Effects of low temperature and gibberellic acid on floral gene expression and floral determinacy in 'Washington' navel orange (Citrus sinensis L. Osbeck)

Low temperature (LT) and gibberellic acid (GA3) were used, respectively, to promote and inhibit flowering in 'Washington' navel orange to identify relationships between floral gene expression and floral intensity. All trees were maintained under warm temperature (WT) (24/19 °C, day/night) from April to September. For trees receiving 11 additional weeks of WT, buds expressed low levels of FLOWERING LOCUS T (FT), SUPPRESSOR OF OVEREXPRESSION OF CONSTANS 1 (SOC1), LEAFY (LFY), APETALA1 (AP1) and APETALA2 (AP2), but never expressed SEPALLATA1 (SEP1), PISTILLATA (PI) and AGAMOUS (AG) and did not flower. In contrast, trees transferred to LT (15/10 °C, day/night) for 8 weeks followed by WT through week 11 had significantly greater bud expression of FT and SOC1 in week 2, AP1 and AP2 during weeks 8 through 10, and SEP1, PI and AG after transfer to WT; 84% of buds flowered. Foliar-applied GA3 (50 mg L−1) in weeks 2 through 8 of LT treatment did not affect FT, SOC1 or LFY expression, but significantly reduced transcripts of AP1 by week 8, AP2, SEP1, PI and AG after transfer to WT and inflorescence number to values equal to 11-week WT-treated trees. Delaying GA3 application to weeks 4 through 8 of LT treatment increased AP1, AP2, SEP1, PI and AG expression equal to LT-treated trees and significantly greater than WT-treated trees; inflorescence number was significantly greater than trees in the WT treatment or receiving seven GA3 applications, but less than LT-treated trees. The results provide evidence suggesting that in adult citrus buds determinacy and subsequent floral development are controlled by AP1 and AP2 transcript levels, which regulate downstream floral organ identity gene activity and the effect of GA3 on citrus flower formation.