Gynura bicolor DC is widely distributed in certain areas of southeast Asia. It is a member of the Asteraceae family and has several local names, e.g., Suizenjina and Kinjisou in Japan (Hayashi et al., 2002). Its leaves are characterized by green adaxial and reddish purple abaxial surfaces. In Ishikawa Prefecture, G. bicolor is cultivated in the field from early spring to late fall. However, the reddish purple abaxial sides of leaves harvested in midsummer often lack this characteristic color (Yoshiaki et al., 2010).
Several growers say that the loss of red color of G. bicolor in midsummer is less in leaves from shaded locations than in those in sun-exposed locations. Many studies have reported on the effects of shading on the content of anthocyanins in fruit skin and on fruit color (Koyama and Goto-Yamamoto, 2008; Wang et al., 2000). In most fruits, light is a prerequisite for anthocyanin synthesis (Mancinelli, 1983) and shading during fruit development results in decreased anthocyanin content (Koyama and Goto-Yamamoto, 2008; Wang et al., 2000). On the other hand, temperature is an important factor that affects anthocyanin biosynthesis in various fruits such as apples (Ubi et al., 2006) and grapes (Yamane et al., 2006). The expression of genes involved in anthocyanin biosynthesis is repressed by high temperatures in a red-wine grape cultivar (Mori et al., 2007). Because G. bicolor plants are often exposed to high temperature in midsummer, the loss of red color of the leaves may largely be dependent on a repression of anthocyanin biosynthesis genes caused by high temperature.
Chlorogenic acid is the most abundant phenolic compound in G. bicolor leaves (Fukuoka et al., 2012). The contents increase in response to environmental stresses (Oh et al., 2009). In lettuce, leaves of sun-exposed plants contained more CGA than leaves from plants growing under shaded locations (Grace et al., 1998). Generally, phenolic compounds including anthocyanin can scavenge ROS (Kanatt et al., 2005; Nara et al., 2006). Chlorogenic acid was the most efficient scavenger of free radicals in G. bicolor leaves (Fukuoka et al., 2012). Because stress conditions enhance the production of ROS (Asada and Takahashi, 1987), leaves from sun-exposed locations in midsummer may contain a larger amount of CGA to scavenge abundant ROS.
However, because G. bicolor is a minor crop, there has been little research on this plant and no reports of changes in polyphenol biosynthesis of leaves induced by shading. In this article, we show that artificial shading in midsummer is an effective method to promote anthocyanin accumulation but reduces ROS scavenging capacity as a result of lower production of CGA.
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