GLYCOALKALOIDS CONTENT IN POTATO TUBERS AS AFFECTED BY FERTLILIZATION DURING VEGETATION AND STORAGE.

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2 GLYCOALKALOIDS CONTENT IN POTATO TUBERS AS AFFECTED BY FERTLILIZATION DURING VEGETATION AND STORAGE. Andrzej C. Zolnowski, Zdzislaw Ciecko, Miroslaw Wyszkowski Department of Environmental Chemistry University of Warmia and Mazury, Plac Lodzki 4, Olsztyn, Poland Introduction Glycoalkaloids are naturally occurring toxins and are found in a number of solanaceous plants accumulating in all organs of the plant (Jadhav et al., 1981). High concentration of these substances found in parts of plant where the metabolic activity is much higher than normal, especially in leaves, flowers, and sprouts (Wünsch & Munzert, 1994). In commercial potato cultivars (Solanum tuberosum L.) the primary glycoalkaloids are alphasolanine and alpha-chaconine, which are structurally similar compounds, glycosides of the same steroidal alkaloid solanidine (Friedman & Dao, 1992). The potato glycoalkaloids are one of the most poisonous compounds of the human diet with two toxic actions: first a membrane disruption activity affecting the digestive system and the second anticholinesterase activity on the central nervous system. Many human poisonings and even deaths have been associated with the consumption of potato tubers with higher than normal glycoalkaloid level (Morris & Lee, 1984). Glycoalkaloid levels in tubers of commercial potato cultivars are generally low. The safe level for human health is 20 mg per 100 g of fresh tissue (Kamasa & Borys, 1991; Styszko, 1999). Actually the obligatory level of glycoalkaloids for all new cultivars registered in Poland is 8 mg per 100 g of fresh tissue. However the National Institute of Agriculture Botany (NIAB-Cambridge) has recommended that the concentration of glycoalkaloids have to be lower than 6 mg per 100 g of fresh tissue of tuber for all cultivated potatoes (Panovska et al., 1994). The glycoalkaloid concentrations can be influenced by environmental conditions during growth, harvest and storage (Sinden et al., 1984; Percival et al., 1996). The objective of the presented study was to determine the effect of NPK, Mg and K fertilization and methods of N and Mg application on the total glycoalkaloid (TGA) content during vegetation and storage time. Materials and methods Cultivar Mila ( medium early maturity group) was grown as a three-year experiment planted in 1995, 1996 and 1997 at the Experimental Station in Tomaszkowo, which belongs to the University of Warmia and Mazury in Olsztyn. Experiment No. 1 comprised objects with increasing levels of NPK fertilization (N 40 P 17 K 50, N 80 P 34 K 100, N 120 P 51 K 150, N 160 P 68 K 200 kg ha -1 ), in which half the dose of nitrogen was applied either as foliage dressing or as soil amendment. The second experiment comprised the effect of magnesium (Mg) applied to leaves 12 kg ha -1, and to soil 24 kg ha -1, and was tested using the same NPK fertilization levels. Third experiment comprised the effect of potassium elevated doses (0, 65, 130, 195, and 260 kg ha -1 ) with N 80 P 34 (all fertilizers applied to soil). During the vegetative period plants fertilized through leaves were sprayed three times with nitrogen and six times with magnesium. Potatoes were grown on manure (25 t ha -1 ). The representative sample of fresh potatoes from each combination, the size about 3-5 cm including peel, was analysed before, after, at harvest time and after six months of storage. Potato tubers stored in chamber with controlled humidity RH ± 95%, and temperature at 4ºC. The TGA content 27

