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  • Author or Editor: Bruce W. Wood x
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The relatively low incidence of axillary shoot development in pecan [Carya illinoensis (Wangenh.) C. Koch] and its detrimental impact on nut productivity prompted: 1) an evaluation of methods to induce axillary shoot morphogenesis, and 2) estimation of levels of endogenous phytohormones in abscising and nonabscising axillary shoots. Retention of developing shoots from 1-year-old terminals was increased three-fold by ringing. Application of benzyladenine (BA) delayed, but did not prevent, shoot abscission, whereas abscisic acid (ABA), indoleacetic acid (IAA), and gibberellic acid (GA3) applications had no detectable effect. Normal morphogenesis of axillary shoots was independent of the presence of developing catkins. Phytohormones tentatively identified in pecan axillary shoots and catkins were IAA, ABA, dihydrozeatin riboside (DHZR), t-zeatin riboside (ZR), trans-zeatin (Z), and dihydrozeatin (DHZ). A possible influence of these substances on budbreak and shoot morphogenesis is discussed. Axillary shoots eventually abscising exhibited a substantially higher level of IAA-like and ABA-like substances than did shoots completing morphogenesis. Chemical names used: N-(phenylmethyl)-1H-purin-6-amine(BA); abscisic acid (ABA); 1H-indole-3-acetic acid (IAA); gibberellic acid (GA,).

Open Access
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Paclobutrazol (PBZ) was applied in one or more formulations as a basal trunk drench (0, 19, 38, 76, 152 mg·cm−2 trunk cross-sectional area) or orchard floor application (0, 52, 104, and 208 mg·cm−2 trunk cross-sectional area) to 3- and 10-year-old pecan [Carya illinoensis (Wangenh.)C. Koch] trees. Vegetative growth was highly sensitive to PBZ for at least 3 to 4 years after a single treatment. There were no differences due to formulation for any of the parameters evaluated. Shoot growth and production of in-shell nuts were negatively related in a curvilinear manner to PBZ rate. Nut size increased the first two seasons after treatment, and yield efficiency increased curvilinearly with PBZ rate. Excessive vigor of Iammas shoots coincided with the cessation of vegetative growth inhibition by PBZ. There was a differential cultivar response in nut yield for 3-year-oid ‘Cheyenne’ and ‘Desirable’ trees; however, there was no cultivar differences in 10-year-old trees of ‘Shoshoni’, ‘Desirable’, and ‘Cape Fear’. Use of PBZ appears to allow for the manipulation of tree size of young pecan trees. Chemical names used: β-[(4-chlorophenyl)-ethyl] -α-(1, l-dimethylethyl)-1H-1,2,4-triazole-1-ethanol (paclobutrazol).

Open Access
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Abstract

Seasonal measurements of net photosynthesis (Pn), stomatal conductance (Cs), and endogenous senescence-related chemicals were made on pecan [Carya illinoensis (Wang.) C. Koch] trees growing in the orchard environment. Leaves from terminal shoots of mature, 75-year-old ‘Moneymaker’ trees in the “on” alternately bearing phase maintained 10% to 40% higher Pn rates than did those of “off” phase trees. These “on” leaves also maintained higher levels of chlorophyll, protein, amino acids, and RNA, but less K. than “off” leaves. The presence of developing fruit suppressed the rate of late summer and autumnal leaf senescence, with “on” trees retaining their leaves in a higher state of vigor than those of “off” trees. Pn and Cs rates between leaflets of adjacent fruiting and nonfruiting shoots of 7-year-old ‘Desirable’ trees were also 10% to 15% higher for the fruiting shoots throughout the season. Pn and Cs rates were influenced such that gas exchange of leaves increased as sink demand increased. These data provide evidence that this sink-demand associated stimulus operates in a localized rather than a generalized manner and that it retards the rate of leaf senescence.

