Induction of phenylalanine ammonia‐lyase in hypocotyls of sunflower seedlings by light, excision and sucrose

Light, excision and sucrose increased extractable phenylalanine ammonia‐lyase (PAL) activity from hypocotyl tissue of sunflower (Helianthus annuus L. cv. Peredovik) to 2–6 times the basal level. Intact sunflower seedlings or whole hypocotyls incubated in water or 0.1 M sucrose exhibited, in continuous light, a pattern in which PAL peaked 4 and 28 h after the beginning of the illumination. When 0.5 cm long hypocotyl segments were incubated in water or 0.1 M sucrose, they exhibited, both in continuous light and in the dark, a pattern in which PAL rose during an initial period of 10 h (assay in sucrose and light) to 48 h (assay in water and dark) and then remained nearly constant at a high value for at least the next 10 h. When whole hypocotyls were incubated in 0.1 M sucrose, a third pattern in PAL activity was found in which PAL peaked after 28 h and subsequently declined. In all the above systems the increase in PAL activity was significantly reduced by cycloheximide. Furthermore, the subsequent decay of PAL activity following illumination was prevented by delayed transfer to cycloheximide. It is suggested that the results can be explained on the basis of a turnover mechanism involving continued de novo enzyme synthesis and subsequent synthesis of a PAL‐inactivating system.