Timing of the initial action of phytochrome with regard to protochlorophyll synthesis in the mustard seedling.

Significant accumulation of photoconvertible protochlorophyll(ide) in the cotyledons of the mustard seedling takes place from 24 h after sowing onwards (25° C). The rate of accumulation in darkness is greatly increased by a pretreatment with red or far-red light. The strong effect of continuous red light, given from the time of sowing, remains fully reversible by a 756 nm-light pulse up to about 18 h after sowing. On the other hand, the effect of continuous far-red light which can be detected at 15 h after sowing is not influenced by a subsequent application of 756 nm-light pulses. An interpretation of the data requires the concept that continuous red light and continuous far-red light act from different sites. This conclusion is based on a comparison of the present data with the earlier published data on phytochromemediated anthocyanin synthesis in the mustard seedling cotyledons.