Soil drench applications of paclobutrazol (0, 0.5, 1, 2, 4, 8, 16, and 32 mg a.i·pot-1) to greenhouse grown pecan seedlings reduced plant height, plant dry weight, organ dry weight, in ter node length, leaf thickness, leaf area, and chlorophyll content. Carbohydrate levels (mg·g dry weight-1 and mg·g plant-1) in treated plants increased. Total plant carbohydrate levels were unchanged at levels ≤ 2m g a.i.·pot-1, but plants of reduced size showed increased levels of carbohydrates per mg of tissue. Seedlings treated with high levels of paclobutrazol had a slight tendency for increased net photosynthesis.
Exposure of 13-year-old trees of several pecan [Carya illinoensis (Wangenh.) C. Koch] cultivars to severe cold in the winters of 1983-84 and 1984-85 resulted in the death of several healthy bearing trees of alternate-bearing cultivars (‘Chickasaw’, ‘Cheyenne’, ‘Cherokee’, and ‘Shoshoni’), while less tree death occurred in moderately bearing and relatively minor alternate-bearing cultivars (‘Cape Fear’ and ‘Desirable’). ‘Chickasaw’ trees entering winter after bearing a heavy nut crop the previous season experienced greater tree death and reduced midwinter trunk tissue levels of starch, sugars, and K than did trees with a light nut crop the previous season. The increased susceptibility of heavily bearing trees, especially of alternate-bearing cultivars, to extreme winter cold may be due to the effect of heavy fruiting on tree reserves and subsequent cold acclimation.
The problems of excessive vegetative growth and tree-size control of young pecan [Carya illinoensis (Wangenh.) C. Koch] trees prompted the evaluation of three tirazole analogs [paclobutrazol (PBZ), uniconazole (UCZ), and flurprimidol (FPD)] for their growth suppression efficacy and horticultural usefulness on pecan. A one-time soil application of these growth regulators at 132, 264, and 588 µmol·cm–2 trunk cross-sectional area suppressed shoot elongation by 50% to 90% for up to 3 years after treatment. Growth suppression greater than about 60% reduced nut yield; presumably due to drastically reduced leaf area and internal shading among leaves within compacted shoots. Relative efficacy in terms of shoot growth was UCZ > PBZ > FPD; however, all three chemicals exhibit commercial potential for controlling tree size. Chemical names used: β-[(4-chlorophenyl)methyl]-α-(1,1-dimethylethyl)-1H-1,2,4-triazole-1-ethanol (paclobutrazol); (E)-(p-chlorophenyl)-4,4-dimethyl-2-(1,2,4-triazol-1-yl)-1-penten-3-ol (uniconazol, name pending); α-(1-methylethyl)-α-[4-(trifluoromethoxy)phenyl]-5-pyrimidinemethanol (flurprimidol).
Economic loss resulting from nickel (Ni) deficiency can occur in horticultural and agronomic crops. This study assesses whether excessive iron (Fe) can induce Ni deficiency. Both chelated Fe and diethylenetriaminepentaacetic acid (DPTA; a commonly used Fe-chelant) induces Ni deficiency in pecan [Carya illinoinensis (Wangenh.) K. Koch]. Foliar sprays of Fe [Fe-DPTA (1.1995 g·L−1)] during early post-budbreak shoot growth can trigger, or increase in severity, Ni deficiency symptoms in the emerging pecan canopy. Deficiency is also inducible in greenhouse-grown ‘Desirable’ seedlings at budbreak by Fe-DPTA application to soil and to a much lesser extent by DPTA alone. Endogenous Fe, just after budbreak, triggers Ni deficiency-associated distortions in pecan seedling leaf growth and morphology when the Fe:Ni is ≈150 or greater with subsequent severity being proportional to the Fe:Ni ratio and Fe:Ni ≈1200 or greater triggering extreme dwarfing of canopy organs. Timely treatment of symptomatic organs with foliar-applied Ni-sulfate restores normal growth, whereas foliar treatment with salts of other transition metals (titanium, vanadium, chromium, cobalt, copper, zinc, and molybdenum) of possible metabolic significance is ineffective. Results indicate that excessive endogenous Fe, and DPTA to a lesser extent, in organs and tissues during early post-budbreak growth can trigger Ni deficiency. A similar Fe on Ni antagonism may also occur with the Ni-associated nutritional physiology of other crops; thus, excessive exposure to chelated Fe not only triggers Ni deficiency in pecan, but may also occur in other horticultural and agronomic crops.
