The aim of this study was to determine concentrations and critical levels ofN03-N in tomato petiole sap by a quick test using a portable meter, and the relationship between these values and those of N03- N on petiole dry matter, and of organic and total N on leaf blade dry matter. Tomato (Lycopersicon esculentum Mill.) plants, cv. Santa Clara, grown in pots of two soil samples (clay = Vi and sand-course = TM), were submitted to five N rates (25, 125, 225, 325 and 425 mg/dm3) sidedress applied as nutrient solutions. Leaves opposite to 1st and 3 rd clusters were sampled at the onset of anthesis of each cluster, and petiole sap N03-N concentrations were determined with a portable meter equipped with a microelectrode selective to N03- N. Additionally, organic N (org-N) and N03-N were determined in leaf blade dry matter, and N03-N was determined in petiole dry matter. Total-N (tot-N) concentrations in blade dry matter were the sum of N03-N and organic-N. Critical N levels were highest on the TM soil, and for both soils, highest on the 1st cluster. N03-N critical levels in petiole sap ofleaves opposite to 1st and 3"1 clusters were 2,581 and 1,085 mg/L on soil Vi, and 2,616 and 1,690 mg/L on soil TM, respectively. Concentrations of N03-N in petiole dry matter and of org-N and tot-N in blade dry matter were well estimated by sap N03-N concentrations in leaf samples taken from plants grown on soil TM, and in leaves opposite to the 3d cluster from plants grown on soil Vi.
Lycopersicon esculentum; critical level; nitrate.