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Bruce W. Wood

The factors regulating pecan [Carya illinoinensis (Wangenh.) K. Koch] pollen grain germination are poorly understood for both in vitro pollen viability tests and on receptive stigmatic surfaces of pistillate flowers. Potential regulating factors include flavonols, calcium (Ca), Ca-like alkali earth elements (AEEs), and rare earth elements (REEs). When various concentrations of certain naturally occurring simple flavonols (e.g., quercetin, kaempferol, myricetin, naringenin, and hesperetin) were tested in vitro by adding to standard pecan pollen germination medium, hesperetin, myricetin, and kaempferol functioned as a strong agonist at low concentration (0.12–2.0 µm for hesperetin and kaempferol, and 0.25 µm for myricetin), increasing pollen germination 2- to 3.9-fold over flavonol-free media. Hesperetin and myricetin were antagonistic at 16 µm. Kaempferol was not antagonistic at any concentration up to and including 16 µm. Naringenin was an antagonist at concentrations from 0.12 to 16 µm; whereas, quercetin was an antagonist at 8–16 µm, but tended to function as an agonist at low concentration (0.12–0.50 µm). The equal molar replacement of Ca2+ in standard pecan pollen germination media by single REEs, resulted in certain REEs [e.g., yttrium (Y), gadolinium (Gd), and thulium (Tm)] partially replacing the obligate need for Ca2+; thus, functioning as agonists in absence of Ca. All non-Ca AEEs [beryllium (Be), magnesium (Mg), strontium (Sr), expect for barium (Ba)], also partially substituted for Ca2+ at equivalent molar concentrations, but none were as efficacious as Ca2+. Results are suggestive that a) pollen germination in in vitro test can be improved by incorporation of certain flavonols, and b) pollen germination on stigmatic surfaces of flowers in orchards might be influenced or regulated by flavonol composition and Ca-like metals in the liquid matrix of the wet (receptive) stigmatic surface.

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Bruce W. Wood

Pecan [Carya illinoinensis (Wangenh.) K. Koch] nursery transplants performed best on establishment in nonirrigated orchards when using large trees planted early in the dormant season. After 6 years, growth and survival of bare-root transplants were equal to that of containerized transplants when established during the dormant season. Reducing transplant trunk height by ≤75% at planting did not affect subsequent tree survival, although rate of height growth and tree vigor increased such that there was no difference between pruned and nonpruned trees after 3 years, except that pruned trees appeared to possess greater vigor. There also were no differences in growth or survival between augured and subsoil + augured planting sites within 6 years of transplanting, and there were no differences between root pruned (severe tap or lateral root pruning) and nonpruned trees.

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Bruce W. Wood

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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Bruce W. Wood

There is increasing evidence of substantial pollination related crop losses by pecan [Carya illinoensis (Wangenh.) K. Koch] orchards. These most likely occur in block-type orchards consisting of only one or two cultivars, but can also occur at locations with a great number of different genotypes nearby. Main crop cultivars should generally be within about two rows of pollinizers to ensure cross-pollination. Thus, block widths exceeding about four rows between pollinizers are especially likely to exhibit serious pollination problems. Scattered trees of off-type genotypes are potentially of major importance as backup orchard pollinizers. Tree age/size and spring temperatures influence the characteristics of flower maturity windows and are probably primary factors contributing to pollination-related fruit-set losses in many block-type orchards. Flower maturity tends to be earlier in older/larger trees while warmer springs accelerate catkin development relative to that of pistillate flowers. Because of substantial variability in relative differences associated with initiation and duration of flower maturity windows within either protandrous or protogynous flowering types (i.e., Type I or II), selection of complementary pollinizers should be based on the relatively high resolution 30-class flowering classification system rather than the traditional low resolution 2-class system. Other factors sometime causing pollination related crop losses are either abnormally wet weather or strong dry winds during the pollination period or abnormally warm or cool springs. Pollination problems can be visually detected by noting premature non insect related post pollination fruit drop or diminishing fruit set with increasing distance from pollinator trees or off-type genotypes within the orchard.

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Bruce W. Wood

The United States pecan [Carya illinoinensis (Wangenh.) K. Koch] industry is based on about 10,107,170 trees (about 15% nonbearing) comprising about 492,137 acres (199,168 ha) of orchards (34% in Texas, 27% Georgia, and 17% Oklahoma) dispersed among about 19,900 farm operations (36% in Texas, 16% Georgia, and 7% Oklahoma) in 24 states. Fifty-six percent of this acreage is on farms with ≥100 acres (40.5 ha) of trees (i.e., 5% of total farms). An evaluation of production related changes over the last decade indicate fundamental changes occurring in the nature of the U. S. industry. These include a) movement toward agricultural industrialization as reflected by fewer small-farms and more large-farms; b) reduced percentage of young (i.e., nonbearing) trees in most major producing states; c) substantial decline in number of farms and acres in the southeastern regionhistorically the primary production area-yet substantial growth in the northern region of production; d) a national 3% increase in the number of pecan farms and 14% increase in acreage; and e) substantial demographic changes, such as the enhanced importance of the southwestern region including New Mexico with diminished importance of many southeastern states. States also drastically differ in degree of biennial bearing, as measured by the biennial bearing index (i.e., K = 0.04 - 0.73; where 0 = no production variation and 1 = maximum variation), average production efficiency of both orchards [Epa = 192 - 1,224 lb/acre (215 - 1,374 kg·ha-1)] and trees [Ept = 19 - 60 lb/tree (8.6 kg/tree)], variation in grower prices (cv = 18 - 36%), and relationship between price and national supply of pecan (r 2 = 0.94 - 0.03). For the pecan industry as a whole, average price received for nut-meats is as closely associated with national supply of pecan nut-meats as that of almond and pistachio and is far better than that of walnut-pecan's primary competitor. The supply of pecan meats on-hand at the beginning of the season, plus supply from the current season's crop, plus the price of walnut meats accounts for 80% of price variation in average United States pecan meat price.

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Bruce W. Wood

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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Bruce W. Wood

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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Bruce W. Wood

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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Bruce W. Wood

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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Bruce W. Wood

Of 18 commonly used adjuvants evaluated on pecan [Carya illinoinensis (Wangenh) K. Koch], a few exhibited potential for substantially suppressing net photosynthesis (A) and the conductance of foliage to water vapor (g sw) when used within their recommended concentration range; however, most provided no evidence of adversely influencing A or g sw. Suppression of gas exchange by certain adjuvants persisted at least 14 days after a single application. The recently developed organosilicone-based surfactants generally exhibited the greatest potential for suppression. These data indicate that orchard managers should consider the potential adverse influence of certain adjuvants when developing orchard management strategies.