Mechanical Pruning Improves Pecan Kernel Antioxidant Capacity over Three Consecutive Harvest Years.

Pecan (Carya illinoinensis) is the most horticulturally important tree nut native to North America. Pecan kernels are an excellent source of important nutrients and bioactive compounds, notably phenolics. There is a growing demand for pecans in domestic, Indian and Asian markets. Alternate bearing is a serious horticultural problem in pecan orchards. In the southwestern US, alternate bearing is primarily managed through mechanical pruning programs. The objective of our study was to investigate the effects of mechanical pruning on kernel antioxidant capacity. Our study was conducted in a bearing ‘Western’ pecan orchard in the Mesilla Valley, NM. Beginning in 2006, trees were subjected to a range of mechanical pruning frequency treatments (unpruned, annual, biannual and triannual pruning) paralleling conventional practices. At kernel maturity, nuts were sampled from three canopy-height zones in each of the 2012-2014 seasons. Phenolic compounds were extracted from defatted nutmeat using an acidified acetone extractant. The Total Phenolics Content (TPC) and Oxygen Radical Absorbance Capacity (H-ORAC_FL) assays were conducted to assess the in vitro phenolics contents and antioxidant capacities of the nutmeats, respectively. Glycoside-linked and ester-linked phenolics were further liberated from crude phenolic preparations by acid and base hydrolysis, respectively. These fractions were analyzed by HPLC with a fused-core C18 column. Results showed that alternate bearing significantly impacts (p<0.05) antioxidant capacities when all samples were grouped by harvest year. Higher TPC and H-ORAC_FL values were associated with pecans sampled during “on” year than “off” year. But when compared with non-pruned, the severity of alternate bearing was alleviated in all samples with pruning treatments applied across the three-year span, regardless of pruning frequency. Close examination of HPLC results for all 3 fractions revealed a standard set of phenolic compound profile in all samples, while a significant escalation for the concentration of ellagic acid and its derivatives was revealed when pruned (379 µg/g crude extract) and unpruned samples (247 µg/g crude extract) were compared. In conclusion, antioxidant capacities of pecan can be greatly boosted by mechanical pruning, even at a triannual frequency. The outcomes of this experiment provide pecan growers with cost-effective options to improve both nut productivity and human health promoting components in kernels.