INTERACTION BETWEEN INDOLE-3-ACETIC-ACID AND ETHYLENE IN THE CONTROL OF TRACHEID PRODUCTION IN DETACHED SHOOTS OF ABIES-BALSAMEA

We investigated the interaction between indole-3-acetic acid (IAA) and ethylene in the regulation of the seasonal periodicity of tracheid pr oduction in 1-year-old balsam fir (Abies balsamea (L.) Mill.) cuttings collected at different times during the dormant period. The cuttings were left with their buds intact or were debudded and treated either a pically with IAA or 2-chloroethylphosphonic acid (Ethrel) in lanolin, laterally with IAA or Ethrel in lanolin, or basally with Ethrel, Co2or Ag+ in deionized water. The treated cuttings were then cultured for up to 5 weeks under controlled environment conditions favorable for c ambial growth. No change in ethylene evolution was detected during the rest-quiescence transition, when IAA-induced tracheid production incr eased. The induction of cambial reactivation by IAA was associated wit h a rise in ethylene evolution, but there was no consistent relationsh ip among IAA concentration, tracheid number and ethylene emission. Nei ther Ethrel, Co2+ nor Ag+ affected tracheid production when applied ba sally, except for 10 and 100 mu M Ethrel and 100 mu M Co2+, which were inhibitory. In contrast, ethylene evolution was promoted by Ethrel an d inhibited by Co2+, whereas Ag+ had no effect. Similarly, applying Et hrel apically or laterally increased ethylene evolution, but did not p romote tracheid production except in the treatment in which 1 mg Ethre l g(-1) lanolin was applied laterally to cuttings treated apically wit h 0.1 mg IAA g(-1) lanolin, and in the treatment in which 10 mg Ethrel g(-1) lanolin was applied laterally to budded cuttings. We conclude t hat (1) ethylene evolution is not specifically associated with IAA-ind uced tracheid production, (2) ethylene does not mimic the promoting ef fect of IAA on tracheid production, and (3) ethylene can promote trach eid production, but only when its application results in a localized u nphysiologically high concentration in the cambial region, which, in t urn, induces an accumulation of IAA.