| Summary: | In a split N-application system, the objective was to quantify N/<sup>15</sup>N in gluten and non-gluten proteins after the late application of 30 or 60 kg N, whereby 10% of the third split was applied as <sup>15</sup>N. This fertilization was combined with a reduced water supply for 21 days (well-watered (ww); water deficit (wd)). German spring wheat cultivars, Elite wheat Taifun, Quality wheat Monsun and cultivars from the Mediterranean territory, Golia, Gönen, were examined. The protein content in gluten was for 30 kg N, ww, similar for Taifun, Golia, and Gönen, but markedly lower in Monsun (231, 245, 247, 194 mg protein/g DM). The water deficit increased the protein content in the gluten of Golia and Gönen and was higher than that of Taifun and Monsun (297, 257, 249, 202 mg protein/g DM). Fertilization of 60 kg N, ww, did not result in any change in the protein content in gluten and differences between the cultivars were not detectable. The <sup>15</sup>N protein in gluten was for 30 kg N, ww, markedly higher in Gönen (2.32 mg <sup>15</sup>N protein/g DM), compared to Golia and Monsun (1.93, 1.50 mg <sup>15</sup>N protein/g DM), and similar in Taifun (1.64 mg <sup>15</sup>N protein/g DM). <sup>15</sup>N fertilizer uptake into gluten was stimulated by water deficit for 30 and 60 kg N, leading to significantly increased <sup>15</sup>N protein in Golia and Gönen, (2.38, 2.99, 4.34, 5.87 mg <sup>15</sup>N protein/g DM). Fertilization of 60 kg N led to a proportional two-time increase in the <sup>15</sup>N gluten protein of the four cultivars, in ww and wd plants. Assessed on the basis of <sup>15</sup>N fertilizer allocation under wd conditions into gluten proteins, Golia and Gönen have a stronger sink activity, compared to Taifun and Monsun.
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