Investigation of endogenous phytohormone contents in male and female litchi flowers

Litchi (Litchi chinensis Sonn.) bears the following three types of flowers: male, functionally hermaphroditic male, and functionally hermaphroditic female flowers. Endogenous and exogenous phytohormones considerably affect the sexual expression of flowers in other monoecious and dioecious species. However, little information is available regarding the relationship between phytohormones and sex differentiation in litchi. Therefore, we analyzed the phytohormone contents of the male and female flowers of several litchi cultivars using a method involving a solid-phase extraction and high-performance liquid chromatography-electrospray ionization tandem mass spectrometry. In all tested cultivars, auxin and abscisic acid contents were lower in female flowers than in male flowers. In contrast, the abundance of cytokinins (e.g., isopentenyl adenine and trans-zeatin) was higher in female flowers than in male flowers. We also tested the effects of cytokinins on litchi flower formation by applying a synthetic cytokinin, forchlorfenuron (CPPU), on 'Hei ye' litchi trees growing in a commercial orchard in Hsinchu City, Taiwan. Inflorescences on trees were treated with 50 ppm CPPU in 0.5% ethanol or 0.5% ethanol alone (control). The results of this preliminary testing suggested a CPPU treatment during the floret anthesis stage may induce abnormal feminization and female flower abscission. The data presented herein may be relevant for future research on the regulation of sex differentiation by exogenous phytohormones.