Cultivar Differences in Response to Reduced Nitrogen Fertilization in Wheat (Triticum aestivum L.)

Nitrogen fertilization is a key factor in wheat crop management, but it has environmental impacts [through nitrate leaching, use of fossil fuels for manufacture and application, as well as important greenhouse gas (N2O) emissions] and represents a significant cost for the growers. Therefore, there is presently an increasing concern to reduce the excessive use of N fertilizers. We analyzed results of seven yield trials with winter wheat cultivars grown on two levels of Nitrogen fertilization, performed during the years 2021-2023 at five sites, where the average yield difference between the two levels of N fertilization was higher than 5%. The analyzed trials included a large variation for grain yield and protein concentration, as well as for the index of tolerance to Nitrogen deficiency. ANOVA for grain yield and protein concentration showed significant effects of cultivars and environments for both yield and protein concentration, but for tolerance index only environments had a significant effect. The effect of lower N availability was stronger and more variable on protein content than on yield. Correlation between grain yield and protein concentration was not significant, but manifested the known negative trend, similar both under recommended and reduced N fertilization. Deviations from regression of protein concentration on grain yield were similar in most cultivars under recommended and reduced N fertilization, but in cultivar FDL Consecvent were higher under lower N availability, suggesting a better adaptation to this stress. Deviations from the regression of performance under Nitrogen deficiency stress on the performance with better N availability, for yield and protein percentage were not significantly correlated, and only three cultivars (FDL Consecvent, Voinic and FDL Miranda) showed positive deviations for both traits. Performance under N deficiency stress is influenced by the potential manifested under no stress and by the response to stress, described by the tolerance index. Analyzing the effect of these two components, we found that, for the analyzed germplasm and environments, performance under no N stress showed higher correlation with performance under reduced N fertilization than the tolerance index. The implications for breeding programs are that (1) genetic progress in grain yield and protein concentration under normal N fertilization, which should be easier to achieve, will be reflected in the performance under reduced fertilization, and (2) a wider diversity of germplasm should be explored, to identify possible differences in the tolerance index, which could further improve performance under reduced fertilization. Deviations from the regressions that describe the general relationship between the levels of N fertilization highlighted a few cultivars with better efficiency in using less available nitrogen, and are worth exploiting in breeding for improved performance under reduced fertilization.