Introduction

Common to all plants, photosynthesis has been well studied for its direct effect on growth, development including essential physiological processes that translates to yield and quality of agricultural produce for both horticultural and agronomic crops.

One of the main factors which affects photosynthesis is light. Its presence, intensity, exposure and quality plays key roles in the regulation of plant growth, survival and adaptation (Naoya et al. 2008; Zhang et al. 2003). Several studies in that deal with light environments and conditions have been numerous for several horticultural crops as in hazelnut (Hampson et al. 1996), begonia (Jeong et al. 2009), shrubs (Stanton et al. 2010), tomato (Fan et al. 2013) and pansy (Koksal et al. 2015) among others. According to the studies of Jeong et al. (2009) and Vendrame et al. (2004), the plant form, flowering, leaf size and its color for herbaceous plant species are affected by light intensity.

Nowadays, the use of artificial light has been considerably used for horticultural crops due to mass production and quality. Thus, it has played a remarkable importance among automation systems in green houses (Koksal et al. 2015). Studies of Soh et al. (2015) have considered the use of LED lights for some succulent species. Studies of Nam et al. (2016) have also studied the effects of intensity but these were various Crassulaceae species which were grown under hydroponic systems of which the light was measured in lux values. However, limited information is available for the effect of its exposure and more importantly to its intensity. According to Low (2007), indoor plants are often classified on the light necessary for growth they may be low (35 μmol･m^-2s^-1), medium (75 μmol･m^-2s^-1) and high (150 μmol･m^-2s^-1).

The popularity and demand of succulents have been constantly rising due to its drought-tolerant and water-efficient characteristics. Most of the Crassulaceae species that can be found in the markets are green in color which have differentiating rosette formation of its leaves (Nyffeler et al. 2008). Echeveria is considered as one of the popular genus among the succulent plants. This genus is comprised of 140 species of which 95% of the total is endemic in Mexico (Meyran and Lopez 2003; Vazquez et al. 2013). Echeveria species are known for its capability to develop pink to reddish leaf edges in certain environmental conditions (Fischer and Schaufler 1981). These pigmentation in plants are due to the presence of anthocyanins (Lo Piero et al. 2005).

The proponents have hypothesized that light intensity is regulates the development of these pigments in Echeveria species as well as may affect the growth performance of these plants which may change its leaf color and structure. This study aimed to determine the effects of light intensity on the growth, development and the anthocyanin content of two succulent Echeveria species.

Materials and Methods

Planting materials

Echeveria agavoides and Echeveria marcus species were procured from Kim Succulent Nursery, Anseong, South Korea. These species have a common trait which under unknown conditions, their leaf margins will turn pinkish to red in color. Young, healthy and disease-free succulents were chosen as experimental plants which were around 60-days old and were placed inside the greenhouse of Sahmyook University, Seoul, South Korea.

Experimental design, treatments and growth conditions

The experiment was laid out in a completely randomized design with four replications with six plants per replication. There was a total of seventy-two plants per species. Three light intensity levels served as the factor which were high (150 μmol･m^-2s^-1), mid (75 μmol･m^-2s^-1) and low (35 μmol･ m^-2s^-1) intensity.

All experimental plants were placed inside three plant growth chambers (KGC-175 VH, Koenic Ltd., South Korea). The relative humidity was set at 65%. There was a 14 hours light period and 10 hours dark period.

Hunter’s CIELAB

Color change was determined using the Hunter’s CIELAB (Konica Minolta Spectrophotometer CM2600d) which makes use of the L*a*b* color space to indicate lightness, hue and saturation of colors. One leaf for each plant was tagged to trace the color changes. The color value was measured through two areas within the tagged leaf; margin of the top and underside of the leaf.

Anthocyanin analysis

Modified quantitative method for anthocyanin (Fuleki and Francis, 1968) was used in this study by gathering an inch from the tip of the tagged leaves of succulent plants. One-gram fresh-cut leaf samples were macerated using a mortar and pestle. The macerated sample was added with 1 mL of 95% ethanol and 1.5 N HCl (85 : 15) which served as the extracting solvent. The mixed solution was transferred to a separate container. Samples were then centrifuged at 13,000 rpm at 4°C using the Micro Refrigerated Centrifuge Smart R17 (Hanil Science Co. Ltd., Seoul, South Korea). This was then stored and refrigerated overnight. Samples were placed in a microplate which was then analyzed for a full-spectrum UV/Vis absorbance at 535 nm using the Fluostar Optima Microplate Reader (BMG Labtech, Ortenberg, Germany).

Statistical analysis

Data gathering was done every two weeks for a month. Aside from the Hunter’s CIELAB and anthocyanin analysis, growth and development parameters were also collected: plant height and diameter. Statistical analyses were conducted using Statistical Product and Service Solutions for Windows, version 16.0 (SPSS Inc., Japan). The data were analyzed using analysis of variance (ANOVA), and the differences between the means were tested using Duncan’s multiple range test (P < 0.05).

