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


METHODS
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.


RESULTS
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 deltaG(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 deltaG(n), with fitting unbiased by fertilization treatment, trimming or genotype.


CONCLUSIONS
In untrimmed culms, deltaG(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.
Bancal
Pierre
P
INRA, UMR 1091 INRA/INAPG Environnement et Grandes Cultures, F-78850 Thiverval Grignon, France. bancal@grignon.inra.fr