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

1.种子萌发的热抑制当种子感觉到温度太高而不能萌发时，它们会促进ABA的生物合成，并保持较高的ABA含量，从而阻止其萌发。温度越高，种子对ABA的敏感性越高，维持ABA生物合成的增强，但不影响生物活性GA_1的含量。因此，为了充分降低内源ABA的含量，使种子萌发免于严重的热抑制，必须将抑制ABA合成与促进外源GA促进ABA分解代谢结合起来。这种由ABA介导的温敏机制将在冬季一年生杂草的生活史控制中起决定性作用。2.萌发前种子春化兼性的冬季一年生植物种子在秋季和春季都能萌发。当冬季一年生草本植物加拿大斑茅的种子被土壤埋藏后暴露在冬季低温下时，从春季萌发的种子中获得的植株表现出早花特性。这种现象称为萌发前种子春化(PGSV)。在103种一年生越冬植物中，有21种表现出萌发前种子春化的特征，没有表现出低温诱导种子二次休眠的现象。我们认为，萌发前种子春化是调节兼性冬季一年生植物生活史的关键生态因素，导致春季萌发的植物提前开花。通过差减筛选，从冷藏加拿大豆科植物种子中分离出30个基因。

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.