Transient effects of light on auxin transport in the Avena coleoptile.

The transport of indole-3-acetic acid-1-(14)C (IAA) through 4 mm segments of etiolated Avena coleoptiles was studied as a function of time by applying IAA in apical agar blocks and measuring the basal IAA export rate at 5-minute intervals. The transport velocity found in this way is at least 15 mm/hour at 26 degrees . Following a 30-minute equilibration period, the export rate is nearly constant for at least 50 minutes at physiological donor concentrations. Exposure to about 3 x 10(5) ergs/cm(2) blue light for 15 minutes leads to a transient reduction in the export. The export rate reaches a minimum about 25 minutes after the onset of illumination, then rises to reach a maximum by 35 minutes, and subsequently declines again. The result is a net export depression during the first 80 minutes, amounting to some 12 to 17%. Its timing closely matches the timing of the light growth response elicited by the same light dosage.At higher IAA concentrations (0.5 and 1.8 mg/l), the export rate reaches a peak about 60 minutes after the initial application of auxin, and thereafter declines rapidly. Light increases this decline in export rate, without causing peaks and troughs, and even at 0.25 mg/l IAA the transient changes in export appear to be superimposed on a gradual decline in export rate after illumination. Blue light is effective in all these phenomena; the red far-red system appears to exert no effect. The results are discussed in relation to the mechanism of action of light both in the light growth response and in phototropism.