Correlations of Crop Load and Return Bloom with Root and Shoot Concentrations of Potassium, Nitrogen, and Nonstructural Carbohydrates in Pecan

The current theory of pecan [Carya illinoinensis (Wangenh.) C. Koch] alternate bearing is the ‘‘growth regulator–carbohydrate theory’’ in which flowering is first controlled by growth regulators produced by fruit and leaves, and then by the size of the carbohydrate pool near budbreak. Lack of nitrogen (N) reserves has also been proposed to be limiting after large crops, thus reducing return bloom. Annual production was determined for 12 individual trees for 3 years. Return bloom was monitored on four previous-season shoot types: 1) vegetative shoots, 2) bearing terminal shoots without a second growth flush, 3) bearing lateral shoots without a second growth flush, and 4) bearing shoots that were primarily in the terminal position with a second growth flush. Nonstructural carbohydrates, organically bound N, and potassium (K) concentrations were determined in roots and shoots. Regression analysis was used to determine the effect of yield on subsequent nonstructural carbohydrates, N, and K in the roots and shoots, and their postyield concentrations on subsequent flowering. Alternate bearing was evident because there were reductions of 18%, 16%, and 18% in the percentage of current season shoots flowering for every 10 kg/tree production increase in the previous season’s yield in 2002, 2003, and 2004 respectively. Flower production in 2002 decreased by 2.6 flowers/ 1-year-old branch and 1.6 flowers/1-year-old branch in 2003 for each 10 kg/tree increase in production. The third year of the study, neither previous season shoot type nor yield affected subsequent flower production. The previous year’s shoot type did not affect the percentage of current season shoots flowering; however, the previous year’s shoots that had a second growth flush producedmore flowers the following year than the other shoot types. Results suggested that crop loadwas not related to nonstructural carbohydrates, N, or K in the roots and shoots during January in these wellmanaged trees. Stored nonstructural carbohydrates, N, and K were also not related to return bloom. These data suggest that the current ‘‘growth regulator–carbohydrate theory’’ may not be valid in these well-managed trees. Nonstructural carbohydrates, K, and organically bound N do not appear to be critical factors regulating flowering. Alternate or irregular bearing is the most significant horticultural problem in pecan production. Alternate bearing is typically synchronized over regions by biotic or abiotic stresses and results in high-amplitude cycling (Gemoets et al., 1976; Wood, 1993). Irregular and often unpredictable production negatively impacts all economic aspects of pecan production and marketing. The currently supported theory for pecan alternate bearing is the ‘‘growth regulator–carbohydrate theory.’’ This theory evolved over years of research and contributions by several scientists. The origins of the theory began when Smith and Waugh (1938) reported that stored carbohydrate levels markedly affected subsequent flowering. The role of carbohydrates in flowering has been supported by several studies (Malstrom, 1974; Sparks and Brack, 1972; Wood, 1989, 1991; Worley, 1979a, b). Barnett and Mielke (1981) suggested that phytohormones may be involved in regulation of pecan alternate bearing. The involvement of phytohormones or growth regulators has been supported by several studies (Amling and Amling, 1983; Smith et al., 1986; Wood, 2003; Wood and McMeans, 1981). There are inconsistencies related to the involvement of stored carbohydrates regulating return bloom. For instance, Wood et al. (2003) reported that there was no association between alternate-bearing intensity and fruit ripening date or nut volume. In addition, as the postripening foliation period increased, alternate bearing increased. Weak or nonsignificant relationships have been reported between cluster size and return bloom (Rohla et al., 2005), suggesting little role for carbohydrates in regulating return bloom. Another study found stored carbohydrates in bearing shoots was greater than in vegetative shoots, although return bloom of bearing shoots was depressed relative to vegetative shoots (Smith et al., 1986). These studies bring into question the role of nonstructural carbohydrates in regulating alternate bearing. Nitrogen applications have generally increased pecan yield (Brooks and Livingston, 1962; Hunter, 1964; Hunter and Hammar, 1947, 1961; Skinner, 1922; Smith and Hamilton, 1937; Smith et al., 1985; Sparks, 1968; Taylor, 1930; Worley, 1974, 1990). Traditionally, nitrogen (N) has been applied as a Received for publication 15 June 2006. Accepted for publication 28 Sept. 2006. Funding for this study was provided by the Oklahoma Agricultural Expt. Sta., USDA Crop Germplasm Committee, and the Oklahoma Pecan Growers’ Association. Approved for publication by the Oklahoma Agricultural Expt. Sta. Regents Professor. Former graduate student. Current address: Samuel Roberts Noble Foundation, Ardmore, OK 73402. Professor. Corresponding author: E-mail: [email protected] 44 J. AMER. SOC. HORT. SCI. 132(1):44–51. 2007. JOBNAME: jashs 132#1 2007 PAGE: 1 OUTPUT: January 24 04:32:56 2007 tsp/jashs/133030/00897