purposes (Lhuillier-Soundé lé et al., 1999). Nitrogen remobilization efficiency (NRE) is therefore estimated as In wheat (Triticum aestivum L.), nitrogen remobilization from the the fraction of stored nitrogen at flowering that is not vegetative organs of the crop to the grains has been shown to depend on environmental factors and genotype. We performed, for a set of 10 recovered in the vegetative parts at maturity (Cox et al., winter wheat genotypes, field experiments at six sites over a 2-yr period. 1985a, 1985b Van Sanford and Mackown, 1987). NRE By measuring nitrogen uptake at flowering (NUF from 32-284 kg depends on growing conditions during the grain filling ha Ϫ1 ), the amount of remobilized nitrogen (REMN from 24-228 kg period and genotype. ha Ϫ1 ) and nitrogen remobilization efficiency (NRE from 0.44-0.92) Palta et al. (1994) showed that NRE was high in the we were able to determine the effect of genotype and environment on Mediterranean-like conditions of western Australia, in the relationship between REMN and NUF. Environment and genowhich plants generally suffer from water stress during type had significant effects on nitrogen remobilization and nitrogen the grain-filling period. This increase in NRE may result remobilization efficiency, which mainly depended on treatment (nitroindirectly from limits on nitrogen uptake during the graingen and fungicide) and site. For environments without limiting factor filling period, forcing the plant to make greater use of during the grain-filling period, we found that REMN was not dependent on the genotype and could be estimated by a single two-parameter its stored nitrogen. Cox et al. (1986) showed that higher linear relationship (REMN ϭ 4.13 ϩ NUF ϫ 0.76, r 2 ϭ 0.97). We levels of nitrogen fertilization before flowering lead to analyzed the effect of drought stress before and after flowering, high a decrease in nitrogen remobilization efficiency as the temperature during these periods, nitrogen availability and disease resulting higher level of nitrogen availability just after pressure on REMN by comparing observed and estimated REMN. flowering renders nitrogen remobilization unnecessary. The effect of the environment on the relationship between nitrogen up-Similar results were obtained by Moll et al. (1982) and take at flowering and nitrogen remobilization depended on nitrogen Przulj and Momcilovic (2001) with barley (Hordeum vuluptake during grain-filling period and disease pressure and was also gare L.). Halloran (1981) reported an increase in NRE affected by genotype. Disease-resistant genotypes seemed to be able in conditions unfavorable for nitrogen uptake before to keep remobilization efficiency stable in conditions of strong disease anthesis, linked to drought or high temperature. Foliar pressure, whereas nitrogen remobilization efficiency decreased strongly in susceptible genotypes under the same conditions. diseases, such as brown and yellow rusts (cause by Puccinia spp.), Septoria blotch (caused by Septoria tritici Roberge in Desmaz), and powdery m...

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We need to understand the effects of N deficiency on yield according to the crop stage at which it occurs In humid temperate climates, N is still a major limiting factor and its duration and intensity, to optimize N fertilization for wheat (Triticum aestivum L.) production. Our objective was to understand and quantify N deficiency effects on the crop grain number under these constraints. In particular, it is essential to to develop optimum N fertilizer management strategies for wheat. In determine whether all N deficiencies are detrimental to this aim, several experiments were conducted on various soil types and yield, or whether some deficiencies can be tolerated climates with 'Soissons' winter wheat. Rates and dates of N fertilizer because they have only minor effects on yield. It is application were varied in each experiment. This resulted in highly already known that in various environmental conditions, variable dynamics of N accumulation in plants, leading to various N including those in which N levels vary (Boiffin et al., deficiencies throughout the crop cycle. Deficiencies were character-1981; Abbate et al., 1995), wheat yield is largely deterized by a N nutrition index (NNI). Seven criteria describing the defimined by the grain number per square meter (GN) of ciency (beginning of deficiency, BD; end of deficiency, ED; duration the crop. However, few studies have quantified differof deficiency, DD; intensity of deficiency, ID; the product ID ϫ DD ϭ ences in GN in response to N nutrition, or in relation IDD; the lack of nitrogen accumulation at anthesis, LNA; and the NNI at anthesis, NNI a ) were estimated for each treatment. Large to the intensity and duration of the N deficiencies. ranges were obtained for each criterion. Treatments also resulted in Little is known about the effects of N nutrition condihighly variable grain numbers. For a N deficient crop, the grain numtions on wheat GN. Abbate et al. (1995) have shown ber decrease relative to the control treatment in the same experiment that if N nutrition is limiting and continuous, i.e., the (RGN) was analyzed according to the deficiency criteria. Whatever wheat crop is subjected to N deficiency from early in the grain number component affected (spike number per per square the crop cycle until after anthesis, GN is strongly and meter or grain number per spike), the RGN appeared to depend on linearly correlated with the amount of N in the spikes the history of the deficiency, the main explicative variable being IDD, at anthesis. However, the effects of fluctuations in N that is, the product of the duration and the intensity of the deficiency. nutrition on GN, such as temporary N deficiency, are The equation RGN ϭ 1.00355-0.00110 ϫ IDD (R 2 ϭ 0.929) allowsunknown. In particular, the effects may vary according the prediction of grain number for wheat crops subjected to various N fertilization strategies.

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