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 measured on plants treated with KCl and KNO 3 at different concentrations (5, 10, 20 mol m −3 ) for 16 h. Whatever the treatment, the exudation pattern was linear for at least 5 h. Compared to KCl, a 2 fold enhancement of exudation was induced by nitrate; the exudation rate increased as a function of KNO 3 incubation time reaching a maximum after 20 h. A sap dilution was always observed after detopping. Nitrate treatment did not increase total osmolarity, but modified the sap composition by increasing the amino acid concentration and decreasing the carbohydrate concentration. Surprisingly, no relationship was found between sap osmolarity and exudation, suggesting that NO 3 − enhancement of this latter process is mediated by changes in hydraulic conductivity rather than osmolarity