Second positive phototropic response patterns of the oat coleoptile

1.During second positive irradiation, bending increases steadily with time. Under optimal conditions, the lag between onset of illumination and beginning of parabolic bending behavior is about 3 min. - 2. Shortly after irradiation ceases, bending becomes linear with time. On a clinostat, bending continues for about 2.5 hr. Auxanometric measurements show that the ultimate cessation of bending is not due to failing growth rate. - 3. The second positive response shows a striking dependence on intensity of irradiation. Inactivation occurs when irradiation approaches the intensity of full daylight. - 4. Induction is linear with duration of illumination, both at purely activating intensities and at partially inactivating intensities. - 5. Induction at 2°, while somewhat slower than at 25°, retains linear dependence on exposure duration. This suggests that the reactions immediately following light reception are slowed but not stopped at low temperature. - 6. Growth, which drops to about 0.5 μ/min at 2°, resumes at about 18 μ min(-1) as soon as plants are warmed to 25°. Curvature does not seem to begin for about 10 min. Combined with information about lag time for primary auxin action, this suggests that lateral auxin transport, as well as growth, is strongly inhibited at near-freezing temperatures. - 7. The induced transport system is highly stable at 2°. - 8. Under optimal conditions, the lag between onset of irradiation and induction of capacity to produce measurable curvature is only a few seconds. The length of the lag is dependent on the rate of induction. The lag is thought to be due to the requirement that enough induction be accumulated to overcome resistance of the coleoptile. - 9. Induction is dependent on the gradient of light across the coleoptile, whether measured for purely activating or partially inactivating intensities. The light received is probably integrated either across individual cells or across the entire width of the coleoptile.