Role of sulphate transporter systems in sulphur efficiency of mustard genotypes

Fourteen genotypes of mustard [Brassica juncea (L.) Czern. & Coss.] were grown for 30 days in complete nutrient solution with 50 μM (S-insufficient) and 1 mM (S-sufficient) sulphur levels, and sulphur efficiency (SE) was analysed. SE, the ability of a plant to maintain good growth on S-limited condition, was calculated as the ratio of dry matter production at low and high S supply. There was 2-10-fold differences in SE of the genotypes. Of the 14 genotypes, Pusa Bold was identified as the most S-efficient, while Pusa Jai Kisan the most S-inefficient. SE of Pusa Bold was about 10-fold higher than Pusa Jai Kisan. To find out the physiological basis of this difference, we investigated the possible role of SO42− influx across the root cell plasma membrane in conferring SE by measuring the short-term 35SO42− uptake in two contrasting genotypes, S-efficient Pusa Bold and S-inefficient Pusa Jai Kisan. This was done by quantifying sulphate uptake over two different concentration ranges: a high-concentration range (50-500 μM) and a low-concentration range (5-50 μM). Uptake experiments revealed the presence of two separate sulphate transporter systems mediating high- and low-affinity sulphate uptake. Interestingly, the sulphate uptake by the roots of Pusa Bold is mediated by both high- and low-affinity sulphate transporter systems, while that of Pusa Jai Kisan by only low-affinity sulphate transporter system. The study suggests that root SO42− uptake rate may play an important role in conferring S efficiency of mustard genotypes under S-limiting condition. This information may be useful in breeding mustard varieties more efficient in acquiring S from soil solution.