Effect of synthetic auxins on fruit development of ‘Bing’ cherry (Prunus avium L.)
Introduction
Small fruit size is one of the limiting factors in marketing cherry (Prunus avium L.) fruit (Sansavini and Lugli, 2005, Whiting and Ophardt, 2005). As consumers prefer large cherries, fruit size is a very important marketing consideration, and the economic benefits of treatments capable of improving average fruit size are potentially very high. Several techniques have been used to improve fruit size of cherry. Among them, bloom and fruit thinning have been proven to be effective (Proebsting, 1990). Whiting and Lang (2004) showed that a negative relationship exists between the ratio of fruit number to leaf area and the size of the sweet cherry fruit. However, the commercial application of chemical bloom or fruitlet thinning of sweet cherry has not yet been reported in the literature and no products are currently registered for this purpose (Byers et al., 2003, Whiting and Ophardt, 2005). Synthetic auxins may be effective in enhancing fruit growth, when applied during the second stage of fruit development (Faust, 1989, Westwood, 1993). These auxins are known for their ability to increase cell enlargement (Westwood, 1993, Arteca, 1996, Davis, 2004), thus enhancing fruit growth in certain species such as Citrus (Agusti et al., 1995), peach (Agusti et al., 1999, Flaishman, 2006), litchi (Stern et al., 2000), apricot (Agusti et al., 1994) and loquat (Agusti et al., 2003). In all species so far studied, synthetic auxin had the potential for increasing fruit size without inducing thinning.
In Citrus, peach and litchi, it was found that application of the synthetic auxin 3,5,6-trichloro-2-pyridyloxyacetic acid (3,5,6-TPA), at concentrations between 10 and 20 mg l−1, considerably increased fruit size, whereas in apricot and loquat, 2,4-dichlorophenoxypropionic acid (2,4-DP) at 25–50 mg l−1 had the optimum effect.
Application of 2,4,5-trichlorophenoxypropionic acid (2,4,5-TP) was in commercial use, in stone fruit and litchi orchards in Israel, until its registration was withdrawn.
Since no report has been published as to the effect of synthetic auxins on fruit size of sweet cherry, the objective of this study was to evaluate the effects of some synthetic auxins on fruit development, size, maturation, quality and yield in ‘Bing’ sweet cherry.
Section snippets
The orchards
Experiments were conducted between 2003 and 2005 on mature cherry ‘Bing’ trees grafted on ‘Mahaleb’ (Prunus mahaleb L.) rootstock, in three orchards:
- (1)
At Merom-Golan, located 900 m above sea level (a.s.l.) on the Golan Heights, where the trees were 2.5 m high, spaced at 2.5 m × 4.5 m (880 trees ha−1).
- (2)
On the Fichman Experiment Station, located 1000 m a.s.l. on the Golan Heights, where the trees were 3.0 m high, spaced at 2.5 m × 4.5 m (880 trees ha−1).
- (3)
At Ortal, located 1100 m a.s.l. on the Golan Heights, where the
Fruit size and yield
In the first year (2003), 2,4-DP at the intermediate concentration of 50 mg l−1 and [2,4-D + NAA] at concentrations of 25 and 30 mg l−1, respectively, [0.3% (v/v) Amigo™], increased fruit size and total yield (Fig. 1, Fig. 2). The increase in the yield of large fruit was 65% with 50 mg l−1 2,4-DP (6.4 kg tree−1 vs. 3.9 kg tree−1 in the control), while with lower or higher concentrations, there were slight decreases in yield (Fig. 1). [2,4-D + NAA], which were applied only as 0.3% Amigo™ (25 mg l−1 2,4-D + 30 mg l−1
Discussion
One of the limiting factors in stone fruit development is the low supply of assimilates to the fruit, especially with a heavy yield (Faust, 1989, Westwood, 1993). The dependence of fruit growth on assimilate availability has been demonstrated with peach, by reducing vegetative growth using growth retardants (Blanco, 1987) or girdling (Dann et al., 1984). Both treatments can modify the source–sink relationship and carbohydrate partitioning between plant organs and favour fruit growth instead of
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