Original research articleThe influence of nitrogen and potassium fertilisation on the content of polyphenolic compounds and antioxidant capacity of coloured potato
Introduction
Potatoes are grown in various climates and on various soil types and are the fourth-most-consumed food crop in the world (USDA, 2014). Germany, Poland, the Baltics and other Central European countries have a high share of potato production in Europe’s agricultural output (European Commission, 2012). The Polish are the second-largest consumers of potatoes among all nations of the European Union, with the Latvians being the greatest eaters. Besides being rich in carbohydrates, high-quality proteins, vitamin C and minerals, potatoes provide consumers with a relatively inexpensive source of biologically active compounds (Brown, 2005). Among different varieties, those with coloured peel and flesh deserve special attention because they have almost double the concentration of phenolic compounds in comparison to white or yellow cultivars (Brown, 2005, Albishi et al., 2013). Potatoes with purple-bluish peel and flesh, botanically belonging to the subspecies of Solanum tuberosum L. subsp. andigenum, originate, as many agricultural varieties of today, from the Andean regions in Peru (Brown, 2005). The distinguishing feature of those cultivars is their relatively high content of anthocyanins (Rodriguez-Saona et al., 1998), which play an important role as antioxidants, known for their ability to efficiently scavenge free radicals, which, if present at higher levels in the human body, enhance the risk of occurrence of chronic and neurological disorders (Prior, 2003). Thus, the regular consumption of potatoes of the subspecies of S. tuberosum L. subsp. andigenum can have a significant health-promoting effect. Damianaki et al. (2000), who studied the inhibitory action of polyphenols on human breast cancer cells, pointed out that a systematic increase in polyphenols in a diet might be an essential part of a healthcare strategy for developing countries.
After tomatoes and red wine, potato tubers are the third most significant source of polyphenols. Potatoes with a purple–blue peel and flesh contain 1.2–4.4 mg/kg of polyphenols (Al-Saikhan et al., 1995), whereas the anthocyanin content has been determined to range within 5.5–38 mg/100 g of fresh tuber weight (Brown et al., 2003, Brown et al., 2005, Brown et al., 2008, Brown, 2005). The content of polyphenols is typical for a variety, but it also depends on habitat conditions. It was proven that a higher elevation above sea level, low temperature, higher air humidity and precipitation positively influence the content of polyphenolic compounds in potatoes (Lachman et al., 2008, Rosenthal and Jansky, 2008). Another factor strongly influencing the quality of potatoes is mineral fertilisation. Nitrogen might increase the content of dry matter and proteins in tubers (Fontes et al., 2010, Öztürk et al., 2010), whereas a decrease in the concentration of starch was observed at the same time (Westermann et al., 1994). With regard to bioactive compounds, stress conditions, such as disrupted access to macronutrients (including nitrogen and potassium) may contribute to an increase in the level of antioxidants. Rosenthal and Jansky (2008) showed the impact of high nitrogenous fertiliser doses (up to 260 kg/ha) and low potassium doses on the total content of antioxidants. This is, however, one of the very few reports on the impact of agronomic practices on the content of polyphenolic compounds in purple-bluish potato tubers, and the effect of mineral fertilisation on the polyphenolic composition and antioxidant properties of purple–blue potatoes has not yet been determined. In view of the above, the aim of the present study was to explore how the availability of nitrogen and potassium through fertilisation stimulates the pathways of the production of polyphenolic compounds, especially phenolic acids and anthocyanins and, consequently, how it influences the antioxidant properties of the purple–blue potato cultivar, ‘Blue Congo’.
Section snippets
Reagents
2,2′-Azino-bis(3-ethylbenzothiazoline-6-sulfonic acid) diammonium salt, potassium persulfate, Trolox, 2,4,6-tris(2-pyridyl)-s-triazine were purchased from Sigma–Aldrich (Switzerland). Delphinidin-3-O-glucoside, malvidin-3-O-galactoside, p-coumaric and ferulic acid were purchased from Extrasynthèse (Lyon, France). Chlorogenic, neochlorogenic and cryptochlorogenic acids were obtained from TRANS MIT GmbH (Giessen, Germany). Acetonitrile for UPLC (Gradient Grade) was from Merck (Darmstadt,
Anthocyanins
The pigments of coloured potatoes belong to the class of anthocyanins and their content varies within a broad range reaching concentrations above 100 mg per 100 g of fresh weight depending on cultivar (Reyes et al., 2005). It was found that red potatoes contained mainly acylated glucosides of pelargonidin, while purple-fleshed ones had additional acylated glucosides of malvidin, petunidin, peonidin and delphinidin (Brown, 2005, Lachman et al., 2009). In the present study, the determination of the
Conclusions
A proper control of nitrogen and potassium fertilisation levels during the growth of purple–blue potatoes cv. ‘Blue Congo’ might be an effective way to increase the content of polyphenolic compounds in their tubers. In general, the highest concentration of anthocyanins, phenolic acids and the highest antioxidant capacity measured by TEAC ABTS and FRAP methods were noted when nitrogen at a concentration of 120 kg/ha was applied. In conclusion, the accumulation of phytonutrients with
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