Abstract: Spray with 0.1 mg·L ^{- 1} 24-epibrassinolide ( EBR) significantly up-regulated the net photosynthetic rates of cucumber (Cucumis sativus) seedlings and the effect lasted for 1 week. EBR had no effect on chlorophyll content and stomatal limitation value (l) , but significantly increased apparent quantum yield(AQY) , carboxylation efficiency (CE) , the maximum carboxylation rate of Rubisco (V_c,max ) and the maximum rate of RuBP regeneration (J_max ). EBR treatment also increased the p roportion of‘open’PSⅡ reaction centers ( qP) , thereby resulting in an increased quantum yield of PSII electron transport (Φ_PSⅡ ) , but had little effects on the maximal photochemical efficiency of PSⅡ ( Fv/Fm) , and efficiency of energy capture by open PSⅡ reaction centres ( Fv’ /Fm’).

Key words: Brassinosteroid, Cucumber, Chlorophyll fluorescence, Photosynthesis, Regulation