Succulents are strategically tried plants that has the capability to live in arid environment as well as extreme habitats. These plants are related to about 12,500 species from 70 flowering plant families with remarkable variations in stem and leaf structure, and their flowers (Nyffeler et al. 2008). The third largest family of succulents is the Crassulacae consisting of plants that have a wide range of habitat adaptation and temperature tolerance. These succulents are popularly known to be called as the ‘stonecrop’ and ‘houseleek’ family with a broadcast appeal for growers, hobbyists and collectors (Rowley 1978; Sevilla et al. 2012). Within this huge family, several subfamilies are popularly known including Sedeveria, Sedum, Crassula, Echeveria, and Graptopetalum. They come in variety of morphological structures and their leaves evidently create a certain unique growth pattern.
Due to busy lifestyles of the new generation, there is a huge demand for plants that can survive indoors with minimum maintenance and watering needs. To add aesthetic beauty and greens inside their homes and other establishments, live plants are the most natural and beautiful decorations. Attention of enthusiasts, businessmen and plant collectors have been attracted due to the bizarre growth-forms and attractive flowers that requires relatively minimum watering and care, and can be able to survive in indoors (Fischer and Schaufler 1981) and extreme outdoor conditions for landscapes, of which can be found in the characteristics of succulent plants (Oldfield 1997).
Succulents are propagated and cultivated in greenhouses to meet demands for landscaping, home decors and, other related products, and purposes. Production during other season proves to be more difficult during colder conditions as well as those months where there are shorter days.
Although, hydroponics has been used intensively to grow fruits, vegetables and some ornamentals (Admane and Sardare 2013), it has not been normally used as a technique to improve quality and growth of succulents. The use of hydroponics also allows the isolation of diseases or insect pests usually found in soil, direct control over the rhizosphere, increased planting distance, maximizing use of land area, efficient use of water and nutrients, ease in cleaning, no cultivation needed, transplanting is easy and ultimately, achieve highest possible yields or produce more number of plants (Rorabaugh 2014).
On the other hand, this technology is only limited to high economic valued crops and is expensive in operation (Resh 1983). Other growers believed that hydroponically growing succulents may exhibit leaf drop or defoliation since there is plenty absorption of water since succulents are already efficient in water storage. The issue is not with the use of water but the extent and efficiency of nutrient absorption.
Technologists have innovated and invented several materials that may be able to solve certain problems in greenhouses or open cultivation of certain crops for mass production. Plant responses to environment are also influenced by various factors including both environment and cultural practices coupled with the innate genetic characteristic of crops (Hartmann and Kester 1975). Among environmental factors include light, temperature and nutrition which may also be enhanced through cultural practices to exemplify better quality of plant growth. Light may have both quantity, which may be distinguished with intensity and duration, and quality, which differs what type of light and/or wavelengths are exposed to crops (Dorais 2003).
Supplemental lighting has been a recognized efficient strategy to maintain, optimize and provide photosynthetic requirement of long-day plants with respect to photoperiodism. It guarantees a better opportunity to meet market demands and improve plant growth and quality (Gottdenker et al. 2010).
Among quality of lighting, the choice between the types of light source is in constant research and there are several studies that reported the use of LEDs in greenhouses enable efficient and affordable lighting system compared to conventional incandescent lighting (Kosal et al. 2015) as well as having long lifespans with tremendous control of spectra distributions (Llewellyn et al. 2015).
Generally, LEDs come in combination of most electrically efficient colors including blue, red and cool white. These provide precision delivery of photons and result studies indicated that they are more cost effective option for supplementary lighting in greenhouses (Nelson 2014). Studies of Li et al. (2016) reported that the use of low-cost and longliving light emitting diode was a good artificial light source to explore the effects of supplementary lighting during dark period especially during the winter on the vegetative characteristics, early yield and physiology of plants grown in a greenhouse without heating. According to Jovanovic et al. (2006), indoor plants are often classified on the light necessary for growth they may be low (200 ft-c or 2,000 lux), medium (500 ft-c or 5,000 lux), high (750 ft-c or 7,500 lux) and very high (1,000 ft-c or 10,000 lux).
