Due to the strong vegetative nature of relatively young pecan trees and the absence of dwarfing rootstocks or cultivars, controlling tree size is a major problem in high-density pecan orchards. Paclobutrazol, an effective inhibitor of gibberellin biosynthesis, offers a superior method of tree size control of pecan compared with traditional and generally unsuccessful pruning or hedging methods (Sparks, 1979). Previous research indicated the strong effect of PBZ on pecan seedlings grown in a greenhouse (Marquard, 1985; Wood, 1984), young trees in the field (Andersen and Aldrich, 1987; Gash and David, 1989; Wood, 1986, 1988a), and large pecan trees (Wood, 1988b; Worley et al., 1996). However, there are differential cultivar responses depending on tree size and age and on the balance between vegetative and reproductive growth (Gash and David, 1989; Wood, 1988a; Worley et al., 1996).
A well-maintained carbohydrate and nitrogen balance within certain levels is essential for pecan flower formation (Gourley and Howlett, 1941). Terminal growth is related to the amount of stored carbohydrates and is an outward expression of the nutritional and physiological condition of the tree (Crane, 1930; Gossard, 1954; Sparks and Brack, 1970). It is also the most common characteristic observed by growers, who associate it with tree vigor and nutrition. There can be a large variation in shoot growth on the same tree or different trees. Old trees tend to produce short shoots while young trees produce longer shoots (Crane, 1930). In mature trees, longer shoots produce more leaf area and carbohydrates (Sparks, 1969), but excess vegetative growth resulted in little or no blossoming and nut production (Gossard, 1954). Trees with short and weak shoots were less likely to fruit the following year than shoots within the optimal growth range for fruiting. Heavy nut production is associated with shoots of medium length (Isbell, 1928).
Different cultivars have a wide shoot-length range for optimal fruit production (Amling, 1959; Isbell, 1928). The optimal shoot length ranges for fruiting in ‘Western Schley’, ‘Burkett’, ‘Stuart’, and ‘Success’ were on shoots 4 to 15, 4 to 18, 4 to 9, and 2.5 to 8 inches, respectively (Taylor, 1959). Gossard (1954) suggested that the best range for terminal growth was 4 to 8 inches. The general recommendation for pecan fertilization is to keep annual terminal growth on mature bearing pecan trees to 4 to 8 inches long. A significant positive relationship was found between yield and percentage of fruiting shoots (Sparks and Heath, 1972; Sparks, 1975). Young pecan trees usually produce few lateral branches due to aggressive vegetative growth, which contributes to relatively low yields (Thompson, 1981). Increased lateral branching may increase the potential number of sites for nut initiation (Andersen and Aldrich, 1987). Chemical applications (Malstrom and McMeans, 1977, 1978) and pruning (Kuykendall and Tate, 1970) may be potential methods to promote lateral shoots.
This experiment was carried out to 1) evaluate the effects of paclobutrazol on inhibiting pecan vegetative growth and increasing the number of different shoot lengths and 2) investigate the relationship between shoot length and number of pistillate inflorescences.
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