Fruit Production Characteristics of ‘Pawnee’ Pecan

Yield and selected nut quality characteristics were monitored on hedge pruned ‘Pawnee’ pecan [Carya illinoinensis (Wangenh.) K. Koch.] trees over a 5-year period to characterize optimum production defined as equal crops 2 successive years. Previous year yield was linearly or quadratically related to current-season yield in three of four instances. Optimum yield ranged from 18 kg/tree to 29 kg/tree among years. Weight/nut, weight/kernel, and percent kernel were negatively related to yield/tree. Weight/nut and weight/kernel were more closely related to yield/tree than percent kernel, indicating that as cropload increased, shell weight and kernel weight were disproportionately affected. Increasing croploads reduced kernel weight more than shell weight, thus reducing the correlation between percent kernel and yield. Twenty-seven percent of the trees in the study produced greater than average yields with a lower than average alternate bearing index. Alternate bearing was recently identified as a key component limiting expansion of the pecan industry by producers, shellers, and processors (Smith and Weckler, 2011). Pecan alternate-bearing theories have undergone modification as additional data have become available. Current theory concerning flowering regulation of angiosperm fruit trees (Bangerth, 2009) and specifically pecan (Wood, 2011) states that the level-one signal of the autonomous floral pathway is florigen [a mobile flowering locus T protein (Yang et al., 2007)]. The level-two floral signal appears to involve long-distance signals that engage histones affecting chromatin configuration and thus accessibility to particular genes for transcription (Kouzarides, 2007; Nelissen et al., 2007). Phytohormones are candidates for long-distance signals such that gibberellins and auxin inhibit floral induction, whereas cytokinins promote floral induction (Bangerth, 2009; Wood, 2011). In pecan, non-structural carbohydrate concentrations in roots were implicated in pistillate flower differentiation (post-floral induction) (Malstrom, 1974; Smith and Waugh, 1938; Sparks and Brack, 1972; Wood, 1989, 1991; Worley, 1979), but later work demonstrated that abundant stored non-structural carbohydrates were not related to return bloom (Rohla et al., 2007a, 2007b; Smith et al., 2007). Work on Arabidopsis indicated that a fundamental mechanism of the photoperiodic and autonomous pathways was delivery of sugar to the shoot apex (van Nocker, 2001). Thus, nonstructural carbohydrates would only function in floral regulation (causing reversion of the induced meristem) when their concentrations were below a threshold capable of supplying requisite sugar to the induced (reproductive) meristem. Vernalization has been suggested as a requirement for floret formation in pecan (Amling and Amling, 1983). In Arabidopsis, vernalization and autonomous pathways converge on the negative regulation of flowering locus C (Aus ın et al., 2004). In pecan, vernalization may enhance sugar transport to induced meristems, avoiding meristem reversion to the vegetative state and favoring pistillate flower differentiation. Cultivars receiving inadequate vernalization may flower through the autonomous pathway; however, reversion of induced meristems should be greater resulting in less production. Several cultural practices have been developed to reduce pecan alternate bearing. These include management of the tree canopy for light interception (Hinrichs, 1961; Lombardini, 2006), mineral nutrition (Smith, 2010; Smith et al., 1985), groundcover vegetation (Smith, 2011), water (drainage, conservation, and irrigation) (Kallestad et al., 2006; Smith and Bourne, 1989), pest control (Mulder et al., 2011), and cropload (Reid et al., 1993; Smith and Gallott, 1990; Smith et al., 1993). Cultivar selection also has a major impact on alternate bearing (Conner and Worley, 2000). ‘Pawnee’ is a popular cultivar grown throughout the entire pecangrowing region because of its early-season maturity and large fruit size (Thompson and Grauke, 2000). However, it has developed a reputation among producers for alternate bearing. Alternate-bearing pecan cultivars typically produced a lower percentage of shoots with fruit if that shoot bore fruit the previous year compared with shoots that were vegetative (Rohla et al., 2007b). The