Development and the components of exudates and bleaching sap in the root of cucumber seedlings were examined to elucidate the response of the rhizosphere to CO2 enrichment treatments (one time in the morning and twice in the morning and afternoon). Results showed that: CO2 enrichment significantly promoted root development shown by higher fresh mass, and increased root secretion of sugars, amino acids, organic acids and phenolic acids. However, the root exudates per unit of root fresh mass did not increase so much, unchanged or even decreased in cases. This indicated that the increase in root exudates per plant was mainly attributed to the increase in root growth. CO2 enrichment improved nutrient uptake, which was indicated by increases in concentrations of minerals and ZT in the bleaching sap. However, the concentrations of GA, ABA and IAA decreased. Twice enrichments a day in the morning and afternoon proved more effective than the morning enrichment only. In conclusion, CO2 enrichment resulted in improvements in the root development, metabolic activities and the consequent shoot growth in cucumber seedlings.