Citrus trees alternate between rich and poor crops and are known to be alternate-bearing crops. Alternate bearing results from suppression of flowering due to bearing of fruits. To understand the molecular mechanism how fruit bearing affects flowering, we investigated the relationship between fruits and a flowering-related gene, citrus FLOWERING LOCUS T (CiFT). On trees with different amounts of fruits, the fruit weight/leaf area ratio at harvest was negatively and highly correlated with CiFT expression in the vegetative shoots during fall and winter, which is the period of floral induction. In addition, CiFT expression levels during fall and winter were positively and highly correlated with the flower number the following spring. These results indicate that fruit growth suppresses CiFT expression and decreases the flower number the next spring. In another experiment conducted to determine the effect of the period of fruit bearing on CiFT expression, trees having 3 primary scaffold branches were analyzed. From each branch in 1 tree, all the flowers or fruits were harvested at different times. In November, CiFT was expressed at different levels in each branch, with a tendency to be low in the stem of vegetative shoots from branches that bore fruits for longer periods. This result indicates that a long fruit-bearing period suppresses CiFT expression in vegetative shoots. CiFT expression was detected at much higher levels in fruit-bearing shoots than in vegetative shoots in September. In January, the high levels of CiFT expression in bearing shoots decreased to levels lower than those found in vegetative shoots. Thus, in fruit-bearing shoots, the CiFT expression of an unknown relationship to floral induction is observed during the period before floral induction and the seasonal change of CiFT expression in fruit-bearing shoots is different from that in vegetative shoots.

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