Critical petiole nitrate concentration of two processing potato cultivars in Eastern Canada

Plant-based diagnostic methods of nitrogen (N) nutrition such as petiole nitrate (NO₃-N) concentration can be used to improve the efficiency of N utilization, and hence decrease the risks of N losses to the environment. Our first objective was to determine the effect of N fertilization and supplemental irrigation on the petiole NO₃-N concentration during tuber growth of two potato cultivars, Russet Burbank and Shepody, widely grown for processing in Eastern Canada. Our second objective was to establish the critical petiole NO₃-N concentration using the relationship between petiole NO₃-N concentration and the N nutrition index (NNI), an index based on the N concentration of shoots and tubers. This on-farm study was conducted at two sites in each of three years, 1995 to 1997. The N fertilization rates ranged from 0 to 250 kg N ha^?1 with three rates in 1995, six rates in 1996, and four rates in 1997. The NO₃-N concentration of petioles from the most recently mature leaves was measured on three sampling dates in 1995 and four sampling dates in 1996 and 1997. The petiole NO₃-N concentration generally decreased with time. At all sites and on all sampling dates, the petiole NO₃-N concentration increased with increasing N fertilization and was significantly greater for Shepody than for Russet Burbank. Irrigation had no consistent effect on petiole NO₃-N concentration. Petiole NO₃-N concentration was related to NNI (0.29<R²<0.62). Critical petiole NO₃-N concentrations required to reach a NNI of 1.0, indicating a situation where N is not limiting growth, were greater for Shepody than for Russet Burbank, and they decreased with time. Critical petiole NO₃-N concentrations (Y) expressed as a function of the number of days after planting (X) are Y = 4.80 - 0.055X for Russet Burbank and Y = 5.03 - 0.054X for Shepody.