RETARDATION OF EXCESSIVE ELONGATION OF CO SMOS PLUGS BY ULTRAVIOLET-IRRADIATION

Seedlings of cosmos (Cosmos bipinnalus Cav. 'Radiance') were irradiate d at 25 degrees C for 72 hr by a blacklight blue lamp (main radiation peak at 352 nm; UV-A), a fluorescent sun lamp (310 nm; UV-B) or a germ icidal lamp (254 nm; UV-C), which were placed 60 cm above plug flats. Irradiation by a fluorescent sun lamp and a germicidal lamp inhibited hypocotyl elongation compared to the control irradiated by a three-ban d fluorescent lamp. Seedlings irradiated by a germicidal lamp died aft er irradiation. Decreasing the distance between the fluorescent sun la mp and the plug flats inhibited hypocotyl elongation. Seedlings irradi ated by a fluorescent sun lamp gained less dry matter, compared to see dlings irradiated by a three-band fluorescent lamp, but this suppressi on was reversed by the supplemental irradiation of a three-band fluore scent lamp with a fluorescent sun lamp. Exposure of seedlings to UV-B provides a useful means of preventing excessive hypocotyl elongation a nd improves the quality of plug seedlings.