Office of Research and Development Publications

Foliar application of zinc sulphate and zinc EDTA to wheat leaves: differences in mobility, distribution, and speciation

Citation:

Doolette, C., T. Read, C. Li, K. Scheckel, E. Donner, P. Kopittke, J. Schjoerring, AND E. Lombi. Foliar application of zinc sulphate and zinc EDTA to wheat leaves: differences in mobility, distribution, and speciation. JOURNAL OF EXPERIMENTAL BOTANY. Oxford University Press, Cary, NC, 69(18):4469-4481, (2018). https://doi.org/10.1093/jxb/ery236

Impact/Purpose:

Zinc (Zn) deficiency affects nearly half of the world’s agricultural soils and one third of the global population. Zinc deficiency in humans is particularly problematic for women and children in developing countries where the majority of food intake consists of cereal grains. Furthermore, with an ever-growing global population, there is a need to provide enough food that is of a sufficient nutritional quality. This leads to a situation of intensified food production, which means nutrients need to be added to food crops in the form of fertilisers. However, Zn deficiency in staple food crops is not only caused by low concentrations of Zn in soils, but also by its low bioavailability in soil. This study aims to provide detailed information on the mobility, distribution and speciation of foliar-applied Zn in wheat using radiolabelled 65Zn and state-of-the-art synchrotron techniques. Specifically, the objectives of this study are to: a) assess and quantify the mobility of Zn applied to leaves as ZnSO4 or ZnEDTA using a radioisotope tracing technique and synchrotron X-ray fluorescence microscopy (XFM), and b) identify the chemical speciation of Zn, within leaf tissues to which Zn has been applied in ionic or chelated form (ZnSO4 and ZnEDTA, respectively) using X-ray absorption spectroscopy (XAS). Ultimately, this experiment will test the hypotheses that ZnEDTA will be taken up more readily than ZnSO4 and be more mobile, and that ZnEDTA will be taken up and redistributed as the chelated complex.

Description:

Foliar application of zinc (Zn) to crops is an effective way to increase the grain concentration of Zn. However, the development of more efficient foliar Zn fertilizers is limited by a lack of knowledge regarding the distribution, mobility, and speciation of Zn in leaves once it is taken up by the plant. We performed an experiment using radiolabelled Zn (65Zn), and in situ time-resolved elemental imaging using synchrotron X-ray fluorescence microscopy (XFM), to investigate the behaviour of two commonly used Zn foliar fertilizers (Zn sulphate and ZnEDTA) in wheat (Triticum aestivum) leaves. Both experiments showed that Zn had limited mobility in leaves, moving <25 mm from the application point after 24 h. Although limited, the translocation of Zn occurred quickly for both treatments; moving more between 3 h and 12 h after application than between 12 h and 24 h. Speciation analysis using synchrotron-based X-ray absorption near-edge structure (XANES) showed that ZnEDTA was in fact taken up in chelated form and not as ionic Zn (Zn2+). The XANES data also showed that Zn, from both treatments, was then complexed by ligands in the leaf (e.g. phytate and citrate), potentially in response to localized Zn toxicity. The results of the present study provide important insights into the behaviour of commonly used foliar-applied Zn fertilizers, and can be used to optimize current fertilization strategies and contribute to the development of more efficient foliar Zn fertilizers.