Carotenoids are the principle pigments responsible for the many colors of leaves, fruits, and flowers in plants (Gross, 1991). They act as photoprotective agents and accessory light-harvesting complexes. Carotenoids also play an important role in human health by acting as sources of provitamin A or by acting as protective antioxidants required for proper reproduction, growth, and development; a normal functioning ocular system; epithelial cell integrity; and immune system functionality (FAO/WHO, 2002; Murkovic and Neunteufl, 2002). In vegetables, common provitamin A carotenoids include β-carotene, α-carotene, and β-cryptoxanthin (ODS/NIH, 2006). Other common carotenoids such as lycopene, lutein, and zeaxanthin do not have vitamin A activity but serve as antioxidants.
Pumpkins and squash (Cucurbita spp.) are excellent dietary sources of carotenoids (Gross, 1991) and, in 2007, ranked 11th among other vegetables produced around the world (FAOSTAT, 2008). The predominant carotenoids found in pumpkins and squash include lutein, α-carotene, and β-carotene (Gross, 1991). Based on reports by Holden et al. (1999) and Murkovic et al. (2002), the concentrations of lutein, α-carotene, and β-carotene found within Curcurbita species and their various fruit types can vary dramatically. In their studies, the fresh weight (FW) range of lutein, α-carotene, and β-carotene in summer-type squash (C. pepo) were 0.0 to 21.3 μg·g−1, 0.3 to 1.7 μg·g−1, and 0.6 to 23.0 μg·g−1, respectively. Within winter-type squash (C. moschata and C. maxima), lutein ranged from 0.8 to 170.0 μg·g−1 FW, α-carotene from 0.0 to 75.0 μg·g−1 FW, and β-carotene from 7.1 to 74.0 μg·g−1 FW.
The flesh colors of pumpkins and squash generally include a wide range of whites, yellows, and oranges (Gross, 1991). This color is based on the particular carotenoid types and concentrations that are influenced by genetic and environmental factors. Over a dozen genes that affect the rind and flesh color of squash have been described (Paris and Nelson Brown, 2005) and include D (dark), l-1 (light coloration–1), l-2 (light coloration–2), and B (bicolor). Tadmor et al. (2005) studied the effects of these particular genes in different genetic combinations within near-isogenic lines (NILs) of C. pepo. In genetic backgrounds that lacked either the dominant D and dominant L-2 alleles, a yellow flesh color developed. In genetic backgrounds with either dominant D or L-2, a yellow–orange flesh color developed and when the dominant allele of B interacts with the dominant allele of L-2, an intense orange flesh color will occur. One additional gene that effects squash flesh color is the dominant Wf (white flesh), which confers a white flesh color by preventing yellow pigment accumulation (Paris and Nelson Brown, 2005).
The broad range in carotenoid types and concentrations among and within Cucurbita species indicates the potential for genetic improvement of these compounds through plant breeding. Accuracy in breeding will require estimates of carotenoid types and their concentrations that are precise enough to distinguish genotypic differences among breeding material. One obstacle is that the extraction and analysis of carotenoid content is time-consuming and expensive. In practical breeding programs, it is not realistic to analyze the carotenoid content of even a small segregating population for selection of genotypes with high levels of carotenoids. An alternative reliable method to estimate carotenoid content and concentration would be beneficial.
High-performance liquid chromatography (HPLC) is used to chemically analyze tissues for carotenoid types and concentrations (Gross, 1991). It is labor-intensive and expensive but a reproducible and highly sensitive process that can separate, identify, purify, and quantify carotenoid levels. In contrast, colorimeters, which objectively measure and describe visible color, are relatively inexpensive and easy to use. The most preferred methods to objectively measure color are the tristimulus Hunter and the CIE L*a*b* systems (Seroczynska et al., 2006). Tristimulus color measurement systems have three color sensors with spectral sensitivity curves similar to that of the human eye and are referred to as color-matching functions (Konica Minolta Photo Imaging, U.S.A., Inc., Mahwah, NJ). These three functions are detected as x, y, and z coordinates, which can be converted into the desired color measurement value system.
Previous studies have correlated color measurement systems with carotenoid content in vegetable crops such as tomato (Arias et al., 2000; D'Souza et al., 1992), sweetpotato (Ameny and Wilson, 1997; Simonne et al., 1993), pepper (Reeves, 1987), and winter-type squash (Francis, 1962; Seroczynska et al., 2006). Interestingly, the authors of these studies differ in their opinions as to how reliable colorimetric analysis would be at estimating carotenoid content and concentrations. If a fair prediction of carotenoid content and concentration could be obtained, this rapid and inexpensive method could be very useful in breeding pumpkins and squash for enhanced carotenoid levels. The objective of this research was to determine if the carotenoid content and concentration of pumpkin and squash (C. moschata and C. pepo) can be correlated with colorimetric analysis using the CIE L*a*b* color value system.
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