Does Foliar Zinc Application Boost Leaf Photosynthesis of ‘Wichita’ Pecan Fertigated with Zinc-EDTA?

Many growers fertigating their orchards with zinc–ethylenediaminetetraacetic acid (Zn-EDTA) are still using supplemental zinc foliar sprays because of a lack of confidence that soil-applied Zn-EDTA is supplying enough Zn to the trees. A field study was conducted in a pecan orchard located near San Simon, AZ, on 8-year-old ‘Wichita’ trees growing in an alkaline, calcareous Vekol loam soil to evaluate the effectiveness of supplemental foliar Zn sprays. All trees were fertigated with 6.0 kg⋅ha–1 Zn in the form of Zn-EDTA in 2018 and 11.0 kg⋅ha–1 Zn in 2019 and did not exhibit visible signs of Zn deficiency. Foliar treatments of 3.75 mL⋅L–1 urea–ammonium nitrate (UAN), 3.6 g⋅L–1 zinc sulfate monohydrate (ZnSO4·H2O), 3.6 g⋅L–1 ZnSO4·H2O with 3.75 mL⋅L–1 UAN, 11 mL⋅L–1 Zn-EDTA, and water alone were applied to individual fruiting shoot terminals of trees on two dates each in 2018 and 2019. Treatments were sprayed directly onto the leaves of the selected terminals. Zn-EDTA was included as a foliar treatment in 2019 only. Leaf photosynthesis was measured to determine the impact of leaf Zn concentrations on plant function. Midday stem water potential (MDSWP) was measured to verify that water stress was not limiting photosynthesis. Both measurements were taken about 2 to 4 weeks after the application of foliar treatments. MDSWP measurements indicated a lack of water stress and therefore no effect on photosynthesis. Leaf samples collected from untreated branches indicated that the average foliar Zn concentration of untreated leaves was 21.3 mg⋅kg–1 in 2018 and 15.7 mg⋅kg–1 in 2019. No differences were observed in photosynthesis rates of treated branches. No additional benefit to leaf photosynthetic function or appearance was observed from spraying Zn on foliage of trees fertigated with Zn-EDTA.