THE NITRATES CONTENT IN EARLY POTATO TUBERS DEPENDING ON GROWING CONDITIONS

Wanda Wadas¹, Romualda Jabłońska-Ceglarek², Edyta Kosterna^1
1 Department of Vegetable Crops, Podlasie University in Siedlce, Poland
² Department of Vegetable Crops, Podlasie University in Siedlce, Poland

ABSTRACT

The effect of the potato cultivation way (without covering and under agrotextile covering in period from planting to the plant heigh of 15 cm) and the level of nitrogen fertilization (without nitrogen, 30, 60, 90 kg N ha^-1) on the nitrates content in tubers of very early potato cultivars ('Aster', 'Drop') was estimated. The potatoes were harvested after 60 and 75 days from planting.

The forceing of plant vegetation by agrotextile covering resulted in a decrease of the nitrates content in tubers, especially at a very early date of potato harvest. After 60 days from planting the nitrates content in tubers of potato cultivated under cover was lower on average by 8.78 mg NaNO₃ kg^-1 f. w. (5.54%), in comparison to the cultivation without plant covering. After two-week delay of potato harvest the result of the use of cover were smaller and non-statistically confirmed.

With increasing the level of nitrogen fertilization followed increase the nitrates content in tubers. Higher increase this form of nitrogen content, as a result of increasing the dose of nitrogen, was found in tubers of 'Aster' cultivar, than of the 'Drop'. The nitrates content in tubers of 'Aster' cultivar was higher on average by 25.33 mg NaNO₃ kg^-1 f. w. (16.87%) after 60 days from planting, and by 25.55 mg NaNO₃ kg^-1 f. w. (18.02%) after 75 days from planting, in comparison of the `Drop´ cultivar.

Key words: early potato, agrotextile cover, nitrogen fertilization, nitrates.

INTRODUCTION

In Poland potato take up important position in human feeding. On the nutritional values of potato to consist the content of nutrition components, especially vitamin C, protein and mineral compounds, and also relatively low content in tubers of unhealthy for human compounds, such as glycoalkaloids and nitrates. The nitrates toxicity for human is not large, but big threat to make the products of their change - nitrites, at of which share can be arise nitrosamines, to belong the strong of cancerigenic substance [4, 8, 11, 18].

The nitrates are the natural components of potato tubers. Their content are related with change of nitrogen. The potato, in comparison with another plants has a little ability to accumulation of nitrates. In potato tubers this compounds perform, on average in quantity 10-22 mg NO₃^- 100 g^-1 [8, 11]. The admissible content of nitrates in potato tubers in accordance with regulation of Ministry of Health and Social Welfare, the 8^th of October 1993 amount to 250 mg NaNO₃ kg^-1 f. w. in other words about 182 mg NO₃^- kg^-1 f. w. tubers.

The early cultivars of potato have a greater inclination to nitrates accumulation, than the later cultivars. In case of this same cultivars the different of nitrates content in tubers depending on agronomical factors (cultivation, fertilization, irrigation) and climatic conditions of the vegetation period find [1, 3, 4, 5, 6, 7, 10, 12, 13, 14, 15, 16, 17, 19]. According to Bečka and Miča (1986 cite Machnacki [13]) vegetation conditions decide till in 85% of intensity to nitrates accumulation. With creation the optimal conditions of growth and the proper choice of the potato cultivar to seem possible obtain the tubers with relatively low this compounds content.

Results of the study show on significant change in the nitrates content of the vegetation period. The physiological earlier tubers the nitrates content in them is higher [3, 4, 7, 9, 16]. With forceing of potato harvest to become danger the excessive content of this compounds in the tubers. Simultaneously find, that use of the perforated foil or agrotextile covers in the potato cultivation for early crop contributed to decrease of nitrates content in tubers, in comparison with the traditional cultivation [9, 16].

The aim of the study was to estimate the effect of the agrotextile covering and different nitrogen doses on the nitrates content in early potato tubers.

MATERIAL AND METHODS

The experiment was carried out in the years 2000-2002 at the Experimental Station Podlasie University in Siedlce, on brown soil. The soil was characterised by the mean to high content of available phosphorus and potassium and by the magnesium content ranging from low to medium, pH = 6.0-6.7. A field experiment was conducted in the split-block-split-plot design in three replications. The area of each plots was 15 m². The factors were analysed as follows:

Factor I - the way of potato cultivation - without covering and under agrotextile covering in period from planting to the plant height of 15 cm;

Factor II - the level of nitrogen fertilization - without nitrogen, 30, 60 and 90 kg N ha^-1 with the constant phosphorus and potassium fertilization in the amount of 90 kg P₂O₅ ha^-1 and 120 kg K₂O ha^-1;

Factor III - the very early potato cultivar - 'Aster' and 'Drop'.