3 was determined using Bergers method (Bergers, 1980). Experimental data were calculated according to the T-Duncan test. Results and discussion The glycoalkaloid content in the presented experiments characterize rather low levels. With elevated doses of NPK and K (tab. 1 & 3) the changes of glycoalkaloid content in analysed tubers characterize parabolic course. Low concentration average 4.6 mg% of TGA at the first sampling time slightly increased after to 4.8 mg% and then reduced to 4.0 mg% at harvest time. In the second experiment with both doses of magnesium applied to leaves and to the soil stated that higher amount of TGA was synthesized at the beginning of vegetation (tab. 2). Highest level of TGA was found before 5.6 mg%. From this stage the concentration of glycoalkaloids was slowly decreased to 3.9 mg% at harvest. Received data showed that fully matured tubers in comparison with immature tubers contain lower concentrations of TGA. This data correspond with changes observed by Hellenas et al. (1995) These authors stated that young tubers especially obtained from early cultivars have often been connected with higher glycoalkaloid levels above the recommended safety limit of 20 mg per 100 g of fresh weight. This high concentration was expected because the original high content of TGA was diluting while accumulating dry mass by potato tubers. The high concentration of glycoalkaloids in immature tubers are especially dangerous in the case of baby potatoes which are sold in supermarkets. These potatoes often are generally not peeled and they are the most significant source of glycoalkaloids in the human diet. The NPK fertilization and method of nitrogen and magnesium application slightly affected the variability of glycoalkaloid content (tab.1). The elevated rates of NPK had affected the tubers TGA content generally. The content of TGA was increased in response to elevated nitrogen fertilization in the all sampling times. Nitrogen applied to the soil caused higher tuber TGA content than applied to the leaves. The NPK+Mg fertilization in the second experiment caused further significant increase of TGA content before and at harvest (tab. 2). The magnesium (Mg) doses of 24 kg ha -1 applied to the soil resulted in slightly higher TGA content than 12 kg ha -1 of Mg applied to the leaves. Obtained data correspond with Evans and Mondy (1984) showed that increased doses of Mg caused increasing tuber TGA content. However, Rogozinska (1991) demonstrated that Mg was a factor that decreased the TGA level. Another very important factor of fertilization causing the TGA level is potassium. The data obtained from third experiment showed that potassium significantly decreased TGA content at and after time (tab. 3). In other sampling times, this influence was not significant. Similar effect of K fertilization obtained by Mazurczyk (1986) using 100 and 240 kg K 2 O in his experiment with five varieties of potatoes. High dose of potassium significantly decreased TGA level in obtained tubers in comparison with objects fertilized with low dose. This reaction author explained that potassium is one of the components that are responsible for reduced-sugar content. If the potassium level is relatively high in tubers then the reduced sugar content is low. These sugars are necessary for glycoalkaloid synthesis. The stored tubers can also accumulate glycoalkaloids above safety limits. The TGA content increases in tubers because of high temperature, low humidity, periodic light exposure and damage of tuber skin during harvest time and transportation. Presented data were obtained from tubers stored at 4ºC and relative humidity (RH) ±95%. These conditions caused the TGA content from all objects after 6 months of storage to be much lower than after harvest. Conclusions 1. The changes of glycoalkaloid content in tubers had a parabolic character. The low concentration determined before and maximum stated after which 28

