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 single application shortly before budbreak or split between budbreak followed by a May or June application. However, scientists suggested that a critical N shortage develops in the fall during years with large crops, “on-years,” that contributes to alternate bearing (Goff et al., 2001; Kraimer et al., 2004; Wood, 2001b).
Potassium (K) could be limiting during on-years because developing fruit may deplete leaf K (Diver and Smith, 1984; Krezdorn, 1955; Sparks, 1977) that simultaneously accumulates in shuck and kernel tissue during fruit ripening (Diver and Smith, 1984). Additionally, leaf K concentration and kernel oil content are closely associated (Hunter and Hammar, 1956). Transport of sugars and amino compounds in the phloem is accomplished by osmotically generated hydrostatic pressure differences between the source and sink (Lalonde et al., 2003). Adequate K favors phloem loading by improving adenosine triphosphate synthesis, a high energy source required for phloem loading (Haeder, 1977; Mengel, 1980). Studies have provided evidence that K increases flux rates without diluting phloem sap content of organic solutes (Mengel, 1980; Mengel and Haeder, 1977), thus transport rates are substantially faster when adequate K is available. Vreugdenhil (1985) demonstrated that the K gradient in the phloem coincided with the direction of flow. Thus, K availability affects both phloem loading and transport (Haeder, 1977; Vreugdenhil, 1985), and consequently fruit development, yield, and potentially alternate bearing.
The objective of this study was to determine the relationship of nonstructural carbohydrates, K, and organically bound N during January in selected tissues with the previous crop yield and subsequent season flowering. Vigorous, exceptionally well-managed ‘Pawnee’ trees that were alternate bearing were chosen for the study to eliminate any casual relationships between nonstructural carbohydrates, K, or organically bound N with crop load or flowering that might be apparent in less vigorous trees. For instance, a low-vigor tree with a large crop is likely to have few stored carbohydrates. If a small crop follows, it is unclear whether the lack of return bloom was associated with depressed nonstructural carbohydrates or was suppressed for other causes and the relationship with stored carbohydrates was casual. This study determined the relationships of 1) crop load with return bloom and the concentrations of nonstructural carbohydrates, K, and organically bound N; and 2) stored carbohydrates, K, and organically bound N with return bloom. Return bloom was characterized for four shoot types.
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