Zinc (Zn) deficiency is common in commercial pecan [Carya illinoinensis (Wangenh.) C. Koch] orchards. Correction by multiple annual foliar spray applications is expensive but effective in eliminating Zn deficiency. Correction by soil application is also expensive and is usually impractical or noneffective. There is a need for more economical and long-lasting methods for satisfying tree Zn nutritional needs. It is reported here that tree foliar Zn needs [(i.e., 50 μg·g−1 dry weight (dw) or greater] are potentially met through one-time “banding” of Zn sulfate (ZnSO4·7H2O) or Zn oxide (ZnO) onto orchard floors. Zinc needs of 4-year-old ‘Desirable’ trees growing on acidic soil were satisfied over a 4-year period by a single-banded soil application of either Zn sulfate or ZnO over underground drip irrigation lines at a Zn rate of 2112 g Zn per tree (giving foliar Zn concentrations of 60–115 μg·g−1 dw). Rates of Zn at 264 to 1056 g per tree are occasionally efficacious, but rates less than 264 g Zn per tree (0, 33, 66, and 132) were always ineffective for meeting a leaf sufficiency threshold of 50 μg·g−1 dw. Sulfate and oxide Zn forms were equally effective in meeting tree Zn needs. Foliar Zn concentrations increased quadratically with increasing soil-banded Zn treatments; however, foliar Zn concentrations did not necessarily increase over the 4-year period within each Zn rate treatment. Increasing amounts of banded Zn per tree also increased foliar Mn concentration (from ≈150 to 269 μg·g−1 dw) of treated trees the fourth year posttreatment but did not affect foliar concentration of other key micronutrients (i.e., Fe, Co, Cu, or Ni). This fertilization strategy offers an efficacious alternative to annual foliar Zn sprays for orchards established on acidic soils and provides a means of ensuring rapid and long-term Zn absorption through soil application. The approach indicates that soil banding of Zn on certain acidic soils can satisfy the nutritional needs of pecan trees for several years after a single application.
Alternate bearing (AB) by individual trees is a major biological problem faced by pecan [Carya illinoinensis (Wangenh.) K. Koch] nut producers. The linkage between flowering and subsequent cropload with xylem sap characteristics at the time of floral bud swelling and expansion is unknown. Multiyear field studies of mature ‘Cheyenne’ and ‘Moneymaker’ trees, in “on” or “off” phases of AB, were evaluated regarding this linkage. Xylem sap flowing from trunks of ‘Cheyenne’ trees just before, and at the time of, budbreak (i.e., “late winter/early spring”) consisted of a variety of simple sugars. These were hexoses (fructose and glucose), a disaccharide (sucrose), polysaccharides (raffinose and stachyose), and sugar alcohols (xylitol and sorbitol). Sucrose was the overwhelmingly dominant simple carbohydrate at this growth stage, comprising 55% to 75% of the total molar composition, regardless of tree bearing status or sampling time during the seasonal transition from late winter to early spring as buds swell, break, and begin to produce shoots and flowers. Both sap flow volume and concentration of individual carbohydrates were much greater in “on” phase than “off” phase trees. “On” phase xylem sap contained ≈19.9-fold more sucrose than sap from “off” phase trees. The concentration of all sap carbohydrates was much greater at flow inception, declining quickly as buds transition from “bud swell” to “budbreak” and subsequent “shoot growth.” Depending on crop year, individual “on” phase ‘Cheyenne’ trees (≈25 years old) exhibited flow volumes 5.5- to 20.2-fold greater than “off” phase trees. In-shell nut yield by both ‘Cheyenne’ and ‘Moneymaker’ trees (110 years old) increased hyperbolically with increasing “late winter/early spring” sap flow volume. Sap flow from ‘Cheyenne’ and ‘Moneymaker’ resulted in near maximum nut yield when flow volume per xylem tap peaked was at ≈10 L/tree and ≈15 L/tree, respectively, over a 16-day sampling period. These findings are suggestive that sucrose, and possibly other simple carbohydrates, moving acropetally toward axillary bud meristems of shoots during “late winter/early spring” at about the time of “bud swelling” influences the final phase of floral development and therefore subsequent cropload.
