Studying the differences in absorption and distribution of foliar-applied zinc in maize and wheat grown with low or adequate Zn supply

Human zinc (Zn) deficiency is an important health problem worldwide, especially in developing countries. It is mainly caused by consumption of modern high-yielding cereal cultivars with low concentrations and bioavailability of Zn. Zinc concentration in cereal grains can be improved by genetic and/or agronomic biofortification. Agronomic biofortification, which is based on optimized applications of soil and foliar Zn fertilizers, is found very effective for some cereals such as wheat and rice. However, maize appeared to be less responsive to foliar fertilizers in term of grain Zn enhancement. The aim of this study is to elucidate the physiological reasons behind the poor response of maize crop to foliar Zn applications compared to wheat. A series of experiments have been conducted to reveal the differences in leaf uptake, root and shoot translocation of foliar-applied Zn in maize and wheat by using stable isotope of Zn (i.e., 70Zn). The results showed that wheat has greater capacity for leaf absorption and remobilization of foliar-applied zinc compared to maize. Increased leaf Zn uptake and localization in wheat leaves was visually confirmed by fluorescence microscopy using Zinpyr, a Zn-responsive fluorescent dye. Although, the plant Zn nutritional status affect the initial absorption and penetration of foliar-applied Zn and it subsequent redistribution within the plant, differences in uptake rate was associated with leaf morphology and physiological characteristics in these species. Poor response of maize to foliar Zn spray was linked to poor absorption of Zn through leaf cells and thus translocation to other plant parts. These results advance our understanding of factors that influence the efficacy of foliar Zn fertilizers in maize and wheat crops.