In this review, recent developments and future prospects of obtaining a better understanding of the regulation of nitrogen use efficiency in the main crop species cultivated in the world are presented. In these crops, an increased knowledge of the regulatory mechanisms controlling plant nitrogen economy is vital for improving nitrogen use efficiency and for reducing excessive input of fertilizers, while maintaining an acceptable yield. Using plants grown under agronomic conditions at low and high nitrogen fertilization regimes, it is now possible to develop whole-plant physiological studies combined with gene, protein, and metabolite profiling to build up a comprehensive picture depicting the different steps of nitrogen uptake, assimilation, and recycling to the final deposition in the seed. A critical overview is provided on how understanding of the physiological and molecular controls of N assimilation under varying environmental conditions in crops has been improved through the use of combined approaches, mainly based on whole-plant physiology, quantitative genetics, and forward and reverse genetics approaches. Current knowledge and prospects for future agronomic development and application for breeding crops adapted to lower fertilizer input are explored, taking into account the world economic and environmental constraints in the next century.

Summary• In wheat ( Triticum aestivum cv. Soissons) plants grown under three different fertilisation treatments, we quantified the effect of leaf rust ( Puccinia triticina ) on flag leaf photosynthesis during the whole sporulation period.• Bastiaans' model: Y = (1 − x ) β was used to characterize the relationship between relative leaf photosynthesis ( Y ) and disease severity ( x ). The evolution of the different types of symptoms induced by the pathogen (sporulating, chlorotic and necrosed tissues) was evaluated using image analysis.• The β -values varied from 2 to 11, 1.4 -2, and 0.8 -1 during the sporulation period, when considering the proportion of sporulating, sporulating + necrotic, and total diseased area, respectively. Leaf nitrogen (N) content did not change the effect of the disease on host photosynthesis.• We concluded that leaf rust has no global effect on the photosynthesis of the symptomless parts of the leaves and that the large range in the quantification of leaf rust effect on the host, which is found in the literature, can be accounted for by considering the different symptom types. We discuss how our results could improve disease assessments and damage prediction in a wheat crop.

The use of kinetic equations of NO 3 Ϫ transport systems in oilseed rape (Brassica napus), determined by 15 NO 3 Ϫ labeling under controlled conditions, combined with experimental field data from the INRA-Châlons rape database were used to model NO 3 Ϫ uptake during the plant growth cycle. The quantitative effects of different factors such as day/night cycle, ontogenetic stages, root temperature, photosynthetically active radiation, and soil nitrate availability on different components of the constitutive high-affinity transport systems, constitutive low-affinity transport systems, inducible low-affinity transport systems, and inducible high-affinity transport systems of nitrate were then determined to improve the model's predictions. Simulated uptake correlated well with measured values of nitrogen (N) uptake under field conditions for all N fertilization rates tested. Model outputs showed that the high-affinity transport system accounted for about 89% of total NO 3 Ϫ uptake (18% and 71% for constitutive high-affinity transport systems and inducible high-affinity transport systems, respectively) when no fertilizer was applied. The low-affinity transport system accounted for a minor proportion of total N uptake, and its activity was restricted to the early phase of the growth cycle. However, N fertilization in spring increased the duration of its contribution to total N uptake. Overall, data show that this mechanistic and environmentally regulated approach is a powerful means to simulate total N uptake in the field with the advantage of taking both physiologically regulated processes at the overall plant level and specific nitrate transport system characteristics into account.Winter oilseed rape (Brassica napus) is an important crop in northern Europe because of its varied utilizations (oil and biofuel). However, yields remain highly variable. As a consequence, oilseed rape has been extensively studied to identify key components of yield and to improve them by more effective nitrogen (N) application with the target of reducing environmental impacts such as N leaching and improving N use efficiency for seed filling (Boelcke et al., 1991;Habekotté, 1993;Schjoerring et al., 1995;Sieling and Christen, 1997;Vos and van der Putten, 1997). Many mathematical models have been built to simulate crop growth, development, and yield (BRASNAP-PH, Habekotté, 1997a; and LINTUL BRASNAP, Habekotté, 1997b). Some of these (DAISY, Petersen et al., 1995; and CERES-Rape, Gabrielle et al., 1998) have been devoted mainly to predicting ecological impacts of N losses from winter oilseed rape. When N nutrition has been taken into account, N uptake usually has been based on the balance of demand and supply. In this context, N availability in the soil solution is modeled using mass flow and NO 3 Ϫ diffusion equations (CERES-Rape, and DAISY), and N demand is often determined using the critical dilution curve determined by Colnenne et al. (1998) for oilseed rape (CERES-rape). In these models, the root system is considered as a "black box." ...

This work initiates a modelling approach that allows us to investigate the effects of canopy architecture on foliar epidemics development. It combines a virtual plant model of wheat (Triticum aestivum L.) with an epidemic model of Septoria tritici which is caused by Mycosphaerella graminicola, a hemi-biotrophic, splashed-dispersed fungus. Our model simulates the development of the lesions from the infected lower leaves to the healthy upper leaves in the growing canopy. Epidemics result from the repeated successions of lesion development (during which spores are produced) and spores dispersal. In the model, canopy development influences epidemic development through the amount of tissue available for lesion development and through its effects on rain penetration and droplets interception during spore dispersal. Simulations show that the impact of canopy architecture on epidemic development differs between canopy traits and depends on climate. Phyllochron has the strongest effect, followed by leaf size and stem elongation rate.