Nitrate use and xylem exudation in detopped wheat seedlings: An early diagnosis for predicting varietal differences in nitrogen utilisation?

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