• Photoassimilate transport from source leaves to the capitulum was investigated in sunflower (Helianthus annuus) during anthesis and seed filling. • Following foliar application of a 13/14 CO2 -pulse, labelled photoassimilates were detected using mass spectrometry, phosphorimaging, HPTLC and HPLC. • The upper 10 (to 15) leaves exported photoassimilates into the capitulum. Photoassimilate distribution patterns were sectorial: each leaf supplied a defined 2/8-3/8 sector of the capitulum. Photoassimilates exported via the midvein accumulated in a 1/8 sector, which aligned exactly with the insertion site of the leaf. The two main lateral veins of the leaf exported photoassimilates into the two adjacent 1/8 sectors of the capitulum. During early and late stages of anthesis, strong sinks were staminate florets and young achenes, respectively. During seed filling, an import maximum and minimum appeared in the intermediate and central whorls, respectively. Sucrose was established as the only phloem transport sugar. Raffinose, although also 14 C-labelled in the path, is not transported in sunflower. • It is concluded that a single floret is typically connected with the leaves of three neighbouring ortostichies in sunflower. Photoassimilate distribution patterns demonstrated here generally may reflect the functional relationships between the phyllotaxy of source leaves and the position of sinks in developing inflorescences like those of Asteraceae.
Keywords: 13C; 14C pulse-chase experiment; Helianthus annuus (sunflower); distribution pattern; phloem transport; phyllotaxy; source-sink relationship.