Excessive dependence on nitrogen (N) fertilizer to achieve high crop yields in modern agricultural production has increased production costs and caused serious environmental problems, such as soil acidification and water eutrophication. Screening low-nitrogen-tolerant wheat cultivars is a key approach to improving nitrogen use efficiency and significantly reducing nitrogen fertilizer input. This study aimed to elucidate the low-nitrogen tolerance characteristics of spring wheat, provide germplasm resources, and establish a theoretical basis for gene mining and breeding of low-nitrogen-tolerant wheat varieties.

A total of 285 spring wheat accessions were used as experimental materials. Seedling-stage germination bag experiments were conducted under low nitrogen (0.05 mmol·L^-1) and normal nitrogen (5 mmol·L^-1) treatments. Nine traits, including seedling length, root length, shoot dry weight, and root dry weight, were measured, and the low-nitrogen tolerance coefficient for each trait was calculated. A comprehensive evaluation of seedling-stage low-nitrogen tolerance was performed using principal component analysis and the membership function method. Selected materials were further subjected to precise field screening and identification. Representative low-nitrogen-tolerant and nitrogen-sensitive materials were used to analyze changes in nitrogen uptake, assimilation, and transport-related enzyme activities under low-nitrogen stress, as well as grain quality traits.

Based on comprehensive evaluation using principal component analysis and the membership function method, the 285 spring wheat accessions were classified into five groups. Group Ⅰ consisted of extremely nitrogen-sensitive materials (5 accessions) with D values ranging from 0.14 to 0.20, including WN-269, WN-247, WN-244, WN-149, and WN-249. Group Ⅱ included nitrogen-sensitive materials (84 accessions) with D values of 0.20 to 0.34. Group Ⅲ comprised moderately nitrogen-efficient materials (162 accessions) with D values of 0.34 to 0.51. Group Ⅳ consisted of low-nitrogen-tolerant materials (29 accessions) with D values of 0.51 to 0.61. Group Ⅴ included extremely low-nitrogen-tolerant materials (5 accessions) with D values of 0.61 to 0.70, namely WN-49, WN-186, WN-237, WN-294, and WN-235. Five extremely nitrogen-sensitive and five extremely low-nitrogen-tolerant accessions were selected for field identification, resulting in the final identification of one nitrogen-sensitive accession (WN-269) and one low-nitrogen-tolerant accession (WN-235). Analysis of nitrate reductase, glutamate synthase, and glutamine synthetase activities showed that enzyme activities in leaves significantly decreased under low-nitrogen stress, while enzyme activities in the low-nitrogen-tolerant material were significantly higher than those in the nitrogen-sensitive material.

One low-nitrogen-tolerant wheat accession and one nitrogen-sensitive accession were successfully identified. A comprehensive evaluation system for low-nitrogen tolerance at both seedling and adult stages in wheat was established, and root surface area, root volume, and root dry weight were identified as core indicators for evaluating low-nitrogen tolerance.