Response of Pea Stem Tissues With Different Phytochrome Contents to Red Light Dosage

Abstract

Partially de-etiolated Pisum stem segment tissues with a 4-fold difference in spectrophotometrically measurable phytochrome react in the same way to the same percentage conversion of phytochrome to the PFR form (1). These results are consistent with the theory that the amount of physiologically active phytochrome is so low that it is spectrophotometrically unmeasurable, while the bulk of the phytochrome is physiologically inactive and quantitatively variable. If this theory is correct, it is nevertheless possible that the bulk form functions as an accessory pigment by absorbing light and transferring the energy to the active centers which are then aible to cause a physiological response, However, the results cited above were not done in a manner that would detect such an effect if it were present. There are 2 methods of establishing various percentages of PFR: in the photostationary state technique, sources of mixed red and far-red light are given to saturation, and the final pigment ratio is dependent on the quality of the light mixture. The alternative (red dosage) method employs very short exposures to pure red or far-red light so that the amount of pigment converted is limited by the incident energy. The latter method is generally much less convenient, and it has been shown that with etiolated Pisum and Avena tissues,