Dynamics of nitrogen uptake and mobilization in field-grown winter oilseed rape (Brassica napus) from stem extension to harvest. II. An 15N-labelling-based simulation model of N partitioning between vegetative and reproductive tissues

Abstract

Background and aims: Oilseed rape (Brassica napus) has often been used as a catch crop to deal with the issue of N leaching, but for this to be effective, prediction of the crop's N uptake capability and N partitioning is required. The aim of this work was to build a compartmental model of N dynamics in oilseed rape, based on the kinetic description of N uptake, partitioning and mobilization in each organ.

Model: In this study, logistic and exponential equations were fitted to the N relations of each compartment, especially the leaf at each node. Data previously obtained from an 15N-labelling field experiment was used to quantify the partitioning of total N content, the allocation of N taken up and subsequent changes in the sink/source status for endogenous N in each tissue throughout the growth cycle.

Key results and conclusions: This modelling approach provides a unique tool for the quantitative estimation of cycling of endogenous N in relation to changes in N uptake at the whole-plant level. Furthermore, as oilseed rape is known to release large amounts of N to the soil during spring through leaf loss, this model was used to identify potential methods for improving the N harvest index of the crop. Simulations showed that N content or yield could be improved by 15% by optimizing N transfer from vegetative to reproductive tissues and by reducing the residual %N (DW) in abscised leaves.

Fig. 1.

N dynamics in each organ during source/sink transition in terms of parameters characterizing N allocation and N mobilization. t_Nmax and t_Nmin, time when organ reaches N_max and N_min, respectively (°Cd); N_max and N_min, highest and lowest total N content value (mg plant⁻¹); k_TNin and k_TNout, slope of equations characterizing N allocation and N mobilization (°Cd⁻¹); k₁, k₂ and k₃, slope of equations characterizing ¹⁵N allocation, endogenous N allocation and mobilization, respectively; N₁, highest N content derived from N uptake; N₂, highest N content derived from mobilization; t_mob, time when endogenous N mobilization starts.

Fig. 2.

Timing of changes in source–sink relationships in different tissues in Brassica napus during its growth cycle and balance of associated N flows.

Fig. 3.

Conceptual overview of the proposed compartmental model used for dynamic calculation of N flows during the growth cycle in Brassica napus. Model is made up of i organs (ranging from 1 to 29) and j leaves at different nodes (numbered from 11 to 36).

Fig. 4.

Cumulated total N contents in different tissues of Brassica napus (pods, leaves at node 20 and node 26, stem and taproot), and N content derived from uptake or from mobilization. Vertical bars indicate ± s.e. for n = 3, when larger than the symbol.

Fig. 5.

Simulated changes of mineral N uptake by Brassica napus during the growth cycle and of the cycling N compounds pool at the whole-plant level, estimated strictly from mobilization of endogenous N, or by considering that either 20 % or 40 % of N taken up is reduced in the roots, and the resulting amino acids increasing the pool of N compounds.

Fig. 6.

Simulated flows of mobilized N from and to each tissue, calculated from the compartmental model, and for the different simulations tested [S0 (control), S1, S2 and S3, as explained in legends of Tables]. Numbers in brackets indicate the amount of endogenous allocated, or mobilized, N as compared with S0.