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Qinglong Zhang and Patrick H. Brown

The characteristics and mechanisms of foliar Zn uptake and translocation in pistachio (Pistachio vera L.) and walnut (Juglans regia L.) were investigated using 68Zn labelling in both intact and detached leaves. Following washing, mature walnut and pistachio leaves retained 8% and 12% of the total Zn applied, respectively. About half of retained Zn (3.5% and 6.5% of total Zn respectively) was absorbed into the leaf and translocated outside the treated area. Leaf age affected the Zn absorption capacity of pistachio but not walnut. Immature pistachio leaves absorbed more Zn than mature leaves. The absorption of Zn by walnut leaves at high concentrations (7.5 to 15 mm Zn) was not significantly affected by the pH of the solution. In pistachio Zn absorption was greatest at pH 3.5 and declined as pH increased to 8.5. The uptake process was not affected by light or addition of metabolic inhibitors. Foliar leaf absorption was only slightly affected by changes in temperature with an average Q10 of 1.2 to 1.4. This study suggests that foliar Zn uptake is dominated by an ion exchange and/or diffusion process rather than an active one. This study also demonstrates the usefulness of stable isotope labelling in studies of foliar Zn absorption.

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Qinglong Zhang and Patrick H. Brown

The distribution and transport of foliar applied Zn were determined for pistachio (Pistachio vera L.) seedlings and mature trees using stable 68Zn isotope. In seedlings, ≈5.4% of Zn adsorbed by the leaf was transported out of the treated leaves and this Zn was detected in all other plant parts to varying extent. In mature trees, the transport of Zn occurred both acropetally and basipetally within the leaflets with more basipetal movement; however, no significant amount of Zn was transported out of the treated leaflets during the first 10 days after application. The total percentage of Zn transported to other plant parts 20 days after application was significantly greater when Zn was applied to immature leaflets (6.5%) than to mature leaflets (2.1%), though the majority of the absorbed Zn remained within the treated leaflets. The limited mobility of foliar-absorbed Zn in pistachio may partially be attributed to the high binding capacity of leaf tissue for Zn.

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Qinglong Zhang and Patrick H. Brown

In this study, we investigated the effectiveness of several Zn formulations applied at various times of the year in increasing Zn status of pistachio and walnut leaves. Formulations included inorganic and organic forms of Zn. Fall sprays was ineffective at supplying Zn to developing leaves even when very high rates (5000 ppm) were used. Late dormant and budbreak sprays were effective at supplying Zn to developing leaves and nuts only when extremely high rates (5000 ppm) were applied. Spring flush sprays were the most effective, while late spring and summer sprays were ineffective. The majority of the Zn applied remained in the epidermis of the sprayed leaves, which resulted in high Zn content of leaves but poor correction of Zn deficiency and little or no translocation of Zn to other plant parts. Many of the Zn formulations sprayed at spring flush at a rate of 1000 ppm effectively increased leaf Zn values by at least 10 μgg–1. Addition of an appropriate organic acid to the spray solution and adjustment of pH to ≈4.5 improves leaf uptake and translocation of Zn. Addition of specific surfactants into the spray solution is also recommended. Use of N- and P-containing Zn spray formulations is less effective than sulfur-based sprays (i.e., ZnSO4). Significantly, there is little residual effect of foliar sprays (even at spring flush), indicating that consecutive sprays for several years are needed to maintain productivity in Zn-deficient regions.