NITRATE USE AND XYLEM EXUDATION IN DETOPPED WHEAT SEEDLINGS - AN EARLY DIAGNOSIS FOR PREDICTING VARIETAL DIFFERENCES IN NITROGEN-UTILIZATION

Differences in nitrate use for two wheat varieties (Soissons and These e) were found from studying nitrogen (N) composition in stumps collect ed at dawn in field experiments-soluble extracts showed that Soissons shoots received less amino acids (mainly glutamine) compared with nitr ate, whereas Thesee shoots received more amino acids compared with nit rate. Amino acid and nitrate supply to Soissons shoots were maintained at a more constant level compared with Thesee. An interpretation of t hese varietal characteristics was attempted by examining properties of young seedlings grown in controlled conditions. Xylem sap rates in re sponse to KCl or KNO3 were determined in detopped seedlings: Soissons exudation was always greater than that of Thesee. Compared with KCI, t he addition of KNO3 in the root medium induced an enhancement of the f lux of xylem exudate for both varieties but the enhancement due to KNO 3 was lower for Soissons (x1.6) than for Thesee (x2.1). The increase o f root nitrate reductase (NR) activity in response to external nitrate concentration or to incubation time was also markedly higher in These e and was associated with higher concentrations of amino acids and NH4 + and lower nitrate in the xylem sap than for Soissons. The faster wat er flux and the greater nitrate concentration in the xylem for Soisson s could explain its higher shoot NR activity. Moreover, a (NO3-)-N-15 tracing showed that Soissons accumulated significantly more new N in t he shoot than Thesee. The characteristics of xylemic sap of young seed lings in response to nitrate could be used as criteria for describing varietal differences in N use.