Lag-phase and rate of synthesis in phytochrome-mediated induction of phenylalanine ammonia-lyase in mustard (Sinapis alba L.) cotyledons

Mustard seedlings were irradiated with continuous far-red light either with or without far-red preirradiation for between 6 and 44 h. The preirradiation increases the rate of light-mediated phenylalanine ammonia-lyase (EC 4.3.1.5) accumulation in the cotyledons up to 5-fold but does not eliminate the apparent lag-phase of induction which lasts between 30 and 60 min, both with and without a preirradiation. No differential effect of inhibitors presumed to be acting close to gene transcription was observed for plants, whether given a preirradiation or not. These results do not support the notion that two different induction mechanisms are involved in the action of phytochrome on phenylalanine ammonia-lyase synthesis during primary and secondary irradiation. There was however a strong promotive effect of preirradiation on P _fr-mediated enzyme accumulation which can be explained by an elevated level of the rate of enzyme synthesis left over from the previous light treatment and a transient increase in the capacity of the system to increase the rate of enzyme synthesis in response to active phytochrome.