Abstract
The rice root system develops a large number of nodal roots from which two types of lateral roots branch out, large L-types and fine S-types, the latter being unique to the species. All roots including S-types are covered by root hairs. To what extent these fine structures contribute to phosphate (P) uptake under P deficiency was investigated using a novel 3-D root growth model that treats root hairs as individual structures with their own Michaelis-Menten uptake kinetics. Model simulations indicated that nodal roots contribute most to P uptake followed by L-type lateral roots and S-type laterals and root hairs. This is due to the much larger root surface area of thicker nodal roots. This thickness, however, also meant that the investment in terms of P needed for producing nodal roots was very large. Simulations relating P costs and time needed to recover that cost through P uptake suggest that producing nodal roots represents a considerable burden to a P-starved plant, with more than 20 times longer pay-off time compared to S-type laterals and root hairs. We estimated that the P cost of these fine root structures is low enough to be recovered within a day of their formation. These results expose a dilemma in terms of optimizing root system architecture to overcome P deficiency: P uptake could be maximized by developing more nodal root tissue, but when P is growth-limiting, adding more nodal root tissue represents an inefficient use of the limiting factor P. In order to improve adaption to P deficiency in rice breeding two complementary strategies seem to exist: (1) decreasing the cost or pay-off time of nodal roots and (2) increase the biomass allocation to S-type roots and root hairs. To what extent genotypic variation exists within the rice gene pool for either strategy should be investigated.
Highlights
Rice consumption in Africa is increasing rapidly, leading to a widening gap between demand and local rice production (Nigatu et al, 2017)
The objectives of our study were (1) to obtain detailed data on root growth and P uptake of upland rice from both field and greenhouse experiments using DJ123 as a representative P-efficient upland rice genotype; (2) to use this data to parameterize and test a root architecture model that includes root hairs; (3) to use this root architecture model to simulate P uptake; (4) to conduct a sensitivity analysis in order to identify factors exerting a strong influence on P uptake; and (5) to conduct a cost-benefit analysis with regard to P needed to grow a certain root class versus the P uptake provided by that class
Tissue P analysis was done by digesting tissue samples in a 4:1 HNO3/HClO4 mixture followed by Modeling Root Growth in OpenSimRoot
Summary
Rice consumption in Africa is increasing rapidly, leading to a widening gap between demand and local rice production (Nigatu et al, 2017) One reason for this widening gap is the relatively low rice productivity in Sub-Saharan Africa (SSA), which is due to a combination of low production inputs and unfavorable production environments (Saito et al, 2019). African farmers producing upland rice are generally lowincome farmers that acquire very limited amounts of fertilizers and other inputs (Tsujimoto et al, 2019). Their economic conditions exacerbate problems inherent to the uplands, where highly weathered soils such as Oxisols tend to bind phosphate (P) in plant-unavailable forms. It is of utmost importance that rice varieties with improved P acquisition and utilization efficiency are developed, as these provide a cost-efficient partial solution to the soil fertility problem in SSA (Vandamme et al, 2015)
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