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

• Organ-specific Ncrit curves and NNI values (leaf, stem, shoot) for wheat, oilseed rape and maize were identified. • Shoot DM based Ncrit curves were comparable to previously published dilution curves. • Plants use primarily the stem than the leaves as temporary N storage at high N availability. The concept of the critical nitrogen (N crit) 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 N crit 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, N crit 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 N crit curves were comparable to previously published dilution curves. Maize leaf N concentration was clearly lower than that of wheat and OSR. Leaf N crit of wheat and OSR decreased only slightly during the growth period, but maize N crit showed a more substantial decrease at low DM. Contrarily, all crops showed similar stem N crit curves. Comparison of organ-specific N nutrition index (NNI) revealed that at lower NNI shoot , the increase in NNI leaf was initially higher, while NNI stem exhibited a steeper increase at higher NNI shoot. 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. [ABSTRACT FROM AUTHOR]