Results and Discussion

Plant height and diameter

Results revealed that the use of light intensity levels have highly affected the growth parameters of both Echeveria species (Table 1).

The use of high light intensity gave the lowest plant height for E. agavoides (44.07 mm) ad E. agavoides (47.84 mm). For E. agavoides, the tallest plants were found in those grown under low light intensity (47.21 mm) and was followed by mid light intensity (45.29 mm) which were significantly different from each other. Comparable results have been observed by those in E. marcus. The tallest plants were found to be those grown under the low light intensity (54.29 mm) which significantly did not differ from those with mid light intensity (54.25 mm).

The plant diameter for both plant species have had the same growth patterns in their height. Results showed that diameter of plants for E. agavoides had higher values for those under low light levels with 94.06 mm which did not significantly differ from those grown in mid light intensity with 89.35 mm. The shortest plants were observed from those treated with high light intensities with 81.59 mm. For E. marcus, the largest plants were recorded from those in low light levels as well with 72.48 mm followed by mid and high with 67.40 mm and 68.90 mm, respectively, which did not significantly differ from each other.

This result is fairy different from the actual photosynthesis theory wherein the higher exposure to light would provide a higher growth rate compared to lower light exposures (Adams and Early 2004). Long et al. (1994) have also reported that plants grown under low light grown plants shown to be more usually susceptible to photo inhibition compared to plants grown under high light intensities.

However, results of the studies of Steinger et al. (2003) and Zhang et al. (2003) also reported that low light levels may lead the plants to increase in height and specific leaf area (SLA) in order to adjust to various light conditions which may change morphological and physiological aspects including the leaf and stem organs. Studies of grasses regarding shading and their growth rate revealed that it has a facilitative effect increasing plasticity and has been considered as an adaptive response which is responsible for the widespread ability to decouple growth from source of availability (Semchenko et al. 2012).

Hunter’s CIELAB

Table 2 shows the CIIELAB values of E. agavoides in response to light intensities. Results shows that the use of high intensity levels for L* value had the deeper lightness with 38.96. This was followed by mid intensity level (40.43) and low intensity level (43.54) for the top portion of the leaves. This trend was also seen to be similar to those of the bottom portion of the leaves.

For a* values, positive values indicate that the color is more inclined to red hues, however negative values tend to show green colors. Based on the above results on the a* values for the top portion of the leaves, high intensities had given -1.03 which is much closer to the positive values compared to mid and low intensities which had -2.85 and -4.26, respectively, which did not significantly differ from each other. This trend was also observed in the bottom portion of the leaves which had more positive values. On the other hand, b* values were not significantly affected by the treatments.

Results for the CIELAB color values for E. marcus is shown on Table 3 and had been found to have affected L* and a* values alone.

Results showed that the use of high intensity of light gave the deepest color hue with an L* value of 43.04 which was followed by mid (47.77) and low (50.69) light intensity levels, respectively. This was consistent for the top and bottom portions of the leaves. Likewise, a* values had a lower negative value indicating a hue closer to red for those succulents grown under high light intensity and was followed by mid and low intensity levels.

Anthocyanin analysis

The average anthocyanin content analysis showed that Echeveria species were significantly affected using different light intensities (Table 4).

High light intensity gave the highest value of 0.93 and 0.31 for E. agavoides and E. marcus, respectively. These were significantly different from those of mid light intensity which gave 0.55 (E. agavoides) and 0.25 (E. marcus), and was followed by those of low light intensities 0.28 (E. agavoides) and 0.12 (E. marcus).

According to studies of Rabino and Mancinelli (1986), light can affect the production of anthocyanin in plants. There is light-dependent anthocyanin production which occurs in the plant that displays high irradiance reaction resulting to photomorphogenic responses. Studies of Beckwith et al. (2004) also had the same results for Pennisetum setaceum, a purple pigmented ornamental grass which was when treated to low-light environments appeared light purple or green colors which resulted to lower aesthetic appeal.

Image analysis

Smart segmentation of images coupled with the original images for light intensities and their corresponding histogram results is presented on Fig. 1 for E. agavoides and Fig. 2 for E. marcus.

Based on the results of the smart segmentation (Table 5), it was observed that the use of high intensity had more pixels of red compared to mid and low intensities for E. agavoides. However, for E. marcus, succulents grown under mid and low intensities were not significantly different from each other and had more or less no red pixels found in smart segmentation. This result is also consistent with their histogram of images.

Increased red pigments or anthocyanin in higher light intensity may be due to one of its function as a protective shield to plants which is exposed to UV light such as Cotinus coggygria (Shamir and Nissim 1997). These red pigmentations contribute an important factor in increasing its increase in marketability and consumer preference (Shvarts et al. 1997).