Response of tissue with different phytochrome contents to various initial photostationary States.

Pretreatment of etiolated pea plants with red light and with red combined with far-red light produced morphologically similar plants having 4-fold differences in spectrophotometrically detectable phytochrome. Stem segments from the variously pretreated plants respond in the same way to different percentage conversions of phytochrome to P(FR). These results suggest that the P(FR)./P(R) ratio, rather than the concentration of P(FR), governs pea stem segment elongation. However, the ratio hypothesis does not explain contradictions between spectrophotometric and physiological assays previously obtained with this tissue, nor does it explain similar contradictions obtained in other systems. The only hypothesis consistent with the data to date is that of the existence of bulk and active phytochrome fractions, with the latter present in insufficient quantities to be spectrophotometrically detectable.