Differences in Photoresponse and Phytochrome Spectrophotometry Between Etiolated and De-etiolated Pea Stem Tissue

Morphologically similar pea plants having a 4-fold difference in spectrophoto-metrically detectable phytochrome can be produced by pretreatment of etiolated plants with red light (R) or with red and far-red light combined (RF). A search for response differences which could be ascribed to differences in phytochrome content has resulted only in the establishment of differences due to de-etiolation. Segments of etiolated plants differ from those of plants de-etiolated by R and RF pretreatments in 2 ways. Segments from etiolated plants appear to respond rapidly to the far-red absorbing form of phytochrome (P(FR)), while segments from de-etiolated plants do not respond rapidly to P(FR). This statement is based upon 2 observations: (i) the red light induced growth inhibition in segments from etiolated plants rapidly escapes reversibility by far-red light, while with segments from R or RF pretreated plants, the red light effect is fully reversed by subsequent far-red light for up to 2 hr; and (ii) segments from etiolated plants were inhibited to a greater degree than were segments from RF pretreated plants when various photostationary state levels of P(FR) were maintained for 30 or 90 min and then removed by photoconversion to P(R). The in vivo nonphotochemical transformation curves of the phytochrome of etiolated and RF pretreated plants appear to differ in 2 related respects: (i) the amount of phytochrome destroyed in de-etiolated tissue is greater than that in etiolated tissue, perhaps as a result of the fact that (ii) the rate and extent of apparent reversion of P(FR) to P(R) in etiolated tissue is about twice that in de-etiolated tissue.