Zinc has a complete 3d104s2 outer electronic configuration and, unlike the other d block micronu-trients such as such as manganese, molybdenum, copper, and iron, has only a single oxidation state and hence a single valence of II. The average concentration of zinc in the crust of the Earth, granitic, and basaltic igneous rock is approximately 70,40, and 100 mg kg"1, respectively (38), whereas sedimentary rocks like limestone, sandstone, and shale contain 20, 16, and 95 mg kg"1, respectively (39). The total zinc content in soils varies from 3 to 770 mg kg"1 with the world average being 64mg kg"1 (40).
There are five major pools of zinc in the soil: (a) zinc in soil solution; (b) surface adsorbed and exchangeable zinc; (c) zinc associated with organic matter; (d) zinc associated with oxides and carbonates; and (e) zinc in primary minerals and secondary alumino-silicate materials (41).
There is evidence that Zn2+ activities in the soil solution may be controlled by franklinite (ZnFe2O4), whose equilibrium solubility is similar to that of soil-held zinc over pH values of 6 to 9 (42,43). The mineral will precipitate whenever zinc concentration in the soil solution exceeds the equilibrium solubility of the mineral and will dissolve whenever the opposite is true. This process provides a zinc-buffering system.
Zinc may be associated with soil organic matter, which includes water-soluble and organic compounds. Zinc is bound via incorporation into organic molecules, exchange, chelation, or by specific and nonspecific adsorption (41).
Zinc is associated with hydrous oxides and carbonates via adsorption, surface complex formations, ion exchange, incorporation into the crystal lattice, and co-precipitation (41). Some of these reactions fix zinc rather strongly and are believed to be instrumental in controlling the amount of zinc in the soil solution (44). Zinc is complexed with CaCO3 in alkaline (pH 8.2) soils in the western half of Texas where most of the pecans are grown in the state (45-47). Soil-incorporated ZnSO4 at 91 kg per pecan tree did not bring the zinc content of the soils to an adequate level because the zinc was transferred from the sulfate form to sparingly soluble ZnCO3 (48).
Five rates of ZnSO4 and three rates of S were supplied to pecan trees in March 1966 in a single application to soil (deep Tivoli sand, pH 8.2; mixed thermic, Typic ustipamments) in Dawson county, Texas (south plains) (49). In the absence of applied sulfur, adding of ZnSO4 in excess of 20 kg per tree was required to raise zinc concentrations in leaflets in June or September 1966 above the minimum optimum of 60mg kg"1. Additions of sulfur reduced the amount of ZnSO4 required to reach 60mg kg"1 to 18.8kg per tree with 4.5 kg S per tree and to 16.2kg per tree with 11.9 kg S per tree. Leaflets collected in September 1967 contained more than 60mg Zn kg"1 if ZnSO4 was applied in March 1966 at rates greater than 4.8 kg per tree. However, in 1967, at any given rate of ZnSO4 (above 1.4 kg per tree), leaflet zinc concentration was reduced by the addition of sulfur, but the concentrations of zinc in the leaflets remained above the minimum optimum level. This study indicates that leaflet zinc of pecan trees in calcareous soils can be increased by soil applications of ZnSO4, but that a larger increase will occur if S is applied with ZnSO4. On the other hand, soil applications seemed impractical considering the fact that with a planting of 86 trees per ha, an application of 120 kg of ZnSO4 ha"1 would be required. In acid soils of the southeastern United States, high rates of soil-applied zinc may be responsible for the elusive mouse-ear symptom in the acid soils of the southeastern United States (50). These results agree with Sommers and Lindsay (51), who reported that in soils with high concentrations of heavy metals, nickel will compete with zinc for chelation in acid soils and that cadmium and lead will do the same in alkaline soils.
The higher phosphorus content in zinc-deficient plants supplied with high phosphorus can to some degree be attributed to a concentration effect (52). However, the main reason for the high concentration in the leaves is that zinc deficiency enhances the uptake rate of phosphorus by the roots and translocation to the shoots (53). This enhancement effect is specific for zinc deficiency and is not observed when other micronutrients are deficient. Enhanced phosphorus uptake in zinc-deficient plants can be part of an expression of higher passive permeability of the plasma membranes of root cells or impaired control of xylem loading. Zinc-deficient plants also have a high phosphorus content because the retranslocation of phosphorus is impaired.
