Foliar boron (B) applications have been observed to promote flowering, fruit set, and yield in a variety of perennial tree crops (Batjer and Thompson, 1949; Hanson et al., 1985; Nyomora et al., 1999; Stephenson and Gallagher, 1987). Because B is passively absorbed and transported through the transpirational stream, deficiencies of B may be transitory (Brown et al., 1996; Hu and Brown, 1997; Raven, 1980). Such deficiencies commonly occur during periods of rapid plant growth, especially during flowering and seed set. Premature flower and fruit drop of tree crops has been attributed to B deficiency, suggesting that B movement to reproductive structures is restricted or that growth and development of floral structures have a higher demand for B than do vegetative structures (Dell and Huang, 1997).
Reductions in crop yield and quality in low B soils potentially result from impaired reproductive development during the flowering/fruiting cycle (Dell and Huang, 1997). Pecan [Carya illinoinensis (Wangenh.) K. Koch] is a wind-pollinated, monoecious crop exhibiting heterodichogamy. Main pecan cultivars in a block require a pollenizer with suitable pollen-release phenology, located within ≈49 m from the main cultivar, to maximize crop potential (Wood, 1997). Although genetic factors account for most of the potential of the pollenizer, other variables, including mineral nutrition of the plant, may influence pollen quality and its subsequent performance (Nyomora et al., 2000).
A natural abortion of pecan fruit occurs during four periods within the growth cycle of the pecan fruit (Sparks and Heath, 1972). The severity of these fruit abortions or “drops” may vary by cultivar (Sparks and Madden, 1985). Under normal conditions, the most widely planted pecan cultivar in the Southeast, ‘Desirable’, experiences an average fruit abortion of ≈40% to 60% during the second drop (Sparks and Madden, 1985). Studies by Yates and Sparks (1995) suggest that embryo sacs of abortive fruit from the second drop were shriveled and contained an egg apparatus similar to that observed in unfertilized eggs. The second drop of pecan fruit, occurring between 14 and 45 d after pollination, coincides with abscission of nonpollinated flowers and is attributed to unsuccessful fertilization of the egg (Sparks and Madden, 1985).
Yield enhancement by foliar B applications in perennial tree crops has long been recognized. Nyomora et al. (1999) demonstrated that foliar B applications resulted in increased fruit set of almond [Prunus dulcis (Mill D.A. Webb)]. Fruit set has also been enhanced with foliar B in ‘Italian’ prune (Prunus domestica L.) and ‘Anjou’ pear (Pyrus communis L.) (Batjer and Thompson, 1949; Hanson et al., 1985). Fruit set of sour cherry (Prunus cerasus L.) was increased by as much as 100% with foliar B applications (Hanson, 1991a, 1991b). Stephenson and Gallagher (1987) found that foliar B sprays enhanced kernel quality in macadamia [Macadamia integrifolia (Maiden and Betche)]. Foliar B sprays also led to a greater total mass and decreased hull percentage of almond (Nyomora et al., 1997).
Soil B availability is influenced by soil texture, pH, liming, organic matter, interrelationships with other nutrients, and, most notably, soil moisture (Wood, 1999). Many of the soils on which pecans are grown in the southeastern United States are coarse-textured, well-drained sands, or sandy loams from which B may leach readily. In addition, excessive soil applications of B can potentially induce leaf scorching and reduced fruit set (Blackmon and Winsor, 1946). These factors combined with the relatively immobile nature of B within the plant indicate that foliar B applications would be an efficacious method of providing B to pecan reproductive tissues. However, the effects of foliar-applied B on pecan in the southeastern United States are poorly understood. The objective of this study was to investigate the effect of foliar B application on leaf tissue B concentration, fruit retention, and kernel quality in pecan.
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