Nitrates content in table potato tubers under the influence of herbicides and biostimulants

ABSTRACT

The objective of the study was to determine the effect of herbicides and biostimulants on the potato tuber content of nitrates. The experimental material consisted of tubers harvested in a field experiment which examined the following factors: three medium early potato cultivars (Bartek, Gawin, Honorata) and five methods of herbicide and biostimulant application (Harrier 295 ZC (linuron + chlomazon), Sencor 70 WG (metribuzin), growth regulator Kelpak SL (algae extract from Ecklonia maxima − auxins and gibberellins), growth regulator Asahi SL (sodium para-nitrophenol, sodium ortho-nitrophenol, sodium 5-nitroguaiacol) and a control unit where weeds were mechanically controlled). The tuber content of nitrates was determined in fresh matter with an ion-selective nitrate electrode and silver–silver chloride reference electrode. Herbicides and biostimulants increased the potato tuber content of nitrates compared with control tubers. However, they did not pose a threat to consumer health. Tubers of plants sprayed with the herbicide Sencor 70 WG (metribuzin) and the biostimulant Asahi SL (sodium para-nitrophenol, sodium ortho-nitrophenol, sodium 5-nitroguaiacol) contained the most nitrates whose accumulation was also affected by cultivars and weather conditions in the study years.

KEYWORDS:

• Chemical protection
• cultivars
• Solanum tuberosum L
• undesirable nitrogen compounds

Introduction

Potato is a food staple in Europe and in Poland (Leszczyński 2012; Hmelak Gorenjak and Cencič 2013; King and Slavin 2013). Per capita potato consumption in Poland is quite high, amounting to 100–121 kg per year in the last decade, and 97 kg in 2017 (Potato Market 2017). Potato tubers are still the main component of dishes eaten for dinner because they are easily digestible, provide considerable amounts of vitamin C, valuable protein, carbohydrates as well as macronutrients and micronutrients (Lisińska et al. 2009; Zarzecka et al. 2015; Trawczyński 2016). In addition to nutrients, potato tubers contain natural adverse substances which include nitrates and glycoalkaloids (Lachman et al. 2005; Hmelak Gorenjak and Cencič 2013; Gugała et al. 2017; Urban et al. 2018). As such, nitrates (V) are not toxic to humans but 5–7%, and sometimes up to 20% of these substances are reduced by intestinal and abdominal flora to toxic nitrates (III) − nitrites (Nitrate in vegetables. Opinion of the Scientific Panel … 2008). The latter are carcinogenic and mutagenic, may cause changes in the brain, impair thyroid functioning and absorption of proteins and fats, reduce contents of vitamins belonging to group A and B and, as a result, decrease the nutritional value of potato tubers and vegetables consumed by humans (Rogozińska et al. 2005; Santamaria 2006; Raczuk et al. 2014).

The variation in the potato tuber content of nitrates is caused by e.g.: genetic factors, cultivation practices, and climatic conditions during the growing season which affect tuber metabolism and chemical composition (Gugała and Zarzecka 2009; Hmelak Gorenjak et al. 2014; Kołodziejczyk 2016; Trawczyński 2016). Lachman et al. (2005), Järvan and Edesi (2009) claimed that the conventional way of cultivation (mineral fertilizer and pesticides were used) increased nitrates content in tubers comparing with the potato cultivated ecologically. Gugała and Zarzecka (2009) did not show the influence of insecticides and Zarzecka et al. (2010) influence of herbicides and tillage systems on nitrates content in tubers. Many authors found that potato tubers chemical composition was in the first place determined by the cultivar (Järvan and Edesi 2009; Hmelak Gorenjak and Cencič 2013; Barbaś and Sawicka 2016; Zarzecka et al. 2016). Inappropriate farming practices or adverse precipitation and thermal patterns may contribute to an accumulation of excessive nitrate amounts in tubers (Pobereżny et al. 2015; Simson et al. 2016). Pursuant to the Minister of Health Regulation of 13 January 2003 and the Commission (EC) Regulation No 1881/2006 of 19 December 2006, nitrate content in potato tubers may not exceed 200 mg/kg fresh matter. Potatoes destined for direct consumption or processing into chips, crisps and dried products, particularly tubers produced using intensive cultivation technologies, should be regularly checked for harmful nitrate compounds. The objective of the present work was to determine nitrate (V) contents in potato tubers as affected by applied herbicides and biostimulants under changeable weather conditions.

