The Effect of Fruit on Floral Gene Expression in Alternate-bearing Citrus reticulata (Blanco)

‘Pixie’ mandarin is alternate bearing. At spring bloom following the high yield on-crop, tree inflorescence number is markedly reduced, resulting in an off-crop. At return bloom following an off-crop, trees flower profusely and set an on-crop. In two separate years, the apical five buds from nonbearing shoots of off-crop and bearing shoots of on-crop ‘Pixie’ mandarin trees were collected from October through post-anthesis and the expression of key genes regulating floral development was determined using quantitative real-time PCR. Off-crop trees (20 and 5 kg/tree, years 1 and 2, respectively) produced 4.8 and 4.7 inflorescences at the apical five buds of nonbearing shoots for the 2 years; whereas on-crop trees (236 and 184 kg/tree, years 1 and 2, respectively) produced no inflorescences. Expression of TERMINAL FLOWER 1 and FLOWERING LOCUS C was not influenced by crop load. SUPPRESSOR OF OVEREXPRESSION OF CONSTANS 1 was expressed in buds from off- and on-crop trees from October to anthesis; expression continued post-anthesis for on-crop trees. FLOWERING LOCUS T was expressed from October to anthesis in buds from off-crop trees; FT was not expressed in buds from on-crop trees after November. In years 1 and 2, maximum LEAFY expression for off-crop trees was in February and greater than on-crop trees (P = 0.0424), for which expression remained low post-anthesis. Similarly, in both years, for off-crop trees maximum expression of SEPALLATA occurred 1 month before anthesis and was greater than on-crop trees (P <0.0001), for which low expression continued post-anthesis. For off-crop trees, APETALA 1 and APETALA 2 expression was greatest at 1 and 2 months before anthesis, respectively, and greater than on-crop trees (P < 0.0001 and P = 0.0069, respectively); expression continued for on-crop trees post-anthesis. For off-crop trees, PISTILLATA (PI) and AGAMOUS (AG) expression was not detected until 2 months before anthesis; for on-crop trees PI and AG were not expressed at this time or in subsequent months. Inhibition of PI and AG expression by the on-crop is not unique to ‘Pixie’. In ‘Nules’ Clementine mandarin, PI and AG were expressed 1 month before anthesis in buds from nonbearing shoots of off-crop trees and not expressed in buds from bearing shoots of on-crop trees at this time or in later months. Inhibition of FT expression by the on-crop has been reported previously in alternate bearing citrus. This is the first report that the on-crop of citrus fruit inhibits expression of PISTILLATA and AGAMOUS.