Open Access
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The problematic alternate-bearing characteristic of pecan [Carya illinoensis (Wangenh.) C. Koch] has prompted an investigation of the role of rootstock and carbohydrate concentrations in tree organs in relation to nut production. Six years of data on the influence of open-pollinated pecan rootstocks on in-shell nut production by 80- year-old trees of ‘Stuart’ of ‘Schley’ scion cultivars provide evidence that rootstocks may influence yield, yield efficiency, and alternate bearing and that yield is closely associated with January root starch concentrations. Several “superior” trees, possessing greater-than-average nut yields and less-than-average alternate-bearing intensity, were identified as candidates for improved pecan rootstocks, indicating a projected frequency of ≈5% for the occurrence of superior rootstocks.

Open Access
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In an attempt to solve the problems of nonuniform and delayed shuck dehiscense of pecan [Carya illinoensis (Wangenh.) C. Koch], ethephon and NAA were evaluated for their efficacy as harvest-aid treatments. A 3-year study under commercial-like orchard conditions using 75-year-old ‘Stuart’ trees resulted in a spray mixture of 9 mm ethephon and 1.5 or 3.0 mm NAA, or just 9 mm ethephon alone, accelerating shuck dehiscence by 1 to 2 weeks relative to that of the nontreated control. While all three treatments induced some degree of leaflet abscission, the two treatments employing the NAA and ethephon combination induced only about one-fourth (21% vs. 75%) as much leaflet abscission as when ethephon was used alone. However, this level of leaflet abscission (21%), plus an associated 50% drop in net photosynthesis for several days post-treatment, was sufficient to reduce in-shell nut yields in subsequent years. This appears to preclude commercial acceptability of such treatments for pecan. Chemical names used: (2-chloroethyl)phosphonic acid (ethephon), 1-napthaleneacetic acid (NAA).

Open Access
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In an effort to identify the primary transfer carbohydrates and to describe their characteristics of uptake by pecan [Carya illinoensis (Wangenh C. Koch)] leaf disks, it was discovered that the major carbohydrates in pecan xylem and phloem exudate and leaf free space leachate were sucrose, fructose, glucose, and inositol. Sucrose was the primary component of each of these systems. Uptake of sucrose, glucose, and fructose by leaf tissues was biphasic with a saturable active (sensitive to CCCP) carrier-like component and a nonsaturating passive (insensitive to CCCP) diffusion-like component. Uptake by the saturable component predominated at levels below 8 and 14 mM for fructose and glucose, respectively, and up to at least 40 mM for sucrose. The observations of a preponderance of sucrose in the free space of leaf disk and a sucrose V max an order of magnitude greater than for fructose or glucose suggests that sucrose is the major uptake carbohydrate moving from the leaf free space into the cytoplasm. The observation that uptake of any of these three sugars was noncompetitively inhibited by the other two sugars and that their uptake was unequally influenced by metabolic inhibitors suggests the possible presence of a separate carrier for each sugar. Sucrose uptake also appeared to be without hydrolysis and was coupled to the co-transport of protons with uptake diminishing with decreasing apoplastic proton levels.

Open Access
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The cyclic, alternate bearing and correlative aspects of U.S. produced pecan [Carya illinoinensis (Wangenh.) K. Koch] nuts are characterized. An attempt to forecast production using stepwise autoregressive techniques identified a national level biennial cycle for cultivar (CV) and seedling (SC) class nuts and a novemennial (9 year) cycle for SG class nuts. The intensity of the biennial cycle at the national level has generally been low to moderate over the last 50 years for CV and SG class nuts with no clear time trend being expressed. During the most recent years (1979-1991), national production of CV class nuts has not exhibited pronounced bienniality, whereas that of SG class nuts exhibited a moderate bienniality. The nature of the the irregularity of cycling of U.S. and state production appears to nullify the use of univariate polynomial equations as a practical tool for accurately forecasting nut production. Nut production within individual states was also cyclic, with 2-, 3-, 5, 6-, 10-, 12-, 14-, 15-, and 16-year cycles, depending on state and nut class. The most intense contemporary biennial cycles for CV class nuts were from Oklahoma, South Carolina, and North Carolina, whereas cycling of SG class nuts was most intense in Texas and Oklahoma. Correlations of production within and among states indicated that most interrelationships are relatively weak; however, national production of CV class nuts are highly correlated (r = 0.96) with the production of CV class nuts in Georgia, whereas that of SG class nuts is most correlated with that of Louisiana.

