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Abstract

Pecan [Carya illinoinensis) (Wangenh.) K. Koch] kernel development was characterized by an initial rapid localized expansion of testa and endosperm, which was closely associated with low levels of free and bound abscisic acid (ABA) and with high levels of gibberllin-like (GL) substances. Rapid cotyledon growth began with the termination of testa and endosperm expansion, which was subsequent to a sharp increase in both free and bound ABA. The rate of change for growth in kernel dry weight was highly correlated with the rate of change in levels of both free ABA (R 2 = 0.86) and bound ABA (R 2 = 0.88). Levels of GL substances (ng/g kernel), as measured by the dwarf pea and cucumber bioassays, were relatively low after the rapid accumulation of kernel dry weight, however, GL substances detected by the barley endosperm bioassay were high during the last 30 days of kernel development. Abscisic acid and GL substances seem to exercise a significant role in seed development.

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Seven open-pollinated pecan [Carya illinoinensis (Wangenh.) K. Koch] stocks were grown in a nursery in blocks. Bud growth of ungrafted seedlings was influenced by rootstock, with growth being more advanced on `Curtis', `Elliott', `Apache', and `Sioux' seedlings than on `Moore', `Riverside', and `Burkett'. Bud growth of grafted trees was influenced by scion, with growth of `Candy' being most advanced, while `Cape Fear' trees were more advanced than `Stuart'. Growth of `Candy' grafted trees was affected by rootstock, with growth being more advanced on `Elliott' and `Curtis' seedling rootstock as compared to `Apache', Sioux', `Riverside', and `Burkett' seedling rootstock. Tree damage caused by a May freeze was directly related to bud growth and was influenced by scion and rootstock.

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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.

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The crop water stress index (CWSI), based on the relationship between the canopy temperature of a well-watered plant in full sunlight and the atmospheric water content, numerically quantifies water stress. A 4-year study was established to determine the long-term effect of water application levels on production, nut quality characteristics, and growth of pecans [Carya illinoinensis (Wangenh.) C. Koch cv. Western Schley]. Highest yields were attained when trees were relatively nonstressed (CWSI ≤ 0.08). Trees subjected to moderate water stress before irrigation (CWSI ≥ 0.20) showed reduced yield, nut weight, and tree growth, although water-use efficiency increased. With water management practices resulting in maximum yield, nut size, and tree growth (CWSI ≤ 0.08), tree water use varied up to 44% in the same orchard, depending on crop load and yearly climatic variations.

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Chilling and heating effects on budbreak of pecan [Carya illinoinensis (Wangenh.) K. Koch] trees were examined by linear regression analyses from experimental data and from records of budbreak dates over a wide geographic range. The results demonstrate that budbreak in pecan is under the interactive control of heating and of chilling. Heat required for budbreak varies inversely with chill accumulation, and budbreak may occur with no chilling once sufficient heat accumulates. Variability in budbreak increases dramatically when there are fewer than ≈ 100 chilling degree days. Heating degree days with daily minima <2.2C are inefficient; 3.9C is the most efficient heating and chilling base. At base 3.9C and the daily minimum heating temperature of 2.2C, heating degree days required for budbreak of a composite of cultivars can be predicted from chilling degree days over a wide geographic range by the relationship, Log Y = 2.7190-0.0216 √X.

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`Dodd' pecan seedlings [Carya illinoinensis (Wangenh.) K. Koch] were chilled at 6C for 0 to 1800 hours in 300-hour intervals and percent budbreak and days to budbreak recorded. Chilling duration required for ≥ 50% budbreak was 900 hours. Chilling > 900 hours increased budbreak percentage and reduced time to budbreak. `Dodd' seedlings chilled at 1, 5, or 9C for 0 to 2500 hours in 500-hour intervals had more lateral budbreak after 1000 hours of chilling at SC than at 1 or 9C. When chilling hours ranged from 1500 to 2500, 1C increased budbreak of the first lateral bud compared with 5 or 9C. As chilling was increased from 1000 to 2500 hours, the days to budbreak declined, and the uniformity of budbreak increased.

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This work is the result of 3 years of collaborative research between Mississippi State Univ. and New Mexico State Univ. Physical, chemical, and sensory characteristics were studied to assess eating quality of popular New Mexico pecan (Carya illinoinensis) cultivars. The force and energy necessary to break (shear) pecan nuts, and Hunter `a' and hue angle values varied with harvest year and cultivar. All other traits, including sensory evaluation results, varied only with cultivar. `Ideal' was of light color, small size, and not as firm as the others, while `Burkett' was soft and slightly rancid. `Wichita' was the cultivar rated best by panelists, despite its slightly darker color. `Western Schley' and `Salopek' were also acceptable, although not as acceptable as `Wichita'.

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Abstract

Inflorescence and staminate flower development in pecan [Carya illinoinensis (Wang.) K. Koch] were examined using scanning electron microscopy. Organogenesis was described from inception to pollen dehiscence. The order of organ initiation was: a single bract, rounded floral apex, 2 lateral bracteoles, and 3-7 stamens. The initiation and time when stages of floral differentiation occur were determined for 1 protandrous and 2 protogynous cultivars. The time of early inflorescence development and the initiation of floral primordia and bracts were similar in both cultivar types, occurring about 12 months prior to staminate maturity. However, the initiation time of later floral development stages was divergent. The floral stages in the protandrous cultivar up to anther lobing occurred during the previous growing season. In the protogynous cultivars, initiation of bracteoles, the floral apex, stamen primordia and anther lobing took place in the spring of their anthesis.

Open Access

More than 93% of pecans [Carya illinoinensis (Wangenh.) K. Koch] produced in the United States are grown in the southeastern and southwestern states. However, the native range of the pecan tree extends northward into Kansas, Missouri, and Illinois. In these northern states, commercial pecan production is expanding as additional acres of native trees are brought under cultivation and orchards of short-season, cold-hardy cultivars are established. Native nut production dominates the northern pecan industry accounting for over 95% of nuts produced in the region. Cultural practices for native pecans have been developed for northern groves that feature low inputs and good yields. Pecan cultivars adapted to the north ripen their fruit in a climate that provides 155 to 200 frost-free days. Few generalizations can be made about northern cultivars. The nuts produced by these cultivars vary in size from small [4 g (0.14 oz)] to medium [8 g (0.28 oz)] with shelling percentages ranging from 44% to 59%.

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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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