Organ-specific critical N dilution curves and derived NNI relationships for winter wheat, winter oilseed rape and maize

The concept of the critical nitrogen (Ncrit) dilution curve and the derived NNI is generally accepted and used for many crops to describe the crop N status during vegetative growth. Based on field trials with different N treatments carried out in northern Germany, we identified Ncrit curves based on shoot DM, leaf DM and stem DM for winter wheat (2003/04 - 2007/08), winter oilseed rape (OSR, 2003/04 - 2005/06, 2012/13 - 2014/15), and maize (2007 + 2008), and related the derived organ-specific NNI (Nitrogen Nutrition Index) values. For each sampling date, Ncrit values were estimated using 'Linear-Plateau' functions, in order to fit a broken function for the shoot and stem fraction consisting of a constant N concentration and a negative power function describing the decrease of the N concentration. For the leaf fractions of wheat and OSR, a linear regression gave the best fit, in maize a negative power function. The relationship between the organ-specific NNI was described by a broken function with two linear branches differing in their slope. Shoot dry matter (DM) based Ncrit curves were comparable to previously published dilution curves. Maize leaf N concentration was clearly lower than that of wheat and OSR. Leaf Ncrit of wheat and OSR decreased only slightly during the growth period, but maize Ncrit showed a more substantial decrease at low DM. Contrarily, all crops showed similar stem Ncrit curves. Comparison of organ-specific N nutrition index (NNI) revealed that at lower NNIshoot, the increase in NNIleaf was initially higher, while NNIstem exhibited a steeper increase at higher NNIshoot. Our results suggest that plants use primarily the stem compared to the leaves as temporary N storage at high N availability; however, the effect in wheat was less pronounced leading to the question of the capacity of N translocation for an adequate grain protein concentration.