Investigating Genetic Progress and Variation for Nitrogen Use Efficiency in Spring Wheat

Improved N use efficiency (NUE) increases wheat (Triticum aestivum L.) yields and reduces N losses in the environment. We investigated genetic variation and correlations among agronomic and NUE traits in Canada Western Red Spring wheat cultivars to further improve NUE. Trials were conducted for 3 yr at two locations in Alberta, Canada, under two levels of N (200 and similar to 50 kg ha(-1)). Genotype x environment interaction was significant for traits associated with vegetative growth, and genotype x N fertilizer treatment interaction was significant for important yield and NUE traits. There were significant positive correlations between total dry matter and N uptake efficiency (NUpE) in the high (r = 0.74, P < 0.05) and low (r = 0.83, P < 0.05) N treatments. The effect of dwarfing Rht-1b allele was more prominent under high N treatment in increasing NUE. However, cultivars with Rht-1b allele showed inconsistent results for NUpE, indicating that Rht alleles might have pleiotropic effects on N uptake. Grain yield, NUE, and N utilization efficiency (NUtE) exhibited genetic improvement over time only under high N treatment. Our results indicated that grain yield increased mainly due to improved harvest index (HI), suggesting improvement in C assimilation rather than N partitioning efficiency. Nitrogen use efficiency may further be improved by intercrossing cultivars with high HI, N harvest index, and NUtE and those with good NUpE, while using total dry matter production as a selection criterion.