Effect of nitrogen and sulphur application on the glucosinolate content in vegetable turnip rape (Brassica rapaL.)

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Abstract
Effect of Nand S application on the total glucosinolate (GSL) content in the edible parts of vegetable turnip rape cv. No. 21 (Brassica rapa L.) was studied. Plants were grown with different amounts and ratios of Nand S in a glasshouse. The individual GSLs present were quantified by HPLC. The total GSL content ranged widely from 28.4 to 80.0 mmol kg−1 DW. In the 10 mm N (N1) treatment with three levels of S (0.5, 1, and 2 mm), the total GSL content linearly increased by larger S application for all harvest dates. The total GSL content in the 10 mm N with 0.5 mm S (N1S1/2) treatment was the lowest. On the other hand, even though the total GSL content in the 30 mm N (N3) treatment also increased by larger S application, there was no significant difference between the 30 mm N with 1 mm S (N3S1) treatment and the 30 mm N with 2 mm S (N3S2) treatment. The total GSL content increased gradually with harvest dates in the N3 treatments. Among the 8 GSLs identified, gluconapin and glucobrassicanapin, which belong to aliphatic GSLs, were the main components. The sum of the relative proportions of five aliphatic GSLs ranged from 79 to 92% while that of two indolyl GSLs (4-hydroxyglucobrassicin and glucobrassicin) ranged from 2.0 to 5.4% of the total GSL content. Indolyl GSL content increased with larger N application. Leaf N content in the N1 treatment was significantly lower (38 g kg−1 DW) than that of the N3 treatment with an average value of 60 g kg−1 DW. However, the variations in the S application did not result in changes in the leaf N content for both the N1 and N3 treatments. In contrast, the S content of the leaves significantly increased with larger S application in both the N1 and N3 treatments. In the 2 mm S (S2) application, the leaf S content decreased by 43% from 13.1 in the N1 treatment to 7.46 g kg-1 DW in the N3 treatment. These results indicated that the GSL content is strongly affected by N application as well as S application.

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