Thus, this study was conducted in order to determine the effects of light intensity and duration as supplementary lighting using LED fixtures on selected Crassulaceae succulent species grown in a hydroponic system.
Materials and Methods
Time and location of study
The study was conducted for a duration of 4 weeks within the months of November to December 2015 of which daylength only lasts for around ten hours from 8 : 00 o’clock in the morning to 6 o’clock in the afternoon. The selected Crassulaceae species, in general, are known to be very responsive or sensitive to its environment and was selected as the species to serve as the experimental unit.
The study was done in a Factorial Arrangement in Randomized Complete Block Design (RCBD) with two factors which includes light duration and intensity. Light duration served as Factor A with 13 and 16 hours of light exposure comprising of three and six hours of supplementary light duration and 10 hours of natural sunlight exposure. Light intensity served as Factor B with a measured value of 4,000 lux representing low intensity value (60 μmol • m-2 • s-1) and 8,000 lux as a high intensity value (120 μmol • m-2 • s-1). Treatments were replicated twice with a total of five treatments including the Control (with no supplementary lighting or having 10 hours of natural light).
Selected species under the Crassula Family were chosen with distinct color variation. There were five selected species from different subfamilies of Crassulaceae. These are the following species that were selected for the study as experimental units namely: Sedeveria letizia, Sedum ‘Sun Red’, Tom Thumb (Crassula rupestris), Echeveria ‘Momotaro’ and, Graptoveria opalina.
Treatment application and set-up
Succulent roots were washed with running water to remove soil contaminants and debris on previous medium. Roots were wrapped with non-woven fabric as rooting medium. Succulents were placed in trays in an upright position. Point of placements were made sure to have more or less the same lux value. Succulents were grown under an Ebb-and-Flow hydroponic system. Korean Standard nutrient solution with an average EC of 2.0mS • cm-2 and the temperature was maintained 10°C or higher. Each hydroponic reservoir had 100 L of prepared solution. Feeding time was only twice a day with a 1- hour ebb at 8-9 am and 5-6 pm. Home light-emitting diode (LED) fixtures were used as artificial lights. Two bulbs with a distance of 12 inches from each other and about 20 inches from the succulents were placed in the hydroponic bed frames creating 4,000 lux value, on the other hand four bulbs were used with the same distance forming a rhombus vortex pattern creating twice the amount or 8,000 lux value. Supplemental light fixtures were installed with timers to automatically coordinate proper lighting duration or three hours and six hours.
Plant height, diameter, visual quality score and color reading were parameters gathered weekly. The visual quality score was based on ornamental hedonic scale which included indicating and taking note of changes appearing as affected by light. The hedonic scale represents numbers 1 - 5 having described as dead, fair but not saleable, fair and saleable, good and excellent, respectively. The color reading was gathered using the Konica Minolta Spectrophotometer CM2600d following the CIELAB which makes use of the L* a* b* color space to indicate lightness, hue and saturation of colors.
The nutrient solution was checked daily to evaluate the level of EC, pH and the temperature. Since hydroponic beds were adjacent to one another, thick black curtains were placed in between these beds to ensure proper responses of succulents to their respective supplementary lighting treatments. The environmental temperature was maintained at 15°C.
Results and Discussion
S. letizia, acquired from an Italian collection (Sedum cuspidatum × Echeveria setosa var. ciliate), belongs to subfamily Sedoidae. Plants under this group are well-known for being in ‘splits’ or having another included within them (Fig. 1). They are drought tolerant species and leaf color may change depending on the shading or lighting (Lee and Kim 2008; Rowley 1978). Results of the analysis revealed that supplemental lighting significantly affected S. letizia height, diameter, and *b among parameters as shown on Table 1.
Among treatment combinations plants under three hours of supplementary lighting with 8000 lux had tallest plants with 68.42 mm which were comparable to both six hours light duration regardless of intensity with 62.02 mm and 62.28 mm. However, Control and three hours supplemental light and low intensity gave the shortest plants which did not significantly differ from each other with 56.07 mm and 60.73 mm, respectively.
