Foliar Nutrient Sprays and Harvest Date Affect Total Phenolics, Antioxidant Activity, and Titratable Acidity of 'Wonderful' Pomegranate, Poster Board #199

Pomegranate (Punica granatum) fruit contain phytochemicals that are believed to provide beneficial effects to human health. To assess the effects of foliar nutrient applications and harvest date on the concentration of potentially beneficial phytochemicals in pomegranate fruit, research was conducted at two commercial 9-year old 'Wonderful' orchards in California's San Joaquin Valley. Zinc sulfate (ZnSO₄ at 3000, 4000, or 5000 mg·L^-1), magnesium sulfate (MgSO₄ at 1%, 2%, or 3%), potassium nitrate (KNO₃ at 1%, 2%, or 3%), or a control treatment of deionized water was applied by backpack sprayer at early fruit set (July; fruit green to breaker with caliper ~ 50 mm). MgSO₄ and KNO₃ treatments were applied again at late fruit set (August; fruit red with caliper of ~ 70 mm). A randomized complete block design with whole-tree experimental units and 7 blocks were utilized at each site to evaluate total phenolics (TP), antioxidant activity (AA), and titratable acidity (TA). Trees at Site 1 were harvested at the beginning of their commercial harvest period (October) and trees at Site 2 were harvested at the end of their season (November). A subsample of up to 5 unsplit fruit per study tree were collected for evaluation of postharvest parameters, including fruit mass and caliper. Arils were removed and stored at –80°C prior to phytochemical analyses. TP were measured in gallic acid equivalents (GAE) and quantified via spectrophotometry utilizing the Folin-Ciocalteu reagent. AA was expressed in percent radical scavenging ability and determined utilizing the DPPH free radical method. TA was quantified using an autotitrator. Early-harvested fruit had lower levels of TP, AA, and TA compared to late-harvested fruit and there was a significant interaction between site and treatment for TP. At both sites, treatments did not result in a significant reduction in any harvest parameter measured. At the late-harvested site, TP were statistically significantly greater in fruit from trees treated with 2% KNO₃ as compared to all other treatments, including the control. Taken together, the results of this study indicate that both foliar applications of KNO₃ and late-season harvests have potential commercial use to increase the levels of desired phytochemicals of ‘Wonderful’ pomegranate fruit.