Total and Protein Nitrogen Content in Potato Tubers under the Influence of Various Care and Nutrition Methods with the Use of Biostimulants

A two-factor field experiment was carried out in 2018–2020 at the Zawady Agricultural Experimental Station (52°03’ N and 22°33’ E) belonging to the University of Natural Sciences and Humanities in Siedlce in Poland. Tubers of two edible potato cultivars were investigated, i.e. Oberon and Malaga – factor one; in turn, factor two included variants of herbicide application with biostimulators: 1. control object – without preparations, 2. herbi - cide clomazone + metribuzin (Avatar 293 ZC), 3. herbicide clomazone + metribuzin and biostimulant PlonoStart, 4. herbicide clomazone + metribuzin and biostimulant Aminoplant, 5. herbicide clomazone + metribuzin and bio-stimulant Agro-Sorb® Folium. The biostimulators used in the research significantly increased the content of total nitrogen and protein nitrogen in tubers, as well as the share of protein nitrogen in total nitrogen in relation to the tubers collected from the control object. The genetic features of the cultivars also determined the concentration of total nitrogen and protein nitrogen in potato tubers.


INTRODUCTION
The potato (Solanum tuberosum L.) is one of the most important food crops after wheat, rice and maize, and is the staple food for the world population. According to the United Nations, potato tubers are a food security crop. The global monthly consumption of potato per capita is 31.3 kg; in Europe it is higher at 87.8 kg/capita (FAO, 2020; Pllana et al., 2018). The advantages potatoes include a rich chemical composition, high nutritional value; moreover, they can grow in any climate and conditions (Silveira et al., 2020). The chemical composition of potato tubers depends on the cultivar, weather conditions, technology of cultivation, fertilization, harvesting and storage conditions (Manjunath et  In modern plant cultivation, various plant growth regulators called biostimulants are increasingly used. They are one of the elements of agrotechnics, which, apart from fertilizing and protection plants, can positively affect the size and quality of crops (Yakhin,  Biostimulants include hormones, enzymes, proteins, amino acids, vitamins, microelements, alga extracts and other compounds (Kunicki, 2010;Rutkowska, 2016). The most common biostimulants are: amino acids, proteins, microelements. Plants are able to take up amino acids and peptides both through the leaves and the root system. In addition, plants are capable of self-synthesis of amino acids, but this process requires much energy (Yakhin et al., 2017;Radkowski, 2018). Amino acids are among the most important organic compounds and play an important biological role as building blocks of proteins, enzymes, nucleic acids, antioxidants, hormones and other components. Therefore, the use of these compounds as biostimulants may save energy and improve dynamics of plant development (Shukla et al., 2014 Zarzecka et al., 2020). Therefore, the aim of this study was to determine the effect of biostimulants based on amino acids and microelements and herbicide on the content of total nitrogen and protein nitrogen in potato tubers.

MATERIAL AND METHODS
The research was carried out in 2018-2020, in the Agricultural Experimental Station Zawady (52°03'N and 22°33'E) which belongs to the University of Natural Sciences and Humanities in Siedlce in Poland. The content of available forms of phosphorus, potassium and magnesium in 2018 was low, and in 2019 and 2020 it was medium. The soil was acidic with a pH of 5.25-5.42 in 1 M KCl (Table 1). Two factors were tested in the split-plot system in three replications: I -two medium-early edible potato cultivars Malaga and Oberon, II -five ways of using herbicide and herbicide with biostimulants: control object -mechanical maintenance, Avatar 293 ZC (clomazone  Table 2 ). The description of the biostimulants and herbicide and their effects are presented in Table 2. The forecrop in all research years was winter triticale. The potatoes were fertilized in autumn with 25 t·ha -1 of manure and the 44.0 kg P·ha -1 (superphosphate, 17.5% P) and 124.5 kg K·ha -1 (potash salt, 50% K) mineral fertilizers were applied. In the spring before planting, 100 kg N·ha -1 (ammonium nitrate 34% N) were applied. Potatoes were planted by 18 to 23 April, and were harvested by 4 th to 17 th September. Pathogens and pests were controlled chemically.

Meteorological conditions
In terms of humidity and thermal conditions, 2018 was the most favorable year for potato development. It was a warm season with evenly distributed rainfall. In 2019, a significant shortage of rainfall was found in relation to the multi-year period. In 2020, the air temperatures were optimal, while the humidity conditions were close to the long-term values, but the rainfall was unevenly distributed during the potato vegetation. Weather data is included in Table 3.

