Abscisic acid in the thermoinhibition of lettuce seed germination and enhancement of its catabolism by gibberellin

Germination of lettuce (Lactuca sativa L. cv. 'Grand Rapids') seeds was inhibited at high temperatures (thermoinhibition). Thermoinhibition at 28 degreesC was prevented by the application of fluridone, an inhibitor of abscisic acid (ABA) biosynthesis. At 33 degreesC, the sensitivity of the seeds to ABA increased, and fluridone on its own was no longer effective. However, a combined application of fluridone and gibberellic acid (GA(3)) was able to restore the germination. Exogenous GA(3) lowered endogenous ABA content in the seeds, enhancing catabolism of ABA and export of the catabolites from the intact seeds. The fluridone application also decreased the ABA content. Consequently, the combined application of fluridone and GA(3) decreased the ABA content to a sufficiently low level to allow germination at 33 degreesC. There was no significant temperature-dependent change in endogenous GA(1) contents. It is concluded that ABA is an important factor in the regulation of thermoinhibition of lettuce seed germination, and that GA affects the temperature responsiveness of the seeds through ABA metabolism.