Yield parameters, antioxidant activity, polyphenol and total soluble solids content of beetroot cultivars with different flesh colours

The two-year vegetation field experiment was established in the Botanical Garden of the Slovak University of Agriculture in Nitra in 2016 and 2017 (Nitra, Slovak Republic, 48°18′ N, 18°05′ E, 144 m a. s. l.). The climate in the experimental area is characterised by warm and dry summer and slightly warm, dry or very dry winter. The basic climatic characteristics of the experimental area in individual experimental years are mentioned in Table 1. The long-term average (1961–2010) was used for the evaluation of air temperature and precipitation within the experimental period (May–August) in 2016 and 2017. Significant differences in precipitation sums in the vegetation period were found between experimental years. The year 2017 can be marked as hotter and drier compared to the year 2016.

According to Petříková and Hlušek (2010), an irrigation amount of 550 mm is required during the vegetation period of beetroot. The water filling to the recommended level was done by sprinkler irrigation.

Field experiments were established on the medium-heavy soil, classified as Fluvisol, with pH 7.14–7.18. The basic agrochemical characteristics of soil in the experimental area are described in Table 2. Based on soil analyses, any fertilisers were applied.

Within this study, 16 beetroot cultivars were tested, including 11 cultivars with typical red colour (‘Boltardy’, ‘Boro’ F₁, ‘Crosby Egyptian’, ‘Cylindra’, ‘Detroit Globe’, ‘Detroit 2’, ‘Egyptian Turnip Rooted’, ‘Opolski’, ‘Pablo’ F₁, ‘Renova’ and ‘Taunus’ F₁), 2 yellow-fleshed (‘Boldor’ F₁, ‘Golden’), 2 white-fleshed (‘Albino’, ‘White Detroit’) cultivars and 1 cultivar with combined white-purple colour (‘Chioggia’). Seeds of individual beetroot cultivars were selected and bought according to their availability on the seed market. The basic characteristics of all beetroot cultivars, evaluated according to the descriptor UPOV (2008), are described in Table 3.

The sowing of beetroot seeds was realised on 19 May 2016 and 25 May 2017. Each beetroot cultivar was sowed into three rows (repetition) with a length of 3 m. The used spacing was 0.30 × 0.10 m. The harvest of beetroot was realised on 9 August 2016 and 15 August 2017. The average sample of beetroot, used for analyses, was prepared from 10 roots. Within sample preparation, roots were quartered and opposite parts were used for qualitative analyses.

The following reagents and chemicals were used for qualitative analyses of beetroot: (±)-6-hydroxy-2,5,7,8-tetramethylchromane-2-carboxylic acid (Trolox; 97%; Acros Organics™, Denmark), 2,2-diphenyl-1-picrylhydrazyl (DPPH; ≤100%; Sigma Aldrich, USA), Folin–Ciocalteu reagent (Merck, Germany), gallic acid (GA; Sigma Aldrich, USA) and sodium carbonate (solution 20% w/w; Merck, Germany).

The AOA of beetroot cultivars was analysed in the dry matter by the method of Hegedűs et al. (2019) using DPPH scavenging activity calculated as inhibition of DPPH radicals in mg · kg⁻¹ Trolox equivalents (TEAC).

The lyophilised beetroot sample was homogenised 1 min and 10 g of mild, material was sequentially mixed with 40 ml of water-methanol solution (70%). Then, 20 g of the prepared mixture was mixed with 30 ml of methanol (96%). The mixture was stored at room temperature for 20 h, and it was sequentially extracted 4 h in the orbital shaker. Finally, the mixture was filtered. From each sample extract, 0.2 ml was pipetted and mixed with 1.8 ml of methanol (70%) and 4 ml of DPPH solution (25 mg · l⁻¹). Simultaneously, 2 ml of methanol and 4 ml of DPPH solution were mixed and used as a reference sample (blind experiment). The absorbance of all beetroot samples was measured after 30 min in wavelength of 517 nm (spectrophotometer Jenway 6301, Bibby Scientific Ltd., UK).

The total polyphenol content (TPC) in beetroot was determined photometrically (spectrophotometer Shimadzu UV/VIS-1240) in a lyophilised sample by the method of Lachman et al. (2005) and expressed as milligram of gallic acid equivalent (GAE) per kilogram of dry weight (d.w.). The GA is usually used as a standard unit for phenolic content analysis because of the wide spectrum of phenolic compounds. The TPC was determined by using Folin-Ciocalteau phenol reagent, which was added to a volumetric flask containing 100 ml of extract. The prepared mixture was mixed. After 3 min, 5 ml of sodium carbonate solution (20%) was sequentially added to the mixture. The mixture was adjusted to 50 ml by adding distilled water. After 2 h, samples were centrifuged for 10 min, and sample absorbance was measured at a wavelength of 765 nm against blank. The polyphenol content was sequentially calculated from a standard curve plotted with a known concentration of GA.