Materials and methods

Experimental and agronomic management

Studies were carried out on potato tubers harvested in a field experiment conducted in the years 2012–2014 in Wojnów (52°12'59''N, 22°34'37''E), Poland. The experiment was designed as a split-plot arrangement with three replicates (Badowski et al. 2001). The experimental factors were as follows: factor I – three table potato cultivars: Bartek, Gawin, Honorata; factor II – five methods of an application of herbicides and biostimulants (Table 1). The experiment was set up on the soil which had the texture of sand and belonged to soil valuation class IVb which corresponds to the rye very good complex of agricultural soil suitability (Marcinek and Komisarek 2011). Selected chemical properties of soil in the study years, before the field experiment had been established, are presented in Table 2. Potato was cultivated after winter wheat each year. Natural manuring and mineral fertilisation were applied at the following rates: 25 t ha⁻¹ farmyard manure (autumn), N – 100 (spring), P − 44 and K − 125 kg ha⁻¹ (autumn). The control of diseases and pests was in accordance with the requirements of agricultural practice. Potatoes were planted at 67.5 × 37 cm spacing in mid-April and harvested in early September, at the stage of physiological maturity (BBCH 97). The area of one plot was 15 m². Climatic conditions in the years of the research period varied as shown in Table 3.

Table 1. Factors in the conducted field experiment.

Factor I – three table potatoes cultivars: Bartek, Gawin, Honorata (Level 3)
Factor II – five methods of application of herbicides and biostimulants (treatments) (Level 5)
1. Control object – mechanical weed control until and after potato emergence – without herbicides or biostimulants 2. Harrier 295 ZC (linuron + chlomazon) at a rate of 2.0 l ha^−1 (BBCH 07) 3. Harrier 295 ZC (linuron + chlomazon) at a rate of 2.0 l ha^−1 (BBCH 07), and growth regulator Kelpak SL* at a rate of 2.0 l ha^−1 (BBCH 13-18) 4. Sencor 70 WG (metribuzin) at a rate of 1.0 kg ha^−1 (BBCH 08) 5. Sencor 70 WG (metribuzin) at a rate of 1.0 kg ha^−1 (BBCH 08) and growth regulator Asahi SL** at a rate of 1.0 l ha^−1 (BBCH 13-18)

*Kelpak SL (algae extract from Ecklonia maxima − auxins and gibberellins)

**Asahi SL(sodium para-nitrophenol, sodium ortho-nitrophenol, sodium 5-nitroguaiacol)

Table 2. Chemical properties of soil before the establishment of the field experiment in the study years.

	Year and value
	2012		2013		2014
Soil parameter	Value	Category	Value	Category	Value	Category
pH, KCl	5.6	Lightly acidic	5.6	Lightly acidic	6.35	Neutral
Organic matter, g kg^−1	15	-	16	-	18.7	-
Available P, mg kg^−1	68.6	High	73.4	High	110	Very high
Available K, mg kg^−1	149.4	Very high	129	High	149.4	Very high
Available Mg, mg kg^−1	50	High	51	High	56	High

Table 3. Meteorological data in the study years according to Zawady Meteorological Station.

Years	Months						April-September
	April	May	June	July	August	September
Air temperature (°C)							Mean
2012	8.9	14.6	16.3	20.7	18.0	14.1	15.4
2013	7.4	15.3	18.0	19.0	18.8	11.7	15.0
2014	9.8	13.5	15.4	20.8	18.1	14.1	15.3
Long-term mean (1987–2000)	7.8	12.5	17.2	19.2	18.5	13.1	14.7
Precipitation (mm)							Sum
2012	29.9	53.4	76.2	43.0	51.0	11.4	264.9
2013	36.0	105.9	98.8	91.3	15.0	94.3	441.3
2014	45.0	92.7	55.4	10.0	105.7	26.3	335.1
Long-term mean (1987-2000)	38.6	44.1	52.4	49.8	43.0	47.3	275.2

Determination of nitrates

Before harvest, tuber samples from ten plants per plot were collected in all experimental units and used for chemical analyses. Chemical analyses were performed using fresh material in three replicates at the Chemical Laboratory of the Department of Agrotechnology. Nitrate content in tubers was determined in fresh matter with an ion-selective nitrate electrode and silver–silver chloride reference electrode (Kolbe and Müller 1987).

Statistical analysis

The results of the study were statistically analysed using ANOVA. The significance of sources of variation was tested with the Fisher-Snedecor ‘F’ test, and significance of differences between the compared means was checked at p = 0.05 using the Tukey test.

