Antioxidant Capacity and Zinc Content of ‘Wichita' Pecan Kernels Is Increased by Soil Zinc Fertilizer Application

Pecans possess higher kernel antioxidant activity than any other tree nut. Antioxidants in the diet function to counteract damaging effects of free radicals and may play an important role in prevention of numerous chronic and age-related diseases. Pecan trees in the western US, where soils are typically alkaline and calcareous, require frequent zinc (Zn) fertilizer applications to maintain normal canopy growth and development as well as nut production. Our objective was to investigate the relationships between the antioxidant content in pecan kernels with tree zinc nutrition. The study was conducted in 2014 in a commercial pecan orchard in San Simon, AZ. Trees were fertilized with Zn (as Zn EDTA injected through a microsprinkler irrigation system) at one of three seasonal rates: 4, 2, or 0 kg·ha-1 Zn. Leaf tissue zinc concentrations for the treatments were 32, 24, and 14 mg·kg-1, respectively. Treatment plots each consisted of 12 ‘Wichita’ pecan trees arranged in a randomized complete block design with four blocks. Nuts were harvested at maturity from all trees within each plot and subsamples were collected for analyses of kernel Zn, oil yield, and antioxidant capacity.  Lipids were quantified on a mass percent basis from hexane extractions of homogenized kernels. Antioxidant capacities (AC) were evaluated by 2,2-diphenyl-1-picrylhydrazyl (DPPH) free radical microplate assay. Extracts were obtained from the defatted kernel meal.  AC was expressed as Trolox Equivalents·g-1 of pecan kernel sample (TE·g-1). Kernel Zn concentrations were 43, 38, and 14 mg·kg-1 for the 4, 2, and 0 kg·ha-1 Zn rates, respectively. Kernel oil content ranged from 68-73%, but was not impacted by Zn fertilizer application.  Defatted kernels of the two Zn treatments had significantly higher AC values than the control (0 kg·ha-1 Zn); average AC was 0.135, 0.132, and 0.116 TE·g-1 for 4, 2, and 0 kg·ha-1 Zn rates, respectively. In conclusion, soil application of Zn fertilizer improved human health-promoting aspects of pecan kernels, which may be valuable for product promotion amongst health-conscious consumers.