Effects on Phytochrome Controlled Germination Produced by Far-Red Irradiation of Seeds Before and During Rehydration

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Abstract
Seeds irradiated with red light and then re-dried will respond to this light treatment on subsequent rehydration in the dark. If such high-Pfr seeds are irradiated in the dry state with far-red light immediately before rehydration the percentage germination is significantly reduced in the case of Plantago major and Sinapis arvensis but increased in Bromus sterilis where Pfr inhibits germination. This effect of far-red light can be reversed by red light despite the fact that red light alone has no effect on dry seed. This is due to the interconversion of Pfr and the red light absorbing phytochrome intermediate complex meta-Fa. If there is a delay between far-red irradiation and rehydration of Sinapis seeds, the inhibitory effect of the far-red irradiation becomes progressively less the longer the delay. This reduction in effectiveness of far-red is interpreted in terms of a dark reversal of meta-Fa to Pfr with a half-life of about 4–6 h. The reappearance of Pfr is either very slow or docs not occur in dehydrated Plantago seeds, as far-red tight given 96 h prior to hydration is just as inhibitory as far-red light given immediately before hydration. Meta-Fa does appear to revert to Pfr in darkness in Bromus seeds, but only very slowly. The rapid increase in effectiveness of red irradiation during rehydration of high-Pfr Plantago seeds suggests that, in this species, the pre-treatment used in preparation of high-Pfr seeds may increase the receptivity or amount of the Pfr reaction partner.