Abstract:The objective of this study was to evaluate the effect of supplemental light quality and durations of illumination on growth and root activity of cucumber (Cucumis sativus L.) seedling. The experiment was conducted to study the effect of three supplemental light duration (2, 4, and 8 h) and six light qualities: white light (W), red light (R), blue light (B), a 7:3 mixture of R and B, and a 8:2 mixture of R and B on the state of aboveground seedling and belowground root, root activity and biomass accumulation of cucumber. The result showed that the light duration and light quality had significant effects on seedling height, root diameter, root length, root surface area, root branch numbers, root activity, shoot dry weight (DW), root DW and seedling index as well as the duration and quality interaction. Growth indexes of cucumber with 4 h supplemental light were significantly higher than that treated with 2 or 8 h supplemental light. Light quality with mixture of R and B had higher growth indexes than with B, R, W and natural light. The treatment with supplemental light duration of 4 h and light quality in 7:3 mixture of R and B had the highest seedling height, stem diameter, root length, root surface area, root branch numbers, and root activity. In addition, the shoot and root DW were significantly improved over treatment with supplemental duration of 2 h and light quality in B, R, W and with supplemental duration of 8 h and light quality in B, R, W and in natural light. Shoot dry weight and root dry weight showed a positive relationship with root activity, respectively. It was conclude that providing 4 h of supplemental light (in 7:3 mixture of R and B) could promote the root activity of cucumber and thereby increase the shoot and root DW. The study laid a physical foundation of sound seedling cultivation.