Zinc isotope fractionation in soil-rice systems constrained by varying water regimes and Zn fertilizer application: implication for Zn biofortification

Abstract

Background and aims Zinc (Zn) deficiency in rice (Oryza sativa L.) induced by flooded conditions is a persistent global issue that limits grain Zn accumulation. Soil drainage and Zn fertilizer can enhance soluble Zn in the soil to address Zn deficiency in rice grains. However, the mechanisms underlying the uptake and transport in rice plants remain unclear. Methods Zn uptake and transport processes in rice constrained by water regimes and Zn fertilizers were elucidated using Zn isotope and gene analyses. Results Compared with bulk soil, the Zn isotope compositions of CaCl2 and HCl-extracted pools were systematically higher, but the soil solution was much lighter. All the soil pools marginally varied with the constraining water regimes. In rice, ZIP family genes were constitutively downregulated in the roots and nodes, whereas OsHMA2 was upregulated under the drainage regime and Zn addition relative to the flooding regime. Soil solution-to-shoot and node I-to-grain isotope fractionation were hardly affected by the water regime. However, the fractionation shifted negatively with Zn addition during uptake and was less pronounced for shoot-to-node I, and node I-to-grain transport. Conclusions Switching water regimes from flooding to drainage negligibly affected soil solution-to-shoot transport and node-controlled allocation of isotopically light Zn to grains. However, rice utilizes a low-affinity transport system to isotopically transport light Zn, and the ZIP family transporters become less important when Zn fertilizer is applied. This study demonstrated that Zn fertilizer is more robust for reinforcing grain Zn relative to the drainage regime.