Zinc interception and absorption in wheat spikes contribute significantly to grain zinc biofortification

Context The spike is a crucial organ for intercepting exogenous zinc (Zn) at the late growth stage of wheat (Triticum aestivum L.). However, the role of spikes in wheat biofortification has drawn little attention, and knowledge gaps exist with respect to absorption and translocation of exogenous Zn by wheat.Aims This study aims to determine the role of spikes in wheat biofortification when Zn is applied to the canopy, and to characterise the absorption and translocation of Zn applied to different wheat organs.Methods In two field experiments and one pot experiment, Zn was applied at different stages (heading or early filling stage) to the canopy or to different organs (flag leaf or spike), and Zn concentrations in various organs were determined. Zn interception, Zn absorption, recovery in grain, and translocation amount and rate were calculated.Key results With application to the canopy, the Zn interception rate of spikes was 13% at heading and 28% at early filling. Grain Zn concentration was improved by 17-33% under flag leaf Zn application and 30-37% under spike Zn application, with absorption accounting for 68-90% and 88-99% of Zn applied, respectively. Zn applied at heading was translocated throughout plants before anthesis, and then to spikes. Zn applied at early filling was entirely translocated to spikes. The amount of Zn translocated after flag leaf application and spike application accounted for 16-40% and 13-14% of absorbed Zn, respectively.Conclusions Spikes are critical for intercepting and absorbing exogenous Zn. The Zn absorbed by the spike was more effective for grain Zn biofortification than the Zn absorbed by the flag leaf.Implications The information generated from this study assists in understanding how wheat plants intercept, absorb and translocate Zn. Understanding of interception, absorption and translocation of exogenous Zn in wheat is important to the goal of improving biofortification efficacy; however, knowledge gaps exist with regard to absorption and translocation of exogenous Zn by wheat and the role of spikes in biofortification. Results show that spikes are critical in intercepting and absorbing exogenous Zn, and translocation of exogenous Zn is dependent on application stage. The outcomes improve our knowledge of how wheat plants intercept, absorb and translocate Zn.