Abstract
Seedlings of cosmos (Cosmos bipinnatus Cay. 'Radiance') were irradiated at 25 °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 germicidal 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-band fluorescent lamp. Seedlings irradiated by a germicidal lamp died after irradiation. Decreasing the distance between the fluorescent sun lamp and the plug flats inhibited hypocotyl elongation. Seedlings irradiated by a fluorescent sun lamp gained less dry matter, compared to seedlings irradiated by a three-band fluorescent lamp, but this suppression was reversed by the supplemental irradiation of a three-band fluorescent lamp with a fluorescent sun lamp. Exposure of seedlings to UV-B provides a useful means of preventing excessive hypocotyl elongation and improves the quality of plug seedlings.
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