Title : Root Uptake Modeling: Present state, and what are the basic obstacles still to overcome?
Prediction ability of nutrients uptake from soils with low availability is limited when it comes to quantify by modeling (Silberbush 2013). It looks like that the models developed so far failed to properly account for the processes at the soil-root interface (Silberbush and Barber 1983). Part of the problem was due to a limited capacity of computers at the time, which led to the use of single-root models of the root system (Cushman 1979). This model and others of this kind accounted for radial flow of water and nutrients through the soil towards a cylindrical root, as a result of demand by the root, by mass-flow and diffusion, and uptake by the root (influx) as a function of nutrient concentration in the soil solution at the soil-root interface, obeying Michaelis-Menten kinetics. When the limitation of computer capacity became relatively less crucial, accounting for as a branched system, resulted in more realistic predictions of uptake of barely-available nutrients like phosphate (P) and micro-elements (Roose and Fowler 2004). It led to the development of models accounting for more realistic root architecture (RA), like SimRoot and OpenSimRoot (Lynch et al. 1997, Postma et al. 2017), or of Dunbabin et al. (2004). Still, when it comes to nutrients, the dynamic processes at the soil-root interface are poorly treated, although some better work was done when it comes to water behavior at the root surfaces (Carminati and Vetterlein 2013).
Recent developments in soil-root biota studies have a potential of overcoming the above obstacle (Ofaim et al., 2017). It seems that bacteria, which actively accumulate at the soil-root interface, play a major role in the root functioning as an absorbing organ. These bacteria were selected due to their mobility, as they compete with other bacteria, to feed on the mucilage and materials actively exudates by the roots at pre-set locations. Moreover, the pattern of such exudation changes along the growing roots so, hypothetically, the root may actively drive the bacterial population location by controlling it by exudations. If this is true, it provides a tool to quantify the influx of phosphate and other barely-soluble nutrients along the growing root.
Nutrient influx along roots were already measured for specific purposes (York et al., 2016). Yet, influx of nitrate (in this case) was performed with plants grown in a nutrient solution, not in the soil, and was limited to root sections of 2 cm root length each (in the above-mentioned study). More detailed measurements along roots were not performed in the past, and not with influx of P. Phosphorus-32 was used as a tracer in certain studies involving root uptake, but for limited purposes like uptake by root hairs (Gahunia and Nielsen, 1998). The idea of using the relative abundance of bacteria along a growing root is, therefore, worth testing: If the abundance of the different types of bacteria along a root is controlled by the root, and is plant- and soil-specific, it may be used to quantify the influx along the root. Furthermore, it may give a new evolutionary perspective on plant adaptation to move from aqueous media to grow in soils.