Sulphur turnover in the developing pods of single and double low varieties of oilseed rape (<i>Brassica napus</i> L)

Changes in the sulphur (S) concentrations in pod walls and seeds of the double low variety Cobra and single low variety Bienveou of winter oilseed rape ( Brassica napus L) were studied during pod development. The S concentration in the seeds of Cobra increased between 47 and 61 days after flowering and thereafter remained stable, whereas in Bienvenu seed S concentration was much higher initially and continued to increase throughout the sampling period (from 57 to 108 days after flowering). These differences in S concentration reflect different patterns of glucosinolate accumulation in the seeds. In contrast, there was a large and progressive build‐up of S in the pod walls of Cobra, such that at maturity the S concentration in the pod walls of Cobra was approximately double that of Bienvenu. A large and progressive accumulation of inorganic sulphate was found to be responsible for the build‐up of S in the pod walls of the double low variety. Short‐term feeding experiments with terminal racemes showed that a smaller proportion of 35 S was translocated from the pod walls to the seeds in Cobra than in Bienvenu. However, the build‐up of S in the pod walls of Cobra did not seem to result from a restricted translocation of glucosinolates, since in both varieties exogenously supplied allylglucosinolate was translocated rapidly from the pod walls to the seeds. These results are therefore consistent with the hypothesis that pod walls are a major site for the biosynthesis of glucosinolates present in the seeds, and a metabolic block in the pathway of glucosinolate biosynthesis is responsible for the low glucosinolate concentration in the seeds of double low varieties, which consequently results in a large build‐up of sulphate in the pod walls.