Irregular flowering and fruiting is a common trait in polycarpic woody species (Kelly and Sork, 2002; Monselise and Goldschmidt, 1982). This phenomenon is usually termed alternate (irregular, biennial) bearing in horticultural plants and masting, defined as intermittent production of large crops of fruit/seed by a population, in forest trees. Both have similar characteristics and have been studied extensively. In forest trees, masting has significant ecological implications on dynamics of both plant and animal populations (Kelly and Sork, 2002); in horticultural plants, e.g., apple, pear, prune, pecan, pistachio, citrus, olive, and others, alternate bearing may be a limiting factor in efficient crop production (Jonkers, 1979; Monselise and Goldschmidt, 1982; Sparks, 1983).
Numerous broadly based studies in apple have centered on the cause and control of alternate bearing, focusing primarily on temporal variation (year-to-year) of selected trees or tree populations considered to be in an “on” or “off” cropping year. The results of many of these studies have been extensively reviewed by Davis (1957), Dennis (2000), Jonkers (1979), Luckwill (1977), Monselise and Goldschmidt (1982), and Singh (1948a). Briefly, as young apple trees mature, depending on cultivar, some naturally develop an alternate bearing habit. However, climatic factors, like frost or severe drought, and biotic stresses (insect, disease), resulting in deflowering, defruiting, or defoliation (or significant loss of leaf function) induce pronounced irregular cropping (Buban and Faust, 1982; Jonkers, 1979).
Early studies also established that for a given cultivar and orchard, nutrition, including carbohydrate–nitrogen relationship, pruning, rootstock, vigor, shoot growth, and most climatic factors, except temperature, were not conclusively related to the alternate bearing pattern (Drain, 1924; Singh, 1948b). Irregular cropping was related to cultivar and temperature during flower and fruit development (Jackson and Hamer, 1980) and the tendency was greater in spur- than nonspur-type trees (Jonkers, 1979; Monselise and Goldschmidt, 1982). Early defoliation in “on” years, within 6 weeks after bloom, reduced flower initiation (Aldrich and Fletcher, 1932; Davis, 1957; Fulford, 1960); removal of blossom clusters, flowers, or fruitlets, within 4 to 6 weeks after bloom, in “on” years increased flower initiation in the “on” year and fruiting in the following “off” year (Bobb and Blake, 1938; Harley et al., 1935, 1942).
The finding that removing flowers or fruit early in the “on” year increased flower initiation was significant because the observation not only provided the basis for commercial blossom and fruit thinning practices for minimizing the alternate bearing problem, but also confirmed that lack of flower initiation, and not fruit set, was the most frequent limiting factor (Singh, 1948b). Also, these studies focused new research on the role of developing fruit and fruit seed content on flower initiation, which has provided convincing data that the seeds are the source of hormones (e.g., gibberellins) that inhibit flower initiation (Dennis and Neilsen, 1999; Hedden et al., 1993; Hoad, 1978; Luckwill, 1977). The observation by Guttridge (1962) that exogenously applied gibberellic acid inhibited flower initiation in apple further supported the proposed role of gibberellin. Exactly how the seed-produced gibberellins are involved in alternate bearing remains to be clarified.
With few exceptions, most apple studies have been short term (mostly one “on/off” cropping cycle) and have focused on temporal variation of irregular cropping (yield) (Brown, 1942; Harley et al., 1942; Jackson and Hamer, 1980). However, variation in flowering and fruiting in a given alternate bearing population has at least two major components: 1) variation within a single year (synchrony) and 2) year-to-year variation (temporal). Within-tree variation is potentially a third component; alternate bearing can be induced in selected limbs on the same tree, and this condition may persist for several years (Drain, 1924; Harley et al., 1935). Also, not all spurs on a given tree are synchronized. Some portion of the spur population may be flowering, whereas others are barren.
Surprisingly, in contrast to masting studies in forest trees (Lombardo and McCarthy, 2008; McCarthy and Quinn, 1989), within-year variation of flowering/fruiting has received little attention in apple (Monselise and Goldschmidt, 1982). The within-year variation component may be significant in irregular flowering because not all trees in an alternate-bearing apple orchard are synchronized (Davis, 1957; Drain, 1924; Jonkers, 1979). Flowering is generally a characteristic of an individual tree. Thus, in any given population (orchard), the “on”-year flower density may vary markedly.
The objectives of our long-term study were to: 1) define both temporal and within-year variation of flowering and fruiting in a highly uniform, spur-type ‘Delicious’ apple population growing under identical soil, climatic, and cultural conditions; 2) relate effects of irregular cropping on flower initiation, fruit set, and partitioning into selected fruit size classes; and 3) discuss the importance of the variance of trees with selected flower densities as experimental units for research purposes.
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