EFFECT OF THREE SUBSTRATES ON FRUIT AND LEAF CHEMICAL COMPOSITION OF HIGHBUSH BLUEBERRY 'SIERRA' CULTIVAR

Ireneusz Ochmian¹, Józef Grajkowski², Katarzyna Skupień^3
1 Department of Horticulture, West Pomeranian University of Technology in Szczecin
² Department of Pomology, Agricultural University of Szczecin, Poland
³ Laboratory of Plant Raw Materials Processing and Storage, Agricultural University, Szczecin, Poland

ABSTRACT

The objective of the study was to assess the influence of peat, sawdust and cocoa husk substrates on leaf and fruit chemical composition of blueberry 'Sierra' cv. Regarding minerals content, the plants grown in cocoa husk substrate displayed the highest content of N, P, K both in leaves (25.35, 1.22, and 9.19 g kg^-1, respectively) and berries (16.22, 1.21, and 7.90 g kg^-1 respectively) as well as Mn in the leaves (115.5 mg kg^-1). The berries obtained from bushes cultivated in sawdust showed the highest Ca (1.73 g kg^-1), Mg (1.25 g kg^-1), Zn (10.75 mg kg^-1), and Fe (60.49 mg kg^-1) content in the leaves, and Cu in the leaves and berries (1.97 and 2.40 mg kg^-1, respectively). On the other hand, fruit originating from bushes grown in peat sub-strate had greatest leaf and berry S status (2.18 and 1.48 mg kg^-1, respectively) and Ca and Mg content in fruit (0.14 and 0.42 g kg^-1, respectively). In contrast, the substrates did not affect considerably Fe and Mn content in berries. Comparing nutritive value of fruit it could be concluded that the usage of sawdust substrate yielded the berries of highest soluble solids (15.3%), total sugar (12.58 g 100g^-1), titratable acidity (1.16%) and antioxidant capacity (42.35 µmol Trolox g^-1). However, the greatest amount of vitamin C (34.1 mg 100g^-1) was found in berries originating from the plants grown in peat substrate. No effect of substrate was observed regarding juice extraction efficiency (88.1-88.7%). The all berries did not surpass allowed nitrate and nitrite levels.

Key words: Vaccinium corymbosum, fruit chemical composition, macro- and microelements, mulch.

INTRODUCTION

Due to dietary value and antioxidant activity the highbush blueberry fruit are regarded one of the healthiest food products [10,17]. Because of its value, the species has been cultivated for years in North America. The first breeding project was launched in 1908 in Florida [15] and from USA highbush blueberries were imported to Europe [33]. In Germany, the highbush blueberries were introduced in the twenties [14]. In Poland, first experiments on highbush blueberry cultivation were undertaken in 1946, however, the development of a large-scale production methods started in 1976 [30]. In recent years, in Poland, a considerable increase of blueberries production has been observed. In 2006, blueberries production in Poland was 8,000 t and growing area covered 1300 ha [7]. Despite of low calorie content, the blueberries contain vitamins, minerals, sugars, organic acids, pectins and aromatic compounds [1,23]. In the blueberries 25 different anthocyanins were identified [5]. Hakkinen et al. [9] report that fresh fruit contain 400 mg of phenolics per 100 g depending on ripeness, fruit size, and cultivar. According to Mainland et al. [17] antioxidant capacity is not a constant fruit feature. It is conditioned mainly by phenolics content and varies in dependence on genetic factors (variety, cultivar) [3] and environmental conditions [2].

A common feature of Ericaceae family species are specific soil requirements, different from that of other orchard plants. Blueberries are generally regarded as very sensitive to excessive fertility [8]. The bushes are ubiquitous in pine forests occurring on acid and moisture soils [24]. Because, such soils are less and less available establishing new plantations has become more and more difficult. Thus, a search and developing of modified soil system has become a necessity. In the United States, for mulching purpose the cotton by-products, pecan hulls [12], leaf-mould compost, a pit-coal ash, the sludge of sewage-treatment plant, a litter of conifer needles [4] as well as a peat [21] are used.

From the practical and economical point of view, the components used for mulching should be relatively cheap, easily accessible, and should meet habitat requirements of the species. Therefore, the usage of agricultural and forest by-products for this purpose seems to be especially justified. The objective of the present study was to evaluate the different substrates (a cocoa hull, a sawdust and a peat) on chemical composition of fruits and leaves of highbush blueberry cv 'Sierra'.

