The Influence of Nitrogen Supply on the Development of Nitrate Reductase Activity in Leaves of Potato (Solanum tuberosum L.)

Abstract

Changes in nitrate reductase activity in the youngest expanded leaves of the potato plant have been monitored in the field under a range of N regimes. Highest activity ( in vivo assay, substrate non-limiting) occurred with the highest rate of N application, as did the highest leaf protein content. In high-N leaves the onset of the decline in nitrate reductase activity and protein content, which commenced around 80 days after planting, was not prevented by the application of high levels of nitrogen. This could not be explained by a fall in the concentration of nitrate in the petiole xylem sap supplying the leaves in question. The ratio of nitrate-N to amino acid-N in petiole xylem sap remained relatively constant throughout the growing period in plants receiving 8 g N m −2 at 14 day intervals, but declined under lower N inputs. Only in the highest N treatment did the ratio remain greater than unity. The reasons for these observations are discussed. Nitrate reductase activity was also determined in leaves of field-grown plants with all of the N (20 g m −2 ) applied pre-planting. A comparison of in vivo assays carried out with and without nitrate in the assay medium showed that nitrate reductase was likely to limit nitrate assimilation only during the earliest stages of post-emergence growth.