Auxin Transport within Intact Dormant and Active White Ash Shoots

Abstract

Transport of indoleacetic acid-1(14)C following application to the buds of intact white ash (Fraxinus americana L.) shoots proceeds at a velocity of about 1.3 centimeters per hour in actively growing seedlings, but only 0.3 centimeter per hour in dormant seedlings. The rapid movement is metabolically controlled, and at 1 C or in a nitrogen environment it is reduced to 0.2 centimeter per hour, suggesting that the slower movement is due to diffusion. The transport profile for growing shoots shows a logarithmic decrease in activity in stems treated for 3 hours. However, over longer treatment intervals, especially after 12 hours, a steady state of recoverable activity occurs in the more basal stem segments. Cold-treated shoots acquire the capacity for rapid transport 7 days after they are placed into favorable growing conditions, at which time dormancy callose disappears from the phloem, respiratory activity of the stem tissue increases, and mitotic reactivation occurs in the bud. Following shoot reactivation, the velocity and amount of exogenously supplied indoleacetic acid transported remained relatively uniform until the onset of the succeeding dormant period. Five per cent, or less, of the applied tracer moves into the shoot, with substantial portions remaining as indoleacetic acid.