Chemical analysis -determination of nitrogen
During the harvest, 10 potato plants were dug up by hand from each plot and tuber samples (35-60 mm in diameter) were taken for chemical analysis. The tubers taken from the field experiment were washed thoroughly and cut into thin slices. The total nitrogen was determined with Kjeldahl's method on a 2300 Kjeltec Analyser Unit (ISO. ISO 1871, 2009). Protein nitrogen was determined by using the Bernstein method (AOAC 2006; Barbaś and Sawicka, 2017). The determined values was expressed in g·kg -1 of dry matter of tubers. The total nitrogen uptake with the tuber yield was calculated as the product of tuber dry matter yield and total nitrogen concentration.

Statistical analysis
Test results were developed statistically with the method of the variance analysis. The significance of the sources of variation was tested with the 'F' Fischler-Snedecor test, and the assessment of the significance of differences with the significance level P ≤ 0.05 between the compared averages using the multiple Tukey ranges (Trętowski and Wójcik, 1991).

Content of total nitrogen
The nutritional value of a potato is due to its chemical composition. Potato tubers contain approx. 17% of complex carbohydrates (starch), approx. 0.5% of sugars, 2% of proteins, 2.3% of dietary fiber, approx. 0.1% of lipids and a wide The authors' own research has shown that the Oberon variety contains more total nitrogen than the Malaga variety (Table 4).
Other authors also showed a different con-  Table 4. Content of total nitrogen, protein nitrogen in potato tubers (g kg -1 dry matter) and uptake of total nitrogen with the yield of tubers (kg ha -1 )

Content of protein nitrogen
Chemical analyses have shown, and statistical calculations have confirmed, a significant influence of cultivars, application of herbicide and biostimulants, as well as weather conditions in the years of research on the content of protein nitrogen in potato tubers. Under the conditions of the experiment, the average protein nitrogen content in potato tubers ranged from 12.45 to 19.71 g·kg -1 dry matter (Tables 4, 5). The Oberon cultivar had a statistically higher concentration of protein nitrogen than the Malaga cultivar. According to Mystkowska (2018) Table 5. Content of total nitrogen, protein nitrogen in potato tubers (g kg -1 dry matter) and uptake of total nitrogen with the yield of tubers (kg ha -1 ) in study years  In the human diet, potato is one of the main sources of nutrients (Flis et al., 2012). Hence, an important feature, from the consumer's point of view, is the share of proper protein in total protein, and thus protein nitrogen in total nitrogen, which in the experiment was quite high and ranged from 68.35% to 73.4% (Table 6).
In the conducted research, both the genetic characteristics of the cultivars and the application of herbicide and biostimulants significantly influenced the share of protein nitrogen in total nitrogen ( Table 6). The cultivar Oberon had a greater share than the cultivar Malaga. The applied biostimulants increased the share of protein nitrogen in total nitrogen compared to the control object and to the variant where only the herbicide was applied. In the studies by Barbaś and Sawicka (2017), this share was within the range of 68.03%-80.48% depending on the cultivar, and 59.20-89.65% in dry matter, depending on the year of the study. In Bárta and Bártová (2008), this share was in the range of 44.5-62.1%. Many authors emphasize that potato protein is unique, free from allergens, gluten and lactose, is rich in exogenous amino acids and has a high functional value (Beals,

Uptake of total nitrogen with the yield of tubers
The uptake of nutrients with the yield is determined mainly by the harvested tuber yield and the content of macroelements in tubers, but may also be determined by agrotechnical and environmental factors (Mystkowska and Rogóż-Matyszczak, 2019; Zarzecka et al., 2019). The conducted analysis and calculations showed that the uptake of total nitrogen with the yield of tubers depended significantly on cultivated cultivars, application of herbicide and biostimulants and the weather conditions during plant growth (Table 4, 5). Cultivar Oberon (211.8 kg ha -1 ) was characterized by higher mean total nitrogen uptake (120.5 kg ha -1 ) than cultivar Malaga. The highest total nitrogen uptake capacity was recorded in the object where the Avatar 293 ZC herbicide and the Agro-Sorb® Folium biostimulant were used (195.1 kg·ha -1 ). In the other objects with herbicide and biostimulants (objects 2-4), the intake of this component was significantly higher compared to the control object. The highest total nitrogen uptake was found in 2018, when it was warm, and the sum of rainfall was similar to the values from the long-term period. It was a favorable year for potato yielding and total nitrogen accumulation. Similar results were obtained by Wierzbowska et al. (2015).

CONCLUSIONS
New cultivation technologies with the use of biostimulants reduce the negative environmental impact and at the same time improve the quality of the harvested crops. The present research shows that the application of herbicide and biostimulants on potato plantation increased the content of total nitrogen and protein nitrogen in tubers. The best effects were obtained with the use of the Avatar 293 ZC herbicide as well as PlonoStart and Agro-Sorb® Folium biostimulants, compared to the control object. At the same time, it was found that the share of protein nitrogen in total nitrogen was the highest after spraying potato plants with biostimulants.