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Upland rice (Oryza sativa) is adapted to strongly phosphorus (P) sorbing soils. The mechanisms underlying P acquisition, however, are not well understood, and models typically underestimate uptake. This complicates root ideotype development and trait‐based selection for further improvement. We present a novel model, which correctly simulates the P uptake by a P‐efficient rice genotype measured over 48 days of growth. The model represents root morphology at the local rhizosphere scale, including root hairs and fine S‐type laterals. It simulates fast‐ and slowly reacting soil P and the P‐solubilizing effect of root‐induced pH changes in the soil. Simulations predict that the zone of pH changes and P solubilization around a root spreads further into the soil than the zone of P depletion. A root needs to place laterals outside its depletion‐ but inside its solubilization zone to maximize P uptake. S‐type laterals, which are short but hairy, appear to be the key root structures to achieve that. Thus, thicker roots facilitate the P uptake by fine lateral roots. Uptake can be enhanced through longer root hairs and greater root length density but was less sensitive to total root length and root class proportions. Summary Statement: Upland rice increases phosphorus uptake in strongly sorbing soils by placing fine S‐type lateral roots inside the solubilization zones of thicker roots. [ABSTRACT FROM AUTHOR]

Iron toxicity is a major constraint to rice production, particularly in highly weathered soils of inland valleys in sub‐Saharan Africa where the rice growing area is rapidly expanding. There is a wide variation in tolerance of iron toxicity in the rice germplasm. However, the introgression of tolerance traits into high‐yielding germplasm has been slow owing to the complexity of the tolerance mechanisms and large genotype‐by‐environment effects. We review current understanding of tolerance mechanisms, particularly those involving below‐ground plant–soil interactions. Until now these have been less studied than above‐ground mechanisms. We cover processes in the rhizosphere linked to exclusion of toxic ferrous iron by oxidation, and resulting effects on the mobility of nutrient ions. We also cover the molecular physiology of below‐ground processes controlling iron retention in roots and root‐shoot transport, and also plant iron sensing. We conclude that future breeding programmes should be based on well‐characterized molecular markers for iron toxicity tolerance traits. To successfully identify such markers, the complex tolerance response should be broken down into its components based on understanding of tolerance mechanisms, and tailored screening methods should be developed for individual mechanisms. Progress in identifying markers for Fe toxicity tolerance in rice has been slow because of the complexity of tolerance mechanisms and the importance of below‐ground plant‐soil interactions. We review current understanding of these below‐ground processes, including recent advances in the molecular physiology of tolerance. [ABSTRACT FROM AUTHOR]

Zinc (Zn) deficiency is a major constraint to rice production and Zn is also often deficient in humans with rice-based diets. Efforts to breed more Zn-efficient rice are constrained by poor understanding of the mechanisms of tolerance to deficiency. Here we assess the contributions of root growth and root Zn uptake efficiency, and we seek to explain the results in terms of specific mechanisms. We made a field experiment in a highly Zn-deficient rice soil in the Philippines with deficiency-tolerant and -sensitive genotypes, and measured growth, Zn uptake and root development. We also measured the effect of planting density. Tolerant genotypes produced more crown roots per plant and had greater uptake rates per unit root surface area; the latter was at least as important as root number to overall tolerance. Tolerant and sensitive genotypes took up more Zn per plant at greater planting densities. The greater uptake per unit root surface area, and the planting density effect can only be explained by root-induced changes in the rhizosphere, either solubilizing Zn, or neutralizing a toxin that impedes Zn uptake (possibly HCO- 3 or Fe2+), or both. Traits for these and crown root number are potential breeding targets. [ABSTRACT FROM AUTHOR]

ABSTRACT Rice ( Oryza sativa L.) secretes far smaller amounts of metal-complexing phytosiderophores (PS) than other grasses. But there is increasing evidence that it relies on PS secretion for its zinc (Zn) uptake. After nitrogen, Zn deficiency is the most common nutrient disorder in rice, affecting up to 50% of lowland rice soils globally. We developed a mathematical model of PS secretion from roots and resulting solubilization and uptake of Zn, allowing for root growth, diurnal variation in secretion, decomposition of the PS in the soil, and the transport and interaction of the PS and Zn in the soil. A sensitivity analysis showed that with realistic parameter values for rice in submerged soil, the typically observed rates of PS secretion from rice are sufficient and necessary to explain observed rates of Zn uptake. There is little effect of diurnal variation in secretion on cumulative Zn uptake, irrespective of other model parameter values, indicating that the observed diurnal variation is not causally related to Zn uptake efficiency. Rooting density has a large effect on uptake per unit PS secretion as a result of overlap of the zones of influence of neighbouring roots. The effects of other complications in the rice rhizosphere are discussed. [ABSTRACT FROM AUTHOR]