On a shoot basis, the threshold value of leaf zinc has been established so that leaf deficiencies and decreases in either leaf photosynthesis or fruit growth will be prevented. Leaf deficiency symptoms, as judged by rosette, are not apparent until leaf zinc is 6 ppm (Finch, 1936; Finch and Kinninson, 1934; Hu and Sparks, 1990). Changes in leaf photosynthesis and fruit growth can be detected at higher leaf zinc. Maximum leaf photosynthesis and fruit growth require that the leaves on the supporting shoot have zinc at about 14 ppm (Hu and Sparks, 1990, 1991).

On an orchard basis, the threshold value of leaf zinc that prevents leaf zinc deficiencies is much higher, about 43 ppm (Sparks and Payne, 1982), than that on a shoot basis. The differences in the values on a shoot and orchard basis are explained by variability of deficiency among and within pecan trees. During leaf sampling, leaflets are taken from both normal and zinc-deficient branches or trees. Within a tree, only one small branch may show deficiency. Symptoms often appear first and/or are more severe in the upper portion of the canopy. One tree may have leaf deficiency symptoms, but an adjacent tree in the same orchard will be normal (Sparks and Payne, 1982) as reflected in concomitant variability in the concentration of leaf zinc between trees in the same orchard (Sparks and Payne, 1982; Worley et al., 1972). Leaflet samples typically are taken from the lower canopy and, in practice, are composite samples of leaflets from both normal and zinc-deficient branches and/or trees. As a result of the masking effect of the composite sample and the sampling position, the threshold zinc value for a shoot is much less than on an orchard or tree basis. The net result is that the leaf zincconcentration required to prevent leaf deficiency symptoms on an orchard basis is much higher than on a shoot basis.

On an orchard basis, a leaf zinc threshold value below which zinc is limiting and above which zinc is not limiting to either nut yield or vegetative growth has not been determined. Many reports propose leaf zinc levels to maximize nut yield (Amling, 1965; Brookshire, 1989; Jones, 1972; Storey, 1989; Storey et al., 1975). However, these proposed values vary greatly from 35 (Brookshire, 1989) to 80 ppm (Storey, 1989). Lack of an established threshold value of leaf zinc for optimum pecan vegetative growth and nut yield on an orchard basis is due to investigators failing either to induce deficiency or to examine intermediate zinc levels between severely limiting and nonlimiting leaf zinc concentration. For example, nut yield was reduced by 70% at 19 ppm relative to the maximum at 45 ppm leaf zinc in Georgia. However, leaf zinc concentrations that would cause nut yield reduction between 0 and 70% were not investigated (Brooks, 1965; Lane et al., 1965). Similarly, Smith and Storey (1979) determined nut yield was reduced by 45% at 29 ppm and not affected at 200 ppm, but they did not establish zinc levels between the extremes of 45 and 0% reduction in nut yield. From these and other investigations, the conclusion can be drawn that nut yields and vegetative growth will be limited by low leaf zinc, but are unaffected at high leaf zinc. However, no single study has established the leaf concentrations at which zinc becomes limiting. This study was designed to use mathematical modeling to combine data from published and unpublished sources. The objective was to establish the threshold concentration of leaf zinc on anorchard basis at which zinc begins to limit pecan vegetative growth and nut yield as compared with the value for leaf deficiency symptoms.

Four indices of tree performance were examined in relation to leaf zinc concentration from a low of 19 ppm to a high of 250 ppm by mathematical modeling using a modification of Mitscherlich's plant growth equation (Ware et al., 1982). The four performance indices were percentage of trees without deficiency symptoms, vegetative growth, nut yield, and the combination of leaf deficiency symptoms and nut yield. The maximum value reported for a particular index in any single study was set at 100% and all other values calculated relative to that value. In all cases, the maximum value was nonlimiting with respect to leaf zinc concentration.

Leaf and trunk growth data were combined to derive a mathematical expression for vegetative growth. Combining these data is valid because pecan leaf and trunk growth are highly correlated with total tree growth (Sparks and Baker, 1975). Measurements for leaf growth vs. zinc are from studies conducted on basic soils in western Texas (Malstrom et al., 1984), central Texas (Smith and Storey, 1979), and Arizona (Kilby, 1985). Leaf growth was analyzed following zinc uptake from the soil in each of the three studies, as well as from foliar applied zinc sulfate in the latter two studies. Zinc sulfate, the commonly used zinc source, was applied at the commercial dosage of 3 and 2 pounds of zinc sulfate per 100 gallons of water in the Arizona and central Texas study, respectively. Trunk growth data were from trees growing on an acid soil near Waycross, Ga., taken by me. Growth was determined for controls and treatments 5 years after onesoil application of zinc in an experiment replicated six times with two trees per experimental unit.

Measurements for nut yield vs. leaf zinc are from studies of Brooks (1964), Lane et al. (1965), Malstrom et al. (1984), Smith and Storey (1979), and Worley et al. (1972). Investigations by Malstrom et al. and Smith and Storey were conducted in Texas on basic soils and all others in Georgia on acid soils. Yield response was determined following soil-applied zinc sulfate in all studies except that of Smith and Storey which was for the control compared to foliar treatment of zinc sulfate at 2 pounds of zinc per 100 gallons of water. The relationship of visible zinc deficiency symptoms to leaf zinc was from a study (Sparks and Payne, 1982) with large mature trees growing in Georgia on acid soils. Measurements for this study were restricted to those obtained after broadcast treatments of zinc. The leaf deficiency symptoms were expressed as percent of trees per treatment without visible zinc deficiency symptoms. The leaf zinc concentration in all studies and for all performance indices is comparable because the leaves were taken within a comparable sampling interval, as prescribed (Sparks, 1970).

