Phytochrome in cucumber seeds.

Phytochrome was found by direct spectrophotometry to be present in whole dry seeds of cucumber. This pigment is spectroscopically different from the pigment found in etiolated plants. It shows the phenomenon of inverse reversion; in darkness, the red-absorbing form (Pr) reverts slowly to the far-red-absorbing form (Pfr). This may explain why 75% of the "dry-seed" phytochrome is in the Pfr form.After imbibition, total phytochrome in the seeds starts to increase. The newly-formed pigment is all in the Pr form and has properties similar to those of "classical" phytochrome of etiolated plants. The relationship of this newly-formed phytochrome with control of germination is presently not known. The dry-seed phytochrome remains unchanged during imbibition and appears to retain its capacity for inverse reversion. This may explain the requirement for continuous or intermittent far-red irradiation in the suppression of germination of cucumber seeds. A similar form of phytochrome may be responsible for control of germination in other seeds which are similarly affected by far-red radiation.