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). In addition, this alternation in crop load, especially among trees within a block, makes orchard management difficult. 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). Many marketing problems result from alternate bearing ( Hield and Hilgeman, 1969 Moss et al., 1974). Alternate bearing is a major problem in citrus ( Citrus spp.) production worldwide, especially with mandarin cultivars ( Wheaton, 1992). 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 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). The phenomenon is widespread, occurring in deciduous and evergreen trees ( Monselise and Goldschmidt, 1982). Keywords: Citrus reticulata floral intensity fruit removal on- and off-crop trees vegetative shoot growthĪlternate 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). Taken together, the results of this research provide evidence that fruit of the ‘Pixie’ mandarin reduce floral intensity of the return bloom by inhibiting budbreak, which reduces summer/fall shoot growth and thus the number of nodes that can bear inflorescences and development of spring shoots, which are predominantly floral. For the branch and whole tree experiments, flower number was significantly correlated with the percentage of spring budbreak on parent + summer/fall shoots ( r 2 = 0.88, P ≤ 0.0001 and r 2 = 0.71, P ≤ 0.0001 respectively). Removing all fruit from on-crop trees in December increased the percentage of budbreak in spring and flower number on parent shoots to that of off-crop trees, whereas the number of summer/fall shoots and the number of flowers the parent shoots contributed to bloom were both less than that of off-crop trees. This reduced floral intensity to that of on-crop trees. The importance of summer/fall shoots to return bloom was confirmed by removing all summer/fall shoots from off-crop trees. Removal of all fruit in July from on-crop trees resulted in 2-fold more flowers in spring compared with off-crop trees due to the increased number of flowers contributed by both parent shoots (75% of the total) and the increased number of summer/fall shoots. In the whole tree experiment, parent + summer/fall shoots of off-crop trees produced more flowers the following spring than on-crop trees due to greater flower production by both parent shoots and their greater number of summer/fall shoots. Removal of fruit from individual shoots of on-crop trees after July had no effect on flower number. Removal of fruit from individual shoots on on-crop trees in June or July had no effect on the number of flowers contributed by parent (current spring) shoots to return bloom, but increased total flower number 4-fold because summer/fall shoot number increased more than 8-fold. The number of summer and fall (summer/fall) vegetative shoots that developed on parent shoots with and without fruit and the contribution of spring shoots (floral and vegetative) made by parent shoots alone (now 1 year old) and by their summer/fall shoots to return spring bloom was quantified. Parent shoots (current spring flush shoots) were tagged on on-crop trees and fruit were removed from individual shoots or whole trees. ‘Pixie’ mandarin ( Citrus reticulata) was used as the model system to investigate when and how fruit perpetuate cyclic differences in floral intensity. Whereas it is well documented for citrus that fruit number in the current crop inversely affects flower number in the return bloom, when in the phenology of the tree and how fruit exert an effect on floral intensity the following spring remained unresolved. Alternate bearing trees produce a heavy (on) crop followed by a light (off) crop.
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