The effect of nitrogen and sulphur fertilization on yield and quality of kohlrabi (Brassica oleracea, L.)

Efeito da adubagem com o nitrogénio e enxofre ao rédito e à qualidade das couves-rábanos (Brassica oleracea, L.)

SEÇÃO IV - FERTILIDADE DO SOLO E NUTRIÇÃO DE PLANTAS

The effect of nitrogen and sulphur fertilization on yield and quality of kohlrabi (Brassica oleracea, L.)

Efeito da adubagem com o nitrogénio e enxofre ao rédito e à qualidade das couves-rábanos (Brassica oleracea, L.)

Tomáš Losák^I; Jaroslav Hlušek^II; Stanislav Kráěmar^III; Ladislav Varga^IV

^IAssistente do Professor, Instituição da química agrária, perícia dos solos, microbiologia e alimentação das plantas, Universidade de Mendel de Agricultura e Silvicultura em Brno, Zemedelská 1, 61300 - Brno - República Checa (CZ). E-mail: [e-mail]

^IIProfessor, Instituição da química agrária, perícia dos solos, microbiologia e alimentação das plantas, Universidade de Mendel de Agricultura e Silvicultura em Brno,Zemedelská 1, 61300 - Brno - República Checa (CZ). E-mail: [e-mail]

^IIIProfessor, Instituição da engenharia de alimentação, Universidade de Tomás Bata em Zlín, Námìstí T.G. Masaryka 275, 76272 - Zlín - República Checa (CZ). E-mail: [e-mail]

^IVAssistente do Professor, Instituição da química agrária e alimentação das plantas, Universidade Eslovaca de Agricultura em Nitra, Tr. A. Hlinku 2, 94976 - Nitra - República Eslovaca (SK). E-mail: [e-mail]

SUMMARY

In a greenhouse pot experiment with kohlrabi, variety Luna, we explored the joint effect of N (0.6 g N per pot = 6 kg of soil) and S in the soil (25–35–45 mg kg^-1 of S) on yields, on N, S and NO₃^- content in tubers and leaves, and on alterations in the amino acids concentration in the tubers. S fertilisation had no effect on tuber yields. The ranges of N content in tubers and leaves were narrow (between 1.42–1.48 % N and 1.21–1.35 % N, respectively) and the effect of S fertilisation was insignificant. S concentration in the tubers ranged between 0.59 and 0.64 % S. S fertilisation had a more pronounced effect on the S concentration in leaf tissues where it increased from 0.50 to 0.58 or to 0.76 % S under the applied dose. The NO₃^- content was higher in tubers than in leaves. Increasing the S level in the soil significantly reduced NO₃^- concentrations in the tubers by 42.2–53.6 % and in the leaves by 8.8–21.7 %. Increasing the S content in the soil reduced the concentration of cysteine + methionine by 16–28 %. The values of valine, tyrosine, aspartic acid and serine were constant. In the S₀, S₁, and S₂ treatments the levels of threonine, isoleucine, leucine, arginine, the sum of essential amino acids and alanine decreased from 37 to 9 %. The histidine concentration increased with increasing S fertilisation. S fertilisation of kohlrabi can be recommended to stabilize the yield and reduce the undesirable NO₃^- contained in the parts used for consumption.

Index terms: ammonium sulphate, dose, tuber, leaves, nitrates, amino acid concentrations.

