Studies of the Biochemical Background to Differences in Glucosinolate Content in Brassica napus L. II. Administration of Some Sulphur-35 and Carbon-14 Compounds and Localization of Metabolic Blocks

The aim of the investigation was to study the differences in the metabolism of substances that are utilized in the synthesis of glucosinolates between the Brassica napus cv. Bronowski, which is very low in glucosinolate content, and a cultivar (cv. Regina II) that contains approximately average amounts of these compounds. By experiments in which the plants were grown in nutrient solutions supplied with sulphate-³⁵S, it was shown that the rate of sulphate uptake was similar in the two cultivars. No accumulation of intermediate metabolites could be demonstrated by autoradiography in Bronowski. Sulphate-³⁵S, methionine-³⁵S, methionine-2-¹⁴C, 2-amino-6-(methylthio)caproic acid-2-¹⁴C, and S-(β-d -glucopyranosyl)-4-pentenethiohydroximic acid (desulpho-3-butenylglucosinolate) (glucose-U-¹⁴C or ³⁵S) were fed to shoots of the two cultivars. After incubation, the plant material was extracted with methanol. The extracts were separated into various fractions, and in some experiments glucosinolates or derivatives of their degradation products were isolated. When measuring the radioactivity of the various fractions or isolated products, the incorporation of radioactivity into glucosinolates was found to be poor in Bronowski from sulphate, methionine, and 2-amino-6-(methylthio)caproic acid. Desulpho-3-butenyl-glucosinolate was an efficient precursor of 3-butenylglucosinolate in Bronowski, but a poor precursor of 2-hydroxy-3-bute-nylglucosinolate, which suggests a metabolic block at the hydroxylation step in this cultivar. In Regina II desulpho-3-butenylglucosinolate was a good precursor of both 3-butenylglucosinolate and of 2-hydroxy-3-butenylglucosinolate, which demonstrates that these glucosinolates may be synthesized without prior formation of the corresponding co-methylthioalkyl glucosinolates and that the hydroxylation can take place after the formation of desulpho-3-butenylglucosinolate. The results indicate that the low glucosinolate content of Bronowski is caused by block(s) in the separate pathway leading to the biosynthesis of glucosinolates.