Phytochrome Action during Prechilling Induced Germination of Betula papyrifera Marsh

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Abstract
Seeds of paper birch (B. papyrifera Marsh.) were induced to germinate by prechilling at 3.degree. C or by red light. The light requirement was mediated by phytochrome and the action of phytochrome during prechilling was investigated. Red irradiation (R) prior to prechilling markedly enhanced the effectiveness of the prechilling treatment in inducing subsequent germination at 18.degree. C. Reversal of this enhancement by far-red irradiation (ER) was more effective when FR was supplied after a 1 wk prechill treatment than after a 2 wk treatment. The R enhancement effect exhibited a sharp drop as precilling temperature was increased from 5 to 7.degree. C. This decline is consistent with a membrane phase transition at about 7.degree. C where Pfr [phytochrome far-red] action is diminished by a loss in sensitivity of its receptor sites. Although phytochrome action was observed during prechilling treatments the seeds failed to germinate at prechilling temperatures. It was concluded that while potentiation of germination by Pfr occurred during prechilling, some other reaction(s) leading to radicle protrusion requires higher temperatures. In one seed source loss of germination potential was observed with protracted storage at 3.degree. C. This was prevented by R supplied during the prechilling treatment. The action of phytochrome during prechilling apparently is accentuated in these seeds by 2 factors: an increase in the sensitivity (or number) of Pfr receptor sites and preservation of Pfr by deferment of thermal reversion.