Absorption and translocation of Zn foliar fertilisers

Abstract

Crop Zn deficiency is associated with low Zn availability in soils and is a widespread problem, also resulting in human Zn deficiency. Foliar Zn fertilisation has been shown to be an efficient approach for overcoming this problem. However, the mechanisms by which Zn moves across the leaf surface (penetration across the cuticle and epidermal cells and into the underlying tissues) and is subsequently translocated remain unclear, with this being the main focus of the present research. Specifically, using ZnSO4 and nano-ZnO, the roles of leaf cuticle, trichomes, and stomata in foliar Zn absorption were examined, with the distribution and the speciation of the absorbed Zn in the leaves determined. In addition, the influence of plant Zn status on foliar Zn absorption was also examined. The first experiment aimed to obtain a general understanding of the effects of leaf properties on the foliar absorption of Zn. Methyl jasmonate (MeJA) (0, 0.1, 0.5, 1, and 2.5 mM) was applied to leaves of soybean (Glycine max), sunflower (Helianthus annuus), and tomato (Solanum lycopersicum) to alter leaf properties. It was found for all three plant species that treating of MeJA caused substantial increases (1.3-3.5-times higher) in leaf trichome density, and the hydrophobicity of the adaxial leaf surface. The changes in stomatal density and leaf cuticle thickness varied with plant species and MeJA concentration. Based upon these results, the second experiment used 0 (control) and 1 mM MeJA treatments to examine the effects of changes in leaf properties on the absorption of foliar-applied Zn, Mn, and Fe, using synchrotron-based X-ray fluorescence microscopy (µ-XRF) to examine the distribution of elements in situ within hydrated leaves. Interestingly, foliar absorption of Zn, Mn, and Fe increased up to 3-to 5-fold in sunflower leaves treated with 1 mM MeJA, but decreased by 0.5-to 0.9-fold in leaves of tomato treated with 1 mM MeJA. It was found that these changes were related to the thickness of the cuticle and epidermal cell wall, suggesting that the cuticle is important for absorption of foliar-applied nutrients. However, subsequent translocation of the absorbed Zn, Mn, and Fe within the leaf tissues was limited (moving < 1.3 mm in 6 h) irrespective of the plant species and treatment. Of the three nutrients, Zn was translocated a greater distance within the vein than in the interveinal tissues (being translocated ca. twofold further). The third experiment gave specific consideration to the role of trichomes in foliar Zn absorption. Using µ-XRF for the in situ analyses of nutrient distribution in leaves of soybean and tomato, it was found that upon the foliar-application of ZnSO4, Zn accumulated in some spectroscopy (XAS) analyses showed that the Zn in the leaves following foliar application was mostly present as Zn phytate (37-53%) in Zn sufficient leaves, but as Zn phosphate (about 55%) in Zn deficient leaves. Across several of the experiments described above, the foliar absorption of ZnSO4 and nano-ZnO were compared. It was found tha...

Zinc Hyperaccumulation in Plants: A Review

“…Zn is largely adsorbed in changeable forms, mainly as Zn 2+ , ZnOH + , and ZnCl + , onto clay surfaces and organic matter [11]. Roots generally absorb zinc as Zn 2+ ions except at high pH, where it is absorbed as ZnOH [128].…”