Pecan alternate bearing remains a major industry problem (Smith and Weckler, 2011), although basic information regarding flowering regulation has expanded (Bangerth, 2009; Wood, 2011). Current theory of angiosperm fruit tree flowering regulation supports a level-one autonomous floral pathway with signaling by a mobile flowering locus T protein (Yang et al., 2007) and a level-two long-distance floral signal that engages histones affecting chromatin configuration and thus accessibility to particular genes for transcription (Kouzarides, 2007; Nelissen et al., 2007).
Premature defoliation, i.e., before a normal killing frost, of the entire tree markedly reduces pistillate flowers the next year (Hinrichs, 1962; Worley, 1979a). Early defoliation may negatively impact pistillate flower production by reducing available non-structural carbohydrates (Worley, 1979b) implicated in pistillate floral production (Wood, 1989; Worley, 1979a) or decreasing certain phytohormones or growth regulators involved in level-two long-distance floral signaling (Bangerth, 2009; Wood, 2011).
Typically current-season’s reproductive shoots end in a pistillate flower cluster. Occasionally, lateral buds initiate growth; thus, the distal portion of a reproductive shoot is vegetative. Next year’s pistillate flowers develop from lateral buds (bearing shoots) or terminal and lateral buds (vegetative shoots) on one-year-old branches. These buds produce current-season shoots that end in pistillate flowers or remain vegetative. Pistillate flower induction takes place in early August (Amling and Amling, 1983), but differentiation is delayed until bud swell in March (Wetzstein and Sparks, 1983). Vernalization has been suggested as a requirement for floret formation (differentiation) in pecan (Amling and Amling, 1983). In Arabidopsis, a fundamental mechanism of the photoperiodic and autonomous pathways was delivery of sugar to the shoot apex (van Nocker, 2001). Vernalization and autonomous pathways converge on the negative regulation of flowering locus C (Ausín et al., 2004). In pecan, vernalization may enhance sugar transport to induced meristems, avoiding meristem reversion to the vegetative state and favoring pistillate flower differentiation. Cultivars receiving inadequate chilling may have greater reversion of induced meristems resulting in less production.
Unraveling environmental and cultural conditions that influence flowering is complex, but research combined with persistence has yielded positive results. Pistillate flower induction and eventual differentiation appears to be influenced both locally (shoot or branch) and at the whole tree level. For instance, fruit removal on an individual shoot positively impacts return bloom of the same shoot on certain cultivars (Smith et al., 1986). Shoots with fruit have a greater likelihood of flowering the next year than vegetative shoots if the cultivar has a moderate alternate bearing index, but the likelihood is less if the cultivar severely alternate bears (Rohla et al., 2007a). The tree’s total cropload appears to have more impact on return bloom than the fruit load on an individual shoot, even if stored non-structural carbohydrates are abundant (Rohla et al., 2007b).
Several cultural practices have been developed that mitigate irregular bearing. These include management of the tree canopy for light interception (Hinrichs, 1961; Lombardini, 2006), mineral nutrition (Smith et al., 2012), groundcover vegetation (Smith, 2011), water (drainage, conservation, and irrigation) (Kallestad et al., 2006; Smith and Bourne, 1989), pest control (Mulder et al., 2011), and crop load (Reid et al., 1993; Smith and Gallott, 1990; Smith et al., 1993). Certain arthropod pests and disease occasionally cause partial defoliation on individual shoots [for example, Hyphantria cunea (Drury), Datana integerrima Grote and Robinson, Melanocallis caryaefoliae (Davis), Cladosporium caryigenum (Ellis and Langl.) Gottwald]. The degree of defoliation varies from a few leaflets on a compound leaf to entire shoots defoliated. This localized defoliation may affect subsequent pistillate flower production.
The objective of this study was to evaluate the effect of localized defoliation (shoot level) on return bloom. Specifically, the location and amount of defoliation on a shoot and defoliation time were investigated to determine the effect of subsequent flowering on the same shoot.
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