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, 10-718 Olsztyn, Poland
anzol@uwm.edu.pl
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 (N40P17K50, N80P34K100, N120P51K150, N160P68K200
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 N80P34 (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
flowering, after flowering, 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
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 flowering 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 flowering – 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 flowering 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 flowering and after flowering 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 K2O 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 flowering and maximum stated after flowering which
slowly decreased during time. The magnesium fertilization increased TGA content before
flowering 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
flowering and after flowering 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
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potatoes and industrial potato protein. Potato Research 23: 105-110
2. EVANS D, MONDY N.I., 1984. Effect of magnesium fertilization on glycoalkaloid formation
in potato tubers. Journal of Agricultural and Food Chemistry 32: 465-466
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commercial potato products. Journal of Agricultural and Food Chemistry 40: 419-423
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tubers and leaves of Solanum species used in potato breeding. Euphytica 48: 147-158
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glycoalkaloids, particularly those of the potato (Solanum tuberosum). Food Technology in
Australia 36: 118-124
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Mater. Inter. Euro. Food Tox. IV Konfer. Bioactive substances in food of plant origin,
Olsztyn 22-24. 09. 1994
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tubers following exposure to daylight. Journal of the Science of Food and Agriculture 71:
59-63
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Kartoffelbau 42, 6: 257-259
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potato glycoalkaloids. American Potato Journal 61: 141-156
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ziemniaczanego a elementami jakości ziemniaka jadalnego. Mat. z Konf. Nauk. Ziemniak
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warunkujące jakość. Radzików 23-25 lutego, IHAR Jadwisin: 9-12
15. WÜNSCH A., MUNZERT M., 1994. 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: 3-10
Table 1
Glycoalkaloid content in potato tubers, mean for 1995-1997. Experiment No. 1
Objects
without fertilization
N40P17K50
N80P34K100
N120P51K150
N160P68K200
Average:
N40P17K50
N80P34K100
N120P51K150
N160P68K200
Average:
LSD for methods of N application:
LSD for NPK fertilization:
LSD for interaction:
½ N applied to ½ N applied to
soil
leaves
Control
before
flowering
after
flowering
at harvest
time
2.82
3.83
4.62
5.05
4.77
4.56
5.19
4.52
4.36
4.46
4.63
n.s.
n.s.
n.s.
3.40
4.10
4.89
5.15
5.03
4.79
3.49
4.65
5.51
5.89
4.88
n.s.
1.34
n.s.
2.24
2.59
2.92
4.50
4.56
3.64
3.50
4.43
5.31
3.32
4.14
n.s.
0.97
n.s.
after six
months of
storage
2.07
1.61
2.26
2.54
2.77
2.29
3.00
3.18
3.20
3.41
3.20
n.s.
0.49
n.s.
Table 2
Glycoalkaloids content in potato tubers, mean for 1995-1997. Experiment No. 2
Objects
without fertilization
N40P17K50
N80P34K100
N120P51K150
N160P68K200
Average:
N40P17K50
N80P34K100
N120P51K150
N160P68K200
Average:
LSD for methods of N application:
LSD for NPK fertilization:
LSD for interaction:
24 kg Mg
applied
to soil
12 kg Mg
applied
to leaves
Control
before
flowering
after
flowering
at harvest
time
2.82
4.21
5.28
5.50
6.23
5.30
4.79
5.68
6.34
6.72
5.88
n.s
1.42
n.s
3.40
4.47
4.62
5.17
5.80
5.01
4.70
5.31
5.92
5.94
5.47
n.s
n.s
n.s
2.24
3.62
4.48
3.39
3.90
3.84
4.48
4.99
3.37
3.17
4.00
n.s
1.02
n.s
after six
months of
storage
2.07
2.41
2.39
3.24
3.39
2.86
2.26
2.46
3.03
3.26
2.75
n.s.
0.91
n.s.
Table 3
Glycoalkaloid content in potato tubers, mean for 1995-1997. Experiment No. 3
Objects
before
flowering
after
flowering
at harvest
time
N80P34K0
N80P34K65
N80P34K130
N80P34K195
N80P34K260
Average:
LSD for K fertilization:
5.66
5.08
4.48
4.12
3.67
4.61
0.91
5.44
5.30
4.67
4.19
3.85
4.69
1.12
4.40
4.29
3.91
3.95
4.20
4.15
n.s.
after six
months of
storage
3.31
3.06
3.97
4.11
2.83
3.46
n.s.