The discovery of auxins provided the basis for current chemical fruit thinning of apples. The promotion of postbloom fruit abscission was first reported over 60 years ago (Davidson et al., 1945), and since then, synthetic auxins, NAA and its amide (NAAm), have become dominant apple fruit-thinning compounds (Dennis, 2000). Numerous studies have documented their effectiveness for fruit thinning, but their performance has been inconsistent and is subject to environmental conditions, particularly temperature and relative humidity during and for a short period after application (Luckwill and Lloyd-Jones, 1962; Wertheim, 2000; Westwood and Batjer, 1960; Williams, 1979).
Also, NAA has been reported to cause a transitory depression in the rate of fruit growth for a brief period after treatment and thus, the increase in size of the persisting fruit may not be commensurate with the reduction in crop load (Greene and Autio, 1994; Luckwill, 1953; Marsh et al., 1960). In some cultivars, e.g., ‘Delicious’ and ‘Fuji’, the population of small fruit (<65 mm diameter) may be increased and small, seedless fruit (pygmy) may develop that persist until harvest. These effects are usually more pronounced in late postbloom applications and, more so, with NAAm than with NAA (Hoffman et al., 1955; Wise et al., 2007).
The discovery of the cytokinins and their pronounced effects on cell division and morphogenesis (Letham, 1963; Miller et al., 1995) provided an opportunity to increase fruit growth and perhaps overcome the negative effects of NAA on fruit development.
N6-Benzyladenine [BA, N-(phenylmethyl)-1H-purine-6-amine, bz6Ade], a synthetic cytokinin, and zeatin are the most widely used cytokinins in research and commercial application (Horgan, 1984). BA has increased fruit size in several apple cultivars (Elfving and Cline, 1993; Ferree, 1996; Greene, 1993; Greene et al., 1990; Stover et al., 2001), promoted enlargement of the cortical tissue in the calyx lobes in ‘Delicious’, and increased cell division in apple and pear fruit (Flaishman et al., 2005; Greenhalgh et al., 1977; Letham, 1968; Williams and Stahly, 1960). Numerous thinning studies have confirmed that cytokinins induce fruit abscission and increase fruit size in apple and pear and that the fruit size is often greater than can be accounted for by the thinning effect (Bukovac et al., 2000; Flaishman et al., 2005; Greene et al., 1992; Wismer et al., 1995), thus suggesting a direct, positive effect on fruit growth.
In attempts to improve fruit thinning in hard-to-thin cultivars, combinations of chemicals have been used for aggressive thinning (Wertheim, 2000). An early, perhaps first, study using NAA and BA on ‘Delicious’ identified a strong interaction leading to excessive production of pygmy fruit (Miller, 1985). We observed a similar increase, not only in pygmy fruit, but also in the population of small fruit (<65 mm) in a grower study when NAA was used at ≈8 mm king fruit diameter (KFD) to thin ‘Delicious’ that were previously treated with Promalin at 80% king bloom (KB) to improve fruit typiness (E. Wittenbach, unpublished data). This response was similar, but of a greater magnitude, to that obtained with late or high concentrations of NAA (Hoffman, et al., 1955; Williams, 1979). This observation has been confirmed (Bound et al., 1991) and effects on pygmy and small fruit in ‘Delicious’ have been observed also when both chemicals were used for postbloom thinning on the same trees in the same season (Bukovac et al., 1995; Greene and Autio, 1994). In contrast, no significant negative cytokinin + NAA interaction has been observed on fruit growth in several other cultivars (Basak, 2006; Bukovac et al., 1995; Dennis, 2000; Stopar and Lokar, 2003).
Interactions between auxins and cytokinins on growth and differentiation are well established. Skoog and Miller (1957) demonstrated that the ratio (and concentration) of cytokinin to auxin in the culture media had a dramatic effect on organogenesis of cultured, undifferentiated tobacco tissue. In the presence of low auxin level (≈2 mg·L−1), increasing the cytokinin concentration (≈0.02 to 10 mg·L−1) induced root formation. On increasing the auxin:cytokinin ratio further, the tissue went through a rapid undifferentiated, proliferation stage and then developed buds. A high cytokinin:auxin ratio resulted in an undifferentiated callus and growth was inhibited. This interaction between auxins and cytokinins has been demonstrated for numerous plant tissues and, once the culture medium is optimized, provides the basis for regeneration and micropropagation of plants. Such findings have led to the suggestion that the ratio of these two hormones plays a defining role in controlling differentiation and growth in intact plants. Both auxins and cytokinins are present in apple fruitlets (Letham and Williams, 1969; Luckwill, 1953). However, the role of neither endogenous nor exogenous cytokinins in apple fruit development is understood.
The objective of our study was to provide a better understanding of this interaction by characterizing how the addition of BA modifies the effect of NAA on apple fruit abscission and development.
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