Transient Effects of Light on Auxin Transport in the Avena Coleoptile

Abstract
The transport of IAA-1-14C through Avena coleoptile segments was studied by applying IAA apically and measuring the basal IAA export rate at 5-min. intervals for 85 min. A transport velocity of 15 to 26 mm per hr. was found. With 0.25 ppm IAA or less in the agar donor blocks, i.e. within the physiological range, the export rate reached a plateau by 30 min. Strong blue light starting at 35 min. reduced the total export for the whole period by 12-17%. The export rate was maximally depressed 25 min. after the onset of Illumination. The rate then rose, peaked 10 min. later, and again declined. This response closely matched the transient inhibition of growth by the same light dose. Above 0.25 ppm IAA, the IAA export rate showed no plateau, but peaked in 60 min. and declined rapidly. Blue light accelerated this decline without accompanying peaks or troughs. Red and far red light appeared to elicit none of these responses. The light growth response probably arises from transient withdrawal of auxin from the basipetal stream while the phototropic lateral transport is developing.