Sweet cherry trees exhibit strong apical dominance, producing vigorous, upright shoot growth with limited lateral branching, particularly in young trees (Elfving and Visser, 2006; Jacyna, 2002; Jacyna and Puchała, 2004; Jacyna et al., 2005; Miller, 1983). The interaction of endogenous auxins and cytokinins is thought to play a role in the control by apical dominance of lateral bud activity in the spring and during shoot development (Sachs and Thimann, 1967; Theron et al., 1987; Wickson and Thimann, 1958). Pruning can interrupt the hormonally based apical dominance control system and encourage buds that otherwise might remain quiescent to form shoots. For this reason, commonly used training systems for sweet cherries rely on pruning as a principal means for stimulation of lateral branching during canopy development (Long, 2001, 2005; Long et al., 2005; Nugent et al., 2001). However, pruning removes buds that might otherwise have developed into shoots or spurs and delays cropping (Elfving, 1990; Robinson et al., 2005). Therefore, alternative means for stimulation of bud growth have been examined in apple and, to a lesser extent, in sweet cherry. Scoring (a circumferential knife cut through the phloem tissue around a shoot or stem) and notching (removal of a small piece of phloem above a bud) have been used to interfere with hormone movement and to promote shoot development in apple (Cook and Strydom, 2000; Greene and Autio, 1994; Greene and Miller, 1988; Niu et al., 1998; Ouellette et al., 1996). Bud removal (disbudding) can affect and may improve shoot growth in apple and other tree fruit species (Mika et al., 1983; Peano et al., 2005), and may show promise for shoot growth stimulation in sweet cherry as well (Perry, 1999; Robinson et al., 2005). Very limited information suggests scoring or notching in the spring may positively affect lateral shoot development in sweet cherry (Long, 2005; Long et al., 2005).
Proprietary mixtures containing the cytokinin 6-benzyladenine (BA) and gibberellic acid isomers GA4+7 [Promalin (PR) or Perlan (PER)] can induce sylleptic shoot formation on the current season's shoots when applied during active shoot growth in both apple and cherry (Cody et al., 1985a,b; Elfving and Visser, 2005, 2006; Hrotkó et al., 1999; Jacyna, 2002; Neri et al., 2003; Wustenberghs and Keulemans, 1999), but few reports document the efficacy of such products for inducing shoot development from preexisting buds on 1-year-old or older wood in sweet cherry. Miller (1983) and Veinbrants and Miller (1981) reported that painting PR onto the entire surface of 1-year-old shoots of young sweet cherry trees produced some stimulation of branch development from lateral buds. Jacyna and Puchała (2004) and Jacyna et al. (1989) reported improvement in branch development and in shoot distribution when BA combined with GA3 or GA4+7 was painted on the lower two-thirds of ‘Regina’ sweet cherry branches or the central 30 cm of ‘Bing’ 1-year-old branches. Jacyna and Brown (1989a,b) briefly described the use of PR for one system of training young sweet cherry canopies.
Interrupting apical dominance by disrupting or altering hormonal movement to buds should lead to both increased number of lateral shoots as well as an improved spatial distribution of those shoots along the main stems or leaders on which they are borne (Jacyna and Puchała, 2004; Robinson et al., 2005). Sweet cherry growers in Washington state have reported variable results on branching from the application of cytokinin-containing products such as PR at high concentrations to individual buds at or near spring budbreak or the treatment of sweet cherry 1-year-old wood by scoring or notching above buds. The research described here was undertaken to explore in greater detail the shoot growth responses of young sweet cherry trees under Washington state conditions in terms of both shoot numbers and shoot distribution to alternative shoot-inducing treatments including scoring, nicking (a small cut with a sharp knife into the phloem without regard to location relative to a bud) or notching, or cytokinin applications for improvement of canopy development. Bud removal was also evaluated in some trials.
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