Nitrogen use efficiency (NUE) has not improved over the last 20 years, making it crucial to identify wheat genotypes with high NUE and the traits that contribute to higher NUE. In this study, we conducted two field experiments in the high-yielding environment of Valdivia, Chile. In Experiment 1, we evaluated 20 wheat genotypes: 6 from Chilean breeding programs plus 14 from CIMMYT's Wheat Association Mapping Initiative (WAMI) population under two contrasting nitrogen (N) availability levels: a control without N fertilization and a fertilized treatment supplying 280 kg N ha-1.

Wheat genotypes were sown in a split-plot block design, where the main plot was assigned to the nitrogen availability level and the subplots to genotypes in three blocks. Genotypes showed different grain yield ranking between both nitrogen treatments. Two contrasting genotypes were selected: WBLL1*2/KUKUNA (WBLL1), which achieved high yield in both fertilization treatments, and ATTILA/3*BCN*2///BAV92 (ATTILA), which showed contrasting yields depending on the N treatment.

In Experiment 2, these two contrasting genotypes were evaluated under two nitrogen availabilities as in Experiment 1. As expected, WBLL and ATTILA demonstrated contrasting responses between the nitrogen treatments. In control treatments, grain yield of WBLL was 13% lower than in the high nitrogen treatment, while this difference reached 36% in ATTILA (WBLL1: 1294 and 1122 g m-2; ATTILA: 1126 and 725 g m-2). WBLL and ATTILA showed similar (p> 0.05) N uptake (28.6 and 31.1 g m-2, respectively) under the fertilized treatment.

However, in the control treatment N uptake was higher in WBLL than in ATTILA, because of a higher nitrogen uptake efficiency (NUpE).

This difference allowed higher grain yield by WBLL in low N availability. In addition, WBLL1 exhibited a lower depletion in grain N concentration than ATTILA in the control treatment, regarding his lower grain N concentration (WBLL1: 1.81 and 1.73 %; ATTILA: 2.36 and 2.07 %), which could be attributed to a greater ability to translocate N under lower N availability and its higher N utilization efficiency (NUtE). Therefore, WBLL demonstrated higher NUE (54% higher) than ATTILA.

Wheat breeding programs focused on enhancing NUE could benefit from these differences allowing higher N uptake, especially in restricted N conditions.