RESUMO

Deficiência aguda de S no solo tem sido observada na Europa desde os anos 1980. O couve-rábano é uma das plantas com maior exigência nesse nutriente e sua interação com o N é frequentemente relatada na literatura. Este trabalho foi conduzido em casa de vegetação visando testar o efeito da aplicação de S, na presença de N, na produção e qualidade de couve-rábano (Brassica oleracea, L., variedade Lua). As plantas foram cultivadas em vasos contendo 6 kg de solo, aos quais foram aplicados 0,6 kg de N e S para se obter os teores no solo de: 25 (teor natural), 35, ou 45 mg kg^-1. Foram avaliadas a produção e o teor de N, S e NO₃^- nas raízes e nas folhas e as alterações no teor de aminoácidos nas raízes. A produção de raízes do couve-rábano não afetada pela adubação com S. O teor de N nas raízes e nas folhas variou de 1,42–1,48 % e de 1,21–1,35 % de N, respectivamente, não sofrendo efeito significativo da adubação com S. O teor de S nas raízes oscilou entre 0,59–0,64 % de S. A aplicação de S influenciou positivamente o seu teor no tecido foliar, que apresentou estreita relação com os teores do nutriente no solo, chegando a 0,50; 0,58 e a 0,76 % de S, respectivamente, para os três teores testados. O teor de NO₃^- foi maior nas raízes do que nas folhas. O aumento das doses de S reduziu o teor de NO₃^- nas raízes, em 42,2 a 53,6 %, e nas folhas em 8,8 a 21,7 %. O aumento do teor de S no solo reduziu o teor de cisteína + metionina em valores que variaram de 16 a 28 %, mas não afetou os teores de valina, tirosina, ácido aspártico e serina. Com a variação dos teores de S entre 25 e 45 mg kg^-1 , houve decréscimo de treonina, isoleucina, leucina, arginina, e a soma de amino ácidos esenciais e alanina na faixa de 37 a 9 %. O teor de histidina aumentou com a elevação do teor de S no solo. A adubação do couve-rábano com S é recomendada sobre tudo para a estabilização da produção e para a redução dos teores de NO₃^-, cuja presença em alimentos é indesejada.

Termos de indexação: sulfato de amônio, dose, raízes, folhas; nitrato; aminoácidos.

INTRODUCTION

An acute deficiency of sulphur (S) in the soil has been observed in Europe since the 1980ies. In the Czech Republic this phenomenon was intensified after 1990. Vegetables requiring a higher supply of S began to respond very sensitively. One of the most demanding vegetables is kohlrabi, which absorbs 1.5 kg S per ton yield. It is the most S–dependent of all brassicas. The typical visual S deficiency symptom in kohlrabi is inhibited leaf formation and a light colour of the youngest leaves. The reduced soil content of available S is the result of reduced applications of mineral and organic fertilisers and fungicides (Hlusek et al., 2003).

The substantial decrease in SO₂ emission to less than 10 kg ha^-1 of S further intensified S deficiency in plants, because as much as 30 % of its total amount can be absorbed from SO₂ in the air. The S cycle and its effect on plants are often compared to N (oxidation in soil and reduction in plants). The main difference is that S from organic compounds can be re-oxidised to SO₄–S in plants (Vanìk et al., 2001).

Much like N, S is highly mobile in the soil with a limited sorption capacity. Plants take it up from the soil solution mainly in the form of sulphates (SO₄^2-) (Marschner, 1995). After reduction in the plant S participates in various primary and secondary compounds, such as the amino acids cysteine and methionine, vitamins B₁ and H and enzymes and coenzymes (Haneklaus et al., 1997). Other S-containing compounds are, e.g., tripeptide glutathione (antioxidant and precursor of phytochelatins, which are able to influence the detoxification of some heavy metals), ferredoxine, sulpho-lipids, glucosinolates and others (Schubert, 2002).

One of the essential amino acids, Methionine (C₅H₁₁O₂NS) is, together with cysteine (C₃H₇O₂NS), an indispensable component of proteins – their bonds play an important role in the protein structure where they often form intra or inter-chain disulphide bridges. The S content in proteins is relatively stable and in a certain proportion to N (Vank et al., 2001).

The balance of all macro and micro biogenic elements in the soil is crucial when growing kohlrabi, particularly in the case of N, which determines the yield level as well as the NO₃^- content. The production of one ton of kohlrabi draws 5 kg N. In terms of its use and losses it would be suitable to split the total amount of N in mineral fertilisers in doses and apply it in the soil prior to sowing (Hlusek et al., 2002). Schenk et al. (1991) reported that for kohlrabi the content of mineral N should be contained in the 0–0.3 m layer. The use of fertiliser N is inhibited in S-deficit crops. This could increase the escape of N into the environment, particularly by NO₃^- leaching into the hydrosphere or via gas losses into the atmosphere. Every missing kilogram of S, which limits plant growth, causes N losses of 4–15 kg (Schnug, 1991). Under S deficiency higher N doses increase the deficiency (Janzen & Bettany 1984), resulting in further yield reduction.