The content of TSS in beetroot was analysed by using the refractometric method according to Hegedűsová et al. (2018). The juice from the prepared homogenised sample of beetroot was squeezed on the dry block of a digital hand-held refractometer (Kern ORD 45BM, Balingen, Germany). The value of soluble solids was directly read. The measurement was performed at room temperature.

Statistical analyses were performed by using Statgraphic Centurion XVII (Statgraphics Technologies, Inc., The Plains, Virginia, USA). Obtained results were evaluated by analysis of variance (ANOVA), and average values were tested by the LSD test performed at the significance level of 95% (p < 0.05).

The statistical analysis of obtained results showed significant differences in the root yield among tested beetroot cultivars and experimental years (Figure 1). The yield of beetroot was ranged from 20.74 (‘Cylindra’, 2017) to 41.91 t · ha⁻¹ (‘Albino’, 2016). Generally, white-fleshed beetroot cultivars were characterised by the highest yield potential (35.56–41.91 t · ha⁻¹). The root yield in typical, red-fleshed cultivars of beetroot was ranged from 20.74 (‘Cylindra’, 2017) to 41.09 t · ha⁻¹ (‘Detroit Globe’, 2016). The beetroot yield in yellow-fleshed cultivars reached 22.95 (‘Golden’, 2017) and 32.57 t · ha⁻¹ (‘Boldor’ F₁, 2016). The root yield of beetroot cultivar with combined red-white colour (‘Chioggia’) was 28.89 (2017) and 34.31 (2016) t · ha⁻¹.

Figure 1

The statistically significant effect of the experimental year on the root yield was found among most beetroot cultivars (Table 4). In white beetroot cultivars (Albino, White Detroit), differences in root yield among experimental years were evaluated as statistically non-significant (NS). Thus, these beetroot cultivars can be characterised as plastic cultivars in relation to changing climate conditions.

Within the study from India, Patel et al. (2017) tested the effect of plant spacing on the yield of beetroot. The yield of beetroot in the variant with spacing 0.30 × 0.15 m was ranged from 24.34 to 31.75 t · ha⁻¹, depending on the cultivar. In the Ukrainian study of Vdovenko et al. (2018), the yield of beetroot varied from 33.9 to 37.3 t · ha⁻¹. Values of beetroot yield in the mentioned studies are corresponding with the results of our study.

Compared to obtained results, Wruss et al. (2015) found the significant variability of beetroot yield in the range from 9.32 to 32.5 t · ha⁻¹, depending on the cultivar. Most of the used beetroot cultivars were shown by lower root yield compared to cultivars tested in our study. The lower yield of beetroot was also reached by Felczyński and Elkner (2008) in Poland where its values were ranged from 13.1 to 25.4 t · ha⁻¹, depending on the cultivar and experimental year. Within another Polish experiment, Szopińska and Gawęda (2013) tested the effect of the different growing system (conventional, integrated, organic) on the yield potential of beetroot (‘Regulski Cylinder’), which was also lower (19.86–25.70 t · ha⁻¹) in comparison with our experimental results.

Compared to the previously mentioned results, a significantly higher yield of beetroot was achieved by Irving (2012) in the experiment in Australia. Depending on the cultivar and planting date, root yield was ranged from 29 to 49 t · ha⁻¹. Compared to the results of our study, the author found a significantly higher yield of beetroot in the cultivars ‘Pablo’ F₁ (40–47 t · ha⁻¹) and ‘Boro’ F₁ (39–45 t · ha⁻¹).

Obtained results revealed and confirmed the statistically significant effect of cultivar on the yield of beetroot. Except for cultivar, beetroot yield can be affected by other factors, for example, different growing system (Szopińska and Gawęda, 2013), irrigation and soil salinity (da Silva et al., 2016), fertilisation strategy (Agic et al., 2016), or changing climate conditions during vegetation period (Nizioł-Łukaszewska and Gawęda, 2014).

Figure 2 shows a 3D surface graph with average yields of all cultivars in relation to root shape (with two main shape types) and flesh colour. The highest yield is achieved by white cultivar. In the case of red cultivars, the highest yield was marked in those with circular or elliptic shape instead of oblong or ovate shape.

Figure 2

The average weight (AW) of beetroot was ranged from 187.8 (‘Golden’, 2017) to 412.1 g (‘White Detroit’, 2016). Statistically significant differences of AW were shown among tested beetroot cultivars and years (Figure 3). The statistically significant effect of the experimental year on the root weight was found among most beetroot cultivars (Table 4). In the red-fleshed cultivar (‘Opolski’), the difference of AW was found as statistically NS. Thus, this beetroot cultivar can be characterised as a plastic cultivar in relation to changing climate conditions.

