Alternate bearing (AB) poses a major challenge for the pecan [Carya illinoinensis (Wangenh.) K. Koch.] industry (Wood, 2003). AB refers to a tendency for wide season-to-season fluctuations in cropping intensity. This is often expressed as a heavy (on) cropload season followed by a light (off) cropload season (i.e., a biennial bearing pattern). Whether the season is an on or off year, productivity and yield are directly dependent on the success or failure of the flowering process (Sedgley, 1990).
Flowering control is complex and involves several environmental (e.g., light and temperature) and endogenous cues (e.g., carbohydrates and phytohormones). Wood (1983) showed for pecan that levels of the endogenous phytohormone gibberellic acid (GA) examined in bud tissues peak dramatically in the days immediately surrounding budbreak. Exogenously applied PGRs, such as GA and ethylene, among others, have been studied for the intention of mitigating AB intensity in several fruit tree species, including apple (Malus ×domestica Borkh.), citrus (Citrus spp.), mango (Mangifera spp.), walnut (Juglans spp.), and pecan (Greene, 2000; Hagemann et al., 2015; Hassankhah et al., 2018; Muñoz-Fambuena et al., 2012; Wood, 2011a, 2011b; Wood et al., 2009). Application of ethephon-suppressed endogenous gibberellin GA3 (GA3) production in apple when applied in early spring increased subsequent season flowering and fruit set (Ebert and Bangerth, 1981). Timing is key for PGR treatment, and GA application at full bloom was the most effective timing for reducing return bloom in apple (Tromp, 1982). In a study of GA4+7 exogenous applications on apple trees of varying cropload, researchers found the reduction in subsequent season return bloom to be dependent on the intensity of the cropload, and the effect was most pronounced on trees with a moderate cropload vs. either a heavy or light cropload (Schmidt et al., 2009).
Ethephon, a synthetic ethylene-generating compound, was found to control flowering in an AB apple orchard both by increasing flowering in off years by 33% when applied the previous year and by decreasing flowering in on years by 17% when applied in the previous year in the weeks after full bloom (WAFB) (Bukovac et al., 2006). Wood (2011b) studied effects of various GAs, prohexadione-ca (a GA biosynthesis inhibitor), ethephon, and other PGRs on immature and mature pecan trees, both at the level of whole trees and individual shoots, when applied 3, 6, and 9 WAFB. After GA treatment, the percentage of subsequent season fruiting shoots decreased in comparison with the control group, as did the number of nuts per cluster. These results were consistent whether the whole tree was treated or just individual shoots, and he concluded that GA treatments may be useful for the mitigation of AB in pecan.
Exogenous application of ethephon to whole immature trees in Wood’s (2011b) study increased both the subsequent season’s percentage of new shoots with flowers and the number of nuts per cluster compared with the control group. The ethylene inhibitor AVG was proposed as a mitigation tool for AB because it increases fruit retention in off years and has a carryover effect in which subsequent season on-year production was reduced (Wood, 2011a, 2011b; Wood et al., 2009).
Our objective was to better understand the effects of exogenous PGR applications on return bloom in mature and immature protandrous pecan cultivars grown in New Mexico. We studied three different PGRs (ethephon, two concentrations of GA3, and AVG) applied to individual shoots that were either fruiting or nonfruiting in trees of different maturity and cultivars.
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