The nitrate (NO₃^-) concentration in vegetables is an important quality criterion (Schuphan, 1976; Vetter, 1988). In plant chloroplasts S is a component of the enzyme nitrite reductase which is responsible for the reduction of NO₂^- to NH₃ (Mengel & Kirkby, 1978). Paulsen (2001) pointed out the risk of increased levels of non-protein N in plants. Under S deficiency, the NO₃^- content can increase and after reduction to NO₂^-, block the ability of haemoglobin to transfer oxygen, or form the first degree of carcinogenic nitrosamines. Schnug (1990) also stated that the NO₃^- content in vegetable tissues increased under acute S deficiency.

The objective of the greenhouse pot experiment with kohlrabi was to test the effect of joint N and S fertilisation on kohlrabi yields, NO₃^- content in tubers and leaves, and changes in the amino acid concentrations.

MATERIALS AND METHODS

The greenhouse pot experiment with kohlrabi was established on 24 March 2005 in the greenhouse of the Department of Agrochemistry, Soil Science, Microbiology and Plant Nutrition at the Mendel University of Agriculture and Forestry in Brno. Mitscherlich pots were filled with 6 kg soil (45 % clay) characterised as fluvial soil. The agrochemical properties are given in Table 1.

The soil was analyzed for CH₃COOH, NH₄NO₃, NH₄F, HNO₃ and EDTA by Mehlich III extraction (Mehlich, 1984). Colorimetry was used to determine the content of available P in the extract and atomic absorption spectrophotometry (AAS) to determine available K, Mg and Ca. The micronutrients Zn and Fe were extracted using the Lindsay-Norwell procedure (Richter et al., 1999) and the AAS method was used for their determination. Extraction with de-mineralised water at a ratio of 1:5 for 16 h on a rotation shaker preceded the determination of SO₄–S in the soil. For the actual measurements we used capillary zone electrophoresis (CZE) with silica capillary. The activity of hydrogen ions was detected in a CaCl₂ (c = 0.01 mol dm^-3) soil extract by a potentiometer with a glass electrode against a reference electrode. The soil showed alkali soil reaction (pH) and the content of available P was high. The K, Ca, Mg, Zn, and Fe levels represented a good nutrient level.

The experiment involved three treatments, all of which were fertilised at the same level (0.6 g N per pot) in the form of ammonium nitrate (34.5 % N). Table 2 shows the experimental treatments. Treatment 1 (S₀) was not fertilised with S (natural content in the soil-25 mg kg^-1 of SO₄–S). In treatments 2 (S₁) and 3 (S₂) the SO₄–S in the soil was increased to the required level (35 and 45 mg kg^-1 of SO₄–S, respectively) by applying ammonium sulphate (20.5 % N and 24 % S, respectively). This is the most widespread nitrogen-mineral fertiliser with a physiological acid reaction, which is particularly suitable for neutral to alkali soils.

Kohlrabi variety Luna was sown in pots on 29 March 2005. Fertilisers were applied via irrigation solution on 9 May 2005, after previous thinning to 3 plants per pot at the 4-leaf stage. The experiment involved 3 treatments, each of which was repeated 4 times. Plants were regularly irrigated with de-mineralised water during the growing season to 60 % of the maximal capillary capacity. The pots were monitored for water content every day. The top soil layer (0.02 m) was loosened with a special garden fork five times, together with weeding. Plants were protected against aphids (Aphis) with Pirimor (0.05 %).

The tubers were harvested at full maturity on 27 June 2005. Immediately after harvest the individual tubers without leaves were weighed. Nitrate concentration (mg kg^-1 of NO₃^-) was determined in the fresh matter tubers and leaves by a potentiometer using ion selective electrode (ISE). The raw tubers were chopped, then lyophilised and homogenised in a fragmentation mill. The methods for laboratory feeds testing of were applied according to regulations of the Ministry of Agriculture of the Czech Republic (Ministerstvo, 2001).

