INTRODUCTION

Extreme weather conditions such as local torrential rainfall, drought, and heavy snowfall caused by climate changes have long-been a global issue and these environmental changes are having their impact on the growth of plants (Kim et al., 2011).

The plant factory that allows cultivation of crops under controlled settings with a maximized use of space is being popularized as the current climate changes can cause a serious environmental impact on the growth and propagation of plants grown in the openness (Kim et al., 2011). A plant factory is a system integrated with a computer that allows the cultivation of plants irrespective of the weather and spatial conditions with its ability to artificially set its own environmental factors like light, temperature, humidity, concentration of CO₂ and culture medium (Yoon, 2012).

LED refers to a semiconductor device that glows when applied with a forward current (Yoon, 2012) and has many benefits in the plant cultivation within the plant factory settings. With the usage LED light source, which has monochromatic diodes, one is able to choose the wavelength that is effective in photosynthesis and the photomorphogenesis for optimization of plant cultivation unlike the natural light which consists of various colors of mixed lights (Yoon, 2012). Also in comparison to the high pressure sodium and metal halide lamp, which are a type of discharge lamp, LEDs produce less heat which results to less plant damages during cultivation (Lee, 2010). LEDs are environmental-friendly as their production does not require mercury, their maintenance cost is low with their long lifespan of 10,000 to 50,000 hours and a high power saving efficiency (Hwang et al., 2004; Yoon, 2012). Thus the LED light sources are often used in the field of photobiology (Robin et al., 1994; Tripathy and Brown, 1995). Studies on the growth characteristics of plants grown under LEDs in plant factory are being actively done locally with culinary vegetables like red and green lettuces, horseradish, and bokchoy (Kim et al., 2014; Kim and You, 2013; Lee et al., 2012).

The LEDs of red, blue, white, far-red, red+blue, red+blue+white, red+far-red using this study were often used to research on the plant factory or plant physiology (Kim et al., 2014; Kim and You, 2013).

The S. capitata is Korean endangered plant established by Ministry of environment (Ministry of environment, 2015). It is herbaceous perennial belong to Caryophyllaceae. This plant’s peduncles fall in the ground because they grow too long. Its flower blooms between October and September. Pink-colored small flowers are gathered at the end of peduncle. The height of S. capitata is relatively small with an average of 30cm. It can be cultivated in limited space like a plant factory due to its capacity to grow under low light conditions (Lee, 2003).

Specially, confirming the number of flowers S. capitata was needed for its artificial proliferation because of forming a number of seeds per flower. For this reason, we studied physiological characteristics and the number of flowers of S. capitata. Until today, the physiological response and reproduction on a variety of plants have studied (Goins et al., 1997; Zhou Y et al., 2002; Choi, 2003; Govert et al., 2010; Choi et al., 2013; Bernier et al., 1993; Corbesier and Coupland, 2005) except them of S. capitata.

Thus this study was done to confirm S. capitata can be stable cultivated and multiplied in the plant factory even if climate change continued and to find optimal light source for their optimal physiological response and flowering.

MATERIALS AND METHODS

1.Cultivation and Maintenance

S. capitata was disseminated in a seed bed (width 60 ㎝, length 30 ㎝, and height 3.5 ㎝) filled with bed soil (horticultural bed soil with mineral supplements, Han Areum Co.) on March 20, 2013.

Water was supplied every 2 to 3 days from dissemination until germination. Germinated seedling was transplanted into a round container (diameter 12cm, and height 15 ㎝) filled with bed soil. Each light condition of the plant factory was allotted with 3 plants and 6 different light conditions were looked at while cultivation took place for 10 months from June 2013 to March 2014.

The fertilizer (GoldSoil, KGChemical Co.) which consists of 70% organics, 4.3% nitrogen, 1.7% phosphoric acid, 1% potassium, was diluted in water by 3% and supplied every 7 days as a soil nutrient.

