Root Uptake

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 vitroderived shoots soaked in 1 to 3 mg indolebutyric acid (IBA) per liter produced adventitious shoots in vitro; when soaked for 8 days in 10 mg 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.).