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  • Author or Editor: Bruce W. Woods x
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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).

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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).

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Inadequate cross-pollination of pecan [Carya illinoinensis (Wangenh.) K. Koch] occurred in block-type orchards generally thought exempt from pollination-related crop losses because of an abundance of nearby potential pollinizers. “Off-genotypes” appeared to be potentially major assets in such orchards due to their role as backup pollinizers; hence, their presence insures against crop losses due to poor pollination. Fruit-set in `Desirable' main crop rows declined sigmoidally as distance from 'Stuart' pollinizer rows increased. For 15.4-m row spacings, rate of decrease was maximum between 49 and 78 m, depending on crop year. Maximum fruit-set was in rows immediately adjacent to the pollinizer. Tree age/size and spring temperature influences on the characteristics of flower maturity windows are probably primary factors contributing to pollination-related fruit-set losses in block-type orchards relying upon pollen from a single complementary pollinizer or from neighborhood trees. For example, flower maturity was earlier in older/larger trees, and higher spring temperatures accelerated catkin development relative to that of pistillate flowers. Maximum fruit-set occurred when pistillate flowers received pollen around 1 day or less after becoming receptive, whereas no fruit-set occurred when they were pollinated around four or more days after initial receptivity. These findings indicate that many block-type orchards in the southeastern United States are exhibiting pollination-related crop reductions and that future establishment of such orchards merits caution regarding the spatial and temporal distribution of pollinizers.

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Mitigation of alternate bearing (AB) through regulation of floral initiation of pistillate flowers is central to improving cropload management of pecan [Carya illinoinensis (Wangenh.) K. Koch] trees and orchards. The present study examines the influence of key bioregulators {i.e., an auxin [as B-napththaleneacetic acid (NAA)], a cytokinin [6-benylamino purine (6-BA)], an ethylene generator (ethephon), and an auxin transport inhibitor [2,3,5-triiodobenzoic acid (TIBA)]} on subsequent season pistillate flowering. Gibberellic acid (i.e., GA3) and NAA inhibited, whereas prohexadione–calcium (P-Ca; calcium 3-oxido-5-oxo-4-propionylcyclohex-3-enecarboxylate), ethephon, and BA + TIBA promoted floral initiation when topically applied to canopies before the kernel filling stage of seed development. These bioregulators exhibit potential for integration into a bioregulator-based strategy to mitigate pecan AB by selective and timely use in “off” or “on” cycle years, depending on the bioregulator. Field studies provide evidence that a “cytokinin–gibberellin balance,” with partial modulation by auxin and ethylene, acts in the endogenous primordial environment of floral meristems as a “second-level signal” regulating a key step in a three-step process for initiation of pistillate flowers in pecan. This establishes a new model for explaining pistillate flower initiation in pecan and a basis for designing future research on the control and management of pistillate flowering and AB.

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Long-term productivity of commercial pecan [Carya illinoinensis (Wangenh.) K. Koch] enterprises in relatively low-light environments such as the southeastern United States are limited by excessive tree crowding as orchards age. An effective horticultural strategy for countering this problem in relatively high-light environments is mechanical hedge-type pruning; however, uncertainty persists regarding the best strategies in low-light environments. This report describes the results of a 4-year study regarding the response of ≈25-year-old ‘Desirable’ pecan trees to different mechanical hedgerow-type, moderate canopy width (i.e., 2.43-m cuts from tree axis) pruning strategies. Canopy treatments were nonpruned control (NPC), annual dormant season side-hedge pruning on two faces, annual summer season side-hedge pruning on two faces, and alternating annual dormant season side-hedge pruning on a single alternating face. Relative to the NPC treatment, all three pruning strategies: 1) reduced in-shell nut yields by roughly 19% to 38%; 2) reduced marketable nut-meat yield by ≈19% to 36%; 3) failed to stimulate shoot development or fruiting within the central interior zone of tree canopies; 4) increased kernel percentage of nuts; 5) increased nut-meat grade; 6) substantially reduced alternate bearing intensity (I = 0.51 to ≈0.20); and 7) reduced orchard crowding. Pruning-associated reductions in nut yield appear sufficient to limit the commercial usefulness of annual or biennial mechanical hedgerow-type pruning of ‘Desirable’ pecan orchards at moderate canopy widths in relatively low-light environments such as the southeastern United States.

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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).

Open Access
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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.

Open Access
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Abstract

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.

Open Access
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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.

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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.

Free access