The average photosynthetic photon flux density (PPFD) was measured during the cultivation period, and the average PPFD of each plant factory chamber was 150.59±27.29 μmolm^-2s^-1 (Table 1). Temperature was set at 19.42±5.10℃ on average and adjusted using a cold and hot-air circulator (SS-2000, Zero engineering Co, Korea). Humidity was kept at an average of 71.81±9.22% using a humidifier (Fox-1H, Parus Co, Korea). The CO₂ concentration within the plant factory was at 401.59±86.87 ppm during cultivation. Above-mentioned data were collected in computer and were measured every 10 minutes using LCSEMS (PARUS Co., Korea). Photoperiod was set at 16 hours out of 24 hours.

2.Light Conditions within the Plant Factory

Red, blue, white, red+blue mixed, red+blue+white mixed, red+far-red mixed (R; B; W; R+B; R+B+W; R+fR respectively hereafter) were used as the light sources within the plant factory (Table 1), using ‘the LED grow light’ system (Parus Co. 2010). Each of light qualities was selected by ones mainly used in plant factory. Red and blue light are commonly used photosynthesis in plants and white light similar to daylight. Infrared light relate to photomorphogenesis.

The power consumption of each light source is 200watt and the dimensions of the chamber installed in the plant factory were 120㎝ in width, 52㎝ in length and 50㎝ in height.

The optimal spectrum of each light source reached its peak when the wavelengths of the light were measured at 660nm with R, 450nm with B and 780nm with fR. White LED light was used that its wavelength is 450-540nm.

3.Measurements of the Physiological Response

Physiological response measured in October, 2013 when growth activity is vigorous. To investigate the physiological response of S. capitata, photosynthetic rate (umolCO₂g^-2s^-1), transpiration rate (mmolg^-2s^-1), stomatal conductance (mmolH₂Om^-2s^-1), water use efficiency (umolCO₂mmolH₂O^-1), chlorophyll content (CCI), minimum and maximum chlorophyll fluorescence (Fo, Fm) and the photochemical efficiency of photosystem II (Fv/Fm) were measured.

Photosynthetic rate, transpiration rate and stomatal conductance were measured by Photosynthesis measuring apparatus (Lci Ultra compact Photosynthesis System, ADC Co.) between 10:00AM to 12:00PM when the solar radiation was higher than the light saturation point. When measured, leaves were inserted that’s chamber and wait 5 min for stabilizing. Water use efficiency was calculated from dividing the photosynthetic rate by the transpiration rate (Lee et al., 2012).

Chlorophyll contents was measured using the Chlorophyll contents Measuring Apparatus (CCM-200, Chlorophyll Content Meter, ADC Co.). CCI(chlorophyll content index), its unit, is quantified value in a non-destructive manner as comparing two lights absorption when irradiating the lights of 940nm and 660nm in the leaves.

Fo, Fm and Fv/Fm were measured using a Chlorophyll Fluorometer (OS30p, Continuous source Chlorophyll Fluorometer, ADC Co.) between 02:00PM to 04:00PM. When measuring, leaves were done dark adaptation for 20 min then irradiated the light of 660nm in a moment.

3.Measurements of Growth and Flowering Response

S. capitata is the plant that form rosette has rosette leaves and also its flower stalk has leaves (Lee, 2003). In this study, we counted total leaves per plant.

Multiples of tiny flowers grow on the tip of the flower stalk or the axil of the S. capitata (Lee, 2003). In this experiment, the number of inflorescence which flowered on each plant under different light conditions was counted. On each plant, total number of inflorescences per plant was counted. Then, 15 inflorescences per plant were selected and the number of flowers on each of the selected inflorescence was counted on November 2013. Based on this, we calculate total number of flowers per plant.

3.Statistical Analysis

The effects of different LED irradiation patterns on the physiological and flowering responses of S. capitata were tested using one-way ANOVA. The statistical differences between the treatment groups were evaluated via Fisher’s least significant difference test as post-hocs, with significance at P=0.05. The data processing was conducted using STATISTICA 8 (Statsoft Inc. 2006, Tulsa, USA).

