Survey of glucosinolate variation in leaves of Brassica rapa crops

The breakdown products of glucosinolates (gsl) are biologically active secondary metabolites involved in plant defense and human nutrition. We identified and quantified 14 different gsl present in the young leaves of 82 different varieties of Brassica rapa, including the following crops: Chinese cabbage, broccoleto, Pak choi and other leafy vegetables, turnip, sarson and rapeseed. We did not find crop specific gsl, but their quantity varied extensively among varieties and crops, except that the Chinese cabbage accessions tended to have similar gsl profile and amount. Gluconapin, glucobrassicanapin (aliphatic), neoglucobrassicin, glucobrassicin (indolic), and gluconaturtiin (aromatic) are the predominant gsl in most of the varieties surveyed. We also found two gsl not commonly found in B. rapa, 2-methyl-2-propenyl and n-butyl. Their identities were confirmed by HPLC-MS. Most of the Chinese cabbages contain lower amount of aliphatic than indolic gsl, whereas broccoleto, turnip and rapeseed all have much higher aliphatic gsl content than indolic gsl content. The predominant aliphatic gsl in most of the varieties contain 4-carbon side-chains. The lack of significant correlation observed between the conversion of 3- to 4-carbon side-chain gsl and the conversion of 4- to 5-carbon side-chain gsl suggests that these two elongation cycles are probably under the control of two independent genes in B. rapa. The absence of glucoraphanin in all accessions indicates that only functional Brgsl-Alk alleles are present in B. rapa.