Plant root nutrient uptake revisited

Trans-membrane transport proteins facilitate the internalisation of nutrients into plant roots either by active transport against or passive transport along an electrochemical gradient. Plant nutrient uptake from the external solution is a kinetic process that is dependent on the external nutrient concentration, formalised as Michaelis-Menten root ion uptake kinetics. A number of recent studies have demon- strated deviations from the classical Michaelis-Menten model. In contrast to the supposition that the plant uptake flux is solely determined by transporter properties, it has been shown that unstirred fluid layers, forming in the vicinity of plant root surfaces, even in well-stirred nutrient solutions, can cause ion diffusion to control the nutrient internalisation flux by limiting the ion resupply to the root cell surface. Moreover we recently showed that P efflux is highly localised to the tips of maize roots and is not, as previously assumed, homogeneously distributed across the entire root surface. In this research project we aim at obtaining a better understanding of plant nutrient uptake by gathering a large set of data on diffusion limitations in root nutrient uptake and on the localisation of ion efflux from roots, as well as at evaluating their impact on nutrient uptake modelling by applying a set of highly novel and innovative chemical imaging, nutrient solution sampling, and numerical simulation techniques. We expect the outcomes of this project to enhance the current understanding of plant nutrient uptake significantly, especially at the sub-mm scale, which is very challenging to investigate experimentally.

The overall aim of this project was to re-evaluate nutrient uptake by plant roots regarding the underlying conceptual understanding. In many studies, the nutrient uptake capacity of plant roots has received the majority of the scientific attention, while the capacity of the soil to resupply nutrients to a root, which constantly depletes the soil nutrient pool, has often received little attention. Moreover, the measurement of the affinity of roots to take up nutrients is experimentally challenging, and may have contributed a too strong emphasis of the root nutrient uptake properties and an undervaluation of the capacity of soils to resupply nutrients in plant nutrient acquisition. During nutrient uptake, small nutrient amounts are also lost from the roots to the surrounding soil, which is termed nutrient efflux. The localisation of nutrient uptake (influx) and nutrient efflux are experimentally especially challenging. In this project we developed methods capable of localising nutrient influx and efflux along the root axis on a sub-mm spatial scale, showing that nutrient influx and efflux show similar spatial patterns along plant root axes, an aspect of nutrient acquisition on which only little information was available so far. Moreover, we utilised these nutrient efflux localisation techniques to investigate complex nutrient uptake and efflux cycling around plant roots growing in soil and marine sediments. In another part of the project, we developed a novel approach to measure the nutrient release kinetics (i.e. time course of nutrient release) of soils, which are needed in the mathematical simulation of nutrient uptake by plant roots. Utilizing more complex, but therefore also more realistic mathematical models for soil nutrient release kinetics lead to a better agreement of experimental and simulated data. In two independent studies we could also show that soil nutrient release kinetics are often controlling plant nutrient uptake. This project has extended the current understanding of plant nutrient acquisition regarding the role of nutrient influx and efflux, as well as on soil nutrient resupply (kinetics) for plant nutrient acquisition.