Sweet potato leaf is also rich in vitamin B, β-carotene, iron, calcium, zinc and protein, and as a crop is more tolerant of diseases, pests, and high moisture than many other leafy vegetables grown in the tropics (AVRDC, 1985; Woolfe, 1992). Depending on the genotypes and growing conditions, sweet potato leaves are comparable with spinach in nutrient contents (Woolfe, 1992; Yoshimoto et al., 2002a; Ishiguro et al., 2004b). The new cultivar ―Suioh‖ is a bushy plant that is easy to harvest (Ishiguro et al., 2004a). This cultivar is harvested from nursery beds six times a year, from April through October. Total yield of greens is 4.16 kg/kg of seed roots. Sensory evaluation shows that the taste of the cooked leaves and petioles is very good, and that the hot water extract from greens can be used as a substitute for green tea.
The average contents of minerals and vitamins are 117 mg calcium, 1.8 mg iron, 3.5 mg total
carotene, 7.2 mg total vitamin C, 1.6 mg total vitamin E, and 0.56 mg vitamin K/100 g fresh weight of leaves. Levels of iron, calcium and carotene rank among the top, as compared with other major vegetables (Ishiguro et al., 2004a).
In sweet potato leaves, average content of β-carotene from five varieties was 6.2 mg/100 g fresh weight, while that from six leafy vegetables was 2.1 mg/100 g fresh weight. Ratio of 9-cis and 13-cis isomers to total β-carotene content in the leaves of five varieties was 21.5%, while that of six commercial vegetables was 14.7%. These results suggest that sweet potato leaves contain higher content of 9-cis and 13-cis isomers than the commercial vegetables (Yoshimoto et al., 2003). The 9-cis isomer has a higher antioxidant potency than that of the all-trans one (Levin and Mokady, 1994), as mentioned previously.
Lutein is a member of the xanthophylls family of carotenoids and is found in vegetables and fruits (Mangels et al., 1993). Lutein is believed to mitigate eye disease such as age- related macular degeneration and cataracts. Green leafy vegetables like spinach and kale have high lutein content (Alves-Rodrigues and Shao, 2004). Purified lutein has been used in dietary supplements and as a food ingredient for eye health in recent years. ―Suioh‖, a sweet potato cultivar is developed specifically for the use of its top at KONARC. Lutein content in
―Suioh‖ leaves grown in the field, ranges from 31.5 mg to 42.6 mg/100 g fresh weight (average content, 36.8 mg/100 g fresh weight) (Ishiguro and Yoshimoto, 2006). The average content in ―Suioh‖ leaves is higher than Ipomoea aquatica leaves (11.9 mg/100 g fresh weight) and exceeds that in other fruits and vegetables listed in a carotenoid database of 120 fruits and vegetables (Ishiguro and Yoshimoto, 2006).
Polyphenols
Phenolic acids are bioactive compounds and a diverse group of secondary metabolites universally present in higher plants (Rhodes et al., 1986; Meyer et al., 1998), where they play important roles in the structure of plants and are involved in a number of metabolic pathways (Harborne, 1980). Phenolics also have attracted special attention because they may protect the human body from oxidative stress, which in turn is associated with many diseases including cancer and cardiovascular diseases, as well as aging (Huang and Ferraro, 1991; Peluso et al., 1995; Stevens et al., 1995; Shimozono et al., 1996; Shahrzed and Bitsch, 1996; Kaul and Khanduja, 1998; Prior et al., 1998; Hagerman et al., 1998; Yoshimoto et al., 1999a; Robards et al., 1999). Sweet potato leaves are an excellent source of antioxidative polyphenolics, such as CA, ChA, diCQA, and triCQA (Islam et al, 2002a, 2003a, 2003b, 2003c), superior in this regard to other commercial vegetables, including sweet potato roots and potato tubers (Lugasi et al., 1999; Islam et al., 2002b; Yoshimoto et al., 2001, 2002a, 2003; Ishiguro et al., 2002a, 2004b).
Total leaf polyphenol contents of 1389 genotypes, collected from all over the world, were analyzed and characterized in 2000 and 2001 (Islam et al., 2003c). A highly significant (P >
0.001) liner correlation is found between the polyphenol contents of the genotypes from 2000 and the genotypes from 2001 (r = 0.812; n = 700).
This result indicates that the yearly variations of total phenolics in leaves of sweet potato genotypes are less. Furthermore, previous data revealed that the highest polyphenol concentration was in leaves (6.19 ± 0.14 g/100 g dry weight), followed by petioles (2.97 ± 0.26 g/ 100 g dry weight), stems (1.88 ± 0.19 g/100 g dry weight), and finally roots (<1.00
g/100 g dry weight), indicating that polyphenolic concentrations are organ-dependent (Islam et al., 2002b). The frequency distribution of total leaf polyphenolic content of 1389 genotypes collected from world wide is shown in Figure 5. The highest content found is 17.1 g/100 g dry weight and the lowest is >6.00 g/100 g dry leaf powder of total polyphenolics; the concentration is very high compared to other commercial vegetables (Yoshimoto et al., 2003;
Ishiguro et al., 2002, 2004b).
CA and five CQA derivatives: ChA, 3,4-diCQA, 3,5-diCQA, 4,5-diCQA, and 3,4,5- triCQA are found in sweet potato leaves (Islam et al., 2002a).ChA and diCQA derivatives have been isolated from various plants including sweet potato (Shimozono et al., 1996;
Walter et al., 1979), as mentioned above, but there are very few reports on 3,4,5-triCQA. All caffeoylquinic acid derivatives (except CA; P < 0.05) are positively correlated (P > 0.001) with total polyphenol contents of sweet potato leaves. The correlation of total phenolics with ChA (r = 0.84), 3,4-diCQA (r = 0.78), 3,5-diCQA (r = 0.81), 4,5-diCQA (r = 0.85) and 3,4,5- triCQA (r = 0.59) are positive and significant.
The results indicate that the correlation between total phenolics with different CQA derivatives is an important aspect, which should be kept in mind for better planning for improvement of the desired parameters. ChA, di- and triCQA are esters of quinic acid (QA) and bear one-, two-, and three-caffeoyl groups. Isolation of 3,4,5-triCQA is reported in Securidaka longipedunculata (Polygalaceae) (Mahmood et al., 1993) and Tessaria integrifolia (Asteraceae) (Peluso et al., 1995). Several varieties of sweet potato contain a high content (>0.2%) of 3,4,5-triCQA (Islam et al., 2002b; 2003c), suggesting that the sweet potato leaf is a source of not only mono- and diCQA derivatives but also triCQA.
Figure 5. Frequency distribution of total polyphenol content of 1389 sweet potato genotypes.
Anthocyanins
Fifteen anthocyanin compounds have been identified and characterized in sweet potato leaves (Islam et al., 2002b). The content of cyanidin in leaves was much higher than that of peonidin (Oki et al., 2002), suggesting that the cyanidin type is dominant (Islam et al., 2002b;
2005). The cyanidin type anthocyanins are superior to the peonidin type in antimutagenicity (Yoshimoto et al., 1999a, 1999b) and antioxidative activity (Rice-Evans et al., 1995).