Determination of a critical nitrogen dilution curve for winter oilseed rape

Several controlled environmental and field experiments were carried out to define the critical nitrogen dilution curve for winter oilseed rape, cultivar Goeland. This curve is described by the following power equation: N = 4.48 W-0.25, where N is the total nitrogen concentration in the shoot biomass and W the shoot biomass. This curve has been validated over the range of shoot dry matter of 0.88 to 6.3 t ha(-1). For lower shoot biomasses this equation overestimated the critical nitrogen concentration; we propose a constant value of 4.63 (N is expressed in reduced N, which is a more stable N fraction in the shoot at these stages of development). These results have been validated in several pedoclimatic conditions in France on a single variety in 1994 and 1995. The higher position of this curve relative to the C-3 species reference curve (Greenwood et al., Annals of Botany 67: 181-190, 1990) can be explained by the experimental conditions obtained by Greenwood et al. (1990); therefore, all their rape data are rather close to the critical curve that we propose. The differences found between wheat and winter oilseed rape critical N dilution curves correspond to their respective leaf:stem dry matter ratio and the specific leaf loss phenomenon occuring in rape. Winter oilseed rape has a higher capacity of N accumulation in its shoot than wheat for the same aerial dry matter. The proportion of nitrate in shoots rises with the nitrogen nutrition index (N.N.I.) and is more important for rapeseed than for wheat for the same N.N.I. This difference is especially high at the beginning of flowering when the shade provided by the canopy of rapeseed flowers decreases nitrate reductase activity. (C) 1998 Annals of Botany Company.