Introduction

Phenolic compounds mainly contain anthocyanins, flavanols, flavonols, stilbenes and phenolic acids (Gougoulias 2010; Kok and Bal 2017a).

Phenolic compounds profile is linked to grape composition, influencing by various factors such as ecological factors (Downey et al. 2006), different viticultural practices (Kok 2011; 2016; Kok et al. 2013; Kok and Bal 2016, 2017b) and grape cultivar (Kok 2017; Kok and Bal 2017a).

Phenolic compounds have important roles as components of wine colour, conducing wine sensory quality. Inside of this family of compounds, anthocyanins are the most considerable molecules, finding in the skins of coloured grapes (Bouzas-Cid et al. 2016).

There have been numerous efforts to rise phenolic compounds composition and anthocyanin content of wine grapes (Pardo-Garcia et al. 2014). Foliar oak applications have recently been used for improving contents of phenolic compounds and anthocyanin.

Newly published scientific papers about utilization of aqueous oak extracts point out that how foliar applications of oak extract modulate the aroma and phenolic compounds in wine grapes (Martinez-Gil et al. 2011, 2012; Pardo-Garcia et al. 2014).

Study results about oak applications indicate that foliar oak applications increase phenolic compounds composition and anthocyanin content in wine grapes (Martinez-Gil et al. 2011, 2012, 2013; Pardo-Garcia et al. 2014).

One of the other viticultural practices, affecting wine grape and wine composition is foliar or soil fertilization (Bell and Henschke 2005). There are different favourable (Delgado et al. 2004) or unfavourable (Soubeyrand et al. 2014) research findings about effects of high nitrogen supply on phenolic compounds content of wine grapes. The point that should not be forgotten here is that these conflicting findings seem to be related to nitrogen dose, application time and application numbers, prevailing ecological factors in vineyards (Portu et al. 2015).

Reasons mentioned above, the aim of current study was to determine effects of foliar oak application doses combined with low urea concentration on phenolic compounds composition and anthocyanin content of cv. Cabernet Sauvignon.

Material and Methods

Plant material and characteristics of research area

The research was performed in 2016 growing season in a 7-year old commercial vineyard (40°56′40,81′′N; 27°20′15,74′′E; 243 m a. s. l) in Tekirdag, Turkey.

The vineyard was planted with cv. Cabernet Sauvignon grafted onto SO4 (Vitis berlandieri × Vitis riparia No. 4, Selection Oppenheimer No. 4). Grapevine spacing was 1.5 m and row spacing was 3.2 m and grapevines were trained to a guyot training system. In current vineyard, disease and pest controls and other all vineyard operations were conducted according to local standard practices.

The climate of the region where the research was performed is temperate. In 2016 year, annual mean temperature, relative humidity and total precipitation of Tekirdag were successively recorded as 15.54 °C, 80.95% and 791.20 mm.

Yield and wine grape quality characteristics considered in research

In available study, grape length (cm), grape width (cm), grape weight (g) and cluster length (cm), cluster width (cm), cluster weight (g) were measured as yield parameters. Furthermore, total soluble solids content (°Brix), titratable acidity (g/L), pH, total soluble solids content × pH2, total phenolic compounds content (mg GAE/kg fw) and total anthocyanin content (mg GAE/kg fw) were found out as wine grape quality parameters.

Foliar applications of oak and urea and application times

In present study, uniform grapevines were selected for the foliar applications and it was utilized from different doses of foliar oak applications with urea, consisting: 1–0 ppm oak (Oa), 2–1250 ppm oak (Oa), 3–2500 ppm (Oa), 4–5000 ppm (Oa), 5–0 ppm oak (Oa) + 0.6% Urea (Ur), 6–1250 ppm oak (Oa) + 0.6% Urea (Ur), 7–2500 ppm oak (Oa) + 0.6% Urea (Ur), 8–5000 ppm oak (Oa) + 0.6% Urea (Ur).

In order to prepare aqueous extracts of both oak and urea, powdered Oak (Artu Chemical®, 65% tannin, 24% non-tannin, 5% insoluble solids and 6% humidity) and urea (Sigma-Aldrich, 98%) were used. Foliar spraying applications of oak and urea on leaves of grapevines were carried out two times at verasion period and 15 days after verasion by using a back pump.

