Glycine-chelated zinc lowered foliar phytotoxicity than excess zinc sulfate and improved zinc use efficiency in two sweet potato cultivars

Although applying widely used ZnSO4 at high spraying rates provides high Zn concentration in some crops, it usually causes leaf burn and yield and fertilizer losses. Whether chelated Zn fertilizers can improve Zn con-centration in edible roots of sweet potato while concomitantly avoiding apparent adverse effects deserves detailed study. Field experiments were conducted in 2019 and 2020 on two sweet potato cultivars to study the effects of foliar spraying of three typical Zn sources (i.e. zinc sulfate heptahydrate [ZnSO4], EDTA-chelated zinc [ZnEDTA], and glycine-chelated zinc [ZnGly]) at normal to high spraying Zn rates of 1.19, 2.39, 4.78 and 7.16 kg ha(-1). The results showed that excess ZnSO4 spray caused greater damage to the leaves of cultivar Tianfeng1 than to those of cultivar Pushu90, whereas both chelated Zn fertilizers reduced the extent of foliar burn on the two cultivars. ZnGly achieved a higher biofortification level than ZnEDTA and higher yield level than ZnSO4. ZnGly facilitated the most efficient storage of Zn in the edible roots, and the highest Zn use efficiency was observed under ZnGly treatment at spraying rates of 1.19 or 2.39 kg ha(-1). In summary, the high fertilizer efficiency and safety threshold spraying rate of ZnGly are potentially useful for Zn biofortification of sweet potato.