Fruits and vegetables are important for human health; people with diets low in fruits and vegetables are more likely to have chronic diseases (Rao and Rao, 2007). Furthermore, dietary fruits and vegetables, rich in antioxidants, are highly recommended. Cellular damage, associated with aging, chronic diseases, and cancers often caused by free radicals, can be prevented with antioxidants (Ames et al., 1993; Valko et al., 2007).
Carotenoids are lipid-soluble pigments that are synthesized in plants, algae, fungi, and bacteria. Specific carotenoids are orange, yellow, and red pigments present throughout the plant and in high concentrations in certain fruits and vegetables (reviewed in Farre et al., 2010). Carotenoids play a key role in photosynthesis; in the pigment–protein complex of photosystems, they have an essential role for harvesting light energy and transferring the energy to chlorophyll (Malkin and Niyogi, 2000). Carotenoids are responsible for the color of many fruits and flowers; these pigments are important for attracting pollinators and seed dispensers.
In humans and other animals, specific carotenoids provide essential vitamin precursors; β-carotene and β-cryptoxanthin are pro-vitamin A forms of carotenoids (Yeum and Russell, 2002). Diets deficient in vitamin A cause night blindness in humans (Rao and Rao, 2007). Human macula pigments are a mixture of carotenoids, lutein, and other xanthophylls. These compounds prevent free radical production in the retina, protecting the macula from blue light phototoxication and other damage caused by blue light (reviewed in Stringham and Hammond, 2005).
All members of the Capsicum genus accumulate carotenoids in the pericarp of the fruit with cultivar-specific abundances and compositions (Guzmán et al., 2010; Hornero-Méndez et al., 2000; Howard et al., 2000; Rodriguez-Uribe et al., 2012; Wahyuni et al., 2011; Wall et al., 2001). In addition to their nutritional value, the red pigments (capsanthin and capsorubin) are extracted and used as a non-carcinogenic red dye for cosmetics and to color different foods (Wall and Bosland, 1998). Quantification methods based on high-performance liquid chromatography (HPLC) are in place for the most common carotenoids: lutein, capsanthin, capsorubin, β-carotene, β-cryptoxanthin, violaxanthin, and zeaxanthin (Guzmán et al., 2010) among others.
Light plays a key regulatory role for genes and gene products related to photosynthesis including carotenoids (Pizarro and Stange, 2009). Phytochrome regulates expression of phytoene synthase, a key step on the carotenoid biosynthetic pathway in tomato fruit (Alba et al., 2000; Schofield and Paliyath, 2005). Similar results were observed for carotenoids in pepper leaves (Simkin et al., 2003). However, the accumulation of carotenoids in Capsicum fruit does not always increase with increasing light. Russo and Howard (2002) showed that seven of 10 red-fruited cultivars of C. annuum increased total carotenoid accumulation in fruit when grown under greenhouse conditions relative to the field. Lee et al. (2005) report increased levels of lutein and zeaxanthin in Capsicum fruit grown in a greenhouse vs. two different field settings. Light levels were ≈20% reduced in the greenhouse setting.
In this study, we investigated the role of light on carotenoid content in three orange-fruited C. annuum cultivars known to have unique carotenoid profiles (Guzmán et al., 2010). We compared the carotenoid accumulation in leaf and fruit tissues of these cultivars in plants grown under three different light levels to evaluate the role of light in two different developmental contexts.
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