Genetic improvement of agronomic crops is necessary to cope with chilling stress. To identify the physiological factors responsible for this genotypic difference in chillinduced inhibition of photosynthesis, leaf CO 2 assimilation, the electron flux in the chloroplast and the antioxidant metabolism in isolated chloroplasts were examined in two genotypes of cucumber (Cucumis sativus) plants with distinct chilling tolerance. Cucumber plants were exposed to 100 µmol m -2 s -1 at 9/7°C (day/night)

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... or 10 d and were then returned to optimal conditions for 2 d. Chilling resulted in more significant reductions in rbcL and rbcS transcripts, ribulose-1,5-bisphosphate carboxylase-oxygenase (Rubisco) content and initial Rubisco activity, leading to higher electron flux to O 2 in the chilling-sensitive genotype than in the chilling-tolerant genotype. The chilling-tolerant genotype showed lower H 2 O 2 contents in the chloroplasts by maintaining higher H 2 O 2 -scavenging activity in the chloroplasts than in the chilling-sensitive genotype. H 2 O 2 accumulation in chloroplast was negatively correlated with the initial Rubisco activity and photosynthetic rate. Keywords: CO 2 assimilation -Cucumis sativus -Electron transport flux -Reactive oxygen species -Rubisco -Water-water cycle. Abbreviations: APX, ascorbate peroxidase; AsA, ascorbic acid; A sat , light-saturated rate of CO 2 assimilation; DHAR, dehydroascorbate reductase; GR, glutathione reductase; Ja(O 2 -dependent), O 2dependent alternative electron flux; Ja(O 2 -independent), O 2 -independent alternative electron flux; Jc+o, electron flux for photosynthetic carbon reduction and photorespiratory carbon oxidation; J PSII , total electron flux in PSII; rbcL, large subunit of Rubisco; rbcS, small subunit of Rubisco; Rubisco, ribulose-1,5-bisphosphate carboxylaseoxygenase; ROS, reactive oxygen species; SOD, superoxide dismutase.