The influence of nitrogen (N) nutrition on tuber quality of potato (Solanum tuberosum L.), i.e., specific gravity, starch, nitrate, and protein content as well as ascorbic acid content was investigated in a pot experiment. As expected, specific gravity and starch content decreased with increasing fertilizer rates. Protein and especially nitrate content were positively related to nitrogen fertilization, while the effect on l-ascorbic acid content was relatively small. Only at the highest N ORDER REPRINTS level, which led to drastic yield depressions (3.8 g N pot À1 ), l-ascorbic acid content was significantly reduced. Since NO À 3 is considered phloem-immobile and xylem transport into the tubers is restricted (because of the tuber's low transpiration rate), the pathway of nitrate translocation into the tubers is unclear. For Ca 2þ , a direct uptake from the soil solution by the tuber surface is well documented. In order to check whether this pathway may also be of importance for NO À 3 , potato plants were cultivated in solution culture. The growing tubers were wrapped with paper tissue soaked with Ca( 15 NO 3 ) 2 -labelled tuber medium, while contact of the tubers with the nonlabeled liquid growth medium was strictly avoided. 15 N-content of tubers was measured on a mass spectrometer. The experiment demonstrated that with high NO À 3 concentration at the tuber surface, up to 40% of total tuber NO À 3 content can be taken up through the tuber skin.