 24-epibrassinolide(EBR) resulted in increases in CO 2 assimilation,hydrogen peroxide(H 2 O 2) accumulation,and leaf area in cucumber.H 2 O 2 treatment induced increases in CO 2 assimilation whilst inhibition of the H 2 O 2 accumulation by its generation inhibitor or scavenger completely abolished EBR-induced CO 2 assimilation.Increases of light harvesting due to larger leaf areas in EBR-and H 2 O 2-treated plants were accompanied by increases in the photochemical efficiency of photosystem II(Φ PSII) and photochemical quenching coefficient(q P).EBR and H 2 O 2 both activated carboxylation efficiency of ribulose-1,5-bisphosphate oxygenase/carboxylase(Rubisco) from analysis of CO 2 response curve and in vitro measurement of Rubisco activities.Moreover,EBR and H 2 O 2 increased contents of total soluble sugar,sucrose,hexose,and starch,followed by enhanced activities of sugar metabolism such as sucrose phosphate synthase,sucrose synthase,and invertase.Interestingly,expression of transcripts of enzymes involved in starch and sugar utilization were inhibited by EBR and H 2 O 2.However,the effects of EBR on carbohydrate metabolisms were reversed by the H 2 O 2 generation inhibitor diphenyleneodonium(DPI) or scavenger dimethylthiourea(DMTU) pretreatment.All of these results indicate that H 2 O 2 functions as a secondary messenger for EBR-induced CO 2 assimilation and carbohydrate metabolism in cucumber plants.Our study confirms that H 2 O 2 mediates the regulation of photosynthesis by BRs and suggests that EBR and H 2 O 2 regulate Calvin cycle and sugar metabolism via redox signaling and thus increase the photosynthetic potential and yield of crops. Brassinosteroids (BRs) are potent regulators of photosynthesis and crop yield in agricultural crops; however, the mechanism by which BRs increase photosynthesis is not fully understood. Here, we show that foliar application of 24-epibrassinolide (EBR) resulted in increases in CO 2 assimilation, hydrogen peroxide (H 2 O 2) accumulation, and leaf area in cucumber. H 2 O 2 treatment induced increases in CO 2 assimilationpite willing inhibition of the H 2 O 2 accumulation by its generation inhibitor or scavenger completely abolished EBR-induced CO 2 assimilation.Increases of light harvesting due to larger leaf areas in EBR-and H 2 O 2 -treated plants were accompanied by increases in the photochemical efficiency of photosystem II (Φ PSII) and photochemical quenching coefficient (q P) .EBR and H 2 O 2 both activated carboxylation efficiency of ribulose-1,5-bisphosphate oxygenase / carboxylase (Rubisco) from analysis of CO 2 response curve and in vitro measurement of Rubisco activities. Coreover, EBR and H 2 O 2 in creased contents of total soluble sugar, sucrose, hexose, and starch, followed by enhanced activities of sugar metabolism such as sucrose phosphate synthase, sucrose synthase, and invertase. transiently, expression of transcripts of enzymes involved in starch and sugar utilization were inhibited by EBR and H 2 O 2.However, the effects of EBR on carbohydrate metabolisms were / by H 2 O 2 generation inhibitor diphenylene oxide (DPI) or scavenger dimethyl thio-urea (DMTU) pretreatment. All these results 2 secondary messenger for EBR-induced CO 2 assimilation and carbohydrate metabolism in cucumber plants. Our study confirms that H 2 O 2 mediates the regulation of photosynthesis by BRs and suggests that EBR and H 2 O 2 regulate Calvin cycle and sugar metabolism via redox signaling and thus increase the photosynthetic potential and yield of crops.