Absorption of foliar applied Zn is decreased in Zn deficient sunflower (Helianthus annuus) due to changes in leaf properties

Aims Despite the importance of foliar Zn fertilisation, it remains unclear how the Zn status of the plant itself influences the efficacy of foliar applied Zn, with this forming the focus of the present study. Methods Sunflower (Helianthus annuus) was grown in nutrient solutions with either 0.5 (sufficient) or 0 μM Zn (deficient) to relate the absorption of foliar applied Zn to leaf properties. The distribution and speciation of Zn within the leaf were examined in situ using synchrotron-based X-ray fluorescence microscopy and X-ray absorption spectroscopy. Results Zinc deficiency decreased foliar absorption of Zn as ZnSO4 by 50–66% compared to Zn sufficient sunflower, despite μ-XRF analysis showing the pattern of Zn absorption was similar for both plants. Rather, Zn deficiency decreased the leaf trichome density and likely altered the adaxial leaf surface composition and structure, with these presumably being the main causes for the reduced Zn absorption. The Zn status of the plant also influenced the speciation of the absorbed Zn, with 37–57% as Zn phytate in Zn sufficient leaves but 55% as Zn phosphate in Zn deficient leaves. Conclusions Zinc status of the plants influenced leaf surface properties, with Zn deficiency leading to decreased absorption of foliar applied Zn.