Alternate bearing (also called biennial or uneven bearing) is the tendency of a fruit tree to produce a heavy crop (on-crop year) followed by a light crop or no crop (off-crop year). The phenomenon is widespread, occurring in deciduous and evergreen trees (Monselise and Goldschmidt, 1982). Alternate bearing may occur over an entire region or block of trees, for an individual tree, part of a tree, or even for one branch (Monselise and Goldschmidt, 1982). Alternate bearing is initiated by an environmental trigger that is favorable or unfavorable to crop production, resulting in excessive fruit set or extreme thinning of reproductive structures, respectively (Hield and Hilgeman, 1969). Alternate bearing is a major problem in citrus (Citrus spp.) production worldwide, especially with mandarin cultivars (Wheaton, 1992). Many marketing problems result from alternate bearing (Hield and Hilgeman, 1969; Moss et al., 1974). On-crop trees produce a large number of small size fruit of little commercial value (Hield and Hilgeman, 1969) and off-crop trees produce a small number of large size fruit, a high proportion of which have unattractive, thick coarse rinds and are culled in the packinghouse (Moss et al., 1974). In addition, this alternation in crop load, especially among trees within a block, makes orchard management difficult.
The alternate bearing habit in citrus is known to be due to a lack of flowering in the spring following a heavy on-crop year (Goldschmidt and Golomb, 1982; Hield and Hilgeman, 1969), and not due to a negative effect of the heavy on-crop on fruit set (Goldschmidt and Golomb, 1982). Floral intensity (number of flowers) and yield are inversely proportional to the number of fruit produced the preceding year (Becerra and Guardiola, 1984; Moss, 1971, 1973). The inhibitory effect of fruit on flowering was reported to extend to adjacent branches, but not to adjacent limbs (Mullins et al., 1989, Plummer et al., 1989).
The severity of alternate bearing is reported as the alternate bearing index (I), which is equal to
When I is 1, alternate bearing is 100%, and when I is 0, there is no alternate bearing (Pearce and Dobersek-Urbanc, 1967). For many citrus cultivars, the crop is typically still on the tree during floral induction (Plummer et al., 1989) and sometimes during anthesis and initial fruit set (Monselise and Goldschmidt, 1982). Delaying harvest in an on-crop year is known to further reduce return bloom (El-Otmani et al., 2004a; Hilgeman et al., 1967a, 1967b). Thus, it was suggested that fruit reduce flowering by inhibiting floral induction (Garcia-Luis et al., 1995, Koshita et al., 1999; Plummer et al., 1989). Consistent with this proposal, fruit removal during this period increased floral shoot number in spring with a concomitant decrease in vegetative shoot number (Garcia-Luis et al., 1995). Similarly, earlier fruit removal in summer or 1 month before harvest in October increased the number of buds that sprouted per shoot (Monselise et al., 1981), the number of leafless (one apical flower with or without additional lateral flowers and no leaves on a single shoot) and leafy (one apical flower with or without additional lateral flowers with one or more leaves on a single shoot) floral shoots, and the number of flowers, but reduced the number of vegetative shoots (Garcia-Luis et al., 1995).
Monselise and Goldschmidt (1982) suggested that biennial bearing was induced through a loss of flowering positions after a heavy on-crop year. The authors cited the inhibitory effect of the heavy on-crop on summer vegetative shoot growth (Monselise and Goldschmidt 1982; Monselise et al., 1981). Others also noted the inhibitory effect of fruit on shoot emergence and return bloom (Ehara et al., 1981; Plummer et al., 1989). However, no studies have quantified the contribution of summer and fall vegetative shoot growth to return bloom.
The objective of the research presented herein was to determine when in the phenology of the tree the setting on-crop negatively impacts floral intensity of the return bloom, hence reducing return yield to perpetuate alternate bearing in ‘Pixie’ mandarin. The following three hypotheses were tested: 1) the on-crop reduces summer and fall vegetative shoot growth and thereby decreases the number of nodes that can bear flowers the next spring; 2) the heavy on-crop reduces the transition of vegetative buds to floral buds, resulting in more vegetative shoots and less floral shoots; and 3) the on-crop inhibits budbreak in the spring. ‘Pixie’ mandarin is a strong alternate bearer, which might be due in part to the fact that the mature crop is normally still on the tree during floral induction through anthesis and early fruit set. For navel orange (Citrus sinensis), and presumably other Citrus species in California, the transition from vegetative to reproductive development occurs about in mid-December to mid-January (Lord and Eckard, 1987). Thus, if fruit reduce floral intensity at return bloom by inhibiting phase transition, removal of fruit from about November to early January of an on-crop year should increase the number of floral shoots and reduce the number of vegetative shoots formed during spring bloom. Alternatively, if fruit reduce floral intensity by inhibiting summer and fall shoot growth, summer through fall fruit removal should increase the number of vegetative shoots in these flushes and, hence, the number of nodes that bear flowers in spring. Thus, removing the summer and fall shoots from off-crop trees should greatly reduce the floral intensity of these trees. To determine when crop load exerts an effect on return bloom, fruit were removed from shoots of on-crop trees from June through January or from entire on-crop trees in July and in December before irreversible commitment to flowering. The number of floral shoots, flowers, and vegetative shoots contributed by parent shoots (current spring flush shoots) alone or including their summer/fall shoots to bloom the following spring was quantified in the two separate experiments, respectively.
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