Effect of chilling on the gravitropic response of 'Poinsett 76' cucumber (Cucumis sativus L.) radicles.

Germinating 'Poinsett 76' cucumber (Cucumis sativus) seeds are chilling sensitive, and subsequent radicle elongation is inhibited by exposure to nonfreezing temperatures below 10 degrees C. Reorienting germinated seeds with 5-mm-long radicles from a vertical to a horizontal position induced gravitropic curvature within 2 hours at 10 to 25 degrees C without significantly affecting the rate of radicle elongation. However, neither elongation nor curvature occurred in similar seeds held at 2.5 or 5 degrees C. Chilling seeds with 5-mm-long radicles at 2.5 degrees C for 18 hours significantly reduced the subsequent rate of radicle elongation at 25 degrees C by 47% compared with nonchilled control, while gravitropic curvature was reduced by only 34%. After 36 hours of recovery at 25 degrees C, the difference was even more pronounced; elongation was reduced by 26% while curvature was reduced by only 6%. Clearly, gravitropic curvature was less chilling sensitive than radicle elongation, despite the fact that differential elongation of cells in the radicle are needed to produce curvature. Exposing seeds with 5-mm-long radicles to a heat shock of 45 degrees C for 4 to 10 minutes significantly diminished the chilling-induced reduction in radicle elongation and gravitropic curvature.