Interest in interiorscaping with plants has increased rapidly over the past four decades with the need for green in the urban environment (Manaker, 1997). House plants provide people with aesthetic appreciation and other beneficial effects. Plants can effectively improve the indoor air quality by reducing volatile organic compounds, such as formaldehyde, benzene, toluene, ethylene, and xylene (Thomsen et al., 2011; Wolverton, 1988), thus reducing the risk of sick building syndrome (Kim et al., 2011). House plants also fulfill psychological needs of people by providing green color and comfort and enhance the indoor environment to make it more aesthetically pleasing (Bringslimark et al., 2007).
Foliage plants are often used as house plants because of their attractive foliage and their ability to survive and grow under limited indoor light (Chen and Henny, 2008). Among the characteristics of foliage plants, variegation is an important trait, which provides unique visual appearance and aesthetic variation in interior design, making it one of the considerations of consumers’ preferences in purchasing decisions (Chen et al., 2004). Variegation is the occurrence of patterns, especially as a result of differences in the amount or composition of the green pigment chlorophyll, although other pigments such as anthocyanin and carotenoids may also be involved in a wide variety of multicolored leaf patterns. Variegation can occur through differential gene expression, leaf blisters, virus, or genetic mosaics such as chimeras (Marcotrigiano, 1997), and the variegation appearance can be altered by environmental factors, particularly light intensity (Tilney-Bassett, 1986).
Among the environmental conditions for indoor plants, the most limiting factor for plant growth is reduced light intensity (Manaker, 1997). Although the light intensity outdoor is generally higher than 1000 μmol·m−2·s−1 of PPF at sunny days, typical indoor light intensity is less than 40 μmol·m−2·s−1 (Manaker, 1997). Because plant growth depends on light (i.e., photosynthesis), plants require a particular light environment for proper growth and development (Maloof et al., 2001). However, plants also can adjust to varying light environments through physiological and morphological changes (i.e., acclimation), resulting either in increased light capture or improved light utilization. General acclimation responses to low light include higher shoot to root ratio (whole-plant level), increased leaf size per unit leaf dry weight, increased total chlorophyll content, and a decrease in the chlorophyll a:b ratio (individual leaf level) (Allard et al., 1991; Evans and Poorter, 2001; Nemali and van Iersel, 2004). However, the effect of low light intensity on variegation is not consistent among species (either with increase, decrease, or no changes in variegation), thus the information of appropriate light levels is critical for producing and maintaining variegated foliage plants with their best performance in the interiorscape (Chen and Henny, 2008; Li et al., 2007).
Previously, several attempts for measuring variegation were made (Marcotrigiano and Hackett, 1993; Shen and Seeley, 1983; Smith et al., 1988). Shen and Seeley (1983) measured leaf variegation using leaf area meter, and Marcotrigiano and Hackett (1993) measured leaf variegation using a photocopy machine, transparencies, and a portable area meter. However, these methods were time-consuming and appeared relatively unreliable (Kwack et al., 1998; Li et al., 2007). Kwack et al. (1998) and Li et al. (2007) used a commercial scanner and digital image analysis software (Photoshop; Adobe Systems, San Jose, CA) to quantify the variegation, and they reported this method to be reliable and effective compared with previous methods.
Although color is one of the most important visual plant characteristics, the analysis of color was often neglected mostly because of lack of an appropriate method (McGuire, 1992; Voss, 1992). Digital image analysis also provides the ability to quantify coloration of the leaves, which can improve understanding of changes in variegation (Kwack et al., 1998). CIE L*a*b* (CIELAB) value is the most complete color space specified by the Commission Internationale de l'Éclairage (1978). CIELAB color space is suitable for standardization of colorimetric practice in science, which is numerically coordinated to locate individual colors, where L* represents darkness and brightness, a* represents green and red, and b* represents blue and yellow, as the value increases from negative to positive, respectively (Voss, 1992). McGuire (1992) indicated that chroma [C* (degree of departure from gray toward pure chromatic color)] and hue angle [h° (red, green, blue, and yellow)] calculated from a* and b* are somewhat analogous to color saturation or intensity and describe a more straightforward and appropriate measurement of color for horticultural research.
The objective of this study was to investigate the optimum light intensity for two foliage plants in an indoor light environment. Along with the morphological changes in the leaves of two foliage plants, digital image analysis could give a good indication of the changes in variegation and coloration under different light intensities. These results will improve understanding of the light requirements for better performance of foliage plants under limited indoor light environments.
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