Grape sampling and storing of grapes until analyses

When the clusters on control grapevines of cv. Cabernet Sauvignon achieved approximately 23%, the grapes were harvested at technological maturity. After the grapes were harvested, standard measurements and chemical analyses for grapes were immediately managed. However, it was taken advantage of 300-grape samples to appraise total phenolic compounds content and total anthocyanin content. All grape samples were stored at −25 °C up to analyses of total phenolic compounds and total anthocyanin. Prior to these analyses, grape samples were thrown out from −25 °C, allow to thaw overnight at 4 °C and then homogenized in a commercial laboratory blender for 20 s.

Analyses of total phenolic compounds and total anthocyanin compounds by spectrophotometer

In current study, spectrophotometric methods explained by Singleton et al. (1978) and Di Stefano and Cravero (1991) were successively made use of defining total phenolic compounds content and total anthocyanin content in grapes of cv. Cabernet Sauvignon. Results of both analyses were expressed as milligrams of gallic acid equivalent per kilogram of fresh weight (mg GAE/kg fw).

Statistical analysis

All statistical analyses were performed by using TARIST statistical software program. Differences among the means were compared by using Fisher’s Least Significant Difference (LSD) multiple comparison test at 5% level.

Results and Discussion

Yield characteristics

As shown in Table 1, grape length and grape weight among the yield parameters were significantly affected by different doses of oak application combined with urea application (p < 0.05).

Table 1 Yield parameters of cv. Cabernet Sauvignon grapes from different doses of foliar oak and urea applications
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Grape cultivars differ in terms of grape sizes (Amone and Simonetti-Bryan 2013). In this study, the different doses of oak and urea applications had significant effects on grape length (p > 0.05) and the highest means were obtained from applications of 2500 ppm Oa + 0.6% Ur (13.51 mm) and 5000 ppm Oa + 0.6% Ur (13.55 mm) than 0 ppm Oa (12.24 mm) and 0 ppm Oa + 0.6% Ur (12.44 mm) (Table 1).

As far as grape width is concerned, there were no significant differences among the different doses of oak and urea applications (p < 0.05) and means ranged from 12.05 (0 ppm Oa) to 12.92 mm (5000 ppm Oa + 0.6% Ur) (Table 1).

Concerning grape weight, significant differences were observed among the different doses of oak and urea applications (p < 0.05) and applications of 2500 ppm Oa + 0.6% Ur and 5000 ppm Oa + 0.6% Ur brought about the highest increases in grape weight (1.55 and 1.57 g) than 0 ppm Oa (1.33 g) (Table 1).

Cluster sizes in grape cultivars can be vary depending on grape cultivar and ecological factors and viticultural practices (Gougoulias and Masheva 2010). In available study, cluster characteristics were not significantly affected by the different doses of oak and urea applications (p < 0.05). In point of cluster length, means changed from 15.22 (0 ppm Oa) to 17.12 mm (5000 ppm Oa + 0.6% Ur) (Table 1).

As displayed in Table 1, different doses of oak and urea applications had not significant effects on cluster width (p < 0.05) and the highest mean was recorded for 5000 ppm Oa + 0.6% Ur (11.17 mm) compared to 0 ppm Oa (9.37 mm) (Table 1).

As regard to cluster weight represented in Table 1, significant differences were not observed among the different doses of oak and urea applications (p < 0.05). While 0 ppm Oa application was causing the lowest mean (179.19 g); the highest mean was obtained from 5000 ppm Oa + 0.6% Ur application (200.27 g).

Wine grape quality characteristics

Taking into account the wine grape parameters, it was seen that total soluble solids content and total soluble solids content × pH2 were significantly affected by different doses of oak and urea applications (Figs. 1 and 4; p < 0.05).

Fig. 1
figure 1

Effects of different doses of oak and urea applications on total soluble solids content of cv. Cabernet Sauvignon

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Grapes are harvested when they achieve a crucial stage for sale or processing. It is utilized from percentage soluble solids by weight as main measure of the grape’s suitability for harvest (Janick and Paull 2008). In this study, different doses of oak and urea applications significantly affected total soluble solids content (p < 0.05) and 5000 ppm Oa + 0.6% Ur application led to the highest mean (25.58 °Brix), whereas applications of 0 ppm Oa and 0 ppm Oa + 0.6% Ur were the lowest means as 23.00 and 23.32 °Brix (Fig. 1).

