Shoot initiation in light‐ and dark‐grown tobacco callus: the role of ethylene

Shoot‐forming tobacco (Nicotiana tabacum L. cv. Wisconsin 38) callus produces less endogenous ethylene than non‐shoot‐forming tissue cultured in the light (16 h photoperiod) or the dark. In shoot‐forming tissue more ethylene is produced early in culture (days 0–5) than later. Also dark‐grown tissue produces much more ethylene than light‐grown. On the basis of experiments in which (1) gaseous ethylene was added to or (2) CO2 removed from the flasks, (3) Ethrel (an ethylene releasing agent) and (4) 1‐aminocyclopropane 1‐carboxylic acid (an ethylene precursor) were added to the medium, it was determined that this gaseous phytohormone had two contrary effects on shoot initiation (shoot primordium formation). Early in culture (days 0–5) endogenous or exogenous ethylene inhibited organogenesis, but later (days 5–10) exogenous ethylene or increased endogenous ethylene production speeded up primordium formation.