MATERIAL AND METHODS

The experiment was established in spring 2001 at the Experimental Station Rajkowo belonging to Agricultural University of Szczecin. The purpose of the field trial was to evaluate highbush blueberry cultivation in a neutral reaction (pH 7.1) heavy soil using three different substrates. The bushes of 'Sierra' cultivar were planted on the hill plasticulture system with raised beds 35 cm high and 100 cm wide. The bushes were spaced 1.5 m apart in the row and 2.5 m apart between the centers of the beds. In this culture system, three types of substrates were used: the acid muck soil (peat), the conifer sawdust obtained from a local sawmill, and the cocoa husk a by-product obtained from Chocolate Confectionary Plant 'Gryf' in Szczecin. The experiment was carried out in 2004-2006. Physical properties of the substrates utilized in the experiment are shown in Table 1.

Among the substrates tested, the greatest field water capacity was found for peat (44.8%) and the least for sawdust (31.3%). Regarding full water capacity, cocoa husk substrate was predominant (85.3%). These differences necessitated diversifying in substrates watering.

The fertilization of the plants was limited to nitrogen supply only, because chemical analyses both of the soil and the substrates showed high and/or middle content of other nutrients. Each type of media used in the raised bed system was fertilized with the ammonium nitrate on three occasions at a total dose of 30 kg N ha^-1. The fertilizer was spread evenly on the bed tops at the width of 1 m. The content of nutrients in particular substrates is presented in Table2. In comparison to mineral soils [22], the organic media used in this trial were especially rich in K, Ca and Mg.

The supplemental irrigation was applied through the drip line type T-Tape with acidified water (with H₂SO₄ up to pH 2.5-3.5 measured in H₂O). The intensity of water supply was adjusted to soil moisture by means of the tensiometer Nieuwkoop B.V. (AALSMEER Holand) manual monitoring twice a week. Measuring tubes (30 cm) were installed 15 cm below the soil surface and 2.2 PF was used as a threshold value for irrigation. Having reached the threshold, the soil was irrigated to approximately 1.0 PF.

The water used for peat substrate irrigation had higher pH (3.72) because peat reaction was suitable for blueberry cultivation. Whereas, for cocoa husk and sawdust substrate a water of pH 2.36 was used due to decrease these substrates reaction (Table 3). Among the substrates tested, the peat maintained a constant pH 3.3-3.5 during field trial, while, the highest reaction throughout the all experiment was observed for cocoa husk substrate (4.6-6.4) (Table 4).

One hundred leaves out of plants growing at particular substrate were sampled each year at the beginning of August for chemical analyses. The fruit of each harvest in the season were packed in polyethylene bags (250 g) and kept frozen (-25°C) until analyzed. Having completed fruit collecting, the all fruit samples were combined, thawed at room temperature, and then dried (initial temperature 70°C, the final 105°C). The dried fruit were pulverized (Lab mill WŻ-1). The macro- and microelements content was determined according to Polish Standards guidelines. The dried plant material was mineralized in the mixture of H₂SO₄ and H₂O₂ for nitrogen determination, whereas, mineralization in the mixture of HNO₃ and HClO₄ (3:1) was performed for P, K, Ca, Mg, S, Cu, Zn, Fe and Mn estimation. After mineralization total nitrogen content was determined with the Kjeldahl method. The content of K and Ca was measured with the atomic emission spectrometry, whereas Mg, Cu, Zn, Fe and Mn content with the flame atomic absorption spectroscopy using Solaar S AA. Phosphorus content was determined with the Barton method at wavelength 470 nm, whereas sulphur content with the turbidimetric method at wavelength 490 nm employing spectrophotometer Marcel s 330 PRO.

Moreover, in fresh fruit right after the harvest titratable acidity, total sugar, soluble solids, L-ascorbic acid, nitrate and nitrite content and antioxidant capacity was determined. The titratable acidity was determined by titration of water extract of blueberry homogenate with 0.1 N NaOH to the end point of pH 8.1 (measured with an Orion 720 A pH meter; Orion Research Incorporated, Boston, MA, USA) according to PN-90/A-75101/04. The total sugar content was determined according to the Loof-Schoorl method. The soluble solids content was determined in the berry juice by means of an Abbé refractometer (PN-90/A-75101/02). The L-ascorbic acid content was determined with the iodometric method [27] by titration of metaphosphoric acid fruit tissue extract with 0.001 M KJO₃ towards alkaline starch solution (5% starch in 5% NaOH) and 2 M KJ. The nitrate and nitrite content was measured by means of requantometer RQflex 10 (Merck). The fruit antioxidant capacity was evaluated by metmioglobine oxidation inhibition by antioxidants present in fruit extract compared to that of Trolox (a synthetic antioxidant, a vitamin E analogue) [18]. Total antioxidant capacity was expressed as µmol Trolox per 1 g fruit tissue.

