Ethylene production and enzyme induction in excised plant tissues.

Excising plant tissues has been found to increase the activity of phenylalanine ammonia-lyase (EC 4.1.3.5) (1, 4, 11, 12). This increase is probably due to de novo synthesis of this enzyme, since inhibitors of protein synthesis block the rise of PAL' activity (4, 11, 12). We suggested earlier (4) that the increase of PAL in gherkin hypocotyl segments was due to the release into the medium of reaction products (hydroxycinnamic acids) which by a feedback mechanism caused repression of PAL synthesis. Other hypotheses that have been advanced include the release of proteinaceous inhibitors of enzyme synthesis (2) and ethylene, which is produced in response to tissue wounding (7, 10). Support for the latter hypothesis stems from the work by Imaseki et al. (7) with sliced sweet potato roots and by Riov et al. (10) who worked with flavedo tissue excised from mature grapefruits. This paper presents data which indicate that there was no correlation between ethylene production and PAL development in gherkin hypocotyl segments, and that ethylene did not increase PAL activity in intact hypocotyl tissue but did so in excised tissue.