A DIMERIC MECHANISM FOR THE ACTION OF PHYTOCHROME: EVIDENCE FROM PHOTOTHERMAL INTERACTIONS IN LETTUCE SEED GERMINATION*

Abstract

Abstract— Sensitization of the phytochrome‐mediated germination at 20°C of lettuce seeds (Lactuca sativa L. cv. Grand Rapids) by pretreatment at 4°C, 28°C, or on 1% ethanol, was studied. The 660 nm fiuence‐response characteristics were similarly biphasic for all sensitizing treatments and displayed responses at very low fluences (VLFR) as well as responses characteristic of non‐sensitized seeds at 10000‐fold higher, low fluences (LFR). Maximum VLFR increased with the duration of sensitizing treatments. However, the fluence ranges required for the two types of responses remained relatively constant. These and additonal responses of sensitized seeds to 730 nm fluences were compared to simulations of a mechanism involving a receptor, X , and based on the dimeric structure of phytochrome in which each monomer is independently phototransformed from the inactive (P r ) to the active (P fr ) form. The fluence requirements for phytochrome photoconversion in seeds were determined to be similar to those of purified Avena phytochrome in vitro, on which photochemical parameters for the simulations were based. The analyses suggest that P r :P fr ‐ X and P fr :P fr ‐ X are responsible, respectively, for the VLFR and the LFR, and that sensitization involves membrane influences on the activity of P r :P r ‐X. They also suggest the concentration of X to be about 0.001 that of total phytochrome dimer in this system.