GERMINATION AND DORMANCY OF ABSCISIC ACID-DEFICIENT AND GIBBERELLIN-DEFICIENT MUTANT TOMATO (LYCOPERSICON-ESCULENTUM) SEEDS - SENSITIVITY OF GERMINATION TO ABSCISIC-ACID, GIBBERELLIN, AND WATER POTENTIAL

Germination responses of wild-type (MM), abscisic acid (ABA)-deficient (sit(w)), and gibberellin (GA)-deficient (gib-1) mutant tomato (Lycopersicon esculentum Mill. cv Moneymaker) seeds to ABA, GA4+7, reduced water potential (psi), and their combinations were analyzed using a population-based threshold model (B.R. Ni and K.J. Bradford [1992] Plant Physiol 98: 1057-1068). Among the three genotypes, sit(w) seeds germinated rapidly and completely in water, MM seeds germinated more slowly and were partially dormant, and gib-1 seeds did not germinate without exogenous GA4+7. Times to germination were inversely proportional to the differences between the external osmoticum, ABA, or GA4+7 concentrations and the corresponding threshold levels that would either prevent (phi(b), log[ABA(b)]) or promote (log[GA(b)]) germination. The sensitivity of germination to ABA, GA4+7, and psi varied widely among individual seeds in the population, resulting in a distribution of germination times. The rapid germination rate of sit(w) seeds was attributable to their low mean psi(b) (-1.17 MPa). Postharvest dormancy in MM seeds was due to a high mean psi(b) (-0.35 MPa) and a distribution Of psi(b) among seeds such that some seeds were unable to germinate even on water. GA4+7 (100 muM) stimulated germination of MM and gib-1 seeds by lowering the mean psi(b) to -0.75 MPa, whereas ABA inhibited germination of MM and sit(w) seeds by increasing the mean psi(b). The changes in psi(b) were not due to changes in embryo osmotic potential. Rather, hormonal effects on endosperm weakening opposite the radicle tip apparently determine the threshold psi for germination. The analysis demonstrates that ABA- and GA-dependent changes in seed dormancy and germination rates, whether due to endogenous or exogenous growth regulators, are based primarily upon corresponding shifts in the psi thresholds for radicle emergence. The psi thresholds, in turn, determine both the rate and final extent of germination within the seed population.