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

Background: Direct plant uptake of organic nitrogen (N) may be important for plant N nutrition, but we lack knowledge of how the concentration and form of external N influence organic N uptake and plant N status.

Aims: We investigated the uptake of the amino acid asparagine (Asn) in white clover in the presence of different inorganic, organic and total N concentrations.

Methods: Actively N2‐fixing white clover seedlings were for one week exposed to combinations of NO−3 (3–30 µmol N kg−1 sand DW) and Asn (3–30 µmol N kg−1 sand DW), whereafter the Asn uptake rate was determined by addition of 13C4‐Asn. Shoot and root amino acid profiles were also analyzed.

Results: Increasing NO−3 and total N concentrations decreased 13C4‐Asn uptake rates and internal clover Asn content. In addition, total N and NO−3 also affected amino acid profiles, with Asn, Asp, Glu, Gln, Cys, Gly, Pro, Ser, and Ala being more related to the low N doses, and Thr, Val, Ile, Leu, Phe, Tyr, Trp, and Met being more abundant at increasing N doses.

Conclusions: Asn uptake rate in white clover is reduced by increasing inorganic N. Plant amino acid profiles are likely to be a more sensitive indicator of N supply.