Glucosinolates in Turnip Tops and Roots: Cultivars Grown for Greens and/or Roots

Fourteen cultivars of turnip (Brassica rapa, rapifera group, also B. campestris L. ssp. rapifera (Metzg.) Sinsk.) recommended for human consumption of either tops or tops and roots and five cultivars recommended for consumption of roots were selected to compare glucosinolate (GS) levels in tops and roots. Also, two cultivars used for animal feed were included. The study revealed significantly lower levels of 1-methylpropyl-GS and 2-hydroxy-3­ butenyl-GS in tops and roots of cultivars grown for greens, compared to those used for animal feed. Contents of 1­ methylpropyl-, 3-butenyl-, and 4-pentenyl-GSs were higher in turnip tops than in roots, while 2-hydroxy-3-butenyl-, 4-(methylthio)butyl-, 4-(methylsulfinyl)butyl-, 2-hydroxy-4-pentenyl-, 5-(methylthio)pentyl-, 2-phenylethyl-, 3-indo­ lylmethyl-GSs and total GS were all higher in the roots. GS patterns for seeds tended to correlate with those of the tops. Early workers noted that turnips were goitrogenic in rabbits (7). Glucosinolates (GSs), whose breakdown products (Figs. 1 and 2) may be goitrogenic, are present in turnips (1). Some of these products have been tested in animals. The product 5-vinyl­ oxazolidine-2-thione (Fig. 2) increases thyroid and liver weights in rats (11), and thiocyanate (SCN) ion inhibits uptake of iodine by the rat thyroid (16). On the positive side, the GS hydrolysis products benzyl isothiocyanate (Table 1) and 2-phenylethyl iso­ thiocyanate inhibit chemical carcinogenesis in rats (20). The 1983, U.S. production of 5400 t of frozen turnip greens and 10,600 t of frozen turnip greens plus roots (15) points to the consumption of the entire plant and to the need for definitive GS analysis of the greens and roots of common cultivars. In prior work on turnips (1), data suggested a tendency for an absence of oxazolidine-2-thiones (OZTs) in cultivars rec­ ommended for human consumption, and, for two cultivars, the GS levels of the green tops differed significantly from the levels in the roots. The present study was undertaken to study further the GS levels in turnips by analyzing tops and roots of 20 cul­ tivars to ascertain differences in the respective plant parts. In addition. seeds of six cultivars were examined to ascertain cor­ relations between seed GSs and those of vegetative parts.