O presente estudo objetivou determinar teores e níveis críticos de nitrogênio na forma de nitrato (N-N03) na seiva do pecíolo de tomateiros utilizando-se medidor portátil, e estabelecer a relação destes valores com os teores de N-N03 na matéria seca dos pecíolos, e de N-orgânico e de N-total na matéria seca do limbo. Plantas de tomateiro (Lycopersicon esculentum), cv. Santa Clara, cultivadas em vasos contendo amostras de dois solos (argiloso = Vi; areia-franca = TM), foram submetidas a cinco doses de N (25, 125,225, 325 e 425 mg/dm3) aplicadas parceladamente, via solução nutritiva. As folhas opostas ao 1° e ao 3° cacho foram amostradas por ocasião do início da antese das flores do respectivo cacho, e as concentrações de N- N03 na seiva dos pecíolos foram determinadas utilizando medidor portátil equipado com microeletrodo seletivo ao N-N03. Determi nou-se, também, as concentrações de N-N03 na matéria seca do pecíolo e do limbo, e as concentrações de N-orgânico na matéria seca do limbo, ambas por colorimetria. Os valores de N-total na matéria seca do limbo foram obtidos a partir da soma dos teores de N-N03 e de N-orgânico. Os níveis críticos das formas de N foram maiores no solo TM, e em ambos os solos, maiores na folha oposta ao 1° cacho. Os níveis críticos de N-N03 nas seiva das folhas opos tas ao 1° e ao 3° cacho foram, respectivamente 2.581 e 1.085 mg/L para o solo Vi, e 2.616 e 1.690 mg/L para o solo TM. Os teores de N-N03 na matéria seca dos pecíolos, e de N-orgânico e de N-total na matéria seca do limbo foram estimados com relativa precisão à partir dos teores de N-N03 na seiva, em ambas as amostragens fei tas nas plantas cultivadas no solo TM, e nas folhas opostas ao 3° cacho das plantas cultivadas no solo Vi.

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.