Substantial change may be observed in photosynthesizing plants as to their chloroplast movement leading to differential height of plants. This movement may be affected by the amount of light absorbed. Under low photon flux which saturate the light responses in photosynthesis, chloroplasts tends to gather at the cell surfaces parallel to the plane of the leaf and perpendicular to the incident light which causes the plant to bend down. However, at maximum or higher lux value, the movement of the chloroplasts are perpendicular and parallel to the indecent light creating an upright position (Taiz and Zeiger 1991).
Diameter had similar results of that of height, wherein the highest diameter was recorded from plants grown in 3 hours supplementary lighting with high intensity value of 59.03 which were comparable to those of 6 hours and high intensity value of 53.30.
Based on the color reading, supplementary lighting did not significantly affect the L* and a* values of the color reading. However, it may be noted that the L* value had a high difference between treatments having Control with 35.62 indicating a lightness of the color spectrum. Fig. 2
This indicated that color appears to be bland compared to those treated with supplementary lighting ranging from 19.66- 21.18. Studies of Park et al. (2013) revealed that the use of supplementary lighting significantly increased chlorophyll content in S. wallisii. Chlorophyll absorbs mist of the light for the plant as a source of energy through photosynthesis and thus increasing the green pigmentation of foliage (Adams and Early 2004).
On the other hand, b* values were highly affected by treatments. Results revealed that six hours of high intensity value had darker hues with 14.10 while the rest of the treatments did not significantly differ from each other.
Sedum ‘Sun Red’
‘Sun Red’ is a new developed hybrid which has increase quality when red colors are evident (Fig. 3). This flaunty redcolored succulent is a member of the Sedum subgroup. They are often being studied as plants used for green roofing due to their fire prevention characteristics and dry resistance tolerance compared to other plants, it has a low transpiration value during the day (Al-Busaidi et al. 2010; Zinkan 2010). Data on the response of ‘Sun Red’ to supplementary lighting are shown on Table 2.
Results revealed that the plant height and diameter of this species were not significantly affected. However, its visual quality rating was found to be highly affected by supplementary lighting. Among treatments, plants grown in supplementary lighting regardless of duration and intensity had significantly affected the visual score of the succulent while those of the Control had the lowest VQR rating of 3.50 described as good. The use of poor lighting especially during winter affected the growth of Sun Red succulents.
Results of Park et al. (2013) on the study of foliage plants that ornamental value deteriorates when plants are exposed in poor light conditions and also inhibits growth. This will lead to lower quality of aesthetic worth of succulents. According to researches of Lopez (2013), the use of supplementary lighting in green houses in months of winter or months approaching winter could lead to the increase of daily light integral which affects both quality and yield depending on which crop. These also include rooting, thicker stems and reduction of defoliation. Studies of Hicklenton (1989) also revealed that the use of supplementary lighting provided increased quality and vegetative growth on root and shoot dry weight and increased leaf area at the end of its vegetative stage while at maturity, the use of supplemental lighting in the early stage gave significant improvement on the flower and vegetative dry weight, stem length, leaf area and number of flowers.
Among color reading data, only a* was significantly affected by the treatments. Results revealed that those succulents treated with supplemental lighting were significantly similar with each other except for those with the Control with 1.89 indicating a low hue towards red color. Thus, the use of supplemental lighting have significantly increased the red hues of Sun Red succulent variety.
These results were also observed in the studies of Koksal et al. (2015) of Viola curmota. It was found that the use of supplementary LED lighting significantly affected all vegetative parameters including root and shoot fresh weight and ratio, leaf number, flower number and quality of plants compared to those of the control. The use of LED lighting had more than 52% growth rate compared to plants which were deprived of supplemental lighting regardless of duration and intensity.
Also known as Tom Thumb, C. rupestris or rosary plants due to the formation of leaves on the stem which are small globular to triangular leaves whirling around the center of stem with multiple branches from the center of the main stem. They are capable of taking partly shady areas to full sun. In the event of excess light, the plant gives of a seemingly red tinge to the edge of its leaves or pale pink (Zinkan 2010). This succulent belongs to the Crassula subgroup where plants classified to this group are capable of tolerating wide range of environments (Jones 2011). Data on the effects of supplemental lighting on hydroponically grown C. rupestris are shown on Table 3.
