Tracheid Production in Response to Changes in the Internal Level of Indole-3-Acetic Acid in 1-Year-Old Shoots of Scots Pine

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Abstract
Different concentrations of indole-3-acetic acid (IAA) were applied in lanolin to 1-year-old shoots of Pinus sylvestris (L.) in a manner known to stimulate cambial activity. The internal concentration of free IAA was measured at a distance below the application point by combined gas chromatography-selected ion monitoring-mass spectrometry using [13C6]IAA as a quantitative internal standard, and related to the production of tracheids at the same site. The experiment was performed with: (a) debudded cuttings, where the major source of endogenous IAA, the apical buds, were replaced with exogenous IAA, and (b) intact, attached shoots, where endogenous IAA was supplemented by applying IAA around the circumference of the shoot. In both experimental systems, an increase in the internal IAA level was positively related to increased tracheid production. It was also demonstrated that the concentration of internal IAA measured at the sampling site was comparable with endogenous IAA levels found in intact control shoots, and that a wide range of applied IAA concentrations was associated with a relatively small range of internal IAA levels.