Co-occurrence of organic and inorganic N sources influences asparagine uptake and internal amino acid profiles in white clover

Direct plant uptake of organic nitrogen (N) is important for plant N nutrition, but we lack knowledge of how the concentration of external N forms (organic and inorganic) -influence organic N uptake and plant N status. We investigated the uptake of the amino acid asparagine (Asn) in white clover in the presence of different nitrate (NO3-), Asn, and total N concentrations. White clover seedlings were for one week exposed to combinations of NO3- (3-30 {micro}mol N kg-1 sand DW) and Asn (3-30 {micro}mol N kg-1 sand DW), where after the Asn uptake rate was determined by addition of 13C4-Asn. Shoot and root Asn content and amino acid profiles were also analyzed. Increasing external NO3- and total N concentrations decreased 13C4-Asn uptake rates and internal clover Asn content. Furthermore, total N affected clover amino acid profiles from non-essential amino acids at low N doses to the dominance of essential amino acids at increasing N doses. Asn uptake rate in white clover is reduced by increasing inorganic N, but not by increasing organic N concentrations. Furthermore, plant amino acid profiles are likely to be a more sensitive indicator of N supply and descriptor of the N status.