Titratable acidity measurement is one of the key determinants on when to harvest grapes and how to treat the harvested grapes and later the wine (Linskens and Jackson 1988). The range for titratable acidity in wine grapes is quite broad from 6 to 10 g/L (Butzke 2010). In actual study, different doses of oak and urea applications had not significant effects on titratable acidity of grapes (p < 0.05). Fig. 2 show the data for titratable acidity of grapes applied with different doses of foliar oak and urea. Titratable acidity in grapes of grapevines applied with oak combined with urea had lower than other applications. The lowest mean of titratable acidity was 9.16 g/L for 5000 ppm Oa + 0.6% Ur and the highest mean was obtained from 0 ppm Oa application (11.00 g/L).

Fig. 2
figure 2

Effects of different doses of foliar oak and urea applications on titratable acidity of cv. Cabernet Sauvignon

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Another way used to express acidity in grape must is pH. Cooke (2004) inform that suitable ranges for pH for quality winemaking vary between 3.30 and 3.50 in red wine grapes. In present study, although pH means were not affected by the different doses of oak and urea applications (p < 0.05); means ranged from 3.26 (0 ppm Oa) to 3.46 (5000 ppm Oa + 0.6% Ur) (Fig. 3).

Fig. 3
figure 3

Effects of different doses of foliar oak and urea applications on pH of cv. Cabernet Sauvignon

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Coombe et al. (1980) declare that equation of total soluble solids × pH2 is proper indicator of optimum ripeness in wine grapes. The best wines are made at index values ranging from 200 to 270 in red wine grapes. In this study, different doses of oak and urea applications significantly affected total soluble solids content × pH2 (p < 0.05) and while the lowest mean of total soluble solids content × pH2 was 233.05 for 0 ppm Oa application, the highest means were successively recorded for applications of 1250 ppm Oa + 0.6% Ur (261.25), 2500 ppm Oa + 0.6% Ur (266.02), 5000 ppm Oa + 0.6% Ur (270.64) and 5000 ppm Oa (270.90) (Fig. 4).

Fig. 4
figure 4

Effects of different doses of foliar oak and urea applications on total soluble solids content × pH2 of cv. Cabernet Sauvignon

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Phenolic compounds are an integral part of grapes and wines, contributing to colour, taste and stability of wines (Botes 2009). There can be important varietal differences within V. vinifera L. in terms of phenolic compounds content (Packer 2002; Kok 2017; Kok and Bal 2017a) and phenolic compounds level in grapes is also affected by environmental factors (Downey et al. 2006) and viticultural practices (Kok 2011; 2016; Kok et al. 2013; Kok and Bal 2016, 2017b). In current study, it was observed that the different doses of oak and urea applications had no significant effects on total phenolic compounds content (p < 0.05). Among the applications, 5000 ppm Oa + 0.6% Ur application gave rise to the highest total phenolic compounds content (3659.05 mg GAE/kg fw) than 0 ppm Oa application (2999.39 mg GAE/kg fw) (Fig. 5).

Fig. 5
figure 5

Effects of different doses of foliar oak and urea applications on total phenolic compounds content of cv. Cabernet Sauvignon

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Anthocyanins are red compounds, responsible for the colour of red grapes and wines. These compounds are chiefly located in the vacuoles of grape skins (Tokuşoğlu and Hall 2010). Results of total anthocyanin content are displayed in Fig. 6 and there were no significant differences among the different doses of foliar oak and urea applications (p < 0.05). However, 5000 ppm Oa + 0.6% Ur application led to the highest total anthocyanin content (1954.85 mg GAE/kg fw) compared to 0 ppm Oa (1494.40 mg GAE/kg fw).

Fig. 6
figure 6

Effects of different doses of foliar oak and urea applications on total anthocyanin content of cv. Cabernet Sauvignon

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Conclusion

Phenolic compounds and anthocyanins are very crucial substances for wine grape quality. There have been various applications to improve grape quality and one of these applications used to increase wine grape quality is foliar oak applications.

In present study, it was tried to increase efficiency of foliar oak application’s doses by using low concentration of urea application. The research results shown that increasing doses of foliar oak applications combined with low urea concentration enhanced quality parameters of wine grape. From the study findings, it could be concluded that foliar application of 5000 ppm Oa + 0.6% Ur seems to be more effective in wine grape quality of cv. Cabernet Sauvignon.