Abstract
We investigated the interaction between indole-3-acetic acid (IAA) and ethylene in the regulation of the seasonal periodicity of tracheid production 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 apically with IAA or 2-chloroethylphosphonic acid (Ethrel) in lanolin, laterally with IAA or Ethrel in lanolin, or basally with Ethrel, Co(2+) or Ag(+) in deionized water. The treated cuttings were then cultured for up to 5 weeks under controlled environment conditions favorable for cambial growth. No change in ethylene evolution was detected during the rest-quiescence transition, when IAA-induced tracheid production increased. The induction of cambial reactivation by IAA was associated with a rise in ethylene evolution, but there was no consistent relationship among IAA concentration, tracheid number and ethylene emission. Neither Ethrel, Co(2+) nor Ag(+) affected tracheid production when applied basally, except for 10 and 100 micro M Ethrel and 100 micro M Co(2+), which were inhibitory. In contrast, ethylene evolution was promoted by Ethrel and inhibited by Co(2+), whereas Ag(+) had no effect. Similarly, applying Ethrel apically or laterally increased ethylene evolution, but did not promote tracheid production except in the treatment in which 1 mg Ethrel g(-1) lanolin was applied laterally to cuttings treated apically with 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 that (1) ethylene evolution is not specifically associated with IAA-induced tracheid production, (2) ethylene does not mimic the promoting effect of IAA on tracheid production, and (3) ethylene can promote tracheid production, but only when its application results in a localized unphysiologically high concentration in the cambial region, which, in turn, induces an accumulation of IAA.