Canopy morphology of 83 pecan [Carya illinoinensis (Wangenh.) K. Koch] cultivars differed in structural, size, and form characteristics. Cluster analysis identified two to five distinct classes for canopy height and diameter and their ratio, inclination angles for both major limbs and young shoots with lower-order structures, branch types, and canopy form and volume. Cultivar-related variability in these traits may have the potential for the improvement of pecan cultivars for factors such as light interception, cooling, air movement, and fruiting; thus, there is potential for identifying the development of canopy characteristics adapted to specific site conditions or cultural/management strategies.
Pecan is wind pollinated, exhibits heterodichogamy and are either protandrous (I) or protogynous (II). Orchards are typically established using two complimentary flowering types but with no further scrutiny as to the degree of compatibility of these two types. Additionally, orchards are sometime established with a very low frequency of pollinator. An evaluation of several orchards revealed that yield losses are due to poor pollination is likely common. Data indicate that trees beyond about 46 m (150 feet) from a complementary pollinator exhibit substantial reductions in fruit-set; therefore, large block-type plantings are disadvantaged. Flowering data over several years show that Type I and Type II cultivars are often functionally noncomplementary, suggesting that pecan cultivars should also be identified with a seasonal identification (i.e., early, mid, and late). Data also indicate that dichogamy patterns substantially change as trees age or with abnormally warm or cool springs; hence, pollination patterns will vary depending upon orchard age. Data indicate that orchards should be comprised of 3+ cultivars. RAPD-DNA analysis of “hooked-nuts” indicates that this trait is not reliable as an indicator of selfing.
The desirability of controlling growth of large pecan [Carya illinoensis (Wangenh.) C. Koch] trees prompted the evaluation of paclobutrazol (PBZ) for growth suppression. PBZ was applied to 75-year-old ‘Stuart’ pecan trees via trunk injection (rates of 0, 50, 100, and 200 mg·cm–1 trunk diameter) or as a spray to the orchard floor (rates of 0, 19, 38 and 76 g/tree). Terminal-shoot growth and leaf area were reduced during 4 years after treatment in both studies. In-shell nut yield was reduced the third and fourth years after PBZ injection, but was increased the second year after soil application. PBZ can reduce terminal-shoot growth in large trees, but higher doses may produce a decline of nut production. Chemical name used: β-[(4-chlorophenyl)methyl]-α-(1,1-dimethylethyl)-1H-1,2,4-triazole-1-ethanol (paclobutrazol).
The lack of satisfactory methods for clonal propagation of pecan [Carya illinoensis (Wangenh.) C. Koch] rootstocks resulted in the examination of mound stooling as a propagation technique. Semi-hardwood shoots from severed stumps received several treatments involving phloem girdling and IBA. Rooting occurred only in girdled or girdled plus IBA (3000 and 6000 ppm) treatments. Girdling triggered, whereas IBA enhanced, rooting. Roots per clone was related to shoot diameter but not height. Clones were able to survive harsh field conditions, thus providing a method for cloning rootstocks and facilitating rootstock research. Chemical name used: 1H-indole-3-butyric acid (IBA).