Figure 3

Similar results of root weight were presented in several studies with beetroot. Within the Austrian experiment (Wruss et al., 2015), the average root weight of seven beetroot cultivars ranged from 214 to 366 g. Székely et al. (2014) found root weight variability of beetroot, depending on its cultivar, from 298.5 to 395.5 g in Hungary. The significant variability of root weight, depending on the beetroot cultivar, was presented by Nizioł-Łukaszewska and Gawęda (2014) in the study realised in Poland (83.7–336.7 g). The authors found a slightly higher value of AW in the beetroot cultivar ‘Boro’ F₁ (336.7) in comparison with the same cultivar in our study.

On the other hand, significantly lower root weight of beetroot was found in experiments in Poland (149.4–202.4 g; Szopińska and Gawęda, 2013), Ukraine (170.4–180.2 g; Vdovenko et al., 2018) or Brazil (161.6–180.7 g; da Silva et al., 2016).

The statistical analysis of experimental results showed statistically significant differences of AOA among tested beetroot cultivars (Table 5). Values of AOA varied from 314.73 (‘White Detroit’, 2016) to 972.50 mg TEAC · kg⁻¹ d.w. (‘Pablo’ F₁, 2017). Generally, red-fleshed beetroot cultivars were characterised by significantly higher AOA (645.88–972.50 mg TEAC · kg⁻¹ d.w.) than beetroot cultivars with non-typical flesh colour (314.73–513.60 mg TEAC · kg⁻¹ d.w.). The statistically significant effect of the experimental year on the AOA of beetroot was also found (Table 4).

Compared to obtained results, Guldiken et al. (2016) found higher AOA of beetroot (1,370 mg TEAC · kg⁻¹ d.w.). Kovarovič et al. (2017) tested the effect of cultivar on the AOA of beetroot, determined by the DPPH method. The AOA of red-fleshed cultivars (20.70–21.83% inhibition of DPPH) was statistically significantly higher compared to the red-white cultivar of beetroot (8.37%), as it was shown by results obtained in our study. Within the study of Wruss et al. (2015), the significant variability of AOA in relation to beetroot cultivars was found and confirmed by using two different methods of AOA determination (ORAC - oxygen radical absorbance capacity, and FRAP - Ferric reducing antioxidant power). The difference between beetroot cultivars with the lowest and highest AOA content was approximately double in both used analysis methods. The significant effect of cultivar on the AOA of beetroot was also shown in the study of Nizioł-Łukaszewska and Gawęda (2014) who found the variability of its values from 25.60% to 37.99% DPPH. Significant variability of AOA in relation to beetroot cultivar was also found and presented by Székely et al. (2014) or Czapski et al. (2009). Obtained results revealed and confirmed the statistically significant effect of cultivar on the AOA of beetroot. Except for cultivar, AOA of beetroot can be affected by other factors, for example, fertilisation strategy (Babagil et al., 2018), growing locality (Kavalcová et al., 2015), or changing climate conditions during the vegetation period (Nizioł-Łukaszewska and Gawęda, 2014).

Obtained results showed the significant effect of flesh colour on the AOA of beetroot, which was decreased in the following order of flesh colour: red > yellow > red-white > white. Results indicate that beetroot cultivars with darker flesh colour are characterised by higher AOA because of polyphenol content. A similar trend was also shown in the experiments with other crops, for example, potato (Gupta et al., 2015), sweet potato (Ji et al., 2015), or carrot (Scarano et al., 2018).

Experimental results revealed a ver y strong correlation between AOA and TPC in tested beetroot cultivars (Figure 4). The increasing TPC content was expressed by higher AOA values in individual beetroot cultivars. This fact was also shown in several studies with beetroot (Koubaier et al. 2014; Kavalcová et al., 2015; Kovarovič et al., 2017), carrot (Yoo et al., 2020; Scarano et al., 2018), red cabbage (Leja et al., 2010), potatoes (Šulc et al., 2008), or sweet potatoes (Salawu et al., 2015).

Figure 4

The statistically significant differences of TPC among tested beetroot cultivars were found (Table 6). Values of TPC were ranged from 717.27 (‘White Detroit’, 2016) to 2,731.00 mg GAE · kg⁻¹ d.w. (‘Pablo’ F1, 2017). The highest TPC was found in red-fleshed cultivars (1,537.64–2,731.00 mg GAE · kg⁻¹ d.w.), followed by beetroot cultivars with yellow (995.45–1,183.72 mg GAE · kg⁻¹ d.w.), red-white (756.93–941.41 mg GAE · kg⁻¹ d.w.) and white (717.27–933.17 mg GAE · kg⁻¹ d.w.) colour of root flesh. The statistically significant effect of the experimental year on the TPC in beetroot was also found (Table 4).

