Optimization of lighting spectrum for photosynthetic system and productivity of lettuce by using light-emitting diodes

Impact of controlled illumination spectrum on photosynthetic system and productivity of lettuce (Lactuca sativa L. cv. Grand Rapids) grown in phytotron was investigated. The variable-spectrum lighting modules were designed using four types of high-power light-emitting diodes (LEDs) with emission peaked in red at the wavelengths of 660 nm and 640 nm, in blue at 455 nm, and in far-red at 735 nm. Biometric characteristics, pigments content and photosynthesis intensity in lettuce grown under eight different light irradiance were measured and compared. A corresponding experiment under a conventional high-pressure sodium lamp was also performed for reference. The treatments were carried out under photoperiod of 14 h and 21/15 degrees C (day/night) temperature. Lettuce was grown for 29 days after sowing in a phytotron chamber. Stomata size of lettuce grown under LED was larger than that of the plants growing under high-pressure sodium lamp. The lowest number and largest size of stomata were observed under light without the red component, peaked at 660 nm. Elimination of the blue component (455 nm) resulted in an enhancement of fresh mass production and increased leaf area, but the photosynthetic productivity did not show similar effect. The chlorophylls content in lettuce leaves was high during the entire growth period, but strongly decreased at the end of the treatment without blue light. The photosynthesis in lettuce leaves was most intensive under irradiance without the far-red component (735 nm). We conclude that productivity of lettuce can be optimized by adjusting the light spectrum and flux density.