Pecans [Carya illinoinensis (Wangenh.) K. Koch] require more Zn than many crops and can tolerate applications of Zn that would cause toxicity in other plants (Worley, 2002). High-pH calcareous soils are common in the semiarid southwestern United States. In these alkaline soils, Zn binds with hydroxyls and carbonates to form low-solubility compounds, making it less available to plants (Udo et al., 1970). Lack of Zn availability frequently leads to Zn deficiency in desert-grown pecans.
Low concentrations of leaf chlorophyll are one result of Zn deficiency. Hu and Sparks (1991) found that leaf chlorophyll content was lower in leaves containing less than 14 mg⋅kg–1 Zn. Zn deficiency can also shorten palisade cells, increase intercellular space, and reduce leaf thickness and surface area (Ojeda-Barrios et al., 2012). Hu and Sparks (1991) noted that stomatal conductance (gS) and net photosynthesis (Pn) were reduced concomitantly by low Zn levels. Heerema et al. (2017) determined a threshold leaf Zn concentration between 14 and 22 mg⋅kg–1, below which Pn declined and above which Pn did not increase, supporting the findings of Hu and Sparks (1991). Measurements taken in June and July showed leaf Zn concentration thresholds on the upper end of this spectrum, whereas measurements acquired in August showed leaf Zn concentration thresholds on the lower end. Zn concentrations close to or less than 14 mg⋅kg–1 prevent normal fruit production on the supporting shoot (Hu and Sparks, 1990).
For field production, the minimum leaf Zn concentration recommended to avoid loss of yield or nut quality, reduction in vegetative growth, and visible symptoms of Zn deficiency is usually reported to be at least 40 to 60 mg⋅kg–1 (Heerema, 2013; Robinson et al., 1997; Smith et al., 2012; Sparks, 1993; Sparks and Payne, 1982). Zn-EDTA applied to the soil through fertigation over a 5-year period at rates of 2.2 and 4.4 kg⋅ha–1 Zn largely eliminated foliar Zn deficiency symptoms and increased rates of Pn (Heerema et al., 2017; Walworth et al., 2017), but these treatments were not sufficient to attain the recommended minimum concentrations. The greatest leaf Zn concentrations obtained during this study were 35 mg⋅kg–1, but Pn showed no significant increase when foliar Zn concentrations exceeded ≈22 mg⋅kg–1, indicating that Pn is not Zn limited beyond this point.
Foliar application of Zn is a common practice to alleviate Zn deficiency. However, managing Zn with foliar applications is costly and time intensive, and foliar-applied Zn is poorly distributed within the plant (Wadsworth, 1970). Although foliar application is effective for increasing leaf tissue Zn concentrations, only a small fraction of applied Zn is actually absorbed. Wadsworth (1970) indicated that only 0.6% to 1.2% of Zn applied to immature pecan leaves was absorbed. In walnuts Brown et al. (1995) found that ≈2% to 4% Zn was absorbed by mature leaves, whereas more than 8% was absorbed by immature leaves. Foliar Zn absorption may be dependent on the form of Zn applied. Frequently used spray materials include ZnSO4 and zinc nitrate [Zn(NO3)2]. In Zn spray-tank mixes that contain nitrate, the nitrate ion aids in the uptake of Zn (Worley, 2002). Urea has been reported to enhance the penetration of nutrients into foliar tissues (Hsu and Ashmead, 1984). Storey (1977) observed that foliar absorption of Zn from either ZnSO4 or Zn(NO3)2 sprays was enhanced by including UAN in the spray mixture. In other crops, a mixture of manganese sulfate, ZnSO4, and iron sulfate salts were applied to soybean, fava bean, pea, and wheat with and without the addition of 1% urea. In all cases, the addition of urea enhanced the uptake of these metals (El-Fouly et al., 1990).
Ferrandon and Chamel (1988) found that the cuticular sorption of Zn was significantly greater when Zn was applied to the cuticles of tomato leaves in the inorganic forms of ZnSO4 or zinc chloride than in the organic Zn-EDTA form. Cuticular sorption of Zn-EDTA was ≈5 nM⋅cm–2 after 72 h vs. ≈41 nM⋅cm–2 for ZnSO4. In pea plants, Zn uptake rates were ≈1.45 times greater when Zn was applied as ZnSO4 vs. Zn-EDTA. More of the Zn applied in the form of Zn-EDTA was translocated away from the point of foliar contact than that applied as ZnSO4 (Ferrandon and Chamel, 1988). Brown et al. (1995) found that walnut tree leaves sprayed with Zn-EDTA did not contain significantly more Zn than leaves sprayed with ZnSO4. However, Zn concentrations of unsprayed leaves on branches adjacent to those sprayed with Zn-EDTA were increased significantly compared with the control, whereas leaves on branches adjacent to ZnSO4 treatments were not, suggesting greater mobility of Zn-EDTA within the tree than ZnSO4. In pecans, foliar applications of 50, 100, and 150 mg⋅L–1 Zn-EDTA resulted in an increase in leaf Zn concentrations, chlorophyll content, and leaflet area (Ojeda-Barrios et al., 2014); however, Zn-EDTA sprays generally did not bring foliar Zn concentrations to the desired level of at least 40 to 60 mg⋅kg–1. They did, however, achieve the 14- to 22-mg⋅kg–1 concentrations suggested by the data of Heerema et al. (2017).
The identification of adequate minimum pecan foliar Zn concentrations for commercial field-level recommendations is still open to question. Although the results of Heerema et al. (2017) suggest that 14 to 22 mg⋅kg–1 is adequate to maximize rates of Pn, recommendations for commercial orchards are generally much greater (at least 40 to 60 mg⋅kg–1) (Heerema, 2013; Robinson et al., 1997; Smith et al., 2012; Sparks, 1993; Sparks and Payne, 1982). In part, these greater mean leaf Zn concentration recommendations are the result of the tree-to-tree variability in leaf Zn that exists within an orchard (Sparks, 1993; Sparks and Payne, 1982). Finally, it is possible that when Zn is applied to foliage, as continues to be the common practice, Zn affects the relationship with Pn in fundamentally different ways than when it is soil applied. In this study, we explore whether foliar Zn applications (in the form of Zn-EDTA, ZnSO4·H2O alone or in combination with UAN) will increase leaf Pn of ‘Wichita’ pecan trees that are already receiving soil-applied Zn-EDTA.
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