Koubaier et al. (2014) found extraordinary, statistically high-significant differences of TPC in beetroot (700–6,600 mg GAE · kg⁻¹ d.w.). Within the study of Guldiken et al. (2016), TPC in beetroot was 2,550 mg GAE · kg⁻¹ d.w. This value is comparable to the polyphenol-richest cultivars of beetroot tested in our study. Kavalcová et al. (2015) tested the effect of growing locality and cultivar on the TPC in beetroot in the Slovak Republic with values ranged from 820.10 to 1,280.56 mg GAE · kg⁻¹ d.w. In the red-fleshed beetroot cultivar ‘Renova’, significantly lower TPC was found (820.10–1,280.56 mg GAE · kg⁻¹ d.w.) compared to our study. In the study of Kovarovič et al. (2017) in the Slovak Republic, TPC in beetroot was ranged from 368.75 to 887.75 mg GAE · kg⁻¹ d.w., depending on cultivar. Values of TPC in beetroot cultivars ‘Cylindra’ (887.75 mg GAE · kg⁻¹ d.w.), ‘Crosby Egyptian’ (882.40 mg GAE · kg⁻¹ d.w.) and ‘Chioggia’ (373.80 mg GAE · kg⁻¹ d.w.) were significantly lower compared to results of our study for the same tested cultivars. Within the study of Ninfali et al. (2013), the lower TPC in beetroot cultivars, except one cultivar, was determined (720–1,726 mg GAE · kg⁻¹ d.w.) in comparison with our study. The lower TPC in beetroot was also shown by Wootton-Beard et al. (2011) and Číž et al. (2010). Its values were ranged from 617.8 to 1,450.3 mg GAE · kg⁻¹ d.w. in the mentioned studies. On the contrary, significantly higher TPC in beetroot (3,764 mg GAE · kg⁻¹ d.w.), in comparison with our study, was found by Čanadanovič-Brunet et al. (2011). The significant variability of TPC in beetroot, depending on its cultivar, was also shown by Wruss et al. (2015), Székely et al. (2014), Czapski et al. (2009), or Felczyński and Elkner (2008).

Obtained results revealed and confirmed the statistically significant effect of cultivar on the TPC in beetroot. Except for cultivar, TPC of beetroot can be affected by other factors, for example, growing system (Heimler et al., 2017; Kazimierczak et al., 2014), fertilisation strategy (Babagil et al., 2018), growing locality (Kavalcová et al., 2015), or water stress (Stagnari et al., 2014).

According to Cejpek (2012), the estimation method of TSS is used for testing sugar content in fruit and vegetable juices or other food products and the total concentration of monosaccharides and disaccharides in any solutions. Hegedűsová et al. (2018) defined TSS as additive quantity, which expresses the content of dissolved substances, mainly sugars, in vegetable or fruit extracts. As the unit of TSS, Brix degrees (BRIX) are used.

The obtained results showed statistically significant differences in TSS among tested beetroot cultivars (Table 7). Values of TSS were varied from 7.1 (‘Chioggia’, 2016) to 10.8 ºBRIX (‘Detroit 2’, 2016). The TSS of beetroot cultivars was decreased in the following order of flesh colour: red (8.1–10.8 ºBRIX) > white (7.8–8.8 ºBRIX) > yellow (6.8–8.2 ºBRIX) > red-white (6.4–7.7 ºBRIX). The statistically NS effect of the experimental year on the TSS in beetroot was found (Table 4).

Székely et al. (2014) found similar content of TSS in beetroot in a Hungarian experiment (6.0–10.7 ºBRIX). In the cultivar ‘Cylindra’, higher TSS content was found (10.7 ºBRIX) compared to the same cultivar in our study. On the other hand, Stagnari et al. (2014) determined significantly higher TSS content in beetroot (13.9 ºBRIX) compared to our study. The significantly higher content of TSS (14.2–21.5 ºBRIX) in beetroot was also shown in the Australian study of Irving (2012). Values of TSS in cultivars ‘Pablo’ F₁ (15.5–18.4 ºBRIX) and ‘Boro’ F₁ (15.1–18.3 ºBRIX) were significantly higher than values of the same cultivars tested in our study.

The significant variability of TSS or sugar content, depending on the beetroot cultivar, was also shown by Wruss et al. (2015) or Nizioł-Łukaszewska and Gawęda (2014). Besides cultivar, TSS in beetroot can be also affected by the growing system (Szopińska and Gawęda, 2013) or water stress (Stagnari et al., 2014).