Potatoes were cultivated in the fields after cereals. Farmyard manure in the dose of 30 t ha^-1 was applied in autumn, and the mineral fertilizers in the spring tillage. The six-week pre-sprouted potato tubers were planted on 12, 11 and 9 April in the successive years of the study. The potatoes were harvested after 60 and 75 days from planting. For laboratory studies, 50 tubers of different size, according to proportional participation in the yield of each treatment were taken. The nitrates content estimated the colorimetric method (PN-92/A-75112). The results gave in mg NaNO₃ in kilogramme of fresh weight in tubers.

The results of the experiment were analysed statistically by means of analysis of variance. The significance of differences was verified using the Tukey's test.

The weather conditions in the years of the research period was varied. Only the year 2002 can to admit as favourable for the proper growth and development of potato plants. In the year 2000, the third decade of April was very warm (19°C), but the temperature dropped at the beginning of May, by 3.5°C. In the year 2001, very warm was the first decade of May (18.6°C), but the significant drop in temperature was observed at the end of this month (12.4°C). Such temperature distribution limited the growth and development of potato plants. In the years 2000 and 2001, occurred precipitation shortages in May and June, especially in the year 2000. The total precipitation sum was appropriate for the proper growth and development of plants, only in the year 2002 (table 1).

RESULTS AND DISCUSSION

The nitrates content in tubers in large degree depends from weather conditions in the vegetation period of potato, which was confirmed by the presented studies. Seasonal precipitation shortages and high temperature, as also precipitation excess and low temperature during of the vegetation period decrease the speed reduction of nitrates in plant and in result follows their accumulation [2, 3, 4, 6, 7, 13, 15, 19]. In the discussed studies, the least content of nitrates in tubers were found in the year 2002, which was warm and wet. In this year, the nitrates contents in potato tubers harvested after 60 days from planting was lower on average by 22,10 mg NaNO₃ kg^-1 f. w. (14.92%), in comparison to the previous years with precipitation shortages, and after 75 days from planting by 29.60 mg NaNO₃ kg^-1 f. w. (21.46%), in comparison to the dry vegetation season the year 2000 and by 20.24 mg NaNO₃ kg^-1 f. w. (14.68%), in comparison to the drought vegetation season the year 2001 (tables 2, 3). The study carried out by Hlušek et al. [7] showed that, in the years with the higher amout of precipitations in the plants vegetation the nitrates content in tubers of very early potato cultivar was smaller on average by 31.63%, in comparison to the years, which were warm and dry.

The forceing of plants vegetation as a result of use the agrotextile cover had significantly influence on the decrease the nitrates content in potato tubers, which was confirmed by the studies carried out by Lachman et al. [9]. With this method of potato cultivation for early crop, nitrates content in the tubers harvested after 60 days from planting was lower on average by 8.78 mg NaNO₃ kg^-1 f. w. (5.54%), in comparison with the cultivation without plant covering. After two-week delay of potato harvest the effect of the use of agrotextile cover were smaller and was not statistically confirmed (tables 2, 3). The slight higher favourable effect of the use of agrotextile covers, resulted in a decrease the nitrates content in tubers, was observed in case of the `Aster´ cultivar than the 'Drop' cultivar, however, the interaction of the cultivation way and potato cultivar was not statistically confirmed. In the study carried out by Lachman et al. [9], the use of agrotextile cover caused decrease the nitrates content in tubers on average by 29.00 mg NO₃^- kg^-1 f. w. (14.30%), in comparison to the control object without covering. In other author´s studies, showed also smaller accumulation of nitrates in potato tubers cultivated under polyethylene perforated foil [16]. According to Prośba-Białczyk and Mydlarski [17] the agrotextile covering favoured accumulation of nitrates in physiological earlier tubers, harvested after 56-58 days from planting, however, did not have any effect on this compounds content in the older tubers, harvested after 70-72 days from planting.

