THERMO-INHIBITION OF SEED GERMINATION AND PRE-GERMINATION SEED VERNALIZATION IN WINTER ANNUAL WEEDS

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1.Thermo-inhibition of seed germinationWhen the seeds sense that the temperature is too high to germinate, they enhance ABA biosynthesis and maintain high ABA contents to prevent their germination. If the temperature is much higher, the seeds increase their sensitivities to ABA, maintaining the enhanced ABA biosynthesis, but not affecting the bioactive GA_1 contents. Therefore, for reducing the endogenous ABA contents sufficiently to rescue seed germination from the severe thermoinhibition, the inhibition of ABA synthesis must be combined with the enhancement of ABA catabolism by exogenous GA. This temperature-sensory mechanism mediated by ABA will play a decisive role in the control of life cycles in winter annual weeds.2.Pre-germination seed vernalizationSeeds of facultative winter annuals germinate both in autumn and spring. When soil-buried seeds of a winter annual, Conyza canadensis, had been exposed to low temperatures in winter, plants from spring-germinating seeds showed early flowering traits. This phenomenon was defined as pre-germination seed vernalization (PGSV). Out of 103 winter annuals, 21 species expressed the characteristic of pre-germination seed vernalization, showing no induction of secondary seed dormancy by the chilling. We conclude that pre-germination seed vernalization is a key ecological factor regulating life cycle of facultative winter annuals, resulting in early flowering of the spring-germinated plants. Using subtractive screenings, thirty genes were isolated from the chilled seeds of C.canadensis.