Effects of light and temperature on gibberellin (GA(-induced seed germination were studied in Arabidopsis thaliana (L.) Heynh. with the use of GA-deficient (gal) mutants, mutants with a strongly reduced sensitivity to GA (gai) and with the recombinant gai/gal. Seeds of the gal mutant did not germinate in the absence of exogenous GAs, neither in darkness, nor in light, indicating that GAs are absolutely required for germination of this species. Wild-type and gai seeds did not always require applied GAs in light. The conclusion that light stimulates GA biosynthesis was strengthened by the antagonistic action of tetcyclacis, an inhibitor of GA biosynthesis. In wild-type, gal and gai/gal seeds light lowered the GA requirement, which can be interpreted as an increase in sensitivity to GAs. In gai and gai/gal seeds light became effective only after dormancy was broken by either a chilling treatment of one week or a dry after-ripening period at 2-degrees-C during some months. The present genetic and physiological evidence strongly suggests that temperature regulates the responsiveness to light in A. thaliana seeds. The responsiveness increases during dormancy breaking, whereas the opposite occurs during induction of dormancy (8 days at 15-degrees-C pre-incubation). Since light stimulates the synthesis of GAs as well as the responsiveness to GAs, temperature-induced changes in dormancy may indirectly change the capacities to synthesize GAs and to respond to GAs. GA sensitivity is also directly controlled by temperature. It is concluded that both GA biosynthesis and sensitivity to GAs are not the primary controlling factors in dormancy, but are essential for germination.