The most distinct zinc deficiency symptoms are 'little leaf' and 'rosette' in pecans and peaches (Figure 15.1 and Figure 15.2). These symptoms have long been considered to represent problems in indole acetic acid (IAA, auxin) metabolism. However, the mode of action of zinc in auxin metabolism is unidentified. Retarded stem elongation in zinc-deficient tomato (Lycopersicon esculentum Mill.) plants was correlated with a decrease in IAA level, but resumption of stem elongation and IAA content occur after zinc is resupplied. Increased IAA levels preceded elongation growth upon resupply of zinc (54), which would be expected if growth was a response of increased supply of auxin caused by application of zinc. Low levels of IAA in zinc-deficient plants are probably the results of inhibited synthesis of IAA (55). There is an increase in tryptophan content in the dry matter of rice (Oryza sativa L.) grains by zinc fertilization of plants grown in calcareous soil (56). The lower IAA content in zinc-deficient leaves may be due to the biosynthesis of IAA tryptophan (57). Lower IAA contents may be the result of enhanced oxidative degradation of IAA caused by superoxide generation enhanced under conditions of zinc deficiency (55).
Zinc absorbed by pecan seedlings was translocated predominately to the youngest, physiologically active tissue, in agreement with the results of Millikan and Hanger (35), who worked with subterranean clover (Trifolium subterraneum L.). Autoradiograph and radio assays revealed variation between seedlings of open pollinated pecans with respect to rate of Zn absorption (37). For example, one set of seedlings absorbed extremes from 0.7 to 91 mg Zn kg"1 if roots were exposed to 65Zn in a beaker of water for 96 h.
Grauke et al. (58) detected the highest concentration of zinc in pecan seedlings originating from west Texas populations compared to those populations indigenous to east Texas, regardless of whether they were grown in central Texas or Georgia. Selecting hard woodcuttings from the best of the west Texas populations would appear to be an ideal way to use clonal rootstocks as a means of establishing pecan orchards on uniformly zinc-absorbing rootstocks in place of the very heterozygous seedlings used in the last 100 years. McEachern (59) consistently was able to root 40% of the juvenile stem cuttings that he treated, whereas less than 10% of the adult cuttings survived. However, the juvenile growth of a pecan tree is confined to the bottom 3 m of the trunk up from the ground line (60). This portion of the trunk is intermediate in rooting response, and all distal trunk and branches are adult. Heavy pollarding of the trees produce only adult compensatory growth that will not root. Juvenile tissue tends to have a high IAA / low ABA ratio, whereas adult tissue tends to have low IAA / high ABA (59). Only about 12% of juvenile pecan stem cuttings developed viable root systems in greenhouse studies, and none of the adult cuttings initiated roots (59). Only the lower 2 m of the trunk of the original seedling tree of a pecan cultivar is juvenile and eligible to produce cuttings that are capable of rooting (59).
Tissue culture became the popular means of clonal propagation in the 1960s because of the work of Skoog and Miller (61). Smith (62) was unsuccessful after trying most of the known plant growth regulators because of endogenous fungi that defied all sanitation procedures. Pecan tissue culture was plagued with Alternaria spp. in another study (63). This contamination is more severe in orchard-grown than in greenhouse-grown pecan seedlings but was still present under the most sterile growing conditions. Knox's attempt to culture pecan was unsuccessful. Knox advanced the theory that Alternaria is an endophyte or resident fungus. Knox (63) stated that the host pecan tree does not appear to be disadvantaged or diseased. If the vigor of the tree is essentially unaltered, then the fungus cannot be considered a pathogen and is more appropriately described as an endophyte or resident. The vigor of cultured pecan tissues apparently is enhanced by the fungus, perhaps implying a mutualistic relationship between Alternaria and pecan trees. There has been a long precedence for resident fungi in pecan roots because ectomycorrhizal fungi are prominent in native pecan groves and are considered to enhance zinc absorption by pecan roots from leaf mulch. Native pecan trees on fence lines, separating a cultivated field from a native pecan grove that is not tilled, will inevitably be rosetted on the side of the tree where the soil has been disturbed by disking compared to normal healthy growth on the untilled side of the tree.
Pecan tissue finally was cultured successfully by using single-node cuttings obtained from 2-month-old seedlings of pecan (64). Cuttings were induced to break buds and form multiple shoots in liquid, woody plant medium and 2% glucose supplemented with 6-benzylamino purine. In vitro-derived shoots soaked in 1 to 3 mg indolebutyric acid (IBA) per liter produced adventitious shoots in vitro; when soaked for 8 days in 10mg IBA per liter, they were rooted successfully in soil and acclimated to greenhouse conditions. Etiolation of stock plants did not improve shoot proliferation or rooting under in vitro culture (64).
Absorption of zinc varies with species. For example, Khadr and Wallace (65) reported that rough lemon (Citrus aurantium L.) absorbed more 65Zn and 59Fe from the soil than trifoliate orange (Poncirus trifoliate Raf.).
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