Results and discussion

The study demonstrated that nitrate content in potato tubers ranged from 117.7 to 166.4 mg kg⁻¹ fresh matter and was significantly affected by the herbicides and biostimulants applied in the experiment (Tables 4 and 5). The concentration of nitrates in tubers was similar to values reported by other authors (Lachman et al. 2005; Rytel 2012). The herbicides and biostimulants contributed to an increase in nitrates compared with control where only mechanical cultivation had been performed (Table 4). Nitrate content was the highest when herbicides were applied in combination with biostimulants, which indicates that growth regulators increased nitrate uptake by potato tubers. Also Sawicka and Mikos-Bielak (2009) found a significant increase in the concentration of nitrates due to an application of Asahi SL.

Table 4. Nitrate content (mg kg⁻¹ FM) of potato tubers in response to herbicide and biostimulant application.

	Cultivar			Mean
Herbicide and biostimulant	Bartek	Gawin	Honorata
Control	151.3	146.0	140.8	146.0
Harrier 295 ZC	152.9	149.5	147.6	150.0
Harrier 295 ZC + Kelpak SL	154.0	151.6	149.5	151.7
Sencor 70 WG	155.3	147.7	150.5	151.1
Sencor 70 WG + Asahi SL	156.7	149.9	151.8	152.8
Mean	154.0	148.9	148.0	150.3

Note: LSD_0.05 for: cultivar = 3.6; for herbicide and biostimulant = 4.0; cultivar × herbicide and biostimulant interaction was non significant.

Zarzecka et al. (2016) in another experiment with the use of herbicides: Plateen 41.5 WG (metribuzin + flufenacet), Racer 250 EC (fluorochloridone) and Sencor 70 WG (metribuzin) found an increase of nitrates in tubers in comparison to the control object. Järvan and Edesi (2009) reported an increase in nitrate content in conventionally grown (pesticide-treated) potato compared with traditional cultivation. In their earlier study, Zarzecka et al. (2010) applied herbicides and observed no significant effect of the chemicals, and only a tendency for nitrates to increase. Also Lachman et al. (2005) observed an increase in the tuber content of nitrates in the conventional tillage method including herbicide application, comparing with the organic tillage method. However, the differences were not statistically confirmed. An increase in nitrate content was recorded in all the cultivars assigned to treatments 2–5 but no cultivars x herbicide and biostimulant interaction was observed (Tables 5 and 6).

Table 5. Nitrate content (mg kg⁻¹ FM) of potato cultivars as affected by temperatures and rainfall in 2012, 2013 and 2014.

Herbicide and biostimulant	Year and cultivar
	2012			2013			2014			Mean
	Bartek	Gawin	Honorata	Bartek	Gawin	Honorata	Bartek	Gawin	Honorata
Control	161.9	160.2	159.7	143.7	130.0	117.7	148.3	147.7	145.0	146.0
Harrier 295 ZC	162.5	161.8	161.0	146.3	136.7	135.0	150.0	150.0	146.7	150.0
Harrier 295 ZC + Kelpak SL	163.6	163.2	163.0	146.7	140.0	137.3	151.7	151.7	148.3	151.7
Sencor 70 WG	165.8	161.4	164.0	150.0	131.7	140.7	150.0	150.0	146.7	151.1
Sencor 70 WG + Asahi SL	166.4	164.6	165.0	150.3	135.0	142.7	153.3	150.0	147.7	152.8
Mean, cultivar	164.0	162.2	162.5	147.4	134.7	134.7	150.7	149.9	146.9	150.3
Mean, year	162.9			138.9			149.2			150.3

Note: LSD_0.05 for: cultivar = 3.6; for herbicide and biostimulant = 4.0; for year = 3.6; cultivar × herbicide and biostimulant interaction was non significant.

Table 6. Analysis of variance for nitrate content in three potato cultivars subjected to five herbicide and biostimulants applications in 2012, 2013 and 2014.

Sources of variation	Degrees of freedom	Sum of squares	Variance	F	F Critical value	LSD0.05
Cultivars	2	947.9484	473.9742	11.4913	3.880	3.615*
Years	2		6545.0516	158.683	3.880	3.615*
Cultivars × Years	4	822.2524	205.5631	4.9838	3.260	6.261*
Error I	12	494.9529	41.2461
Herbicide and biostimulants	4	737.0677	184.2669	6.5147	2.500	4.67*
Cultivars × Herbicide and biostimulants	8	261.8701	32.7338	1.1572	2.070	6.017
Herbicide and biostimulants × Years	8	314.1043	39.2630	1.3881	2.070	7.045
Cultivars × Herbicide and biostimulants × Years	16	384.1179	24.0074	0.8487	1.790	10.422
Error II	72	2036.5000	28.2847

*Significant at the 0.05 level.

