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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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Data about gene escape by seeds and volunteers were compiled for the first time in one study for several crops, i.e. wheat (Triticum aestivum), sugar beet (Beta vulgaris), oilseed rape/canola (Brassica napus) and maize (Zea mays). These species represent important genetically modified (GM) crops with herbicide tolerance (HT) or insect resistance (Bt), show different levels of autogamy and allogamy and are grown in different climatic zones of the world. Post-harvest measures and strategies were identified for minimizing gene escape from these crops. All species were found to cause problems in terms of gene escape by seed and volunteers though there are important differences between species and climatic zones. Post-harvest tillage was identified as a key factor for reducing the soil seed bank and volunteers. Timing and intensity of tillage has to be specifically adapted to the dormancy characteristics of each species. Furthermore, there is a close interaction between gene escape and the cropping system. Rotations should avoid the same crop or other critical crops in temporal vicinity to the GM crop in order to keep volunteer populations below a critical density. In no-till systems with use of HT varieties, HT volunteers can reduce the efficiency of the whole system if additional herbicides have to be applied. Seed impurities and admixtures during seed production are another major source of gene escape. Since seed lots of certified growers present less adventitious presence of other varieties, these should be preferred to farm-saved seeds. Education of farmers, cleaning of equipment, control measures and separate production and supply chains are additionally important to minimize gene escape.

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