Effect of fruit on floral gene expression and floral intensity in alternate bearing ‘Nules Clementine’ and ‘Pixie’ mandarin

Authors: Tang, Lisa; LOVATT, CAROL - University Of California, Riverside

Submitted to: Journal of the American Society for Horticultural Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/26/2026
Publication Date: 4/21/2026
Citation: Tang, L., Lovatt, C. 2026. Effect of fruit on floral gene expression and floral intensity in alternate bearing ‘Nules Clementine’ and ‘Pixie’ mandarin. Journal of the American Society for Horticultural Science. 151(3):272-282. https://doi.org/10.21273/JASHS05545-25.
DOI: https://doi.org/10.21273/JASHS05545-25

Interpretive Summary: 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 were compared in early-maturing ‘Nules Clementine’ and late-maturing ‘Pixie’ mandarin. 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 range (4 °C < daily minimum temperature = 10 °C) at 4 and 6 MBFB for ‘Nules Clementine’ and ‘Pixie’ mandarin, respectively. Despite being on different calendar dates, maximum FT transcription occurred during a similar low-temperature accumulation (72 days at 1 MBFB and 68 days at 3 MBFB) at respective orchards. LEAFY (LFY) and APETALA1 (AP1) were upregulated to maximum levels after a similar warm-temperature accumulation (2 and 5 days with daily maximum temperature = 24 °C following the floral-inductive low-temperature period) at 1 MBFB for ‘Nules Clementine’ and 2 MBFB ‘Pixie’ mandarin. This was accompanied by the maximum expression of downstream APETALA2 (AP2), SEPALLATA1 (SEP1), PISTILLATA (PI), and AGAMOUS (AG) at 1 MBFB for both cultivars. The results are 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 low- or warm-temperature accumulations under the OFF-crop condition. In contrast, ON-crop trees of both cultivars did not produce any inflorescence during return bloom. Buds of these trees never expressed FT during the 6 MBFB; LFY expression dropped below the detection limit at 3 MBFB through bloom; upregulation of AP1 expression did not occur. Consequently, downstream gene expression was significantly lower in ON-crop trees than OFF-crop trees at 1 MBFB for both cultivars. Early harvest of ON crop of fruit at 3 or 2 MBFB failed to restore floral gene expression to the level of OFF-crop trees or spring flowering, but instead led to vegetative shoots during return bloom, indicating buds were not determined as late as 2 MBFB (February). Taken together, the results suggest that repression of FT in 4 MBFB through bloom by the ON crop prevented successful completion of the induction process in floral meristem determinacy by inhibiting LFY and AP1 upregulation and thereby preventing activation of floral organ identity genes AP2, SEP1, PI and AG and flower formation.

Technical Abstract: In study, we assessed the effects of light (OFF) and heavy (ON) crop load on floral gene expression and flowering the following spring in two mandarin cultivars with an alternative bearing behavior. For both cultivars, trees bearing an OFF crop flowered profusely in April whereas ON-crop trees did not flower. In comparison with OFF-crop trees, the presence of fruit suppressed bud expression of gene FLOWERING LOCUS T during the 6 months before bloom, inhibited the increase in LEAFY/APETALA1 expression, and prevented downstream floral genes from being activated in ON-crop trees just before bloom. Removal of the heavy ON crop a month before the standard harvest time did not restore flower intensity or gene expression in either cultivar. The results suggest that to recover key gene expression and spring flowering, fruit may need to be harvested prior to December. Since such early harvests are not commercially feasible for some cultivars produced in certain regions, strategies that reduce ON bloom, which prevent an ON crop and thereby break the ON/OFF cycle, might be more viable for managing alternate bearing mandarin compared to the attempt to increase OFF bloom.