The application, in the middle of April, of a mixture of IAA (10−4 M) and GA3 (10−4 M), to apical stem of young beeches, disbudded more than 1 month ago, brings about the formation of an unusual early wood, composed of small xylary elements only, on a short distance. If the phytohormones are applied immediately on the following day after disbudding, the young trees produce normal early wood, on a long distance. GA3 applied alone has hardly any effect on cambial reactivation; IAA applied alone stimulates less cambial reactivation and xylogenesis than the mixture of IAA and GA3. The study of [3H]-IAA transport in apical shoots of the same trees shows that beeches disbudded 1 month ago have lost the ability to activate the auxin transport. Disbudding, carried out in the middle or at the end of the preactivation phase, affects the IAA transport system within a few days; a further exogenous phytohormone application cannot then replace buds and promote the renewal of cambial activity. In young beeches, the resumption of tissue ability to polarize the auxin transport, which in February characterizes the beginning of the preactivation phase, depends on buds; the maintenance and development of this ability are related to their presence.
Photoreceptors Regulate Plant Developmental Plasticity through Auxin
