Vascular connections between the receptacle and empty achenes in sunflower (Helianthus annuus L.)

Empty achenes in sunflower, particularly in the centre of the capitulum, may be caused by poor vascularization. This hypothesis was tested by microscopic examination and translocation experiments. Phloem and xylem were identified by fluorescence of aniline-blue-stained callose and autofluorescence, respectively. Vascular strands that extended from the receptacle into empty achenes were regularly found in longitudinal sections. The phloem-mobile probe, carboxyfluorescein, was translocated from the receptacle to the pericarp and the testa of empty achenes. Similarly, (14)CO(2)-derived (14)C-photoassimilates moved into empty achenes. The observations suggest that empty achenes are both structurally and functionally connected with the vascular system of the receptacle. Hence, deficient vascular connections do not prevent seed filling in sunflower.