Free access
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Dormant season sprays of hydrogen cyanamide applied to pecan [Carya illinoinensis (Wangenh.) K. Koch] trees advanced budbreak, flowering, and shuck dehiscence. Hydrogen cyanamide was applied to dormant branches at ≈60, 45, 30, and 15 days before normal vegetative budbreak at rates of 0, 120, 240, 480, and 960 mm (corresponding to ≈0%, 0.5%, 1%, 2%, and 4%, solutions for 3 years). Depending on treatment, hydrogen cyanamide advanced budbreak by as much as 17 days, female and male flower maturity by up to 15 days, and nut ripening by as much as 14 days without reducing nut yield or causing phytotoxicity. Hydrogen cyanamide applied at 480 to 960 mm ≈60 days before expected budbreak possibly may be used commercially to advance ripening, manipulate time of pollen dispersal, and substitute for chilling when pecan is grown in mild environments.

Free access
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Abstract

Ethephon at 3, 6, and 12 mm induced abscission of mature foliage on pecan [Carya illinoensis (Wangenh) K. Koch] seedlings. This effect was offset to varying degrees using NAA at 3, 6, or 12 mm in which NAA acted as an effective antidote to ethephon and presumably ethylene. Abscission varied with leaf age and with the relative timing of leaf treatment with ethephon and NAA. Leaves treated with NAA 6 days prior to ethephon treatment were much more sensitive to ethephon induced abscission than those treated one day prior to, at time of, or one day after ethephon sprays. There were no abscission response differences among the 3 latter NAA treatment times. Ethephon greatly reduced 14C-IAA transport in leaf midrib tissue and increased the amount of IAA conjugated in leaf tissue without changing free IAA levels. It is suggested that ethephon treatment reduces the amount of IAA transported from the leaf blade to the abscission zone by inhibiting auxin transport in vascular tissue. IAA inactivation by conjugation appears to have no influence on the IAA level reaching the cells of the abscission zone. Chemical names used: (2-chloroethyl)phosphonic acid (ethephon); 1-naphthaleneacetic acid (NAA).

Open Access
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Abstract

Field observations indicate that conjunctive use of ethephon (Ethrel) and NAA (Fruitone-N) can induce early pecan [Carya illinoensis (Wangenh.) C. Koch] shuck (involucre) dehiscence, while greatly reducing undesirable leaflet abscission. Comparisons of the efficacy of 2,4-D and NAA in preventing undesirable leaflet abscission revealed that the comparative molar protective activity of 2,4-D greatly exceeds that of NAA, providing leaflets absolute protection against ethephon-induced abscission, but it was functionally inferior to NAA due to the induction of leaflet necrosis. Single ethephon treatments accelerated shuck dehiscence 3 to 6 weeks for several cultivars. Treatment of ‘Stuart’ and ‘Moneymaker’ pecan fruit and foliage, several weeks prior to the completion of natural shuck dehiscence, with a NAA-ethephon mixture accelerated shuck dehiscence by 5 and 3 weeks, respectively, without severe leaflet abscission or loss of nut quality. These data indicate possible development of harvest-aid technology for early harvesting of pecan without severe leaflet abscission. Chemical names used: (2-chloroethyl)phosphonic acid (ethephon); (2,4-dichlorophenoxy)acetic acid (2,4-D); and 1-naphthaleneacetic acid (NAA).

Open Access