Phenologic and Tissue Distribution of Sesquiterpene Lactones in Cultivated Sunflower (Helianthus annuus L.)

Phenolic and tissue distribution of seven sesquiterpene lactones (STLs), 4,5-dihydroniveusin A, argophyllin B, argophyllin A, 15-hydroxy-3-dehydrodesoxytifruticin, niveusin B, 1,2-anhydridoniveusin A and an unidentified epoxide, in cultivated sunflower (Helianthus annuus L.), have been determined by a high-performance thin-layer chromatography and UV-reflectance scanning densitometry analysis. Leaves and flower heads were the major tissues in sunflower accumulating STLs, while the roots, stems, cotyledons, pollen, and achenes did not contain detectable STLs. The total STL amount per leaf increased from bottom to top leaves, while STL concentration (per gram fresh mass basis) was highest in top followed by bottom and then middle leaves. The total STL amounts in floral tissues were less than those in leaves but were highly concentrated on the top surface of disk florets forming a chemical defense to protect achene development from insect herbivores. Non-polar STLs (15-hydroxy-3-dehydrodesoxytifruticin, niveusin B, and 1,2-anhydridoniveusin A) were more abundant in early growth stages but were then dominated by polar STLs (4,5-dihydroniveusin A, argophyllins A and B, and an unidentified epoxide), especially argophyllin A, after four weeks of growth. Age-related expression of STL content in sunflower reveals an interesting phenomenon of secondary metabolism where 15-hydroxy-3-dehydrodesoxytifruticin may be the first stable STL and argophyllin A a final metabolite of STL biosynthesis.