Induction of acropetal 14C-photosynthate transport and radial growth by indole-3-acetic acid in Pinus sylvestris shoots

Abstract

The relationship between radial growth and assimilate movement was determined in one-year-old Pinus sylvestris (L.) cuttings collected during the dormant period and reactivated for 1-27 days under environmental conditions favorable for growth. The cuttings were either left with buds intact, defoliated along the distal 8 cm, or debudded and treated apically with 0 or 1 mg g(-1) indole-3-acetic acid (IAA) in lanolin. Radial growth was measured as tracheid production and bark radial width. The distribution of (14)C-photosynthate 1 or 5 days after exposure to (14)CO(2) was used to indicate assimilate movement. Debudding inhibited tracheid production and decreased acropetal (14)C-photosynthate transport, whereas applying IAA to debudded cuttings promoted both these processes and, in addition, induced vigorous callus growth within the bark immediately below the application point. Distal defoliation markedly increased the amount of (14)C-photosynthate transported toward the apex without altering how debudding and exogenous IAA affected tracheid production and the distribution of (14)C-photosynthate. The production of tracheids induced by apically applied IAA in debudded and distally defoliated cuttings increased progressively in material collected on September 9, October 14 and November 24, whereas bark radial width and acropetal (14)C-photosynthate movement increased only between the first two collection dates. Ringing midway between the IAA source and the bottom of the distally defoliated region with a lanolin mixture of 10 mg g(-1)N-1-naphthylphthalamic acid (NPA), 1 mg g(-1) methyl-2-chloro-9-hydroxy-fluorene-9-carboxylic acid (CF) or 10 mg g(-1) phenylacetic acid (PAA) reduced radial growth and [1-(14)C]IAA transport below the ring and locally promoted radial growth and accumulated radioactivity above. (14)C-Photosynthate transport into the region above the ring either was not altered (CF, PAA) or was increased (NPA). The results indicate that apically applied IAA induces the acropetal movement of (14)C-photosynthate in debudded P. sylvestris shoots by locally promoting activity correlated with tracheid and callus production rather than by affecting radial growth or phloem transport processes, or both, at a distance below the IAA source.