Foliar application of zinc (Zn) is one of the important agronomic approaches to improve and fortify Zn nutrition in crops and fruits, particularly when soil factors limiting Zn availability and/or soil Zn supply could not satisfy the peak Zn demand during the maximal vegetative or early reproductive growth stages. Conventional foliar Zn fertilisers are primarily formulated from soluble inorganic Zn chemicals and Zn-chelates, but new generations of foliar Zn fertilisers of low phytotoxicity are much needed to provide long-lasting efficacy for matching the peak Zn demand in intensive cropping and horticulture systems. The present objective was to characterise foliar uptake of Zn from a newly synthesised zinc hydroxide nitrate [Zn5(OH)8(NO3)2•2H2O] nanocrystals (ZnHN) on leaf surfaces of different characteristics. The purposely engineered ZnHN suspension with controlled solubility for low phytotoxicity and the morphology of positively charged sheets for high surface adherence will provide the basis for developing a new generation of Zn foliar fertiliser with a long-lasting and consistent Zn2+ supply at leaf surfaces.
For accurately quantifying the net Zn uptake from foliar Zn application, new leaf-washing protocols using dilute nitric acid (2%) ± ethanol (3%) were established to remove residue particles of suspension-based Zn chemicals (low solubility), such as ZnO and ZnHN on the youngest fully expanded leaves of three plant species representing 3 major types of leaf surface characteristics (citrus, capsicum and tomato). A half-leaf loading method was evaluated and established for rapid comparison of Zn uptake efficacy of foliar Zn fertilisers under controlled conditions, without the stringent requirement for control leaves (without foliar Zn treatment) with similar background Zn concentrations to those treated leaves at the beginning. This was based on the fact that there was no significant horizontal movement of the Zn absorbed in the Zn-treated half across the mid-rid into the other untreated half.
By using these established methods, short-term foliar Zn uptake from ZnHN suspension at tomato leaf surfaces were firstly investigated, in response to different environmental factors and surface structures (adaxial vs. abaxial). Foliar Zn uptake linearly increased with ZnHN application dose or concentration, with a preferentially higher uptake at the abaxial surface than the adaxial, presumably due to higher densities of stomata and trichomes. The role of stomata in Zn uptake was indirectly demonstrated by the higher Zn uptake during the light phase than the dark phase. Foliar Zn uptake from ZnHN was significantly limited by the ambient relative humidity at 80% or lower, due to high point of deliquescence of ZnHN (~87%).
The synchrotron-based X-ray fluorescence microscopy (µ-XRF) has been firstly used to spatially and non-destructively map the characteristics of short-distance diffusion of the absorbed Zn in fresh leaves of tomato and citrus. It was found that the magnitude of the absorption of Zn was influenced by the form of Zn applied and the leaf surface to which it was applied (abaxial or adaxial). Once the Zn had moved through the leaf surface it appeared to bind strongly with limited further redistribution. Regardless, in these underlying tissues, Zn moved into the lower-order veins, with concentrations 2- to 10-fold higher than in the adjacent tissues. However, even once in higher-order veins, the movement of Zn was still comparatively limited, with concentrations decreasing to levels similar to the background within 1-10 mm.
The efficacy of the ZnHN in whole plants was investigated in glasshouse experiments to examine the relationship between foliar Zn uptake and ZnHN concentration, and the distribution and redistribution patterns of foliar absorbed Zn from the ZnHN in tomato plants. Total Zn absorption from ZnHN suspension positively increased with increasing ZnHN application rate, but the relative efficacy started to decline at > 400 mg Zn L-1. Excluding the consideration of rainfastness, application of ZnHN can last longer than Zn(NO3)2, since around 16% of the total ZnHN was taken up in 3 weeks, in comparison with near 90% of the applied Zn(NO3)2. Foliar-absorbed ZnHN-Zn was distributed from the fed leaves into other growth parts and preferentially translocated into roots with less retention in the fed leaves compared to Zn(NO3)2.
Overall, the newly synthesised ZnHN nanocrystals can gradually dissolve and continually replenish the Zn2+ pool in the aqueous phase on leaf surfaces, thus maintaining a relatively stable Zn2+ concentration gradient at the surfaces for a long-lasting foliar Zn uptake over an extended growth period. The efficacy of the prolonged foliar Zn supply could be enhanced if the ZnHN suspension be sprayed over a large leaf surface area at peak vegetative or early flora development stage. However, further research is required to investigate the effects of humectants, which are required to lower the POD of ZnHN for its use under a wide range of climatic conditions in practice.