Contrasting grain-Zn response of biofortification rice (Oryza sativa L.) breeding lines to foliar Zn application

•Heading stage foliar-applied Zn was remobilized to rice grain in most genotypes.•Foliar Zn application did not overcome Zn deficiency.•Foliar Zn application increased grain Zn by 1–10mgkg−1 depending on genotype.•IR68144 and IR83668 were Zn-deficiency tolerant and consistently high in grain Zn. Enriching grain Zn concentration in rice through Zn fertilization is one approach to Zn biofortification efforts. Experiments to understand the effectiveness of foliar Zn application at different growth stages in enriching grain Zn of biofortification breeding lines were conducted during the dry season at an experiment station and in a farmer's field with moderately and severely Zn-deficient soils, respectively. Foliar Zn applications at 4kgZnha−1 as zinc sulfate were given at different growth stages: active tillering, heading, and both active tillering+heading. Foliar Zn application at active tillering had no effect on brown rice Zn concentration, but, at heading, there was a range in genotype response from 1 to 10mgkg−1 increase, with an average increase of 4mgkg−1 at the moderately Zn-deficient site and 7mgkg−1 at the severely Zn-deficient site. At the moderately Zn-deficient site, two biofortification breeding lines (IR83668, IR91152AC) reached the target of 30mgkg−1 Zn in brown rice without Zn fertilization at heading stage, while two other genotypes (IR68144, IR91143AC) reached the target with heading-stage foliar Zn application. At the severely Zn-deficient site, only one of the tested genotypes (IR68144) reached the target Zn concentration even after foliar Zn application at both active tillering and heading stage and the same genotype recorded the greatest response to foliar Zn (10mgkg−1 increase). Greater total leaf area at spraying time increased the effectiveness of foliar Zn application at the severely Zn-deficient site only. Foliar Zn application at the two tested growth stages failed to overcome agronomic Zn deficiency. Three of the biofortification breeding lines (IR68144, IR85800, and IR83668) had high grain Zn content that was independent of grain yield. In a separate experiment to test a wider range of spraying times at the severely deficient site with IR64, the spraying at the early milk stage emerged as the most effective stage for increasing brown rice Zn concentration. Our results show that agronomic Zn biofortification through foliar Zn application is likely to be much more effective at increasing grain Zn concentration of genotypes with str