Aspects of phytochrome decay in etiolated seedlings under continuous Illumination

The rate of total phytochrome decay in the dicotyledons Amaranthus caudatus, Mirabilis jalapa and Pisum sativum under continuous illumination with red, incandescent, and blue light depends on the PFR/Ptotal maintained by each source. Amaranthus is an exception to this in that there is a deviation from firstorder decay kinetics under continuous illumination with incancdescent light. This deviation is probably not related to the chlorophyll present in the Amaranthus sample since chlorophyll-rich Pisum buds have the same phytochrome decay rate as epicotyl tissue under continuous incandescent light. Reports of a prolonged lag phase before the onset of first-order decay kinetics of phytochrome in Pisum have not been confirmed and the small lag phase observed in the present work can be accounted for by the time required to attain the PFR/Ptotal ratio characteristic of blue light in a carotenoid rich tissue. In the monocotyledon, Avena sativa, and perhaps monocotyledons in general, decay rate is maximal at a low PFR concentration and the decay curve is the same under continuous red, incandescent and blue light. This dicotyledon/monocotyledon difference with respect to saturation of phytochrome decay does not correlate with the other dicotyledon/monocotyledon difference, the presence or absence of dark reverions of PFR to PR, since the dicotyledons Amaranthus and Mirabilis that lack reversion still show no saturation of decay. Possible growth control by the PFR/Ptotal ratio is discussed in relation to environmental changes in light quality.