Early nitrogen deficiencies favor high yield, grain protein content and N use efficiency in wheat

Nitrogen fertilization has been widely studied in wheat (Triticum aestivum L.), with a view to maximizing local yields and obtaining high grain protein contents. It has long been accepted that nitrogen nutrition must be non-limiting throughout the crop cycle for these targets to be reached. However, studies over the last 20 years have shown that some periods of N deficiency are detrimental, whereas others have no impact on grain yield. There is, therefore, still a need to define the precise N deficiency path that can be tolerated. Nitrogen nutrition index (NNI) is an appropriate indicator of N deficiency. Based on experiments with wheat crops showing various patterns of NNI dynamics from the start of stem elongation to flowering, we aimed to identify a minimum nitrogen nutrition path, including periods of N deficiency, defining the threshold above which there is no detrimental impact on wheat crop yield. We used experimental data from 18 site-year experiments, each including 1-14 cultivars and 2-8 fertilization strategies, with determinations of crop NNI at four growth stages (Z30, Z32, Z39 and Z60 on the Zadoks scale). Experimental treatments were assigned to two groups: those with and without yield loss due to N fertilization strategy, relative to the maximum yield in each trial. Using receiver operating characteristics curve analysis, we identified the NNI path best distinguishing between the two groups of treatments. We found that the lowest acceptable NNI value (i.e. the lowest value for which there was no yield loss), increased during the crop cycle. We characterized, for the cultivars studied, periods of N deficiency during vegetative growth that did not lead to a decrease in yield or grain protein content, and even some periods in which the deficiency improved nitrogen use efficiency. Finally, we concluded that references in NNI should be revised for more efficient N management and the threshold NNI path could be used to determine timing of N fertilizer application on the basis of real-time crop N status monitoring.