Effect of Fruit on Floral Gene Expression and Floral Intensity in Alternate Bearing ‘nules Clementine’ and ‘pixie’ Mandarin

For alternate bearing mandarin trees (Citrus reticulata), high ON-crop yields alternate almost annually with low OFF-crop yields. In this study, effects of crop load on floral gene expression during the 6 months before full bloom (MBFB) and inflorescence number the following spring in early-maturing ‘Nules Clementine’ and late-maturing ‘Pixie’ mandarin were compared. The OFF-crop trees of both cultivars flowered profusely in April. In these trees, bud FLOWERING LOCUS T (FT) expression was first detected only when the air temperatures decreased to the floral-inductive low temperature (LT) range (4 °C < minimum temperature ≤10 °C) at 4 and 6 MBFB for ‘Nules Clementine’ and ‘Pixie’ mandarin, respectively. Despite occurring on different calendar dates, maximum FT transcript levels corresponded to a similar number of accumulated LT degree-days (56 days at 1 MBFB and 52 days at 3 MBFB) in respective orchards. LEAFY (LFY) and APETALA1 (AP1) were upregulated to maximum levels at a similar number of accumulated warm temperature (WT) degree-days (2 days and 5 days with a maximum temperature ≥24 °C following sufficient LT accumulation) at 1 MBFB for ‘Nules Clementine’ and at 2 MBFB for ‘Pixie’ mandarin. Subsequent maximum expression of downstream APETALA2 (AP2), SEPALLATA1 (SEP1), PISTILLATA (PI), and AGAMOUS (AG) occurred at 1 MBFB for both cultivars, consistent with the initiation of floral organogenesis just before bloom as a result of successful meristem determinacy. The results also provided evidence supporting that the regulation of FT and LFY/AP1 in citrus buds is quantitatively dependent on LT and WT accumulation under the OFF-crop condition. In contrast, ON-crop trees of both cultivars did not produce any inflorescences the following spring. Buds of these trees never expressed FT during the 6 MBFB, LFY expression dropped below the detection limit at 3 MBFB through bloom, and upregulation of AP1 expression did not occur. Consequently, downstream floral organ identity gene expression was significantly lower in ON-crop trees than in OFF-crop trees at 1 MBFB for both cultivars. Early harvest of ON-crop fruit at 3 MBFB or 2 MBFB failed to restore floral gene expression or spring flowering; instead, it increased vegetative shoot number during return bloom, indicating buds were not determined as late as 2 MBFB (February). Collectively, the results suggest that repression of FT during 4 MBFB by the ON crop prevented successful completion of the induction process in floral meristem determinacy by inhibiting LFY and AP1 upregulation, thereby preventing activation of the floral organ identity genes AP2, SEP1, PI, and AG and flower formation.