Introduction

Lettuce (Lactuca sativa L.) belongs to a member of the compositae, which is one of the major vegetable in the world. Lettuce is representative rice wrapped vegetable in Korea [5]. In Korea, leaf lettuce is commonly used as “wrap-up vegetable” with perilla (Perilla frutescens), kale (Brassica oleracea L. var. acephala), and pak-choi (B. rapa L. var. chinensis) [9] and is widely grown commercially in greenhouses using hydroponic culture [6].

Light emitting diodes (LEDs) have been proposed as a photosynthetic radiation source for space flight growing systems and as a more efficient source for terrestrial controlled- environment agriculture facilities [2]. It showed that lettuce and other dicotyledon plants developed excessive hypocotyl elongation when grown under high-intensity red-emitting LEDs as the sole source of irradiance. LEDs are a promising electric light source for space-based plant growth chambers because of their small mass and volume, solid-state construction, superior safety, and longevity [1, 2]. The electrical efficiency (μmol·J-1) of gallium aluminum arsenide (GaAlAs) red LEDs is reportedly greater than that of fluorescent lamps and comparable to high-pressure sodium lamps [1].

Commonly available red LEDs emit a narrow-spectrum of light (660 nm with 25 nm bandwidth at half peak height), which closely matches a peak absorbance of chlorophyll [7]. Although red LEDs have great potential for use as a light source to drive photosynthesis, plants are adapted to utilize a wide-spectrum of light to control photomorphogenesis. Previous reports indicate that a minimum amount (20-30 μmol·m−2·s−1) of blue light in broad-spectrum lamps is necessary to achieve normal growth and morphology. This blue light requirement appears to be a species-specific response [12]. Tests with the addition of blue fluorescent (BF) light to narrow-band red LEDs (660 nm) support the findings with broad-spectrum lamps. Despite the increasing popularity of red LEDs as a radiation source for growing plants, information is available for only a few plant species that directly compares growth and yield under red LEDs (with and without supplemental blue light) with those under the broad spectrum light. Hence, the purpose of this study is to evaluate the effects of different LEDs on early seedling growth, leaf color, inorganic elements, ascorbic acid and SPAD value in lettuce

Materials and Methods

Lettuce growing

Lettuce (Lactuca sativa L., cv. 'Oak Leaf') seeds were incubated at 4℃ on moistened germination paper for 2 days and germinated seeds were grown in peat substrate with a with a 6 h/18 h (day/night) photoperiod and an average ambient temperature of 20/16°C (day/night) . Each plastic culture tray (10 cm W × 35 cm L) contained three plants spaced 12 cm apart which is an effective planting density of 75 plants per m2). Evapotranspiration losses of water were replenished daily by adding deionized water to the nutrient solution reservoir. Air temperature, relative humidity, and CO2 levels were maintained in growth chambers at 20℃, 60%, and 500 mg·l-1 respectively. The referred ambient parameters of the room were maintained throughout the experiment duration. Each treatment comprises a 60×60cm illuminated growth area.

LED treatment

Lettuce seedlings were grown for 30 day with photosynthetic photon flux (PPF) maintained at near 300 μmol·m-2·s-1. Levels of PPF were measured with a quantum sensor (Model LI-180, LI-COR, Lincoln) calibrated with a spectro-radiometer (Model LI-1800, LI-COR) to enable accu-rate readings for LEDs. The photosynthetic photon flux (PPF) in Red, Blue, Red+Blue, Fluorescent, Ultra violet, Far red was 172, 192, 169, 163 μmol·m-2·s-1, 5.1-3, 497.7 w·m-2. The maximum peak of each LED radiation showed red (450-660 nm), far-red (730 nm), blue (450 nm), blue-red (450+660 nm), Ultra violet (350 nm), Fluorescent as control light.

Leaf surface were measured with a Minolta CR-200b colorimeter (Minolta, Ramsey, NJ) Three matured leaves per plant, in the upper third of the canopy, were measured on half of the plants of each plant under grown different LED radiation treatments. Leaf L*a*b* values were measured at the midpoint between the distal and basal ends of leaf. Chlorophyll meter readings (SPAD values) were repeatedly taken ten times at the centre of the leaves throughout the experiments.

For determining inorganic elements, about 2 g lettuce powder was directly weighed in a porcelain crucible. The sample was preheated on a Bunsen burner until smoke was no longer observed. The crucible was placed in a cold muffle furnace. The temperature of the muffle was raised from 50℃/30 min to 450℃, and kept at this temperature for 4 hr. Mineralization was considered completion when ash was white or slightly colored, with no charred particles. The ash was dissolved in 1 ml HCl and transferred with deionized water into 10.0 ml volumetric flask. For ICP-OES spectrometric measurements (Optima 3000 DV; Perkin Elmer Corp., Norwalk, CT). Details on ICP-OES instrumental operating conditions and measuring parameters used are given in Table 1.

Table 1.ICP-OES operating conditions

Analysis

The working multi-elemental standard solutions were made with 10% HCl. All working standard solutions were stored in Nalgene polypropylene bottles. The calibration curve was made from 5 points and the blank. The multi-elemental standard solutions ranged from 2.5 to 100.0 mg/l for Ca, K, and P; 0.25-10.0 mg/l for Fe, 0.625-25.0 mg/l for Mg; and 0.0125-0.5 mg/l for Mn. The collected data were analyzed by using SAS package (version 8.0, SAS Institute Inc., Cary, NC) for Duncan's multiple range tests.

Results and Discussion

Effect of growth

The wavelength of each LED used in this study was 450 and 660 nm in red, 450 nm in blue, 450 and 660 nm in red+blue, 350 nm in ultraviolet, and 730nm in far-red light (Fig. 1).

