Pecan [Carya illinoinensis (Wangenh.) K. Koch] trees are native to the United States and Mexico with a range that extends from floodplains in Illinois and Iowa through Texas to Mexico (Fig. 1). Trees originating from northern populations mature seeds within a growing season of ≈170 d and survive in areas that receive average annual minimum winter temperatures of –26 to –29 °C. Trees at the southern extent of the range in Oaxaca, Mexico, may experience no freezing temperature in some years and new growth occurs before dehiscence of the previous season's foliage (Grauke and Thompson, 2008). Observations of seedlings from across the range, grown in a common orchard, allow native populations to be divided into two main provenances. Seedlings originating from southern sources (from Texas south) break bud earlier in the spring, retain foliage later in the fall, and grow larger in height and trunk diameter than seedlings originating from more northern sources (Wood et al., 1998).
To survive in the north, pecan trees must be adapted to withstand the onset of cold temperatures in the fall, to survive extremely cold temperatures in midwinter, and to begin growth only after the danger of a spring freeze is over. In the fall, families of seedlings in provenance orchards can be distinguished by the inception of dormancy with seedlings from southern sources retaining leaves longer in the fall than seedlings grown from northern sources. However, leaf drop is typically initiated for all seedlings after the first frost (0 °C), which is received at the Brownwood, TX, worksite by 16 Nov. (5 of 10 years) and at the College Station, TX, worksite by 30 Nov.
The progression of pecan cultivars through winter dormancy has not been well characterized. When pecan seedlings grown from open-pollinated ‘Dodd’ seeds were given 900 h of chilling at 6 °C and were transferred to a greenhouse at 23 °C, greater than 50% began growth within 80 d (Smith et al., 1992). Longer periods of chilling reduced time to and increased uniformity of budbreak. Seedlings chilled at 5 °C had higher levels of budbreak in both first and second lateral buds after 1000 h than seedlings chilled at either 1 or 9 °C. All terminal buds broke at 1000, 1500, 2000, and 2500 h regardless of chilling temperature. However, first lateral buds receiving 1 °C chilling had the highest recorded levels of budbreak after 1500 and 2000 h, whereas second lateral buds receiving 5 °C continued to show the highest levels of budbreak at those time periods. The severity of the winter may influence duration of dormancy to different degrees in different areas of a tree, influencing uniformity of budbreak within the canopy. Heating requirements for pecan budbreak have been identified for pecan cultivars that experience minimal chilling (Sparks, 1993). Northern cultivars have greater chilling requirements for budbreak than southern cultivars (Sparks, 1993, 2005; Wood et al., 1998).
Generally, seed development occurs faster and results in early seed maturation in ecotypes from northern latitudes (Daws and Pritchard, 2008; Sparks, 1991). Northern pecan parents have conferred that trait on their progeny in several cultivars such as ‘Pawnee’ (Thompson and Hunter, 1985), ‘Osage’ (Thompson et al., 1991), ‘Kanza’ (Thompson et al., 1997), and ‘Lakota’ (Thompson et al., 2008). Evaluating seasonal phenology of controlled cross-progeny families is easily done and is useful in predicting the extent to which a genotype fits a targeted environment, but more accurate methods of characterizing hardiness may improve the recognition of critical limitations and influence decisions of cultivar release and deployment.
Differential thermal analyses (DTA) have been used to imply cold hardiness in woody tissues of some species. In apple, pear, and azalea, a low temperature exotherm (LTE) of dormant woody stem sections, detected by DTA, correlated with injury to both xylem and pith that occurred during cooling (Graham and Mullin, 1976; Montano et al., 1987; Quamme et al., 1972a, 1972b). The LTE is an indication of the temperature at which supercooled water, presumably in the xylem, freezes. DTA profiles also show broad exothermic events at higher temperatures, which are usually interpreted as water freezing in extracellular spaces of the xylem and pith. The magnitude of the LTE decreases with slow cooling rates (less than 5 °C/h) or increased exposure times at subzero temperatures (Quamme et al., 1972a, 1972b, 1973). The correlation between LTE temperatures and mortality led some to suggest that the DTA technique could be used to indicate cold hardiness and Quamme (1991) proposed using LTE temperature as a measurement of cold hardiness in breeding programs for some species. LTE temperatures have previously been shown to correlate with lethal cooling in winter and early spring-harvested pecan apical floral buds and stem samples (Rajashekar and Reid, 1989).
Many factors contribute to cold hardiness in pecan trees: rootstock, crop load, tree age, nutritional status, seasonal growth and weather conditions, cultivar, and ecotype (Grauke and Pratt, 1992; Sanderlin, 2000; Smith, 2000, 2002; Smith et al., 2001; Sparks and Payne, 1977). Our interest here was to survey a broad range of pecan diversity (112 cultivars) to determine if differences in ecotype could be detected by LTE profile.
Daws, M.I. & Pritchard, H.W. 2008 The development and limits of freezing tolerance in Acer pseudoplatanus fruits across Europe is dependent on provenance Cryo Letters 29 189 198
George, M.F., Burke, M.J., Pellett, H.M. & Johnson, A.G. 1974 Low temperature exotherms and woody plant distribution HortScience 9 519 522
Graham, P.R. & Mullin, R. 1976 The determination of lethal freezing temperatures in buds and stems of deciduous azalea by a freezing curve method J. Amer. Soc. Hort. Sci. 101 3 7
Grauke, L.J. 2008 Monitoring bud growth 12 Sept. 2008 <http://extension-horticulture.tamu.edu/carya/Manual/BUDBRK.html>.
Grauke, L.J. & Thompson, T.E. 1997 Pecan. In: Register of new fruit and nut varieties. Brooks and Olmo, List 38 HortScience 32 793 796
Ketchie, D.O. & Kammereck, R. 1987 Seasonal variation of cold resistance in Malus woody tissue as determined by differential thermal analysis and viability tests Can. J. Bot. 65 2640 2645
Montano, J.M., Rebhuhn, M., Hummer, K. & Lagerstedt, H.B. 1987 Differential thermal analysis for large-scale evaluation of pear cold hardiness HortScience 22 1335 1336
Quamme, H., Stushnoff, C. & Weiser, C.J. 1972a The relationship of exotherms to cold injury in apple stem tissues J. Amer. Soc. Hort. Sci. 97 608 613
Quamme, H.A., Stushnoff, C. & Weiser, C.J. 1972b Winter hardiness of several blueberry species and cultivars in Minnesota HortScience 7 500 502
Quamme, H., Weiser, C.J. & Stushnoff, C. 1973 The mechanism of freezing injury in xylem of winter apple twigs Plant Physiol. 51 273 277
Romberg, L.D. 1966 Notes on varieties used in breeding of which progeny were fruited in the years through 1966 US Pecan Field Station, Brownwood, TX Unpublished records. USDA-ARS
Smith, M.W., Cheary, B.S. & Carroll, B.L. 2001 Rootstock and scion affect cold injury of young pecan trees J. Amer. Pomological Soc. 55 124 128
Sparks, D. 1991 Geographical origin of pecan cultivars influences time required for fruit development and nut size J. Amer. Soc. Hort. Sci. 116 627 631
Vogel, K.P., Schmer, M.R. & Mitchell, R.B. 2005 Plant adaptation regions: Ecological and climatic classification of plant materials Rangeland Ecol. Manag. 58 315 319