For juice extraction efficiency fruit were homogenized with a blender and heated up to 50°C. Then, after cooling, 3 ml of pectinase (Rapidase Super, BE, NC, USA) per kg of pulp were added. The pulp was left to stand at a room temperature for 1 hour. Afterward, the pulp was pressed for 10 min at the final pressure of 300 kPa by means of a laboratory hydraulic press [20].

The results obtained were subjected to statistical analysis using Statistica 7.1 (Statsoft, Poland). Each year the study trial was a randomized block design with three replications (4 bushes per replication). Analysis of variance 3-year synthesis for fixed model was used. The values were evaluated by the Duncan test and the differences at P < 0.05 were considered significant.

RESULTS AND DISCUSSION

In general, the leaves exhibited higher accumulation of macro- and microelements compared to berries (Table 5). Both fruits and leaves originating from bushes grown in cocoa husk substrate were characterized by the highest total N, P and K content. According to Hanson [8] the sufficient range for leaves collected in mid-summer is between 1.7-2.1% N. In this assay similar values were observed in leaves of plants grown in peat and sawdust (~2.2% N after recalculation). The leaves obtained from cocoa husk bedding displayed 2.5% N (after recalculation). Nitrogen content in fruit varied 1.5-1.6% (after recalculation); whereas, Skupień [29] obtained 1.7-2.8% N divergence in four cultivars of blueberries. In this study, P leaf status ranged 0.97-1.22 g kg^-1 and K leaf status 5.56-9.19 g kg^-1. Stępień and Mercik [32] determined similar status of P in blueberry leaves of NPK trial (1.2 g kg^-1 after recalculation) and lower for K (5.1 g kg^-1 after recalculation).

Further, the greatest amount of Ca and Mg was found in the leaves of plants cultivated in sawdust (1.73 and 1.25 g kg^-1, respectively) and in berries grown in peat substrate (0.14 and 0.42 g kg^-1, respectively). For 'Spartan', 'Bluecrop', 'Jersey' and 'Blueray' fruit Skupień [29] determined similar Ca (1.27-1.91 g kg^-1 after recalculation) and lower Mg content (0.16-0.18 g kg^-1 after recalculation).

The usage of peat in the assay favored S accumulation in the leaves and berries (2.18 and 1.48 g kg^-1, respectively) compared to other substrates. Glonek and Komosa [6] found 1.1-1.2 g S kg^-1 leaf d.w. (after recalculation), nevertheless, fertilization rate applied.

The highest level of Cu was measured in leaves and fruit when sawdust medium was utilized (1.97 and 2.40 mg kg^-1, respectively). Regarding Zn content, the greatest amount was observed in leaves of plants grown in sawdust (10.75 mg kg^-1) and berries of cocoa husk raised bed system (10.89 mg kg^-1). Skupień [29] found much lower fruit Cu (0.17-0.30 mg kg^-1) and Zn range (1.08-1.30 mg kg^-1). On the other hand, Levula et al. [13] determined comparable Zn concentration (11 mg kg^-1) in control lingonberries.

The leaves of bushes grown in sawdust showed the highest Fe content (60.49 mg kg^-1), whereas, the usage of cocoa husk enhanced Mn (115.50 mg kg^-1) level in the leaves. Glonek and Komosa [6] found similar Fe (53.9-57.7 ppm) and Mn (107.6-128.0 ppm) leaf status that was slightly increased in fertilized plants compared to control ones. In contrary, none of media tested in this assay exerted considerable, in practical terms, influence on Fe and Mn content in fruits.

Data on chemical composition of blueberry fruit are listed in Table 6. The berries obtained from plants grown on sawdust substrate showed the highest soluble solids (SS) content (15.3%). On the other hand, berries originating from plants cultivated on cocoa husk had the lowest SS content (13.7%), despite of the highest amount of K, Zn and Mn – the minerals stimulating sugars synthesis in the plants [31]. Apart from agronomic conditions, genetic factors also exert influence on SS content. Skupień [28] determined SS content ranging from 11.6% ('Spartan') to 13.80 ('Blueray') for berries of bushes mulched with sawdust. Much higher divergence was found by Prior et al. [23] 10.0% ('Duke') – 19.0% ('Rancocas') reflecting both cultivar, and geographical site, and harvest date influence. From the practical point of view the type of substrate had only negligible effect on total sugar content in blueberries. However, the amounts observed in this study (12.22-12.51 g 100 g^-1) overrated the quantities reported by Ostrowska and Ściążko [19] for 'Bluecrop' (8.36 g 100 g^-1) and 'Jersey' (9.57 g 100 g^-1) mulched with sawdust and peat.