4 slowly decreased during time. The magnesium fertilization increased TGA content before time. 2. Used in the first and in the second experiments, elevated doses of NPK caused increasing TGA concentration. Higher concentrations stated after NPK fertilizers used with magnesium. Potassium used as a fertilizer significantly decreased TGA content at and after time. 3. The temperature 4ºC and relative humidity (RH) ±95% caused TGA content from all objects after 6 months of storage to be much lower than after harvest time. References 1. BERGERS W. A., A rapid quantitative assay for solanidine glycoalkaloids in potatoes and industrial potato protein. Potato Research 23: EVANS D, MONDY N.I., Effect of magnesium fertilization on glycoalkaloid formation in potato tubers. Journal of Agricultural and Food Chemistry 32: FRIEDMAN M., DAO L., Distribution of glycoalkaloids in potato plants and commercial potato products. Journal of Agricultural and Food Chemistry 40: GELDER W.M.J. VAN, VINKE J.H., SCHEFFER J.J.C., Steroidal glycoalkaloids in tubers and leaves of Solanum species used in potato breeding. Euphytica 48: HELLENÄS K.E., BRANZELL C., JOHNSSON H., SLANINA P., Glycoalkaloid content of early potato varieties. Journal of the Sciences of Food and Agriculture 67: Jadhav S.J., Sharma R.D., Salunkhe D.K., Naturally occurring toxic alkaloids in food. Critical Reviews in Toxicology 9: KAMASA J., BORYS J., Ocena wartości gospodarczej odmian ziemniaka w Polsce. Synteza materiałów wyjściowych dla hodowli ziemniaka dorobek i perspektywa. Inst. Ziemn. Bonin: MAZURCZYK W., Wpływ nawożenia azotowego i potasowego na poziom glikoalkaloidów w bulwach ziemniaka. Mat. Konf. Wpływ nawożenia na jakość plonów Olsztyn, czerwca 1986 r: MORRIS S.C., LEE T.H., The toxicity and teratogenicity of Solanaceae glycoalkaloids, particularly those of the potato (Solanum tuberosum). Food Technology in Australia 36: PANOVSKA Z., HAJSLOVA J., KOTAL F., KOSINKOVA P., Glycoalkaloids of potato. Mater. Inter. Euro. Food Tox. IV Konfer. Bioactive substances in food of plant origin, Olsztyn PERCIVAL G.C., DIXON G.R., SWORD A., Glycoalkaloid concentration of potato tubers following exposure to daylight. Journal of the Science of Food and Agriculture 71: ROGOZIŃSKA I., Effect of magnesium fertilizer on quality traits of potatoes. Kartoffelbau 42, 6: SINDEN S.L., SANFORD L.L. & WEBB R.E., Genetic and environmental control of potato glycoalkaloids. American Potato Journal 61: STYSZKO L., Relacje pomiędzy odpornością na ważniejsze patogeny i mątwika ziemniaczanego a elementami jakości ziemniaka jadalnego. Mat. z Konf. Nauk. Ziemniak jadalny i dla przetwórstwa spożywczego czynniki agrotechniczne i przechowalnicze warunkujące jakość. Radzików lutego, IHAR Jadwisin: WÜNSCH A., MUNZERT M., Effect of storage and cultivar on the distribution of glycoalkaloids in potato tubers. Einfluss von Lagerung und Sorte auf die Verteilung der Glykoalkaloide in der Kartoffelknolle. Potato Research 37:

5 24 kg Mg applied to soil 12 kg Mg applied to leaves ½ N applied to soil ½ N applied to leaves Glycoalkaloid content in potato tubers, mean for Experiment No. 1 Table 1 Objects before after at harvest time after six months of storage Control without fertilization N 40 P 17 K N 80 P 34 K N 120 P 51 K N 160 P 68 K Average: N 40 P 17 K N 80 P 34 K N 120 P 51 K N 160 P 68 K Average: LSD for methods of N application: n.s. n.s. n.s. n.s. LSD for NPK fertilization: n.s LSD for interaction: n.s. n.s. n.s. n.s. Glycoalkaloids content in potato tubers, mean for Experiment No. 2 Table 2 Objects before after at harvest time after six months of storage Control without fertilization N 40 P 17 K N 80 P 34 K N 120 P 51 K N 160 P 68 K Average: N 40 P 17 K N 80 P 34 K N 120 P 51 K N 160 P 68 K Average: LSD for methods of N application: n.s n.s n.s n.s. LSD for NPK fertilization: 1.42 n.s LSD for interaction: n.s n.s n.s n.s. Glycoalkaloid content in potato tubers, mean for Experiment No. 3 Table 3 Objects before after at harvest time after six months of storage N 80 P 34 K N 80 P 34 K N 80 P 34 K N 80 P 34 K N 80 P 34 K Average: LSD for K fertilization: n.s. n.s. 30

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