Samples for amino acid determination were adjusted using acidic and oxidative acidic hydrolysis HCl (c = 6 mol dm^-3). The chromatographic analysis of sample hydrolysates was performed in the analyzer AAA 400 (manufacturer INGOS Prague, CR) and using Na-citrate buffers and ninhydrin detection (Official Journal..., 1978; Krámar et al., 1998).

All results were evaluated using the analysis of variance (ANOVA). Correlation matrices and regression functions were calculated according to Snedecor & Cochran (1967) by the statistical software package Unistat, v.5.1 - UNISTAT® Limited, London, England (Minaík & Hofbauer, 2003).

RESULTS AND DISCUSSION

A typical characteristic of kohlrabi is the high withdrawal of N from the soil (Feller & Fink, 1997) and any NO₃-N deficiency in the soil reduces yields (Steingrobe & Schenk, 1991). Sharof & Weir (1994) studied the minimum amount of N required for vegetable crops including kohlrabi in relation to components of N balance in the soil and calculated that N requirement values were invariably lower than N soil levels. Results of yields are listed in table 3.

The tuber weight in fresh matter was not significantly affected by S application with a slight variation between 106.8 and 108.5 g (Table 3). Nevertheless Prásková (2006) reported that in a pot experiment with kohlrabi the yields increased from 2.7 to 8.5 % under S doses increasing from 13.7–20–40 mg kg^-1. Likewise, Hlusek et al. (2003) stated that kohlrabi tuber yields increased by 10.5 % when the S soil content increased from 13.7 to 40 ppm and the N level was 0.75 g N per pot. Contrastingly, Sanderson & Carter (2002) discovered that yields of brassicas were not stimulated by S application.

The NO₃^- concentration in plants is affected primarily by the vegetable species, level of N fertilisation, the respective plant organ, growth stage and S concentration in the tissues. A negative linear correlation was discovered between NO₃^- in the tissues and the S concentration in plants (Schnug, 1990). Our results proved that the NO₃^- content was highest in the control treatment without S fertilisation, both in tubers and leaves (Table 3). More NO₃^- was stored in the tubers than in leaves and with an increasing soil S level the concentrations dropped significantly by 53.6 % in the tubers and by 21.7 % in the leaves (Treatment S₁). Together with an increasing dose of S the tuber/leaf ratio in the NO₃^- content narrowed down from 1.76 (Treatment S₀) to 1.12 (Treatment S₂). In a kohlrabi experiment Hlusek et al. (2003) also discovered that the NO₃^- content in tubers decreased by 14.2 % at a level of 40 ppm S in the soil and 0.75 g N per pot. Losák (2005) reported a drop in NO₃^- contents in onion by 13.1 % related with an increase in the SO₄–S level in the soil to 40 ppm.

The S doses did not affect the N concentration in kohlrabi tubers, which varied in a narrow range between 1.42 and 1.48 % N (Table 4). For the S contents the differences among treatments were not significant either (0.59–0.64 % S) and the N/S ratio was also narrow, i.e. 2.22–2.50. In contrast, Smatanová et al. (2004) reported in increase in the S content in peppers from 0.08 % S to 0.24 % S associated with an increased S content in the soil from 7.8 to 30.6 ppm.

The N content in kohlrabi (Table 5) was lower in leaves than in tubers and increased to 1.25–1.35 % N at higher S doses. Higher S soil doses were reflected in an increase of leaf concentration from 0.50 % S (Treatment S₀) to 0.76 % S (Treatment S₂) while simultaneously the N/S ratio decreased from 2.42 to 1.77; see Table 5. S deficiency symptoms on the youngest leaves were only observed towards the end of the growing season – treatment S₀. Rosen et al. (2005) stated that the N dose has no effect on S concentration in cabbage tissues, which depended only on the applied dose of 110 kg ha^-1 of S. Sanderson & Carter (2002) also discovered that the S content in brassica tissues increased after S fertilisation in the form of gypsum. Hlusek et al. (2003) detected that the S content increased in Chinese cabbage after S application to 0.89 % S compared to 0.71 % S in the control treatment without S fertilisation, and simultaneously a lower N/S ratio, decreasing from 4.84 to 3.40. Likewise Sanderson & Ivany (1996) observed that after the application of potassium sulphate and gypsum the S concentration in cabbage leaves increased to 0.70 % and 1.45 % S respectively.

