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  • Author or Editor: Bruce W. Wood x
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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.

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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.

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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.

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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.

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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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Pecan growers often receive substantially higher prices for nuts if they can be marketed early in the harvest season, sometime doubling their profit as a result. Time of nut ripening (shuck dehiscence) is the primary limiting factor to the realization of early harvesting. It is now possible to advance shuck dehiscence by 10 days or more using hydrogen cyanamide (formulated as Dormex). A three year study using young 'Cheyenne' trees (6th leaf) indicated that budbreak, flowering, and shuck dehiscence could be advanced when treated with hydrogen cyanamide during the winter dormant season. The degree of advancement varied with the application date and concentration utilized. Results we most desirable when treatments were applied about 60 days before budbreak and application was at the 2% and 4% (480 and 960mM) levels. Hydrogen cyanamide had no detectable adverse effects on any growth, quality or production parameter.

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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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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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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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Excessive fruit drop (i.e., June drop) can limit orchard profitability of certain pecan [Carya illinoinensis (Wangenh.) K. Koch] cultivars. The present study examines efficacy of aminoethoxyvinylglycine (AVG; formulated as ReTain®; Valent BioSciences, Libertyville, IL), a natural ethylene inhibitor, for increasing nutmeat yield in a commercial ‘Desirable’ pecan orchard over a 2-year period. The 30-ha experiment consisted of two treatments (nontreated versus ReTain) in the first year, an “off” year in the orchard's alternate bearing cycle. The second year's study, an “on” year, consisted of four treatments (i.e., “08 nontreated + 09 nontreated,” “08ReTain + 09 nontreated,” “08 nontreated + 09 ReTain,” and “08ReTain + 09 ReTain”). AVG, as ReTain [132 mg·L−1 a.i. (11.7 oz/acre)], was applied as two post-pollination canopy sprays (937 L·ha−1) 2 weeks apart in both years. During the “off” year, ReTain increased nut yield parameters with ReTain increasing kernel yield by 36% (704 kg·ha−1 versus 516 kg·ha−1) over that of nontreated trees. In the subsequent “on” crop year, the trees exhibiting a ReTain-associated previous year yield increase of ≈36% exhibited a reduction in yield of ≈25%, thus largely negating the previous season's yield increase over a 2-year alternate bearing cycle. Additionally, ReTain-treated trees during the “on” year failed to exhibit an increase in yield parameters over that of the nontreated control. As a result of a lag effect on subsequent year yield parameters, ReTain offers potential as a crop-load management tool for ‘Desirable’ orchards in “off” years such as a year of relatively high nutmeat price followed by a year of relatively low price. There appears to be no positive effect on yield when used in a heavy crop-load “on” year of an alternate bearing cycle. Thus, ReTain might have benefit for stabilizing alternate bearing in ‘Desirable’ pecan orchards. Kernel quality (defined as percentage of nut weight as kernel) of individual nuts from “on” year trees was not as sensitive to units of yield increase as for individual nuts of “off” year trees, thus implying that the rate of assimilate partitioning to individual reproductive structures in “off”-year trees is not as great as that in “on”-year trees.

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