Photoinduction of phenylalanine deaminase in gherkin seedlings : III. Effects of excision and irradiation on enzyme development in hypocotyl segments.

1. If hypocotyl segments from gherkin seedlings are floated on water the level of the enzyme phenylalanine deaminase (PADAse) increases for about 30 hours (25°) and then remains constant. The pattern of these changes differs from that of the photoinduced changes in the PADAse level in intact seedlings, where an initial increase in enzyme level is followed by a decline that starts at 25° about 3 hours after the beginning of irradiation. 2. The development of PADAse in the segments is inhibited by cycloheximide and puromycin, inhibitors of protein synthesis, and by the end products of its reaction, namely, cinnamic acid and p-coumaric acid. Repression due to the accumulation of p-coumaric acid is the likely reason why the irradiation gradually loses its effect on the induction of PADAse in intact seedlings whereas the derepressing effect of excision could be explained by the disappearance of p-coumaric acid from the segments. 3. Application of cinnamic acid or p-coumaric acid to the segments 16 hours after excision results in a drop in the PADAse level. Cycloheximide prevents this decline. These findings indicate the induction of a PADAse inactivating system requiring de novo protein synthesis, probably the same system found to be responsible for the drop in enzyme level in intact seedlings. 4. Irradiation of the segments with blue light stimulates the PADAse synthesis, and so does the addition of glutathione to the incubation medium. These agents must have different sites of action, since irradiation antagonizes the repressing effect of cinnamic acid, whereas glutathione does not.