Influence of Humidity on Foliar Absorption

The method of zinc application is critical. Growers are tempted to use custom-fixed-wing aircraft instead of investing in hydraulic or air-mist ground sprayers. An application of ZnSO₄ at 11.2 kg Zn ha^-1 produced leaves containing 117 mg Zn kg^-1 on ground-sprayed trees compared with 34 mg Zn kg^-1 in aerially sprayed trees (34). A typical airplane application is 52 L ha^-1 (5 gal per acre), whereas a ground application is typically 1728 L ha^-1 (200 gal per acre). The limited spray volume of water from air application evaporates before adequate absorption occurs, particularly in arid climates.

Pecan leaves treated either with ZnSO₄ or NZN at 80% relative humidity showed increased zinc absorption relative to those treated at 40% RH (76). This result is consistent with observations made by Rossi and Beauchamp (90) of increased absorption of ZnSO₄ and ZnCl₂ at high humidity. Leaves treated under high humidity conditions maintained substantial amounts of surface moisture for 24 h. The increase in sorption is a reflection of the increased hydration, which permitted a longer period of uptake. The inclusion of humectants in foliar soybeans increased leaf nitrogen contents (91). Stein and Storey (91) evaluated 46 different adjuvants in a variety of classes, including alcohols, amines, carbohydrates, esters, ethoxylated hydrocarbons, phosphates, polyethylene glycols, proteins, silicones, sulfates, sulfonates, and alcohol alkoxylates. Glycerol was the only adjuvant that increased the percentage of nitrogen and phosphorus in leaves over the foliar fertilizer controls, which had no adjuvant.

A simple demonstration often used in classroom lectures utilizes a Petri dish of dry ZnSO₄ that remains dry throughout a 50-min class period, whereas a Petri dish containing dry Zn(NO₃)₂ will contain large drops of water at the end of the class period. The facts that ZnSO₄ is hydrophobic and Zn(NO₃)₂ is hydrophilic makes the latter more appropriate for arid climates. Relative humidity normally rises to 30% within 30 min after sunrise and rapidly falls to as low as 5% in the El Paso and Mesilla Valleys of Texas and New Mexico (34).

Addition of surfactants reduced hydration time of aerially applied zinc solutions to one third of those without surfactant. The hydration time of a chelated zinc fertilizer alone was 34 min and that of the fertilizer with surfactant was only12 min in the arid climate of the El PasoValley (37). With aerial application at 4 kg Zn ha^-1 (in 76 L of water), foliar zinc content was significantly different at 43 mg kg^-1 without surfactant and 31 mg kg^-1 with surfactant. In another experiment, zinc absorption from chelated zinc was reduced from 43 mg kg^-1 without surfactant to 31 mg kg^-1 with surfactant. Likewise, zinc accumulation from ZnSO₄ treatments containing no surfactant was reduced from 59 to 38 mg kg^-1 with surfactant. Accelerated evaporation rate was probably due to the surfactants reducing the surface tension of the solution droplets, thus allowing the droplets to spread more evenly over the leaf and thus accelerated loss of spray solution. With the treatment solutions devoid of surfactants, the droplets stood higher thereby decreasing the evaporative surface, allowing additional time for Zn absorption (80). Likewise, pecan trees treated with ZnSO₄, via a ground sprayer, at the rate of 5.6 kg Zn per acre in 1892 L of water, at 40% RH, produced leaves containing 189 mg Zn kg^-1 with a surfactant and 301 mg kg^-1 without a surfactant (37).

Fully expanded mature pecan leaves were inefficient in foliar absorption of ZnSO₄. Abaxial pecan leaf surfaces are only slightly more absorptive than adaxial surfaces (37). The differences were much greater than those reported by Malavolta et al. (92) but were similar to those reported by Heymann-Herschberg (93) for citrus, who further concluded that absorption through the stomata was unimportant. Franke (80) pointed to the cuticular leaf surface as the controller of ion absorption. Wadsworth (37) noted that the immature leaves with thinner cuticles absorbed more zinc than mature leaves. He also found that abaxial surfaces with thinner cuticles were more absorptive than adaxial surfaces. Acropetal transport of zinc was the primary direction of movement. Fourteen percent of the zinc was translocated from auxiliary buds compared with 1% from zinc applied to leaf midribs. This difference suggests that the tender buds had less cuticle than a fully expanded leaf.

Zinc accumulates in the young, expanding leaves. Translocated ⁶⁵Zn was found predominately in the stem, midrib, and lateral veins with relatively small amounts in the mesophyll (37). Resistance of movement was in the abscission zone. Millikan and Hanger (36) determined that ⁶⁵Zn accumulated in the nodes. Histological studies would probably confirm a concentration of small cells in the abscission zone, thus accounting for the accumulation of zinc.