REASSESSMENT OF THE RELATIONSHIP BETWEEN NITROGEN SUPPLY AND XYLEM EXUDATION IN DETOPPED MAIZE SEEDLINGS

Root pressure characteristics were studied on detopped 7 day old maize seedlings (Zea mays L.) grown on different root media. Exudation rate after detopping, sap osmolarity and sap nitrogen compounds were measu red on plants treated with KCl and KNO3 at different concentrations (5 , 10, 20 mol m(-3)) for 16 h. Whatever the treatment, the exudation pa ttern was linear for at least 5 h. Compared to KCl, a 2 fold enhanceme nt of exudation was induced by nitrate; the exudation rate increased a s a function of KNO3 incubation time reaching a maximum after 20 h. A sap dilution was always observed after detopping. Nitrate treatment di d not increase total osmolarity, but modified the sap composition by i ncreasing the amino acid concentration and decreasing the carbohydrate concentration. Surprisingly, no relationship was found between sap os molarity and exudation, suggesting that NO3- enhancement of this latte r process is mediated by changes in hydraulic conductivity rather than osmolarity.