The effect of N and S fertilisation on the spectrum of amino acids in kohlrabi tubers is presented in Table 6. shows that with the given level of N nutrition and increasing S level in the soil (S₀, S₁ and S₂), the percentage sum (g/16 g N) of the content of sulphurous amino acids å cysteine + methionine decreased by 16–28 % Table 6. Cysteine (S₀:S₁ = by 23 %; S₀ : S₂ = 26 %) and methionine deceased in the same order by 9 and 30 %, resp., and glycine also decreased by 6–29 %. These results correspond to the findings of Smatanová et al. (2004) who also stated that the content of methionine in spinach decreased from 0.76 to 0.66 g kg^-1 when the levels of S in the soil increased from 7.8 to 30.6 mg kg^-1 of SO₄–S, simultaneously reducing the cysteine content from 0.58 to 0.45 g kg^-1 as a result of the increasing S level in the soil from 20.6 to 30.6 ppm. Losák & Ducsay (2005) also observed a statistically significant decrease in the methionine concentration in onion tubers by 6.8 % at the highest level of S in the soil (60 mg kg^-1 of SO₄–S). On the contrary, Eppendorfer & Eggum (1995) reported that in plant tissue under S deficiency the cysteine concentrations (g/16 g N) declined more than methionine.

In other essential amino acids at different levels of S fertilisation virtually the same values of amino acids valine and tyrosine and of the non-essential amino acids aspartic acid and serine were observed. No substantial differences between treatments S₀ and S₁ were found in threonine, isoleucine, leucine, arginine, åEAA and alanine, however between treatments S₀, S₁ and S₂ the content dropped in a range of 37–9 % in all cases. The content of amino acid lysine increased between treatments S₀ and S₁ by 28 %, that of proline by 10 %; on the contrary, from S₀ to S₂ lysine decreased by 37 % and proline by 26 %, and between treatments S₁and S₂ lysine dropped 50 % and proline 33 %. The only essential amino acid content that increased with increasing S doses was histidine. The increase observed on S₁ and S₂ treatments as compared with S₀ was 21 % and 44 %, respectively; from S₁ to S₂ the in increase was 9 %. In treatment S₂ the glutamic acid content increased extremely, by more than 150 %, compared to the S₀ and S₁ treatments. The high content of åNEAA under treatment S₂ was probably affected by the high concentration of glutamic acid. The phenylalanine concentrations ranged between 0.07 and 0.39 %. Eppendorfer & Eggum (1994) also explored the effect of different levels of N, P, K and S on the amino acid content in potato tubers. They stated that increased S doses had a negative effect on cysteine, methionine, lysine and leucine concentrations, but raised the glutamic acid concentration.

CONCLUSIONS

The result of a one-year greenhouse pot experiment was that S fertilisation contributed to stabilise kohlrabi tuber yields of while simultaneously reducing NO₃^- contents both in the consumable parts and leaves considerably. The concentrations of amino acids, with exception of cysteine, methionine, å cysteine + methionine and glycine, did not change. It would also be appropriate to apply S-containing mineral fertilisers in doses of 45–90 kg ha^-1 of S for kohlrabi cultivation.

ACKNOWLEDGEMENTS

This study was supported by the Research plan No. MSM6215648905 "Biological and technological aspects of sustainability of controlled ecosystems and their adaptability to climate change", which is financed by the Ministry of Education, Youth and Sports of the Czech Republic.

LITERATURE CITED

Received for publication in june of 2007 and approved in november of 2007.

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Publication Dates

• Publication in this collection
  01 July 2008
• Date of issue
  Apr 2008

History

• Accepted
  Nov 2007
• Received
  July 2007