RESULTS AND DISCUSSION

1.Physiological response

In the increasing orders, the photosynthetic rates ranged from R+B+W≥B, R, W, R+fR≥R+B (Figure 1A). Similarly, the photosynthetic rate of rice (Oryza sativa L.) is higher in the R+B than R (Matsuda et al., 2004), that of Cocklebur (Xanthium strumarium L.) is higher in the R than B (Sharkey and Raschke, 1981).

Specially, photosynthetic rate under R+B+W was higher than R+B because the wavelength of W including the wavelength of green and yellow as well as red and blue compensates the defect of R and B. Its cause is considered by the pigments absorbing green and yellow wavelength in the plants (Hopkins and Huner, 2004; Robert, 2010).

The transpiration rate for S. capitata ranged from W≥R ≥R+B+W, R+fR≥B≥R+B (Figure 1B) while water use efficiency was no difference (Figure 1D) because the increasing patterns of photosynthetic rate and transpiration rate are too similar.

The stomatal conductance ranged in the following increasing order, W>R≥R+B+W, R+fR≥B, R+B (Figure 1C). This increasing pattern is similar to transpiration rate than photosynthetic rate. It seems like that photosynthetic rate has dissimilar pattern by the difference of carbon assimilation ability (Ball et al., 1987) while stomatal conductance affects on the transpiration rate.

The chlorophyll concentrations ranged in the following increasing order, R+B≥R+B+W≥W>B≥R+fR≥R (Figure 1E). In a similar experiment, the chlorophyll contents of P. ginseng is higher in W than B (Park et al., 1989) and red lettuce is higher in B than R (Johkanm et al., 2010). Also, Fragaria x ananassa cv. akihime, the cultivar of strawberry, is higher in R+B than R and B. In the case of plants, which are adapted to low light conditions, it is known that the most of the energy is spent on the manufacture and maintenance of the light-harvesting machinery (Chavan et al., 2011). In the case of S. capitata, the photosynthetic rate was the lowest under R+B. Hence it can be deduced that the most of its acquired energy was invested on the maintenance of chlorophyll content for efficient photosynthesis.

In the increasing orders, the minimum chlorophyll fluorescence (Fo) ranged from B≥R+B, R+fR≥W≥ R+B+W≥R (Figure 2A), the maximum chlorophyll fluorescence (Fm) ranged from B, W, R+B, R+B+W>R+fR>R (Figure 2B) and photochemical efficiency of photosystem II (Fv/Fm) ranged from R+B+W≥W≥B, R+B, R+fR>R (Figure 2C). Wang et al. (2009) reported that the Fv/Fm of Cucumis sativa is increased in the order of W≥B>R.

In this experiment, R+B+W which is the most close in 0.83, is the light condition that S. capitata feels low stress but R is the most stressful light condition because of low value of Fv/Fm. Under R+B+W, photosynthetic rate is the highest because photoinhibition did not happen nearly. W includes the wavelengths of green and yellow lights, which are absorbed by anthocyanin, carotin, and xanthophylls (Hopkins and Huner, 2004; Robert, 2010). These pigments have functions protected leaves from photoinhibition (Hopkins and Huner, 2004; Robert, 2010).

2.Growth response

The number of leaves per plant of S. capitata has no difference (Figure 3). This result means that it’s the number of leaves was not affected by physiological responses on the different light qualities.

3.Flowering response

The number of flowers per plant ranged in the following increasing order, R, R+fR>R+B>B, W, R+B+W (Figure 4). In a similar experiment with Cyclamen persicum Mill. Cv. ‘Dixie White’, the number of flowers is more in R than B (Jeong et al., 2003). The number of flowers of cranberry (Vaccinium macrocarpon Ait) is more in R than W. Generally, phytochrome affected dominantly by red or far-red light induce flower bud. In this our study red light enhanced the flowering rate (Figure 4). But this result is opposed to flowering phenomenon of Arabidopsis suppressed by red light (Hopkins and Huner, 2004). These results show that the change of physiological response on the light quality affects on the number of flowers except the number of leaves of S. capitata. Specially, R and R+fR seem that it’s light stress and the number of flowers increased than the others. In terms of climate change, these results mean if global warming increased, stable artificial cultivation and proliferation of S. capitata will be possible using plant factory.