Quantitative methods for estimating foliar uptake of zinc from suspension-based Zn chemicals

The present study developed methods for quantifying foliar Zn uptake from suspension-based Zn chemicals of low solubility, which were ZnO (particle size: 0.15(-)1.34 m) and a newly synthesized Zn hydroxide nitrate crystal (Zn-HNC) (50(-)100 nm thickness and 0.2(-)1 m lateral dimension). Recently matured leaves of citrus (Citrus aurantium L. cv. Valencia), capsicum (Capsicum annume L. cv. Giant Bell), and/or tomato (Solanum lycopersicum L. cv. Roma) were in vitro-treated with microdroplets (5 L per droplet) of Zn-HNC- and ZnO-suspension solutions on the adaxial surface and incubated under controlled conditions for up to 72 h. Leaf-washing protocols were compared, including: dilute ethanol (3%), dilute nitric acid (2%), and their combination. The methods for quantifying Zn uptake were: (1) whole-leaf loading by which droplets of the Zn suspension solutions were loaded onto central regions of both left and right sides of leaf blades and (2) half-leaf loading by which soluble-Zn (ZnSO4) droplets were loaded onto only one side of leaf blades while the other was used as the background Zn control. Foliar-surface characteristics of the plant species affected the effectiveness of the washing methods. The dilute nitric acid (2%; +/- 3% ethanol) was required to remove residue particles of the ZnO and Zn-HNC suspensions from foliar surfaces of capsicum and tomato (highly trichomatic), but the residue Zn chemicals on citrus leaves (nontrichomatic and highly waxy) were similarly and effectively removed by the three washing methods. For quantifying Zn uptake by the leaves, the half-leaf loading method showed its advantages over the whole-leaf loading method, because it did not stringently require similar background Zn concentrations in the control and treated leaves at the start and had little risks of secondary absorption of soluble Zn in the washing solution.