Traits and trade-offs: dissecting nitrogen use efficiency in maize using plant modeling

Abstract

Excessive nitrogen (N) can be lost to the ecosystem and result in acidification and/or eutrophication of natural ecosystems. Reducing N application in agricultural systems is necessary to mitigate environmental pollution. Breeding for crops with a high N use efficiency can contribute to maintaining or even increasing yield at lower amounts of nitrogen fertilizer applied. Nitrogen use efficiency is co-determined by N uptake and physiological use efficiency (PE, grain biomass per unit of N taken up), to which soil processes as well as plant architectural, physiological, and developmental traits contribute. This thesis aims to quantify the effects of relevant maize traits and mechanisms to nitrogen use efficiency to support maize breeding programs for sustainable maize production.I developed and validated a functional-structural plant (FSP) model which simulates nitrogen uptake and the consequences for plant growth, as a function of a range of plant traits. I show that it is primarily the architectural and developmental traits that are relevant for N uptake and plant N use; not so much the physiological traits. Furthermore, I show that plasticity can be important not only under nitrogen-limiting conditions but also under non-limiting conditions, notably by improving nitrogen uptake. Importantly, this increase in uptake was linked to changes in the vertical distribution of roots rather than to total root system size. In addition, I show that using maize cultivars that plastically respond to plant N status by improving N uptake may mitigate any negative effects of stay-green on grain nitrogen content. Finally, I combined cluster analysis with FSP modelling to identify and quantify root system phenotypes that allow for both high yield and high N uptake. The results demonstrated trade-offs in root phenotypes for high yield and for high N uptake, between root sink strength for carbon, and root-to-leaf biomass partitioning. Using cluster analysis I identified two root phenotypes that give both high yield and high N-uptake, the combination of low root biomass and high root length density at 15 cm to 45 cm depth allowed maize to combine high yield and high N uptake.Overall, this thesis shows combining ecological principles with current breeding criteria can add new dimensions to the process of selection for new cultivars. However, we still v need to understand more about the role of trade-offs between traits to maximize plant growth while being efficient in resource use, as well as the role of phenotypic plasticity in the relationship between productivity and resource capture over a wide range of environments. Therefore, combining in silico and in vivo approaches can allow a better and more solid understanding of how individual physiological mechanisms influence both plant and crop performance.