Plant height and diameter were not significantly affected by parameters (Fig. 4). This may be due to the fact that this species have a wide range of adoptability to change in environment especially on its range of areas to grow in terms of lighting. However, visual quality score was highly affected by the treatments.
Among treatments, those with the highest visual score was attained by those grown under the longest supplementary hours and high intensity with 4.75 described as excellent. In the study of Park et al. (2013) the use of a 12 hour period on the Hedera helix plant significantly increased plant height, node number and its leaf number and length were found greatest under this light exposure compared to those of 0, 4, and 8.
Based on the color reading parameter, hues a* and b* were significantly affected by supplementary lightning duration and intensity. Lower hues a* were observed from succulents treated with six hours and high intensity value with -5.94 indicating a darker shade of green color while those exposed to lower light duration and intensity value, including the Control, had the highest a* value with 8.17 and 7.99, respectively which did not significantly differ from each other. Tom Thumb with the use of higher light intensity and duration is able to produce a more striking green color compared to those of the control which led to a higher rate on the visual score.
Succulent plants which were exposed to six hours with high intensity value had the lowest hue value of 18.44 which significantly differed from the rest of the treatments. A low hue b* indicates a position near that of red shades. Accordingly, with the increase of light, Tom Thumb would produce visible red to light pink edges of its leaf. Results of the study showed that the higher the intensity and longer the duration of supplementary lighting using LED fixtures, this specific succulent species increased the visible production of its unique light pink to red edge color, thus scoring a high rating with its visual score as well.
‘Momotaro’ belongs to the subgroup Echeverioideae or also known as Echeveria which are highly valued for their colors and amazing variations with a stunning leaf that come in attractive rosettes. Some variations include development of red colors on edges depending on light reception (Low 2007). Results of the effects of supplementary lighting of E. ‘Momotaro’ is found on Table 4.
Results revealed that plant diameter was affected by supplemental lighting (Fig. 5). The widest diameter was observed by succulents exposed with six hours and highest value of intensity of supplemental lighting (80.50 mm). As the diameter of the leaves increased the uprightness of succulents gave an extra visual quality point to the plant. This result was consistently seen with the visual score which was highly affected by the treatments and was followed by those that were exposed to supplemental lighting as compared to a low visual score to those of the control.
Among color reading values, a* was significantly influenced by supplemental lighting. The highest a* value was found in plants exposed to six hours + 4000 lux with 1.42 indicating a nearness to reddish hues.
Graptoverias are hybrid crosses between Graptopetalum and Echeveria in which G. opalina is classified with. They are sun-loving succulents that changes their leaf opening based on the perception and absorption of light (Graham 1987; Zinkan 2010). Presented on Table 5 is the response of G. opalina on the supplemental lighting duration and intensity.
Diameter was highly affected by supplemental lighting duration and intensity. Among treatments those that were exposed to no lighting (Control) and with three hours + 4000 lux had the largest diameter with 133.29 mm and 138.38 mm which did not significantly differ from each other. All succulents with supplemental lighting, aside from the minimal exposure of duration and intensity, had statistically similar diameter ranging from 125.81 mm-115.50 mm. Opening and widening of diameter is not ideal with this species. Once there is a larger opening, the color of the succulent fades and leaving it with spaces and forming loose rosette (Fig. 6).
Among color reading data, lightness (L*) was highly affected by supplemental lighting. Similar results were observed as to there is a darker expression of colors when succulents were exposed to supplementary lighting. Treatments did not significantly differ from each other except those that were treated with no light (Control) which had the highest lightness of 37.22 indicating a faded color.
Based on the results of the study, recommended use of the following light intensity for selected succulent species to enhance growth performance and quality are three hours + 8000 lux for S. letizia, six hours + 8000 lux for C. rupestris and E. ‘Momotaro’ and G. opalina.