EFFECT OF WHITE-LIGHT ON MERISTEMATIC ACTIVITY IN DEVELOPING SUNFLOWER HYPOCOTYLS

To determine whether hypocotyl elongation in sunflower seedlings (Heli anthus annuus L.) is dependent on cell divisions (meristematic activit y), 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 o n cell reproduction. In the next step an immunocytochemical method was used to detect the percentage of nuclei in S-phase (meristematic acti vity) in different regions and tissues of the hypocotyls. In the perip heral cell layers (epidermis, cortex) meristematic activity was much g reater than in the pith of the organ. In rapidly growing (etiolated) h ypocotyls meristematic activity is largely restricted to the closed ap ical hook of the stem. After transfer to WL the hook opens and hypocot yl elongation is inhibited. In the epidermis and cortex of the apical hook a large WL-induced enhancement in the percentage of nuclei in S-p hase occurred, which was followed by a light-mediated retardation of m eristematic activity. Our data show that WL exerts a transient stimula tory effect on meristematic activity during photomorphogenesis of the sunflower seedling.