UC Riverside

In September, buds of ‘Washington’ navel orange trees (Citrus sinensis) grown under warm, well-irrigated conditions for five months expressed FLOWERING LOCUS T (FT), SUPPRESSOR OF OVEREXPRESSION OF CONSTANS1 (SOC1), LEAFY (LFY), APETALA1 (AP1) and APETALA2 (AP2); SEPALLATA1 (SEP1), PISTILLATA (PI) and AGAMOUS (AG) were not expressed and no inflorescences developed. Subjecting the trees to low-temperature or water-deficit treatment had no effect on FT, SOC1 and LFY expression, but increased AP1 and AP2 expression with concomitant activation of SEP1, PI and AG and significant flowering. Gibberellic acid (GA3) applied to buds of low-temperature- or water-deficit-treated trees did not affect FT, SOC1 or LFY expression, but dramatically reduced AP1 and AP2 transcription, repressing SEP1, PI and AG and flowering. Similarly, buds of field-grown low-yield, off-crop ‘Pixie’ and ‘Nules Clementine’ mandarin trees (C. reticulata) collected in October expressed FT, SOC1, LFY, AP1 and AP2, with SEP1, PI and AG expression delayed until March, one month before the intense return bloom. For high-yield, on-crop trees, which failed to flower, bud FT transcripts were not detected in October through March, LFY, AP1, AP2 and SEP1 expression were lower than off-crop trees by March, and PI and AG transcripts never exceeded the limit of detection. Removing the on crop from ‘Pixie’ mandarin trees in November increased FT expression above detectable levels, AP1 to the level of off-crop trees, and activated PI and AG, resulting in a small number of inflorescence (21% of off-crop trees). To mitigate the negative effects of the on crop on return bloom and yield, fruit would need to be removed before October. Results of this research demonstrated: (i) citrus buds initiate floral development prior to exposure to low temperature in fall-winter, consistent with transition from vegetative to floral development in summer, but leaving open the possibility all buds on adult citrus trees are induced to flower; (ii) sustained FT expression is required for increased AP1 and AP2 expression to levels sufficient for floral determinacy, activation of downstream floral organ identity genes and flowering; and (iii) GA3 and the on crop of citrus fruit inhibit flowering by preventing bud determinacy, not floral induction.