Alternate bearing is the most significant horticultural problem facing pecan producers. Several reviews have been published on the subject (Barnett and Mielke, 1981; Monselise and Goldschmidt, 1982; Smith, 2005; Sparks, 1974, 1975, 1979, 1986, 2000, 2003a; Wood, 1991; Wood et al., 2004). Initial research (Smith and Waugh, 1938) and later work suggested that stored carbohydrate concentrations during the winter markedly affected subsequent flowering (Malstrom, 1974; Wood, 1989, 1991; Worley, 1979a, 1979b). Other work suggested that inhibition of return bloom by developing fruit was incited by phytohormones or other growth regulators (Amling and Amling, 1983; Smith et al., 1986; Wood, 2003; Wood and McMeans, 1981; Wood et al., 2003). Hypotheses for alternate bearing have undergone modification as more data became available. The current theory supports a two-level control with inhibitors and promoters determining induction during the previous growing season and the dormant season carbohydrate pool influencing pistillate flower development (Smith et al., 1986; Sparks, 2000, 2003a; Wood, 2003; Wood and McMeans, 1981; Wood et al., 2003, 2004).
Nitrogen application has 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); however, Sparks (2003b) has pointed out that alternate bearing tendency was frequently increased. A recent N management strategy is an early spring N application followed by a late summer or fall N application during years with large crops (Goff et al., 2001; Wood, 2001a). The hypothesis guiding this strategy is that the N demand by large crops creates a critical N shortage during the late summer and fall as the crop matures. This N shortage that develops in the fall when crops are large may contribute to alternate bearing.
There are conflicting reports regarding the benefit of fall N application. Acuña-Maldonado et al. (2003) reported the greatest N absorption between budbreak and the end of shoot expansion, with little absorption between the end of leaf expansion and leaf fall. Kraimer et al. (2004) later reported that N applied during the kernel filling stage was strongly absorbed. The newly absorbed N replenished endogenous N reserves and might moderate pecan alternate bearing.
Nonstructural carbohydrates and organically bound N must be transported from a source or storage site (net exporter) to a sink (net importer) via the phloem. Adequate K favors phloem loading by improving ATP synthesis, a high-energy source required for phloem loading (Haeder, 1977; Mengel, 1980). Potassium also increases flux rates without diluting phloem sap content of organic solutes (Mengel, 1980; Mengel and Haeder, 1977), thus transport rates are faster when adequate K is available. Thus, it appears that K may affect both phloem loading and phloem transport (Haeder, 1977; Vreugdenhil, 1985).
In pecan, K accumulated quickly during the final 30 d before the fruit ripened, when the cotyledons were rapidly developing (Diver and Smith, 1984). Most of the kernel weight was accumulated during this period. Kernel oil content was strongly correlated with leaf K concentration (Hunter and Hammar, 1956). Developing fruit substantially depressed leaf K concentration during the latter part of the growing season (Diver and Smith, 1984; Krezdorn, 1955; Sparks, 1977), suggesting that K may become limiting when crop loads are large. Adequate K supplies appear critical to the transport and delivery of sugars and N compounds and are more likely to be limiting during years with large crops.
One method to combat alternate bearing and improve nut quality during large crop years is mechanical fruit thinning (Smith and Gallott, 1990). Reid et al. (1993) demonstrated that fruit thinning while the fruit were between one-half and full ovule expansion in the liquid endosperm developmental stage reduced alternate bearing. Crop load thresholds based on the percentage of fruiting shoots and fruit size have been established for various cultivars (Smith et al., 1993). However, information is lacking on the effect of cluster size on alternate bearing and nut quality. Uncertainty remains regarding the need to account for cluster size in crop load threshold recommendations for mechanically thinning pecan trees.
One objective in this study was to determine the effect of thinning trees to different cluster sizes on return bloom and nut quality. Another objective was to determine the relationship of trees thinned to different cluster sizes on dormant-season nonstructural carbohydrates, organically bound N, and K concentrations. Also characterized was the fruiting potential of different shoot types based on their previous year's fruiting, position on the branch, and the presence or absence of a second growth flush.
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