Effect of white light on meristematic activity in developing sunflower hypocotyls

Summary

To determine whether hypocotyl elongation in sunflower seedlings (Helianthus annuus L.) is dependent on cell divisions (meristematic activity), we used a specific inhibitor of DNA synthesis (fluorodeoxyuridine). The seedlings were either grown for 6 days in darkness or continuous white light (WL). Under both conditions hypocotyl growth was retarded by 30–70% in the presence of the inhibitor. Because the nuclei do not become endopolyploid we conclude that hypocotyl growth is dependent on cell reproduction. In the next step an immunocytochemical method was used to detect the percentage of nuclei in S-phase (meristematic activity) in different regions and tissues of the hypocotyls. In the peripheral cell layers (epidermis, cortex) meristematic activity was much greater than in the pith of the organ. In rapidly growing (etiolated) hypocotyls meristematic activity is largely restricted to the closed apical hook of the stem. After transfer to WL the hook opens and hypocotyl elongation is inhibited. In the epidermis and cortex of the apical hook a large WL-induced enhancement in the percentage of nuclei in S-phase occurred, which was followed by a light-mediated retardation of meristematic activity. Our data show that WL exerts a transient stimulatory effect on meristematic activity during photomorphogenesis of the sunflower seedling.