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

Abstract

Background and aims: 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.

Key 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.

Fig. 1.

Relationships between culm N at anthesis (including roots) and grain number. (A) Results after selected plants were trimmed at anthesis by removing one longitudinal half of the ear. (B) Genotype-dependant correlations between initial parameters before trimming. Each point is the mean of three pots containing two main culms. Closed symbols represent the genotype ‘Apache’ and open symbols the genotype ‘Autan’; large symbols represent culms with whole ears and small symbols culms with trimmed ears. The various symbols refer to level of N fertilization before/after anthesis: low/low, circles; low/high, stars; medium/low, squares; medium/high, diamonds; high/low, triangles; high/high, inverted triangles. In (A) the regression line and correlation coefficient apply to the whole data set (d.f. = 71); in (B) two correlations are presented for the genotypes ‘Apache’ (closed symbols) and ‘Autan’ (open symbols).

Fig. 2.

Relationship between the amount of remobilized nitrogen from vegetative parts, including roots (N_rem) and (A) nitrogen in vegetative parts at anthesis, after trimming (N_ant), and (B) actual grain number (G_n). Each point is the mean of three pots containing two main culms. Symbols represent the various genotypes and treatments as in Fig. 1. In (A) the regression line and correlation coefficient apply to the whole data set (d.f. = 71), while in (B) two correlations are given for whole ears (large symbols) and trimmed ears (small symbols).

Fig. 3.

Contribution to N remobilization in main culm (N_rem) for the various treatments as detailed in Table 2. The various terms used in eqn (5) are shown, as well as the residuals of the multiple correlation to N_rem of N_ant, N_up and δG_n. N_ant is the amount of N in vegetative parts at anthesis (after trimming), N_up is the N uptake after anthesis, and δG_n refers to the difference between G_n and grain number as predicted from N_ant,i (before trimming) by eqn (3). Bars indicate the s.e.m. for three replicates.