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PLANT PHYSIOLOGY , Vol 101, Issue 2 607-617, Copyright © 1993 by American Society of Plant Biologists
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DEVELOPMENT AND GROWTH REGULATION |
Germination and Dormancy of Abscisic Acid- and Gibberellin-Deficient Mutant Tomato (Lycopersicon esculentum) Seeds (Sensitivity of Germination to Abscisic Acid, Gibberellin, and Water Potential)
B. R. Ni and K. J. Bradford
Department of Vegetable Crops, University of California, Davis, California 95616-8631
Germination responses of wild-type (MM), abscisic acid (ABA)-deficient
(sitw), 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, sitw 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 ([psi]b,
log[ABAb]) or promote (log[GAb]) 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 sitw 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 [mu]M) 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 sitw 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.
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