CHAPTER 5: EVALUATION OF SELECTED NUTRIENTS AND PHYTOCHEMICALS OF
5.4 Discussion and conclusions
88 Figure 5.1: Stanch content of different sweet potato cultivars grown at different locations during 2012 and 2013 growing season.
89 3.8°C and 5.1°C, respectively, higher than that of Umbumbulu. An increase in temperature is known to affect partitioning of dry matter in plants (Rotter and van de Geijn, 1999). The low starch content in storage roots from these areas may have been affected by increases in temperature thus starch partitioning and allocation to storage roots was decreased. A similar trend was observed in β-carotene content.
Secondary metabolites, such as phenolics, are generally expected to increase under drought stress conditions and this is believed to be a response to an increase in oxidative damage (Podsedek, 2007; Oh et al., 2009). Richards Bay had high ETo and high temperatures suggesting that there was a water deficit (drought stress). Storage roots harvested from that location recorded high antioxidant activity but the leaves did not follow the same trend as the storage roots. Instead total antioxidant activity was high in the leaves from Deepdale and Umbumbulu. This means that the roots were suffering from prolonged deprivation of water which led to a saturated redox chain and decline of generated adenosine triphosphate (ATP).
This is called oxidative stress and it released toxic free radicals which can inactivate enzymes involved in oxidative systems in the plant (Lin et al., 2006). High antioxidant activity means that toxic radicals were being removed to protect plant cells against oxygen toxicity (Perata and Alpi, 1993). Comparison of antioxidant activity of storage roots and leaves revealed that sweet potato leaves had a generally high antioxidant activity. According to Debarry et al.
(2005), antioxidant defence is not completely efficient in protecting plants from oxidative burst, free radical damage must be constantly repaired.
Carotenoids are plant pigments that can confer plants with resistance to adverse effects of drought (Jaleel et al., 2009). The concentration of carotenoids was high at Umbumbulu followed by Deepdale then Richards Bay. This was the opposite of total antioxidant activity which increased under drought stress conditions. Results of this nature were also reported by Lin et al. (2006) on sweet potato leaves. These authors discovered that carotenoids were not affected by drought stress, they further reported that increases in total antioxidants may have compensated for the need for carotenoids. Increases in the content of these phytochemicals (carotenoids and total antioxidants) in sweet potato leaves is beneficial to human health since they offer protection against common diseases such as cerebrovascular events, cancer and other age related degenerative diseases (Scalzo et al., 2005; Islam, 2006). All along total antioxidants have been associated with the leaves (especially green leafy vegetables), but
90 results from this experiment indicate that even the storage roots can have appreciable amounts of antioxidants.
High chlorophyll content in sweet potato leaves and other substances presumably present are likely to have a health-promoting potential since chlorophyll has also been reported to have numerous beneficial biological properties to humans (Ferruzzi et al., 2002; Kizhedath and Suneetha, 2011). High concentration of this phytochemical was also recovered from Umbumbulu planted sweet potato, followed by Deepdale then Richards Bay. This further emphasises that utilization of sweet potato leaves as a vegetable could increase food and nutrition security (micro-nutrient access (vitamin A)) and health. It is beneficial to food security in that it could add to the number of seasonal leafy vegetables used by rural communities who have limited access to produce markets (Vorster et al., 2007). Continuous availability of sweet potato leaves is guaranteed since they (leaves) can be harvested several times during the year even during off-season when storage roots are not available due to unfavourable environmental conditions. The continuous availability and high nutritional benefit will satisfy two of the important pillars of food security.
Storage roots of orange-fleshed cultivars yielded higher β-carotene content than the cream- fleshed sweet potatoes as was shown in previous studies (Rautenbach et al., 2010; Laurie and van Heerden, 2012; Laurie et al., 2012). The deeper the orange colour, the greater the β- carotene concentration. Sweet potato cultivar A45 contained more than 15 mg/100g β- carotene, which can provide 100% of the recommended daily allowance (RDA) of vitamin A for children between the ages of 4 – 8 years (IOM 2006).
It is concluded that starch, β-carotene and phytochemicals content and antioxidant activity in sweet potato can be increased or reduced by environmental conditions. Moist coast hinterland and ngongoni veld were more favourable to sweet potato nutritional and phytochemical conditions. Sweet potatoes grown in this agro-ecological area can store starch and phytochemicals (secondary metabolites) better than those grown under harsh environments (Moist coast forest, thorn and palm-veld). Drought stress increased total antioxidant activity in storage roots. Increases in carotenoids may not be due to drought stress. Nutrients and phytochemical concentration is also affected by growth season. The study also demonstrated that sweet potato leaves have a health-promoting potential as they have high content of phytochemicals and antioxidant activity. They can also play a vital role in improving food and
91 nutrition security as a leafy vegetable while farmers are still waiting for the storage root to mature.
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