The bushes grown on cocoa husk substrate yielded berries with lowest total acid content (0.70 % citric acid). This effect was paralleled by the highest pH of cocoa husk substrate (4.6-6.4). However, berries grown on sawdust (pH 3.9-4.9) showed the highest titratable acidity (1.16 % citric acid), though, the lowest pH had peat substrate (3.3-3.5). Rosenfeld et al. [25] determined similar acidity 0.82% for 'Blueberry' cv stored at 4°C.

In this study, berries obtained from bushes grown on peat and sawdust showed higher vitamin C content (34.1 and 31.8 mg 100 g^-1, respectively) compared to that of cocoa husk (22.4 mg 100 g^-1). Apart from cultivar and type of substrate, weather conditions also affect ascorbic acid content. Łata et al. [16] observed for 'Darrow' and 'Bluecrop' in 2001 16.6 and 16.6 mg 100 g^-1 (recalculated), whereas, in 2002 30.6 and 25.2 mg 100 g^-1, respectively.

Juice extraction efficiency was high 88.7% (cocoa husk) – 88.1% (sawdust) and substrate type had no practical effect. Rossi et al. [26] report blueberry juice yield 79-81%.

Similarly, the substrates did not affect considerably antioxidant properties of berries. TEAC (Trolox equivalents antioxidant capacity) values ranged from 37.63 (peat) to 42.35 µmol Trolox g^-1 (sawdust). For 'Bluecrop' cv Kalt et al. [11] determined higher antioxidant activity (60.1 µmol Trolox g^-1 f.w.). Ścibisz et al. [34] found significant differences in antioxidant capacity depending on the harvest date. The berries of 'Earlyblue'cv from 1^st crop showed 29.9 µmol Trolox g^-1, whereas, that of 3^rd harvest 61.8 µmol Trolox g^-1. Along with crop season a progressive lessening of berries size is a common occurrence. Taking into account that anthocyanins and other phenolics are mainly concentrated in blueberry skins smaller fruit show higher antioxidant capacity because of higher surface:volume ratio.

The substrate had a substantial effect on nitrate accumulation in blueberries, however, the values were very low. The highest level of NO₃^- was determined in berries obtained from the plants grown on sawdust (37.8 mg kg^-1), while, the lowest for peat (19.7 mg kg^-1). Ostrowska and Ściążko [19] also found very low nitrate content in blueberries varying from 15.5 to 22.7 mg kg^-1. Regarding fruits, Polish law regulates permissible nitrate amount only for bananas (up to 200 mg kg^-1). On the other hand, for low-accumulation vegetables aimed for babies and young children the allowed content must not exceed 200 mg kg^-1.

In contrast, the substrates did not affect considerably nitrite content in blueberries 0.80 mg kg^-1 (peat) – 1.00 mg kg^-1 (sawdust). However, the values obtained also met Polish regulations allowing up to 1 mg NO₂^- per kg for apple juice, fruit-vegetable juices, and banana-containing products.

CONCLUSIONS

1. The usage of cocoa husk substrate resulted in highest content of N, P, K in blueberry ‘Sierra’ leaves and berries and Mn in the leaves.

2. The plants grown in sawdust substrate showed the highest level of Ca, Mg, Zn, and Fe in the leaves as well as Cu both in the leaves and berries.

3. The fruit originating from plants grown in sawdust exhibited enhanced soluble solids and titratable acidity content, and highest antioxidant capacity.

4. The fruit obtained from plants cultivated in peat showed the greatest Ca and Mg content. Also, these plants yielded fruit and leaves with elevated amount of S and berries with the highest vitamin C content.

5. The media affected neither Fe, nor Mn level in fruit, nor juice extraction efficiency. Nevertheless the substrate used, nitrate and nitrite content in berries met food law regulations.

6. Three-year study carried out on 5-year old highbush blueberry plantation showed possibility of cultivating this species in a compact soil with an alkaline reaction provided that the plants were grown in peat or sawdust media and drip irrigation with an acidified water was applied. The hill plasticulture system made of cocoa husk showed lesser suitability for this purpose because of too high substrate reaction and nutrients excess.

ACKNOWLEDGEMENTS

The study was supported by the grant of the Scientific Research Committee No.0395/P06/2004/26.

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Accepted for print: 23.10.2008

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