Zinc agronomic biofortification in wheat and its drivers: a global meta-analysis

Zinc (Zn) deficiency affects 17% of the world's population, particularly those consuming large cereal grains with low Zn concentration and poor bioavailability. Here, we synthesize data from four field trials along with 139 studies from the literature, to evaluate the impact of management practices and soil properties on improving grain Zn concentration in wheat through applying Zn fertilizers. Soil application of Zn fertilizer improves grain Zn concentration by only 29.1%, below the biofortification target of 40 mg kg^-1 required for human nutrition. Foliar Zn application and soil+foliar applications increase grain Zn concentration by 55.2% and 62.3%, respectively, surpassing the 40 mg kg^-1 threshold. Factors such as the rate of Zn application, initial grain Zn concentration (prior to Zn application), and timing of Zn spraying are identified as principal factors influencing the response of grain Zn concentration to Zn fertilization. In search of desirable grain Zn concentration in wheat while minimizing potential environmental risks, our study proposes a method for assessing rates of Zn fertilizer application based on the Zn biofortification target, the Zn agronomic biofortification index, and the contribution of principal factors influencing the improvement in grain Zn concentration.