Fig. 1.Absolute PPF spectral distribution balance of LEDs and fluorescent light at the centre of growth areas(The wavelength of each LED used in this study was 450 and 660 nm in red, 450 nm in blue, 450 and 660 nm in red+blue, 350 nm in ultraviolet, and 730 nm in far-red light).

The PPF irradiance in red, blue, red+blue, fluorescent, ultraviolet, and far-red was 172, 192, 169, 163 μmol· m-2· s-1, and 5.1-3, 497.7 w · m-2 respectively. The plant height, hypocotyl length, leaf characteristics, and fresh weight cultivated by various LEDs light radiation are shown in Table 2. Plant height of lettuce grown under red, blue, red-blue, ultraviolet, far-red and fluorescent light radiation were 13.3, 8.8, 9.1, 10.8, 6.2 and 14.2 cm, respectively.

Table 2.zMeans separation within columns by Duncan's multiple range test at 5% level.

Plant height of length cultivated by red and fluorescent showed the highest score compared with other samples. Hypocotyl length was measured following 30 days of growth. Elongation of the hypocotyl was significantly affected by only red light irradiation. There was no measurable change in five light irradiation except for red light. No elongation of hypocotyl was observed in blue, red+blue, ultraviolet, far-red and fluorescent light irradiations.

A large number of leaves were founded when the seedlings were grown under red+blue and fluorescent. Very small number of leaf was produced by far-red light irradiation. Leaf length of lettuce seedlings were decreased by far-red light irradiation and meanwhile it was significantly increased by red and fluorescent.

Lee et al. (2010) reported that plant growth like a leaf length and fresh weight is more stimulated in red+blue light than that of fluorescent light under same photon flux, although the photosynthetic photon flux of fluorescent light irradiation (163 μmol · m-2· s-1) was similar to red+blue light (169 μmol · m-2· s-1). Similarly to the leaf length, the longer leaf diameter of lettuce plants grown under red, red+blue, and fluorescent light irradiation has resulted in more compact foliage and improved morphology in comparison with plants grown in ultraviolet and far-red light irradiation.

Pinho et al. (2007) showed that red+blue light irradiation causes an increased leaf diameter and number in lettuce plants [11]. The higher increment of fresh weight grown in LEDs light irradiation was observed in red+blue, red, and fluorescent light irradiation, in turn.

Effect of quality

Changes in the Hunter color value of leaf lettuce cultivated by various LED irrigations are shown in Fig. 2. The LED irradiation during the cultivation induced the color change of the samples. Redness (a*) of lettuce leaf in the treatments of red+blue light was higher than that of red, blue, ultraviolet and fluorescent light irradiation. Zhou and Singh (2004) reported that amount of each individual anthocyanin of cranberry increased differently under natural light, red, and far-red light, suggesting that expressions of enzymes that catalyze the anthocyanin biosynthesis are regulated differently by environments [13]. The results were agreement with the results of Lee et al. (2010) that the red+blue light irradiation increased significantly anthocyanin content in leaf lettuce [4].

Fig. 2.Effects of LED radiation light on Hunter's values of 'Oak Leaf' lettuce. Changes in the Hunter color value of leaf lettuce cultivated by various LED irrigations are shown.

Fig. 3 shows the SPAD values of lettuce grown under red, blue, red+blue, u ltraviolet, and fluorescent light irradiation. Highest SPAD values were recorded in red+blue light irradiation. The SPAD values were higher in red, blue, red+blue light irradiation as compared with the fluorescent light except for ultraviolet light. The effect of irradiance on SPAD values are used for the estimation of the nitrogen status of crops. Most crops are sun adapted plants in which leaf transmittance is less affected by irradiance than in shade plants [3, 10] and SPAD values are therefore probably not affected by irradiance in rice plant. However, in lettuce plant grown in various LEDs irradiation, the SPAD values are significantly affected by LEDs light irradiation.

Fig. 3.Effects of LED radiation light on SPAD values of 'Oak Leaf' lettuce. The SPAD values of lettuce grown under red, blue, red+blue, u ltraviolet, and fluorescent light irradiation.

Changes in seven kinds of inorganic elements in lettuce seedlings grown under different LEDs light irradiation were shown in Table 3. The red, blue and ultraviolet light irradiation caused an increased N content in lettuce seedlings.Otherwise the red+blue light irradiation showed 2.5∼2.8 times lower than that of red and blue light irradiation, respectively. In the changes of P content, LEDs light irradiation showed low P level compared with that of fluorescent light irradiation. The Ca and Mg content was only increased by red+blue light irradiation. The Mn content was increased in the treatment of blue and fluorescent light irradiation and decreased red and ulraviolet light irradiation. The Fe content in lettuce seedlings ranged from 3.2 to 6.7 mg · kg-1.

Table 3.zMeans separation within columns by Duncan′s multiple range test at 5% level.

As one of the functional nutrients, ascorbic acid content was measured in leaf of lettuce plants grown under different LEDs light irradiation (Fig. 4). Red+blue and fluorescent light irradiation showed highest ascorbic acid contents, meanwhile red, blue and ultraviolet light irradiation caused lower ascorbic acid content compared with red+blue and fluorescent light irradiation. Nam et al. (2004) reported that far-red radiation significantly increased the antioxidant activity of rice hull extracts [8].

Fig. 4.Ascorbic acid content of 'Oak Leaf' lettuce under different radiation lights. As one of the functional nutrients, ascorbic acid content was measured in leaf of lettuce plants grown under different LEDs light irradiation.

Detailed studies are required regarding the application of LED light for seedling growth, inorganic elements, antioxidative compounds including ascorbic acid in terms of economic utility, nutrition enhancement, and the correlation between light quality and growth of colored lettuce seedling.