The relationship of zinc deficiency symptoms (Fig. 1), vegetative growth (Fig. 2), and nut yield (Fig. 3) to leaf zinc followed the modified Mitscherlich's plant growth model. All tree performance indices are characteristic of a typical micronutrient response (Ware et al., 1982). Thus, the intermediate range of zinc concentrations is narrow between eliciting severe to no effect on the performance indices. The leaf zinc concentration associated with maximum response was 48, 49, and 50 ppm for leaf deficiency symptoms, vegetative growth, and nut yield, respectively. Leaf deficiency symptoms and nut yield have similar slopes and follow a common curve (Fig. 4), but their slope is greater than that for vegetative growth. However, the leaf zinc concentration for maximum responses is similar for all performance indices. Thus, the threshold leaf concentration of zinc for pecan on an orchard basis is about 50 ppm.

The curve for vegetative growth (Fig. 2) and nut yield (Fig. 3) indicate that a leaf zinc concentration of about 250 ppm and 200 ppm is not toxic to vegetative growth and yield, respectively. Both values are from foliar-applied zinc. Results from soil-applied zinc likewise demonstrate that a leaf zinc concentration of at least 200 ppm is not toxic to tree growth (Sparks and Payne, 1982).

Historically, zinc deficiency in pecan has been more of a problem in non-native than in native areas (Sparks, 1987). Cultivation induces and/or enhances zinc deficiency (Allen et al., 1932; Demaree, 1933; Rosborough et al., 1946). Native groves generally are not cultivated, hence the low prevalence of zinc deficiency. Presently, deficient zinc is a potential factor in reduced tree performance mainly when zinc is supplied only via acid soils as may exist in Georgia at some locations. In regions with basic soils as in most areas of Texas, Oklahoma, New Mexico, Arizona, and California, the standard practice is to apply zinc as a foliar spray because the chemistry of basic soils impedes plant uptake of soil applied zinc (Alben and Hammar, 1944; Smith et al., 1980). Over the past decade, foliar zinc sprays have become the standard application method in most pecan orchards, regardless of soil type. Pecan trees supplied zinc through properly timed foliar sprays rarely exhibit limiting zinc because leaf absorption of zinc is extremely efficient. Foliar-applied zinc sulfate can produce leaf zinc values far higher (Kilby, 1985; Smith and Storey, 1979; Storey et al., 1971) than the threshold value of 50 ppm determined in the current study.

Because foliar applied zinc sprays usually results in leaf zinc values in excess of 50 ppm, the threshold value has limited usefulness in orchards in which foliar sprays are the only means of preventing zinc deficiency, mainly orchards in basic soils outside the native pecan range. In contrast, in orchards on acid soils, where zinc is supplied via soil application, and in native pecan groves, the threshold value has significance. An exact threshold value is especially significant in native groves.

Low-input management strategies must be employed to make a pecan grove an economically feasible operation because nut yields in pecan groves, unlike pecan orchards, are inherently low (Sparks, 1980). Any cost associated with culture of the trees in pecan groves, such as spraying zinc, has a greater impact on profit margin than in pecan orchards. Thus, to spray pecan trees in native orchards to increase leaf zinc above the threshold level of 50 ppm is an unnecessary expense and especially if the spray is solely for zinc nutrition; that is, when zinc application does not coincide with a pesticide spray. The current recommended threshold zinc values for the two major native pecan producing states, Oklahoma and Texas, are 60 ppm and 80 ppm (Smith, 1991), respectively. The current results of this study suggest that these values are too high and may cause growers managing native groves to spray zinc unnecessarily. Furthermore, a need to spray zinc on pecan trees growing in groves should be rare as indicated by evidence from this study together with the infrequency of deficiency symptoms in pecan groves under sod culture (M. W. Smith, personal observation). The current investigation clearly documents that the zinc level required for maximum vegetative growth and nut yields coincides with the threshold value for visible leaf symptoms of zinc. Thus, trees without visible leaf deficiency symptoms do not require zinc sprays to be zinc-sufficient.

¹Adapted from a paper originally published in HortScience 28:1100-1102.

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Fig. 1. Pecan trees without deficiency symptoms vs. leaf Zn (zinc) concentration of pecan (derived from Sparks and Payne, 1982). The relationship of deficiency symptoms vs. zinc follows Mitscherlich's plant growth model: Y = 100.2125 [1-0.1e^{-0.1059(x-48.4513)}], r2 = 0.790, P = < 0.05. The broken line indicates the threshold value (48 ppm) of zinc for maximum response.

Fig. 2. Relative vegetative growth vs. leaf Zn (zinc) concentration of pecan. The relationship of vegetative growth vs. zinc follow Mitscherlich's plant growth model: Y = 98.2347 [1 - 0.1e^{-0.07925(x-49.0380)}], r2 = 0.504, P = < 0.05. The broken line indicates the threshold value (49 ppm) of zinc for maximum response.

Fig. 3. Relative nut yield vs. leaf Zn (zinc) concentration of pecan. The relationship of nut yield vs. zinc follows Mitscherlich's plant growth model: Y = 97.0594 [ - 0.1e^{-0.1147(x-50.3876)}], r2 = 0.852, P = < 0.05. The broken line indicates the threshold value (50 ppm) of zinc for maximum response.

Fig. 4. Relative response of pecan trees based on combining values of leaf deficiency symptoms (Fig. 1) plus nut yield (Fig. 3) in relation to leaf Zn (zinc) concentration of pecan. The relationship of relative response to zinc follows Mitscherlich's plant growth model: Y = 98.0988 [1 - 0.1e^{-0.1053(x-49.0058)}], r2 = 0.818, P = < 0.05. The broken line indicates the threshold value (49 ppm) of zinc for maximum response.