Rehydration of Phytochrome in Imbibing Seeds of Amaranthus retroflexus L

It is commonly recognized that light-sensitive seeds require a period of imbibition before full promotion of germination by a light stimulus can be attained. Yet little work describes the nature of these preliminary events. Of the several major limitations that may be involved, clearly the rehydration (or activation) of phytochrome and the system with which it interacts are the chief ones to consider. The object of this communication is to describe the rehydration of inactive phytochrome (Pr) in Amaranthus retroflexus L. seeds as determined by a method by which phytochrome rehydration can be physiologically separated from other events occurring during dark imbibition. Recently, Tobin and Briggs (1 1) presented evidence for rapid rehydration of phytochrome in isolated embryos of Pinus palustris Mill. Using spectrophotometric methods, they detected low levels of phytochrome in dry seeds and much larger amounts 2 min after the addition of water. However, physiological changes in germination connected with the phytochrome system were not displayed until seeds had imbibed for approximately 3 hr. Nyman (6) found that irradiation of dry seeds of Pinus sylvestris L. with red light could give subsequent promotion of germination. Recent studies by several authors using spectrophotometry traced the appearance and increase of phytochrome levels in various seeds (1, 4, 5, 7, 8). Some found phytochrome only in imbibed seeds (4, 5), while others could detect the pigment in dry seeds as well (1, 8). Kendrick et al. (4) found that water uptake of Amaranthus caudatus L. seeds followed the same pattern as the initial increases in phytochrome, but McArthur and Briggs (5) presented evidence that appearance of phytochrome in peas was not strictly correlated with water uptake. Most of these reports do not attempt to associate the early appearance of phytochrome in imbibing seeds with any physiological role. In a previous paper (9), we presented some evidence that rehydration of the inactive and active (Pfr) forms of phytochrome could be detected physiologically during prechilling of A. retroflexus seeds. This paper supports and extends these findings.