Of the experimental cultivars, Bartek accumulated the greatest amount of nitrates, it being significantly lower in Gawin and Honorata. An effect of cultivar on the concentration of nitrates has also been reported by Gugała and Zarzecka (2009), Järvan and Edesi (2009), Sawicka and Mikos-Bielak (2009), Simson et al. (2016), Zarzecka et al. (2016). Variance analysis demonstrated that weather conditions in the study years significantly affected nitrate content in the tubers of Solanum tuberosum (Table 5). The highest values of this characteristic were obtained in 2012 when precipitation during the growing season was lower compared with the remaining years whereas air temperature was the highest. Also Kołodziejczyk (2016) recorded the highest nitrate content in the growing season which saw precipitation shortages in April-June and in August. By contrast, nitrates were the lowest in 2013 which was wet and the coldest compared with the long-term mean, as confirmed in the study by Trawczyński (2016). Analysis of variance components revealed that nitrate content was in 78% determined by weather conditions, in 16% by cultivar and in 2.5% by methods of an application of herbicides and biostimulants. Similar changes were reported by Trawczyński (2016).

Herbicides and biostimulants applied in the study contributed to an increase in the potato tuber concentration of nitrates. Yet, consumption of 266 g tubers examined in the study did not exceed the permissible daily intake of these compounds and, as a result, did not pose a threat to human health (Table 7). Similar changes following an application of potato chemical control and fertilisation were reported by Pobereżny et al. (2015).

Table 7. Daily human consumption of nitrates (mg) with 266 g potato^a as affected by herbicides, biostimulants and cultivars. Means across 2012–2014^b.

Herbicide and biostimulant	Cultivar			Mean
	Bartek	Gawin	Honorata
Control object	40.2	38.8	37.5	38.8
Harrier 295 ZC	40.7	39.8	39.3	39.9
Harrier 295 ZC + Kelpak SL	41.0	40.3	39.8	40.4
Sencor 70 WG	41.3	39.3	40.0	40.2
Sencor 70 WG + Asahi SL	41.7	39.9	40.4	40.7
Mean	41.0	39.6	39.4	40.0

^aMean per capita consumption of potato tubers is 97 kg year⁻¹ in Poland in 2017 (Potato Market. Status and perspectives 2017).

^bAcceptable daily intake (ADI) of nitrates is 200 mg day⁻¹ (Regulation of the Minister of Health 2003, Commission Regulation (EC) 2006).

Conclusions

The content of nitrates in potato tubers depended significantly on the cultivar, herbicides and biostimulants as well as weather conditions during vegetation. Herbicides and biostimulants increased the potato tuber content of nitrates compared with control tubers. Most of these compounds were cumulated by sprayed with Sencor 70 WG herbicide and Asahi SL biostimulant, but this did not pose a threat to the health of the consumer.

Acknowledgements

We thank Iwona Mystkowska for the chemical analyses carried out.

Disclosure statement

No potential conflict of interest was reported by the authors.

Notes on contributors

Krystyna Zarzecka is a professor of agricultural sciences in the agronomy discipline. Krystyna Zarzecka’s research interest is potato agrotechnics.

Marek Gugała is a habilitated doctor in agricultural sciences in the agronomy discipline. Marek Gugała’s research interests are potato agriculture and the influence of biostimulators on arable crops. Marek Gugała is the author of over 400 scientific and popular science works and co-author of over 20 inventions, including patents.

Anna Sikorska is a doctor of agricultural sciences in agronomy discipline. Anna Sikorska’s research interests are the influence of biostimulants on arable crops and winter rape. Anna Sikorska is the co-author of over 50 scientific and popular science works.

ORCID

Krystyna Zarzecka http://orcid.org/0000-0002-7792-6448

Marek Gugała http://orcid.org/0000-0001-5048-3432

Anna Sikorska http://orcid.org/0000-0003-4465-1609

Additional information

Funding

The results of the research carried out under the research theme No. 214/04/S and No. 363/S/13 were financed from the science grant granted by the Ministry of Science and Higher Education in Poland.