In Situ Analysis of Nickel Uptake from Foliar Application in Pecan Using Instrumental µXRF Analysis

Nickel is an essential element as the cofactor in the enzyme urease, and pecan ( Carya illinoinensis (Wangenh.) K. Koch) is known to suffer from commercially important nickel deficiency manifested in “mouse-ear” disease. Foliar application of nickel has been shown to increase foliar nickel concentrations and foliar urease activity and to cure the nickel deficiency syndrome. However, there is currently no detailed knowledge on the uptake pathways of nickel into pecan leaves and subsequent internal redistribution of nickel after foliar application. In this study, we cultivated pecan seedlings for a period of 6 months after germination (growing to 250–350 mm tall with mature compound leaves) under low nickel availability in hydroponics solutions. The solution was purified to < 0.001 mg L −1 of nickel, and plants displayed severe “mouse-ear” deficiency symptoms. Half of the plants were then treated with a foliar nickel spray (100 mg L −1 nickel), and leaves were analysed using instrumental micro-X-ray fluorescence (µXRF) analysis, and total leaf nickel concentrations and urease activities were determined. The results show that nickel is rapidly taken up in the leaf and distributed into the vascular bundles. Application of nickel leads to a major increase in urease activity as well as in leaf nickel concentrations, and young emerging growth no longer display deficiency symptoms. The uptake pathway of nickel following foliar application is via the main vein into the secondary veins towards other tissues.