The factor, significantly influenced on the nitrates content in tubers is nitrogen fertilization. In general high nitrogen doses caused increase the nitrates content in tubers [1, 3, 5, 7, 10, 12], which was confirmed by the presented studies. With increasing the nitrogen doses followed increase the nitrates content in tubers. Increased the dose of nitrogen caused higher increase this compounds content in tubers of `Aster´ cultivar, than of the `Drop´, but interaction the level of nitrogen fertilization and the potato cultivar was statistically confirmed only at the later date of potato harvest (tables 2, 3). After 60 days from planting, independently from cultivar, significant increased the nitrates content in tubers on average by 14.97 mg NaNO₃ kg^-1 f. w. (9.56%), in comparison to the control object without the nitrogen fertilization were found only just than at the dose of 90 kg N ha^-1. After two-week delay of potato harvest already the dose of 30 kg N ha^-1 caused significant increase the nitrates content in tubers of 'Aster' cultivar, on average by 12.44 mg NaNO₃ kg^-1 f. w. (8.14%), in comparison with the cultivation without the nitrogen fertilization, however, significant increased this compounds content in tubers of 'Drop' cultivar, on average by 10.00 mg NaNO₃ kg^-1 f. w. (7.28%), occured at the dose of 60 kg N ha^-1. In case of both cultivars increased the dose of nitrogen from 60 to 90 kg N ha^-1 not caused significant increase the nitrates content in tubers (tables 2, 3). Similar tendency of chance the nitrates content in tubers of very early potato cultivars as a result of increasing the doses of nitrogen were observed by Machnacki [13] and Mikos-Bielak et al. [16]. In the study carried out by Prośba-Białczyk and Mydlarski [17], increased the dose of nitrogen from 40 to 80 kg N ha^-1 caused increase the nitrates content in tubers of 'Aster' and 'Drop' cultivars, on average by 69.90 mg NaNO₃ kg^-1 f. w. (69.52%) after 56-58 days of vegetation and by 42.50 mg NaNO₃ kg^-1 f. w. (74.04%) after 70-72 days of vegetation, however, the study carried out by Lis [12] not showed the significant difference in the nitrates content in tubers of the same potato cultivars at using the dose of nitrogen 40 and 80 kg N ha^-1. It show, that except the nitrogen fertilization a large influenced on the nitrates accumulation can to have another agronomical and environmental factors. In other author´s studies, increased the dose of nitrogen from 60 to 120 kg N ha^-1 caused increase the nitrates content in tubers of early potato cultivars, on average by 37% [7].

The nitrates content in tubers depends from genetic characters of cultivar [3, 4, 6, 7, 12, 14]. In the potato cultivation for very early crop a large meaning have choice of cultivar with a little inclination to accumulation this form of nitrogen. The study carried out by Lis [12] showed, that even after the application dose of nitrogen 200 kg N ha^-1 the nitrates content in tubers of 'Aster' and 'Drop' cultivars was lower, in comparison to all group the early cultivars investigated. Independently from using cultivation way and the level of nitrogen fertilization more nitrates accumulation the tubers of 'Aster' cultivar, than 'Drop', which was confirmed by the study carried out by Frydecka-Mazurczyk and Zgórska [3], and Prośba-Białczyk and Mydlarski [17]. After 60 days from planting the nitrates content in tubers of `Aster´ cultivar were higher, in the three-year period, on average by 25.33 mg NaNO<sub>3</sub> kg<sup>-1</sup> f. w. (16.87%), and after 75 days from planting by 25.55 mg NaNO<sub>3</sub> kg<sup>-1</sup> f. w. (18.02%) (tables 2, #table3 ). In the very early date of potato harvest differentiation of the nitrates content in tubers of 'Aster' cultivar were higher, than in tubers of `Drop´ cultivar, however, in the later date of potato harvest the variability coefficient of this compounds content in tubers of 'Drop' cultivar were higher (table 4). The higher variability of the nitrates content in mature tubers of 'Drop' cultivar, than in tubers of 'Aster' cultivar showed the study carried out by Mazurczyk and Lis [2000].

CONCLUSIONS

1. The nitrates content in tubers was higher in years with less rainfall in the vegetation period of potato.

2. The forceing of plant vegetation by use the agrotextile covers contributed to improvement the tubers quality by decreased the nitrates content, especially at a very early date of potato harvest.

3. With increase the level of nitrogen fertilization, followed the increase of nitrates content in tubers. Increased the dose of nitrogen caused higher increase this form of nitrogen content in tubers of 'Aster' cultivar, than of the 'Drop'.

4. The more nitrates accumulation the tubers of `Aster´ cultivar, than the tubers of 'Drop' cultivar.

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