Azodyn-rape : a simple model for decision support in rapeseed nitrogen fertilisation

*UMR d'Agronomie, Bibliothèque, 78850 Thiverval-Grignon Diffusion du document : UMR d'Agronomie, Bibliothèque, 78850 Thiverval-GrignonInternational audienc

A species-specific critical nitrogen dilution curve for sunflower (Helianthus annuus L.)

For annual and perennial crops, mathematical models have been developed to describe tissue nitrogen (N) dilution during crop growth and to estimate the plant N status applying the N nutrition index (NNI), the ratio between the actual tissue N concentration ([N]) and the tissue N concentration needed to obtain the maximum instantaneous crop growth rate (critical tissue N concentration, [N] ). The relationship between shoot [N] and shoot dry matter (DM, tha ) can be described by an allometric power equation: [N] =aDM , where a and b are crop-specific parameters. Critical N dilution curves (CNDC) have been determined for several C crops but not specifically for sunflower (Helianthus annuus L.). The objectives of this work were to (i) determine and validate the N dilution curves for critical, minimum, and maximum [N] for sunflower from the juvenile stages to the end of flowering, (ii) compare the critical curve with published CNDCs for other C crops, and (iii) estimate the range of variation of NNI for different levels of N fertilization and irrigation. A wide range of field experiments from Argentina, Australia, France, Italy, and Spain was used to establish the dilution curve for sunflower and to independently validate it. The fitted CNDC [N] =4.53DM yielded lower values for [N] than references used until now for diagnosis and decision making in sunflower. The value of parameter a was generally similar to that of other C species, but the value for parameter b differed. This was possibly associated with species differences in dry mass partitioning, and justified the development of a sunflower-specific CNDC. A preliminary reference curve for maximum [N] suggested an evolution from the juvenile stages to the end of flowering similar to that of [N] . Minimum [N], in contrast, appeared to be more constant over time. Relationships between relative grain yield and NNI across a range of locations indicated that in general, maximum grain yield was reached around NNI=0.8, although at one location this was around NNI=1.0. The CNDC can provide useful applications for crop modeling, N status diagnosis, and N fertilization decision

A model of pollinator-mediated gene flow between plant populations with numerical solutions for bumblebees pollinating oilseed rape

We present a model that predicts the level of gene flow mediated by animal pollinators from a source population to a sink population. The model requires specification of three elements: (1) the paternity that originates from a single flower, the paternity shadow; (2) the mean number of flowers that pollinators visit during stays in the sink population, the residence; (3) the proportion of pollinators arriving at the sink that carry pollen from the source population. Provided that pollinators visit enough flowers in the sink to exhaust the paternity shadows from the source, the general results are that gene flow is inversely proportional to the mean pollinator residence in the sink population, and is proportional to the fraction of pollinators arriving with pollen from the source. These results are used to propose explanations for two of the widely observed patterns in gene flow among plant populations. Numerical solutions to the model are derived using experimentally determined values of elements (1) and (2) that represent bumblebees, Bombus spp., visiting agricultural fields of oilseed rape, Brassica napus L. In B. napus, the paternity shadow attenuates rapidly over approximately 20 recipient flowers. Mean bumblebee residences in the fields studied varied between 490 and 720 flowers. In the absence of a direct measurement of element (3), we calculated the maximum level of bumblebee-mediated gene flow by assuming that all bees arrived at the sink saturated with pollen from extrinsic sources. In this case, the model predicts that bumblebee-mediated gene flow accounted for between 0.1% and 0.5% of the progeny in the agricultural fields studied. A likelihood analysis Of Our observations is unable to reveal convincingly the proportion of bees arriving at the sink via a source population, but the literature suggests that bumblebees have high site fidelity, which implies that bee-mediated gene flow may be substantially less than our estimated maximum. We consider the role of various factors, including wind pollination, in accounting for the differences between the model's predictions and the generally higher levels of gene flow observed in previous studies of oilseed rape