Decorrelating source and sink determinism of nitrogen remobilization during grain filling in wheat

Nitrogen (N) remobilization is the major source of N for grain filling in wheat, the other being N uptake after anthesis (N-up); however, variations in remobilization efficiency are not fully understood. It is hard to tell whether the source or the sink effects predominate, because N in the culm at anthesis (N-ant) correlates strongly with both N remobilization (N-rem) and grain number (G(n)), respectively the main source and the main sink. A pot experiment was thus designed to assess the relative contributions of the source and sink to N-rem regulation. Using two cultivars of winter wheat (Triticum aestivum, 'Apache' and 'Autan'), three pre-anthesis and two post-anthesis N fertilization levels were applied in order to vary the N sources, while ear trimming at anthesis reduced sink size. Unlike results observed at a scale of m(2), the equation binding N-ant to N-rem exhibited a negative intercept, challenging the concept of nitrogen remobilization efficiency. Before ear trimming, G(n) fitted well to N-ant, with a slope dependent on genotype. To obtain a sink variable that was less correlated with N-ant, the difference delta G(n) was calculated between actual grain number and that which could be predicted from culm N before trimming. A multiple regression then predicted N-rem (r(2) = 0.95) from N-ant, N-up and delta G(n), with fitting unbiased by fertilization treatment, trimming or genotype. In untrimmed culms, delta G(n) had a negligible effect, so that N-rem could be fitted to N-ant and N-up only: grain N filling appeared to be determined by sources only (N-ant and N-up), not by sink, and the reduction of N-rem by N-up was quantified. In these 'normal' cases, the regulation of N-rem should thus be located within the N sources themselves. In contrast, ear-trimming needs to be considered with caution as it introduced a sink limitation on N-rem; moreover one with an important genotype effect.