AOBPreview published online on April 1, 2009
Annals of Botany, doi:10.1093/aob/mcp077
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Decorrelating source and sink determinism of nitrogen remobilization during grain filling in wheat
INRA, UMR 1091 INRA/INAPG Environnement et Grandes Cultures, F-78850 Thiverval Grignon, France
* For correspondence. E-mail bancal{at}grignon.inra.fr
Received: 8 December 2008 Returned for revision: 21 January 2009 Accepted: 23 February 2009
Background and Aims: Nitrogen (N) remobilization is the major source of N for grain filling in wheat, the other being N uptake after anthesis (Nup); 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 (Nant) correlates strongly with both N remobilization (Nrem) and grain number (Gn), respectively the main source and the main sink.
Methods: A pot experiment was thus designed to assess the relative contributions of the source and sink to Nrem 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.
Key Results: Unlike results observed at a scale of m2, the equation binding Nant to Nrem exhibited a negative intercept, challenging the concept of nitrogen remobilization efficiency. Before ear trimming, Gn fitted well to Nant, with a slope dependent on genotype. To obtain a sink variable that was less correlated with Nant, the difference 
Gn was calculated between actual grain number and that which could be predicted from culm N before trimming. A multiple regression then predicted Nrem (r2 = 0·95) from Nant, Nup and Gn, with fitting unbiased by fertilization treatment, trimming or genotype.
Conclusions: In untrimmed culms, Gn had a negligible effect, so that Nrem could be fitted to Nant and Nup only: grain N filling appeared to be determined by sources only (Nant and Nup), not by sink, and the reduction of Nrem by Nup was quantified. In these ‘normal’ cases, the regulation of Nrem 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 Nrem; moreover one with an important genotype effect.
Key words: Triticum aestivum, winter wheat, source/sink, grain filling, nitrogen uptake, grain number, nitrogen harvest index, nitrogen remobilization efficiency, genotype x environment