Use of labeled sulphur-35 for tracing sulphur transfers in developing podsof field-grown oilseed rape

This study was conducted to better understand the dynamics of sulphur (S) t ransfer between pod walls and seeds of field-grown oilseed rape by using su lphur (S-35) as an investigative tool. Labeling experiments with S-35 were carried out to determine the effects of nitrogen (N) and sulphur fertilizat ion on these transfer mechanisms. Sixty-four plants from field trials ferti lized with 200 kg N ha(-1) in the forms of ammonium nitrate (AN) or urea (U ), with or without 75 kg S ha(-1) in the forms of ammonium thiosulphate and MgSO4 were sampled. At 30, 43, 56, and 77 days after flowering (DAF), term inal racemes were cut and labeled with S-35-SO42-. After labeling, pods and seeds were separated into 3 groups according to their position on stem, an d measurements of S-35 levels were performed accordingly. This short-term l abeling experiment showed that the pod walls retained from 39 to 61% of lab eled S-35, according to the different treatments, whereas seeds accumulated from only 1 to 16% of applied S-35. On average, when S was added, a sharp decrease of S-35 in seeds from 2.6 to 1.7%, 9.0 to 5.4%, and 14.8 to 7.7% w as observed at 30, 43, and 56 DAF, while progressively the percentage value s in pod walls increased from 49.6 to 50.5%, 43.1 to 52.2%, and 41.7 to 63. 5%, respectively. The increase of S-35 in pod walls was found to be tied to the glucosinolate concentration of seeds. By artificially increasing the r atio values of external N-NO3- to S-SO42-, these results demonstrated that the transfer of S-35 to seeds was more affected by the higher level of N-NO 3- in plant tissues than S-SO42- levels. The N/S ratio value above which th e transfer of S was disrupted was around 6.