Tracking Foliar Zinc Absorption in Apple Trees

09/01/2020

Zinc deficiency in the soil can affect plant health, leading to poor yields, especially in fruit crops. Zinc is a vital micronutrient involved in various cellular processes and plays a critical role in maintaining the structure of various plant proteins, such as transcription factors. Therefore, an interruption or decline in zinc supply substantially affects vegetative development and reproductive success in subsequent years.

Zinc fertilization is standard agricultural practice for fruit trees, but achieving successful mitigation can be challenging, particularly in alkaline soils characterized by strong zinc fixation capacity. Nutrient supplementation usually requires application of large quantities of fertilizer, which is cost prohibitive. It is also frequently ineffective for fruit tree crops because the root systems penetrate deep into soil layers and zinc mobility is often poor in soils.

In contrast to soil amendments, foliar application of nutrients guarantees fast and targeted uptake through direct delivery of nutrients to plant tissues during vital growth stages. Foliar nutrient penetration is a complex process which relies on the leaf surface characteristics of plants, the physicochemical properties of the chemical nutrients, supplement type and concentration, and environmental conditions.

In a study using synchrotron-based X-ray fluorescence microscopy (XRF) on SSRL’s beam line 2-3, Tian et al. tracked the in vivo localization of zinc after foliar fertilizer treatment of apple plants (Malus domestica Borkh.). Apple was selected because of its high sensitivity to zinc deficiency and the fact that apple orchards in areas with calcareous and salt-affected soils suffer from zinc deficiency, resulting in significant yield loss and deterioration of fruit quality.

The researchers found that the absorption of foliar-applied zinc was largely dependent on plant leaf surface characteristics. In particular, abaxial leaf surfaces—the underside of the leaf—absorbed significantly greater zinc concentrations than adaxial leaf surfaces.

High-resolution elemental maps revealed that the cell wall’s high binding capacity for zinc contributed to its limited penetration across epidermal cells. Trichome density and stomatal aperture had opposite effects on zinc fertilizer penetration: a relatively high density of trichomes increased the hydrophobicity of leaves, whereas the presence of stomata facilitated foliar zinc penetration.

The results indicates that the extent of zinc mobilization is a key factor in its foliar uptake. Low levels of zinc promoted the accumulation of other mineral elements in treated leaves, and the complexation of zinc with phytic acid likely occurred due to exposure to high-zinc conditions.

The study provides direct visual evidence for the zinc penetration process across the leaf surface, which is important for the development of strategies for zinc biofortification in fruit-bearing crops.

References

Xie, R., et al. 2020. “Penetration of Foliar-Applied Zn and Its Impact on Apple Plant Nutrition Status: In Vivo Evaluation by Synchrotron-Based X-Ray Fluorescence Microscopy,” Horticulture Research 7, 147. [DOI:10.1038/s41438-020-00369-y]