The SPAD index critical value (NC) in the terminal leaflet of the fourth potato plant leaf (Solanum tuberosum L.) was determined and, the nitrogen (N) rate to obtain the maximum 'Monalisa' yield, at dry and wet seasons. The experiment was carried out in Viçosa, Minas Gerais State, Brazil in a soil Podzolic red-yellow. Five pre-planting nitrogen rates were evaluated (0; 50; 100; 200 and 300 kg ha-1) applied in the planting furrow with four sidedress N rates (0; 100; 200 and 300 kg ha-1) using ammonium sulfate as the N source. The experiment was installed in a randomized complete block design with split-plots and four repetitions. In the plots the rates of N were applied in pre-planting, and in the split-plots in sidedress. 21 days after plant emergence, immediately before the N fertilizer sidedressing, SPAD NC was determined. At dry and wet seasons, SPAD NC were 42.2 and 35.2. These values were associated with the pre-planting N rates which led to the highest commercial tuber yields, 39.88 and 44.46 t ha-1, respectively. In the dry season, only the application of N in PP provided increments in the production the highest commercial production of tubers being obtained with 189,98 kg ha-1 of N; In the wet season, the applications of N should be accomplished in pre-planting and in sidedress, with the rate of 255,85 kg ha-1 of N, being 20% applied in pre-planting and the remaining in sidedress.
Solanum tuberosum L; planting seasons; fertilization; chlorophyllometer
