INTERNODE LENGTH IN PISUM GIBBERELLINS AND THE RESPONSE TO FAR-RED-RICH LIGHT

The hypothesis that photoperiod extensions with far-red-rich light promote stem elongation by increasing gibberellin A1 (GA1) levels is examined. GA1 and GA20 levels were measured by gas chromatography-selected ion monitoring (GC-SIM) using di-deuterated internal standards. The present results from both tall (Le) and dwarf (le5839 peas (Pisum sativum L.) do not support this hypothesis, since GA1 levels do not correlate with the elongation observed after photoperiod extensions. However, dwarf (le5839) plants possessed substantially reduced GA1 levels and elevated GA20 levels compared with isogenic tall (Le) plants. The response to a photoperiod extension with incandescent light requires the presence of GA, since reduction of GA levels by either genetic (genes na and lh) or chemical (PP333) means reduced the response. The response in na plants may be restored by GA1 application. Furthermore, in na plants a far-red-rich photoperiod extension increased the response to applied GA1. However, the metabolism of [3H]GA1 was not modified by a photoperiod extension with incandescent light. These results suggest that a photoperiod extension with far-red-rich light may enhance elongation by increasing the ability of the plant to respond to endogenous GA1. © 1990, Gustav Fischer Verlag, Stuttgart. All rights reserved.