opposite was true for cultivars that tend to bear uniform crops among years. Data indicate that the fruit-bearing characteristics of ‘Pawnee’ represent a cultivar that should have relatively uniform production (Rohla et al., 2007a). However, even cultivars with low alternate-bearing tendencies will produce an excessive crop resulting in few flowers and fruit the next year unless fruit were adequately thinned. An objective of this study was to determine the maximum Fig. 1. Correlations between the previous year yield with the subsequent year yield of ‘Pawnee’ pecan. Correlations were calculated for (A) 2002 with 2003 through (D) 2005 with 2006. Coefficients of determination were nonsignificant (NS) or significant at 1% (**) or 0.1% (***). Dotted lines represent 95% confidence bands. Received for publication 20 Jan. 2012. Accepted for publication 23 Feb. 2012. Funded by the Oklahoma Agricultural Experiment Station, Samuel Roberts Noble Foundation, Hauani Creek Ranch, Montz Pecan Orchard, Oklahoma Pecan Growers’ Association, Texas Pecan Board, and the USDA Crop Germplasm Committee. I gratefully acknowledge the support and assistance of Becky Cheary, Tim Montz, and Jake Montz and the use to the Montz Pecan Orchard for this project. Approved for publication by the Oklahoma Agricultural Experiment Station. To whom reprint requests should be addressed; e-mail [email protected]. HORTSCIENCE VOL. 47(4) APRIL 2012 489 cropload ‘Pawnee’ trees in this study could carry and return a similar crop the next year. In addition, selected nut quality characteristics were correlated with cropload. Materials and Methods ‘Pawnee’ on open-pollinated seedling ‘Elliott’ rootstocks were 14 years old when the study was initiated. Tree were spaced 12.2 m · 12.2 m, irrigated by microsprinkler, and were planted on a Teller sandy loam soil (fine-loamy, mixed, thermic, Udic Argiustoll). The soil pH was 7.9 in the upper 15 cm and 7.2 in the 15to 30-cm level. Rows were oriented north–south and were hedge-pruned annually on the east and west sides 2.5 to 3 m from the trunk and the top 8.5 to 9.5 m tall. The orchard floor was maintained vegetationfree throughout the growing season with multiple applications of glyphosate [N(phosphonomethyl) glycine] in the form of its potassium salt applied at 408 g ha a.i. No winter cover crops were used, but coolseason vegetation was allowed to grow during the winter. Trees received 160 kg ha nitrogen (N) annually from nitrate-contaminated irrigation water and had not received supplemental N application since 2002. Foliar applications of zinc sulfate (36% zinc; 6.7 kg ha material) were applied annually beginning when the first leaf began to unfurl then at 2-week intervals until applied five times during each growing season. Forty-nine trees were monitored annually for 5 years. Trees were harvested, yield measured, and a random 40-nut sample was collected from each tree. Nut samples were collected from field-run nuts (before cleaning to remove light weight nuts) and analyzed for weight/nut, weight/kernel, and kernel percentage. Trunk diameter was measured annually at 1.4-m height while trees were dormant. An alternate-bearing index was calculated for each tree using the equation: I = 1= n 1 ð Þ a2 a1 ð Þ j j = a2 + a1 ð Þ 1⁄2 + a3 a2 ð Þ j j = a3 + a2 ð Þ + an an 1 ð Þ j j= an + an+1 ð Þ where I was the alternate-bearing index; n was the number of years; and a1, a2, and an were yields of corresponding years (Pearce and Dobersek-Urbanc, 1967). The alternatebearing index ranged from 0 to 1; a higher index indicates more alternate bearing. Yield efficiency was calculated by dividing the yield by the trunk cross-sectional area. Linear and quadratic equations were fitted to data using least squares techniques with 95% confidence bands calculated (Draper and Smith, 1966). Previous season yield/tree was correlated with current-season yield/tree from 2002 through 2006. Current-season yield/tree was also correlated with weight/ nut, weight/kernel, and percent kernel. Previous season yield efficiency (yield/trunk cross-sectional area) was correlated with current-season yield efficiency. Superior trees among the 49 observed trees were determined by those with above average cumulative yield and below average alternate-bearing index. Trees that occurred in both categories they were identified as superior (Wood, 1989).