NEMA, a functional-structural model of nitrogen economy within wheat culms after flowering. I. Model description

Abstract

Background and aims: Models simulating nitrogen use by plants are potentially efficient tools to optimize the use of fertilizers in agriculture. Most crop models assume that a target nitrogen concentration can be defined for plant tissues and formalize a demand for nitrogen, depending on the difference between the target and actual nitrogen concentrations. However, the teleonomic nature of the approach has been criticized. This paper proposes a mechanistic model of nitrogen economy, NEMA (Nitrogen Economy Model within plant Architecture), which links nitrogen fluxes to nitrogen concentration and physiological processes.

Methods: A functional-structural approach is used: plant aerial parts are described in a botanically realistic way and physiological processes are expressed at the scale of each aerial organ or root compartment as a function of local conditions (light and resources).

Key results: NEMA was developed for winter wheat (Triticum aestivum) after flowering. The model simulates the nitrogen (N) content of each photosynthetic organ as regulated by Rubisco turnover, which depends on intercepted light and a mobile N pool shared by all organs. This pool is enriched by N acquisition from the soil and N release from vegetative organs, and is depleted by grain uptake and protein synthesis in vegetative organs; NEMA accounts for the negative feedback from circulating N on N acquisition from the soil, which is supposed to follow the activities of nitrate transport systems. Organ N content and intercepted light determine dry matter production via photosynthesis, which is distributed between organs according to a demand-driven approach.

Conclusions: NEMA integrates the main feedbacks known to regulate plant N economy. Other novel features are the simulation of N for all photosynthetic tissues and the use of an explicit description of the plant that allows how the local environment of tissues regulates their N content to be taken into account. We believe this represents an appropriate frame for modelling nitrogen in functional-structural plant models. A companion paper will present model evaluation and analysis.

Fig. 1.

Overview of the model of N economy within a wheat (Triticum aestivum) culm after flowering. (A) Structure and functioning of N acquisition and distribution within a culm. Each lamina (la), sheath (sh) and internode (in), all chaff, all grains (grain), all roots (r) and the peduncle are represented by different modules, but only the uppermost phytomer (n), grains and roots are shown on the figure. Symbols in black represent the N-related model variables: PAR intercepted by each entity, photosynthetic N mass (N^ph) and area (A^green), remobilizable N mass in roots (N_r^rem) and total N mass in grains (N_grain^tot); N^struct, the structural N mass, shown in grey, is constant throughout the grain-filling period. A supplementary module describes the mobile N pool (N_c^mob), which is enriched by root N uptake and assimilation (U_r) and N^ph, N_r^rem degradation (D^Nph, D_r^rem), depleted by grain N filling (dN^tot_grain/dt) and N^ph, N_r^rem synthesis (S^Nph, S_r^rem). N fluxes between modules are represented by arrows. (B) Calculations within one model time step. Each box represents one meta-process, with the calculation principle indicated as well as the numbers of the equations; the number in the box corresponds to the calculation step; the dependence between one meta-process and the others is indicated by arrows. In addition, the common pool of N_c^mob is explicitly represented because of its central role in the regulation of N fluxes. The PAR intercepted by each entity is calculated once at flowering.

Fig. 2.

Botanical structures of a phytomer (A) and a culm (B) in wheat after flowering as implemented in the model. Laminae are defined by planar shapes characterized by their orientation, θ_la; sheaths and internodes are represented by vertical cylinders. Parts of internodes are surrounded by sheaths, and grains are surrounded by chaff; enclosed parts are represented by dotted lines. Phytomers are numbered according to their rank from the top of the culm, n being the uppermost phytomer.