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 hand 21115·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 sbow similar effect. Tbe cblorophylls content in lettuce leaves was high during tbe entire growtb period, but strongly decreased at the end of tbe treatment without blue ligbt. Tbe 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. INTRODUCTION Spectral quality affects various physiological processes in plants. Red light is important for the development of the photosynthetic apparatus (Saebo et aI., 1995). Blue light has' effect on the formation of chlorophyll, stomata opening and photomorphogenesis (Senger, 1982; Dougher and Bugbee, 1998; Schuerger et aI., 1997; Heo et aI., 2002). Lighting with a fixed spectrum is neither spectrally optimal nor . energetically effective and remains probably the most conservative technological factor in plant cultivation. Recently developed high-brightness light emitting diodes (LEOs) ground the future lighting technology (solid-state lighting). These optoelectronic devices feature high radiant efficiency; longevity, relatively narrow emission spectra and small . switching time and contain no mercury as most conventional light sources do. Using the new-generation light sources offers tremendous untapped reserves in increasing of the efficiency of photophysiological processes in plants, accelerating of the selection cycles, improving of quality of vegetable food. saving of energy resources, and elimination of the , impact of mercury on the environment (Bula et aI., 1991; Zukauskas et aI., 2002). Lettuce is an important greenhouse vegetable, intensively grown during the i seasons of low solar irradiation. The supplementation by artificial light is relatively . expensive. Therefore the search for alternative lighting sources is highly important. LEDProc. v,n IS on Artificial Lighting Ed. R. Moe Acta Hort. 711, ISHS 2006 183 based lighting presented herein might result in accelerated growth and increased quality oflettuce and other similar vegetables. The aim of the present study is to determine the spectrum of light that is optimal for growth, photosynthetic system and productivity of lettuce. MA TERIALS AND METHODS Lettuce was grown in growth chambers in Reat substrate (pH 6.0-6.5) prepared with fertilizes PG MIX (NPK 14-16-18; I.3 kg/m ). All experiments were carried out under 14 h photoperiod and 21/l5°C (day/night) temperature. LED-based lighting facility for plant cultivation under different spectral composition was designed using high-power LEOs with the useful light emitting area of 0.22 m• Four types of LEOs were used: 6 LEOs LuxeonTM type LXHL-LR3C (peak wavelength A = 455 nm) and 100 LEOs LuxeonTM type LXHL-MOID (A = 640 nm) of LUMILEDS LiGHTfNG, USA, and 9 LEOs L660-66-60 (A. = 660 nm) and 22 LEOs L73505-AU of EPITEX, Japan (A = 735 nm). The spectral and circadian characteristics for all experiments are presented in Table I. For comparison, the plants were growing under of high-pressure sodium lamps SON-T-Agro (PHILIPS). Leaf area of lettuce was measured by a CI-202 Leaf Area Meter (CID Inc., USA). Photosynthesis intensity was measured using a CI-3IO Portable Photosynthesis System (CID Inc., USA). The content of chlorophylls was determined in 100% acetone extract using a spectrophotometer Genesys 6 (ThermoSpectronic, USA). Samples were taken from the 2 or 3'd fully expanded shoot leaves. The dry weight of lettuce leaves was estimated after drying at the temperature of I05°C. For stomata measurements, the epidermal layer was removed and examined by means of optical microscopy. Stomata were counted on the lower leaf side in 10 visual fields of 0.17 mm; the average length was determined from 30 stomata. All measurements were performed at the end of experiments, except the chlorophyll content that was determined 4 times during each experiment. RESULTS Elimination of the blue component (455 nm) resulted in an enhancement of the fresh mass production and in an increase ofleafarea (Fig. 1), while elimination offar-red component (735 run) decreased both the indices. In general, irradiation by the 735 nm component at night had a tendency to decrease the fresh mass production compared to treatments without nocturnal break. The highest photosynthetic productivity was observed in EXP 2, where 735 nm component was applied for I h at night. In contrast, the lowest values were observed in EXP 8 (without 735 nm component) as well as under high-pressure sodium lamps (Fig. 2). The stomata length in lettuce leaves varied in the range of 51-59 !lm. This index was higher in plants that were grown under LEOs as compared to SON-T -Agro treatment (Fig. 3). The number of stomata in visuaJ field of 0.17 mm ranged from 4 to 10. The treatment without blue (445 run) component resulted in the most elongated stomata and in the least number of those per visual field (4-5 units). 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 (Fig. 4). The chlorophyll content in other treatments was development-dependent, although no significant tendencies were observed. The photosynthesis in lettuce leaves was most intense under illumination without the far-red 735 nm component (Fig. 5). DISCUSSION A number of investigators indicated that the blue component in small quantities is essential for plant growth and development, although an overexposure may reduce the photosynthesis intensity and yield (Yanagi et aJ., 1996; Goins et aI., 1997; Yorio et at,