The SPAD critical level (Soil Plant Analysis Development) in potato leaves was established to verify the possibility of its utilization in Monalisa potato tuber yield prognosis. The experiment was installed in a Red-Yellow Argisoil having 39 mg kg-1 of mineral nitrogen content. The experiment was laid out in randomized complete blocks with split plot design and four replications. In the plot were set five N rates (0; 25; 50; 100 and 200 kg ha-1), applied in the furrows before planting. In the split plot were set four side-dressed N rates (0; 90; 180 and 360 kg ha-1), applied in bands 10 cm apart plant side at 23 days after plant emergence, before hilling. SPAD indices were obtained from the terminal leaflet of the fourth leaf from the apex (QF) for six times. The first one (SPAD1) was measured three days before the side-dress N fertilization and the others at 7; 17; 27; 37 and 47 days after the side-dress N application. SPAD indices were also determined in leaves older (FM) than the QF. N content in the QF was determined at the first sampling time. SPAD1 index and N content in the QF increased as preplanting N rates increased following a quadratic function, attaining the critical values of 44.9 and 6.15 dag.kg-1, respectively. The correlation between SPAD1 indices and tuber yield was linear and positive, showing that SPAD1 could be used as a prognostic index of potato yield. At all sampling dates, SPAD indices in the QF and FM leaves were affected by N rates but they reached different values, showing the necessity to standardize the time and leaf to be sampled.
Solanum tuberosum; chlorophyll; tuber; critical level
