Recent molecular and histochemical analyses of nitrate transporters have cast doubt on the ability of the Enzyme–Substrate interpretation of analysis of nitrate influx isotherms to improve modelling of N uptake in agronomic models. Le Deunff and Malagoli advocate the use an alternative formalism, the Flow–Force theory, to describe ion isotherms based upon biophysical ‘flows and forces’ relationships of non-equilibrium thermodynamics. This formalism can be combined easily with changes in the nitrate influx rate induced by climatic and in planta factors formalized by polynomial curves. They argue that application of the Flow–Force formalism allows nitrate uptake to be modelled in a more realistic manner, and allows scaling-up in time and space of the regulation of nitrate uptake across the plant growth cycle.
Using a thermodynamic flow–force interpretation of nitrate uptake isotherms, Malagoli and Le Deunff develop a functional– structural model to predict N uptake in winter oilseed rape, Brassica napus. The structural component of the model, the active root biomass, is derived from a combination of root mapping in the field, the relationship between specific root length and external nitrate concentration, and the assignment of an absorption capacity related to integrated root system age. They find that model simulations are well matched to measured data for N uptake under field conditions at three different levels of fertilizer application. Model ouputs indicate that the topsoil layers contain about 80 % of the total root system and account for 90–95 % of N taken up at harvest.
Difficulties in linking the various regulations of nitrate transport acting at different levels of time and on different spatial scales have hindered the development of models for nitrogen uptake. Le Deunff and Malagoli substitute the more usual enzyme–substrate interpretation for a ﬂow–force approach of nitrogen uptake isotherms and combine it with experimentally determined regulation in order to model nitrate in winter oilseed rape, Brassica napus. This approach avoids the use of unique nitrate uptake reference kinetics and allows root plasticity in response to environmental and in planta factors to be taken into account. Furthermore, it allows the regulation of nitrate uptake by roots to be scaled up relatively easily in time from hours to months.
Spontaneous male sterility is an advantageous trait for both producing hybrids and understanding the developmental process of the male reproductive unit in many crops. Lu et al. use a map-based cloning strategy together with comparative mapping to study the triallelic genetic male-sterile locus BnMs5 in Brassica napus, and delimit it to a 21-kb fragment of the A8 chromosome. Sequence analysis suggests that BnMs5 originates from the B. rapa MF2 subgenome, and may reside in a hotspot region of chromosomal evolution. This work paves the way for further cloning of BnMs5, and presents a powerful method for mapping loci in plants with complex genomic architecture.
Low levels of phosphorous in soils seriously limit seed production of Brassica napus. Ding et al. investigate phenotypic variation of seed yield and yield-related traits in B. napus plants grown with contrasting supplies of P, and identify a total of 74 putative quantitative trait loci (QTLs). The results suggest that different genetic determinants are involved in controlling yield and yield-related traits under normal and low P conditions, and the QTLs detected under reduced P supply may prove useful in marker-assisted selection.