Abscisic acid in the thermoinhibition of lettuce seed germination: enhancement of abscisic acid catabolism by exogenous gibberellin

Abstract

The thermoinhibition of lettuce seed germination at 28 C was prevented by an ABA biosynthesis inhibitor, fluridone. At 33 C, however, the combined application of fluridone and GA₃ was necessary to overcome the thermoinhibition. The exogenous GA₃ lowered endogenous ABA contents in the seeds, enhancing catabolism of ABA. Fluridone also decreased the ABA contents. Nevertheless, neither fluridone nor GA₃ alone induced the germination at 33 C, because sensitivity of the seeds to ABA was further increased at the elevated temperature. Consequently, the combined application of fluridone and GA₃ decreased the ABA contents to a sufficiently low level allowing the seeds to germinate at 33 C. We conclude that ABA regulates thermoinhibition of lettuce seed germination and that GA affects temperature responsiveness of the seeds through ABA metabolism.