Natural variation allows the investigation of both the fundamental functions of genes and their role in local adaptation. As one of the essential macronutrients, sulfur is vital for plant growth and development, and also for crop yield and quality. Selenium and sulfur are assimilated by the same process, and although plants do not require selenium, plant-based selenium is an important source of this essential element for animals. Here, we report the use of linkage mapping in synthetic F2 populations, and complementation to investigate the genetic architecture of variation in total leaf sulfur and selenium concentrations in a diverse set of Arabidopsis thaliana accessions. We identify in accessions collected from Sweden and the Czech Republic two variants of the enzyme adenosine 5'-phosphosulfate reductase 2 (APR2) with strongly diminished catalytic capacity. APR2 is a key enzyme in both sulfate and selenate reduction and its reduced activity in the loss-of-function allele apr2-1 and the two A. thaliana accessions Hodonín (Hod) and Shahdara (Sha), leads to a lowering of sulfur flux from sulfate into the reduced sulfur compounds, cysteine and glutathione, and into proteins, concomitant with an increase in the accumulation of sulfate in leaves. We conclude from our observation, and the previously identified weak allele of APR2 from the Sha accession collected in Tadjikistan, that the catalytic capacity of APR2 varies by four orders of magnitude across the A. thaliana species range, driving significant differences in sulfur and selenium metabolism. The selective benefit, if any, of this large variation remains to be explored.