THE MECHANISM OF LIFE CYCLE REGULATION IN WINTER ANNUAL PLANTS VIA TEMPERATURE RESPONSEVENSS OF SEEDS

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Winter annual plants have adapted their life cycles to seasonally changing temperature. To know how they have evolved the life cycles we investigated the eco-physiological mechanisms controlling the germination timing, and proposed a model as follows.Winter annuals escape their growth in summer as soil-buried seeds. Germination of the seeds is inhibited during summer because the maximum temperatures for germination were just below the minimum temperatures in the soil. The seeds germinate in autumn when the temperature range for germination and those in the soil overlap. That is, the buried seeds detect appropriate germination timing by sensing the high temperatures.When seeds sense the temperature is too high to germinate, they enhance ABA biosynthesis and maintain a high ABA level to prevent their germination. Endogenous GA level is low, but it will play a positive role on ABA degradation. This situation, thermo-inhibition at the lower supraoptimal temperature, is overcome by inhibition of ABA synthesis. If the temperature is much higher, the seeds raise their sensitivities to ABA in addition to the enhanced ABA synthesis, but not affect the GA level. Therefore, for restoring the seed germination at the supraoptimal temperature, a combined action of the inhibition of ABA synthesis and the enhancement of ABA degradation is necessary.