The higher kernel number in bread wheat compared with durum wheat is independent of nitrogen nutritional status

Context and objective Considering the constitutively higher number of spikes and grains per square meter in bread wheat compared with durum wheat, this study investigates whether nitrogen fertilization affects grain yield and yield components differently in bread wheat and durum wheat. Methods A three-year field experiment was conducted using two bread wheat and two durum wheat cultivars under three nitrogen application rates (0, 80, and 160 kg N ha⁻¹) in a Mediterranean environment. Results Across the three years, bread wheat produced a higher kernel number per square meter (15,903 on average) but a lower kernel weight (41.5 mg), whereas durum wheat exhibited the opposite pattern (11,463 kernels m⁻² and 51.7 mg per kernel on average). Both species showed similar nitrogen accumulation capacities; however, bread wheat allocated more nitrogen to the stems, while durum wheat allocated more to the grains, resulting in a higher Nitrogen Harvest Index for durum wheat (0.75 vs. 0.68 in bread wheat). Species differences in Nitrogen Nutrition Index (NNI) emerged only under high N supply: bread wheat approached optimal N status (>0.95) at N160 in favorable seasons, whereas durum wheat plateaued at lower values (≈0.86), suggesting structural limitations in achieving full N sufficiency. Significant relationships were found between NNI, yield and yield components, with the exception of kernel weight. At any given NNI level, bread wheat consistently produced more kernels per spike, as well as more spikes and kernels per square meter, than durum wheat; however, these differences were independent of NNI. Conclusions and significance Because nitrogen fertilization rate did not directly correspond to nitrogen nutritional status, accurate assessment of species or cultivar sensitivity to nitrogen should rely on NNI rather than fertilization rate, and different nitrogen application strategies should be adopted for bread and durum wheat cultivars when high nitrogen inputs are required.