Abstract: In the present work, transgenic cucumber seedlings over expressing CsRCA and wildtype cucumber seedlings ‘08-1’at three-leaf stage exposed to high temperature (40 ℃, PFD 600 μmol· m^-2 · s^-1) were used to study the regulatory mechanism of photosynthesis by CsRCA. The results showed that the mRNA abundance of rbcL and rbcS as well as the activities of ribulose bisphosphate carboxylic enzyme (Rubisco) and Rubisco activase (RCA) were significantly higher in CsRCA overexpressing cucumber seedlings than in wild type (WT). Following 2h exposure to high temperature, a notable decrease was observed in photosynthetic rate (P_n), photochemical performance index based on the absorption of light energy (PI_ABS), activities of Rubisco and RCA as well as the relative expression of rbcL, rbcS and CsRCA in both wildtype cucumber seedlings and transgenic cucumber seedlings. It was found that high temperature stress led to higher Wk, a parameter of chlorophyll (Chl) a fluorescence OJIP curve. Furthermore, high temperature greatly reduced the efficiency of electron transfer along the electron transport chain beyond Q_A(ψ₀) and the quantum yield for electron transport (φ_E0), indicating that PSII oxygen complexes (OEC) and electron transport chain downstream Q_A were inhibited by high temperature. However, the inhibition could be alleviated by over expressing CsRCA in cucumber seedlings. Taken together, our data suggested that over expressing CsRCA improves photosynthesis in cucumber seedlings under high temperature stress by enhancing activities of the Rubisco and RCA, and maintaining the number of active reaction centers.

Key words: PSⅡ activity, CsRCA, high temperature, over expression, photosynthetic rate, cucumber.